Effects of thermodynamics, dynamics and aerosols on cirrus clouds based on in situ observations and NCAR CAM6

Patnaude, Ryan; Diao, Minghui; Liu, Xiaohong; Chu, Suqian

Cirrus cloud radiative effects are largely affected by ice microphysical properties, including ice water content (IWC), ice crystal number concentration (inline-formulaNi) and mean diameter (inline-formulaDi). These characteristics vary significantly due to thermodynamic, dynamical and aerosol conditions. In this work, a global-scale observation dataset is used to examine regional variations of cirrus cloud microphysical properties, as well as several key controlling factors, i.e., temperature, relative humidity with respect to ice (RHi), vertical velocity (inline-formulaw) and aerosol number concentrations (inline-formulaNa). Results are compared with simulations from the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 6 (CAM6). Observed and simulated ice mass and number concentrations are constrained to inline-formula≥62.5inline-formulaµm to reduce potential uncertainty from shattered ice in data collection. The differences between simulations and observations are found to vary with latitude and temperature. Comparing with averaged observations at inline-formula∼100 km horizontal scale, simulations are found to underestimate (overestimate) IWC by a factor of 3–10 in the Northern (Southern) Hemisphere. Simulated inline-formulaNi is overestimated in most regions except the Northern Hemisphere midlatitudes. Simulated inline-formulaDi is underestimated by a factor of 2, especially for warmer conditions (inline-formula−50 to inline-formula−40inline-formulaC), possibly due to misrepresentation of ice particle growth/sedimentation. For RHi effects, the frequency and magnitude of ice supersaturation are underestimated in simulations for clear-sky conditions. The simulated IWC and inline-formulaNi show bimodal distributions with maximum values at 100 % and 80 % RHi, differing from the unimodal distributions that peak at 100 % in the observations. For inline-formulaw effects, both observations and simulations show variances of inline-formulaw (inline-formulaσw) decreasing from the tropics to polar regions, but simulations show much higher inline-formulaσw for the in-cloud condition than the clear-sky condition. Compared with observations, simulations show weaker aerosol indirect effects with a smaller increase of IWC and inline-formulaDi at higher inline-formulaNa. These findings provide an observation-based guideline for improving simulated ice microphysical properties and their relationships with key controlling factors at various geographical locations.

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Patnaude, Ryan / Diao, Minghui / Liu, Xiaohong / et al: Effects of thermodynamics, dynamics and aerosols on cirrus clouds based on in situ observations and NCAR CAM6. 2021. Copernicus Publications.

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