New Insights on the North American Monsoon from the 2004 Name Field Campaign

Abstract:

The 2004 North American Monsoon Experiment (NAME) was an international field campaign conducted over northwestern Mexico, the Gulf of California, and the surrounding region. The overarching goal of NAME was the determination of the sources and limits of predictability of warm season precipitation over North America, with emphasis on time scales ranging from seasonal-to-interannual. The observational component of NAME involved measurements of a range of monsoon phenomena using aircraft, soundings, profilers, research radars, ships, surface stations, rain gauges, and surface flux sites.

The NAME enhanced observational network has confirmed a number of previous theories and hypotheses with respect to North American Monsoon (NAM) processes, but also yielded new insight and details regarding specific phenomena of the NAM region. Several new findings stemming from NAME observations are:

1. The 12-14 July 2004 overall gulf surge event was comprised of several stages: an initial disturbance identified as a Kelvin shock (an internal bore modified by rotation) followed by a general acceleration of the flow by multiple convectively generated gravity currents and bores along the span of the Gulf, then ending again in the northern Gulf as a Kelvin shock disturbance;
2. A tropical upper-tropospheric trough (TUTT) associated with the 12-14 July Gulf Surged enhanced precipitation on its western flank as it passed over the mountains of northern Mexico. It is found that these upper-level disturbances modify the low-level flow and slightly increase CAPE so as to cause more highly organized mesoscale convective systems to move off the Sierra Madre Occidental toward the west and in doing so, strengthen moisture surges over the Gulf of California; and 
3. The diurnal cycle of convection determined from sounding-based heat and moisture budgets over the Sierra Madre Occidental (SMO) is characterized by shallow convection around noon, deep convection at 1800 LT, evolving to stratiform precipitation by midnight. The upslope flow along the western slope of the SMO is delayed until late morning, likely in response to early morning clouds over the SMO and reduced morning insolation over the west-facing slopes.