Warm season Gulf Stream Lightning: Convective Structure and Forcing

Climatologies of NLDN Cloud-to-Ground (CG) and TRMM-LIS total lightning flash densities demonstrate an off shore maximum in lightning activity over the Gulf Stream region oriented parallel to the east coast of the U.S. This lightning maximum is associated with convection exhibiting characteristically continental vertical structure in fields of both radar reflectivity and ice water path, as observed by the TRMM Precipitation Radar.

From a forcing perspective, the diurnal cycle of lightning and precipitation over the Gulf Stream exhibits an extended nocturnal and early morning peak, reaching a maximum near 12 UTC (7 AM local). NLDN lightning data suggest that systems forming over land the previous day may undergo an extended period of resurgence in intensity and thus their ability to generate lightning as they pass over the warm waters of the Gulf Stream (28 C). Analysis of climatological variables in active periods of Gulf Stream lightning indicate the presence of increased surface temperatures, increased surface and mid-level humidity and broad scale ascent. That is, available thermodynamic reanalysis data suggests that the warmer, more moist conditions at the surface combined with favorable synoptic features lead to increased CAPE over the Gulf Stream, promoting stronger vertical motion and enhanced mixed-phase microphysics, leading to higher lightning flash rates relative to adjacent coastal waters. However, concomitant background aerosol optical depths were also analyzed using MODIS data and found to be larger (smaller) during periods of enhanced (reduced) lightning over the Gulf Stream. Thus the influence of aerosols on cloud microphysical properties, and in particular mixed phase microphysics, may also play a role in providing a favorable environment for the development of lightning-producing convection over the Gulf Stream.