Ice cloud microphysical trends observed by the Atmospheric Infrared Sounder

Abstract

Abstract. We use the 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 (T ci ), ice optical thickness (τ i ) and ice effective radius (r ei ). The trends in ice cloud properties are shown to be independent of trends in information content and χ 2 . Statistically significant decreases in ice frequency, τ i , and ice water path (IWP) are found in the SH and NH extratropics, but trends are much 15 smaller and statistically insignificant in the tropics. However, statistically significant increases in r ei 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 χ 2 trends. Values of r ei are larger at the tops of opaque clouds and exhibit strong dependence on surface wind speed, column water vapour (CWV) and surface temperature (T sfc ) with changes up to 10-12 µm. Transparent clouds exhibit a much smaller change in r ei for CWV, while none is 20 observed for T sfc . Comparisons between DARDAR and AIRS suggest that r ei 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.