EXTENDING TWOMEYS ANALYTICAL ESTIMATE OF NUCLEATED CLOUD DROPLET CONCENTRATIONS FROM CCN SPECTRA

A fundamental but approximate formula has been established by Twomey i n order to compute the nucleated cloud droplet number concentration as a function of the vertical velocity and the CCN (cloud condensation n uclei) characteristics expressed by the C and k parameters. The deriva tion of such a relationship employs the widespread power-law dependenc e N-CCN = Cs-vw(k) of the total activable CCN number N-CCN at a given supersaturation of water vapor over cloud droplets s(v,w). In this pap er the authors follow Twomey's theoretical approach but using a more r ealistic four-parameter CCN activation spectrum as shaped by the physi cochemical properties of the accumulation mode in a natural aerosol po pulation. The analytical scheme is shown to improve the estimate of cl oud droplet number concentration because it accounts for the limited a vailability of small-sized CN (condensation nuclei) with increasing su persaturation. A first validation and calibration of the new generic C CN activation spectrum is made by comparison with activation spectra, either produced by growing realistic lognormal distributions of CN or measured by a CCN spectrometer over the mid-Atlantic Ocean.