Propagation of Convective Complexes Observed by TRMM in the Eastern Tropical Atlantic

Precipitation maxima during the West African summer monsoon propagate generally westward in tandem with African easterly waves. A heretofore unreported, repeating pattern of northward drift of precipitation maxima is detected on Tropical Rainfall Measurement Mission (TRMM satellite) time-latitude distributions of daily accumulations over the eastern tropical Atlantic. Corresponding 3-hourly TRMM accumulations show that the northward drifting envelopes of precipitation during August 2006 are often comprised of individual swaths propagating towards the southwest, presumably as mesoscale squall lines. The implied northward drift on the time-latitude distribution is a component of a resultant northwestward movement. The study examines the entire available record of TRMM precipitation observations, 1998-2010, to summarize TRMM maxima propagation over the eastern tropical Atlantic. Meridional displacements of precipitation maxima are most prevalent in June-September 2006, occurring less frequently during other summers. An investigation of geopotential and circulation fields, limited to two case studies, suggests mechanisms to explain some of the observed propagation of TRMM maxima. In one event, northward drift of the precipitation envelope is consistent with the corresponding displacement of the intertropical convergence zone trough, although the southwest propagation of individual mesoscale convection maxima does not correspond to any synoptic feature on reanalysis circulation or reanalysis downscaled by a regional model. One speculation is that southwestward propagation of precipitation maxima could be caused by regeneration of convection at outflow boundaries of mature thunderstorms.

A Climatological Perspective of Transient Synoptic Features during NAME 2004

This note provides a first look at a recently developed long-term climatology of transient synoptic features in northern Mexico. Key features investigated include inverted troughs, cutoff lows, cold fronts, and open troughs (westerly short waves). This 35-yr analysis of transient systems crossing northern Mexico (1967–2001) was developed to help place the summer climatology of the 2004 North American Monsoon Experiment (NAME) into a broader perspective. Inverted troughs are found to be the most commonly occurring transient synoptic feature during the monsoon with a mean frequency of occurrence of 55 days per summer season (June–September). Inverted troughs are found to contribute from 20% to 25% of the average summer rainfall observed in northern Mexico. Rainfall doubles during inverted trough days compared to days without transient systems being present. In 2004 the monsoon season was greatly shortened due to a poorly developed subtropical high. Compared to long-term means, inverted troughs contributed less rainfall to the region in 2004 and this was, in part, associated with the shortened monsoon season. In contrast, frontal penetration into the region was almost double the 35-yr mean. These climatologies are designed to provide NAME researchers with benchmarks to assess model performance relative to how these models handle these systems and their associated rainfall. The work presented is a small portion of a much larger study that aims to determine the impact of all of these rain-bearing transient systems on the monsoon in northern Mexico.