Documentation of improvements in tropical cyclone (TC) track forecasting by nearly 50% have been noted by the National Hurricane Center since 1990, however a concurrent lack of improvement has been noted in intensity forecasting. Particularly troubling to predict have been episodes of rapid intensification (RI) that are poorly anticipated by forecast models, possibly due to the influence of microphysical processes driving TC intensity change. Passive microwave sensors such as the Special Sensor Microwave Imager (SSM/I) and the Tropical Rainfall Measuring Mission Microwave Imager (TMI) are able to retrieve microphysical information about precipitation features in TCs through emission by liquid water at low frequency channels and ice-scattering at higher frequencies, which may detect key precursors to TC intensification.
A dataset consisting of every SSM/I and TMI TC overpass globally from the launch of each instrument through 2008 is used to examine the predictive potential of TC intensity and intensity change based on annular statistics of brightness temperatures and retrieved rain rates. Correlation of these parameters with storm intensity proves to be highly skillful, while much less skill seen in intensity change evaluation with particular lack of skill noted for storms undergoing RI or rapid weakening. Thus, the magnitudes of brightness temperature depressions, as used in operational schemes, do not yield useful relationships with intensity change.
However, when the morphology of the storms is considered, processes related to RI suggest improved predictive skill. Composite analyses of the satellite overpasses reveal fundamental differences between RI overpasses and other intensity change classifications, with a ring feature surrounding the core and stronger convective signal evident in the RI classification. Extending 24 hours before and after RI onset a pattern is noted of the ring feature developing 6 hours prior to the onset of RI and over time contracting and intensifying through 18 hours after RI begins. These composites are then broken down according to wind shear magnitude to evaluate the role of shear in the precipitation structures observed for systems undergoing RI, and evaluate other potential modes of RI in addition to the axisymmetric route.
Daniel Harnos came to the University of Illinois after receiving a B.S. in Meteorology with an Environmental Sciences minor from Rutgers University. Before starting graduate school he worked at the Mount Washington Observatory, site of the highest recorded wind speed in the Northern Hemisphere. Currently he is a recipient of a NASA Earth and Space Science Fellowship supporting his work with Steve Nesbitt. He recently returned from several weeks of acting as a forecaster for NASA’s Genesis and Rapid Intensification Processes field campaign.