THE EFFECTS OF DIFFERENT RADIATION PARAMETRIZATIONS ON CLOUD EVOLUTION

Citation
Pe. Loughlin et al., THE EFFECTS OF DIFFERENT RADIATION PARAMETRIZATIONS ON CLOUD EVOLUTION, Quarterly Journal of the Royal Meteorological Society, 123(543), 1997, pp. 1985-2007
Citations number
19
Categorie Soggetti
Metereology & Atmospheric Sciences
ISSN journal
00359009
Volume
123
Issue
543
Year of publication
1997
Part
A
Pages
1985 - 2007
Database
ISI
SICI code
0035-9009(1997)123:543<1985:TEODRP>2.0.ZU;2-0
Abstract
With the use of the microphysical stratus model (MISTRA), investigatio ns into the effects of variations in the radiative-transfer parametriz ations on cloud development have been undertaken. Two radiative-transf er schemes were coupled with the microphysical-thermodynamical section of MISTRA, one based on the exponential sum-fitting method and the ot her based on the correlated k-distribution method of determining gaseo us absorption properties. Model runs were initiated with parameter val ues in accordance with measurements made over the North Sea and compar isons made between model runs where only the radiation schemes were al tered. 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 resul ts from the original radiation method and so determine why there were differences between the two sets of results. The results showed that i nfrared 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 offse t solar heating during daytime. Exclusion of infrared scattering produ ced a large increase in total cloud-top cooling rates, almost 100%, bu t produced only minimal changes in other bulk cloud properties of typi cally 3%, partially due to the net increase in heating immediately bel ow cloud top. The inclusion of e-type water-vapour absorption was also investigated. Results showed that by reducing the wavelength range wh ere this component was included from 8.0-35.7 mu m to 8.0-12.5 mu m pr oduced only a small change, 3%, in the downward infrared fluxes above the cloud. However, this was matched by larger changes in bulk cloud p roperties of typically 7%. This indicates that cloud development is ve ry sensitive to changes in the downward infrared radiation field above the cloud. Most importantly, the changes in the radiation fields repo rted are smaller than the values given as experimental errors and what could be considered reasonable theoretical uncertainty.