The reasons for the shift in tropical cyclone formation regions between El Nino and La Nina will be investigated. TRMM has observed two strong La Nina events (1998 and 1999) and one moderate El Nino event (2002). The distribution of tropical cyclogenesis during the May to November peak of the tropical cyclone season varied dramatically during those three years. TRMM 3B42 gridded data will identify precipitation signatures associated with both equatorial waves and the penetration of middle latitude troughs into the subtropics. The equatorial waves will be isolated by space-time filtering of the gridded data. The results of these calculations will be combined with the seasonal mean zonal wind distribution to evaluate the following hypotheses: that the storm formation region shifts eastward during El Nino because (i) the climatological convergence region shifts eastward; (ii) middle latitude troughs reach the subtropics further east, and the precipitation they produce excites equatorial waves; (iii) the equatorial waves propagate westward and grow within the shifted convergence region; and (iv) storms form within these stronger, more convectively active waves. This work will be led by PI Molinari, who has published several papers on equatorial waves and tropical cyclogenesis.
In addition to the gridded precipitation data, TRMM orbital data will be utilized to define the convective-stratiform split, the depth of convection, and the organization of convection within equatorial waves. The objective is to understand the evolution of convective intensity and convective system area within strong equatorial waves that produce tropical cyclones versus those that do not. Convective systems will be analyzed using the TMI 85 GHz calibrated brightness temperature product (1B11). The height and extent of the cloud tops in these convective systems will be diagnosed from the VIRS channel 4 radiances (1B01). This work will be led by co-I Mohr, who has extensive experience and publications on the interpretation of convective systems in the tropics using remote sensing data, including TRMM.
The proposed research addresses two goals of the Precipitation Science program: Analysis of TRMM precipitation information for evidence of climatically significant precipitation variations; and Space-time properties of precipitation variations and their relationship to variations in the climate system.
