POLDER OBSERVATIONS OF CLOUD BIDIRECTIONAL REFLECTANCES COMPARED TO APLANE-PARALLEL MODEL USING THE INTERNATIONAL SATELLITE CLOUD CLIMATOLOGY PROJECT CLOUD PHASE FUNCTIONS

This study investigates the validity of the plane-parallel cloud model and in addition the suitability of water droplet and ice polycrystal phase functions for stratocumulus and cirrus clouds, respectively. To do that, we take advantage of the multidirectional viewing capability of the Polarization and Directionality of the Earth's Reflectances (PO LDER) instrument which allows us to characterize the anisotropy of the reflected radiation field. We focus on the analysis of airborne-POLDE R data acquired over stratocumulus and cirrus clouds during two select ed flights ton April 17 and April 18, 1994) of the European Cloud and Radiation Experiment (EUCREX'94) campaign. The bidirectional reflectan ces measured in the 0.86 mu m channel are compared to plane-parallel c loud simulations computed with the microphysical models used by the In ternational Satellite Cloud Climatology Project (ISCCP). Although clou ds are not homogeneous plane-parallel layers, the extended cloud layer s under study appear to act, on average, as a homogeneous plane-parall el layer. The standard water droplet model (with an effective radius o f 10 mu m) used in the ISCCP analysis seems to be suitable for stratoc umulus clouds. The relative: root-mean-square difference between the o bserved bidirectional reflectances and the model is only 2%. For cirru s clouds, the water droplet cloud model is definitely inadequate since the rms difference rises to 9%; when the ice polycrystal model chosen for the reanalysis of ISCCP data is used instead, the rms difference is reduced to 3%.