Abstract
Abstract. We use the Atmospheric Infrared Sounder (AIRS) version 6 ice cloud property and thermodynamic phase retrievals to quantify variability and 14-year trends in ice cloud frequency, ice cloud top temperature (Tci), ice optical thickness (τi) and ice effective radius (rei). The trends in ice cloud properties are shown to be independent of trends in information content and χ2. Statistically significant decreases in ice frequency, τi, and ice water path (IWP) are found in the SH and NH extratropics, but trends are of much smaller magnitude and statistically insignificant in the tropics. However, statistically significant increases in rei are found in all three latitude bands. Perturbation experiments consistent with estimates of AIRS radiometric stability fall significantly short of explaining the observed trends in ice properties, averaging kernels, and χ2 trends. Values of rei are larger at the tops of opaque clouds and exhibit dependence on surface wind speed, column water vapour (CWV) and surface temperature (Tsfc) with changes up to 4–5 µm but are only 1.9 % of all ice clouds. Non-opaque clouds exhibit a much smaller change in rei with respect to CWV and Tsfc. Comparisons between DARDAR and AIRS suggest that rei is smallest for single-layer cirrus, larger for cirrus above weak convection, and largest for cirrus above strong convection at the same cloud top temperature. This behaviour is consistent with enhanced particle growth from radiative cooling above convection or large particle lofting from strong convection.
Highlights
While our understanding of ice cloud microphysics has greatly advanced from targeted in situ campaigns during the past several decades (Baumgardner et al, 2017), global distributions obtained from satellite observing systems remain highly uncertain (e.g. Stubenrauch et al, 2013)
The Atmospheric Infrared Sounder (AIRS) Version 6 (V6) cloud properties are used from 1 September 2002 until 31 August 2016 (Kahn et al, 2014; K14 hereafter). As this investigation addresses ice microphysics, we focus on the 26.5 % of AIRS pixels containing ice thermodynamic phase, and their ice cloud top temperature (Tci), ice optical thickness, and rei retrieval parameters (K14)
The histograms each contain one of several Advanced Microwave Scanning Radiometer (AMSR) variables on the x-axis and the AIRS upper layer Tcld on the y-axis. The intent of these diagrams is to reveal the physical response of cloud top rei and τi to precipitating and nonprecipitating cloud types and meteorological variability inferred from AMSR
Summary
While our understanding of ice cloud microphysics has greatly advanced from targeted in situ campaigns during the past several decades (Baumgardner et al, 2017), global distributions obtained from satellite observing systems remain highly uncertain (e.g. Stubenrauch et al, 2013). The aforementioned observational, theoretical, and numerical modelling studies motivate the development of additional constraints on rei and its covariability with other ice cloud, thermodynamic, and dynamic fields This investigation is a first attempt to quantify secular changes in rei from AIRS over its decade and a half observational record with well-characterised radiometric stability (Pagano et al, 2012). Kahn et al (2015) describe pixel-level comparisons between AIRS and MODIS ice cloud properties and show that overlapping sensitivity for both τi and rei is observed for optically thicker pixels containing four positive ice phase tests with spatial maps resembling those described in King et al (2013). The total column water vapour (CWV) cloud liquid water path (LWP), rain rate (RR) (Hilburn and Wentz, 2008), sea surface temperature (Tsfc) (Gentemann et al, 2010), and direction-independent near surface wind speed (u) are obtained from AMSR-E (Wentz et al, 2014a) and AMSR-2 (Wentz et al, 2014b) using Version 7 data
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