Abstract: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.