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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, D24205, doi:10.1029/2005JD006916, 2006

Microphysical and dynamical controls on cirrus cloud optical depth distributions

Jennifer E. Kay

Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA


Marcia Baker

Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA


Dean Hegg

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


Abstract

We assess microphysical and dynamical controls on cirrus cloud optical depth distributions [P(σ)] along idealized air parcel trajectories. We find P(σ) shape depends primarily on the ratio of the ice crystal fallout timescale to timescales of other microphysical and dynamical processes. With homogeneous freezing only, two P(σ) regimes emerged. In the limited fallout regime, relatively slow fallout allows complete depletion of the ice supersaturation, and P(σ) has a peak at large optical depth values (σ > 1). In contrast, in the fallout-dominated regime, relatively rapid fallout results in persistent high-ice supersaturation and multiple freezing events, and P(σ) has a monotonically decreasing shape dominated by small optical depth values. The addition of heterogeneous freezing alters the homogeneous-freezing P(σ) shape only in the limited fallout regime. Here glaciated ice nuclei (IN) do not inhibit homogeneous freezing but can change P(σ) by reducing the optical depth of the P(σ) peak and adding a monotonically decreasing tail at low optical depth values. Surprisingly, glaciated IN do not significantly change P(σ) values or shape in the fallout-dominated regime. Fluctuations in vertical velocity and accompanying temperature changes have relatively little impact on P(σ) unless the fluctuation timescales are shorter than fallout timescales, but longer than ice crystal growth timescales. As temperature fluctuations increase in amplitude, new freezing events affect P(σ) as long as fluctuation timescales approach or exceed freezing timescales. Our modeled P(σ) qualitatively resemble observed P(σ), indicating these results could aid in GCM cirrus P(σ) parameterization and help diagnose the controls on cirrus P(σ).

Received 22 November 2005; accepted 12 July 2006; published 22 December 2006.

Keywords: cirrus; optical depth; climate models.

Index Terms: 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 3311 Atmospheric Processes: Clouds and aerosols; 1626 Global Change: Global climate models (3337, 4928); 3359 Atmospheric Processes: Radiative processes.

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Citation: Kay, J. E., M. Baker, and D. Hegg (2006), Microphysical and dynamical controls on cirrus cloud optical depth distributions, J. Geophys. Res., 111, D24205, doi:10.1029/2005JD006916.