Radar-observed characteristics of the diurnal cycle of precipitation during NAME 2004

A multi-radar network operated in the southern Gulf of California (GoC) region of northwest Mexico, during the 2004 North American Monsoon Experiment, is used to analyze the diurnal cycle of precipitation in this region. It is found that terrain played a key role, as the diurnal cycle was dominated by convective triggering during the afternoon over the peaks and foothills of the Sierra Madre Occidental (SMO) mountain range. Precipitating systems gradually grew upscale and moved WNW toward the Gulf, propagating weakly ahead of steering winds. Distinct precipitation regimes within the monsoon are identified. The first, Regime A, corresponded to enhanced precipitation over the southern portions of the coast and GoC, typically during the overnight and early morning hours. This was due to precipitating systems surviving the westward trip from the SMO after sunset, likely because of enhanced environmental wind shear as diagnosed from local soundings. The second, Regime B, corresponded to significant northward/coast-parallel movement of systems phase-locked to the diurnal cycle, and often overlapped with Regime A. During undisturbed regimes (i.e., neither A nor B), precipitating systems are mainly confined to the SMO and do not last beyond local midnight; that is, they decay before they move significantly off the SMO. Mesoscale convective systems and other organized precipitation features were responsible for most of the rainfall in this region, particularly during the disturbed regimes. Organized systems are most important during the early evening hours over the SMO, and during the early morning hours over the southern GoC and adjacent coastal plain. Other characteristics of the diurnal cycle also will be addressed in this report.