A Study of the Macrophysical and Microphysical Properties of Warm Clouds over the Northern Hemisphere Using CloudSat/CALIPSO data

This study investigates the general macrophysical and microphysical properties of single-layer warm clouds over the oceanic and land areas in Northern Hemisphere using four standard CloudSat products during 2008. The yearly averaged occurrence frequency of hydrometeor is 77.5% over the ocean and 65.8% over the land, to which the single-layer warm clouds contribute 20.9% and 9.5%, respectively. Using the liquid water path (LWP) as a statistical indicator for the life stages of warm clouds, we form ensemble averages of different retrieved variables as a function of LWP values in the following five groups 0-0.5, 0.5-1.0, 1.0-1.5, 1.5-2.0, and 2.0-2.5 mm, respectively. The cloud base heights (CBH) of single-layer warm clouds are located near 1 km with few changes, and the geometrical thicknesses over the land are 0.5-1 km thicker than those over the ocean. Vertical profiles of liquid water content (LWC) show that the maximum values of LWC over the land are about 10-30% smaller and the occurring altitudes are nearly 0.5 km higher as compared to those over the ocean with the same LWP, which is likely related to the weak upward motions and large surface evaporation over the ocean. The CFADs of radar reflectivity reveal the growth processes of cloud to rain via drizzle within warm clouds with increasing LWP (Figure 1). When LWP is less than 0.5 mm, the cloud droplets at the base of clouds grow mostly by condensation. With increasing LWP up to 1.0 mm, the radar reflectivity gradually increase from cloud top downward, indicating the drizzle growth by collection of small cloud droplets. When LWP is great than 1.0 mm, sufficient raindrops begin to develop. Note that the CFAD distributions of radar reflectivity over the land always appear later and dBZ values always a little smaller than those over the ocean with the same LWP. This is probably due to the influence of continental aerosol in this region, suggesting that the high aerosol concentrations may induce a suppression of drizzle and delay precipitation process in warm clouds. However, the particle sizes in the upper part of clouds over the land are generally larger than those over the ocean, somewhat inconsistent with our basic understanding of the aerosol effects. It may be attributed to stronger vertical air motion over the land, which transports large particles to the upper part of clouds. Moreover, the cloud particle spectral shapes imply a balance between breakup and coalescence processes when LWP is greater than 2.0 mm. The results of this study can be used to provide a valuable target for cloud-resolving models simulations. Finally, we find a significant difference between oceanic and land areas in the contoured frequency by optical depth diagrams (CFODDs) of radar reflectivity when LWP is 0.1-0.2 mm and optical depth exceeds 30. Under these conditions, the reflectivity values over the ocean increase rapidly downward with increasing optical depth (decreasing height). The faster growth of particles might be caused by the evaporation-condensation mechanism, which can occur under a certain water vapor environment. However, few large droplets are present in the early stage of continental warm clouds because of the aerosol effects, preventing the evaporation-condensation mechanism to occur. This may partially explain delayed drizzle formation over land relative to ocean. Figure 1. CFADs of radar reflectivity for all cloudy profiles grouped according to LWP with an interval of 0.5 mm over (top) NH ocean and (bottom) NH land. The radar reflectivity in the ranges of less than -20, -15 to 0, and 0 to 10 dBZ, are respectively interpreted as corresponding to cloud, drizzle, and rain.