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Title:
Further Results On The Iris Effect
Authors:
Lindzen, R. S.; Chou, M.-D.; Hou, A. Y.
Publication:
EGS XXVII General Assembly, Nice, 21-26 April 2002, abstract #2687
Publication Date:
00/2002
Origin:
EGU
Bibliographic Code:
2002EGSGA..27.2687L

Abstract

Over the past year, our original work on the iris effect as an important negative feed- back for global climate has been subject to substantial criticism. We briefly review the iris effect in order to explain that it is, in fact, a property associated with cumulus convection, and must, therefore, be scaled by a measure of cumulus activity. What it depends on is the degree to which cumulus towers detrain ice, which, in turn, depends on precipitation efficiency within the cumulus themselves. Its observation depends on considering scales and times which allow for significant numbers of cumulus clouds under a variety of circumstances. Such conditions do not demand global measure- ments. In brief, the original results suggested that precipitation efficiency increased with temperature, and that, therefore, upper level cirrus coverage diminished with temperature. The negative feedback depended on the claim that the infra red impact of upper level cirrus exceeded its impact on the visible. The name, "iris," stems from the analogy between the opening and closing of clear areas in response to temperature to the opening and closing of the eye's iris in response to light.

Our current efforts extend earlier efforts and respond to various criticisms:

1. Our data set now extends to 4 full years rather than the 20 months used in the original study. 2. We have considered versions of our full data with degraded spatial and temporal resolution in order to determine exactly what resolution is necessary for a study of this phenomenon. 3. We have devised a better measure of cumulonimbus activity than the simple infrared threshold used originally. 4. We have shown that uncertainties in infrared properties do not substantially alter feedback estimates. 5. We have used CERES data in order to investigate how albedo varies with area of cirrus (scaled by cumulus activity) in order to separate the albedo change due to fluctuations of cirrus areas from mean albedos which are biased by the high albedos associated with thick anvils near cumulus cores. It is, of course, the former which are relevant to the feedback.

Results from each of the above studies will be presented.

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