A Study of Aerosol Impacts on Clouds and Precipitation Development in a Large Winter Cyclone

Abstract

Aerosols influence cloud and precipitation development in complex ways due to myriad feedbacks at a variety of scales from individual clouds through entire storm systems. This paper describes the implementation, testing, and results of a newly modified bulk microphysical parameterization with explicit cloud droplet nucleation and ice activation by aerosols. Idealized tests and a high-resolution, convection-permitting, continental-scale, 72-h simulation with five sensitivity experiments showed that increased aerosol number concentration results in more numerous cloud droplets of overall smaller size and delays precipitation development. Furthermore, the smaller droplet sizes cause the expected increased cloud albedo effect and more subtle longwave radiation effects. Although increased aerosols generally hindered the warm-rain processes, regions of mixed-phase clouds were impacted in slightly unexpected ways with more precipitation falling north of a synoptic-scale warm front. Aerosol impacts to regions of light precipitation, less than approximately 2.5 mm h−1, were far greater than impacts to regions with higher precipitation rates. Comparisons of model forecasts with five different aerosol states versus surface precipitation measurements revealed that even a large-scale storm system with nearly a thousand observing locations did not indicate which experiment produced a more correct final forecast, indicating a need for far longer-duration simulations due to the magnitude of both model forecast error and observational uncertainty. Last, since aerosols affect cloud and precipitation phase and amount, there are resulting implications to a variety of end-user applications such as surface sensible weather and aircraft icing.

Uncertainty from the choice of microphysics scheme in convection-permitting models significantly exceeds aerosol effects

“…Studies of summertime convective precipitation in Germany found that increased aerosol concentrations had a strong effect on cloud microphysical (and therefore radiative) properties but that the combined effects of microphysical and dynamical processes resulted in relatively little effect on surface precipitation (Seifert et al, 2012). This is similar to the findings of Thompson and Eidhammer (2014) in idealised and continental-scale simulations.…”

Simulating North American mesoscale convective systems with a convection-permitting climate model

“…The main physics packages used in our WRF simulation are the Thompson aerosol-aware microphysics (Thompson and Eidhammer 2014), the Yonsei University (YSU) planetary boundary layer scheme (Hong et al 2006), the Rapid Radiative Transfer Model (RRTMG) (Iacono et al 2008), and the improved Noah-MP land surface model (Niu et al 2011). Additionally, an upgraded lake water temperature treatment is implemented and spectral nudging (von Storch et al 2000;Miguez-Macho et al 2004) of temperature, horizontal wind speed, and geopotential height is applied.…”

Aerosol midlatitude cyclone indirect effects in observations and high-resolution simulations

“…the synoptic-scale atmospheric processes and much smallerscale cloud microphysical processes play a role in regulating the cyclone life cycle (Naud et al, 2016(Naud et al, , 2017Grandey et al, 2013;Deng, 2015, 2016;Thompson and Eidhammer, 2014;Igel et al, 2013;Zhang et al, 2007).…”