Relationships between Tropical Cyclone Intensity and Satellite-Based Indicators of Inner Core Convection: 85-GHz Ice-Scattering Signature and Lightning
A key component in the maintenance and intensification of tropical cyclones is the transverse circulation, which transports mass and momentum and provides latent heat release via inner core convective updrafts. This study examines these updrafts indirectly, using satellite-borne observations of the scattering of upwelling microwave radiation by precipitation-sized ice particles and satellite-borne observations of lightning. The observations are then compared to tropical cyclone intensity (defined here as maximum sustained wind speed) and the resulting relationships are assessed. Substantial updrafts produce large ice particles aloft, which in turn produce microwave ice-scattering signatures. The large ice, together with supercooled liquid water also generated by substantial updrafts, is a necessary ingredient in charge separation, which leads to lightning. Various parameters derived from the inner core ice-scattering signature are computed for regions encircling hurricanes and typhoons, and observations of lightning activity or inactivity are analyzed. High correlations with future tropical cyclone intensity result from the ice-scattering signature parameters most closely associated with the areal extent of at least moderate precipitation rates. As expected, the relationship reveals increasing intensity with increasing ice-scattering signature. Indicators of more intense convection yield less information concerning tropical cyclone intensity. Correlations tend to be of the same sign for both present cyclone intensity at the time of the satellite overpass and subsequent intensity change. Correlations are higher for future cyclone intensity than for either of these. The lightning observations are much more limited than the microwave observations, because the short amount of time in which lightning can be detected may not adequately represent a particular storm’s electrical activity. The inner core lightning observations show no clear relationship to tropical cyclone intensification. However, the lightning observations do suggest an increased likelihood of inner core lightning in weak tropical storms and strong hurricanes/typhoons. In the examination of case studies, the paradoxical situation of much greater lightning frequency in rainbands than in eyewalls is noted.