Diagnostic Analysis On A Spring Unusual Cold Wave Rainstorm (Snowstorm) Process

From March 2 to March 6, 2007, Hebei province suffered an unusual rainstorm (snowstorm) and cold wave, while snowstorm took place in the northern region and in southern region rainstorm occured, process cooling of 10～28℃, some of the waters of the Bohai Sea suffered storm tide and gale, the disaster of snow and frost caused significant damage to the north-central areas of industrial and agricultural production and transportation and the term begins of primary and secondary school in some northern cities delayed 7-10 days.This process is an extreme weather event in the context of very high temperature, with broad scope precipitation (snow), intensity, strong wind, steep descent of temperature and many place experienced conversion of rainfall to the snow, in some areas also experienced phase transformation of rain→sleet→snow→sleet→rain, which was worthy of in-depth study. Analysis of conventional observation Data, NCEP 1°x 1°Data, FY2 meteorological satellite images, Doppler weather radar data, Automatic Weather Station (AWS)data, and WRF model, snow storm (rain storm) were investigated .First the cause of strong winds and steep descent of temperature was analyzed, the results showed that the Jianghuai cyclone was moving northeast into the sea to strengthen the strength of process and the interaction of cold air southward from the Northeast China Plain, increased pressure gradient leading to the strong wind in the Bohai Sea. West and northwest cold air mess led to the strengthening cold air with north and central cooling weather and strong winds.Then the cause of formation of the rare snow storm (rain storm), satellite images, characteristic of Doppler radar and physical characteristics were studied. Which showed: by the effect of deep trough in 500hPa, wind shear line in 700hPa and 850hPa, Jianghuai cyclone, the east wind on the ground, big snow storm weather was caused in Hebei; low vortex shear line in 700hPa, 850hPa strengthen the lower convergence, forming a strong momentum of increase and pieces of water vapor; strong airflow below 850hPa and northeast to the southwest warm airflow above 700hPa strongly interacted, strengthening the vertical shear and a stronger impetus uplift; when Jianghuai cyclone went northeast into the sea , the cold air southward along the Northeast Plain interacted, which made surface pressure gradient to strengthen, east winds intensified south-central, which played an important role in the big storm (snow) formation; the water vapor caused the storm weather had two channels: one from the Bay of Bengal, another one from the East China Sea; the MPV high-value areas in 700hPa and rainfall center had a well correlation.Next the cause of formation and the forecast of multi-phase precipitation were studied. The results showed that: in 850hPa and below, by the effect of the Mongolian cyclone together with the Jianghuai cyclone, easterly winds led to the lower atmosphere temperature continued to drop, when the layer of 0℃ went below than 950hPa, nature precipitation will change to snow from rain. With the eastward shift of the system, Jianghuai cyclone entered to the sea and wind directions changed, from northeast to northwest, coupled with the Taihang Mountains terrain, eastern parts of the Taihang slightly got warm, and then 0℃-height layer elevation, resulted to snow to rain. In Doppler radar map, 0°C height layer rapid decline could be seen as one of the criteria of precipitation from liquid to solid. Meteorological analysis indicated, when 1000hPa temperature below 2℃, while 925hPa temperature below 2℃, precipitation converted from solid to liquid.Final application of WRF model, some physics parameters that caused snow storm and cold storm were deeply analyzed, the cause of formation was investigated more. Which showed that: the region of precipitation located in front of the jet stream, the formation of which was earlier than precipitation; diagnosis of MPV in 700 hPa indicated that the strong convective precipitation process took place in stable condition, the wet barotropic element was positive, while the wet baroclinic element always negative, but both absolute experienced the first increasing and then decreasing, the process precipitation occurred mainly in the front of the contour of the intensive center and the center of negative MPV₂ section; heavy rainfall occurred near the center of wet Q-vector convergence in 850hPa.