Can ice nucleating aerosols regulate seasonal Arctic cloudiness:

Observations, parameterization and implications from the M-PACE

Paul J. DeMott and Anthony J. Prenni, PI’s

Through funding by the Department of Energy’s Atmospheric Radiation Measurements (ARM) program, we collected ice nuclei data during the M-PACE (Mixed-Phase Arctic Cloud Experiment) in 2004 from late September 2004 through October 2004 in the vicinity of the Department of Energy North Slope of Alaska field site. This research was done in collaboration with scientists from Penn State, NCAR, the University of North Dakota, the University of Illinois and others. M-PACE) successfully documented the microphysical structure of Arctic mixed-phase clouds (Verlinde et al., 2007). Liquid was found in clouds with cloud-top temperatures as cold as -30^oC, the coldest cloud-top temperature warmer than -40^oC sampled by the aircraft. Observations in widely different forcing conditions suggest that the cause of the persistent liquid in these cold, ice-precipitating clouds is not in their dynamical characteristics, but must be microphysical in origin. The prevalence of liquid down to these low temperatures potentially could be explained by the relatively low ice nuclei concentrations measured (Prenni et al., 2007).

Ice nuclei (IN) concentration measurements were made using a continuous flow diffusion chamber (CFDC) on the University of North Dakota Citation II aircraft. IN data are presented as IN concentrations, binned into unit ranges of processing ice supersaturation. These averaged concentrations include a substantial contribution (~87%) from measurements for which no IN were detected. Also shown is the parameterization of Meyers et al. (1992) that is used in many models, often without regard to the location, season or altitude being modeled. It is clear that this parameterization is not representative of average IN behavior as assessed during M-PACE flights in the vicinity of lower level Arctic stratiform clouds, and the use of this parameterization will impair our ability to predict cloudiness and related radiative forcing in this region.

Several recent studies suggest that the Arctic climate is more sensitive to changes in climate forcing than other regions on Earth, while global climate models are less reliable in this region (ACIA, 2004). Clouds play a particularly important role for the surface energy balance in the Arctic and are difficult to model. One possible reason for this is the difference in the aerosol properties of the Arctic atmosphere compared to lower latitudes. The global climate models that form the basis for assessments, such as the ACIA report, use the same cloud and aerosol descriptions in the Arctic as anywhere else on Earth, and are calibrated to provide a reasonable global climate. For the global climate, ice clouds such as found in tropical cirrus anvils are probably more important; however, applying formulations optimized for mid-latitude and tropical conditions to the Arctic, where conditions are clearly different, results in a poor representation of this apparently very sensitive region. This was demonstrated in Prenni et al. (2007) using simulations with RAMS (see below). This means that global models which under-predict liquid-water clouds may feature a larger shift from ice to liquid clouds as the model climate warms. This constitutes an enhanced, unrealistic, positive feedback on climate change and may partly explain some of the models very large sensitivity to such features as projected ice cover. An important conclusion from these results is a necessity to include a realistic treatment of aerosols and aerosol/cloud interactions in future climate simulations (Prenni et al., 2007).

References:

ACIA, 2004: Impacts of a warming Arctic: Arctic Climate Impact Assessment. Cambridge University Press, 1020 pp.

Meyers, M. P., P. J. DeMott, and W. R. Cotton, 1992: New primary ice-nucleation parameterizations in an explicit cloud model. J. Appl. Meteor., 31, 708-721.

Prenni, A.J., J.Y. Harrington, M. Tjernström, P.J. DeMott, A. Avramov, C.N. Long, S.M. Kreidenweis, P.Q. Olsson, J. Verlinde, 2007: Can Ice-Nucleating Aerosols Affect Arctic Seasonal Climate? Accepted to Bull. Amer. Meteor. Soc.

Verlinde and Co-Authors, 2007: The Mixed-Phase Arctic Cloud Experiment (M-PACE). In press, Bull. Amer. Meteor. Soc.