Numerical Simulation And Diagnostic Analysis Of A Low-level Jet Weather Process In The Yangtze River Basin

Yangtze River valley experienced a heavy rainfall with Low-level jet?LLJ?from 30th June 2016 to 6th July 2016.The heavy rainstorm has the characteristics of high intensity,long duration and concentrated area which caused severe flooding and huge economic losses.In order to deepen the understanding of the LLJ and improve the forecast ability,the Weather Research and Forecast?WRF?model with a high resolution is used to simulate the weather process.Based on the successful simulation of LLJ,the LLJ was divided into parts while the longest and most complete phase was selected to conduct a concrete analysis:analyze the possible mechanism of LLJ and a sensitivity test of the terrain height was performed to analyze the possible effect of topographic factors on the LLJ.Meanwhile,perform a diagnostic analysis of the process from the aspect of water vapor,power,heat and energy.The results are showed as follows:?1?This LLJ occurred with the western Pacific subtropical high on the east and the southwest vortex on the west.The situation of“East High and west low in the pressure field”provides a favorable background for the formation and development of low-level jets.The coupling of high-level jets and LLJ is an important background condition for the development of LLJ.The constant vertical momentum downward from high-altitude provides momentum for LLJ that also an important condition for the occurrence of LLJ.The positive feedback between temperature inversion and vertical wind shear is one of the factors that strengthen LLJ.?2?The mountain body had friction and blocking effect during the formation and development of LLJ which enhances with the increase of the mountain height.Meanwhile,the direction of the mountains will change the original wind direction which making LLJ leading or lagging.Tibetan Plateau has strong lee wave effect,it's flow and squeezing effects will make the air flow appear as a narrow band,which will accumulate the momentum and increase the air speed to form a low-level jet stream.?3?Diagnostic analysis of moist potential vorticity?MPV?theory showed that the configuration of MPV1“positive at upper and negative at low”is conducive to the occurrence of precipitation.The area where MPV1<0 and MPV2>0 in the lower troposphere can be used to predict the location of the storm center.During the precipitation period,the region of MPV1<0 and the dense belt of isoline in the positive and negative transition region of MPV1at 850hPa was consistent with the precipitation zone.Due to the development of the LLJ in later precipitation,the precipitation zone is consistent with the large negative MPV2 zone at850 hPa.?4?Diagnostic analysis showed that the Bay of Bengal and the South China Sea are the main sources of moisture for the heavy rainfall while the LLJ carried water vapor fully which formed a strong water vapor conveyor belt.The distribution of water vapor flux divergence can correspond well to the precipitation area.When the rainfall passes through,the precipitation intensity would be further enhanced when the positive vorticity center developed upwards.The distribution of₍?7? in 900hPa¹000hPa layer completely matched the region of LLJ.Combined the results of diagnostic analysis with index calculation,the forecast decision tree of rainfall affected by LLJ in Yangtze River vally was proposed initially.