Broken Cumulus Bases and Modeling

D L McNaughton

Abstract


Convective cumuliform clouds are sustained by thermal upcurrents produced by solar heating of the ground. Under these conditions, the slice of atmosphere between the ground and the cumulus base is thoroughly mixed by the thermals, so that the water vapor content is the same at all levels in this slice, and the temperature lapse rate is constant, being almost the same as the dry adiabatic value. Thus, the level where condensation begins (i.e. the base of the cumulus cloud) can be predicted fairly accurately from ground-level measurements of air temperature and humidity, provided that the convective updraughts are strong and sustained. If this is so, then the cumulus base will be flat and at least as wide as the main body of the cloud. Special thermodynamic diagrams (tephigrams and others) can be used to estimate this cloud-base level by means of a quick and simple construction.

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