Convective and Rainfall Properties in the Inner Core and Tropical Cyclone Intensity Change Using 11-yr TRMM Data

The Tropical Rain Measuring Mission (TRMM) satellite has observed more than 1000 tropical cyclones since its launch in late 1997. The unique suite of instruments, including Precipitation Radar (PR), TRMM Microwave Imager (TMI), Lightning Imaging Sensor (LIS), and Visible/Infrared Scanner (VIRS), allows examination of convective and rainfall characteristics of these storms. Over 0.1 million Tropical Cyclone related Precipitation Features (TCPFs) have been identified from the 11 year Florida International University/University of Utah TRMM Precipitation Feature (PF) database. In this study, we use the TCPF database during December 1997 through December 2008 to document the observed passive microwave ice scattering magnitudes, radar reflectivity, IR cloud top height, and derived rainfall parameters including rain rate, raining area, and volumetric rain. A semi-manual method is used to divide TC raining area into inner core (IC), inner rainband (IB), and outer rainband (OB) regions. Precipitation features (PFs) within the IC region are examined for their convective vigor and rainfall characteristics based on passive microwave, IR, radar, and lightning properties in terms of different TC intensity change categories. The 24-h TC intensity change categories include weakening (W), neutral (N), slowly intensifying (SI), and rapidly intensifying (RI).

At the weaker end of the convective spectrum, there is a minimum threshold of convective properties in the IC region for storms undergoing RI. The necessary conditions for RI are that the minimum 85 GHz PCT in the IC region must be less than 256 K, the minimum 37 GHz PCT in the IC region must be less than 275 K, and the maximum 20 (30, 40) dBZ echo height in the IC region must be equal to or greater than 8 (6, 4) km. Over 98% of RI minimum 11μm Tbs in the IC region are less than 212 K. In the middle of the convective spectrum (indicated by the median values of the distributions), the convective intensity in the IC region is higher for features associated with RI storms than those in other intensity change stages. However, at the stronger end of the convective spectrum, RI features do not necessarily have stronger convective intensity in the IC region than features in other intensity change stages. Extremely intense convection, therefore, is not the sufficient condition for RI. Modest ice scattering signatures, cloud top heights, and radar reflectivities are, however, necessary conditions for RI.

Although IC conditional mean rain rate is not a good indicator of RI, both IC raining area and IC volumetric rain are. From both TMI 2A12 and PR 2A25 data, it is found that RI storms always have larger raining area and volumetric rain in the IC region than storms in other intensity change stages. This implies that the chance of RI increases when a storm's raining area in the IC region increases. In most cases, this means the storm becomes more symmetric.