Dissertations / Theses on the topic 'Meteorology. Atmospheric physics'

Prior to the development of the multiscale numerical methodology, one-year of sonic anemometer and wind LiDAR measurements from the FINO1 offshore platform are analyzed. A comprehensive database of offshore measurements in the lowest 250 m of the boundary layer is developed after quality data check and correction for flow distortion effects by the measurement mast, allowing the characterization of the offshore conditions at FINO1. Spectral analysis of high frequency sonic anemometer measurements is used to estimate a robust averaing time for the turbulent fluxes that minimizes non-universal contributions from mesoscale structures but captures the contribution from boundary layer turbulence, employing the Ogive function concept. A stability classification of the measurements is carried out based on the Obukhov length. Results compare well to other surface layer observational studies while vertical wind speed profiles exhibit the expected stability-dependency.

Although NWP models have been extensively used for weather forecasting purposes, a comprehensive analysis of its suitability to meet the wind energy requirements needs to be carried out. The applicability of the WRF mesoscale model to reproduce offshore boundary layer characteristics is evaluated and validated against field measurements from FINO1. The ability of six planetary boundary layer (PBL) parameterizations to account for stability effects is analyzed. Overall, PBL parameterizations are rather accurate in reproducing the vertical structure of the boundary layer for convective and neutral stabilities. However, difficulties are found under stable stratifications, due to the general tendency of PBL formulations to be overdiffusive and therefore, not capable to develope the strong vertical gradients found in the observations. A low-level jet and a very shallow boundary layer cases are simulated to provide further insights into the limits of the parameterizations.

Large-eddy simulations (LES) based on averaged conditions from a convective episode at FINO1 are conducted to understand the mechanisms of transition and equilibration that occur in turbulent one-way nested simulations. The nonlinear backscatter and anisotropy subgrid scale model with a prognostic turbulent kinetic energy equation is found to be capable of providing similar results when performing one-way nested large-eddy simulations to a reference stand-alone domain using periodic lateral boundary conditions. A good agreement is obtained in terms of velocity shear and turbulent fluxes of heat and momentum, while velocity variances are overestimated. A considerable streamwise fetch is needed following each domain transition for appropriate energy levels to be reached at high wavelengths and for the solution to reach quasi-stationary results. A pile-up of energy is observed at low wavelengths on the first nested domain, mitigated by the inclusion of a second nested domain with higher resolution that allows the development of an appropriate turbulent energy cascade.

As the final step towards developing the multiscale capabilities of WRF, the specific problem of the transition from meso- to micro-scales in atmospheric models is addressed. The challenge is to generate turbulence on inner LES domain from smooth mesoscale inflow. Several new methods are proposed to trigger the development of turbulent features. The inclusion of adequate potential temperature perturbations near the inflow boundaries of the LES domain results in a very good agreement of mean velocity profiles, variances and turbulent fluxes, as well as velocity spectra, when compared to periodic stand-alone simulations. This perturbation method allows an efficient generation of fully developed turbulence and is tested under a broad range of atmospheric stabilities: convective, neutral and stable conditions, showing successful results in all the regimes.
Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

This works presents the results for the first study to ever attempt to analyze the full potential and limitations of incorporating aerosols within a truly physical SST retrieval for operational weather forecasting purposes. This is accomplished through the application of a satellite sea surface temperature (SST) physical retrieval for satellite split-window and hyperspectral infrared (IR) sensors that allows a better representation of the atmospheric state under aerosol-laden conditions. The new algorithm includes 1) accurate specification of the surface emissivity that characterizes the surface leaving radiance and 2) transmittance and physical characterization of the atmosphere by using the Community Radiative transfer model (CRTM). This project includes application of the NEMS-Global Forecasting System Aerosol Component (NGAC) fields, which corresponds to the first global interactive atmosphere-aerosol forecast system ever implemented at NOAA’s National Center for Environmental Prediction (NCEP).

A number of limiting factors were identified by analysing brightness temperatures and SST outputs biases as a function of latitude, zenith angle, wind and moisture for cases in January and November 2013. SST ouputs are validated against a bulk SST (Reynolds SST) and a parameterized SST derived from operational products and partly against observed measurements from the eastern Atlantic Ocean, which is dominated by Saharan dust throughout most of the year and that is also a genesis region for Atlantic tropical cyclones. These observations are obtained from the NOAA Aerosols and Ocean Science Expeditions (AEROSE). The improved physical SST methodology has the potential to allow for improved representation of the geophysical state under dust-laden conditions.

Natural disasters can be very devastating to the public during their impact phase. After a natural disaster impacts a region, the response and recovery phases begin immediately. Weather conditions may interrupt emergency response and recovery in the days following the disaster. This study analyzes the climatology of weather conditions during the response and recovery phases of hurricanes and tornadoes to understand how weather may impact both environment and societal needs. Using specific criteria, 66 tornadoes and 16 hurricane cases were defined. National Weather Service (NWS) recognized weather stations were used to provide temperature, precipitation, snowfall, relative humidity, wind speed, and wind direction data. Regional and temporal groups were defined for tornado cases, but only one group was defined for hurricanes. By applying statistical analysis to weather observations taken in the week following the disasters, a climatology was developed for the response and recovery phase. Tornado and hurricane post-disaster climate closely mimicked their synoptic climatology with cooler and drier weather prevailing in days 2-4 after a disaster until the next weather system arrived about 5 days later. Winter tornadoes trended to occur in the Southeast and were associated with more extreme temperature differences than in other regions and season. The results of this study may help governmental and non-governmental organizations prepare for weather conditions during the post-disaster phase.

Since their development in the 1990s, gridded reanalysis data sets have proven quite useful for a broad range of synoptic climatological analyses, especially those utilizing a map pattern classification approach. However, their use in broad-scale, surface weather typing classifications and applications have not yet been explored. This research details the development of such a gridded weather typing classification (GWTC) scheme using North American Regional Reanalysis data for 1979-2010 for the continental United States.

Utilizing eight-times daily observations of temperature, dew point, pressure, cloud cover, u-wind and v-wind components, the GWTC categorizes the daily surface weather of 2,070 locations into one of 11 discrete weather types, nine core types and two transitional types, that remain consistent throughout the domain. Due to the use of an automated deseasonalized z-score initial typing procedure, the character of each type is both geographically and seasonally relative, allowing each core weather type to occur at every location, at any time of the year. Diagnostic statistics reveal a high degree of spatial cohesion among the weather types classified at neighboring locations, along with an effective partitioning of the climate variability of individual locations (via a Variability Skill Score metric) into these 11 weather types. Daily maps of the spatial distribution of GWTC weather types across the United States correspond well to traditional surface weather maps, and comparisons of the GWTC with the Spatial Synoptic Classification are also favorable.

While the potential future utility of the classification is expected to be primarily for the resultant calendars of daily weather types at specific locations, the automation of the methodology allows the classification to be easily repeatable, and therefore, easily transportable to other locations, atmospheric levels, and data sets (including output from gridded general circulation models). Further, the enhanced spatial resolution of the GWTC may also allow for new applications of surface weather typing classifications in mountainous and rural areas not well represented by airport weather stations.

The Intertropical Convergence Zone (ITCZ) is a feature that results from the ocean-atmosphere interactions in the tropics around the world. The ITCZ is characterized by surface wind convergence, tall storm clouds, and it forms a belt of high time-averaged precipitation around the globe. The ITCZ undergoes seasonal migrations between 5°S and 15°N roughly following the subsolar point on Earth with the seasons, with a mean annual position located slightly above the Equator, between 2° and 5°N.

This study tested the hypothesis that there was a northward shift in the median position of the ITCZ in the first decade of the 2000’s relative to the 1900’s. This hypothesis has been posed in the literature given a weakening in the intensity of the Trade Winds observed in the southern Caribbean Sea during the first decade of the 2000’s, with concomitant ecological impacts due to weakening in coastal wind-driven upwelling. The hypothesis was tested by analyzing variations in the monthly latitudinal position of the ITCZ over the Atlantic Ocean relative to the median position computed for the period 1987–2011. The position of the ITCZ was derived from satellite-derived ocean surface wind measurements collected from 1987 to 2011. A Mann-Kendall analysis and a Monte Carlo simulation were used to test for trends in the median cross-basin latitudinal position of the ITCZ. The study included an analysis of regional changes across the tropical central Atlantic (50°W to 15°W), the Western Atlantic (50°W to 30°W), and the Eastern Atlantic (30°W to 15°W) within the tropics. The results show a slight southward trend in the median position of the ITCZ over the central Atlantic and also in the Eastern Atlantic in the first decade of the 2000’s relative to the 1990’s. While this trend is barely significant, it is likely simply due to interannual variation in the average annual position of the ITCZ.

The data were also examined for the timing and persistence of a double ITCZ in the Atlantic. The double ITCZ over the Atlantic appeared every year in February or March, with the largest separation between the northern and southern branches of the ITCZ observed in June and July.

The possible effects of changes in the average latitudinal position of the ITCZ on the upwelling in the Cariaco Basin (southeastern Caribbean Sea off Venezuela) were also examined. Anomalies of the median of the latitudinal position of the ITCZ in the Atlantic were compared with anomalies of in-situ temperature collected during the 1990’s and the first decade of the 2000’s by the CARIACO Ocean Time-Series program and with anomalies of satellite SST (from the Advanced Very High Resolution Radiometer satellite; AVHRR) from 1995 to 2016. Correlation analysis were performed between anomalies of water temperatures at various depths and anomalies of satellite SST with anomalies of the monthly mean ITCZ position with lags up to 3 months for the time series, and also just for the Cariaco basin upwelling months (December-April).

For the whole Cariaco time series there were no significant correlations between the anomalies of the ITCZ position and anomalies in subsurface temperatures in the Cariaco Basin. However, during the upwelling period, the central Atlantic and Western Atlantic ITCZ position anomalies were directly correlated with Cariaco Basin temperature anomalies with no-lag (r = 0.20), and the central and Eastern Atlantic ITCZ position anomalies were inversely correlated with Cariaco Basin temperatures (r ~ -0.22 to -0.28) with ITCZ leading Cariaco temperatures by 3 months. However, these correlations were low, indicating that other factors than the position of ITCZ latitudinal position play bigger role on the Cariaco basin upwelling variability.

Interannual variability in oceanographic and meteorological characteristics of the Atlantic Ocean are expected as a result of large-scale changes in other regions of the world, including due to changes such as the El Niño Southern Oscillation (ENSO) and the Atlantic Multidecadal Oscillation (AMO). Six oceanic-atmospheric variables are used to monitor ENSO over the tropical Pacific, while the AMO is determined by monitoring SST over the Atlantic. Correlations with lags of up to ± 6 months were conducted with those climate indices and the anomalies of the median monthly latitudinal position of the ITCZ. Significant direct correlations with ENSO (Multivariate ENSO Index) were seen in the Atlantic and Western Atlantic (r = 0.15), with ENSO leading the position of the ITCZ anomalies by 3 months. This implies that within three months after an El Niño event (warm ENSO anomaly in the Pacific) the ITCZ over the mid-Atlantic and Western Atlantic Ocean tends to shift to a more northerly position. The AMO also had a direct influence on the anomalies of the ITCZ position (r = 0.13) in the Central and the Western Atlantic, with the AMO leading ITCZ anomalies by 1 month (i.e. a warming of the North Atlantic led to a northward shift in the ITCZ one month later). Correlations between AMO and the ITCZ anomalies in the Eastern Atlantic were also direct but with no lag. Although significant, these correlations were low.

An inverse correlation (~ -0.35) was found between ENSO and anomalies of water temperature of the Cariaco Basin. ENSO lagged ocean temperature anomalies by 3 to 4 months for both the whole Cariaco time series and for the upwelling months of CARIACO data. Correlations with AMO were direct (~ 0.4); for the whole time series AMO led Cariaco temperature anomalies by 3 months, but for the upwelling months AMO lagged Cariaco temperature anomalies by one month.

Urban meteorology has developed in parallel to other sub-fields in the science, but in many ways remains poorly described. In particular, the study of urban rainfall modification remains behind compared to other comparable features. Urban rainfall modification refers to the change of a precipitation feature as it crosses an urban area. Typically, this manifests as rainfall initiation, local suppression, local invigoration, and/or storm morphology changes. Research in the prior decades have shown urban rainfall modification to arise from a combination of land-atmosphere and aerosol-cloud interaction. Urban areas create a greater surface roughness, which produces local convergence and divergence, modifying local thunderstorm inflow and morphology. The land surface also generates vertical velocity perturbations which can act to initiate or modify existing convection. Urban aerosols act as CCN to perturb existing cloud and precipitation characteristics. Higher CCN narrows the cloud droplet distribution, creating more smaller cloud droplets, and initially reducing precipitation efficiency by keeping more liquid water in the cloud than what would form into rain. The CCN-cloud interaction eventually increasing heavy rainfall production as graupel riming is enhanced by the narrower cloud droplet distribution, leading to more larger raindrops and higher rain in areas.

This dissertation addresses the observation and modeling of urban thunderstorm interaction from both the land surface and aerosol perspective. It reassesses the original urban rainfall anomaly: The La Porte Anomaly. First analyzed in the late 1960s, the La Porte Anomaly was ultimately dismissed by 1980 as either a temporary, biased, or otherwise unexplainable observation, as the process level understanding had yet to be explained. The contemporary analysis utilizes all existing data and objective optimal interpolation to show that a rainfall anomaly downwind of Chicago has indeed existed at least since the 1930s. The current rainfall anomaly exists as a broad region of warm season rainfall downwind of Chicago that is 20-30% greater than the regional average. Using synoptic parameters, the rainfall anomaly is shown to be independent of wind direction and most closely associated with local land surface forcing. Weekdays, where local aerosol loading has been measured at 40% or more greater than weekends, have up to 50% more warm season rainfall than weekends. The analysis is able to show that there is a land surface and aerosol contribution to the rainfall anomaly, but cannot unambiguously separate them.

In order to separate the land surface and aerosol effects on urban rainfall distribution, a numerical model was improved to better handle urban weather interaction. The Regional Atmospheric Modeling System (RAMS 6.0) was chosen for its base land surface and cloud physics parameterization. The Town Energy Budget (TEB) urban canopy model was coupled to RAMS to handle the urban land surface. The Simple Photochemical Module (SPM) was coupled with the cloud physics to handle conversion of surface emissions to CCN. The model utilized an external traffic simulation to create a realistic diurnal and weekly cycle of surface emissions, based on human behavior. The new Urban RAMS was used to study the land surface sensitivity of city size and of aerosol loading in two studies using the Real Atmosphere Idealized Land surface (RAIL) method, by which all non-urban features of the land surface are removed to isolate the urban effects. The city size study determined that the land surface of a given city eventually has a maximum effect on thunderstorm modifying potential, and that rainfall does not continue to increase or decrease locally for cities larger than a certain size based on that storm’s own motion. The aerosol-cloud analysis corroborated previous observations on the non-linear effects of aerosol loading on clouds. It also demonstrated that understanding the aerosol effect in an urban environment requires high resolution observations of precipitation change. In a single thunderstorm, regions can be both impacted by local rainfall rate increases and decreases from urban aerosols, leading to little total change in precipitation. But the rainfall rate changes can significantly affect soil moisture and drought potential in and around urban areas.Following the idealized studies, the historical and current La Porte Anomaly was simulated to separate the land surface from the aerosol factors near the Chicago area. The Urban RAMS model was deployed on a real land surface with full model physics. Simulations with 1932, 1962, 1992, and 2012 land covers were run over an exceptionally wet Aug. 2007 to approximate the rain variability for an entire summer season. Surface emissions were also varied in the 2012 land cover for variable aerosol loading. The simulations successfully reproduced the location of the downwind rainfall anomaly in each land cover scenario: farther east toward La Porte in 1932, moving southwestward to its current location by 2012. Doubling surface emissions eliminated the downwind anomaly, as was observed during the highest pollution decade of the 1970s. Eliminating surface emissions also decreased the downwind anomaly. As the land cover at the upwind edge of Chicago became more connected from the 1932 to 2012 land cover scenarios, a local upwind rainfall anomaly developed, moving westward with urban expansion. The results of these simulations enabled the conclusions that a) at the upwind edge, the land surface dominates urban rainfall modification, b) the aerosol loading sustains and increases the locally downwind rainfall increase, and c) that the total modification distance is static on given day and given urban footprint. A more expansive city does not produce a rainfall anomaly more distantly downwind, but rather the distance of rainfall modification moves to where the upwind edge of the city begins.

The modeling work ends with a two-city simulation in the southeast United States, of a bow-echo forming near Memphis, TN and crossing Birmingham, AL before splitting. Simulations were performed on different surface emissions rates, land covers where Birmingham did not exist, and a novel approach with two inner emitting grids over both Birmingham and Memphis. A storm tracking algorithm enabled one-to-one comparisons of point simulated storm characteristics between scenarios. The results of most scenarios only corroborated previous research, showing how increased aerosol loading changes cloud and rainfall characteristics until the highest aerosol loading shuts down riming and rainfall enhancement. However, the two most accurate simulations, where the storm forms and splits over Birmingham, were a non-urban higher rural aerosol scenario and the scenario with Memphis also emitting pollution. In order to split the storm over Birmingham, the upwind cloud characteristics were primed by higher upwind aerosols, either from a realistic city upwind or unrealistically high rural aerosols. The conclusions produced by this study demonstrated the importance of aerosol cloud interaction, perhaps equal with land surface, but also the need for far upwind information for a storm in a given city. Memphis and Birmingham are separated by over 300km, far exceeding the threshold thought to connect two cities by mutual rainfall modification.

The overall conclusions of the research presented in this dissertation shows a more unified approach to the effects of urban rainfall modification. The upwind edge of a city is a fixed location, and a thunderstorm begins modifying at that point. The thunderstorm usually produces a local rainfall maximum at the upwind edge, due to the vertical velocity of the urban land surface. The urban aerosols proceed to narrow the cloud droplet distribution, locally reducing rainfall as the storm passes over the urban area. Eventually the enhanced rainfall from enhanced riming produces a maximum somewhere downwind. However, “downwind” is a location relative to the storm’s motion and could exist anywhere over the urban footprint or downwind in a rural region. The climatological location of increased rainfall is an average of every storm in a season and beyond. The results of each part of the study provide a way to continue the research presented here.