Abstract
Abstract. The relationship between precipitation rate and accumulation mode aerosol concentration in marine stratocumulus-topped boundary layers is investigated by applying the precipitation susceptibility metric to aircraft data obtained during the VOCALS Regional Experiment. A new method to calculate the precipitation susceptibility that incorporates non-precipitating clouds is introduced. The mean precipitation rate R over a segment of the data is expressed as the product of a drizzle fraction f and a drizzle intensity I (mean rate for drizzling columns). The susceptibility Sx is then defined as the fractional decrease in precipitation variable x = {R, f, I} per fractional increase in the concentration of aerosols with dry diameter >0.1 μm, with cloud thickness h held fixed. The precipitation susceptibility SR is calculated using data from both precipitating and non-precipitating cloudy columns to quantify how aerosol concentrations affect the mean precipitation rate of all clouds of a given h range and not just the mean precipitation of clouds that are precipitating. SR systematically decreases with increasing h, and this is largely because Sf decreases with h while SI is approximately independent of h. In a general sense, Sf can be thought of as the effect of aerosols on the probability of precipitation, while SI can be thought of as the effect of aerosols on the intensity of precipitation. Since thicker clouds are likely to precipitate regardless of ambient aerosol concentration, we expect Sf to decrease with increasing h. The results are broadly insensitive to the choice of horizontal averaging scale. Similar susceptibilities are found for both cloud base and near-surface drizzle rates. The analysis is repeated with cloud liquid water path held fixed instead of cloud thickness. Simple power law relationships relating precipitation rate to aerosol concentration or cloud droplet concentration do not capture this observed behavior.
Highlights
IntroductionDrizzle 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)
In this study we investigate the factors controlling precipitation susceptibility in warm, stratiform clouds using aircraft observations of precipitating and non-precipitating marine stratocumulus observed in the southeastern Pacific during the VOCALS Regional Experiment (REx, Wood et al, 2011b)
The physical argument that increased aerosol concentrations leads to reduced precipitation is dependent on the assumption that Passive Cavity Aerosol Spectrometer Probe (PCASP) aerosol concentrations N at 150 m correlate well with cloud droplet concentrations Nd in the overlying cloud
Summary
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). Terai et al.: Drizzle susceptibility from VOCALS structural and radiative properties of stratocumulus in general circulation models, we need a better understanding of the macrophysical and microphysical properties that determine the frequency and intensity of drizzle (Wyant et al, 2007)
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