Reflectivity and rain rate in and below drizzling stratocumulus

Abstract

SUMMARYShip-based radar measurements obtained during the East Pacific Investigation of Climate 2001 stratocumulus (Sc) cruise are used to derive characteristics of the rainfall from drizzling Sc. Reflectivity to rain rate (Z-R) relationships are determined from shipboard raindrop-size distribution measurements obtained from observations using filter-paper, and compared to Z-R relationships derived from aircraft probe data from below north-east Atlantic drizzling Sc and stratus. A model for the evaporation and sedimentation of drizzle is combined with reflectivity profiles from a millimetre-wavelength cloud radar to derive information on the mean raindrop radius and drizzle drop concentrations at cloud base, and to show how Z-R relationships change with height below the cloud base. The Z-R relationships are used in conjunction with shipborne C-band radar reflectivity data to estimate areal average precipitation with uncertainties at cloud base and at the surface. In the Sc drizzle Z-R relationship, Z = aR b (where a and b are constants), the estimated exponent b = 1.1 to 1.4 is lower than commonly observed in deep convective rain (b = 1.5). Analyses indicate that variations in Sc rain rates and reflectivities are influenced both by fluctuations in drizzle drop concentration and in mean radius, but that number concentration contributes more to the modulation of rain rate in Sc. Rain rates derived using the scanning C-band radar are found to be spatially variable, with much of the accumulation originating from a small fraction of the drizzling area. The observations also suggest that rain rate in marine Sc is strongly dependent on cloud liquidwater path, and inversely correlated with cloud droplet concentration.

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

“…Unless otherwise noted, precipitation rates R are obtained from radar reflectivities Z at cloud base using the Z − R relationship R (mm day −1 ) = 2.01 Z 0.77 , proposed by Comstock et al (2004). A reflectivity threshold of −15 dBZ, corresponding to a drizzle rate of 0.14 mm day −1 , is used to distinguish between drizzling and non-drizzling clouds.…”

“…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].…”

Investigating potential biases in observed and modeled metrics of aerosol-cloud-precipitation interactions

“…Values of x 2 range widely between −0.8 and −1.75 based on numerous aircraft-based studies in stratocumulus regions (Pawlowska and Brenguier, 2003;Comstock et al, 2004;van Zanten et al, 2005;Wood, 2005;Lu et al, 2009). Such a broad range leads to widely varying simulated strengths of the second aerosol indirect effect in climate models, some of which use x 2 values ranging between 0 and −1.79 (Quaas et al, 2009).…”