SIMULTANEOUS OBSERVATIONS OF AEROSOL AND CLOUD DROPLET SIZE SPECTRA IN MARINE STRATOCUMULUS

Simultaneous field measurements of aerosol and cloud droplet concentra tions and droplet diameter were performed at a maritime site on the co ast of Washington State. The aerosol and droplet spectra were compared for estimating cloud condensation nucleus concentration (N-ccn) as th e number of particles with diameters greater than 80 nm. that is. N-cc n equivalent to N(D-p > 80 nm). Several analytical approaches were dev eloped and applied to the data, including a stratification of the obse rvations 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 a nd droplet size; this correction improved the measured relationships b etween N-ccn and droplet number concentration (N-d). These measurement s, when coupled with the range of possible aerosol chemical compositio ns, imply a cloud supersaturation of 0.24%-0.31% at the Cheeka Peak sa mpling site during periods of high LWC. The observations of droplet an d aerosol spectra supported Twomey's cloud brightening hypothesis in t hat 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, < 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 c m(-3) < N-ccn < 400 cm(-3), but not very sensitive to variation in aer osol 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-ccn(-0.33) and LWC. Predicted values for D-eff agreed (r(2) = 0.8) with D-eff de termined directly from the cloud droplet spectra, suggesting that this approach should be useful in climate modeling for predicting cloud dr oplet size from knowledge of N-ccn and LWC.