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
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.