A polarimetric radar analysis of convection observed during NAME and TiMREX

The mountainous regions of northwestern Mexico and southwestern Taiwan experience periods of intense rainfall associated with the North American and Asian monsoons. Prediction of warm-season rainfall in these regions is limited by the lack of understanding of the nature of precipitating features, including the diurnal variability and elevation-dependent trends in microphysical processes. Using data from NCAR's S-band, polarimetric radar (S-Pol), deployed during the North American Monsoon Experiment (NAME) and the Terrain-influenced Monsoon Rainfall Experiment (TiMREX), individual convective elements were identified and tracked, allowing for an analysis of hydrometeor characteristics within evolving cells. Furthermore, isolated convection was compared to cells embedded within organized systems.

Polarimetric observations in isolated cells over a range of topographical elevations during NAME revealed large drops being lofted above the melting level, where subsequent freezing and growth by riming led to the production of graupel and hail along the western slopes of the Sierra Madre Occidental (SMO) and adjacent coastal plain. Melting of large ice hydrometeors was also noted over higher terrain, leading to short-lived yet intense rainfall despite truncated warm-cloud depths compared to cells over the lower elevations. Cells embedded within MCSs during NAME also displayed the combined roles of warm-rain and ice-based microphysical processes as convection organized along the terrain. In addition to enhancing precipitation along the western slopes of the SMO, melting ice contributed to the production of mesoscale outflow boundaries, which provided an additional focus mechanism for convective initiation over the lower elevations and aided in the propagation of these systems toward the coast.

Intense rainfall was also observed along the Central Mountain Range (CMR) in Taiwan; however, in contrast to the NAME systems, this enhancement occurred as MCSs moved onshore within southwesterly flow and intercepted the CMR's steep windward slopes. Elevated maxima in polarimetric variables, similar to the case in NAME, indicated a substantial contribution from melting ice to rainfall at these higher elevations. Vertical profiles of ice mass, however, revealed greater amounts throughout the entire vertical depth of convection during NAME. Nonetheless, instantaneous rain rates were comparable during both experiments, suggesting efficient warm-rain processes within convection observed in the TiMREX radar domain. In addition, the greatest contribution to hourly accumulated rain mass in these regions was associated with deep organized systems along the western slopes, posing threats along the steep topography due to flash flooding and subsequent landslides and emphasizing the need for improved understanding of the processes leading to intense rainfall in these vulnerable regions.