Mechanism Research For Rainstorm By An Autumn Tc

Autumn typhoon's rainstorm always causes invaluable loss to people's life and property, so researching their mechanism is of far reaching importance. Two main features of autumn typhoons are:(1)they are always invaded by cold air. (2)for subtropical high ridge is relatively further north, it is special that the water vapor delivered off the subtropical high.0716 super typhoon Krosa is a typical autumn tropical cyclone. Using NCEP reanalysis data by 1°×1°, general surface station and regional meso-scale automation station data, the paper analyzed the Krosa's motion mechanics, precipitation distribution and the possible reason of its effect on large areas'torrential rain in east China. The mechanism of rainstorm caused by Krosa is discussed by comprehensive diagnostic analysis on kinds of physical quantities, especially moisture potential vorticity, helicity and wet Q-vector. And some regulation is obtained, which is hoped to have benefits in real typhoon's forecasting. The conclusions of the thesis are below:(1)Precipitation analysis shows rain distribution on mainland caused by Krosa turns to be 4 phases, they are before second landing of I-Lan county in Taiwan, section between second landing of I-Lan county and re-landing on mainland, section between re-landing on mainland and the beginning of cold flow's affection, combination with the cold air. The period of rain focus are the second, third, and the first half of the forth section.(2)During the section of its influence on mainland, Krosa is in the pressure col, that makes its path complex, and its affect on mainland lasts long. This is the large scale background conditions for wide torrential rain areas and huge accumulation precipitation. There is a thousand-miles water vapor deliver belt lies from the South China Sea to east of Taiwan and East China all times along. Strong water vapor flux field of more than 20 g/(s-hPa-cm) is steadily located in east of East China. Those are essential conditions of continuous rainstorm of Krosa. It is in favor of heavy rain's developing and keeping by profitable vertical upward motion in rainstorm area, low-level convergence and high-level divergence, and steadily continuous positive vorticity. At the later of the typhoon, cold air combined with Krosa, leading to frontogenesis in north part of Zhejiang province, that brought large area's heavy rain there.(3)There is a negative correlation between negative center of moisture potential vorticity on 850hPa and the strength of the typhoon. The smaller the negative MPV centre is, the stronger the typhoon behaves. Moving of the negative MPV centre indicates the motivation of the tropical cyclone well 6 hours in advance.(4)Being compared with the moisture potential vorticity field of Morakot, which has no clear clod air effect, it shows torrential rain always situating in negative MPV areas. When there is cold air incursion, heavy rain locates in largest negative gradient of low-level moisture potential vorticity.(5)MPV1 and MPV2 of Krosa are discussed, that reveals torrential rain lies in negative MPV1 field and positive MPV2 field when no cold air intrudes, while in largest positive gradient MPV1 and close negative gradient of MPV2 with cold air incursion. MPV1 contributes greatly to MPV. (6)Since the occurrence and development of typhoon is a establishing and developing process of helicity structure in deed, through the analysis of the evolution of helicity can study the evolution of the typhoon. Great helicity value center on 850hPa changes with the location of typhoon center. Observed precipitation areas coordinated well with the large helicity value area.(7)The evolution of negative center of ageostrophic wet Q-vector divergence on 850hPa had some correspondence to the change of Krosa's precipitation areas. Any time negative center of ageostrophic wet Q-vector divergence had positive correlation with intense of heavy rain 6 hours after that, which could used to forecast the distribution area of heavy rain. The larger high-level positive center of ageotrophic wet Q-vector divergence that responds to lower-level negative center of ageostrophic wet Q-vector divergence is, the heavier the rain in that latitude the same with the negative center in future 6 hours flows.