Evidence of Microphysical Buffering of CCN Impacts on Wintertime Orographic clouds

There is increasing evidence that cloud responses to increasing concentrations of cloud nucleating aerosols is by no means simple and linear. Stevens and Feingold(2009) introduced the term “buffering” to represent those processes whereby CCN pollution can lead to no change or little change in precipitation owing to complex microphysical and dynamical processes that can alter the seemingly simple response of clouds to aerosols. They gave several examples where this can occur as did Levin and Cotton(2009). An example of a simple cloud system is wintertime orographic clouds in which increased concentrations of cloud condensation nuclei is expected to increase the cloud droplet number concentration, reduce cloud droplet diameters, reduce riming growth of ice, and thereby reduce precipitation. However, Saleeby et al.,(2011, 2012) showed through modeling studies that while riming is indeed reduced in the presence of high CCN concentrations, the impact on total precipitation is quite small. The major impact is a “spillover” effect in which pristine and lightly-rimed ice particles drift further downstream of a mountain barrier. While the spillover effect can have major repercussions in certain watersheds, the impact on total precipitation is quite small. Saleeby et al.(2012) showed that the reason total precipitation is only slightly altered is due to the buffering influence of the Bereron-Feindeisen-Wegener effect. That is when aerosol pollution leads to reduced cloud droplet sizes, not only are riming efficiencies reduced, but droplet evaporation is enhanced(owing to their greater net surface area) in the presence of growing ice crystals. Thus ice crystal growth by vapor deposition is enhanced when more numerous smaller droplets are present. Thus the expected response is buffered and total precipitation is altered only slightly.

Another example that has been explored on a more limited basis is a recent study by Muhlbauer et al., (2010) who suggested that enhanced CCN concentrations in wintertime orographic clouds can lead to no change or even an increase in precipitation under certain conditions. We hypothesize that the presence of an active warm-rain drizzle formation process is the important factor controlling whether aerosol pollution decreases or increases precipitation. With an active drizzle formation process in clean conditions the addition of aerosol pollutants (which suppress collision and coalescence of cloud droplets) will result in enhanced production of supercooled water aloft which will promote enhanced ice particle riming rather than suppress it as in our Colorado studies. As consequence of enhanced ice particle riming, orographic precipitation can be increased or be unchanged by the presence of aerosol pollution. In other words microphysical buffering of precipitation process can occur. We are currently performing two-dimensional simulations of drizzling wintertime orographic clouds over the California Sierra Nevada to explore this hypothesis further.