A prolonged and widespread thin mid-level liquid cloud layer as observed by ground-based lidars, radiosonde and space-borne instruments

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A ~ 50-h-lasting mid-level thin liquid cloud layer was present over south and central China on December 13–15, 2018. Its characteristics were revealed by the observations from a polarization lidar and a water vapor Raman lidar at a mid-latitude site together with two geostationary satellites and the space-borne lidar CALIOP, as well as the conventional radiosondes. This layer was optically thick with cloud top at ~3.5-km altitude and a thickness of ~0.7 km. It resided on a warm-front-related inversion layer with the cloud top extending slightly above the inversion top and having a temperature of ~ − 4 to 1 °C. We found that the relative humidity over water peaked (~82–98%) near the liquid cloud center where the inversion layer had the temperature minima (−7 to −5 °C). This suggests that the moisture condensation into liquid droplets proceeded around the temperature minima of the inversion layer. The drizzle-sized water drop virga (no ice) occurred beneath the thin liquid cloud layer only during the first 7 h of its presence over our lidar site. The warm cloud top temperature (−3 °C) and strong wind shear at the cloud altitudes might be responsible for its formation. The Raman lidar observed a discernible increase in the subcloud water vapor mixing ratio due to the evaporation of the water drop virga. Satellite data revealed that the thin cloud layer covered a huge area over southern and central China. We found that its cloud top altitude increased overall with increasing latitude. This is consistent with the well-known warm-front cloud structure. It is shown through examining ECMWF reanalysis data that in the presence of the thin liquid cloud layer, a tropical cyclone located in the Bengal bay was uplifting continuously a large amount of moisture from the Indian Ocean. The resultant mid-level warm moist air masses were transported to China by the Indian monsoon, and then dispersed to the entire southern and central China under the influence of the southern branch of the Westerlies. Our findings provide a new insight into the formation and persistence of thin mid-level supercooled liquid cloud layers.