Wednesday, 11 July 2018
Regency A/B/C (Hyatt Regency Vancouver)
Deep convective updraft properties predicted by cloud-resolving model simulations are known to be highly uncertain, which increasing model resolution unfortunately cannot resolve owing to its roots in microphysical scheme parameterizations. Because in situ measurements capable to well evaluate microphysics schemes are likely to remain rare especially over land and because such measurements will always remain profoundly sparse for well sampling the complex structure of evolving updrafts, it is incumbent on modelers to better harness remote-sensing measurements going forward. Satellite and ground-based remote sensing measurements both offer excellent strengths; of these, ground-based polarimetric radars offer a superior combination of rapid volume scanning and richness of data obtained throughout updraft volumes. It has now been several years since the National Weather Service's Next-Generation Radar (NEXRAD) network was widely upgraded to operational polarimetric capabilities. Here we use data from the NEXRAD KHGX radar in Houston, Texas, to establish foundations for a proposed field campaign dedicated to study of isolated convective updrafts under onshore flow conditions. Under such conditions, some convective cells form in air polluted by emissions from Galveston Bay and Houston areas, unlike upwind and adjacent cells within the KHGX domain, offering an aerosol perturbation that is advantageous both to better establish aerosol impacts on weakly forced convection and to offer cloud modelers observational targets over a relevant variable range. For modelers, isolated cells are also substantially simpler than organized systems, where updrafts commonly entrain varying amounts of previous outflows that substantially complicate robust observational constraints. In one effort presented here, isolated convective cells are objectively identified and tracked in four years of KHGX data, allowing to establish the optimal time of year and location for viewing as many cells as possible, as well as the typical cell track and lifetime. Results indicate that distance from the radar may substantially influence properties of conventionally derived specific differential phase (Kdp) and differential reflectivity (Zdr) column properties. In a companion effort, several tracked cells are selected from observations and from a Weather Research and Forecasting model simulation of 8 June 2013. Kdp and Zdr are forward calculated from simulated rain properties for rough comparison with observations, and 3D polarimetric retrievals of collocated rain rate and size distribution properties are compared with simulated values. Results indicate that the proposed experimental targets are robust for model evaluation, despite uncertainties in forward simulation, and that Lightning Mapping Array observations provide unique added value. A field campaign would ideally add at least one local sounding station, ground-based aerosol size distribution and hygroscopicity measurements from at least two sites, and rapid-scanning radars with capabilities and flexibilities beyond operational NEXRAD.
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