Mesoscale Cloud Inhomogeneity and Climatology
Clouds vary on all spatial scales from planetary down to about 30m,
but practical considerations limit representation of cloud variability
in global climate and weather models to spatial scales larger than
about 100-300 km. Since the relationship between cloud properties and
radiative fluxes is not linear, the presence of cloud variability at
smaller scales (we call scales < 300 km, mesoscale) creates biases in
the modeled radiative fluxes if it correctly predicts the cloud properties
averaged over the smaller scales.
The effects of mesoscale cloud optical thickness variations on solar radiative transfer can be accounted for approximately by re-scaling the area-mean optical parameters (e.g., optical thickness, single scattering albedo, asymmetry parameter) using a simple parameter, ϵ (Cairns et al. 2000), that is given by the expression (Rossow et al. 2002):
ϵ = 1 - τ̂ / τ̅
where τ̅ is the linear average of the varying optical thickness over the area and τ̂ is the "radiative-average" that gives the correct cloud albedo. Thus, the "average" optical thickness value that gives the correct albedo for a spatially inhomogeneous cloud is τ̂, given by
τ̂ = (1 - ϵ) τ̅
Smaller effects of the three-dimensional structure of clouds can be estimated by scaling the other optical parameters using ϵ (see Cairns et al, 2000). A similar parameter, ϵIR, can be defined for the infrared optical parameters.
A climatology characterizing the mesoscale cloud optical thickness inhomogeneity (scales < 300 km) is available here for each month of the year, for each season and for the annual mean. The global maps provide results for all clouds together and for low-level clouds (top pressure > 680 mb), middle-level clouds (680 mb > top pressure > 440 mb) and high-level clouds (440 mb > top pressure). This information is given in terms of the inhomogeneity parameter, ϵ for solar wavelengths and ϵIR for thermal infrared wavelengths (Cairns et al. 2000, Rossow et al. 2002). These two parameters can be used to re-scale the plane-parallel cloud optical thickness and emissivity to account for the effects of the mesoscale inhomogeneity. Also, the average radiative effects are presented in the form of average scene albedo and emissivity biases, where scene values are obtained by weighting the effects by cloud fraction. Also, thermal infrared fluxes are affected by mesoscale cloud top temperature variations, which are described here by the spatial standard deviations. Your selections may be viewed as GIF images or downloaded to your local disk. If you are downloading data, see the browse data format description and sample FORTRAN read subroutines. The full explanation of this climatology can be found in
Rossow, W.B., C. Delo and B. Cairns, 2002: Implications of the observed mesoscale variations of clouds for Earth's radiation budget. J. Climate, 15, 557-585.
Cairns, B., A.A. Lacis, and B.E. Carlson, 2000: Absorption within inhomogeneous clouds and its parameterization in general circulation models. J. Atmos. Sci., 57, 700-714.
These same files are also available from the ISCCP download site.