Cloud supersaturations and the Hoppel minimum

Hudson et al. (2010; H10) showed higher supersaturations (S > 0.3%) than conventional wisdom for stratus clouds. Bimodal dry particle size distributions often found below stratus have been a reason for previous low stratus cloud S estimates. The Desert Research Institute (DRI) CCN spectrometers (Fig. 1) provide more direct estimates of cloud S because they do not need to assume particle composition. These S estimates from minima of Sc distributions are correlated (Fig. 2) with estimates that match CCN spectra with mean cloud droplet concentrations (Nc; H10). Although these CCN Hoppel S are higher than earlier Hoppel S estimates, they are lower (mean 0.37%) than S from CCN spectral matches (H10) (mean 0.48%) (Fig. 2). These S discrepancies could be due to later evaporation of small droplets, which causes and could be related to less participation of small droplets in processes that decrease CCN critical S (Sc). These chemical (gas-to-particle conversion; e.g., sulfate) and physical (coalescence and Brownian capture) processes produce Hoppel minima. The DRI spectrometers have not only observed Hoppel minima in stratus (POST) 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 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 very low Sc. Accurate stratus cloud S estimates are needed to determine the anthropogenic particles that cause the aerosol indirect effect. Hudson, J.G., S. Noble and V. Jha, 2010: Geophys. Res. Lett., 37, L21813, doi:10.1029/2010GL045197. Fig. 1. Differential S distribution in stratus. Fig. 2. Comparison of S estimates in stratus