Orographic Modification of Precipitation Processes in Hurricane Karl (2010)

Abstract

AbstractAirborne radar data collected within Hurricane Karl (2010) provide a high-resolution glimpse of variations in the vertical precipitation structure around complex terrain in eastern Mexico. Widespread precipitation north of Karl?s track traced the strong gradient of terrain, suggesting orographic enhancement. Although the airborne radar did not sample the period of peak precipitation, time series of surface rainfall at three locations near the inner core show greater precipitation where flow was oriented to rise over the terrain. In regions of upslope flow, radar observations reveal reflectivity enhancement within 1?2 km of the surface. The shallow nature of the enhancement points to orographically generated cloud water accreted by falling drops as a mechanism consistent with prior studies, while the heterogeneous nature of the enhancement suggests shallow convection was playing a role. In contrast, regions of downslope flow were characterized by uniform reflectivity above the ground and fallstreaks originating above the melting level. Unlike most previously studied tropical cyclones passing over topography, Karl made landfall on a mountainous continent, not an island. As Karl weakened and decayed over land, the vertical structure of the radar echo deteriorated north of the storm center, and infrared satellite imagery revealed a strong reduction in the upper-level cloud coverage; however, a small region of intense convection appeared and produced locally heavy rainfall as Karl was close to dissipation. These results indicate that orographic modification processes in a landfalling tropical cyclone are not static, and surface precipitation is highly sensitive to the changes.