Abstract
Abstract. Cirrus cloud radiative effects are largely affected by ice microphysical properties, including ice water content (IWC), ice crystal number concentration (Ni) and mean diameter (Di). 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 (w) and aerosol number concentrations (Na). 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 ≥62.5 µm 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 ∼100 km horizontal scale, simulations are found to underestimate (overestimate) IWC by a factor of 3–10 in the Northern (Southern) Hemisphere. Simulated Ni is overestimated in most regions except the Northern Hemisphere midlatitudes. Simulated Di is underestimated by a factor of 2, especially for warmer conditions (−50 to −40 ∘C), 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 Ni show bimodal distributions with maximum values at 100 % and 80 % RHi, differing from the unimodal distributions that peak at 100 % in the observations. For w effects, both observations and simulations show variances of w (σw) decreasing from the tropics to polar regions, but simulations show much higher σw 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 Di at higher Na. These findings provide an observation-based guideline for improving simulated ice microphysical properties and their relationships with key controlling factors at various geographical locations.
Highlights
Cirrus clouds represent one of the most ubiquitous cloud types with an estimated global coverage of approximately 20 % to 40 % (Mace and Wrenn, 2013; Sassen et al, 2008)
Community Atmosphere Model version 6 (CAM6)-nudg data show a similar trend of average ice water content (IWC), number concentration (Ni) and Di with respect to temperature as seen in observations; that is, the average IWC increases with increasing temperature, consistent with previous observational studies (Krämer et al, 2016; Luebke et al, 2013; Schiller et al, 2008), average Ni shows no clear trend with temperature, and average Di increases with increasing temperature
We investigate the statistical distributions of cirrus cloud microphysical properties (i.e., IWC, Ni and Di) as well as several key controlling factors using a comprehensive in situ observational dataset and global climate models (GCMs) simulations
Summary
Cirrus clouds represent one of the most ubiquitous cloud types with an estimated global coverage of approximately 20 % to 40 % (Mace and Wrenn, 2013; Sassen et al, 2008). Regarding the effects of aerosols, Cziczo et al (2013) and Cziczo and Froyd (2014) investigated ice crystal residuals from in situ observations and discovered that the majority of midlatitude cirrus clouds form via heterogeneous nucleation on mineral dust and metallic particles. Using a global-scale dataset of multiple flight campaigns, Patnaude and Diao (2020) isolated individual effects on cirrus clouds from temperature, RHi, vertical velocity (w) and aerosol number concentrations (Na). They found that when Na is 3–10 times higher than average conditions, it shows strong positive correlations with cirrus microphysical properties such as IWC, Ni and numberweighted mean diameter (Di).
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