Cloud Halos and their Roles in the Atmospheric Radiation Balance and Climate

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Recent observations indicate that the air surrounding growing cumulus clouds is sometimes nearly saturated with water vapor. Calculations show that these vapor shrouds or "halos" around clouds can account for a significant increase in the absorption of solar radiation. The purpose of this project is to measure the vapor field around cumulus clouds with high accuracy and resolution and to use the information in calculations of radiative transfer and in assessing the consequences of cloud-environment interactions on cloud microphysics. The specific objectives are as follows:- To measure the vapor field in and around convective clouds with sufficient detail to characterize the 3-dimensional vapor structure relative to cloud edges;- To measure concurrently the size distribution, composition, and optical properties of aerosols within the cloud halo;- To analyze the resulting measurements in order to provide several completely documented cases for use in radiative transfer calculations;- To carry out radiative transfer analyses including absorption and heating rate calculations for the observed distributions of water vapor, aerosols, and cloud;- To determine the implications of the radiation analysis for climate modeling.The measurements will be conducted with the CIRPAS Twin Otter aircraft, equipped with a new instrument called the Fast Water Vapor Monitor developed by Aerodyne Research along with instruments for measuring aerosols, CCN, cloud droplets, and liquid water content. Computer codes with different degrees of complexity will be used for the radiative transfer calculations. The results bear on the question of anomalous cloud absorption, which refers to the discrepancies that have been observed between the observed solar absorption by clouds and the absorption calculated using Mie scattering theory applied to populations of cloud drops. This is a collaborative research project between Cal Tech and Aerodyne Research, Inc. The Aerodyne component of the collaboration is supported by ATM-9910744, Dr. John T. Jayne, PI.