Retrieving cirrus vertical cross sections of extinction, effective particle size, and ice-water content using GOES imager data

Global information of ice water content (IWC) in ice clouds is urgently needed for testing of global climate models (GCMs) and other applications, but satellite retrievals of IWC and the column-integrated IWP are still in the developing stages, and tend to have large uncertainties (e.g. factor of 3 or more). A new retrieval method is presented here which may have relatively low uncertainties, using one or more thermal channels routinely available on operational meteorological and environmental satellites.

Recent research indicates that ice cloud radiative properties depend on more than effective diameter (Deff) and ice water content (IWC). The size distribution shape, or dispersion, such as the degree of bimodality, is also an important factor at thermal wavelengths. Failure to account for this can lead to substantial retrieval errors. Hence, we have developed a retrieval scheme that accounts for the size distribution (SD) shape in mid-latitude and tropical cirrus.

Previously, ice-particle shape appeared to represent a large source of uncertainty in IWP retrievals. Based on the methodology and new information on ice particle mass- and area-dimensional relationships, shape uncertainties now appear considerably less. By using a channel centered near 3.9 microns, absorption is largely volume dependent. This reduces particle-shape uncertainties, which are associated with area dependent absorption.

For a given ice-particle shape, preliminary tests varying particle size, IWC and cloud depth, as well as accounting for surface emission and instrument noise, suggest the scheme is accurate to about ±15% over an IWP range of 1 to 100 g m-2, and about ±25% over an IWP range of 100 to 200 g m-2, except for very thin cirrus having unusually large crystals or multiple cloud layers. This is based on thousands of radiation transfer model retrieval simulations.

By far the greatest contribution to retrieval uncertainty is from ice-particle size and shape. These uncertainties combined produce an estimated IWP uncertainty (standard deviation) of ±20%, based solely on the 3.9 micron channel and an IWP range of 1 to 100 g m-2. Since the AVHRR instrument has a channel near 3.9 microns, and was carried on current and previous NOAA satellites, global trend analyses of IWP can be carried out over the last 20 years.

Coupled with cloud top and cloud base retrievals, normalized IWC-profile shapes, and an integrated IWP retrieval, it is possible to infer vertical profiles of cirrus radiative and microphysical properties such as the extinction coefficient and (an absolute-magnitude) IWC. The retrieval scheme will be further tested with in-situ data and ground-based retrievals from several ARM cirrus IOPs, where GOES-8 channels at 6.7 and 10.8 microns were used. These cirrus IOP cases were sampled with the Counterflow Virtual Impactor (CVI), which gives a direct measurement of IWC and IWP.

Finally, this scheme is equally well suited for water clouds for determining LWP, although uncertainties should be lower since absorption in water clouds at 3.9 microns is about half that for ice clouds, and there is no uncertainty due to particle shape.