Another Look at Stochastic Condensation for Subgrid Cloud Modeling: Adiabatic Evolution and Effects
Open Access
- 1 November 2007
- journal article
- Published by American Meteorological Society in Journal of the Atmospheric Sciences
- Vol. 64 (11), 3949-3969
- https://doi.org/10.1175/2006jas2147.1
Abstract
The theory of stochastic condensation, which models the impact of an ensemble of unresolved supersaturation fluctuations S′ on the volume-averaged droplet-size distribution f (r), is revisited in the modern context of subgrid cloud parameterization. The exact transition probability density for droplet radius driven by independent, Gaussian S′ fluctuations that are periodically renewed is derived and shown to be continuous but not smooth. The Fokker–Planck model follows naturally as the smooth-in-time approximation to this discrete-in-time process. Evolution equations for the moments of f (r) that include a contribution from subgrid S′ fluctuations are presented; these new terms are easily implemented in moment-based cloud schemes that resolve supersaturation. New, self-consistent expressions for the evolution of f (r) and mean supersaturation S in a closed, adiabatic volume are derived without approximation; quite appropriately, these coupled equations exactly conserve total water mass. The behavior of this adiabatic system, which serves as a surrogate for a closed model grid column, is analyzed in detail. In particular, a new nondimensional number is derived that determines the relative impact of S′ fluctuations on droplet spectral evolution, and the contribution of fluctuations to S is shown to be negative definite and maximal near the accommodation length and has a direct correspondence to the analysis of Cooper. Observational support for the theory of stochastic condensation is found in cloud droplet spectra from cumulus cloud fields measured during the Rain in the Cumulus over the Ocean (RICO) and Small Cumulus Microphysics Study (SCMS) campaigns. Increasing spectral broadening with increasing spatial scale is discovered and compares well with theoretical predictions. However, the observed spectra show evidence of non-Gaussian S′ fluctuations and inhomogeneous mixing, processes neglected in the current theory.Keywords
This publication has 21 references indexed in Scilit:
- A Study of Cloud Mixing and Evolution Using PDF Methods. Part I: Cloud Front Propagation and EvaporationJournal of the Atmospheric Sciences, 2006
- Droplet condensation in turbulent flowsEurophysics Letters, 2005
- Spectral (Bin) Microphysics Coupled with a Mesoscale Model (MM5). Part I: Model Description and First ResultsMonthly Weather Review, 2005
- Near-Global Survey of Effective Droplet Radii in Liquid Water Clouds Using ISCCP DataJournal of Climate, 1994
- Variability of the Supersaturation in Cumulus CloudsJournal of the Atmospheric Sciences, 1988
- Small-Scale Variability in Warm Continental Cumulus CloudsJournal of the Atmospheric Sciences, 1985
- Comments on the paper ‘on the broadening of a droplet distribution by turbulence near cloud base’ By M. J. Manton (Q. J. (1979), 105, 899‐914)Quarterly Journal of the Royal Meteorological Society, 1981
- The influence of entrainment on the evolution of cloud droplet spectra: I. A model of inhomogeneous mixingQuarterly Journal of the Royal Meteorological Society, 1980
- On the broadening of a droplet distribution by turbulence near cloud baseQuarterly Journal of the Royal Meteorological Society, 1979
- On the dispersion of the sizes of droplets growing by condensation in turbulent cloudsQuarterly Journal of the Royal Meteorological Society, 1972