A climatology of single-layer cirrus clouds and associated heating rates from NASA satellite observations and RTM calculations

High-level cirrus clouds are recognized as one of the major sources of uncertainty in addressing any potential future climate change.  High-level clouds are transparent to the incoming solar radiation and emit longwave (LW) radiation back to the Earth’s surface, which induces a warming effect.   Correctly representing cirrus clouds in global climate models (GCMs), or at least their bulk radiative effects, is a necessity not only for the global energy budget but also for assessing changes in large-scale general circulation patterns.

In this study single-layer optically thin (τ < 3.6) high-level (cloud top pressure < 440 hPa) cirrus clouds are identified from the CloudSat 2C-ICE satellite data set during the period 2007-2010.  2C-ICE provides the necessary details (e.g., cloud top and base height, ice water content (IWC), effective radius (Re), and optical depth (τ)) required to calculate cloud radiative heating rate (K/day) profiles.  Information, such as atmospheric temperature, ozone, and water vapor profiles, and aerosol properties are also required as input to the one-dimensional Fu-Liou radiative transfer model (RTM).  As such, the vertical profiles are obtained from the collocated CERES, CloudSat, CALIPSO, MODIS (C3M) Meteorological, Ozone, and Aerosol (MOA) analysis, which come from the Global Modeling and Assimilation Office (GMAO) Goddard Earth Observing System Model (GEOS) Version 4/5. 

For the four years considered in this study, the cirrus clouds of interest occur most frequently in the Southern Hemispheric high-latitudes (75 – 90 °S) and are least frequent in the southern mid-latitudes (30 – 60 °S).  Clouds of this type exhibit strong seasonal and latitudinal variations of IWC, Re, τ, and cloud top pressure (P_ct).  The cirrus cloud properties and atmospheric details are used to calculate radiative fluxes and cloud heating rate profiles.  The calculated surface and top-of-atmosphere (TOA) fluxes are constrained by Atmospheric Radiation Measurement (ARM) Program and Clouds and the Earth Radiant Energy System (CERES) observed instantaneous fluxes, respectively.  It is our hope that these results can then be used as a tool for evaluating heating rate profiles in climate model simulations.