Polarimetric Radar Observations of Convection in Northwestern Mexico during the North American Monsoon Experiment

Abstract

The spatial and temporal variability of convection during the North American Monsoon Experiment (NAME) was examined via analysis of three-dimensional polarimetric radar data. Terrain bands were defined as the Gulf of California (over water) and elevations of 0?500 m above mean sea level (MSL; coastal plain), 500?1500 m MSL, and >1500 m MSL. Convective rainfall over the Gulf typically featured the smallest values of median volume diameter (D0) regardless of rain rate. Gulf convection also contained reduced precipitation-sized ice water mass but proportionally more liquid water mass compared to convection over land. These maritime characteristics were magnified during disturbed meteorological regimes, which typically featured increased precipitation over the Gulf and adjacent coastal plain. Overall, the results suggest increased reliance on warm-rain collision and coalescence at the expense of ice-based precipitation growth processes for convective rainfall over the Gulf, relative to the land. Over land D0, ice, and liquid water mass all increased with decreasing terrain elevation, suggesting intensification of convection as it moved off the Sierra Madre Occidental. The results are consistent with the hypothesis that both warm-rain and ice-based rainfall processes play important roles in precipitation formation over land. Coastal-plain convection underwent microphysical modifications during disturbed meteorological regimes that were similar to Gulf convection, but the changes were less dramatic. High-terrain convection experienced little microphysical variability regardless of meteorological regime.