Evaluation of the impacts of ice cloud vertical inhomogeneity on spaceborne passive submillimeter‐wave simulations

Abstract

Spaceborne passive submillimeter instruments can provide higher sensitivity to a broader size range of ice hydrometeors than traditional millimeter and infrared observations. However, a plane‐parallel cloud assumption is commonly applied for simplicity in passive cloud retrieval algorithms, while observations from active instruments have already demonstrated the ice particle variabilities in the vertical dimension. This study is conducted to assess the impacts of ice cloud vertical inhomogeneity on passive submillimeter‐wave simulations. Specifically, a simplified two‐layer scheme for distributions of particle size and ice water content in ice clouds is utilized to explore the inhomogeneous effects on radiative‐transfer process via the brightness temperature differences at the top of the atmosphere. Then the effect is quantitively evaluated based on a generated synthetic ice cloud and compared under the assumption of five particle size distributions to simulate homogeneous and heterogeneous clouds. The simplified two‐layer model simulations show that before saturation, the upper layer generates a stronger brightness temperature depression due to scattering than does the lower layer, given the same ice water content (IWC). However, with increasing IWC or effective diameters, the saturation effect would lead to the opposite impact, especially for high‐frequency channels. The generated synthetic ice cloud simulations reveal that the average values and the root‐mean‐squared errors of the brightness temperature differences are between −6 and 8 K and within 10 K, respectively. The results in this study are of special significance for remote‐sensing ice hydrometeors from passive submillimeter observations for the upcoming Ice Cloud Imager (ICI), as well as for future assimilations of submillimeter radiances in all‐sky conditions.