一次华北极端降雪天气过程的中尺度涡旋特征

Characteristics of the Mesoscale Vortex in an Extreme Snowfall Process in North China

利用ERA5逐小时再分析数据和实况观测资料,对2020年2月13—14日华北极端雨雪天气过程中的中尺度涡旋进行特征分析,并采用相对涡度的环流积分变化方程对中尺度涡旋的发展机制进行诊断。结果表明:中尺度涡旋活动区域(简称涡旋区)的平均涡度垂直廓线呈“S”型分布,正负涡度极值分别位于800 hPa及400 hPa。涡旋区内800 hPa正涡度强度与上升运动大值区一致。900~700 hPa平均涡度强度与平原地区降水强度同相位变化,涡旋强度的峰值较降水强度峰值早约2 h,二者相关系数为0.76,说明低层涡度强度对降雪预报具有指示性。从涡旋发展的动力机制来看,涡旋区800 hPa以下正涡度主要源自水平风辐合;800~700 hPa上升运动与风随高度顺转对涡度的贡献更大。对比无中尺度涡旋的降雪过程(2019年2月14日),涡旋区内垂直方向涡度无“S”型结构,且中低层涡度较弱,北京降雪量明显偏小。涡旋区是否存在中尺度涡旋发展,对北京冬季降雪量级和极端性的预报有指示意义。

Using hourly ERA5 reanalysis data and observation data,the characteristics of mesoscale vortices during the extreme rain and snow event in North China from 13 to 14 February 2020 are analyzed.Besides,the development mechanism of the mesoscale vortices is diagnosed by the circulation integral equation of relative vorticity.The results show that the average vorticity vertical profile of the mesoscale vortex activity area(referred to as the vortex zone)exhibits an“S”shaped distribution,with extreme positive and negative vorticity values at 800 hPa and 400 hPa,respectively.The average vorticity intensity from 900 hPa to 700 hPa varies in the same phases with the precipitation intensity in the plain area.The peak of vortex intensity is about two hours earlier than the peak of precipitation intensity,and the correlation coefficient between the two is 0.76,suggesting that the intensity of lower-level vorticity has indicative significance for snowfall forecasting.From the perspective of the dynamical mechanisms driving vortex development,the positive vorticity below 800 hPa in the vortex zone mainly originates from horizontal wind convergence.The upward motion from 800 hPa to 700 hPa and the clockwise rotation of wind with increasing altitude contribute more to vorticity.Compared to the snowfall process without mesoscale vortex on 14 February 2019,the structure of vorticity in the vertical direction has no“S”shape,and the intensity of vorticity in the middle and lower layers is weak,resulting in obviously lower snowfall amount over Beijing.The existence or absence of mesoscale vortex development in the vortex zone has indicative significance for predicting winter snowfall magnitude and extremity in Beijing.