Cumulus-Type Flows in the Laboratory and on the Computer: Simulating Cloud Form, Evolution, and Large-Scale Structure

Abstract

Cumulus clouds, which are among the largest sources of uncertainty in climate change science and tropical circulation, have to-date resisted the numerous attempts made during the last six decades to unravel their cloud-scale dynamics. One major reason has been the lack of a convincing fluid-dynamical model and the difficulty of making repeatable measurements in an inherently transient flow. This article summarizes recent work showing that cumulus-type f lows can be generated in the laboratory by releasing volumetric heat into a plume above a height analogous to cloud condensation level and in quantities dynamically similar to the release of latent heat in the natural cloud. Such a transient diabatic plume (TDP) seems to mimic cumulus clouds with adiabatic/pseudoadiabatic processes of latent heat release. With appropriate heating profile histories, the TDP simulates a variety of cumulus-cloud forms, from cumulus congestus to cumulus fractus, and permits tracking their evolution through a complete life cycle. Selected examples of such laboratory simulations are supported by preliminary results from direct numerical simulations based on the Navier-Stokes-Boussinesq equations. These simulations suggest that the baroclinic torque plays an important role in the dynamics of both large- and small-scale motions in cloud-type flows.