| Abstract: The sensitivity of the near-infrared spectral atmospheric
and surface fluxes to the vertical location of clouds is investigated,
including a study of factors (drop-size distribution, drop optical depth,
solar zenith angle, cloud geometrical thickness, atmospheric profiles)
which govern this dependence. Because of the effects of the above-cloud,
in-cloud and below-cloud water vapor the atmospheric absorbed flux in each
spectral band depends critically on the cloud location, with a high cloud
resulting in lesser absorption and greater reflection than a low one having
the same drop optical depth. The difference between a high and a low cloud
forcing of atmospheric absorption increases with drop optical depth. For
any optical depth, clouds with larger drops cause a greater forcing of
the spectral atmospheric absorption than those with smaller ones, so high
clouds can even cause an increase rather than a decrease of the atmospheric
absorption relative to clear skies. In contrast, the spectral and total
surface fluxes are relatively insensitive to cloud vertical location. Instead,
they are determined by the drop characteristics, notably drop optical depth.
This near-invariance characteristic is attributable to the fact that most
of the insolation reaching the surface is in the weak water vapor spectral
absorption regions; here drops dominate the radiative interactions and
thus there is little dependence on cloud height. In addition, the overlap
of the drop spectral features with the moderate-to-strong vapor absorption
bands ensures that insolation in these regimes fails to reach the surface
no matter where the cloud is located; instead, these bands contribute the
most to atmospheric absorption. The near-invariant behavior of the spectral
and total surface flux holds separately for a wide variety of conditions
studied. As a consequence, the difference in reflection, between two columns
containing clouds with the same optical depth but located at different
altitudes, is approximately balanced in magnitude by the difference in
the atmospheric absorption; this holds for every spectral interval whether
it be a weak, moderate, or strong vapor/drop absorption band. It also follows
that the net fluxes at the top and surface of overcast atmospheres do not
have a general, unambiguous relationship; this is in sharp contrast to
a linear relation between them in clear skies. However, under certain overcast
conditions (e.g., specific vertical location of clouds and solar zenith
angle), a simple linear relationship is plausible. |