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RESEARCH PRODUCT

The effects of different radiation parametrizations on cloud evolution

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With the use of the microphysical stratus model (MISTRA), investigations into the effects of variations in the radiative-transfer parametrizations on cloud development have been undertaken. Two radiative-transfer schemes were coupled with the microphysical-thermodynamical section of MISTRA, one based on the exponential sumfitting method and the other based on the correlated k-distribution method of determining gaseous absorption properties. Model runs were initiated with parameter values in accordance with measurements made over the North Sea and comparisons made between model runs where only the radiation schemes were altered. Results indicated that differences between the two schemes had a significant effect on cloud evolution. Alterations were then made to the correlated k-distribution method in an attempt to match the results from the original radiation method and so determine why there were differences between the two sets of results. The results showed that infrared scattering played an important role in cloud development as it extended the infrared cooling rates into the cloud top when compared with a case with no infrared scattering, and therefore helped to offset solar heating during daytime. Exclusion of infrared scattering produced a large increase in total cloud-top cooling rates, almost 100%, but produced only minimal changes in other bulk cloud properties of typically 3%, partially due to the net increase in heating immediately below cloud top. the inclusion of e-type water-vapour absorption was also investigated. Results showed that by reducing the wavelength range where this component was included from 8.0-35.7 μm to 8.0-12.5 μm produced only a small change, 3%, in the downward infrared fluxes above the cloud. However, this was matched by larger changes in bulk cloud properties of typically 7%. This indicates that cloud development is very sensitive to changes in the downward infrared radiation field above the cloud. Most importantly, the changes in the radiation fields reported are smaller than the values given as experimental errors and what could be considered reasonable theoretical uncertainty.