Locally Formed Thin Supercooled Liquid Stratiform Clouds on Mid-Level Moisture Advection Layers as Observed by Ground-Based Lidars, Radar, and Radiosondes at a Subtropical Site

Abstract

The local formation characteristics of thin supercooled liquid stratiform clouds on mid-level moisture advection layers have been revealed by the ∼2-year observations from routinely operating ground-based water vapor Raman Lidar, polarization Lidar, cloud radar, and conventional radiosondes at a subtropical site. These moisture advection layers were commonly observed during all seasons over our subtropical site, whereas thin supercooled liquid stratiform clouds occurred mainly in winter when the 0°C isotherm level occurred at lower altitudes. It was determined that these clouds mostly formed on the top of moisture advection layers capped by strong temperature inversions. The temperature-moisture structure collocation (the layer's upper edge with modest/low water vapor mixing ratio of ∼2–5 g kg⁻¹ occurred at the base of the temperature inversion) therein produced a narrow cool-moist water saturation layer. This collocation with a temperature inversion but no moisture inversion was peculiar to the upper boundary of the moisture advection layers. A relative minority (∼33%) of thin supercooled liquid stratiform clouds formed on the bottom or middle part of the moisture advection layers, where a moisture inversion coincided with a temperature inversion. Within the corresponding water saturation layer, the relative humidity peaked at the top of the coincided moisture inversion and temperature inversion, indicating warm-moist air mass advection. Two detailed case studies illustrated how shifts in the mesoscale meteorological environment pertinent to moisture advection layers determined the presence and absence of thin supercooled liquid cloud layers.