Simultaneous field measurements of aerosol and cloud droplet concentrations and droplet diameter were performed at a maritime site on the coast of Washington State. The aerosol and droplet spectra were compared for estimating cloud condensation nucleus concentration ( N ccn ) as the number of particles with diameters greater than 80 nm, that is, N ccn ≡ N ( D p > 80 nm). Several analytical approaches were developed and applied to the data, including a stratification of the observations into periods of high and low liquid water content (LWC) based on a threshold value of 0.25 g m −3 . The aerosol data were corrected for inertial losses of cloud droplets at the inlet using wind speed and droplet size; this correction improved the measured relationships between N ccn and droplet number concentration ( N d ). These measurements, when coupled with the range of possible aerosol chemical compositions, imply a cloud supersaturation of 0.24%–0.31% at the Cheeka Peak sampling site during periods of high LWC. The observations of droplet and aerosol spectra supported Twomey’s cloud brightening hypothesis in that N ccn was highly correlated ( r 2 = 0.8) with N d in apparent 1:1 proportions. For the investigated range (50 cm −3 < N d < 600 cm −3 ) droplet effective diameter ( D eff ) was very sensitive to variation in N ccn for 50 cm −3 < N ccn < 200 cm −3 , somewhat sensitive for 200 cm −3 < N ccn < 400 cm −3 , but not very sensitive to variation in aerosol number for N ccn > 400 cm −3 . A model was applied to the aerosol and droplet data to predict droplet size, as D eff , from N −0.33 ccn and LWC. Predicted values for D eff agreed ( r 2 = 0.8) with D eff determined directly from the cloud droplet spectra, suggesting that this approach should be useful in climate modeling for predicting cloud droplet size from knowledge of N ccn and LWC.
Journal of the Atmospheric Sciences – American Meteorological Society
Published: Mar 6, 1997