Publication: Representing cloud overlap with an effective decorrelation length: An assessment using CloudSat and CALIPSO data

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Title	Representing cloud overlap with an effective decorrelation length: An assessment using CloudSat and CALIPSO data
Authors/Editors*	Howard W. Barker
Where published*	JGR
How published*	Journal
Year*	2008
Volume	113
Number
Pages	17
Publisher	AGU
Keywords
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Abstract	This study commenced testing the hypothesis that a vertically constant, effective, decorrelation length L_{cf}^{∗} can be used to represent overlap of cloud for the purpose of performing radiative transfer calculations in global climate models. It was assumed that total cloud fraction C resulting from multiple layers of overlapping fractional cloud can be described as a linear combination of maximum and random overlap, with the weight defined by exp(-Δz/L_{cf}^{∗}) where Δz is distance between layers. Cloud masks and water contents (CWC) obtained from CloudSat and CALIPSO satellite data, for January 2007, were used to solve for L_{cf}^{∗}. Benchmark shortwave (SW) and longwave (LW) broadband flux profiles were computed for 500 km long retrieved cross-sections via the independent column approximation (ICA). Then, L_{cf}^{∗} was found and used, along with corresponding profiles of cloud fraction and CWCs, in a stochastic cloud generator suitable for use in GCMs, and ICA fluxes computed for the generated fields. When clouds were homogenized horizontally, zonal mean bias errors for SW cloud radiative effect (CRE) at the TOA were generally < ±3 W m⁻², with LW counterparts < 2 W m⁻²; similar to changing cloud particle size by ∼15%. For inhomogeneous clouds, SW CRE biases jumped to typically -5 W m⁻² due partly to limitations with the generator. When L_{cf}^{∗}=2 km (near global median) was used ubiquitously in the generator, C was overestimated slightly, mostly by clouds above ∼10 km, and CRE errors grew by just ∼10% to 20%. Exposing too much high cloud to space produced local SW heating rate biases of ∼15%. While optimal effective decorrelation lengths differ for SW and LW radiation, which in turn generally differ from L_{cf}^{∗}, it appears that use of L_{cf}^{∗} will suffice for both bands. The impact of using L_{cf}^{∗} in GCMs remains to be seen.

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