What does reflection from cloud sides tell us about verticaldistribution of cloud droplet sizes?

[摘要] Cloud development, the onset of precipitation and the effect of aerosol onclouds depend on the structure of the cloud profiles of droplet size andphase. Aircraft measurements of cloud profiles are limited in their temporaland spatial extent. Satellites were used to observe cloud tops not cloudprofiles with vertical profiles of precipitation-sized droplets anticipatedfrom CloudSat.The recently proposed CLAIM-3D satellite mission (cloudaerosol interaction mission in 3-D) suggests to measure profiles of cloudmicrophysical properties by retrieving them from the solar and infraredradiation reflected or emitted from cloud sides.

Inversion of measurements from the cloud sides requires rigorousunderstanding of the 3-dimentional (3-D) properties of clouds. Here wediscuss the reflected sunlight from the cloud sides and top at twowavelengths: one nonabsorbing to solar radiation (0.67 μm) and one withliquid water efficient absorption of solar radiation (2.1 μm). Incontrast to the plane-parallel approximation, a conventional approach to allcurrent operational retrievals, 3-D radiative transfer is used forinterpreting the observed reflectances. General properties of the radiationreflected from the sides of an isolated cloud are discussed. As a proof ofconcept, the paper shows a few examples of radiation reflected from cloudfields generated by a simple stochastic cloud model with the prescribedvertically resolved microphysics. To retrieve the information about dropletsizes, we propose to use the probability density function of the dropletsize distribution and its first two moments instead of the assumption aboutfixed values of the droplet effective radius. The retrieval algorithm isbased on the Bayesian theorem that combines prior information about cloudstructure and microphysics with radiative transfer calculations.

[学科分类] 大气科学