Solar Radiation in the Atmosphere - Short-Wave Solar Radiation in the Earth's Atmosphere - page 34

The albedo of the surface characterizes the reflection process of radiation
only as a description of the energy transformation, but doesn't tell us about
the dependence of the radiance upon the reflection angle and azimuth. If
the surface were an ideal plane, such dependence would be defined with the
well-known laws of reflection and refraction (Sivukhin 1980). However, all
natural surfaces are rough, i. e. they have different scales of the roughness and
even the water surface is practically always not smooth. Therefore, considering
the incoming parallel beam is more complicated in reality, notwithstanding
the reflection from every micro-roughness is ordered to the classical laws
of geometric optics. In particular, reflected radiation extends to all possible
directions and not only to the direction according to the law: “the reflection
angle is equal to the incident angle”. This light reflection from natural surfaces
is usually called the diffused .
It is possible to select threemain types of diffused reflection. The orthotropic
(or isotropic ) reflection , when the diffused reflected radiance does not depend
on the direction. The mirror reflection , when the maximum of the reflected
radiance coincides with the direction of the mirror reflection (the reflection
angleisequaltotheincidentangle)and the backward reflection when the
maximum is situated along the direction opposite to the incident radiation
direction. The mirror reflection evidently characterizes the surfaces close to
theideallysmoothsurfaceandotherwisethebackwardonecharacterizesthe
surfaces close to the strongly rough surface because it is formed by a large
amount of the micro-grounds oriented perpendicular to the incident direction
ofradiation.TheobservationssomeofthemwewillconsiderindetailinChap.3
indicate that the cloud and snow are the closest to the orthotropic surface, the
water is the most mirror surface and others are mainly backward reflected
surfaces. However, the reference to the observation is excessive because of the
mirror reflection of the banks from the water that everybody has seen and the
backward reflection maximum is clearly observed from the airplane board.
The orthotropic surfaces are especially convenient for the theoretical anal-
ysis and practical calculations because they are characterized with only one
parameter - the albedo and because of the simplicity of the mathematical
description. We would like to point out that the assumption concerning the
orthotropic reflection is an approximation and its accuracy is necessary to
evaluate in a concrete problem. It is said that the anisotropic reflection from
other surfaces needs some additional values for its description. The rather
variable characteristics of the anisotropic reflection are considered in differ-
ent studies, however here we are describing the general problem without its
concretization. Note also that the reflection processes depend on the incident
radiation polarization accompanying its change (Sivukhin 1980). Therefore,
the consideration of the reflection without an account of polarization is an
into account the atmosphere, the other characteristics of the system “atmosphere plus surface” are
analogously defined. For example, the incoming irradiance to the surface fromthe diffusing atmosphere
depends itself on the surface albedo (true) because it contains the part of the reflected radiation that
scattered back to the surface. Mark that on the one hand, only true values are used in formulas of
the transfer theory for the reflection characteristics, and on the other hand, only characteristics of the
system“atmosphereplussurface”areavailablefortheobservation.