Abstract: Using the conventional meteorological data,NCEP (National Centers for Environmental Prediction) reanalysis data with resolution of 1°×1°,FY-2C satellite cloud images,and Doppler radar data in Hohhot,the circulation situations,physical quantity fields,characteristics of infrared cloud images and radar echoes during the two snowstorm events caused by reflow and inverted rough on November 5-6 and on November 21-22,2015 in Inner Mongolian Autonomous Region were analyzed.The results show that the westerly trough at 500 hPa,the jet and shear line at 700 hPa and the surface inverted rough are the main influencing systems and mesoscale clouds are the direct cause for the two snowstorm events.Warm and wet airflows transported by jets at 700 hPa and strong convergence of water vapor provide plenty of water vapor for snowstorms.The stronger and longer duration of jets at 700 hPa result in the longer duration and lager amount of snowfall.Systematic ascending motions provide the dynamic conditions for snowstorms,and the coupling of jets at the upper and lower levels favors the enhancement of ascending motion.Cold air forms a cold pad at the lower levels,temperature difference and vertical shears induce dynamical frontogenesis,and strong ascending motion is caused by the frontogenesis secondary circulation.Warm and wet airflows climb up along the cold pad,which provides the thermal conditions for snowstorms.The most severe snowfall occurs when the frontal zone is strongest and begins to weaken,and the snowstorm area occurred at the low-value center of equivalent blackbody brightness temperature (TBB≤220 K),the convergence center of water vapor at 850 hPa,and the ascending motion center.The composite reflectivity factor reaches 30-35 dBz,which means local strong snowfall.The zero velocity line in radial velocity field shows an “S” pattern,and the “bovine eye” exists for a long time,which makes snowfall continue or strengthen.

Key words: Snowstorm, Reflow, Inverted rough, Physics diagnosis, Radar echo