Cell Merger Processes and their Influence on Cloud Microphysics, Kinematics, and
Electrification over Houston

 

Abstract

One very interesting hypothesis advanced during radar studies of convection during METROMEX (Metropolitan Meteorological Experiment) and highly relevant to HEAT (Houston Environmental Aerosol Thunderstorm Project) objectives, concerns the influence of the urban environment on convective frequency and mesoscale organization. Specifically, coarse radar observations from METROMEX suggested enhanced convective cell merging and associated intensification of radar echoes generated over and downwind of the urban corridor. The UHI (Urban Heat Island) along with other mesoscale boundaries (e.g., the sea breeze) provide a mechanism to trigger and release instability in the air passing overhead. The UHI and sea breeze induced convergence boundaries then interact favorably with cold pool boundaries generated by the downdrafts of dissipating convection to trigger new convective cells, which then merge with older cells, resulting in upscale development. To the extent that UHI, sea breeze, or interaction of the two features acts to initiate convection over a coastal city and create a favorable environment for enhanced system merger downwind, we hypothesize that convective intensity (kinematic and microphysical) will adjust symbiotically to these mesoscale features to create a local maximum in rainfall and lightning flash density over and just downwind of the Houston metropolis. Most importantly, we hypothesize that this juxtaposition of convective intensity and lightning relative to the urban corridor takes place regardless of the aerosol content of the boundary layer.

We propose to use both NCAR S-Pol and CSU-CHILL dual-polarimetric radar observations in coordination with SMART-R Doppler measurements to retrieve Lagrangian 4-D kinematic (e.g., updraft strength) and microphysical (e.g., ice and mixed phase precipitation mass) parameters in convection as it initiates, evolves, and moves over both urban and non-urban environments. High resolution, low-level SMART-R dual-Doppler observations of cold pool, UHI, and sea-breeze interactions will provide important details of the enhanced merger process downwind of the urban environment. Our study will provide both statistics on environmental flow and cell evolution over the urban area for model validation, and concrete physical evidence supporting or refuting our hypothesis regarding the urban (i.e., UHI/land surface and sea breeze) influence on convective merger and intensity downwind of Houston.