Short-Wave Solar Radiation in the Earth’s Atmosphere Part 5 pptx

116 Spectral Measurements of Solar Irradiance and Radiance in Clear and Cloudy Atmospheres

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

the clear atmosphere could be explained with the radiation escaping

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). A detailed 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

The Problem of Excessive Absorption of Solar Short-Wave Radiation in Clouds 117

has shown that the horizontal transport in the case of a stochastic cloud

top structure is revealed as stronger than in the case of the cloud inner

parameter variations. To estimate the absorption in the layer correctly,

the scale of the reflected and transmitted irradiances averaging over the

cloud horizontal extension should be 30 km for case 1 and 6 km for case 2

correspondingly. The case of the stochastic cloud top structure corre￾sponds to real cumulus clouds and the case of the cloud inner parameter

variations corresponds to real stratus clouds. Different combinations of

the absorption and scattering coefficients in the cloud layer and different

scales of the horizontal and vertical heterogeneity have been considered

in the study by Hignett and Taylor (1996) and the authors has revealed

that “the internal inhomogeneity in the cloud microphysics and in the

macrophysical structure in terms of cloud thickness are both important

in the determination of the cloud radiative properties”.

4. In addition to other reasons the anomalous absorption in clouds is

suggested to be explained with the water vapor absorption within the

absorption bands in the NIR spectral region, which has not been ac￾counted for before (Evans and Puckrin 1996; Crisp and Zuffada 1997;

Nesmelova et al. 1997; O’Hirok and Gautier 1997; Savijarvi et al. 1997;

Harshvardhan et al. 1998; Ramaswami and Freidenreih 1998). However,

while computing, the detailed and careful accounting of the molecular

absorption in the NIR region has not provided the observed magnitude

of the cloud absorption (Kiel et al. 1995; Ramaswami and Freidenreih

1998). Besides, the results of spectral observations (Titov and Zhuravleva

1995) have demonstrated the strongest effect of the anomalous absorp￾tion in the visual spectral region, where the water vapor absorption is too

weak. Thus, it is seen that the molecular absorption by water vapor in the

NIR region is not enough for an explanation of anomalous absorption.

5. The microphysical properties of clouds have been implied as a reason

of the excessive absorption in various studies (Ackerman and Cox 1981;

Wiscombe et al. 1984; Hegg 1986; Ackerman and Stephens 1987). Very

large drops of the cloud are considered in the studies by Ackerman and

Stephens (1987) and Wiscombe et al. (1984); it is suggested the presence

of them actually increases the radiation absorption within clouds, but it

is too weak and insufficient to explain the anomalous absorption. The

authors of another study (Hegg 1986) have calculated in detail the optical

and radiative parameters of clouds containing two-layer particles with

absorbing nuclei and a nonabsorbent shell and have not obtained high

enough values of the absorption by clouds either. In all considered mod￾els, the noticeable absorption by clouds succeeds only when assuming

a significant amount of the atmospheric aerosols (Wiscombe 1995; Bott

1997; Vasilyev A and Ivlev 1997).

6. The authors of three studies (Kiel et al. 1995;Hignett and Taylor 1996; Ra￾maswami and Freidenreich 1998) have considered the above-mentioned

reasons in different combinations and they conclude that with certain

118 Spectral Measurements of Solar Irradiance and Radiance in Clear and Cloudy Atmospheres

assumptions the calculated and observed values of the cloud radiation

absorption turns out to be close to each other. Nevertheless, it is safe to

say that there is no exhaustive explanation for the total set of observa￾tions. Thus, the problem has not been solved yet as the authorsWiscombe

(1995), Lubin et al. (1996), Bott 1997, Ramanathan and Vogelman (1997),

and Collins (1998) point out.

3.5.2

Comparison of the Observational Results of the Shortwave Radiation Absorption

for Different Airborne Experiments

In the above-mentioned studies of radiation absorption by clouds (confirm￾ing or denying the excessive absorption), the satellite data and the data of

the meteorological network have been mainly used. These observations were

accomplished with different instruments during a long period that called for

complicated statistical data processing. As a result, an averaging picture includ￾ing different types of clouds has been obtained. The absence of either uniform

data or a common methodology for data choice and processing is likely to lead

to the contradictory conclusions in the studies hereinbefore described.

Let the airborne observations considered in the previous section be ana￾lyzed in terms of factor fs. Absorption R = (F↓ − F↑)top − (F↓ − F↑)base in the

atmospheric layer with and without clouds is computed with the airborne mea￾surements of SWR. Table 3.2 demonstrates the conditions and results of the

airborne experiments and the values of factor fs for the total (within spectral

region 0.3–3.0 µm) and spectral (for wavelength 0.5 µm) radiation measure￾ments as values of the total absorption in the layer of the clear or cloudy

atmosphere. The results of the airborne observations are seen to allow fixing

of the effect of the strong shortwave anomalous absorption (fs > 1) in a set of

cases. In other cases there is no influence of clouds on the radiation absorption

(fs = 1) and in some cases the strong reflection of solar radiation by clouds

even prevents its absorption by the below cloud atmospheric layer and by the

ground surface (fs < 1).

3.5.3

Dependence of Shortwave Radiation Absorption upon Cloud Optical Thickness

In accordance with the results of the experiments either in pure and dust clear

atmosphere or under overcast conditions the relative value of SWR absorption

b(µ0,τ) = R|πSµ0 is presented as a function of the optical thickness in the

studies by Kondratyev et al. (1996, 1997a, 1997b) and Vasilyev A et al. (1994).

The approximation of the experimental points has elucidated the linear de￾pendence of function b(τ) that is confirming the analytical expression for SWR

absorption presented in the book by Minin (1988). Table 3.2 demonstrates dif￾ferent magnitudes of factorfs. It is close to unity for the thin clouds with optical

thicknessτ ≤ 7 especially in the pure atmosphere in the Arctic region. In cases

with a high content of sand and black carbon aerosols it is valid fs ≥ 2.5 at