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

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.

Evaluation of the CAM6 Climate Model Using Cloud Observations at McMurdo Station, Antarctica

“…Since no INP measurements were provided by the AWARE campaign, acquiring additional observations of INPs in the future are recommended for this remote region in order to quantify the impacts of various types of INPs over Antarctica. Another study showed that the simulated IWC and ice crystal mean diameter in CAM6 are underestimated compared with in situ observations, possibly due to weaker aerosol indirect effects and insufficient amounts of ice particle growth in the model (Patnaude et al., 2021). Therefore, to increase ice mass concentration and the amount of supercooled liquid water, further modifications are recommended for CNT, CLUBB and MG2 parameterizations, INP number concentrations and ice particle growth.…”

“…A threshold of 10 −4 was applied to model cloud fraction output as the limit of in‐cloud condition. In addition, in‐cloud condition in the simulation also needs to satisfy CWC > 10 −7 g m −3 , which is a typical limit used for defining in‐cloud condensed water mass concentrations in GCMs (e.g., Patnaude et al., 2021).…”

Ice and Supercooled Liquid Water Distributions Over the Southern Ocean Based on In Situ Observations and Climate Model Simulations

“…Therefore, the sum of simulated ice and snow mass concentrations for a specific size range (i.e., 0-5,000 μm in this study) is used to compare with the observed IWC derived from measurements of a similar size range. A similar method was previously used in several model-observation comparison studies, such as Fridlind et al (2007), Eidhammer et al (2014), and Patnaude et al (2021). Table S1 in Supporting Information S1 shows the contributions of various discrete size ranges of ice and snow to their mass concentrations in the CAM6 simulation.…”

Idealized Simulations of the Tropical Climate and Variability in the Single Column Atmosphere Model (SCAM): Radiative‐Convective Equilibrium

“…SCAM RCE also exhibits time-mean saturation over ice (RH ≥ 100) near the tropopause. While ice supersaturation is frequently observed by aircraft (e.g., Patnaude et al, 2021) as well as in satellite data (e.g., Gettelman et al, 2006), and occurs in the RCE simulations of other single column models, cloud-resolving models, and large eddy simulations (Figure 8 in Wing et al [2020]), its prevalence in the grand mean is an artifact of the strict single column geometry: in 3D RCE with CAM5 and CAM6, dry regions make the space-time mean unsaturated (Figure 3 in Reed et al [2021]). In the lower-middle troposphere, mean RH profiles all hover near the 60%-80% range, with SAM being the outlier, on the humid side.…”