The Phase Features of a Cold Vortex over North China

Using the conventional weather data, disastrous weather data, hourly precipitation data observed by automatic weather station and NCEP/NCAR reanalysis data, the phase features over North China during 12—20 July in 2011 are analyzed. The result indicates that the precipitation of the vortex is located in the northeast of Mongolia, North China and the south of Northeast China. Thunderstorm, gale and hail occur in the development stage, while short-time strong rainfall occurs in the weakening stage. Strong warm ridge is located at 850 hPa, a full jet stream exists at 200 hPa, and cold-core structure is presented on the whole troposphere in the development stage. The strong warm ridge isn't obvious in the weakening stage, the east wind is enhanced at the lower layers, and the cold-core also increases. The vortex is baroclinic at 700 hPa, the ascending motion is located in the east area and becomes stronger in the weakening stage, and the southeast wind is also speeded up. The relative humidity is larger at 300 hPa and 700 hPa, but it's dry at 500 hPa, and the south of cold vortex is invaded by dry and cold air in the development stage. The relative humidity is large at the whole layer in the weakening stage, as the vapor is brought by east wind. A northeast—southwest θ_e frontal zone exists in both stages of the cold vortex. The gradient of θ_e becomes significantly strong in the development stage, and the structure is unstable with dry and cold at the upper layer while warm and wet at the lower layer in the vertical direction. The θ_e frontal zone and the unstable structure weakens in the weakening stage. The cold advection which is located at the low and middle layers and the vorticity advection which is located at the middle and high layers play main roles for the cold vortex in the development stage. As the cold advection at the low layer enters the cold vortex center, the cold advection becomes weak at the middle layer, the positive vorticity advection recedes at the medium-to-high level, and the cold vortex weakens.