The Response of Tropical Cyclone Precipitation to Environmental Forcing

This study investigates how environmental factors influence the precipitation field of tropical cyclones. Environmental variables are computed using the ECMWF ERA40 model data from 1987-2002 and the ECMWF initial field values from 2002-2005. The precipitation field dataset is constructed from a large sample size of satellite-derived rain rates via passive microwave radiometers (SSMI, TMI, AMSRE) from 1987 to 2005, using the Remote Sensing Systems retrievals.

Composite rain rates are constructed as a function of varying environmental parameters such as shear, SST, and moisture convergence. These composites illustrate the spatial distribution and magnitude of tropical cyclone rain rates as a function of the environment.

Initial results using shear predictors from the SHIPS model (based on NCEP-NCAR reanalysis and GFS model) agree with previous studies. Weakly sheared tropical cyclones (less than 5 m/s) maintained a reasonably symmetric precipitation field, while moderately sheared (5-10 m/s) cases exhibited an asymmetric appearance. Strongly sheared (greater than 10 m/s) systems boasted the greatest asymmetry. The asymmetry was also a function of storm intensity, which decreased with increasing storm intensity for all shear values. In other words, tropical storms were the most asymmetric while major hurricanes (category 3 and above) were the most symmetric for all shear values. For all cases of storm and shear intensities, the precipitation field was displaced downshear and left of the shear vector.