Extending Twomey’s Analytical Estimate of Nucleated Cloud Droplet Concentrations from CCN Spectra

Jean-Martial Cohard Laboratoire d’Aérologie, UMR CNRS/UPS, Toulouse, France

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Jean-Pierre Pinty Laboratoire d’Aérologie, UMR CNRS/UPS, Toulouse, France

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Carole Bedos Laboratoire d’Aérologie, UMR CNRS/UPS, Toulouse, France

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Abstract

A fundamental but approximate formula has been established by Twomey in order to compute the nucleated cloud droplet number concentration as a function of the vertical velocity and the CCN (cloud condensation nuclei) characteristics expressed by the C and k parameters. The derivation of such a relationship employs the widespread power-law dependence NCCN = Cskυ,w of the total activable CCN number NCCN at a given supersaturation of water vapor over cloud droplets sυ,w.

In this paper the authors follow Twomey’s theoretical approach but using a more realistic four-parameter CCN activation spectrum as shaped by the physicochemical properties of the accumulation mode in a natural aerosol population. The analytical scheme is shown to improve the estimate of cloud droplet number concentration because it accounts for the limited availability of small-sized CN (condensation nuclei) with increasing supersaturation. A first validation and calibration of the new generic CCN 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.

Corresponding author address: Dr. Jean-Pierre Pinty, Laboratoire d’Aérologie, Observatoire Midi-Pyrénées, 14 avenue E. Belin, 31400 Toulouse, France.

Email: pinjp@aero.obs-mip.fr

Abstract

A fundamental but approximate formula has been established by Twomey in order to compute the nucleated cloud droplet number concentration as a function of the vertical velocity and the CCN (cloud condensation nuclei) characteristics expressed by the C and k parameters. The derivation of such a relationship employs the widespread power-law dependence NCCN = Cskυ,w of the total activable CCN number NCCN at a given supersaturation of water vapor over cloud droplets sυ,w.

In this paper the authors follow Twomey’s theoretical approach but using a more realistic four-parameter CCN activation spectrum as shaped by the physicochemical properties of the accumulation mode in a natural aerosol population. The analytical scheme is shown to improve the estimate of cloud droplet number concentration because it accounts for the limited availability of small-sized CN (condensation nuclei) with increasing supersaturation. A first validation and calibration of the new generic CCN 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.

Corresponding author address: Dr. Jean-Pierre Pinty, Laboratoire d’Aérologie, Observatoire Midi-Pyrénées, 14 avenue E. Belin, 31400 Toulouse, France.

Email: pinjp@aero.obs-mip.fr

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