Observing and Modeling Urban Thunderstorm Modification Due to Land Surface and Aerosol Effects

2020-05-12T21:29:16Z (GMT) by Paul E. Schmid

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.