현업예보관점에서 태풍의 온대저기압화 고찰

Abstract

In general, a track of typhoon depends upon the steering flow at the 500hPa, following the edge of the Northwestern Pacific high. In order to forecast the best track of typhoon, a forecaster should analyze the variation of geopotential height at the 500hPa because it determines the location of turning point from easterly to westerly, down-grade of typhoon, and dissipate developing low pressure system reach to mid-latitude. In analysis of the strength of typhoons in the western Pacific, the role of ridge and trough at the 500hPa level are important. The operational typhoon report at the Korea Meteorological Administration (KMA) is ceased at the down-grade time to TD after coming to 30N. In the moment, forecasters make a decision of the end of the typhoon activity from SST, wind speed, satellite pictures and so on. It is difficult to decide a status of developing low in the mid-latitude due to the lack of the surface and upper-air observation data. It only depends on the sequence of satellite pictures, weather radar, and aircraft observation. Therefore, it is very difficult to decide the correct extra-tropical transition. Typhoon is changed and disappeared in two ways. One is extra-tropical transition, another is TD downgraded from TS. But some of typhoons can make a heavy rain, high waves and gusts with a great deal of damage. Like this, typhoon makes a great deal of damage after extra-tropical transition, but it can't keep the character of typhoon. Therefore, the generalization of the ending criteria of typhoon are needed. In other words, typhoon is changed to extra-tropical transition or not, it is one of important matters on the analysis of typhoon events. Nowadays, there is no correct definition to analyze the extra-tropical transition of typhoon. Researchers of typhoons have their own definitions because of their necessity only, therefore there is no enough understanding for the process of extra-tropical transition yet. According to the research. the symmetrical structure of typhoon and its intensity is decreased by the non homogeneous surface effect, vertical wind shear in the synoptic field and relative humidity, and moving speed toward north pole. When winds, thermal structure, moisture field, clouds, convection and precipitation are changed with non-symmetry, this case is called extra-tropical transition. KMA has an unspecified rules of official process formats on the typhoon forecast by the year of 2007, and most events of the extra tropical transition were carried out by the typhoon forecaster's subjective decision. In order to solve this problem, we need more an objective and the correct establishment of processing format based on the science and objectiveness. I am going to analyze the process of the extra tropical transition for the typhoons occurred from 2006 to 2008, and I intend to make more advanced method through this thesis. Most typhoon forecast centers among countries effected by typhoon in the Northwestern Pacific seldom have the correct rules of extra tropical transition. Therefore KMA analyzed the twenty three typhoons occurred in 2006 and the same case study for the extra tropical transition of the United States and Japan. KMA also issued “The Working Manual for the Extra-tropical Transition” in December 2007. As I mentioned already, because of the objective definition is not correct yet, I analyzed about the seven important extra-tropical transition of typhoons occurred in 2006 to 2008 by using the manual developed in 2007. Nowadays we absolutely depend on the analysis for the images of satellite observation, sea surface temperature and wind data to decide the extra-tropical transition in the area of no synoptic data. Hereafter we have to develop the advanced analysis technique for the process of extra-tropical transition or downgrading TD from TS by RADAR and wind profiler which is rapidly increasing its capacity recent years. I referred "The Working Manual for the Extra-tropical Transition" issued in December 2007 when I was as the director of Typhoon and Asian Dust Team, KMA for writing this thesis. The occurrence of typhoons from 2006 to 2008 are not only less than the annual value of normal year also less than typhoons effected the Korean Peninsula. In 2006, twenty three typhoons occurred and seven of them were changed to extra-tropical transition, thirteen extra-tropical transition of twenty four typhoons in 2007 and eleven of twelve in 2008. Like this, the percentage of extra tropical transition was different from each year. The main reason depends on the meteorological environment on the land and ocean at the same time when the typhoon was changed to extra-tropical transition.