Climatology And Case Analysis Of Northern China Elevated Convection

Conventional surface observation,soundings,monitoring data of severe convective weather and NCEP analysis data are used to investigate statistically the spatial and temporal distribution,frontal characteristics and instability mechanisms of cold season(except for June,July and August)elevated convection in northern China(32.5°N-53.5°N,105°E-135°E)during a 16-year period from2000 to 2015.The events are concentrated in Southern Hebei province,western Shandong province and northern Henan province.The annual frequency distribution of elevated convection is bimodal,with a primary peak in February and a secondary peak in November.Cold front is the major frontal system dominating the events.There exists thick cold surface air and strong inversion when elevated convection occurs and more than half of the events are triggered over inversions with a temperature difference greater than 6?.In addition,strong vertical wind shear(20-30m/s)is frequently observed which gives rise to symmetric instability.The result further shows that conditional symmetric instability and frontogenetic forcing are the dominant mechanisms generating cold season elevated convection.Two elevated convention cases are then analyzed and the major characteristics of environmental background,radar echo signature,and instability mechanisms are revealed.The first case was a vertical convection which happened on 27 February 2011 with a MUCAPE of 182J/kg and a maximum updraft of 9.5m/s generated by the release of conditional instability,causing strong rainfall and lightning.Multiple instability mechanisms were included in the second case which mainly occurred in Shanxi,Henan and Hebei Province in China on 19 April 2013.The heavy snowfall happened in Shanxi was triggered by conditional instability with favorable conditions provided by shear line situated above 850 h Pa and synoptic ascending motion due to the coupling of upper-level jet stream andlow-level jet stream.The sleety weather occurred in Henan was likely to be the result of convective-symmetric instability.A diagnosis of conditional instability and conditional symmetric instability has been done and transformation from scattered echo to linear echo was observed.In addition,the secondary circulation produced by frontogenesis is probably one of the instability mechanisms.