Internal Variability of the Dynamically Downscaled Tropical Cyclone Activity over the Western North Pacific by the IPRC Regional Climate Model

Many studies have identified the variations of Tropical Cyclone activities in the past few decades and made future projections on TC behaviors in some climate-change scenarios based on numerical models. However, these variations are also controlled by the internal downscaled dynamics of model. This study attempts to examine such internal variability of dynamically downscaled TCs over the western North Pacific based on four simulations of 29 typhoon seasons (1982-2010) initialized on four successive days using International Pacific Research Center (IPRC) Regional Climate Model (iRAM).

The results show that all simulations can capture the large-scale features in the WNP reasonably well, and can realistically reproduce the observed geographical distributions of TC genesis and the frequency of occurrence. Our results also show that the ensemble mean provides the better downscaled information on seasonal and interannual frequencies of TC genesis and occurrence. However, some sensitivities and uncertainties exist on modeling the statistics of TC activities.

For these four ensemble results, large-spread years and small-spread years are indicated by calculating the variance of TC counts. Some extreme years are chosen to be compared with their large-scale environments, intra-seasonal oscillations and day-to-day model solutions. The time-scales and the spatial-scales are divided into different parts to clarify the most important contributions to the interior variations in the model. The purpose of this study is to find out what are the key factors affecting the variability of TC activities among these ensembles.