Aerosol and terrain effects on winter cloud and precipitation over New York State

Prediction of regional weather and climate regarding precipitation for a region such as the Northeast US with its complex terrain and aerosol environment constitutes a major challenge. Furthermore, complex thermodynamic structures occur in and around New York State (NYS) owing to its complex topography paired with its coastal regions, ultimately affecting cloud and precipitation microphysics. The microphysical processes within the weather systems that produce orographic precipitation are not fully understood. These processes include, but are not limited to, liquid-ice interactions, ice growth through vapor deposition, riming, and aggregation, cloud-aerosol interactions, and melting and refreezing upon sedimentation to the surface. These processes vary with terrain height and width, wind speed, and the thermodynamic profile. And their accurate representation is necessary for effective microphysical modeling and precipitation prediction. Therefore, to better understand a) the impacts of aerosols on winter mixed-phase cloud properties and precipitation, as well as b) the effects of the complex underling terrain over NYS on the aerosol-cloud-precipitation interaction, a winter precipitation event was simulated using Weather Research and Forecasting (WRF) model coupled with a Spectral Bin Microphysical (SBM) scheme. Detailed ice nucleation parameterizations directly linking ice formation with ice nuclei (IN) have been implemented in the SBM model. Mixing ratios of several aerosol species from Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) provide the aerosol initial and boundary condition. On January 6th, 2014, a synoptic frontal system passed through NYS, and was followed by a lake effect event that lasted to January 7th. With a focus of aerosol indirect effects over the complex terrain of NYS, four sensitivity studies on this event were conducted: 1) a Control run where the ice nuclei (IN) and cloud condensate nuclei (CCN) concentration fields were derived from MERRA2 reanalysis, 2) a Clean run with the IN concentration, which was relatively high during the event, decreased to a third, and 3) a CCN run with tripled CCN number than the Clean run, and 4) a Polluted run where the CCN concentration was tripled than the Control run. In the frontal system, the ice-to-liquid partition within the cloud was strongly affected by extra CCN and IN. Particularly, extra CCN contributed to the formation of more numerous but smaller droplets and a higher liquid partition throughout the cloud, promoting the formation of ice crystals especially under high IN concentration condition, and contributed to the riming growth rate of graupels and snow aggregates, resulting in higher surface precipitation. Whereas adding IN to the system largely activated the ice-microphysical processes and promoted cloud glaciation. With higher snow formation and growth rate, snow melting buffered the loss of liquid droplets within the cloud and resulted in increased surface rainfall. For the lake effect cloud event, CCN were found to promote the formation of cloud droplets, which contributed to the riming rate of snow when the updraft in cloud was relatively strong. Extra CCN resulted in higher hydrometer content in cloud and consequently higher snowfall. Whereas IN largely increased the glaciation level of cloud as well as the riming and aggregation growth rate, invigorating the vertical motion, resulting in more precipitation that advected further downwind. Those effects were more significant when the background loading of the other type of nuclei was high. To address the effects of the terrain on winter precipitation, for the synoptic event, three sensitivity studies were designed with different model and terrain resolution. It was found that the large wind speed in coarse resolution simulation brought more vapor and cloud droplets into the domain. However, the warm temperature and inability to fully capture the gravity wave cloud resulted in weaker engagement of the frozen hydrometers, less efficient precipitation formation and therefore less precipitation. Whereas the Smooth terrain allowed more hydrometers in and efficiently translated those into surface precipitation. For the lake effect event, Snow cover over the shores of Lake Ontario intensified the surface convergence, though the effect was contoured by lower hydrometer content and riming growth rate within the cloud, resulted in lower averaged precipitation. Additionally, the convergence zone and cloud band were found to shift to the south when the northern shore was covered. The snow cover over the shores of Lake Ontario, especially the northern shore, played an important role in the amount and distribution of the lake effect snow.

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