Abstract
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.
Highlights
Clouds play an important role in the energy budget of our planet: optically thick clouds reflect the incoming solar radiation, leading to cooling of the Earth, while thinner clouds act as “greenhouse films”, preventing escape of the Earth’s long-wave (LW, see Table A1 in the Appendix; this is not explained in the text for readability’s sake) radiation to space
We have studied the probability of occurrence of these specific ice water content (IWC) profile shapes as a function of ice water path (IWP)
The analysis shows that changes in relative occurrence of IWC profile shapes are only noticeable for strong downdraft within the cloud, while filtering the statistics based on w500 and wbase did not lead to conclusive results
Summary
Clouds play an important role in the energy budget of our planet: optically thick clouds reflect the incoming solar radiation, leading to cooling of the Earth, while thinner clouds act as “greenhouse films”, preventing escape of the Earth’s long-wave (LW, see Table A1 in the Appendix; this is not explained in the text for readability’s sake) radiation to space. IR (infrared) sounders have been observing our planet since 1979: from the TOVS (Television Infrared Observation Satellite) sounders (Smith et al, 1979) onboard the NOAA polar satellites to the AIRS (Atmospheric Infrared Radiation Spectrometer) (Chahine et al, 2006) onboard Aqua (since 2002) and to the IASI (Infrared Atmospheric Sounding Interferometer) (Chalon et al, 2001; Hilton et al, 2012) onboard MetOp (since 2006), with increasing spectral resolution Their spectral resolution along the CO2 absorption band makes IR (infrared) sounders sensitive to cirrus, day and night (Stubenrauch et al, 1999, 2006, 2008, 2010; Wylie et al, 2005).
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