Diagnosis of a Heavy Rain Event Caused by the Intense Development of Yellow River Cyclone in July, 1998

A Yellow River cyclone intensifies rapidly during July 5 to 7, 1998. Its center pressure decreases by 12 hPa over a 24 h period and it produces heavy rain with the maximum rainfall exceeding 350 mm in Beijing. A diagnostic study is conducted from a potential vorticity or "PV thinking" perspective using NCAR/NCEP 6 hourly reanalysis data. The results show that the rapid development of the Yellow River Cyclone is related to the coupling between a surface low system and an upper level positive PV anomaly. When the positive PV anomaly near the tropopause advects over a pre existing surface cyclone, the cyclone deepens dramatically. Warm advection at 850 hPa intensifies the development of cyclone. Heavy rain occurs in the rapid intensification stage of the Yellow River Cyclone. This is a synoptic scale precipitation case, which is caused by the convergence between cold air descending from stratosphere and southwest warm and moisture air flow brought by the Monsoon Surge. On the vertical cross section through the region of heavy rain, abrupt jump of tropopause is shown clearly. The tropopause is near 250 hPa on the cold side and rises dramatically to above 100 hPa on the warm side. The extent of the descent of stratospheric air in the storm can be deduced by the tongue of 1 PVU extending from 200 to 600 hPa.While the cyclone intensifies rapidly, there are strong ascending motions, which lead to the deep moisture air from the south of China transporting to the heavy rain region continually by southwest wind. From the low to mid level of trop osphere, the humidity increases. The amount of precipitable water vapor increases 10.2 mm in 24 hours. The atmosphere is baroclinic over heavy rain region. High level jet with wind speed greater than 30 m·s^-1 and low level jet with wind speed greater than 12 m·s^-1 are found at 200 hPa and 850 hPa, respectively. At surface, a tongue of high θ_se value prevails in North China. At 500 hPa, there is a convergent zone of non geostrophic wet Q vector extending from southwest to northeast, which is caused by the large scale force. The convective cloud bands have a good relationship with the convergent center of Q vector. In the convergence zone, a number of MCSs continuously move to North China alone the southwest wind on the northwest side of the Subtropical High and cause amount of rainfall. It is called "train effect". Furthermore, topography influences the location of heavy rain. East wind prevails at surface over Beijing area when rainfall occurs. The west mountain blocks and lifts the east flow and increases the precipitation on upstream side of the mountain. The maximum precipitation centers occur at Changping, Yanqing.