Geoscience Reference
In-Depth Information
3.5.1
Review of Conceptions for the “Excessive” Cloud Absorption
of Shortwave Radiation
The explanations of the excessive absorption of SWR proposed presently can
be divided into six main groups.
1. The excessive absorption is an artifact caused by observational errors
and imperfectness of data processing (Stephens and Tsay 1990; Pilewskie
and Valero 1995; Poetzsch-Heffter et al. 1995; Yamanouchi and Charlock
1995; Arking 1996; Taylor et al. 1996; Francis et al. 1997). Certain results
of SWR observations under the conditions of cloudy atmosphere have
provided the basis for this conclusion because of providing no signifi-
cant values of the cloud radiative absorption. The optical and radiative
properties of clouds are variable very much depending on the physical
mechanism of their origin and in many cases they don't increase ra-
diation absorption by the system “atmosphere plus surface” but on the
contrary decrease it. It happens because the clouds are reflecting a signif-
icant part of incoming radiation preventing the absorption by the lower
atmospheric layers and ground surface. It also should be mentioned that
in many cases the observations don't provide a data array sufficient for
the qualitative processing. Thus, observations in the cloudy atmosphere
frequently haven't been accompanied with the corresponding observa-
tions in clear atmosphere at the same period, the ground albedo hasn't
been measured every time and only reflected radiation has been reg-
istered. All these factors prevent adequate estimation of the radiative
characteristics of the cloudy atmosphere.
2. The increased absorption in the cloudy atmosphere in comparison with
theclearatmospherecouldbeexplainedwiththeradiationescaping
through the cloud sides in the broken clouds, as it has not been regis-
tered during the observations at the cloud top and bottom. Either field
(Hayasaka et al. 1994; Chou et al. 1995; Arking 1996) or simulated (Titov
1988, 1996a, 1996b; Romanova 1992) experiments could correspond to
this group of studies. The methodology of estimating the radiation es-
caping through the cloud sides proposed in the study by Chou et al.
(1995) a priori assumes the absence of true SWR absorption by clouds.
The authors of another study (Hayasaka et al. 1994) have processed the
observational data according to the method of study proposed by Chou
et al. (1995). The result of this processing is naturally to provide the
conclusion of SWR absorption absence by the cloud.
3. The excessive absorption is an apparent effect caused by the horizontal
transport of radiation in the cloud layer due to the horizontal heterogene-
ity of the layer (stochastic layer structure). Adetailed presentation of this
approach is provided in the studies by Titov and Kasyanov (1997). In ad-
dition, it is necessary to distinguish the cases of the roughness of the top
cloud surface (case 1) and of the heterogeneity of the inner cloud struc-
ture (extinction coefficient variations; case 2). The numerical analysis
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