Modelling Reflecting and Semi-Transparent Cloud for Infrared Sounding
This paper examines the effect of spectrally varying cloud reflectivity and emissivity on infrared radiances. In the first part of the paper the results of two-stream calculations for cloud bulk radiative properties are given. These are directly relevant to the use of high resolution infrared measurements. In the second part, these results are used in a simple way in a non-linear retrieval scheme using TIROS Operational Vertical Sounder (TOVS) data. This scheme is at a research and development stage in the UK Meteorological Office.
It is shown that clouds cannot be assumed to be black-body emitters even when they are completely opaque. Moreover, they cannot be assumed to be grey, especially in the shorter wavelength region 3–6 µm. The non-zero reflectivity of opaque clouds is shown to give rise to significant amounts of reflected solar radiation, again in the short wavelength region.
A parameter to represent the reflectivity of the cloud is included in the non-linear retrieval scheme and values of this parameter estimated from a NOAA-11 case study appear to be in broad agreement with the predictions of the two-stream approximation.
Also presented are results from this case study which show strong directional properties of the reflected radiation — information which is lost in the simplified two-stream calculations. Work is under way utilising more sophisticated discrete ordinates and Monte Carlo scattering calculations to predict the angular reflection properties. It may then become possible to obtain cloud microphysical properties, i.e. effective drop size, from TOVS measurements.
KeywordsBrightness Temperature Water Cloud Solar Zenith Angle Radiative Property Short Wavelength Region
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