Diurnal Cycle of Convection during the 2004 North American Monsoon Experiment

Abstract

The diurnal cycle of summer monsoon convection in the coastal, mountainous region of northwestern Mexico is investigated using data from the 2004 North American Monsoon Experiment (NAME). Data from a special sounding network consisting of research and operational sites have been quality controlled and combined with surface, wind profiler, and pibal observations to create a gridded dataset over the NAME domain. This study concentrates on results from the interior portion of the NAME sounding network, where gridded analysis fields are independent of model data. Special attention is given to surface and pibal observations along the western slope of the Sierra Madre Occidental (SMO) in order to obtain an optimal analysis of the diurnally varying slope flows. Results show a prominent sea-breeze?land-breeze cycle along the western slopes of the SMO. There is a deep return flow above the afternoon sea breeze as a consequence of the elevated SMO immediately to the east. The upslope flow along the western slope of the SMO is delayed until late morning, likely in response to early morning low clouds over the SMO crest and reduced morning insolation over the west-facing slopes. The diurnal cycle of the net radiative heating rate is characterized by a net cooling during most of the daytime except for net heating in the lower and upper troposphere at midday. The diurnal cycle of the apparent heat source Q1 minus the radiative heating rate QR (providing a measure of net condensational heating) and the apparent moisture sink Q2 over the SMO is indicative of shallow convection around noon, deep convection at 1800 LT, evolving to stratiform precipitation by midnight, consistent with the radar-observed diurnal evolution of precipitation over this coastal mountainous region as well as the typical evolution of tropical convective systems across a wide range of spatial and temporal scales. Convection over the Gulf of California is strikingly different from that over land, namely, heating and moistening are confined principally to the lower troposphere below 700 hPa, peaking during the nighttime hours.