Aj. Baran et al., Retrieval of tropical cirrus thermal optical depth, crystal size, and shape using a dual-view instrument at 3.7 and 10.8 mu m, J ATMOS SCI, 56(1), 1999, pp. 92-110
In this paper the authors derive thermal optical depth at 3.7 and 10.8 mu m
for tropical cirrus utilizing Along Track Scanning Radiometer data under n
ighttime conditions. By analytically solving the equation of radiative tran
sfer, inclusive of scattering, a pair of nonlinear equations can be solved
for the optical depth. Stable and unique solutions for the optical depth ar
e found by combining nadir and forward (55 degrees) views. The accuracy of
the analytic solution is compared with solutions from a numerical radiative
transfer model assuming an isothermal cirrus cloud. Numerical solutions fo
r two nonisothermal cirrus clouds are also compared with the analytic solut
ion for an equivalent mean cloud temperature. The numerical model uses appr
opriate hexagonal column scattering phase functions at 3.7 and 10.8 mu m. T
he largest analytic model error is shown to occur at nadir, and it is shown
that this error is not overly sensitive to crystal size and is independent
of cloud-top temperature. A correction to the retrieved optical depth is t
hen applied to obtain the likely true optical depth.
The retrieved true optical depths at 3.7 and 10.8 mu m are combined to form
a ratio that is related to the ratio of extinction coefficients between th
e two wavelengths and thus to the crystal size and shape. Predictions of cr
ystal size and shape are made for tropical cirrus using anomalous diffracti
on theory as geometric ray tracing is nor applicable for typical ice partic
les at thermal wavelengths. Crystal median mass dimension in the range 40 t
o 110 mu m is derived for columns and planar polycrystals, whereas for rose
ttes predicted sizes are much larger than this range and outside the range
found by the Central Equatorial Pacific Experiment (CEPEX) for crystals nea
r the tops of tropical cirrus, The authors therefore conclude that near the
tops of tropical cirrus the crystal habits are most likely to be columns a
nd planar polycrystals, a finding consistent with the CEPEX field results.
In addition to the optical depth ratio between 3.7 and 10.8 mu m being sens
itive to crystal size and shape, it is shown that this sensitivity can be u
sed to locate underlying water clouds below semitransparent cirrus during n
ightime conditions.