Retrieval of two‐layer cloud properties from multispectral observations using optimal estimation

A method to derive two‐layer cloud properties from concurrent visible, near‐infrared, and infrared observations is described. It is a modification of a single‐layer scheme and is applied to Spinning Enhanced Visible Infrared Imager (SEVIRI) observations and validated against coincident A‐Train data, principally to evaluate the accuracy and characterize cloud top pressure (CTP) estimates. CTP values obtained from the single‐layer scheme applied to multilayer clouds are significant overestimates of the upper layer value. The effect is usually larger than that on coincident IR‐only retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS), and this characteristic can be traced to the use of visible wavelength observations. However, the solution cost from the optimal estimation method is found to be especially high in multilayer situations and is a strong indicator of CTP accuracy. Tighter thresholds on the solution cost select, with increasing stringency, scenes with single‐layer or opaque upper layer cloud. High‐cost (presumed multilayer) pixels are reprocessed with the scheme adapted to simulate a two‐layer cloud and with only infrared measurements. The upper cloud is represented by the parameters of the original formulation; the additional lower cloud layer is gray and has a proxy height given by the surface temperature. Despite the simplicity of the cloud‐atmosphere modeling under the upper layer, results obtained from the two‐layer scheme are promising. Upper layer CTPs are of comparable accuracy to the single‐layer cases, lower‐layer CTPs show some useful accuracy, and upper layer optical depths correlate well with radar observations.