Small Ice Crystals in High-altitude Clouds Do Impact Global Circulations


Scientists in DOE’s Atmospheric Radiation Measurement (ARM) Program used a global climate model and found that moderate changes in the concentration of small ice crystals in high-altitude clouds, known as Cirrus clouds, had a large impact on the predicted climate.  Using ARM measurements, new mathematical descriptions of ice particle sizes and concentrations, falling speeds, and radiative properties of cirrus clouds were incorporated into a community global climate model.  Moderate increases in small ice crystal concentrations produced lower fall speeds, more cirrus cloud coverage (a 5.5% global increase) and ice content, and warmer atmosphere temperatures (over 3° C) at high altitudes.  In the tropical regions these changes had an overall cooling effect, but a warming effect elsewhere.  Since the present methods to measure concentrations of small ice crystals in cirrus clouds have a high degree of uncertainty, these modeling results underscore the need to improve measurements for better quantification of climate change prediction.


Mitchell, D.L., P.J. Rasch, D. Ivanova, G.M. McFarquhar, T. Nousiainen (2008), Impact of small ice crystal assumptions on ice sedimentation rates in cirrus clouds and GCM simulations.  Geophys. Res. Lett., 35, L09806, doi:10.1029/2008GL033552