Precipitation response to environmental forcing in tropical cyclones

Environmental vertical wind shear has long been known to have a fundamental effect on tropical cyclone structure. Various case studies and numerical modeling experiments have documented asymmetries that result from shear. A common result is that vertical motion and precipitation is maximized downshear and left of the shear vector.

This study examines the precipitation fields for a large number of tropical cyclones, using all available passive microwave imager overpasses from the last several tropical cyclone seasons. This amounts to thousands of snapshots of hundreds of storms. These data are coupled with estimates of environmental conditions (shear, sea surface temperature, relative humidity, etc.) from NCEP reanalyses.

An initial result confirms that mean rain rates are two or three times greater in the downshear and left-of-shear quadrants than in the upshear and right-of-shear quadrants. This is true throughout the innermost 300 km. Beyond that radius, a downshear maximum dominates without much left-to-right variability.

Initial results suggest that the effects of sea surface temperature on precipitation are greatest at large radius. Between 200-400 km radius, the mean rain rates are nearly twice as high for cases with SST > 29 C than for cases with SST between 27-29 C.