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A comparative analysis between observational data from McMurdo Station, Antarctica and the Community Atmosphere Model version 6 (CAM6) simulation is performed focusing on cloud characteristics and their thermodynamic conditions. Ka‐band Zenith Radar (KAZR) and High Spectral Resolution Lidar (HSRL) retrievals are used as the basis of cloud fraction and cloud phase identifications. Radiosondes released at 12‐h increments provide atmospheric profiles for evaluating the simulated thermodynamic conditions. Our findings show that the CAM6 simulation consistently overestimates (underestimates) cloud fraction above (below) 3 km in four seasons of a year. Normalized by total in‐cloud samples, ice and mixed phase occurrence frequencies are underestimated and liquid phase frequency is overestimated by the model at cloud fractions above 0.6, while at cloud fractions below 0.6 ice phase frequency is overestimated and liquid‐containing phase frequency is underestimated by the model. The cloud fraction biases are closely associated with concurrent biases in relative humidity (RH), that is, high (low) RH biases above (below) 2 km. Frequencies of correctly simulating ice and liquid‐containing phase increase when the absolute biases of RH decrease. Cloud fraction biases also show a positive correlation with RH biases. Water vapor mixing ratio biases are the primary contributor to RH biases, and hence, likely a key factor controlling the cloud biases. This diagnosis of the evident shortfalls of representations of cloud characteristics in CAM6 simulation at McMurdo Station brings new insight in improving the governing model physics therein.

 

Three climate models are evaluated using in situ airborne observations from the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) campaign. The evaluation targets cloud phases, microphysical properties, thermodynamic conditions, and aerosol indirect effects from −40°C to 0°C. Compared with 580‐s averaged observations (i.e., 100 km horizontal scale), the Community Atmosphere Model version 6 (CAM6) shows the most similar result for cloud phase frequency distribution and allows more liquid‐containing clouds below −10°C compared with its predecessor—CAM5. The Energy Exascale Earth System Model (E3SM) underestimates (overestimates) ice phase frequencies below (above) −20°C. CAM6 and E3SM show liquid and ice water contents (i.e., LWC and IWC) similar to observations from −25°C to 0°C, but higher LWC and lower IWC than observations at lower temperatures. Simulated in‐cloud RH shows higher minimum values than observations, possibly restricting ice growth during sedimentation. As number concentrations of aerosols larger than 500 nm (Na₅₀₀) increase, observations show increases of LWC, IWC, liquid, and ice number concentrations (N_liq, N_ice). Number concentrations of aerosols larger than 100 nm (Na₁₀₀) only show positive correlations with LWC and N_liq. From −20°C to 0°C, higher aerosol number concentrations are correlated with lower glaciation ratio and higher cloud fraction. From −40°C to −20°C, large aerosols show positive correlations with glaciation ratio. CAM6 shows small increases of LWC and N_liqwith Na₅₀₀and Na₁₀₀. E3SM shows small increases of N_icewith Na₅₀₀. Overall, CAM6 and E3SM underestimate aerosol indirect effects on ice crystals and supercooled liquid droplets over the Southern Ocean.

 

Stratocumulus cloud top entrainment has a significant effect on cloud properties, but there are few observations quantifying its impact. Using explicit 0‐D parcel model simulations, initialized with below‐cloud in situ measurements, and validated with in situ measurements of cloud properties, the shortwave cloud radiative forcing (SWCF) was reduced by up to 100 W m⁻²by cloud top entrainment in the Southern Ocean. The impact of entrainment‐corrected SWCF is between 2 and 20 times that of changes in the aerosol particle concentration or updraft at cloud base. The variability in entrainment‐corrected SWCF accounts for up to 50 W m⁻²uncertainty in estimating cloud forcing. Measurements necessary for estimating the impact of entrainment on cloud properties can be constrained from existing airborne platforms and provide a first‐order approximation for cloud radiative properties of nonprecipitating stratocumulus clouds. These measurement‐derived estimates of entrainment can be used to validate and improve parameterizations of entrainment in Global Climate Models.

 

In this study, we conduct sensitivity experiments with the Community Atmosphere Model version 5 to understand the impact of representing heterogeneous distribution between cloud liquid and ice on the phase partitioning in mixed‐phase clouds through different perturbations on the Wegener‐Bergeron‐Findeisen (WBF) process. In two experiments, perturbation factors that are based on assumptions of pocket structure and the partial homogeneous cloud volume derived from the High‐performance Instrumented Airborne Platform for Environmental Research (HIAPER) Pole‐to‐Pole Observation (HIPPO) campaign are utilized. Alternately, a mass‐weighted assumption is used in the calculation of WBF process to mimic the appearance of unsaturated area in mixed‐phase clouds as the result of heterogeneous distribution. Model experiments are tested in both single column and weather forecast modes and evaluated against data from the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program's Mixed‐Phase Arctic Cloud Experiment (M‐PACE) field campaign and long‐term ground‐based multisensor measurements. Model results indicate that perturbations on the WBF process can significantly modify simulated microphysical properties of Arctic mixed‐phase clouds. The improvement of simulated cloud water phase partitioning tends to be linearly proportional to the perturbation magnitude that is applied in the three different sensitivity experiments. Cloud macrophysical properties such as cloud fraction and frequency of occurrence of low‐level mixed‐phase clouds are less sensitive to the perturbation magnitude than cloud microphysical properties. Moreover, this study indicates that heterogeneous distribution between cloud hydrometeors should be treated consistently for all cloud microphysical processes. The model vertical resolution is also important for liquid water maintenance in mixed‐phase clouds.

 

Abstract. Aerosols affect cirrus formation and evolution, yet quantificationof these effects remain difficult based on in situ observations due to thecomplexity of nucleation mechanisms and large variabilities in icemicrophysical properties. This work employed a method to distinguish fiveevolution phases of cirrus clouds based on in situ aircraft-basedobservations from seven U.S. National Science Foundation (NSF) and five NASAflight campaigns. Both homogeneous and heterogeneous nucleation werecaptured in the 1 Hz aircraft observations, inferred from the distributionsof relative humidity in the nucleation phase. Using linear regressions toquantify the correlations between cirrus microphysical properties andaerosol number concentrations, we found that ice water content (IWC) and icecrystal number concentration (Ni) show strong positive correlations withlarger aerosols (>500 nm) in the nucleation phase, indicatingstrong contributions of heterogeneous nucleation when ice crystals firststart to nucleate. For the later growth phase, IWC and Ni show similarpositive correlations with larger and smaller (i.e., >100 nm)aerosols, possibly due to fewer remaining ice-nucleating particles in thelater growth phase that allows more homogeneous nucleation to occur. Both200 m and 100 km observations were compared with the nudged simulations fromthe National Center for Atmospheric Research (NCAR) Community AtmosphereModel version 6 (CAM6). Simulated aerosol indirect effects are weaker thanthe observations for both larger and smaller aerosols for in situ cirrus,while the simulated aerosol indirect effects are closer to observations inconvective cirrus. The results also indicate that simulations overestimatehomogeneous freezing, underestimate heterogeneous nucleation andunderestimate the continuous formation and growth of ice crystals as cirrusclouds evolve. Observations show positive correlations of IWC, Ni and icecrystal mean diameter (Di) with respect to Na in both the Northern and SouthernHemisphere (NH and SH), while the simulations show negative correlations inthe SH. The observations also show higher increases of IWC and Ni in the SHunder the same increase of Na than those shown in the NH, indicating highersensitivity of cirrus microphysical properties to increases of Na in the SHthan the NH. The simulations underestimate IWC by a factor of 3–30 in theearly/later growth phase, indicating that the low bias of simulated IWC wasdue to insufficient continuous ice particle formation and growth. Sucha hypothesis is consistent with the model biases of lower frequencies of icesupersaturation and lower vertical velocity standard deviation in theearly/later growth phases. Overall, these findings show that aircraftobservations can capture both heterogeneous and homogeneous nucleation, andtheir contributions vary as cirrus clouds evolve. Future model developmentis also recommended to evaluate and improve the representation of watervapor and vertical velocity on the sub-grid scale to resolve theinsufficient ice particle formation and growth after the initial nucleationevent. 

Aerosols affect cirrus formation and evolution, yet quantification of these effects remain difficult based on in-situ observations due to the complexity of nucleation mechanisms and large variabilities in ice microphysical properties. This work employed a method to distinguish five evolution phases of cirrus clouds based on in-situ aircraft-based observations from seven U.S. National Science Foundation (NSF) and five NASA flight campaigns. Both homogeneous and heterogeneous nucleation were captured in the 1-Hz aircraft observations, inferred from the distributions of relative humidity in the nucleation phase. Using linear regressions to quantify the correlations between cirrus microphysical properties and aerosol number concentrations, we found that ice water content (IWC) and ice crystal number concentration (Ni) show strong positive correlations with larger aerosols (> 500 nm) in the nucleation phase, indicating strong contributions of heterogeneous nucleation when ice crystals first start to nucleate. For the later growth phase, IWC and Ni show similar positive correlations with larger and smaller (i.e., > 100 nm) aerosols, possibly due to fewer remaining ice nucleating particles in the later growth phase that allows more homogeneous nucleation to occur. Both 200-m and 100-km observations were compared with the nudged simulations from the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 6 (CAM6). Simulated aerosol indirect effects are weaker than the observations for both larger and smaller aerosols. Observations show stronger aerosol indirect effects (i.e., positive correlations between IWC, Ni and Na) in the Southern Hemisphere (SH) compared with the Northern Hemisphere (NH), while the simulations show negative correlations in the SH. The simulations underestimate IWC by a factor of 3 – 30 in the early/later growth phase, indicating that the low bias of simulated IWC was due to insufficient ice particle growth. Such hypothesis is consistent with the model biases of lower frequencies of ice supersaturation and lower vertical velocity standard deviation in the early/later growth phases. Overall, these findings show that aircraft observations can capture the competitions between heterogeneous and homogeneous nucleation, and their contributions vary as cirrus clouds evolve. Future model development is also recommended to evaluate and improve the representation of water vapor and vertical velocity on the sub-grid scale to resolve the insufficient ice particle growth. 

Abstract. Cirrus cloud radiative effects are largely affected byice microphysical properties, including ice water content (IWC), ice crystalnumber concentration (Ni) and mean diameter (Di). These characteristics varysignificantly due to thermodynamic, dynamical and aerosol conditions. Inthis work, a global-scale observation dataset is used to examine regionalvariations of cirrus cloud microphysical properties, as well as several keycontrolling factors, i.e., temperature, relative humidity with respect toice (RHi), vertical velocity (w) and aerosol number concentrations (Na).Results are compared with simulations from the National Center forAtmospheric 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 indata collection. The differences between simulations and observations arefound to vary with latitude and temperature. Comparing with averagedobservations at ∼100 km horizontal scale, simulations arefound to underestimate (overestimate) IWC by a factor of 3–10 in theNorthern (Southern) Hemisphere. Simulated Ni is overestimated in mostregions except the Northern Hemisphere midlatitudes. Simulated Di isunderestimated by a factor of 2, especially for warmer conditions(−50 to −40 ∘C), possibly due tomisrepresentation of ice particle growth/sedimentation. For RHi effects, thefrequency and magnitude of ice supersaturation are underestimated insimulations for clear-sky conditions. The simulated IWC and Ni show bimodaldistributions with maximum values at 100 % and 80 % RHi, differing fromthe unimodal distributions that peak at 100 % in the observations. For weffects, both observations and simulations show variances of w (σw) decreasing from the tropics to polar regions, but simulations show muchhigher σw for the in-cloud condition than the clear-sky condition.Compared with observations, simulations show weaker aerosol indirect effectswith a smaller increase of IWC and Di at higher Na. These findings provide anobservation-based guideline for improving simulated ice microphysicalproperties and their relationships with key controlling factors at variousgeographical locations. 

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). The differences between simulations and observations are found to vary with latitude and temperature. Specifically, simulations are found to underestimate IWC by a factor of 5–30 in all regions. Simulated Ni is overestimated in most regions except Northern Hemisphere midlatitude and polar regions. Simulated Di is underestimated, especially for warmer conditions (−50 °C to −40 °C) and higher Na, possibly due to less effective ice particle growth/sedimentation and weaker aerosol indirect effects, respectively. For RHi effects, the frequency and magnitude of ice supersaturation is underestimated in simulations for clear-sky conditions, and the simulated IWC and Ni show maximum values at 80 % RHi instead of 110 % as observed. For w effects, both observations and simulations show variances of w (σw) decreasing from tropics to polar regions, but simulations show much higher σw for in-cloud condition than clear-sky condition. These findings provide an observation-based guideline for improving simulated ice microphysical properties and their relationships with key controlling factors at various geographical locations. 

Cloud phase and relative humidity (RH) distributions at −67° to 0°C over the Southern Ocean during austral summer are compared between in situ airborne observations and global climate simulations. A scale-aware comparison is conducted using horizontally averaged observations from 0.1 to 50 km. Cloud phase frequencies, RH distributions, and liquid mass fraction are found to be less affected by horizontal resolutions than liquid and ice water content (LWC and IWC, respectively), liquid and ice number concentrations (Nc_liqand Nc_ice, respectively), and ice supersaturation (ISS) frequency. At −10° to 0°C, observations show 27%–34% and 17%–37% of liquid and mixed phases, while simulations show 60%–70% and 3%–4%, respectively. Simulations overestimate (underestimate) LWC and Nc_liqin liquid (mixed) phase, overestimate Nc_icein mixed phase, underestimate IWC in ice and mixed phases, and underestimate (overestimate) liquid mass fraction below (above) −5°C, indicating that observational constraints are needed for different cloud phases. RH frequently occurs at liquid saturation in liquid and mixed phases for all datasets, yet the observed RH in ice phase can deviate from liquid saturation by up to 20%–40% at −20° to 0°C, indicating that the model assumption of liquid saturation for coexisting ice and liquid is inaccurate for low liquid mass fractions (<0.1). Simulations lack RH variability for partial cloud fractions (0.1–0.9) and underestimate (overestimate) ISS frequency for cloud fraction <0.1 (≥0.6), implying that improving RH subgrid-scale parameterizations may be a viable path to account for small-scale processes that affect RH and cloud phase heterogeneities. Two sets of simulations (nudged and free-running) show very similar results (except for ISS frequency) regardless of sample sizes, corroborating the statistical robustness of the model–observation comparisons.