Relationship between Tropical Cyclone Size Change and Precipitation Distribution

Size of tropical cyclones (TCs), typically defined as the radius of 15 m/s near-surface wind (R15), is an important property of TCs, along with intensity of TCs. However, far fewer studies have examined TC size than have considered intensity, genesis, and development of TCs. Therefore, the size change mechanism of TCs has not yet been understood well.

In the earlier study of ours (Tsuji et al. 2015, JMSJ, in revision) conducted to understand the mechanism of TC size change, we modelled TC as a simplified TC-like vortex subject to thermal forcing mimicking cumulus heating, and investigated effects of thermal forcing on the size of vortex. We found that the evolution of the vortex size depends on the forcing location. Specifically, when the forcing is applied in the outer part of vortex but still inside R15, the vortex grows in size. Conversely, when the forcing is applied near the center of vortex, the vortex size hardly increases. Moreover, when the forcing is applied outside vortex, the vortex size decreases. We also interpreted these differences in terms of inertial stability near the forcing region and the horizontal extent of the secondary circulation induced by the forcing.

The above results are obtained from the experiments with simplified TCs so that it is unsure whether the mechanism works in real TCs just as it is. However, the findings summarized above are consistent with results of several previous studies about TC size such as Fudeyasu and Wang (2011). Therefore, it is not unreasonable to expect that some relationship between heating (precipitation) distribution and TC size. So we investigate the relationship by using QuickSCAT and TRMM data between August 2001 when the orbital track of TRMM was changed and November 2009 when the observation of QuickSCAT was stopped.

The method is as follows: First, we separate the period of each TC activity into two sub-periods, size increase period and size stationary period which are determined based on R15 evolution obtained by QuickSCAT data. Second we analyze precipitation distributions around each TC. From the analyses, we find that the precipitation is located from inside to outside R15 in the size increase period, whereas it is concentrated near the center of TCs and hardly exist near R15 in the size stationary period. We confirm that the difference of precipitation distribution near R15 is statistically significant.

The above findings are consistent with the results of Tsuji et al. (2015), that is, thermal forcing applied near the center does not change vortex size whereas thermal forcing applied in the outer part of vortex but still inside R15 increases vortex size. On the other hand, the more enhanced precipitation outside R15 in size increase period compared with in size stationary period is inconsistent with the result of Tsuji et al. (2015) that thermal forcing applied outside R15 decreases TC size.