African Easterly Wave Cloud and Precipitation Structure and Implications for Tropical Cyclogenesis

African easterly waves (AEWs) are important for tropical cyclogenesis in the Atlantic and East Pacific.  But, it is not completely clear why some waves spawn tropical cyclones (developing waves [DWs]) while others do not (non-developing waves [NDWs]).  One factor that may a role in tropical cyclone development is the structure and distribution of clouds/precipitation in AEWs.  Another factor that may influence development is the Saharan Air Layer (SAL), a dry, dusty layer of air often blown over the Atlantic from North Africa.  Earlier work has suggested that the dry air and/or enhanced stability associated with the SAL may inhibit convection and tropical cyclogenesis beneath the SAL itself, while other studies have suggested that temperature gradients associated with the SAL may strengthen AEWs to the south of the SAL, potentially enhancing the likelihood of tropical cyclogenesis there.  The first of two objectives of this project is to use the Cloud Profiling Radar on NASA’s CloudSat satellite to examine the detailed cloud and precipitation structure of DWs and NDWs to determine any significant differences between them.  The second objective is to use aerosol information derived from NASA Moderate Resolution Imaging Spectroradiometer (MODIS) data to infer information about the characteristics of the SAL and determine whether these characteristics significantly differ between the two wave types.  Ultimately, this project aims to improve forecasts of tropical cyclogenesis from AEWs.

National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis 700-hPa wind data will be used to separate AEWs into ridge, northerly, trough, and southerly phases for June–November 2006–2015 over a region stretching from 80°W–20°E and 5°–20°N over the tropical Atlantic and West Africa.  Information from National Hurricane Center storm reports will be used to separate DWs from NDWs.  Composites of cloud coverage as a function of wave phase and type will be generated using both CloudSat and MODIS data.  In addition, composites of reflectivity, cloud optical depth, liquid water, etc., will be generated using various CloudSat products.  Aerosol optical depth from MODIS will also be composited as a function of wave phase and type as a proxy for the strength of the SAL.  Finally, the distance between the maximum aerosol optical depth (SAL maximum) and the vorticity maxima in DW and NDW troughs will be composited as an indication of the distance between the SAL and AEWs.

Early results of this work will be available soon.  We hypothesize that these results will indicate that DWs are associated with significantly greater cloud coverage, especially in the trough, compared to NDWs.  The aerosol optical depth may not vary significantly between DWs and NDWs, but we hypothesize that the SAL peak occurs farther north of DW troughs compared to NDW troughs.