HR: 1340h
AN: A13B-0910    [Abstracts]
TI: What Controls Cirrus Cloud Optical Depth Distributions?
AU: * Kay, J E
EM: jenkay@u.washington.edu
AF: University of Washington, Dept. of Earth and Space Sciences, Seattle, WA 98195 United States
AU: Baker, M
EM: marcia@ess.washington.edu
AF: University of Washington, Dept. of Earth and Space Sciences, Seattle, WA 98195 United States
AU: Baker, M
EM: marcia@ess.washington.edu
AF: University of Washington, Dept. of Atmospheric Sciences, Seattle, WA 98195 United States
AU: Hegg, D
EM: deanhegg@atmos.washington.edu
AF: University of Washington, Dept. of Atmospheric Sciences, Seattle, WA 98195 United States
AU: Turner, D
EM: dave.turner@pnl.gov
AF: Pacific Northwest National Labs, PO Box 999, MS K9-24, Richland, WA 99354 United States
AB: Understanding the controls on cirrus cloud optical depth distributions [P(σ)] is critical for calculating cirrus cloud radiative impacts. Using an adiabatic parcel model with binned ice microphysics, we assess the influence of microphysical (nucleation, growth and fallout) and dynamical (constant updraft, idealized waves) processes on P(σ). For various sets of model initial conditions, we find P(σ) shape depends primarily on the ice crystal fallout timescale. At small updraft velocities, short fallout timescales allow ice crystals to fall out before depleting the ice super-saturation (S_i). Thus, regardless of the ice nuclei (IN) concentration, high S_i persists and multiple homogeneous nucleation events occur. In this fallout-dominated regime, P(σ) has a monotonically decreasing shape. In contrast, at large updraft velocities, long fallout timescales resulting from large homogeneous nucleation rates allow complete depletion of the S_i and limited ice crystal fallout. In this limited-fallout regime, P(σ) has a skewed peak at high optical depth values. When glaciated IN are added to the limited-fallout regime evolution, they do not inhibit homogeneous nucleation, but they can reduce the maximum S_i and number concentration of ice crystals. The limited-fallout P(σ) with glaciated IN has an additional monotonically decreasing tail at low optical depth values. Superimposed oscillations in vertical velocity can broaden P(σ) for limited-fallout regime cirrus. With large temperature displacements, vertical velocity waves can also generate the high S_i required for new homogeneous nucleation events that influence P(σ). To complement our parcel model results, we calculate cirrus timescales, thicknesses, and P(σ) using 4000+ hours of raman lidar depolarization and optical depth observations from Lamont, OK (USA). Preliminary results indicate modeled P(σ) resemble P(σ) observations, suggesting P(σ) shapes can be explained in terms of microphysical and dynamical processes included in our parcel model trajectories. Using modeled P(σ) and atmospheric profiles, we will further assess which microphysical and dynamical processes could contribute to the observed inhomogeneity statistics and P(σ).
UR: http://students.washington.edu/jenkay/
DE: 0320 Cloud physics and chemistry
DE: 1626 Global climate models (3337, 4928)
DE: 3311 Clouds and aerosols
DE: 3359 Radiative processes
DE: 3360 Remote sensing
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005