Horizontal Organization of Precipitating Systems and Its Relationship to Synoptic and Thermodynamic Conditions during NAME

One of the main scientific objectives of the North American Monsoon Experiment (NAME) is to achieve a better understanding and more realistic numerical simulations of warm season convective processes in complex terrain (Higgins et al., 2004). Rogers and Johnson (2007) have hypothesized that gulf surges, an important mechanism for moisture transport into the US desert southwest, may be initiated by convective downdrafts impigining on nocturnal inversions in the region. The strength of convective downdrafts, as well as other storm thermodynamic and kinematic properties, such as heat and momentum transport, is strongly correlated with the horizontal structure of precipitating systems. For instance, mesoscale convective systems tend to heat the upper-levels more efficiently due to their larger stratiform areas compared to isolated convective systems; and LeMone et al. (1984) showed that features with linear organization tend to consistently increase momentum at upper and low levels. The frequency of occurrence of mesoscale convective systems and smaller unorganized features over the NAME region is not well known and understanding these statistics is of paramount importance for operational forecasting and cloud modeling.

The horizontal organization of precipitating systems during NAME is examined in this study using composite data from three radars located near the Gulf of California (GoC). The horizontal structure of these precipitating features is examined in terms of horizontal scale (mesoscale or sub-mesoscale) and morphology (linear or non-linear), following Rickenbach and Rutledge (1998). In addition to documenting the frequency of occurrence, rainfall production and convective fractions of these organizational modes, this study focuses on the relationship between the organizational modes and the synoptic and thermodynamic conditions in the south-central part of the GoC using the CSU-NAME upper-air and surface gridded analyses and the North American Regional Reanalysis (NARR). The parameters used to assess the synoptic and thermodynamic conditions in the region include easterly wave phases, general propagation direction, convective available potential energy, vertical wind shear and bulk richardson number. More than 100,000 precipitation features were examined in this study, and while sub-mesoscale non-linear features are the overwhelmingly dominant mode of organization, only 30% of the total rainfall is associated with this group. The majority (63%) of the total rainfall generated during NAME was associated with large and organized, mesoscale linear features. Preliminary results also indicate that different phases of the easterly waves passing south of the NAME radar composite domain do not significantly modulate the amount of either the rainfall in the GoC region or the frequency of occurrence of the different organization modes examined in our study. However, the preliminary results indicated that easterly waves may actually play a role in enhancing precipitation in different parts of this study's domain (i.e. the Baja Peninsula, the coast and the Sierra Madre Occidental).