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This study investigated the impact of land surface heterogeneity on Mesoscale Convective System (MCS) initiations in East Asia, using geostationary satellite data during June-August from 1996 through 2018. The detected MCSs over land exhibited clear diurnal variation with the lowest existence frequency at 10:00 and highest initiation frequency during 12:00-17:00 local time. To quantify land surface heterogeneity, the spatial standard deviation of equivalent Black-Body Temperature (TBB) within a cloud-free 0.35 degrees x 0.35 degrees box (sigma LSTBB:Land Surface TBB) was computed for 10:00 each day. A comparison of the sigma LSTBB and MCS databases revealed that the probability of MCS initiations increased with increasing sigma LSTBB in East Mongolia while the probability was not sensitive to sigma LSTBB in East China. This indicated that MCSs tend to form over heterogeneous land surface conditions in the semiarid region. We found that the impact of land surface heterogeneity on MCS initiations was highest over flat terrain in East Mongolia, where the convection trigger due to topographically-induced circulation was absent. These results suggest that the impact of land surface heterogeneity on MCS initiations during the warm season varies with climate zones and terrain complexities in East Asia, with strongest impact in semiarid and flat regions.

The downscaling framework, which is originally proposed by Emanuel et al. (2008), is modified to improve the interannual variation of the tropical cyclone features in Western North Pacific (WNP). The original framework, which utilizes axisymmetric model driven using monthly mean atmosphere and ocean variables, well reproduced interannual variation of tropical cyclone indices over North Atlantic while there is room for further improvement over WNP. The modified treatment of relative vorticity plays an essential role for improving interannual variation of genesis counts and power dissipation index in WNP. In this update, we introduced probability of cyclogenesis special to WNP which is a priori calculated using monthly mean 850-hPa level relative vorticity and best track. The result suggests that the monthly mean relative vorticity represents a potential of characteristic initiating disturbance in WNP such as monsoon trough and intraseasonal variation. Another experiment shows increased influence of vertical wind shear in middle troposphere improves the distribution of tropical cyclogenesis in WNP while it slightly deteriorates the interannual variation of genesis count.

Global weather forecasts do not have sufficient performance to predict the local severe weather events that are accompanied with cyclones and cold fronts due to their coarse horizontal resolution. This study investigated the performance of dynamical downscaling (DD) using mesoscale model to simulate the severe windstorm in eastern Mongolia which occurred on 26-27 May 2008.Our results revealed that the DD experiments were successful in capturing the general features of the windstorm in terms of wind and temperature patterns. The timing and amplitude of drastic changes in the simulated temperature and wind speed were very similar to that observed than that obtained from the global atmospheric data, suggesting that DD is capable of predicting extreme wind storm events in Mongolia. Analyses on the nested domains indicate that the DD has crucial impact on the performance for simulating severe storm even with a moderate resolution (27 km), and further nesting (9 and 3 km) plays a role to improve it. Furthermore, the maximum wind speed approaches the observed value more closely as the horizontal resolution increases, although it still underestimates the observed wind speed even in the 3 km mesh domain. On the other hand, the abrupt temperature change is captured well even in the low-resolution domain, suggesting a difference in necessary horizontal resolution for temperature change and maximum wind speed.

During the winter monsoon, a convergence line called the Boso Front often appears in the area between the Kanto Plain and the Izu Islands, Japan. Two typical cases of high-level Precipitable Water Vapor (PWV) are observed along the convergence line in the lee of Chubu Mountains by the aid of latest techniques of Global Positioning System (GPS).Statistic analysis of GPS derived Precipitable Water Vapor (GPS-PWV) indicates that the prevailing wind direction controls the position of the high-level PWV in the lee of Chubu Mountains. A numerical model simulates the behavior of water vapor, namely, the temporal variation of simulated PWV and surface wind, which suggest the reasons for the high PWV near the convergence line. The convergence of surface wind gathers moisture, and trapped moisture by reduced wind velocity may also contribute the high-level PWV.

The role of mountain ranges upon the arid climate formation in northwestern China is investigated using regional climate model. Two kinds of numerical experiments are carried out with real topography and with modified topography which does not contain the TianShan Mountains. Simulated rainfall distribution with real topography shows maximum precipitation in upwind slope of the mountain and minimum precipitation in downwind side of it, which coincides with the features of rain-shadow in other regions of the world. However, numerical experiment without the TianShan Mountains also simulates the area of distinct minimum precipitation in the same location. The arid climate, therefore, is formed even though the rain shadow effect is absent. The difference in rainfall amount between these two experiments is very small around downwind arid region. On the other hand, more intense and frequent rainfall events tend to occur in upwind of the mountain range due to forced lifting of environmental flow, which just emphasizes the precipitation contrast between upwind and downwind side of the mountain. The precipitation accompanied with the synoptic scale disturbances is suppressed over the arid region whether the TianShan Mountains exist or not.