Diagnostic Analysis And Numerical Simulation On A Vortex Heavy Rain Occurred In North China

Hebei Province is to the east of the Bohai Sea, west of the Taihang Mountain, and the geographical condition is very complex. In actual weather forecasting, the prediction of storm intensity and rainfall area is one of the problems in forecasting and making decisions, and one important reason is that the meso-scale topography has greater impact on the rainstorm.Firstly, based on routine measuring data, satellite data and radar data in Micaps system, a local severe heavy rain occurred in Hebei Province on July 11-12,2004 was analyzed. Secondly, a large-scale primary diagnostic analysis of the heavy rain has been done from dynamical, thermal to water vapor moisture conditions by using NCEP 2.5°×2.5°re-analysis data. Finally, the meso-scale numerical model MM5 is also used to simulate the heavy rain; the mechanism of the rainfall and vertical structure of the vortex are analyzed in more detail. In addition, a sensitivity experiments on terrain has also been done. And many meaningful results are concluded and helpful for understanding the vortex structure and heavy rain forecasting occurred in North China.The observation shows: (1) The westerly trough in 200hPa, low eddy in North China at 500hPa, and easterly wind in middle and lower levels were the direct influence system of the rainfall. (2) With the blocking effect of Taihang Mountain terrain, the middle-small scale cloud cluster maintained for a long time, so severely heavy rain happened in the east of Taihang Mountain. (3) The characteristic of reflectivity form radar revealed that convection was weak and the heavy rain was mixed precipitation; 3-Hour Precipitation was used to analysis the precipitation intensity and played a positive role in short-range, quantitative and local forecasting.The conclusions in diagnostic analysis are as follows: (1) the low eddy in North China was a very deep and baroclinic system; the three dimensional structure was that in the lower level the curve of temperature was sunken while protuberant in the higher level; the characteristics of the vorticity field was obvious, and that rainfall was corresponded with the deep positive vorticity. Disconvergence field was disorder and upward motion was strong in rainstorm area. (2) The cold air came form northeast area below 500hPa; the height of convection was lower; the source of vapor was from Bohai Sea and the easterly winds in the lower levels may be the main carrier in transporting the water vapor.Numerical stimulations results show that: (1) The MM5 model not only can simulate successfully the heavy rain area in the east of Taihang Mountain, but also can reproduce the evolution of the lower vortex, it implies that the MM5 model is capable for predicting the lower vortex; (2)At the early stage of the rainfall, airflow formed souther and northeaster was blocked by Taihang and gathered; At the mature stage, northeaster airflow played the major role and was blocked by Taihang for a long time when the southerly low-level jet was weaker. The small-scale strong upward currents were below 500hPa and the factor caused the precipitation was mainly occurred in the middle-lower layer; (3) The high value of the local helicity indicated that the rainfall area was consistent with the location of frontal zone. The total helicity had relation with the precipitation; (4) heavy rain area lies in the northeastern of the positive value of the PV, and the movement of the PV would indicate the direction of the heavy rain. The negative value of the PV is a very good forecast index in the rainfall area and intensity. In the MPV, the evolution of MPV₁ and MPV₂ was in accorded with that of the weather system and rainstorms center at the lower levels. The movement of positive MPV2 may imply the activity of the low vortex. The heavy rain will fall in the right-hand front zone along the direction of MPV₂; (5) Divergence term and tilting term in the frontogenetical function were helpful to the frontogenesis. The central position of the front belt approach the rainfall area, the rainfall area occurred mainly in the front of the frontogenisis area, which provided the reference for the future forecast; (6) When the height of the terrain is reduced to the original 40%, the rainfall area moved easterly and the precipitation intensity became weak, the easterly wind in low levle was strengthen, the upward jet was weaken, the weather system and the usual physical field such as vorticity field, divergence field, vertical velocity field moved to east quickly.