Ice water content vertical profiles of high-level clouds: Classification and impact on radiative fluxes

Abstract

In this article, we discuss the shape of ice water content (IWC) vertical profiles in high ice clouds and its effect on their radiative properties, both in short- and in long-wave bands (SW and LW). Based on the analysis of collocated satellite data, we propose a minimal set of primitive shapes (rectangular, isosceles trapezoid, lower and upper triangle), which represents the IWC profiles sufficiently well. About 75 % of all high-level ice clouds (P < 440 hPa) have an ice water path (IWP) smaller than 100 g m-2, with a 10 % smaller contribution from single layer clouds. Most IWC profiles (80 %) can be represented by a rectangular or isosceles trapezoid shape. However, with increasing IWP, the number of lower triangle profiles (IWC rises towards cloud base) increases, reaching up to 40 % for IWP values greater than 300 g m-2. The number of upper triangle profiles (IWC rises towards cloud top) is in general small and decreases with IWP, with the maximum occurrence of 15 % in cases of IWP less than 10 g m-2. We propose a statistical classification of the IWC shapes using IWP as a single parameter. We have estimated the radiative effects of clouds with the same IWP and with different IWC profile shapes for five typical atmospheric scenarios and over a broad range of IWP, cloud height, cloud vertical extent, and effective ice crystal diameter (De). We explain changes in outgoing LW fluxes at the top of the atmosphere (TOA) by the cloud thermal radiance while differences in TOA SW fluxes relate to the De vertical profile within the cloud. Absolute differences in net TOA and surface fluxes associated with these parameterized IWC profiles instead of assuming constant IWC profiles are in general of the order of 1-2 W m-2: they are negligible for clouds with IWP < 30 g m-2, but may reach 2 W m-2 for clouds with IWP > 300 W m-2.