| The dryline is one of the triggering mechanisms of thunderstorm or convective system.Compared with the rich research on the dryline abroad,there are only a few case studies in China,which are insufficiently representative.In this paper,the northeast region(40-53°N,115-135 ° E)is selected as the research area.Based on conventional surface observation data and conventional upper-air sounding observations,the temporal and spatial distributions of the drylines and its meteorological elements of the surrounding environment during the warm season(May-August)in the region from 2003 to 2017 were statistically analyzed;The key parameters of the drylines enviornment of some typical convective drylines which trigger convection and no-convective drylines are dynamically synthesized and compared based on reanalysis data,and the structure of drylines and the mechanism of triggering convection are also studied.Results show that the drylines in northeast China mainly appear in Northeast China Plain and western Liaoning,and the frequency of drylines appears more in the south than in the north.Most of the drylines run southwest to northeast with a width of 90 ? 120 km and a length of 100 ? 800 km.The annual average frequency of the dryline is 15.5% in warm season in northeast China.The frequency of dryline is not obvious in interannual variation,but it is significant and sine-shaped in every ten days,among which the peak is in mid-late May,mid-late June and mid-late August,the frequency of dryline is over 20%,and it is below 10% in July.The atmospheric pressure on the wet side of the dryline is slightly higher than that on the dry side by 1h Pa,the temperature on both sides was more than 24?,and the temperature difference was generally in the range of 1 ? 4?,higher in the West and lower in the east during the day,and the dew point gradient and the specific humidity gradient range are 9.615 ?/ 100 km and 4.583 g/kg/100 km respectively,west is low,east is high.The relative potential temperature gradient is 9.6-19k/100 km,the westerly and northwesterly wind are dominant in the dry side,and the southwest wind is prevailing in the wet side,two streams converge near the dryline.The main factors and theirgradients on both sides of the dryline changed significantly every ten days,the temperature and humidity on both sides and the gradients of humidity increased from May to August,while the absolute value of temperature gradient decreased.In the total 286 cases,40% of the drylines triggered convection,the highest in early and middle June,reaching more than60%.The vertical extension thickness of the dryline,the slope of the dryline interface and the position of the dryline changed significantly from day and night due to the topography of the west high and low east.From Sunrise to 14Bpm(Beijing time),the location of the dryline moves significantly eastward,the thickness of the wet layer increases,the interface of the dryline tends to be vertical,and after 17 pm,the dryline retreats westward,the dryline slopes eastwards and is reduced in height.From 14:00 to 17:00 hours,the dryline became stationary and the Low-level convergence and updraft developed vigorously.Affected by the favorable large-scale circulation situation in the upper and lower layers,the humidity and humidity gradient,low-level convergence intensity and vertical velocity of the convective dryline are stronger than those of the non-convective dryline,the lifting effect is strong;with the strong condition instability and thick wet layer on the wet side of the convection dryline,Convective available potential energy is stronger(CAPE > 1000 J /kg),and the convection is triggered on the wet side of the dryline.However,there is no favorable background circulation condition of the no-convective dryline,the strength and thickness of the wet layer are also poor,and the CAPE value is often small.Even if there is a certain intensity of low-level convergence and updraft on the dry-wet mixing interface of the dryline,the possibility of convection is also very low. |
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