Extended Abstract (292K)P1.11
Parameterization of cloud physics processes in marine stratocumulus based on integral moments of drop spectra
Alexei Belochitski, Univ. of Maryland, College Park, MD; and Y. Kogan
One of the main drawbacks of all popular Kessler-type microphysical parameterizations is the difficulty in defining a threshold between cloud and precipitable water. From both theoretical and experimental standpoints this division is artificial since observational and modeling data, in general, do not show distinctive gap between cloud and rainwater. As a result, the autoconversion and accretion rates are quite sensitive to the value of the threshold. Hence the problem is better posed when artificial division of total water into two parts is avoided altogether (Kogan 1998) and formulation of bulk microphysics is based on full integral moments of cloud drop size distribution (DSD) as opposed to partial moments of Kessler-type parameterizations. Knowledge of six full moments suffices to approximate most of the observed drop size spectra and, hence, the cloud microphysics processes (Belochitski and Kogan 2006). The proposed bulk parameterization is based on five integral moments of the DSD: drop concentration, mean geometrical cross-section of a drop, liquid water content, local drizzle flux and the radar reflectivity. The sixth prognostic variable, the mean drop radius, is parametrized in terms of other moments.
The rates of change of each moment due to various microphysical processes as well as sedimentation rates were calculated using CIMMS LES model with explicit formulation of cloud microphysics. Then, the parametrized expressions for moment tendencies and fall velocities for each moment were obtained using nonlinear regression analysis. This approach allowed to obtain comparativily accurate parameterizations. All parameterized expressions have precisions better than 30%. This is an improvement in comparison with Kessler-type parameterizations which have precisions of 100% at best.
Predictions of the LES model using the new bulk microphysics are compared with the predictions of the explicit microphysics for two cases: non-drizzling and drizzling STBL. The new approach is shown to predict microphysical parameters of a precipitating cloud in the range common in the real atmosphere.
REFERENCES
Belochitski, A. and Y. Kogan: 2006, Relationship Between m-Mode Gamma Type Cloud Drop Size Distributions and Their Integral Moments, Eos Trans. AGU, Jt. Assem. Suppl., 87(36)
Kogan, Y. L.: 1998, Parameterization of cloud physics processes in mesoscale models of stratocumulus cloud layers, AMS Conference on Cloud Physics, Everett, WA
Poster Session 1, Poster Session
Wednesday, 17 January 2007, 2:30 PM-2:30 PM, Exhibit Hall C
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