Office of Biological and Environmental Research Weekly Report

January 12, 2009

Improving the Accuracy of Climate Models by Understanding the Role of Small Ice Crystals on Radiative Transfer. Quantifying the effects of small ice crystals on long and short wave radiation has been a controversial and unsolved problem in cloud microphysics for the last 20 years. This information is needed to model cloud effects on radiative transfer and to better represent feedbacks in General Circulation Models (GCMs). Aircraft data suggests that the contributions of small ice crystals to the total concentration could be overestimated since large ice crystals are shattered into several hundred smaller ones by the scattering probes used to measure small crystals. A study by DOE scientists examined the impact of different assumptions about small ice crystal concentrations using the Community Climate Model (CAM-3). These studies revealed that the formation of ice nuclei from water droplet evaporation can be used to explain why ice crystal concentrations are greater than the concentration of particles that provide the nucleation for the growth of ice concentrations. The data collected has contributed to improved algorithms for satellite and ground lidar observations. Understanding these processes will lead to better representation of cloud processes and more accurate predictive capabilities of climate models particularly relating to cloud processes.

References

Cohen, E.A., G.G. Mace, G. McFarquhar and C. Schwartz, 2006: Observations of cirrus evolution from convective outflow during the Tropical Warm Pool International Cloud Experiment (TWP-ICE). Eos Trans. AGU, 87(52), Fall Meet. Suppl., Abstract A41A-0002.

Freer, M., and G.M. McFarquhar (2008), Development and comparison of cloud particle size distribution fitting and analysis techniques. Proc. 18^th ARM Science Team Meeting, Available from http://www.arm.gov/publications/proceedings/conf18/poster/P00090.pdf

Sednev, I., S. Menon, G. M. McFarquhar, and A. D. Del Genio (2008), Simulating mixed-phase Arctic stratus clouds: sensitivity to ice initiation mechanisms, Atmos. Chem. Phys., 8, 11755-11819.

Media Interest: No

Contact: Rickey Petty and Wanda Ferrell, SC-23.1, (301) 903-5548 and (301) 903-3281

New Deep Cloud Representation Improves Climate Model Simulations during El Niño. Clouds block large amounts of sunlight from reaching the Earth’s surface during El Niño. This shielding effect has largely been missing in the community climate model, CAM3. Scientists in DOE’s Atmospheric Radiation Measurement (ARM) program corrected this long-standing model deficiency by using an improved representation of atmospheric convection based on the ARM observations. The investigators found that the lack of cloud shielding effect in the climate model was caused by poor simulation of low-level cloud cover and water content in the clouds during El Niño. The addition of improved convection representation suppressed what had been overly active deep clouds in the model, making shallow clouds more active, and leading to more low-level clouds than found in the standard model configuration. The improved model also had better representation of water content anomalies in clouds which were higher due to enhanced transport of water vapor by shallow clouds to the lower-middle troposphere. The improvement of the model’s representation of clouds corrected the cloud shielding effect in the community climate model.

Reference:

Li, G, and GJ Zhang, 2008: Understanding biases in shortwave cloud radiative forcing in the National Center for Atmospheric Research Community Atmosphere Model (CAM3) during El Niño. J. Geophys. Res. 113, D02103, doi:10.1029/2007JD008963.

Media Interest: No

Contact: Kiran Alapaty, SC-23.1, (301) 903-3175