Using Fokker-Planck Methods to Model Subgrid Variability in bin Microphysics
Abstract
The complexity of cloud-aerosol interactions has motivated the development of bin microphysical schemes that predict the evolution of the probability distribution functions of cloud/aerosol particle size. In contrast to "physical-space" subgrid turbulence and microphysics schemes that are mature and well-developed, the development of subgrid schemes for "size-space" has received much less attention. Yet, size-space subgrid models will become increasingly important as bin microphysical schemes are applied at larger and larger scales [Lynn et al., MWR, 2005]. The first size-space subgrid model was developed by Levin and Sedunov in the 1960s and is generally referred to as stochastic condensation. In this model, random subgrid fluctuations in supersaturation are assumed and the net ensemble effect of these fluctuations is modeled as a diffusion term in droplet size space. This diffusion term is the Fokker-Planck model of unresolved supersaturation fluctuations and it is the exact analog of the common eddy-diffusivity parameterization in physical space. In this presentation, we first review Fokker-Planck theory and its general applicability to subgrid bin microphysics with an emphasis on the derivation of size-space diffusion terms. We then revisit the original Levin-Sedunov model of stochastic condensation and determine when unresolved supersaturation variability is important and its effect on droplet size spectra. Finally, we present result of observed droplet spectra from the RICO and SCMS campaigns that show evidence of stochastic broadening.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.A33B0976J
- Keywords:
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- 0320 Cloud physics and chemistry;
- 3311 Clouds and aerosols