African Easterly Wave Convection and Tropical Cyclogenesis: A Lagrangian Perspective

Convection resident in African easterly waves (AEWs) has been hypothesized to play an important role in tropical cyclogenesis via thermodynamic and dynamic feedbacks to the larger meso-to-synoptic scale circulation. In particular, convection/rainfall may aid cyclogenesis via a "top-down" approach where a midlevel circulation develops downwards (generally associated with a greater coverage by convection/rainfall) or a "bottom-up" approach where a circulation develops upward from the surface (generally associated with intense convective "hot towers"). In this study, we examine the evolution of convection and cold cloudiness associated with AEWs in the days leading up to tropical cyclogenesis using data from the NCEP Reanalysis and Tropical Rainfall Measurement Mission (TRMM) lightning imaging sensor (LIS), precipitation radar (PR), and microwave imager (TMI).

NCEP Reanalysis 700 hPa meridional winds for the months of June to November for the years 2001–2010 were analyzed for the domain of 5ºN–20ºN and 130ºW–20ºE in order to partition individual AEWs into northerly, southerly, trough, and ridge phases. Subsequently, information from National Hurricane Center (NHC) storm reports was used to identify AEWs which developed tropical cyclones (i.e., developing waves). Composites were created as a function of wave phase and day relative to the day of tropical cyclogenesis (defined as the day for which a tropical depression was identified by the NHC) for five days prior until one day after genesis using TRMM PR, TMI, LIS, and IR brightness temperature data extracted from the NASA global-merged infrared brightness temperature dataset to examine the evolution of convection/cold cloudiness. The full 130ºW–20ºE analysis domain was broken down into five longitude bands, and composites were created for all developing waves over the full analysis domain as well as individual longitude bands in order to examine the evolution in different regions. Finally, similar composites were created using various NCEP variables to assess the associated evolution of the larger scale AEW environment and circulation.

Results suggest that convection/cold cloudiness associated with developing waves valid over the full domain (only data over the ocean was utilized for these composites to eliminate any influence from land/ocean differences) increases in coverage but decreases in intensity from five days prior to genesis (day-5) to the day of genesis (day0). For example, the composite fraction of a 2.5° box covered by IR brightness temperatures less than 210 K for the trough phase more than doubles from 0.014 on day-5 to 0.029 on day0 while the composite lightning flash rate decreases from 52.9 to 18.2 flashes day^-1. The increase in coverage associated with a decrease in intensity of convection may suggest a greater importance for the top-down genesis mechanism. However, different results are obtained over some individual longitude bands. For instance, the convection/cold cloudiness associated with waves that developed over the Caribbean region (95°–70°W) appears to increase in coverage and intensity as genesis is approached (210 K threshold increases from 0.011 on day-5 to 0.041 on day0, and the flash rate increases from 42.5 flashes day^-1 on day-5 to 63.9 flashes day^-1 on day0). This may suggest that the bottom-up mechanism is more important for genesis in some regions than in others. The top-down and bottom-up mechanisms involve the evolution of circulations, not necessarily convection. Hence, further analysis will be conducted of the evolution of the large-scale circulation using NCEP data. However, the resolution of this dataset is probably too coarse to completely and accurately assess the relative importance of the top-down and bottom-up mechanisms (which operate on meso and smaller scales) for tropical cyclogenesis over different regions.