Journal articles: 'Refractivity gradient'

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Constance, Kalu, Idorenyin Markson, and Enyenihi, Henry Johnson. "Determination of Refractivity Gradient and Modified Refractivity Gradient for Cross River State." International Journal of Sustainable Energy and Environmental Research 7, no. 2 (2018): 53–64. http://dx.doi.org/10.18488/journal.13.2018.72.53.64.

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Emmanuel, I., K. D. Adedayo, B. Adeyemi, and O. S. Ojo. "Meteorological parameter anomalies and anomalous radio propagation over Nigeria." Nigeria Journal of Pure and Applied Physics 9, no. 1 (2020): 34–50. http://dx.doi.org/10.4314/njpap.v9i1.7.

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Distribution and variation of anomalous radio propagation, temperature and relative humidity anomalies were obtained and analyzed using six years (2010-2015) Era interim data. Seasonal spatial distribution of refractivity gradient and its components were analyzed. The result showed that about 90% of wet component contributed to the variation of refractivity gradient. Highest range of refractivity gradients and its wet components were obtained during the wet season. The result of correlation between ducting occurrence and temperature showed strong negative correlation except in Lagos where positive correlation of 3% was observed. However, positive correlation which ranges between 39% and 70% exist between duct occurrence and relative humidity except in Lagos in Nigeria. Refractivity gradients, relative humidity anomaly and temperature experience a monthly variation. These variations can be attributed to the seasonal movement of inter-tropical discontinuity (ITD) across Nigeria. Keywords: anomalous, anomaly, correlation, ITD

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Louf, Valentin, Olivier Pujol, and Henri Sauvageot. "The Seasonal and Diurnal Cycles of Refractivity and Anomalous Propagation in the Sahelian Area from Microwave Radiometric Profiling." Journal of Atmospheric and Oceanic Technology 33, no. 10 (2016): 2095–112. http://dx.doi.org/10.1175/jtech-d-14-00208.1.

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AbstractThe Sahelian zone of West Africa is a semiarid area where strong amplitude of the seasonal and diurnal cycles of water vapor and temperature is observed. One year of continuous observation of vertical profiles of water vapor and temperature gathered from Niamey, Niger, with a profiling microwave radiometer is used to analyze the climatology of refractivity and microwave propagation regimes in the low troposphere. Seasonal and diurnal cycles of refractivity and ground-based radar anomalous propagation are emphasized. It is shown that the combined effect of water vapor and temperature vertical gradients is responsible for strong seasonal and diurnal cycles of the ducting propagation regime. Statistics of propagation regimes are given. The probability density functions of the refractivity gradient are found lognormally distributed. Three months of C-band radar data simultaneous with the profiling microwave radiometer observations have also been collected. Relations between the vertical refractivity gradient and the ground-based radar anomalous propagation echoes (APE) are illustrated and discussed. APE spatial distributions are found strongly related to the main features of the orography and topography inside the radar-observed area. Contingency tests show that the probability for APE to be linked to ducting is higher than 95%. In addition, this paper suggests that observing the refractivity vertical profiles from a microwave radiometer profiler located close to a meteorological radar provides information on whether anomalous propagation has to be considered as a potential cause of spurious signal in the measured reflectivity field.

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Xie, F., D. L. Wu, C. O. Ao, A. J. Mannucci, and E. R. Kursinski. "Advances and limitations of atmospheric boundary layer observations with GPS occultation over Southeast Pacific Ocean." Atmospheric Chemistry and Physics Discussions 11, no. 8 (2011): 22857–91. http://dx.doi.org/10.5194/acpd-11-22857-2011.

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Abstract. The typical atmospheric boundary layer (ABL) over the southeast (SE) Pacific Ocean is featured with a strong temperature inversion and a sharp moisture gradient across the ABL top. The strong moisture and temperature gradients result in a sharp refractivity gradient that can be precisely detected by the Global Positioning System (GPS) radio occultation (RO) measurements. In this paper, the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) GPS RO soundings, radiosondes and the high-resolution ECMWF analysis over the SE Pacific are analyzed. COSMIC RO is able to detect a wide range of ABL height variations (1–2 km) as observed from the radiosondes. Whereas, the ECMWF analyses systematically underestimate ABL heights. The sharp refractivity gradient at the ABL top frequently exceeds the critical refraction (e.g., −157 N-unit km−1) and becomes the so-called ducting condition, which results in systematic RO refractivity bias (or called N-bias) inside the ABL. Simulation study using refractivity profiles based on radiosondes reveals that the N-biases are significant and the magnitudes of biases are vertical resolution dependent. The N-bias is also the primary cause of the systematically smaller refractivity gradient (rarely exceeding −110 N-unit km−1) at the ABL top from RO measurement. However, the N-bias seems not affect the ABL height detection. Instead, the very sharp refractivity gradient and the large RO bending angle due to ducting allow reliable detection of ABL height from GPS RO. The seasonal mean climatology of ABL heights derived from a nine-month composite of COSMIC RO soundings over the SE Pacific reveals significant differences from the ECMWF analysis. Both show the deepening of ABL height from the shallow stratocumulus near the coast to a much higher trade wind inversion further off the coast. However, COSMIC RO shows systematically higher ABL heights overall and reveals different locations of the minimum and maximum ABL heights as compared to the ECMWF analysis. The significantly decreasing number of COSMIC RO soundings at lower latitudes along with the lower percentage of RO soundings penetrating into the lowest 500 m above mean-sea-level (a.m.s.l.), result in generally small sampling errors in the mean ABL climatology and will not affect the morphology of RO ABL height climatology. The difference of ABL height climatology between COSMIC RO and ECMWF analysis over SE Pacific is significant and requires further studies.

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Ukhurebor, Kingsley Eghonghon, and Wilson Nwankwo. "Estimation of the refractivity gradient from measured essential climate variables in Iyamho-Auchi, Edo State, South-South Region of Nigeria." Indonesian Journal of Electrical Engineering and Computer Science 19, no. 1 (2020): 276. http://dx.doi.org/10.11591/ijeecs.v19.i1.pp276-284.

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<p>Meteorological variables are crucial constituents in the estimation of refractivity disseminations and the uncharacteristic radio wave propagation situations of the troposphere as a result of their impact on radio wave communication relations over the atmosphere. In this study the measurement and assessment of air temperature, relative humidity and atmospheric pressure was carried out for a period of one year; 2018, so as to estimate the refractivity gradient over Iyamho-Auchi, Edo State, Nigeria using a self-implemented inexpensive portable meteorological monitoring device. The measurements of the essential climate variables were done at the administrative building of Edo University Iyamho by placing the meteorological monitoring device on a fixed position. The results show that the monthly estimated refractivity gradient values which would be useful in the prediction of the local radio propagation range from -20.00 N-units/km to -190.00 N-units/km with an average value of -60.67 N-units/km for the period under consideration. The findings also show that the months with limited relative humidity have greater refractivity gradient values compared to the ones with higher relative humidity. It was also observed from the results that the measured essential climate variables were having significant impacts on the estimated refractivity gradient during all the months in 2018, and these impacts were more noticeable in the months with higher relative humidity compared with the months that were having limited relative humidity. </p>

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Xie, F., D. L. Wu, C. O. Ao, A. J. Mannucci, and E. R. Kursinski. "Advances and limitations of atmospheric boundary layer observations with GPS occultation over southeast Pacific Ocean." Atmospheric Chemistry and Physics 12, no. 2 (2012): 903–18. http://dx.doi.org/10.5194/acp-12-903-2012.

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Abstract. The typical atmospheric boundary layer (ABL) over the southeast (SE) Pacific Ocean is featured with a strong temperature inversion and a sharp moisture gradient across the ABL top. The strong moisture and temperature gradients result in a sharp refractivity gradient that can be precisely detected by the Global Positioning System (GPS) radio occultation (RO) measurements. In this paper, the Constellation Observing System for Meteorology, Ionosphere &amp; Climate (COSMIC) GPS RO soundings, radiosondes and the high-resolution ECMWF analysis over the SE Pacific are analyzed. COSMIC RO is able to detect a wide range of ABL height variations (1–2 km) as observed from the radiosondes. However, the ECMWF analysis systematically underestimates the ABL heights. The sharp refractivity gradient at the ABL top frequently exceeds the critical refraction (e.g., −157 N-unit km−1) and becomes the so-called ducting condition, which results in a systematic RO refractivity bias (or called N-bias) inside the ABL. Simulation study based on radiosonde profiles reveals the magnitudes of the N-biases are vertical resolution dependent. The $N$-bias is also the primary cause of the systematically smaller refractivity gradient (rarely exceeding −110 N-unit km−1) at the ABL top from RO measurement. However, the N-bias seems not affect the ABL height detection. Instead, the very large RO bending angle and the sharp refractivity gradient due to ducting allow reliable detection of the ABL height from GPS RO. The seasonal mean climatology of ABL heights derived from a nine-month composite of COSMIC RO soundings over the SE Pacific reveals significant differences from the ECMWF analysis. Both show an increase of ABL height from the shallow stratocumulus near the coast to a much higher trade wind inversion further off the coast. However, COSMIC RO shows an overall deeper ABL and reveals different locations of the minimum and maximum ABL heights as compared to the ECMWF analysis. At low latitudes, despite the decreasing number of COSMIC RO soundings and the lower percentage of soundings that penetrate into the lowest 500-m above the mean-sea-level, there are small sampling errors in the mean ABL height climatology. The difference of ABL height climatology between COSMIC RO and ECMWF analysis over SE Pacific is significant and requires further studies.

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Bodine, David, Dan Michaud, Robert D. Palmer, et al. "Understanding Radar Refractivity: Sources of Uncertainty." Journal of Applied Meteorology and Climatology 50, no. 12 (2011): 2543–60. http://dx.doi.org/10.1175/2011jamc2648.1.

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AbstractThis study presents a 2-yr-long comparison of Weather Surveillance Radar-1988 Doppler (WSR-88D) refractivity retrievals with Oklahoma Mesonetwork (“Mesonet”) and sounding measurements and discusses some challenges to implementing radar refractivity operationally. Temporal and spatial analyses of radar refractivity exhibit high correlation with Mesonet data; however, periods of large refractivity differences between the radar and Mesonet are observed. Several sources of refractivity differences are examined to determine the cause of large refractivity differences. One source for nonklystron radars includes magnetron frequency drift, which can introduce errors up to 10 N-units if the frequency drift is not corrected. Different reference maps made at different times can “shift” refractivity values. A semiautomated method for producing reference maps is presented, including trade-offs for making reference maps under different conditions. Refractivity from six Mesonet stations within the clutter domain of the Oklahoma City, Oklahoma, WSR-88D (KTLX) is compared with radar refractivity retrievals. The analysis revealed that the six Mesonet stations exhibited a prominent diurnal trend in differences between radar and Mesonet refractivity measurements. The diurnal range of the refractivity differences sometimes exceeded 20 or 30 N-units in the warm season, which translated to a potential dewpoint temperature difference of several degrees Celsius. A seasonal analysis revealed that large refractivity differences primarily occurred during the warm season when refractivity is most sensitive to moisture. Ultimately, the main factor in determining the magnitude of the differences between the two refractivity platforms is the vertical gradient of refractivity because of the difference in observation height between the radar and a surface station.

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Feng, Ya-Chien, Frédéric Fabry, and Tammy M. Weckwerth. "Improving Radar Refractivity Retrieval by Considering the Change in the Refractivity Profile and the Varying Altitudes of Ground Targets." Journal of Atmospheric and Oceanic Technology 33, no. 5 (2016): 989–1004. http://dx.doi.org/10.1175/jtech-d-15-0224.1.

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AbstractAccurate radar refractivity retrievals are critical for quantitative applications, such as assimilating refractivity into numerical models or studying boundary layer and convection processes. However, the technique as originally developed makes some simplistic assumptions about the heights of ground targets () and the vertical gradient of refractivity (). In reality, the field of target phases used for refractivity retrieval is noisy because of varying terrain and introduces estimation biases. To obtain a refractivity map at a constant height above terrain, a 2D horizontal refractivity field at the radar height must be computed and corrected for altitude using an average . This is achieved by theoretically clarifying the interpretation of the measured phase considering the varying and the temporal change of . Evolving causes systematic refractivity biases, as it affects the beam trajectory, the associated target range, and the refractivity field sampled between selected targets of different heights. To determine and changes, a twofold approach is proposed: first, can be reasonably inferred based on terrain height; then, a new method of estimation is devised by using the property of the returned powers of a pointlike target at successive antenna elevations. The obtained shows skill based on in situ tower observation. As a result, the data quality of the retrieved refractivity may be improved with the newly added information of and .

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Abu-Almal, Abdulhadi, and Kifah Al-Ansari. "Calculation of Effective Earth Radius and Point Refractivity Gradient in UAE." International Journal of Antennas and Propagation 2010 (2010): 1–4. http://dx.doi.org/10.1155/2010/245070.

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A large set of 14 years of reliable local radiosonde meteorological data, from 1990 to 2003, has been used to calculate the effective Earth radius and point refractivity gradient in the United Arab Emirates. The obtained values are used to investigate their impact on the design of microwave links. The cumulative distribution of the refractivity gradient in the first 65 meters above the ground surface, the monthly distribution for the median value of thek-factor, as well as their comparison with the ITU-maps are provided. Both experimental and global standard values are applied to specific link budget calculations.

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Park, Shinju, and Frédéric Fabry. "Estimation of Near-Ground Propagation Conditions Using Radar Ground Echo Coverage." Journal of Atmospheric and Oceanic Technology 28, no. 2 (2011): 165–80. http://dx.doi.org/10.1175/2010jtecha1500.1.

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Abstract The vertical gradient of refractivity (dN/dh) determines the path of the radar beam; namely, the larger the negative values of the refractivity gradient, the more the beam bends toward the ground. The variability of the propagation conditions significantly affects the coverage of the ground echoes and, thus, the quality of the scanning radar measurements. The information about the vertical gradient of refractivity is usually obtained from radiosonde soundings whose use, however, is limited by their coarse temporal and spatial resolution. Because radar ground echo coverage provides clues about how severe the beam bending can be, we have investigated a method that uses radar observations to infer propagation conditions with better temporal resolution than the usual soundings. Using the data collected during the International H2O Project (IHOP_2002), this simple method has shown some skill in capturing the propagation conditions similar to these estimated from soundings. However, the evaluation of the method has been challenging because of 1) the limited resolution of the conventional soundings in time and space, 2) the lack of other sources of data with which to compare the results, and 3) the ambiguity in the separation of ground from weather echoes.

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Oluwafemi, Ilesanmi B., and Moses O. Olla. "Estimation of Geoclimatic Factor for Nigeria through Meteorological Data." European Journal of Electrical Engineering and Computer Science 5, no. 3 (2021): 41–44. http://dx.doi.org/10.24018/ejece.2021.5.3.191.

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Geoclimatic factor variable is one of the most important radio climatic variables in the planning of the radio links in any region. A fade margin that takes into account multipath fading has to be incorporated in the link budget in the design of terrestrial line of sight communication system. This work involves the determination of the refractivity gradient over the first 100 m above ground level in Nigeria and by using the determined refractivity gradient, the geo-climatic factor (K) was calculated for typical links in Nigeria. The Geo-climatic factor (K) for the six major cities representing each geopolitical zone in Nigeria is determined in-order to improve future planning of the radio links in the regions. Measurement of meteorological parameters for five years taken in Ikeja, Lagos (Latitude 6º27´11´´N, Longitude 3º23´44´´E), Enugu (Latitude 6º27´35.8704´´N, Longitude 7º32´56.2164´´E), Kaduna (Latitude 10º31´23´´N, Longitude 7º26´25´´E), Port Harcourt (Latitude 4º47´21´´N, Longitude 6º59´54´´E), Kano (Latitude 12º3’N,Longitude 8º32´N) and Abuja (Latitude 9º10´32´´N Longitude 7º10´50´´E) were employed to estimate the country value of K. The pressure, P(hPa), temperature, T(ºC) and the relative humidity, (%), for the six location used were taken for a period of five years (2011-2015). The value of humidity were converted to water vapour pressure, e(hPa). In processing of the data, the average values of each month collected over a period of five years was used. The monthly data was used to calculate the values of the refractivity at the ground level and at 100 m altitude. From the calculated values of refractivity,the values of the refractivity gradient of heights of 65 m and at 100 m was computed and thereafter the geo-climatic factor (K) was calculated for the six geopolitical region of the country.

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Ho, Shu-peng, Liang Peng, Richard A. Anthes, Ying-Hwa Kuo, and Hsiao-Chun Lin. "Marine Boundary Layer Heights and Their Longitudinal, Diurnal, and Interseasonal Variability in the Southeastern Pacific Using COSMIC, CALIOP, and Radiosonde Data." Journal of Climate 28, no. 7 (2015): 2856–72. http://dx.doi.org/10.1175/jcli-d-14-00238.1.

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Abstract The spatial and temporal variability of the marine boundary layer (MBL) over the southeastern Pacific is studied using high-resolution radiosonde data from the VAMOS Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-REx), lidar cloud measurements from the CALIOP instrument on the CALIPSO satellite, radio occultation (RO) data from the COSMIC satellites, and the ERA-Interim. The height of the MBL (MBLH) is estimated using three RO-derived parameters: the bending angle, refractivity, and water vapor pressure computed from the refractivity derived from a one-dimensional variational data inversion (1D-VAR) procedure. Two different diagnostic methods (minimum gradient and break point method) are compared. The results show that, although a negative bias in the refractivity exists as a result of superrefraction, the spatial and temporal variations of the MBLH determined from the RO observations are consistent with those from CALIOP and the radiosondes. The authors find that the minimum gradient in the RO bending angle gives the most accurate estimation of the MBL height.

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Grabner, Martin, Pavel Pechac, and Pavel Valtr. "On horizontal distribution of vertical gradient of atmospheric refractivity." Atmospheric Science Letters 18, no. 7 (2017): 294–99. http://dx.doi.org/10.1002/asl.755.

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Al Ansari, K., and R. A. Kamel. "Correlation Between Ground Refractivity and Refractivity Gradient and Their Statistical and Worst Month Distributions in Abu Dhabi." IEEE Antennas and Wireless Propagation Letters 7 (2008): 233–35. http://dx.doi.org/10.1109/lawp.2008.923204.

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Jicha, Otakar, Pavel Pechac, Vaclav Kvicera, and Martin Grabner. "Estimation of the Radio Refractivity Gradient From Diffraction Loss Measurements." IEEE Transactions on Geoscience and Remote Sensing 51, no. 1 (2013): 12–18. http://dx.doi.org/10.1109/tgrs.2012.2199995.

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Xie, Feiqin, Stig Syndergaard, E. Robert Kursinski, and Benjamin M. Herman. "An Approach for Retrieving Marine Boundary Layer Refractivity from GPS Occultation Data in the Presence of Superrefraction." Journal of Atmospheric and Oceanic Technology 23, no. 12 (2006): 1629–44. http://dx.doi.org/10.1175/jtech1996.1.

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Abstract The global positioning system (GPS) radio occultation (RO) technique has demonstrated the ability to precisely probe earth’s atmosphere globally with high vertical resolution. However, the lowermost troposphere still presents some challenges for the technique. Over moist marine areas, especially in subtropical regions, a very large negative moisture gradient often exists across the thermal inversion capping the marine boundary layer (MBL), which frequently causes superrefraction (SR), or ducting. In the presence of SR, the reconstruction of refractivity from RO data becomes an ill-posed inverse problem. This study shows that one given RO bending angle profile is consistent with a continuum (an infinite number) of refractivity profiles. The standard Abel retrieval gives the minimum refractivity solution of the continuum and thus produces the largest negative bias, consistent with a negative bias often present in the retrieved refractivity profiles in the moist lower troposphere. By applying a simple linear parameterization of the refractivity structure within and just below the SR layer, an analytical relation between the Abel-retrieved refractivity and a continuum of solutions is derived. Combining the Abel retrieval and the analytical relation with some physical constraints, a novel approach is developed to reconstruct the vertical refractivity structure within and below the SR layer. Numerical simulation studies in this paper have demonstrated the great potential of the reconstruction method to provide a much-improved retrieval in the presence of SR, and the method should greatly enhance the ability to measure the MBL structure globally using the GPS RO technique.

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Wang, Sungsik, Tae Heung Lim, Kyoungsoo Oh, Chulhun Seo, and Hosung Choo. "Prediction of Wide Range Two-Dimensional Refractivity Using an IDW Interpolation Method from High-Altitude Refractivity Data of Multiple Meteorological Observatories." Applied Sciences 11, no. 4 (2021): 1431. http://dx.doi.org/10.3390/app11041431.

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This article proposes a method for the prediction of wide range two-dimensional refractivity for synthetic aperture radar (SAR) applications, using an inverse distance weighted (IDW) interpolation of high-altitude radio refractivity data from multiple meteorological observatories. The radio refractivity is extracted from an atmospheric data set of twenty meteorological observatories around the Korean Peninsula along a given altitude. Then, from the sparse refractive data, the two-dimensional regional radio refractivity of the entire Korean Peninsula is derived using the IDW interpolation, in consideration of the curvature of the Earth. The refractivities of the four seasons in 2019 are derived at the locations of seven meteorological observatories within the Korean Peninsula, using the refractivity data from the other nineteen observatories. The atmospheric refractivities on 15 February 2019 are then evaluated across the entire Korean Peninsula, using the atmospheric data collected from the twenty meteorological observatories. We found that the proposed IDW interpolation has the lowest average, the lowest average root-mean-square error (RMSE) of ∇M (gradient of M), and more continuous results than other methods. To compare the resulting IDW refractivity interpolation for airborne SAR applications, all the propagation path losses across Pohang and Heuksando are obtained using the standard atmospheric condition of ∇M = 118 and the observation-based interpolated atmospheric conditions on 15 February 2019. On the terrain surface ranging from 90 km to 190 km, the average path losses in the standard and derived conditions are 179.7 dB and 182.1 dB, respectively. Finally, based on the air-to-ground scenario in the SAR application, two-dimensional illuminated field intensities on the terrain surface are illustrated.

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Falodun, Falodun, Omotoso Omotoso, and Ashidi Ashidi. "Estimation of multipath propagation and fade margin over Coastal area." APTIKOM Journal on Computer Science and Information Technologies 3, no. 3 (2018): 77–83. http://dx.doi.org/10.11591/aptikom.j.csit.124.

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Atmospheric weather parameter is dynamic in nature, hence the need for continuous investigation of the anomalous propagation phenomenon. The coastal region is more unique in its characteristics due to the rapid and continuous variation in the weather parameters. This paper presents the results from 10 years (2006 – 2015) of reanalysis data of meteorological parameters (temperature, relative humidity and atmospheric pressure) obtained from European Center for Medium-Range Weather Forecasts (ECMWF). The data covers some selected stations in the coastal region of Nigeria namely: Port-Harcourt, Warri, Calabar, Arogbo, Oron, Yenagoa and Lagos Island at four synopsies hours of the day (6 hrs, 12 hrs, 18 hrs and 24 hrs). The resolution of the ECMWF data is 0.25 by 0.25. Radio refractivity, refractivity gradient, point refractivity and geoclimatic factor are estimated from the data. Subsequently, the results were used to deduce percentage of fade depth exceedance. The overall resultsa will assist to ascertain the level of signal degradation due to multipath fading and fade depth over the coastal regions of Nigeria

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Ojo, J. S., A. O. Adelakun, and O. V. Edward. "Comparative study on Radio Refractivity Gradient in the troposphere using Chaotic Quantifiers." Heliyon 5, no. 8 (2019): e02083. http://dx.doi.org/10.1016/j.heliyon.2019.e02083.

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Osahenvemwen, Austin O., and Benedict E. Omatahunde. "Impacts of Weather and Environmental Conditions on Mobile Communication Signals." Journal of Advances in Science and Engineering 1, no. 1 (2018): 33–38. http://dx.doi.org/10.37121/jase.v1i1.8.

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The impacts of weather and environmental conditions on mobile communication signals were determined in this study. A Glo mobile communication network operating in the 900MHz band was considered. The Glo fixed base transceiver station (BTS) location at Gloworld in Benin City was considered. A frequency-signal tracker software, version 2.5.1 was installed and configured into a notebook Intel palm top, relative parameters data were obtained from 200 meters from the Glo BTS from 28th of July to 31st of August 2016, with data obtained hourly. Morning, afternoon and evening, and dry weather, fog weather and raining conditions was based on the statistical central tendency parameters. The average refractivity gradient observed was -61.3 N/km. It was observed that dry weather, signal strength variation was within 32 dBm, fog, variation was within 34 dBm range, while the variation of rain was within 38 dBm range indicating higher variation. It was observed that the more the mobile station move away from the BTS the higher the signal loss and that temperature and refractivity gradient has 0.50 and 0.42 positive correlations. In addition, relative humidity and pressure possesses negative correlations of -0.50 and -0.44 respectively.

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Emmanuel, Israel, Babatunde Adeyemi, and Kayode Adedayo. "Estimation of Refractivity Gradient and Geoclimatic Factor for Radio Link Design in Nigeria." Physical Science International Journal 19, no. 2 (2018): 1–9. http://dx.doi.org/10.9734/psij/2018/34489.

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AbouAlmal, Abdulhadi, Raed A. Abd-Alhameed, Kifah Al-Ansari, et al. "Statistical Analysis of Refractivity Gradient and $\beta_{0}$ Parameter in the Gulf Region." IEEE Transactions on Antennas and Propagation 61, no. 12 (2013): 6250–54. http://dx.doi.org/10.1109/tap.2013.2279999.

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Park, Shinju, and Frédéric Fabry. "Simulation and Interpretation of the Phase Data Used by the Radar Refractivity Retrieval Algorithm." Journal of Atmospheric and Oceanic Technology 27, no. 8 (2010): 1286–301. http://dx.doi.org/10.1175/2010jtecha1393.1.

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Abstract The radar refractivity retrieval algorithm applied to radar phase measurements from ground targets can provide high-resolution, near-surface moisture estimates in time and space. The reliability of the retrieval depends on the quality of the returned phase measurements, which are affected by factors such as 1) the vertical variation of the refractive index along the ray path and 2) the properties of illuminated ground targets (e.g., the height and shape of the targets intercepted by radar rays over complex terrain). These factors introduce ambiguities in the phase measurement that have not yet been considered in the refractivity algorithm and that hamper its performance. A phase measurement simulator was designed to better understand the effect of these factors. The results from the simulation were compared with observed phase measurements for selected atmospheric propagation conditions estimated from low-level radio sounding profiles. Changes in the vertical gradient of refractivity coupled with the varying heights of targets are shown to have some influence on the variability of phase fields. However, they do not fully explain the noisiness of the real phase observations because other factors that are not included in the simulation, such as moving ground targets, affect the noisiness of phase measurements.

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Poli, P. "Effects of horizontal gradients on GPS radio occultation observation operators. II: A Fast Atmospheric Refractivity Gradient Operator (FARGO)." Quarterly Journal of the Royal Meteorological Society 130, no. 603 (2004): 2807–25. http://dx.doi.org/10.1256/qj.03.229.

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Boumis, M., D. Rezacova, and Z. Sokol. "Calculation of vertical gradient of atmospheric refractivity making use of 3D objective analysis technique." Electronics Letters 35, no. 18 (1999): 1583. http://dx.doi.org/10.1049/el:19991089.

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Adelakun, A. O., J. S. Ojo, and O. V. Edward. "Quantitative analyses of complexity and nonlinear trend of radio refractivity gradient in the troposphere." Advances in Space Research 65, no. 9 (2020): 2203–15. http://dx.doi.org/10.1016/j.asr.2019.09.055.

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Zhao, Qingyun, Tracy Haack, Justin McLay, and Carolyn Reynolds. "Ensemble Prediction of Atmospheric Refractivity Conditions for EM Propagation." Journal of Applied Meteorology and Climatology 55, no. 10 (2016): 2113–30. http://dx.doi.org/10.1175/jamc-d-16-0033.1.

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AbstractAn ensemble forecast system has been developed at the Naval Research Laboratory to improve the analyses and forecasts of atmospheric refractivity for electromagnetic (EM) propagation with the intention of accounting for uncertainties in model forecast errors. Algorithms for a matrix of ensemble statistics have been developed to analyze the probability, location, intensity, and structure of ducting of various types. Major parameters of ducting layers and their ensemble statistics are calculated from the ensemble forecasts. Their relationships to the large-scale and mesoscale environment are also investigated. The Wallops Island field experiment from late April to early May 2000 is selected to evaluate the system. During the spring season, this coastal region maintains a strong sea surface temperature gradient between cold shelf waters and the warm Gulf Stream, where the boundaries between land, the coastal water, and the Gulf Stream have a strong influence on marine boundary layer structures and the formation of ducting layers. Sounding profiles during the field experiment are used in the study to further understand the structures of the ducting layers and also to validate the ensemble forecast system. While some advantages of the ensemble system over the deterministic forecast for atmospheric refractivity prediction in the boundary layer are studied and demonstrated in this study, the weaknesses of the current ensemble system are revealed for future improvement of the system.

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Wang, Hua, Shipeng Su, Haichuan Tang, Lin Jiao, and Yunbo Li. "Atmospheric Duct Detection Using Wind Profiler Radar and RASS." Journal of Atmospheric and Oceanic Technology 36, no. 4 (2019): 557–65. http://dx.doi.org/10.1175/jtech-d-18-0009.1.

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AbstractA method of detecting atmospheric ducts using a wind profiler radar (WPR) and a radio acoustic sounding system (RASS) is proposed. The method uses the RASS to measure the virtual temperature profile and calculate the Brunt–Väisälä frequency; it also uses the WPR to measure the spectral width of the atmosphere and the atmospheric refractive index structure constant. Then the profile of the atmospheric refractive index gradient and modified refractivity are calculated using virtual temperature, spectral width, and the atmospheric refractive index structure constant. Finally, the height and intensity of the atmospheric duct are calculated to achieve continuous monitoring of the atmospheric duct. To verify the height and intensity of the atmospheric duct, comparison experiments between WPR-RASS and radiosondes were carried out from June 2014 to June 2015 in Dalian, Liaoning Province, China. The results show that the profile of modified refractivity by WPR-RASS has exactly the same trend as the radiosondes, the two methods have a good consistency, and the atmospheric duct value from WPR-RASS is in good agreement with that from radiosondes.

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Wang, Xuping, Bing Liu, Yuguo Yang, et al. "Preparation and laser modulation investigation of quadratic electro-optical crystal Cu:KTN with gradient refractivity effect." Journal of Crystal Growth 468 (June 2017): 356–60. http://dx.doi.org/10.1016/j.jcrysgro.2016.09.052.

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Bettouche, Yamina, Basile Agba, Ammar B. Kouki, et al. "ESTIMATION AND ANALYSIS OF THE RADIO REFRACTIVITY, ITS GRADIENT AND THE GEOCLIMATIC FACTOR IN ARCTIC REGIONS." Progress In Electromagnetics Research M 92 (2020): 181–92. http://dx.doi.org/10.2528/pierm20020709.

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Saleem, Muhammad Usman. "Statistical Investigation and Mapping of Monthly Modified Refractivity Gradient over Pakistan at the 700 Hectopascal Level." Open Journal of Antennas and Propagation 04, no. 02 (2016): 46–63. http://dx.doi.org/10.4236/ojapr.2016.42005.

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Richardson, Lindsey M., Jeffrey G. Cunningham, W. David Zittel, et al. "Bragg Scatter Detection by the WSR-88D. Part I: Algorithm Development." Journal of Atmospheric and Oceanic Technology 34, no. 3 (2017): 465–78. http://dx.doi.org/10.1175/jtech-d-16-0030.1.

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AbstractStudies have shown that echo returns from clear-air Bragg scatter (CABS) can be used to detect the height of the convective boundary layer and to assess the systematic differential reflectivity (ZDR) bias for a radar site. However, these studies did not use data from operational Weather Surveillance Radar-1988 Doppler (WSR-88D) or data from a large variety of sites. A new algorithm to automatically detect CABS from any operational WSR-88D with dual-polarization capability while excluding contamination from precipitation, biota, and ground clutter is presented here. Visual confirmation and tests related to the sounding parameters’ relative humidity slope, refractivity gradient, and gradient Richardson number are used to assess the algorithm. Results show that automated detection of CABS in operational WSR-88D data gives useful ZDR bias information while omitting the majority of contaminated cases. Such an algorithm holds potential for radar calibration efforts and Bragg scatter studies in general.

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Bech, Joan, Bernat Codina, Jeroni Lorente, and David Bebbington. "The Sensitivity of Single Polarization Weather Radar Beam Blockage Correction to Variability in the Vertical Refractivity Gradient." Journal of Atmospheric and Oceanic Technology 20, no. 6 (2003): 845–55. http://dx.doi.org/10.1175/1520-0426(2003)020<0845:tsospw>2.0.co;2.

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Adediji, A. T., M. O. Ajewole, and S. E. Falodun. "Distribution of radio refractivity gradient and effective earth radius factor (k-factor) over Akure, South Western Nigeria." Journal of Atmospheric and Solar-Terrestrial Physics 73, no. 16 (2011): 2300–2304. http://dx.doi.org/10.1016/j.jastp.2011.06.017.

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Mentes, ŞSibel, and Zerefsan Kaymaz. "Investigation of Surface Duct Conditions over Istanbul, Turkey." Journal of Applied Meteorology and Climatology 46, no. 3 (2007): 318–37. http://dx.doi.org/10.1175/jam2452.1.

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Abstract A comprehensive examination of 2 yr of radiosonde data to determine the surface duct conditions over Istanbul (4°N, 29°E), Turkey, was made. The refractivity of the atmosphere is a function of air temperature and water vapor pressure. Any negative gradient in the modified refractivity results in the presence of a duct in the atmosphere. Therefore, the occurrence of ducts strongly depends upon both the synoptic and the local meteorological conditions that prevail over the region. The characteristics of surface ducts occurring over Istanbul were examined statistically. It was found that most of the ducts occur in May and July. The highest occurrence rate of surface ducts was observed in the summer season, and the lowest rate was observed in the winter season. The median duct thickness and duct strength are found to be the highest and the strongest in summer, whereas they are the lowest and the weakest in winter. When the data are separated into stable and unstable atmospheric subgroups, it is seen that surface duct characteristics show clear seasonal differences. Surface ducts in a stable atmosphere are found to be stronger than those in an unstable atmosphere. Also, daytime (1200 UTC) surface ducts occur more frequently than nighttime (0000 UTC) surface ducts in Istanbul. These statistics are discussed in association with local meteorological conditions and weather systems affecting the Istanbul region, and comments are made on the importance of their possible consequences in the region.

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Lopez, Philippe. "A 5-yr 40-km-Resolution Global Climatology of Superrefraction for Ground-Based Weather Radars." Journal of Applied Meteorology and Climatology 48, no. 1 (2009): 89–110. http://dx.doi.org/10.1175/2008jamc1961.1.

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Abstract The propagation of electromagnetic waves emitted from ground-based meteorological radars is determined by the stratification of the atmosphere. In extreme superrefractive situations characterized by strong temperature inversions or strong vertical gradients of moisture, the radar beam can be deflected toward the ground (ducting or trapping). This phenomenon often results in spurious returned echoes and misinterpretation of radar images such as erroneous precipitation detection. In this work, a 5-yr global climatology of the frequency of superrefractive and ducting conditions and of trapping-layer base height has been produced using refractivity computations from ECMWF temperature, moisture, and pressure analyses at a 40-km horizontal resolution. The aim of this climatology is to better document how frequent such events are, which is a prerequisite for fully benefiting from radar data information for the multiple purposes of model validation, precipitation analysis, and data assimilation. First, the main climatological features are summarized for the whole globe: high- and midlatitude oceans seldom experience superrefraction or ducting whereas tropical oceans are strongly affected, especially in regions where the trade wind inversion is intense and lying near the surface. Over land, seasonal averages of superrefraction (ducting) frequencies reach 80% (40%) over tropical moist areas year-round but remain below 40% (15%) in most other regions. A particular focus is then laid on Europe and the United States, where extensive precipitation radar networks already exist. Seasonal statistics exhibit a pronounced diurnal cycle of ducting occurrences, with averaged frequencies peaking at 60% in summer late afternoon over the eastern half of the United States, the Balkans, and the Po Valley but no ducts by midday. Similarly high ducting frequencies are found over the southwestern coast of the United States at night. A potentially strong reduction of ducting occurrences with increased radar height (especially in midlatitude summer late afternoon) is evidenced by initiating refractivity vertical gradient computations from either the lowest or the second lowest model level. However, installing radar on tall towers also brings other problems, such as a possible amplification of sidelobe clutter echoes.

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Wang, X. Y., and K. C. Wang. "Estimation of atmospheric mixing layer height from radiosonde data." Atmospheric Measurement Techniques 7, no. 6 (2014): 1701–9. http://dx.doi.org/10.5194/amt-7-1701-2014.

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Abstract. Mixing layer height (h) is an important parameter for understanding the transport process in the troposphere, air pollution, weather and climate change. Many methods have been proposed to determine h by identifying the turning point of the radiosonde profile. However, substantial differences have been observed in the existing methods (e.g. the potential temperature (θ), relative humidity (RH), specific humidity (q) and atmospheric refractivity (N) methods). These differences are associated with the inconsistency of the temperature and humidity profiles in a boundary layer that is not well mixed, the changing measurability of the specific humidity and refractivity with height, the measurement error of humidity instruments within clouds, and the general existence of clouds. This study proposes a method to integrate the information of temperature, humidity and cloud to generate a consistent estimate of h. We apply this method to high vertical resolution (~ 30 m) radiosonde data that were collected at 79 stations over North America during the period from 1998 to 2008. The data are obtained from the Stratospheric Processes and their Role in Climate Data Center (SPARC). The results show good agreement with those from N method as the information of temperature and humidity contained in N; however, cloud effects that are included in our method increased the reliability of our estimated h. From 1988 to 2008, the climatological h over North America was 1675 ± 303 m with a strong east–west gradient: higher values (generally greater than 1800 m) occurred over the Midwest US, and lower values (usually less than 1400 m) occurred over Alaska and the US West Coast.

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Wakimoto, Roger M., and Hanne V. Murphey. "Frontal and Radar Refractivity Analyses of the Dryline on 11 June 2002 during IHOP." Monthly Weather Review 138, no. 1 (2010): 228–41. http://dx.doi.org/10.1175/2009mwr2991.1.

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Abstract An analysis of a dryline that did not initiate convection during the observational period is presented. The dryline was the weakest kinematic boundary observed during the International H2O Project (IHOP), but was associated with a large moisture gradient. Detailed dual-Doppler wind syntheses from an airborne Doppler radar were combined with radar refractivity measurements providing a rare opportunity to examine both the kinematic and moisture characteristics of this boundary. The radar thin line denotes the approximate kinematic position of the dryline and was quasi-linear on this day. In contrast, the moisture pattern across the dryline was more complex than was suggested by the characteristics of the thin line. Prominent in the horizontal plots was the presence of narrow (few kilometers wide) channels of moisture extending 15–20 km into the dry air mass. Past studies have suggested that echo thin lines observed in the clear air can be used as a proxy for delineating the moisture contrast across the dryline. In contrast, the “moisture extrusions” were present even though the thin line was quasi-linear and were located in weak-echo regions along the thin line. It is hypothesized that transverse rolls developed at an angle to the boundary layer winds and intersected the dryline. The kinematic airflow associated with these rolls could have protected the moist tongues from the eroding effect of the dry flow west of the dryline. The moisture extrusions appear to diminish with time as they mix with the surrounding dry air.

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Wang, X. Y., and K. C. Wang. "Estimation of atmospheric mixing layer height from radiosonde data." Atmospheric Measurement Techniques Discussions 7, no. 2 (2014): 1247–73. http://dx.doi.org/10.5194/amtd-7-1247-2014.

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Abstract. Mixing layer height (h) is an important parameter for understanding the transport process in the troposphere, air pollution, weather and climate change. Many methods have been proposed to determine h by identifying the turning point of the radiosonde profile. However, substantial differences have been observed in the existing methods (e.g., the potential temperature (θ), relative humidity (RH), specific humidity (q) and atmospheric refractivity (N) methods). These differences are associated with the inconsistency of the temperature and humidity profiles in a boundary layer that is not well mixed, the changing measurability of the specific humidity and refractivity with height, the measurement error of humidity instruments within clouds, and the general existence of clouds. This study proposes a method to integrate the information of temperature, humidity and cloud to generate a consistent estimate of h. We apply this method to high vertical resolution (~ 30 m) radiosonde data that were collected at 79 stations over North America during the period from 1998 to 2008; the data are obtained from the Stratospheric Processes and their Role in Climate Data Center (SPARC). The results show good agreement with those from N method as the information of temperature and humidity contained in N; however cloud effects that are included in our method increased the reliability of h. Furthermore, our results agree well with the independent h that was determined from lidar observations. From 1988 to 2008, the climatological h over North America was 1675± 303 m with a strong east–west gradient: higher values (generally greater than 1800 m) occurred over the Midwest US, and lower values (usually less than 1400 m) occurred over Alaska and the US west coast.

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Fashade, O. O., T. V. Omotosho, S. A. Akinwumi, and K. P. Olorunyomi. "Refractivity gradient of the first 1km of the troposphere for some selected stations in six geo-political zones in Nigeria." IOP Conference Series: Materials Science and Engineering 640 (November 13, 2019): 012087. http://dx.doi.org/10.1088/1757-899x/640/1/012087.

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Bianco, Laura, Domenico Cimini, Frank S. Marzano, and Randolph Ware. "Combining Microwave Radiometer and Wind Profiler Radar Measurements for High-Resolution Atmospheric Humidity Profiling." Journal of Atmospheric and Oceanic Technology 22, no. 7 (2005): 949–65. http://dx.doi.org/10.1175/jtech1771.1.

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Abstract A self-consistent remote sensing physical method to retrieve atmospheric humidity high-resolution profiles by synergetic use of a microwave radiometer profiler (MWRP) and wind profiler radar (WPR) is illustrated. The proposed technique is based on the processing of WPR data for estimating the potential refractivity gradient profiles and their optimal combination with MWRP estimates of potential temperature profiles in order to fully retrieve humidity gradient profiles. The combined algorithm makes use of recent developments in WPR signal processing, computing the zeroth-, first-, and second-order moments of WPR Doppler spectra via a fuzzy logic method, which provides quality control of radar data in the spectral domain. On the other hand, the application of neural network to brightness temperatures, measured by a multichannel MWRP, can provide continuous estimates of tropospheric temperature and humidity profiles. Performance of the combined algorithm in retrieving humidity profiles is compared with simultaneous in situ radiosonde observations (raob’s). The empirical sets of WPR and MWRP data were collected at the Atmospheric Radiation Measurement (ARM) Program’s Southern Great Plains (SGP) site. Combined microwave radiometer and wind profiler measurements show encouraging results and significantly improve the spatial vertical resolution of atmospheric humidity profiles. Finally, some of the limitations found in the use of this technique and possible future improvements are also discussed.

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Buban, Michael S., Conrad L. Ziegler, Erik N. Rasmussen, and Yvette P. Richardson. "The Dryline on 22 May 2002 during IHOP: Ground-Radar and In Situ Data Analyses of the Dryline and Boundary Layer Evolution." Monthly Weather Review 135, no. 7 (2007): 2473–505. http://dx.doi.org/10.1175/mwr3453.1.

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Abstract On the afternoon and evening of 22 May 2002, high-resolution observations of the boundary layer (BL) and a dryline were obtained in the eastern Oklahoma and Texas panhandles during the International H2O Project. Using overdetermined multiple-Doppler radar syntheses in concert with a Lagrangian analysis of water vapor and temperature fields, the 3D kinematic and thermodynamic structure of the dryline and surrounding BL have been analyzed over a nearly 2-h period. The dryline is resolved as a strong (2–4 g kg−1 km−1) gradient of water vapor mixing ratio that resides in a nearly north–south-oriented zone of convergence. Maintained through frontogenesis, the dryline is also located within a gradient of virtual potential temperature, which induces a persistent, solenoidally forced secondary circulation. Initially quasi-stationary, the dryline retrogrades to the west during early evening and displays complicated substructures including small wavelike perturbations that travel from south to north at nearly the speed of the mean BL flow. A second, minor dryline has similar characteristics to the first, but has weaker gradients and circulations. The BL adjacent to the dryline exhibits complicated structures, consisting of combinations of open cells, horizontal convective rolls, and transverse rolls. Strong convergence and vertical motion at the dryline act to lift moisture, and high-based cumulus clouds are observed in the analysis domain. Although the top of the analysis domain is below the lifted condensation level height, vertical extrapolation of the moisture fields generally agrees with cloud locations. Mesoscale vortices that move along the dryline induce a transient eastward dryline motion due to the eastward advection of dry air following misocyclone passage. Refractivity-based moisture and differential reflectivity analyses are used to help interpret the Lagrangian analyses.

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Fabry, Frédéric. "The Spatial Variability of Moisture in the Boundary Layer and Its Effect on Convection Initiation: Project-Long Characterization." Monthly Weather Review 134, no. 1 (2006): 79–91. http://dx.doi.org/10.1175/mwr3055.1.

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Abstract An attempt was made to statistically gauge the importance of moisture variability on convection initiation by analyzing data collected by radar, surface stations, soundings, and airborne in situ sensors over the 7 weeks of the International H2O Project (IHOP_2002). Based on radar refractivity data, the spatial structure of humidity near the surface proved to be very anisotropic, crosswind variability being typically twice as large as along-wind variability, in part as a result of the west-to-east climatological gradient in moisture across the Oklahoma panhandle. Variability in humidity was largest from the afternoon to sunset and smallest a few hours before and after sunrise. At the surface, variograms of refractivity increase almost linearly with scale in the crosswind direction, suggesting that the field of moisture shows little in terms of local maxima and minima. Higher in the boundary layer, moisture variability increases at small scales because of the entrainment of dry capping stable layer air as the daytime boundary layer grows, and the rate of that dry-air entrainment could be used to calculate surface moisture variability. The effect of the observed variability in moisture and temperature in the upper boundary layer on convective inhibition was quantified and contrasted with the effect expected from boundary layer updrafts. At synoptic scales and at the upper end of the mesoscale, the location of convection initiation is most sensitive to the variability in temperature. At smaller scales, storm development becomes extremely sensitive to the strength of updrafts; however, those same updrafts also magnify the effect of moisture and temperature variability, as a result of which the effect of small-scale moisture variability cannot be ignored. Some of the consequences of these findings on the representativeness of radiosonde measurements in the boundary layer and instrumentation needs for convection initiation forecasting are surveyed.

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Dan, Eduediuyai, Enyenihi Johnson, and Idorenyin Markson. "Analysis of the Effect of Variations in Refractivity Gradient on Line of Sight Percentage Clearance and Single Knife Edge Diffraction Loss." International Journal of Sustainable Energy and Environmental Research 8, no. 1 (2019): 1–9. http://dx.doi.org/10.18488/journal.13.2019.81.1.9.

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von Engeln, Axel, and João Teixeira. "A Planetary Boundary Layer Height Climatology Derived from ECMWF Reanalysis Data." Journal of Climate 26, no. 17 (2013): 6575–90. http://dx.doi.org/10.1175/jcli-d-12-00385.1.

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Abstract A planetary boundary layer (PBL) height climatology from ECMWF reanalysis data is generated and analyzed. Different methods are first compared to derive PBL heights from atmospheric temperature, pressure, and relative humidity (RH), which mostly make use of profile gradients, for example, in RH, refractivity, and virtual or potential temperature. Three methods based on the vertical gradient of RH, virtual temperature, and potential temperature were selected for the climatology generation. The RH-based method appears to capture the inversion that caps the convective boundary layer very well as a result of its temperature and humidity dependence, while the temperature-based methods appear to capture the PBL better at high latitudes. A validation of the reanalysis fields with collocated radiosonde data shows generally good agreement in terms of mean PBL height and standard deviation for the RH-based method. The generated ECMWF-based PBL height climatology shows many of the expected climatological features, such as a fairly low PBL height near the west coast of continents where stratus clouds are found and PBL growth as the air is advected over warmer waters toward the tropics along the trade winds. Large seasonal and diurnal variations are primarily found over land. The PBL height can exceed 3 km, mostly over desert areas during the day, although large values can also be found in areas such as the ITCZ. The robustness of the statistics was analyzed by using information on the percentage of outliers. Here in particular, the sea-based PBL was found to be very stable.

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Davison, Jennifer L., Robert M. Rauber, and Larry Di Girolamo. "A Revised Conceptual Model of the Tropical Marine Boundary Layer. Part II: Detecting Relative Humidity Layers Using Bragg Scattering from S-Band Radar." Journal of the Atmospheric Sciences 70, no. 10 (2013): 3025–46. http://dx.doi.org/10.1175/jas-d-12-0322.1.

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Abstract Persistent layers of enhanced equivalent radar reflectivity factor and reduced spectral width were commonly observed within cloud-free regions of the tropical marine boundary layer (TMBL) with the National Center for Atmospheric Research S-Pol radar during the Rain in Cumulus over the Ocean (RICO) field campaign. Bragg scattering is shown to be the primary source of these layers. Two mechanisms are proposed to explain the Bragg scattering layers (BSLs), the first involving turbulent mixing and the second involving detrainment and evaporation of cloudy air. These mechanisms imply that BSLs should exist in layers with tops (bases) defined by local relative humidity (RH) minima (maxima). The relationship between BSLs and RH is explored. An equation for the vertical gradient of radio refractivity N is derived, and a scale analysis is used to demonstrate the close relationship between vertical RH and N gradients. This is tested using the derived radar BSL boundary altitudes, 131 surface-based soundings, and 34 sets of about six near-coincident, aircraft-released dropsondes. First, dropsonde data are used to quantify the finescale variability of the RH field. Then, within limits imposed by this variability, altitudes of tops (bases) of radar BSLs are shown to agree with altitudes of RH minima (maxima). These findings imply that S-band radars can be used to track the vertical profile of RH variations as a function of time and height, that the vertical RH profile of the TMBL is highly variable over horizontal scales as small as 60 km, and that BSLs are a persistent, coherent feature that delineate aspects of TMBL mesoscale structure.

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Wang, Hongyan, Gaili Wang, and Liping Liu. "Climatological Beam Propagation Conditions for China’s Weather Radar Network." Journal of Applied Meteorology and Climatology 57, no. 1 (2018): 3–14. http://dx.doi.org/10.1175/jamc-d-17-0097.1.

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AbstractThe vertical refractivity gradient (VRG) is critical to weather radar beam propagation. The most common method of calculating beam paths uses the 4/3 Earth radius model, which corresponds to standard refraction conditions. In the present work, to better document propagation conditions for radar electromagnetic waves, which is essential for hydrology and numerical weather forecast models to more fully benefit from observations taken from the new-generation weather radar network in China, VRG spatial and temporal variations in the first kilometers above the surface are explored using 6-yr sounding observations. Under the effects of both regional climatic and topographic conditions, VRG values for most of the radars are generally smaller than those of the standard conditions for much of the year. There are similar or slightly larger values at only a few radar sites. Smaller VRG values are more frequent and widespread, especially during rainy seasons when weather radar observations are important. In such conditions, beam heights estimated using standard atmospheric refraction are overestimated relative to actual heights for most of the radars. Underestimates are much less common and of much shorter duration. However, height deviations are acceptable for being well within the uncertainty of radar echo height owing to the ~1° beamwidth. In coastal areas and the middle and lower reaches of the Yangtze River, radar observations should be applied with much more caution because of the greater risk of beam blockage and clutter contamination.

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48

Richardson, Lindsey M., W. David Zittel, Robert R. Lee, Valery M. Melnikov, Richard L. Ice, and Jeffrey G. Cunningham. "Bragg Scatter Detection by the WSR-88D. Part II: Assessment of ZDR Bias Estimation." Journal of Atmospheric and Oceanic Technology 34, no. 3 (2017): 479–93. http://dx.doi.org/10.1175/jtech-d-16-0031.1.

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AbstractClear-air Bragg scatter (CABS) is a refractivity gradient return generated by turbulent eddies that operational Weather Surveillance Radar-1988 Doppler (WSR-88D) systems can detect. The randomly oriented nature of the eddies results in a differential reflectivity (ZDR) value near 0 dB, and thus CABS can be used as an assessment of ZDR calibration in the absence of excessive contamination from precipitation or biota. An automated algorithm to estimate ZDR bias from CABS was developed by the Radar Operations Center and can be used to assess the calibration quality of the dual-polarized WSR-88D fleet. This technique supplements existing ZDR bias assessment tools, especially the use of other external targets, such as light rain and dry snow.The estimates of ZDR bias from CABS using a 1700–1900 UTC time window were compared to estimates from the light rain and dry snow methods. Output from the automated CABS algorithm had approximately the same amount of bias reported as the light rain and dry snow estimates (within ±0.1 dB). As the 1700–1900 UTC time window appeared too restrictive, a modified version of the algorithm was tested to detect CABS diurnally on a volume-by-volume basis (continuous monitoring). Continuous monitoring resulted in a two- to fourfold increase in the number of days with CABS detections. Results suggest estimates from CABS are viable for many sites throughout the year and provide an important addition to existing bias estimation techniques.

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Zus, Florian, Jan Douša, Michal Kačmařík, Pavel Václavovic, Galina Dick, and Jens Wickert. "Estimating the Impact of Global Navigation Satellite System Horizontal Delay Gradients in Variational Data Assimilation." Remote Sensing 11, no. 1 (2018): 41. http://dx.doi.org/10.3390/rs11010041.

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We developed operators to assimilate Global Navigation Satellite System (GNSS) Zenith Total Delays (ZTDs) and horizontal delay gradients into a numerical weather model. In this study we experiment with refractivity fields derived from the Global Forecast System (GFS) available with a horizontal resolution of 0.5°. We begin our investigations with simulated observations. In essence, we extract the tropospheric parameters from the GFS analysis, add noise to mimic observation errors and assimilate the simulated observations into the GFS 24h forecast valid at the same time. We consider three scenarios: (1) the assimilation of ZTDs (2) the assimilation of horizontal delay gradients and (3) the assimilation of both ZTDs and horizontal delay gradients. The impact is measured by utilizing the refractivity fields. We find that the assimilation of the horizontal delay gradients in addition to the ZTDs improves the refractivity field around 800 hPa. When we consider a single station there is a clear improvement when horizontal delay gradients are assimilated in addition to the ZTDs because the horizontal delay gradients contain information that is not contained in the ZTDs. On the other hand, when we consider a dense station network there is not a significant improvement when horizontal delay gradients are assimilated in addition to the ZTDs because the horizontal delay gradients do not contain information that is not already contained in the ZTDs. Finally, we replace simulated by real observations, that is, tropospheric parameters from a Precise Point Positioning solution provided with the G-Nut/Tefnut software, in order to show that the GFS 24h forecast is indeed improved when GNSS horizontal delay gradients are assimilated in addition to GNSS ZTDs; for the considered station (Potsdam, Germany) and period (June and July, 2017) we find an improvement in the retrieved refractivity of up to 4%.

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Chen, Shu-Ya, Ching-Yuang Huang, Ying-Hwa Kuo, and Sergey Sokolovskiy. "Observational Error Estimation of FORMOSAT-3/COSMIC GPS Radio Occultation Data." Monthly Weather Review 139, no. 3 (2011): 853–65. http://dx.doi.org/10.1175/2010mwr3260.1.

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Abstract The Global Positioning System (GPS) radio occultation (RO) technique is becoming a robust global observing system. GPS RO refractivity is typically modeled at the ray perigee point by a “local refractivity operator” in a data assimilation system. Such modeling does not take into account the horizontal gradients that affect the GPS RO refractivity. A new observable (linear excess phase), defined as an integral of the refractivity along some fixed ray path within the model domain, has been developed in earlier studies to account for the effect of horizontal gradients. In this study, the error statistics of both observables (refractivity and linear excess phase) are estimated using the GPS RO data from the Formosa Satellite 3–Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC) mission. The National Meteorological Center (NMC) method, which is based on lagged forecast differences, is applied for evaluation of the model forecast errors that are used for estimation of the GPS RO observational errors. Also used are Weather Research and Forecasting (WRF) model forecasts in the East Asia region at 45-km resolution for one winter month (mid-January to mid-February) and one summer month (mid-August to mid-September) in 2007. Fractional standard deviations of the observational errors of refractivity and linear excess phase both show an approximately linear decrease with height in the troposphere and a slight increase above the tropopause; their maximum magnitude is about 2.2% (2.5%) for refractivity and 1.1% (1.3%) for linear excess phase in the lowest 2 km for the winter (summer) month. An increase of both fractional observational errors near the surface in the summer month is attributed mainly to a larger amount of water vapor. The results indicate that the fractional observational error of refractivity is about twice as large as that of linear excess phase, regardless of season. The observational errors of both linear excess phase and refractivity are much less latitude dependent for summer than for winter. This difference is attributed to larger latitudinal variations of the specific humidity in winter.

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Journal articles on the topic 'Refractivity gradient'

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