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GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L10809, doi:10.1029/2007GL032628, 2008

Timescale analysis of aerosol sensitivity during homogeneous freezing and implications for upper tropospheric water vapor budgets

Jennifer E. Kay

Climate and Global Dynamics, National Center for Atmospheric Research, Boulder, Colorado, USA


Robert Wood

Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA


Abstract

Using timescales for the generation and depletion of water vapor, we predict aerosol sensitivity in clouds formed by homogeneous freezing. Our timescale analysis explains why aerosol sensitivity increases dramatically with ice deposition coefficients (α i) ≪ 0.1, and also why aerosol sensitivity increases as vertical velocity increases, temperature decreases, aerosol number decreases, and aerosol size decreases. We combine existing in-situ observations with adiabatic parcel modeling to constrain α i ≥ 0.1 for small ice crystals forming at high ice supersaturations. Two important implications for understanding and modeling upper tropospheric water vapor budgets emerge from our results: 1) aerosol sensitivity can be appreciable at low temperatures and moderate updrafts (∼5 cm/s) in the upper tropical troposphere, 2) reconciling our results with recent laboratory measurements supports theory that α i increases with ice supersaturation and/or decreases with ice crystal size.

Received 13 November 2007; accepted 17 April 2008; published 23 May 2008.

Keywords: homogeneous freezing; aerosols; cirrus.

Index Terms: 3311 Atmospheric Processes: Clouds and aerosols; 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801, 4906).

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Citation: Kay, J. E., and R. Wood (2008), Timescale analysis of aerosol sensitivity during homogeneous freezing and implications for upper tropospheric water vapor budgets, Geophys. Res. Lett., 35, L10809, doi:10.1029/2007GL032628.