Improving the Extreme Rainfall Forecast of Typhoon Morakot(2009) by Assimilating Radar Data from Taiwan Island and China's Mainland

This study examined the impact of an improved initial field through assimilating ground-based radar data from China's Mainland and Taiwan Island to simulate the long-lasting and extreme rainfall caused by Morakot（2009）. The vortex location and the subsequent track analyzed through the radial velocity data assimilation（VDA） are generally consistent with the best track. The initial humidity within the radar detecting region and Morakot＇s northward translation speed can be significantly improved by the radar reflectivity data assimilation（ZDA）. As a result, the heavy rainfall on both sides of Taiwan Strait can be reproduced with the joint application of VDA and ZDA. Based on sensitivity experiments, it was found that, without ZDA, the simulated storm underwent an unrealistic inward contraction after 12-h integration, due to underestimation of humidity in the global reanalysis, leading to underestimation of rainfall amount and coverage. Without the vortex relocation via VDA, the moister（drier） initial field with（without） ZDA will produce a more southward（northward） track, so that the rainfall location on both sides of Taiwan Strait will be affected. It was further found that the improvement in the humidity field of Morakot is mainly due to assimilation of high-value reflectivity（strong convection） observed by the radars in Taiwan Island, especially at Kenting station. By analysis of parcel trajectories and calculation of water vapor flux divergence, it was also found that the improved typhoon circulation through assimilating radar data can draw more water vapor from the environment during the subsequent simulation, eventually contributing to the extreme rainfall on both sides of Taiwan Strait.

RADAR OBSERVATION OF PRECIPITATION ASYMMETRIES IN TROPICAL CYCLONES MAKING LANDFALL ON EAST CHINA COAST

This study explores,for the first time,the asymmetric distribution of precipitation in tropical cyclones(TCs) making landfall along east China coast using reflectivity data collected from coastal Doppler radars at China's Mainland and Taiwan.Six TCs(Saomai,Khanun,Wipha,Matsa,Rananim and Krosa) from 2004 to 2007 are examined.The temporal and spatial evolution of these TCs’ inner and outer core asymmetric precipitation patterns before and after landfall is investigated.The radius of inner-core region is a function of the size of a TC apart from a fixed radius(100 km) adopted in previous studies.All six TCs possessed distinct asymmetric precipitation patterns between the inner-and outer-core regions.The amplitude of asymmetry decreases with the increasing TC intensity and it displays an ascending(descending) trend in the inner(outer) core.In the inner-core region,the heavy rainfall with reflectivity factor above 40 d BZ tends to locate at the downshear side before landfall.Four cases have precipitation maxima on the downshear left side,in agreement with previous studies.As TCs approaching land(~ 2 hr before landfall),their precipitation maxima generally shift to the front quadrant of the motion partly due to the interaction of TC with the land surface.In the outer-core region,the precipitation maxima occur in the front quadrant of the motion in five of the six cases before landfall.After landfall,the precipitation maxima shift from the right-front quadrant clockwisely to the right-rear quadrant of the motion collocated well with the mountainous areas along the coast,which indicates the impact of topography forcing on the precipitation distribution.This study illustrated how the precipitation asymmetry in the inner-and outer-core at different stages of TC landfall is affected by storm motion,vertical wind shear and topography.