Observations of Drizzle in Nocturnal Marine Stratocumulus

Abstract

The development and improvement of cloud microphysical and radiative parameterizations for use in cloud and numerical weather prediction models. OBJECTIVES Detailed study of marine stratocumulus cloud microphysical and radiative processes using a high-resolution large eddy simulation (LES) model with explicit microphysics. Better understanding of interactions between microphysical, radiative and boundary layer thermodynamical processes in order to improve prediction of drizzle, marine stratocumulus cloud base height and visibility. Towards this goal, we investigate: 1) The dependence of drizzle on marine stratocumulus cloud microstructure 2) The effects of aerosol and moisture fluxes on cloud base height, drizzle, and visibility 3) Methods to characterize and formulate variability of cloud parameters for use in numerical forecast models APPROACH The research is based on a high-resolution LES model of marine boundary layer stratocumulus clouds with explicit formulation of aerosol and drop size-resolving microphysics. The LES simulations, as well as observations from ASTEX filed project were used to: 1) develop a drizzle parameterization for marine stratocumulus clouds, and 2) study the effects of aerosol and moisture fluxes on cloud base height and visibility. Measurements obtained by Millimeter Wave Cloud Radar (MMCR) have been used to study the variability of radar reflectivity in boundary layer stratocumulus and low altitude stratiform clouds. WORK COMPLETED The following tasks have been completed this year: 1. The development of a one-term drizzle parameterization for stratocumulus clouds in the range of drop concentration from 10 to 60 cm-3. 2. Analysis of over 50 LES simulations investigating the response of cloud base height, drizzle, and visibility range to the strength of CCN and moisture sources. 3. Analysis of the variability of boundary layer stratocumulus and low altitude stratiform clouds based on radar data collected over two years of observations.

Does precipitation susceptibility vary with increasing cloud thickness in marine stratocumulus?

“…Over parts of the eastern subtropical/tropical oceans dominated by stratocumulus, including the southeastern Pacific, the intensity and frequency of drizzle tends to increase westwards from the coast (Leon et al, 2008;Kubar et al, 2009;Bretherton et al, 2010). The westward increase in drizzle coincides with changes in both aerosol concentrations and macrophysical properties of the stratocumulus deck (e.g., George and Wood, 2010;Bretherton et al, 2010;Allen et al, 2011), thereby raising the question: to what extent does the westward increase in drizzle reflect changes in cloud macrophysical properties, changes in cloud microphysical properties, and changes in aerosol (e.g., Wood et al, 2009)? Of the drizzle falling from stratocumulus, a significant fraction evaporates before reaching the surface vanZanten et al, 2005;Wood, 2005). Whether evaporating or not, drizzle can substantially change the boundary layer characteristics by vertically redistributing energy and water.…”

“…Drizzle with wide ranging intensities and areal extent is a common feature in stratocumulus-topped boundary layers, especially in remote marine environments (Brost et al, 1982;Nicholls and Leighton, 1986;Frisch et al, 1995;Vali et al, 1998;Yuter et al, 2000;Pawlowska and Brenguier, 2003;Bretherton et al, 2004;Comstock et al, 2004;vanZanten et al, 2005;Leon et al, 2008;Kubar et al, 2009). Over parts of the eastern subtropical/tropical oceans dominated by stratocumulus, including the southeastern Pacific, the intensity and frequency of drizzle tends to increase westwards from the coast (Leon et al, 2008;Kubar et al, 2009;Bretherton et al, 2010).…”

“…Precipitation susceptibility estimates for marine stratocumulus can be obtained from previous field studies. Field studies of precipitating stratocumulus (e.g., Pawlowska and Brenguier, 2003;Comstock et al, 2004;vanZanten et al, 2005) identified that simple power law relationships display some skill in relating the cloud base precipitation rate R CB to cloud thickness h (or, alternatively, LWP) and to cloud droplet number concentration N d (Geoffroy et al, 2008). The relationships take the form…”

On the relationship between cloud contact time and precipitation susceptibility to aerosol

“…Although the denominator in equation (1) is drop concentration N d , implicit in our discussion is the direct, although sublinear dependence of N d on aerosol number concentration N a , N d / N c a , where c Ä 1. R has been shown both empirically [e.g., Pawlowska and Brenguier, 2003;Comstock et al, 2004;vanZanten et al, 2005] and theoretically [Kostinski, 2008] [4] Equations (1) and (2) have proven to be useful diagnostics but are somewhat dissatisfying in that they ignore an important parameter that influences rain formation, i.e., the time available for collision-coalescence t c . Cloud lifetime, an upper bound on t c in the case of isolated cumulus clouds, has been explored in warm trade wind cumulus [Seifert and Stevens, 2010;Jiang et al, 2010].…”

Direct Photographic Method for Measuring Droplet Size Distribution in Fog

“…The instrument records the two-dimensional shadows of hydrometeors as they pass through a focused He-Ne laser beam. 2DC is commonly used for aircraft observations with the FSSP (Vanzanten et al 2005). The third method is the impaction method, the oldest method for detecting cloud droplets.…”