Tuesday, 8 August 2000: 2:00 PM
A mixing fraction determines the relative amount of above-cloud-top
air which has been mixed into a cloudy air parcel. A method,
based on the use of a mixing fraction, to calculate radiative and evaporative
cooling terms at cloud top is derived and discussed. We compute cooling terms
for the whole range of the mixing fraction for two cases of
stratocumulus-topped marine boundary layers.
In both cases the total radiative cooling term is found to be the most
dominant contributor to the negative buoyancy excess found at cloud top.
The largest radiative cooling rates are found for clear-air parcels
immediately adjacent to cloud top rather than inside the cloud. With
the help
of a simple longwave radiation model, we show this to be caused by
clear-air cooling due to the temperature jump at cloud top. Further
we argue that the mixing fraction can be used as a vertical
coordinate instead of height. The advantage of this is that the
position of the air parcel relative to cloud top is known and
thus can be used to calculate turbulent fluxes over cloud top from
a horizontal level run.
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