Abstract
Abstract. Anthropogenic aerosols serve as a source of both cloud condensation nuclei (CCN) and ice nuclei (IN) and affect microphysical properties of clouds. Increasing aerosol number concentrations is hypothesized to retard the cloud droplet coalescence and the riming in mixed-phase clouds, thereby decreasing orographic precipitation. This study presents results from a model intercomparison of 2-D simulations of aerosol-cloud-precipitation interactions in stratiform orographic mixed-phase clouds. The sensitivity of orographic precipitation to changes in the aerosol number concentrations is analysed and compared for various dynamical and thermodynamical situations. Furthermore, the sensitivities of microphysical processes such as coalescence, aggregation, riming and diffusional growth to changes in the aerosol number concentrations are evaluated and compared. The participating numerical models are the model from the Consortium for Small-Scale Modeling (COSMO) with bulk microphysics, the Weather Research and Forecasting (WRF) model with bin microphysics and the University of Wisconsin modeling system (UWNMS) with a spectral ice habit prediction microphysics scheme. All models are operated on a cloud-resolving scale with 2 km horizontal grid spacing. The results of the model intercomparison suggest that the sensitivity of orographic precipitation to aerosol modifications varies greatly from case to case and from model to model. Neither a precipitation decrease nor a precipitation increase is found robustly in all simulations. Qualitative robust results can only be found for a subset of the simulations but even then quantitative agreement is scarce. Estimates of the aerosol effect on orographic precipitation are found to range from −19% to 0% depending on the simulated case and the model. Similarly, riming is shown to decrease in some cases and models whereas it increases in others, which implies that a decrease in riming with increasing aerosol load is not a robust result. Furthermore, it is found that neither a decrease in cloud droplet coalescence nor a decrease in riming necessarily implies a decrease in precipitation due to compensation effects by other microphysical pathways. The simulations suggest that mixed-phase conditions play an important role in buffering the effect of aerosol perturbations on cloud microphysics and reducing the overall susceptibility of clouds and precipitation to changes in the aerosol number concentrations. As a consequence the aerosol effect on precipitation is suggested to be less pronounced or even inverted in regions with high terrain (e.g., the Alps or Rocky Mountains) or in regions where mixed-phase microphysics is important for the climatology of orographic precipitation.
Highlights
Orographic clouds form as moist air impinges on mountain ranges thereby potentially forming orographic precipitation which is essential for landscape formation, agriculture and the hydrology of watersheds in many regions of the world (e.g., Roe, 2005)
The complexity of mixed-phase microphysical processes acting in orographic clouds and the sensitivity of orographic precipitation to changes in microphysics are not fully understood yet (e.g., Rotunno and Houze, 2007) and remain challenging to represent in numerical models of weather and climate on various scales
Orographic precipitation is hypothesized to be susceptible to aerosols because the time available to form precipitable hydrometeors in rising air parcels is constrained by the flow over the mountain
Summary
Orographic clouds form as moist air impinges on mountain ranges thereby potentially forming orographic precipitation which is essential for landscape formation, agriculture and the hydrology of watersheds in many regions of the world (e.g., Roe, 2005). Increasing the aerosol number concentration is observed to lead to a shift of the cloud droplet spectrum towards smaller sizes (e.g., Twomey et al, 1984; Lowenthal et al, 2004) and, retards the onset of the coalescence process. Microphysical observations in mixed-phase clouds suggest a decrease in the snowfall rate with increasing aerosol number concentration due to a reduced efficacy of the riming process implied by smaller cloud droplets (Borys et al, 2000, 2003). Both effects together may modulate the amount and distribution of mixedphase precipitation over mountainous terrain
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
