Cloud microphysical relationships in continental stratocumulus clouds measured during the RACORO campaign

Abstract:

Cloud microphysical relationships are determined by various processes that occur in clouds. The simplest and the most important process that occurs during cloud development would be the adiabatic condensational growth of cloud droplets. However, what have been observed in real clouds are often different from the cloud microphysical relationships predicted based on adiabatic condensational growth. There could be various reasons why the measured relationships deviated from adiabatic prediction but turbulent entrainment and mixing of the air above cloud top is considered to be the most important process that modulates cloud microphysical relationships especially in stratocumulus clouds. There were many studies on this issue for marine stratocumulus clouds but studies on continental stratocumulus clouds are rare. Compared to maritime stratocumulus clouds, continental stratocumulus clouds usually have very different thermodynamic and aerosol conditions. So in this study we examined microphysical relationships in continental stratocumulus clouds measured during the Routine AAF CLOUD Optical Radiative Observations (RACORO) campaign as an attempt to reveal the most dominant entrainment-mixing mechanism in these clouds. We used so called the mixing diagram and found that most cases suggested homogeneous mixing of entrained air. We also estimated the scale parameters (i.e., transition length and scale number) because they are indicative of whether entrainment-mixing mechanism is homogeneous or inhomogeneous. The estimated values were found to be critically dependent on turbulent dissipation rate and were suggestive of homogeneous mixing, compared to those estimated for marine stratocumulus clouds in our recent study. We will try to explain why such is the case for continental stratocumulus clouds.