Conventional wisdom stratus cloud supersaturations (S) are < 0.3%. Bimodal dry particle size distributions often found below maritime stratus are a major reason for the low S estimates. The fact that minima between these two peaks are usually < 100 nm and the premise that these Hoppel minima (Hoppel et al. 1986) are a result of cloud processing of the aerosol implies low cloud S. However, Hudson et al. (2010; H10) estimated higher stratus cloud S by matching CCN spectra of a Desert Research Institute (DRI) CCN spectrometer with mean cloud droplet concentrations (Nc). But more recently the DRI instruments have provided another method of estimating cloud S. The high S resolution and resulting differential critical S (Sc) CCN spectra have revealed bimodal distributions below clouds (Fig. 1). The minima of these distributions provide more direct estimates of cloud S than particle size distributions because they do not require particle composition assumptions. These newest cloud S estimates show higher values than previous Hoppel minima but not as high as those of H10 (Fig. 2); mean S 0.37% compared to 0.48%. The discrepancy between these methods may be due to subsequent evaporation of small cloud droplets and/or less participation of small cloud droplets in the processes that decrease CCN Sc and thus cause Hoppel minima. Thus, these chemical (gas-to-particle conversion; e.g., sulfate) and physical (coalescence and Brownian capture) processes may be largely limited to only the larger cloud droplets that had grown on the lower Sc CCN. Thus, Hoppel minima may provide underestimates of cloud S, but these Hoppel estimates may be more pertinent to aerosol-cloud-climate interactions.
The DRI CCN spectrometers have not only observed Hoppel minima in stratus (POST; off central California) but also below RICO small cumuli. As expected both estimates of RICO cloud S were higher than POST since vertical wind is higher in cumuli. The same S discrepancy between methods was found in RICO. The S discrepancy and the fact that Nc are well correlated with CCN concentrations (H10; Hudson et al. 2012) suggests a more homogeneous evaporation process so that CCN that experience increased Sc that result in Hoppel minima tend to be only those with initially rather low Sc. The higher Sc CCN produce only small cloud droplets that either do not persist or do not participate in very much cloud processing. Accurate stratus cloud S estimates are needed to determine the anthropogenic particles that cause the indirect aerosol effect. Higher cloud S implies that smaller more numerous particles are able to produce cloud droplets. Complications for both methods of estimating cloud S include situations when CCN concentrations are different (especially higher) above the stratus clouds (Fig. 2).
Hoppel, W.A., G.M. Frick and R.E. Larson, 1986: Effect of nonprecipitating clouds on the aerosol size distribution in the marine boundary-layer. Geophys. Res. Lett., 13(2, 125-128.
Hudson, J.G., S. Noble and V. Jha, 2010: Stratus cloud supersaturations. Geophys. Res. Lett., 37, L21813, doi:10.1029/2010GL045197.
Hudson, J.G., S. Noble and V. Jha, 2012: Cloud droplet spectral width relationship to CCN spectra and vertical velocity. J. Geophys. Res., Vol. 117, D11211, doi:10.1029/2012JD017546, 2012.