Diagnostic Analysis of a Heavy Rain Event in North China Caused by the Development of Yellow River Cyclone

Yellow River Cyclone is one of the important weather systems that could produce heavy rain in North China. A heavy rain event occurred from 9 to 11 June 1991 in North China and caused by the development of a Yellow River Cyclone is studied in detail through a combined analysis of NCEP/NCAR reanalysis dataset, the temperature of black body (T_BB) data of Geostationary Meteorological Satellite (GMS) and radiosonde data. It is found that the occurrence and development of Yellow River Cyclone are results of intense development of atmospheric baroclinicity not usual in summer season. The joint effects of strong divergence and positive advection of vorticity at high level trigger the Yellow River Cyclone. The warm advection at low-level in the troposphere facilitates the Cyclone's development and leads it to move. The heavy rain region is under the right side of the exit of the high level jet. In the south of the region, from 18:00 (UTC) June 9, low level jet (LLJ) with wind speed greater than 14 m·s^-1 occurs at 850 hPa and transports water vapor to it. Heavy rain occurs at early stage of the Yellow River Cyclone and mainly comes from the shield clouds of the warm section of the cyclone. There are two transportation paths of water vapor in this heavy rain event: one is from southwest, the other is from southeast. And the water vapor flux from southwest is larger. The west extension and intensify of the west Pacific subtropical high help the transportation of water vapor to North China along the LLJ. The total precipitable water vapor increases obviously in North China before the occurrence of heavy rain. It is also found that the contours of relative humidity and potential temperature are dense and decrease with height over heavy rain region. The atmosphere is of intensive convective instability. From the cross sections of vorticity and vertical velocity, it is found that an obvious positive vorticity is transferred from high level vortex to the Yellow River Cyclone. Strong convergence of water vapor flux and ascending motion are found in the Cyclone, and the ascending velocity ahead of the Cyclone is larger than the subsidence velocity behind the Cyclone. Therefore, the vertical circulation is asymmetry, which improves the rapid growing of raindrops. As it is found in other studies of Yellow River Cyclone, there are meso-β-scale convective systems developping and moving along the southwest low level jet in this case. These meso-scale convective systems (MCS) in the frontal clouds cause the inhomogeneous of the precipitation and produces intensive precipitation centers. And the release of latent heat caused by precipitation has positive feed back to the development of the Cyclone.