Variability of convective structure and lightning activity in tropical easterly waves
Walter A. Petersen, University of Alabama, Huntsville, AL; and D. J. Boccippio
Three-to-Five day Tropical Easterly Waves (TEWs) significantly modulate tropical convection and precipitation and provide the requisite background for tropical cyclogenesis. Previous studies have characterized both the TEW signal in precipitation and cloudiness fields over the eastern Atlantic and tropical Pacific, and transitions in these fields across different phases of TEWs. However, prior to the launch of the Tropical Rainfall Measurement Mission (TRMM) there have been only limited observational datasets available for characterizing the vertical structure of convection across TEW wave phases over the global tropics. Importantly, convective vertical structure information provides useful information for remotely inferring latent heating profiles. In this study we take full advantage of a new TRMM combined dataset that includes precipitation radar (PR), lightning imaging sensor (LIS) and microwave imager (TMI) data, NCEP Reanalysis winds and PR reflectivity-profile cluster analysis, to characterize the full 4D structure of convection in TEWs as a function of wave phase over remote regions of continental West Africa and the eastern Pacific Ocean.
In prior analysis of data from the three-week EPIC-2001 ITCZ field experiment held in the tropical east Pacific, we established that in three separate TEWs lightning flash density, convective vertical development, and conditional mean rain rates all varied systematically as a function of wave phase. Specifically, lightning flash counts, convective vertical structure, areas of heavy rain rate, and CAPE were all observed to reach maximum (minimum) values just prior to (after) the passage of TEW troughs. Conversely, convective echo coverage and areas of light rain were an order of magnitude larger after TEW trough passage. These trends suggest similar systematic changes in the diabatic heating structure across the phases of eastern Pacific TEWs.
In order to extend the limited EPIC TEW results, we have analyzed TRMM observations (PR, LIS, TMI) and statistically-partitioned TRMM PR convective structure types for multiple warm seasons (June-October, 1998-2001) over the E. Pacific and over continental W. Africa. The TRMM temporal sampling bias has been mitigated by compositing the data into wave phases (ridge, northerly, southerly and trough) using 700 mb meridional winds. The results reveal: a) reasonable consistency with previous EPIC-2001 results for the eastern Pacific/Mexican warm-pool region; b) the presence of more deep convective vertical structure modes and enhanced lightning activity over W. Africa in the ridge and pre-trough phases, with more area-extensive, but weaker convection, in the trough phase; and c) that the diurnal cycles of lightning and convective vertical structure in W. African TEWs are distinctly different, with pre-trough (trough) conditions characterized by a more (less) peaked afternoon/early evening maximum in deep vertical convective development and lightning activity. To further characterize the observed changes in convective vertical structure as a function of synoptic scale, we are currently examining the TRMM dataset, together with a matched database of NCEP Reanalysis variables. This should enable relationships to be diagnosed between convective structure characteristics of TEWs and TEW wave dynamics.
Extended Abstract (256K)
Session 5B, CONVECTION, waves, and precipitation III
Tuesday, 4 May 2004, 8:00 AM-9:30 AM, Napoleon I Room
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