Radar Observation Of The Boundary Layer Structure And Evolution Of Landfall Typhoon In China

The landfall typhoon in China is one of the major weather systems affecting the Chinese eastern coast, and the boundary layer of typhoon plays an important role in the development and changes of the typhoon structure, therefore the recognition of the structure and evolution characteristics of typhoon’s boundary layer has significant importance for improving landfall typhoon forecasting. In this paper, the coastal ten S-band Doppler weather radars’data are used to analyze statistical characteristics of 17 landfall typhoon boundary layer and changes on typhoon cases between 2004 and 2013 by using Hurricane Velocity Volume Processing (HVVP) and Velocity-Azimuth Display (VAD) methods. Comparing with Sounding Observation, the radial velocity wind retrieval could express accurately the boundary wind structure of the landfall typhoon, the root mean square error of wind speed is less than 1 m s-1 and wind direction less than 5°The composite wind field statistical results of all typhoon shows that:same as the statistical results of the past findings by Gropsonde at sea, there is a maximum tangential wind above the surface layer (100 m) in the boundary similar to the nocturnal jet, and the height of the maximum tangential wind is all beyond 1 km obviously higher than the jet height observed at sea (600 m). The low boundary layer inflow is significantly stronger than the sea, mainly caused by the increased friction on land. Consistent with sea observation, the strength of the inflow is proportional to the intensity of typhoon; meanwhile the inflow depth is proportional to the distance to the typhoon center. The inertial stability decreases with the fewer distance to the typhoon center are the principle reason that leads to the lower inflow depth. One standard deviation error of the mean wind profile in typhoon boundary increases with the decreasing distance to the typhoon center, showing that the distribution is more and more scattered toward the typhoon center. In the same region, the deviation error of the radial wind is much larger than the tangential and full wind speed suggesting that for the different typhoon the inflow varied the most compared with the full and tangential wind. Furthermore, boundary layer profile relative to the movement direction of the typhoon is divided into four quadrants, and the statistical features show that:the inflow layer was significantly higher in the front rather than the rear; the most significant Jet feature exists in the left rear, and the right rear quadrant does not possess significant jet feature, consistent with the previous model to simulate the different asymmetrical distribute feature between land and sea. The surface layer is basically satisfied with the logarithmic law. The estimated drag coefficient distribution with 10 m wind speed demonstrates that:under weak wind speed, the drag coefficient reduces as the wind speed grows, agreed with the trend that directly calculated by using the ground high frequency data. The drag coefficient is obviously larger on land than at sea under the same wind speed, caused by the increase land roughness.Three typical typhoon cases are selected to study the time variation of the boundary layer structure in eyewalK inner and outer rainband. The results show that radial, tangential and full wind profile in boundary layer alter with the different structure as outer rainband, principle rainband, secondary rainband and eyewall. And the strengthend rainband, land-sea contrast and typhoon moving could all contribute to the boundry layer change.To understand better the ability to simulate the structure of typhoon boundary wind profile, The WRF(Weather Research & Forecast Modeling System) model with four different boundary layer parameterization schemes (ACM2, YSU, MYJ, QNSE) is used to simulate the process of the landfall typhoon Utor (1311), with radar observed boundary layer characteristics contrasted. Experimental results show that the four groups of simulated moving path for typhoon center are close to the Best Track center(RMSE around 20 km), but overestimate with the typhoon intensity (RMSE around 13 hPa); the radial wind structure after landfall is especially weakly simulated, indicating that WRF model for the simulation of the typhoon boundary layer structure after landfall needs further more improved. Considering the simulated typhoon path, intensity and boundary layer wind field structure syntheticly, test one with the boundary layer parameterization of ACM2 simulates worst, and other three tests simulate similarly.