Snowstorm Events And Their Associations With The Mesoscale Gravity Waves:Case Studies

Based on data from observations,the surface automated stations,the digital Doppler weather radar,NCEP/NCAR reanalysis and the WRF model simulation,two cases of intense snowstorms with blizzard conditions on 28-29 April 1983 and on 3-4 March 2007 in northeast China are investigated.Mesoscale gravity wave features for the case of 2007 are studied as well.Results show:(1)These snowstorms produced by the explosively deepened cyclones,which developed under the left side of the exit region of the southern jet and the right side of the entrance re-gion of the northern jet at 300 hPa level.The 3D VAPOR visualization of the velocity from the GRIB grids and the WRF simulations show,severe surface winds are connected to the air-flow among the two upper-level jets,suggesting that the surface winds energy originated from the upper troposphere aloft.(2)It is shown by the radar data,the mean horizontal wind velocity in the 4.5-8 km layer of the troposphere tended to be uniform because of the momentum exchange between the wave motion and the basic current.At the same time,a saturated moisture jet stream was es-tablished extending almost throughout the troposphere as seen in the simulation,with the most severe precipitation area located just ahead of the 850 hPa jet center.Waves inspired under the exit region of an upper layer jet stream near the tropopause propagate downwards,resulting in an alternative acceleration or deceleratiand the radar echoes show the gravity wave signature.It is found that considerable increase in on of the horizontal wind velocity in ver-tical over the station.The divergence or confluence caused by the wind change with the height has a substantial effect on the base reflectivity,the intensity of the echoes occurred in the layers with the accelerating flow,but decreased in the layers with the decelerating flow,with about 40 km wide weak precipitation band imbedded in the major cloud bands,in a northwest-southeast orientation.While the waves propagate downwards,it was observed that,the gusty surface northeastly wind appeared as the surface pressure drops rapidly,with an obvious change in the wind direction,and the intensity of the echoes begins to be weakened.The ob-served severe surface winds centers distributed in a southwest-northeast orientation as in the gravity wave signature similar to the features of the echoes.