Evaluating and constraining ice cloud parameterizations in CAM5 using aircraft measurements from the SPARTICUS campaign

摘要

This study uses aircraft measurements of relative humidity and ice crystalsize distribution collected during the SPARTICUS (Small PARTicles In CirrUS)field campaign to evaluate and constrain ice cloud parameterizations in theCommunity Atmosphere Model version 5. About 200 h of data were collectedduring the campaign between January and June 2010, providing the longestaircraft measurements available so far for cirrus clouds in themidlatitudes. The probability density function (PDF) of ice crystal numberconcentration ()derived from the high-frequency (1 Hz)measurements features a strong dependence on ambient temperature. Astemperature decreases from −35 °C to −62 °C, the peakin the PDF shifts from 10–20 L to 200–1000 L,while shows a factor of 6–7 increase.Model simulations are performed with two different ice nucleation schemes forpure ice-phase clouds. One of the schemes can reproduce a clear increase of with decreasing temperature by using either anobservation-based ice nuclei spectrum or a classical-theory-based spectrum witha relatively low (5–10%) maximum freezing ratio for dust aerosols. Thesimulation with the other scheme, which assumes a high maximum freezing ratio(100%), shows much weaker temperature dependence of .Simulations are also performed to test empirical parameters related to watervapor deposition and the autoconversion of ice crystals to snow. Resultsshow that a value between 0.05 and 0.1 for the water vapor depositioncoefficient, and 250 μm for the critical diameter thatdistinguishes ice crystals from snow, can produce good agreement betweenmodel simulation and the SPARTICUS measurements in terms of and effective radius. The climate impact of perturbingthese parameters is also discussed.