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Abstract
Abstract. Mineral dust aerosols often observed over California in winter and spring, associated with long-range transport from Asia and the Sahara, have been linked to enhanced precipitation based on observations. Local anthropogenic pollution, on the other hand, was shown in previous observational and modeling studies to reduce precipitation. Here we incorporate recent developments in ice nucleation parameterizations to link aerosols with ice crystal formation in a spectral-bin cloud microphysical model coupled with the Weather Research and Forecasting (WRF) model in order to examine the relative and combined impacts of dust and local pollution particles on cloud properties and precipitation type and intensity. Simulations are carried out for two cloud cases (from the CalWater 2011 field campaign) with contrasting meteorology and cloud dynamics that occurred on 16 February (FEB16) and 2 March (MAR02). In both cases, observations show the presence of dust and biological particles in a relative pristine environment. The simulated cloud microphysical properties and precipitation show reasonable agreement with aircraft and surface measurements. Model sensitivity experiments indicate that in the pristine environment, the dust and biological aerosol layers increase the accumulated precipitation by 10–20% from the Central Valley to the Sierra Nevada for both FEB16 and MAR02 due to a ~40% increase in snow formation, validating the observational hypothesis. Model results show that local pollution increases precipitation over the windward slope of the mountains by a few percent due to increased snow formation when dust is present, but reduces precipitation by 5–8% if dust is removed on FEB16. The effects of local pollution on cloud microphysics and precipitation strongly depend on meteorology, including cloud dynamics and the strength of the Sierra Barrier Jet. This study further underscores the importance of the interactions between local pollution, dust, and environmental conditions for assessing aerosol effects on cold-season precipitation in California.
Highlights
Precipitation is an important process that regulates atmospheric moisture, heat budgets, and local hydrological cycles
Two winter mixed-phase cloud cases occurring on 16 February (FEB16) and 2 March (MAR02) from the CalWater 2011 field campaign have been simulated using the Weather Research and Forecasting (WRF) model with a spectral-bin microphysical scheme in which ice nucleation parameterizations are updated to connect with ice nucleating particles (INP) so that the effects of aerosols as INP can be explored
By mimicking local pollution from the coastal plains and Central Valley area where particles are effective cloud condensation nuclei (CCN) and including dust and biological aerosol layers that provide effective INP at cold cloud levels, we have examined the effects of local pollution and long-range transported mineral dust on cloud microphysical properties and precipitation
Summary
Precipitation is an important process that regulates atmospheric moisture, heat budgets, and local hydrological cycles. Affected by many factors such as large-scale dynamics, solar heating, and aerosol particles J. Fan et al.: Aerosol impacts on California winter clouds and precipitation during CalWater 2011 et al, 2008), precipitation has always been very challenging to predict. Understanding the factors contributing to precipitation and snowpack formation has important implications for predicting the hydrology and local climate of the region. This has motivated a series of CalWater field campaigns, carried out since 2009, to improve understanding of processes influencing precipitation and water supply in California
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