Analysis and Numerical Simulation of Humidity Halos around Trade Wind Cumulus Clouds

Significant enhancements in humidity above that in the cloud-free atmosphere have been found in the vicinity of isolated cumulus clouds, so-called cloud halos. Cloud halos can reflect features of cloud evolution, increase shortwave atmospheric radiative absorption, and may serve as a predominant region of new particle formation. We report results of an aircraft campaign off Oahu, Hawaii during August 2001, aimed at characterizing the humidity properties around isolated cumulus clouds at 10 m spatial resolution by a novel high-speed spectroscopic water vapor concentration monitor developed by Aerodyne Research, Inc. Analysis of the measurements adds to the existing body of information on cloud halos, their frequency of occurrence, spatial extent, and meteorological characteristics. The dynamic model RAMS (Regional Atmospheric Modeling System) in the LES (Large Eddy Simulation) mode and the SHDOM (Spherical Harmonic Discrete Ordinate Method) radiative transfer model have been implemented for one particularly long-lived cloud to track the temporal evolution of the cloud halos and to calculate the associated spatial and temporal patterns of shortwave absorption. Results of this modeling study are in general agreement with the field measurement data and previous aircraft studies.