Parametrization of the effect of drizzle upon the droplet effective radius in stratocumulus clouds

A method is presented to parametrize the effects of drizzle upon the droplet effective radius in stratocumulus clouds. The cloud-droplet size distribution in stratocumulus is represented by the sum of a modified Gamma distribution to represent the small (radius <20 μm) droplets and an exponential Marsh all-Palmer-type distribution to represent the large (drizzle) droplets. Using this approach a relationship is derived to account for the effect of drizzle upon k, the cube of the ratio between the volume and effective radius. Observational evidence from flights in a range of different air-mass types is presented to validate the approach. The results suggest that the value of k pertaining to the small droplets is better parametrized as a function of volume radius rather than of droplet concentration. The results also suggest that, as the ratio of liquid-water content contained in the large droplets to that in the small droplets increases beyond 0.05, the value of it decreases significantly. This results in an underprediction of the effective radius if commonly used parametrizations for k are used.

[1]  D. W. Johnson,et al.  The Measurement and Parameterization of Effective Radius of Droplets in Warm Stratocumulus Clouds , 1994 .

[2]  J. Hudson,et al.  Droplet Spectral Broadening in Marine Stratus , 1997 .

[3]  H. Gerber,et al.  Microphysics of Marine Stratocumulus Clouds with Two Drizzle Modes , 1996 .

[4]  George A. Isaac,et al.  parameterizations of Marine Stratus Microphysics Based on In Situ Observations: Implications for GCMS , 1996 .

[5]  M. Fujiwara,et al.  The Size Distribution of Cloud Droplets Measured in Small Maritime Cumulus Clouds , 1980 .

[6]  A. Slingo,et al.  Sensitivity of the Earth's radiation budget to changes in low clouds , 1990, Nature.

[7]  D. L. Roberts,et al.  A climate model study of indirect radiative forcing by anthropogenic sulphate aerosols , 1994, Nature.

[8]  C. Pontikis Parameterization of the droplet effective radius of warm layer clouds , 1996 .

[9]  H. Gerber,et al.  Microphysical and short‐wave radiative structure of wintertime stratocumulus clouds over the Southern Ocean , 1996 .

[10]  S. Moss,et al.  Aircraft measurements to validate and improve numerical model parametrisations of ice to water ratios in clouds , 1994 .

[11]  P. R. Jonas,et al.  Turbulence and cloud microphysics , 1996 .

[12]  S. Nicholls,et al.  An observational study of the structure of stratiform cloud sheets: Part I. Structure , 1986 .

[13]  K. Bower,et al.  A parameterisation of the effective radius of ice free clouds for use in global climate models , 1992 .

[14]  Graeme L. Stephens,et al.  Radiation Profiles in Extended Water Clouds. II: Parameterization Schemes , 1978 .

[15]  Robert Pincus,et al.  Precipitation in Stratocumulus Clouds: Observational and Modeling Results. , 1995 .

[16]  E. Hicks,et al.  Contribution to the cloud droplet effective radius parameterization , 1992 .

[17]  Alexei Korolev,et al.  NOTES AND CORRESPONDENCE Airspeed Corrections for Optical Array Probe Sample Volumes , 1997 .