Stratus Cloud Supersaturations

JAMES G. HUDSON (1), Stephen Noble (1), Vandana Jha (1)

(1) Desert Research Institute, Reno, NV

     Abstract Number: 564
     Last modified: May 13, 2010

Abstract
Conventional wisdom holds that supersaturations (S) of stratus clouds are less than 0.3% because of smaller updraft velocities compared to cumulus. Stratus clouds are more important for the indirect aerosol effect because they provide large albedo contrasts with large expanses of ocean. Moreover, low aerosol concentrations make maritime clouds more susceptible to anthropogenic influences.
The Physics of Stratocumulus Tops (POST) in July-August, 2008 off the central California coast provided a wide range of CCN concentrations (at 1% S [N$_(1%)] 70-800 cm$^(-3)) and average cloud droplet concentrations (N$_c of 50-450 cm$^(-3)). These were well correlated for 84 vertical slant cloud penetrations; correlation coefficients exceeded 0.80. Cloud effective S (Seff) is the S for which cumulative CCN concentrations equal N$_c. Seff ranged from more than 1% to below 0.04%. Although there was considerable scatter there was a clear trend of higher Seff with lower CCN concentrations commensurate with suppression of S (Twomey 1959). Even considering mean N$_c, Seff usually exceeded 0.8% for N$_(1%) < 100 cm$^(-3). For N$_(1%) > 500 cm$^(-3) Seff was usually below 0.05%. Seff based on mean N$_c are usually underestimates of true cloud S because N$_c are usually reduced by entrainment. Therefore, attempted representations of undiluted N$_c showed even higher Seff. These were done by considering only clouds with linear profiles of liquid water content with altitude or by using higher percentiles of measured N$_c for each cloud penetration (i.e., 95th percentile of the various one second measurements). Higher N$_(1%) observed just above the stratus for half of the cases seemed to have no influence on N$_c or thus on Seff.
These results indicate that much smaller and less cloud-active particles than previously considered can influence stratus clouds and thus participate in the indirect aerosol effect.

Twomey, S., 1959: Geophys. Pure. Appl., 43, 243-249.