| China is one of the countries over which tropical cyclones (TCs) make landfall most frequently in the world, and thus is seriously affected by TCs. The unusual change of TC intensity, structure and track is a priority and key problem in the operational prediction and scientific research of TC. Two super typhoons in2010, Fanapi(1011) and Megi(1013), are selected as the cases of this study. Fanapi is the only twice-landfalling typhoon over China in the year. During the period of its westward movement and approaching Taiwan, the intensity increases rapidly. Megi is the most powerful typhoon of the year; however, the most remarkable feature is its sudden track change over the South China Sea, which is of great challenge for operational forecast. By using the mesoscale non-hydrostatic numerical model WRF and multiple diagnostic methods, investigations for the intensity and structure changes of Fanapi, and the sudden track change of Megi are carried out. The main sections are as follows:By applying the total force divergence analysis to the intensity and structure diagnosis of Fanapi, it is found that the characteristics of the total force divergence intensity and distribution are able to represent the intensity and structure of TC to a large extent. The analyses of the vertical integrated total force divergence distribution show that the convergence zone occurs firstly in the vicinity of the TC center, then the divergence zone occurs on the periphery of convergence zone. The structure of total force divergence becomes more symmetric with the development of TC. In the vigorous stage of Fanapi, the convergence zone shows as a roughly circular shape with a radius of about150km, with the largest intensity around the TC center and weakens outward. The divergence zone appears as spiral cloud bands shape. Throughout the life history of Fanapi, the average convergence intensity of the total force is generally larger than that of divergence. The average intensity of the convergence and divergence in the TC circulation area tends to be zero, which shows that Fanapi is basically a non-divergent system of total force. In the vertical section, the edge of total force convergence and divergence corresponds with the strong updraft and intense precipitation, indicating the eyewall of TC. The analyses of the four terms in the total force divergence equation provide an explanation to the formation of the above distribution pattern, in which the two horizontal terms are the main contributors of the convergence zone above the TC center and the vertical motion term determines the features of near the eyewall.Observation analyses are carried out on the causes of the sudden track change of super Typhoon Megi. First of all, motion direction angle is defined to identify the five turning and six stages of the track. The angle difference reaches up to135°during the most remarkable northward shift of the track. Furthermore, the main circulation systems are determined in each stage. The steering current in the lower troposphere between800hPa to600hPa changes from easterly to southwesterly first and the steering flow of the whole troposphere changes its direction subsequently, which leads to the northward shift of the track. Quantitative description for the variation of Megi’s thermodynamical structure is present by means of Cyclone Phase Space (CPS) method, focusing on the changes of the parameters around the northward shift of the track. The lower-troposphere thermal asymmetry parameter B changes from negative to positive6hour earlier than the start of the track change, and sustains until the decay of Megi, indicating that the right of motion becomes warmer than the left. The abrupt change of asymmetric thermal structure of a tropical cyclone in weak environmental flow can affect its motion. The warm-towarding movement of typhoon plays a primary part at first, and then the northeastward steering current grows larger, leading to the rightward shift of Megi’s motion. Thus, the sign reversal of parameter B is identified as a key signal for the sudden track change.Further, the wind structure evolution during the northward track shift of Megi is paid close attention with a new scheme of the decomposing wind field technique in a limited area. Results show that the enhancement of south wind within the steering layer (middle and lower troposphere) in a wide area contributes to the sudden track change. The trend of divergent band and its evolution at upper troposphere has close relation with the track shift. The divergent band couples with vigorous ascending motion to the right of Megi’s track, with a remarkable "attraction" to TC’s movement. This exactly explains why the right of motion is warmer than the left.Based on the observational analyses, a132-hour high resolution simulation with a finest grid size of3km is performed focusing on the sudden track change of Megi and its landfalling process over mainland China. The simulation well reproduces the two processes. TC track and intensity are fairly consistent with the observations. The evolution of circulation systems at upper and lower level, the distribution of rainfall over the ocean and land surface are also quite agree with the observations. Using the numerical simulation products, main conclusions from the observation analysis in the previous chapters are verified. |
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