The diurnal cycle of rainfall and the identification of rainfall regimes within the North American monsoon of NW Mexico

During the summer of 2004, a special observing network, including three ground-based Doppler radars, monitored a region of northwest Mexico. Climatologically, this region is at the heart of the North American monsoon and, therefore, is of interest to climate variability studies under the World Climate Research Programme. As part of the North American Monsoon Experiment (NAME), these radars provided a close-up view of precipitation systems for a six week period following monsoon onset in the region north and west of Mazatlan. This region is known to have complex interactions among easterly waves, tropical cyclones, and deep convection over the Sierra Madre Occidental(SMO) and the Gulf of California(GoC). The radars performed rainfall climatology scans at 15 minute intervals nearly 24 hours per day for the period of record.

We present preliminary findings on the diurnal cycle of rainfall and the variability observed in regimes of convection. We also quantify the diurnal sea/land breeze circulations and other environmental flow characteristics. Our approach follows the reduced-dimension analysis methods of Carbone et al (1998, 2002). We examine the onset and movement of convection both parallel and perpendicular to the SMO and the GoC coastline, assuming that thermal forcing strongly influences the observed rainfall patterns. We observe at least two regimes:

A diurnally-forced "undisturbed" regime in which rainfall originates over high terrain of the SMO and subsequently moves westward to the coast during the late afternoon, and

A "disturbed" regime (or regimes), which exhibits considerable variability with respect to diurnal forcing.

The "undisturbed" regime: mainly produces rainfall in the region between the upper SMO and the mid-GoC; is heavily influenced by the elevated heat source and breeze circulations; and exhibits relatively little propagation of organized convection parallel to the mountains and the coast. The "disturbed" regime(s) is (are) often associated with longer-lived precipitation episodes and/or northwestward propagation nearly parallel to the coast. At least some of these events are associated with one or more of the following: so-called Gulf "surges" of southerly momentum; the passage of easterly waves; and the movement of tropical cyclones, which are often located to the south of our domain.

Between the writing of this abstract and the Conference presentation, we will further stratify the diurnal cycle data and quantify the lifecycle of events for both regimes. The statistics should help to define controlling factors in the regional climatology of rainfall and lead to improvements in the represeentation of convection for regional climate and NWP models.