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David G. Lerach and William R. Cotton

Abstract

Four three-dimensional, nested-grid numerical simulations were performed using the Regional Atmospheric Modeling System (RAMS) to compare the effects of aerosols acting as cloud condensation nuclei (CCN) to those of low-level moisture [and thus convective available potential energy (CAPE)] on cold-pool evolution and tornadogenesis within an idealized supercell storm. The innermost grid possessed horizontal grid spacing of 111 m. The initial background profiles of CCN concentration and water vapor mixing ratio varied among the simulations (clean versus dusty and higher-moisture versus lower-moisture simulations). A fifth simulation was performed to factor out the impact of CAPE. The higher-moisture simulations produced spatially larger storms with stronger peak updrafts and low-level downdrafts, heavier precipitation, greater evaporative cooling, and stronger cold pools within the forward and rear flank downdrafts. Each simulated supercell produced a tornado-like vortex. However, the lower-moisture simulations produced stronger, longer-lived vortices, as they were associated with weaker cold pools and less negative buoyancy within the rear flank downdraft. Raindrop and hailstone concentrations (sizes) were reduced (increased) in the dusty simulations, resulting in less evaporative cooling and weaker cold pools compared to the clean simulations. With greater terminal fall speeds, the larger hydrometeors in the dusty simulations fell nearer to the storm’s core, positioning the cold pool closer to the main updraft. Tornadogenesis was related to the size, strength, and location of the cold pools produced by the forward and rear flank downdrafts. Not surprisingly, while the aerosol effect was evident, the influences of low-level moisture and CAPE had markedly larger impacts on tornadogenesis.

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David G. Lerach, Steven A. Rutledge, Christopher R. Williams, and Robert Cifelli

Abstract

This study describes the vertical structure of mesoscale convective systems (MCSs) that characterized the 2004 North American monsoon utilizing observations from a 2875-MHz (S band) profiler and a dual-polarimetric scanning Doppler radar. Both instrument platforms operated nearly continuously during the North American Monsoon Experiment (NAME). A technique was developed to identify dominant hydrometeor type using S-band (profiler) reflectivity along with temperature. The simplified hydrometeor identification (HID) algorithm matched polarimetric scanning radar fuzzy logic–based HID results quite well. However, the simplified algorithm lacked the ability to identify ice hydrometeors below the melting layer and on occasion, underestimated the vertical extent of graupel because of a profiler reflectivity bias.

Three of the strongest NAME convective rainfall events recorded by the profiler are assessed in this study. Stratiform rain exhibited a reflectivity bright band and strong Doppler velocity gradient within the melting layer. Convective rainfall exhibited high reflectivity and Doppler velocities exceeding 3 (−10) m s−1 in updrafts (downdrafts). Low-density graupel persisted above the melting layer, often extending to 10 km, with high-density graupel observed near 0°C. Doppler velocity signatures suggested that updrafts and downdrafts were often tilted, though estimating the degree of tilt would have required a more three-dimensional view of the passing storms. Cumulative frequency distributions (CFDs) of reflectivity were created for stratiform and convective rainfall and were found to be similar to results from other tropical locations.

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