Definition of “cloud top pressure”

[Amirhossein Nikfal]

What is the precise physical definition of cloud top pressure? If I know an accurate definition of this variable, I’ll have a chance to calculate it using available data (e.g. ERA5).

[James Hocking]

Hi,

The idea of the RTTOV simple cloud scheme is to provide a reasonable represention in the IR of a single layer fully opaque cloud. Scattering processes are ignored and only emission from the cloud top is considered. The cloud is treated as an opaque surface with emissivity 1, and the radiance emitted by the cloud top is indirectly interpolated from the atmospheric temperature profile. In practice, RTTOV interpolates radiance%overcast(:,:) computed on the profile pressure levels to the cloud top pressure. So the cloud top pressure should be the pressure at the top of an optically thick cloud. This is only an approximation, and the approximation gets worse as the cloud becomes optically thinner. The simple cloud scheme cannot represent partially transparent clouds well, and indeed as the optical depth decreases there comes a point where “cloud top pressure” is not meaningful for the simple cloud scheme.

Best wishes,
James

[Amirhossein Nikfal]

Thank you very much.
So for a non-simple scheme (e.g. the simulation of visible and IR cloud-affected radiances), cloud water profile is required. This is a standard variable available in most atmospheric datasets.

[James Hocking]

Yes absolutely, scattering simulations are to be recommended over the simple cloud scheme in general. For these you supply full vertical profiles of the relevant cloud fields and also cloud fraction. See section 8.5 in the RTTOV v13.2 user guide for VIS/IR scattering cloudy simulations, or section 8.7 for MW scattering simulations.

Best wishes,
James

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