Geoscience Reference
In-Depth Information
3.5
The Problem of Excessive Absorption
of Solar Short-Wave Radiation in Clouds
Studies of the impact of aerosols and clouds on radiation balance and on radia-
tive flux divergence in the atmosphere are of great importance for the analysis
of various factors contributing to climate formation (Monin 1982; Hobbs 1993).
The data of the surface pyranometric observations of the downward and up-
ward short-wave radiation (SWR) fluxes permit us to calculate the short-wave
radiation budget (SWRB) of the surface, whereas the satellite observational
data characterize the outgoing short-wave radiation (OSWR). The difference
between SWRB and OSWR for conditions of the cloudy and clear sky deter-
mines short-wave radiative forcing at the surface C s ( S ) and at the top of the
atmosphere C s ( TOA ). The mean annual and mean global value of the short-
wave cloud radiative forcing C s ( TOA ), at the top has been determined to vary
from −45 to −50mWcm −2 .Thevalue C s ( TOA )
=
C s ( S )+ C s ( A )isevidently
determined through the yields of the cloud radiative forcings of the surface
C s ( S )andatmosphere C s ( A ). The ratio that describes the cloud forcing of the
system “atmosphere plus surface” has been introduced by the authors of the
study by Cess et al. (1995)
C s ( S )
( F o F c ) bottom
( F o F c ) top
=
C s ( TOA ) =
f s
.
(3.17)
The analysis of the observational data of the cloud radiative forcing of the
city Boulder (USA) has shown that C s ( S )
=
− 92.6mWcm −2 and C s ( TOA )
=
=
− 63.2mWcm −2 that leads to the value f s
1.46. The value calculated on the
basis of numerical simulations should be close to unity: f s
1. Thus, it follows
that the calculations are essentially underestimating the values of the absorbed
SWR by the cloudy atmosphere by magnitude C s ( TOA )− C s ( S )
30mWcm −2
(Hobbs 1993; Cess et al. 1995). It has been called “excessive” (or even “anoma-
lous” ) cloud absorption of shortwave radiation (Stephens and Tsay 1990; Cess
et al. 1995; Pilewskie and Valero 1995, 1996; Ramanathan et al. 1995). This
result has revealed a fundamental gap in present understanding of the cloud
impact on SWRB. This obstacle has led to an emotional scientific discussion
significantly changing the modern ideas about the role of cloudiness in climate
and weather formation (Stephens and Tsay 1990; Cess et al. 1995; Charlock et
al. 1995; King et al. 1995; Pilewskie and Valero 1995; Ramanathan et al. 1995;
Stephens 1995,1996; Titov and Zhuravleva 1995; Yamanouchi and Charlock
1995; Cess and Zhang 1996; Valero et al. 1997; Zhang et al. 1998). The im-
portance of the problem is seen even in the titles of articles (“An absorbing
mystery”, “Shortwave cloud forcing: a missing physics”, “Anomalous absorp-
tion paradox” etc.).
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