Diagnostic Research on Heavy Rain Growth in Shandong Province Influenced by Landing Typhoon Lekima due to the Coupling of Upper- and Low-level Jet Streams

Continuous heavy rainfall occurred in Shandong Province from August 9 to 12, 2019, mainly due to the landing of typhoon Lekima. Local rainfall in Shandong peaked on the night of August 10; thus, this study explored the reasons for the heavy rain peaking on that day using hourly rainfall data, conventional observation data, temperature of black body observed by the FengYun-2G meteorological satellite, and American National Center of Environmental Prediction reanalysis data. Moreover, diagnostic research was employed using the equation of zonal horizontal motion and atmospheric kinetic energy. The conclusions are drawn as follows. (1) The main influence systems of the heavy rain were the westerly trough at 500 hPa, southwesterly upper-level jet, southeasterly low-level jet, inverted trough of typhoon Lekima at 850 hPa, and typhoon circulation of Lekima itself. A large-scale southwesterly upper-level jet at 200 hPa moved southeastward to affect northwest Shandong Province on August 10. Another large-scale southeasterly low-level jet at 850 hPa jumped northward, passing Shandong Province at 0800 BJT (Beijing time) 10 August. Convective clouds of both inverted trough and typhoon Lekima circulation moved northward over central Shandong Province successively, resulting in the top rainfall of that area on the night of August 10 due to the cumulative effect. (2) The fastest east wind increase at 850 hPa emerged northeast of the Zhangqiu radiosonde station around 2000 BJT 10 August during the rainstorm course. Results show that the main reason for east wind strengthening was easterly advection, but the geostrophic deflection force was not conducive to easterly increase. (3) The southwesterly jet emerged at 200 hPa at the Zhangqiu radiosonde station at 2000 BJT 10 August and maintained at 850 hPa at the Qingdao radiosonde station from 0800 BJT 10 to 0800 BJT 11 August. The rain peaked on the night of August 10 in the middle of Shandong Province because the region was at the right and rear of the upper-level jet while also at the left and front of the low-level jet. Both kinetic energies at Zhangqiu at 200 hPa and Qingdao at 850 hPa augmented fastest 12 h before wind speeds reached their maximum value of those days. Results diagnosed using the kinetic energy equation show that the item most favorable to the upper-level wind speed increase at 200 hPa near the Zhangqiu station was the advection of potential energy, and the item most favorable to low-level wind speed increase at 850 hPa near the Qingdao station was kinetic energy vertical flux convergence. (4) Although the typhoon circulation of Lekima acted on Shandong from 2000 BJT 10 to 0800 BJT 13 August, and the dynamic role of mountains in Central Shandong had always been there in the course, the rainstorm there just broke out on the night of August 10. This shows that the coupling of the upper-level jet with the low-level jet at that time might be the major factor in raising the rainfall intensity. Its roles were strengthening ascending motion, raising the whole layer of water vapor transportation, and at least adding atmospheric static instability in Central Shandong.