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This paper (part II) is the second of two papers dealing with cumulus and broader scale flow interaction. The first (part I) is a companion paper that deals with this topic from broad-scale considerations. The second (part II) deals with this topic from the cumulus scale point of view.

A numerical model of individual convective clouds has been used to investigate the effects of a typical population of cumulus clouds on the large-scale features of a tropical disturbance or cloud cluster. The typical cloud population has been determined from U.S. Navy aircraft reconnaissance radar data in the tropical Atlantic Ocean.

This study shows that the detrainment of cloud mass from the population produces net cooling of the air around the clouds. This occurs because the cooling produced by the evaporation of the detrained cloud liquid water over-compensates for the detrainment of sensible heat excess. A heat and moisture budget for a steady-state tropical cloud cluster has been computed using the typical cloud population. A mass budget of the cluster reveals that the vertical circulation required to explain typically observed rainfall rates has a magnitude many times larger than the synoptic mass circulation. The synoptic scale vertical mass circulation is, in fact, a small residual between both the much larger upward mass transport by the clouds and the downward mass transport induced in the cloud-free region around the clouds.

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Raúl Erlando López


It is shown that the lognormal distribution describes the frequency distributions of height, horizontal size, and duration of cloud and radar echo populations in many different regions and convective situations. Two hypotheses are suggested to explain this phenomenon. The first postulates a growth process of cloud parcels, in which growth by entrainment of environmental air occurs by a random process that obeys the law of proportionate effects. The second postulates a formation process for clouds, in which the clouds are formed by the merger of random boundary-layer convective elements.

The information presented in this paper should be useful for the parameterization of cumulus convection in larger scale models, and for the understanding and modeling of cloud formation and development.

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