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The main hypothesis of the Inhibition of Snowfall by Pollution Aerosols (ISPA) project was that an increase in cloud condensation nucleus (CCN) concentration should increase supercooled cloud droplet concentrations while decreasing their size, lower the efficiency of snow growth by accretion (riming), and decrease snowfall rate (Fig. 3). Previous research demonstrated this effect empirically from measurements in mixed-phase orographic clouds at the Desert Research Institute’s Storm Peak Laboratory (SPL, 3210 m MSL) in northwestern CO. The low-level riming process enhances precipitation efficiency, such that rimed ice may comprise 20-50% of the final snow mass that reaches the surface (Mitchell et al., 1990; Borys et al., 2003). This occurs via the seeder-feeder process in which snow crystals falling through a cloud of supercooled liquid water will grow by collection of cloud droplets prior to surface deposition (Reinking et al., 2000). Previous studies by Hindman et al. (1994), Borys et al. (2000), and Borys et al. (2003) suggested that pollution aerosol can delay precipitation formation in winter orographic clouds in the Park Range of Colorado. Their analysis showed that pollution aerosol increases the concentration of CCN and therefore cloud droplets, leading to the formation of smaller droplets which less efficiently rime ice crystals.
Observations at Storm Peak Laboratory (SPL; image) are conducted between 14 Jan – 28 Feb 2010 in order to obtain a broader range of environmental conditions that may support the ISPA effect. These data will be used as a benchmark for model evaluation. In addition, we are proposing to apply an aerosol chemistry model coupled to RAMS to predict daily variability of CCN concentrations not only over the Park Range but for all of western Colorado as well. The Desert Research Institute’s SPL (3210 m AMSL) is located on the west summit of Mt. Werner in the Park Range near Steamboat Springs in northwestern Colorado. This site has been used in cloud and aerosol studies for more than 20 years (Hindman et al., 1994; Borys and Wetzel, 1997). Long-term observations at SPL document the effect of orographically induced mixing and convection on vertical pollutant transport and dispersion. Aerosol concentrations at SPL can vary by two orders of magnitude through the diurnal cycle in winter (Lowenthal et al., 2002).
During snowfall events, the cloud droplet and snow crystal size distributions, and snowfall rate are measured continuously. Crystal habit (shape) and the rimed mass fraction (RMF) are estimated by direct observation. Cloud water and snow are collected for chemical and oxygen isotope (18O) analysis. The cloud droplet (2-47 μm ) and snow crystal (100-6400 μm) size distributions are measured with DMT (Droplet Measurement Technologies, Inc.) SPP-100 (modified PMS FSSP-100) and DMT 2D-P (modified PMS OAP 260Y) spectrometer probes, respectively. Aerosols are sampled through an inlet which excludes cloud droplets and snow and effectively dries the aerosols (Lowenthal et al., 2002). The dry aerosol size distribution from< 0.090 to 20 μm is measured with a TSI, Inc., Scanning Mobility Particle Sizer (SMPS, model 3936) coupled with a TSI model 3022A condensation particle counter (CPC) and a TSI, Inc. model 3321 Aerodynamic Particle Sizer (APS). CCN spectra are measured using a stream-wise continuous-flow DMT CCN-100 counter operated over six user-specified supersaturations down to 0.07% (Lance et al., 2006). The total CN concentration for particles with diameters larger than 10 μm is measured with a TSI model 3010 condensation particle counter (CPC). Snowfall rate will be measured at SPL with a sheltered, high-sensitivity electronic gravimetric balance with a collection pan area of 2564 cm2 which allows the precipitation rate to be resolved to 0.01 mm/hr. Snowfall rate is also measured at Patrol Headquarters, roughly 1 km down slope of SPL with an antifreeze tipping bucket gauge. An acoustic snow depth sensor is also located at this site. Snow data will be collected from SNOTEL sites on the Park Range for areal representative snowfall rates. Snow samples for determination of ice water content (IWC) and chemical and isotopic composition will be collected in polyethylene bags inserted in 15 cm diameter aluminum tubes mounted to a vane which orients the bag openings into the wind. This collects snow before it reaches the ground. The ice water content (IWC) is obtained from the mass of the collected snow, the wind speed, the area of the bag/tube opening, and the time of exposure. The vertical profile of radar reflectivity and Doppler velocity from the surface up to 4 km AGL will be measured every 1 minute by a Ku-band Micro Rain Radar (MRR) manufactured by METEK Inc. (Löffler-Mang et al., 1999; Peters et al., 2002). The Ku-band radar will complement the surface-based in situ instruments by documenting the precipitation profile over SPL at high temporal and vertical spatial resolution. The time-height profiles from the MRR will be used to determine the nature and occurrence of seeder-feeder structures and low-level enhancement or sublimation of snow (Reinking et. al., 2000).