Impact Of Westerly Jet On Two Rainstorms Occurred In Northern China

A diagnostic analysis is made here for two heavy rains,(one happened inShandong province on BST2July to BST3July,2011and the other one happened inBeijing on BST21July to22July,2012), in order to study the influence ofupper-level jet stream on rainstorm being based on the FNL(Final Operational GlobalAnalysis)data provided by NCEP (National Center for Environmental Prediction)andthe GTS(Global Tele-communication System) observation data. The study shows thatupper-level jet stream can exert influence on rainstorm both from synoptic scale andmesocale scales.On the synoptic-scale, there is intense divergence on the junction of south Asiahigh and upper-level jet stream. If there is on the front of high-latitude trough, thedivergence will be more intense. On the other hand, the wind speed is not uniformalong the upper-level jet streak. There will be some wind speed maxima center alongit. On the upper reach side of wind speed maxima center is similar to the entrance ofthe jet streak, suck that the upper-level ageostrophic wind is directed towardhigh-latitude. In the northeastern part of south Asia high exiting ageostrophic winddirected toward low-latitude. Then, intensive divergence exited between them. Thelow-level below the divergence area is convergence line or low pressure center. So onthe southern of upper-level jet and northeastern part of south Asia high, heavy rainfallwill be prone to occur.On the mesoscale, mesoscale gravity wave may occur usually between southAsia high and upper-level jet stream, wavelength is between140km-180km, theperiod is about4hours. Spread along the south side of the jet toward lower reach.Further study show the mesoscale gravity wave is generated through the quickeningof the geostrophic adjustment on the southern of upper-level jet. The imbalance of aircan be measured by ΔNBE (Residual of Nonlinear Balance Equation).(a)Vertical gradient of potential temperature is low in upper-level;(b)Vertical gradient of windspeed is high in low-level are two advantages for the development of mesoscalegravity wave.Before the rainstorm happens, there is potential instability near the convergenceline. The equivalent potential temperature of the lower-level is higher than that of theupper-level. The Water vapor transport belt has been established. The potentialinstability energy can be aroused by mesoscale gravity wave or other conditions. Thenthe rainfall occurs. The WRF(Weather Research and Forecasting) Model is short atsimulation the rainfall in the warm air side before the front arrival. The mesoscalegravity waves are probably the reason to arouse strong convective weather in thewarm air side. It is useful to amend the rainfall forecasted by WRF Model by thestudy of mesoscale gravity wave’s produce and development.