Evaluating and constraining ice cloud parameterizations in CAM5 using aircraft measurements from the SPARTICUS campaign K. Zhang1, X. Liu1, M. Wang1, J. M. Comstock1, D. L. Mitchell2, S. Mishra2,3, and G. G. Mace4 1Pacific Northwest National Laboratory, Richland, WA, USA 2Desert Research Institute, Reno, NV, USA 3Cooperative Institute for Mesoscale Meteorological Studies (CIMMS), Norman, OK, USA 4Department of Meteorology, University of Utah, Salt Lake City, UT, USA
Abstract. This study uses aircraft measurements of relative humidity and ice crystal
size distribution collected during the SPARTICUS (Small PARTicles In CirrUS)
field campaign to evaluate and constrain ice cloud parameterizations in the
Community Atmosphere Model version 5. About 200 h of data were collected
during the campaign between January and June 2010, providing the longest
aircraft measurements available so far for cirrus clouds in the
midlatitudes. The probability density function (PDF) of ice crystal number
derived from the high-frequency (1 Hz)
measurements features a strong dependence on ambient temperature. As
temperature decreases from −35 °C to −62 °C, the peak
in the PDF shifts from 10–20 L−1 to 200–1000 L−1,
while Ni shows a factor of 6–7 increase.
Model simulations are performed with two different ice nucleation schemes for
pure ice-phase clouds. One of the schemes can reproduce a clear increase of
Ni with decreasing temperature by using either an
observation-based ice nuclei spectrum or a classical-theory-based spectrum with
a relatively low (5–10%) maximum freezing ratio for dust aerosols. The
simulation with the other scheme, which assumes a high maximum freezing ratio
(100%), shows much weaker temperature dependence of Ni.
Simulations are also performed to test empirical parameters related to water
vapor deposition and the autoconversion of ice crystals to snow. Results
show that a value between 0.05 and 0.1 for the water vapor deposition
coefficient, and 250 μm for the critical diameter that
distinguishes ice crystals from snow, can produce good agreement between
model simulation and the SPARTICUS measurements in terms of Ni
and effective radius. The climate impact of perturbing
these parameters is also discussed.
Citation: Zhang, K., Liu, X., Wang, M., Comstock, J. M., Mitchell, D. L., Mishra, S., and Mace, G. G.: Evaluating and constraining ice cloud parameterizations in CAM5 using aircraft measurements from the SPARTICUS campaign, Atmos. Chem. Phys., 13, 4963-4982, doi:10.5194/acp-13-4963-2013, 2013.