The aerosol pollution can act as cloud condensation nuclei (CCN) and lead to the formation of more numerous and smaller droplets which reduces riming (Saleeby et al., 2009). The smaller droplets evaporate more readily when ice crystals grow at the their expense (the Bergeron-Findeisen process) depleting liquid water contents and further decreasing riming growth. Saleeby et al., 2011 has found that the reduced riming lowers snow water equivalent precipitation amounts on the windward side of the mountain barrier and increases it on the lee slopes. Overall total precipitation was reduced only a small amount but the spillover effect led to a downstream shift of precipitation from the Pacific watershed to the Atlantic watershed.
The sources of dust in the Rocky Mountains have various origins. The small particles of the order of 10 µm are either regionally produced or are transported from Sahara or Asia. The mineral dust and pollutants transport from Asia to North America can take about 7-10 days (Uematsu et al., 1983). The efficiency is found to be maximum in spring (Yu et al., 2008), which has been attributed to the convective and cyclonic storm activity (Orgill and Sehmel 1976). However, the coarse particles have regional origin. The lofted dust can travel regionally in hours to a week's period (Park et al., 2007). The dust deposited in the San Juan Mountains migrate from the Colorado plateau deserts, which includes northeast Arizona, northwest New Mexico, and southeast Utah. The level of soil disturbance plays a key role in dust transport, as undisturbed soil would need higher wind speeds to be transported. The reason behind enhanced dust events in the US west since the 19th century has been grazing of cattle, farming and humane settlement (Reynolds et al., 2010). Painter et al., (2007) have examined that the snow covered with dust reflects less solar radiation and hence leads to more absorption and therefore, results in twice as faster melting of snow as compared to the fresh snow. This leads to the decrease of seasonal snow cover by several weeks. A record 5% altercation in the water budget in the CRB owing to the early melt of snow has been estimated. With the changing weather dust will have an important contribution in the snow budget of the CRB (Painter et al., 2010).
Mineral dust is the most important constituent by mass in the atmospheric aerosol (Yin et al., 2002). Dust can impact precipitation by acting as CCN, ice nuclei (IN), or giant CCN (GCCN) (Lerach et al., 2013). It is also expected to increase in the future (Prospero, 2003). Dust suspended in the air can create atmospheric instability (Fan et al., 2004) and may lead to the formation of super cells (Lerach, 2012). Convection can be initiated in mesoscale systems through buoyancy and shear in the environment (Klemp and Wilhelmson, 1998). Dust can also affect precipitation processes; it is well known that dust can serve as IN (DeMott et al., 2003; 2009). If the dust becomes coated with sulfates or originates over dry lakebeds, dust can serve as GCCN which when wetted can result in larger cloud droplets and thereby enhance the collision-coalescence process and ice particle riming. Dust serving as GCCN is expected to enhance precipitation. But smaller dust particles coated with sulfates can enhance droplet concentrations, leading to a decrease of ice particle riming. Thus dust functioning as CCN will work in opposition to its activity as GCCN and IN and suppress precipitation much like pollution aerosol. van den Heever et al., 2006 found that dust serving as CCN had a stronger impact on precipitation than its impact as GCCN or IN in deep convection over South Florida. We anticipate that this would be the case for wintertime orographic clouds as well. The focus of this study is on the impacts of dust acting as cloud-nucleating aerosol on wintertime orographic precipitation in the Colorado Mountains.
Lerach (2012) incorporated the dust source and transport module into the Colorado State University Regional Atmospheric Modeling System (RAMS). This module is based on that of Ginoux et al., 2001 which advects lofted dust in two size bins: accumulation mode and coarse mode. The fine mode dust median radius was set to 0.2µm, and the coarse mode dust median radius was set to 3.0 µm. These values were derived from limited AErosol RObotic NETwork (AERONET) observations at Sevilleta, NM (106.885o, 34.35o) from 15 April 2003 at 2200 UTC.
The RAMS bin-emulating two-moment bulk microphysics scheme (Feingold et al., 1998; Saleeby and Cotton 2004, 2008) is used in these simulations. Lerach (2012) implemented a new lookup table in RAMS that activates CCN from a bulk mass-weighted lognormal distribution comprised of both accumulation mode dust and WRF/Chem-based accumulation mode aerosol. In this way, the droplet activation properties of both modes are retained. Dust is assigned a constant κ value of 0.03 (Petters and Kreidenweis, 2007), based on Koehler et al., 2009 for the Arizona test dust. The background median radius is held constant at 0.035 µm, while its κ was set to 0.2. The lookup table activates potential CCN as a function of ambient vertical velocity, temperature, dust and background aerosol number concentrations, background aerosol κ parameter, and the median radii of the dust and background aerosol distributions. A similar approach is used for GCCN (Heymsfield and Sabin, 1989). In this study we set up RAMS 6.0 similar to Saleeby et al., (2011). The 32km North American Regional Reanalysis (NARR) is used for model initialization and boundary nudging for the coarse grid. The cumulative effect of dust acting as cloud-nucleating aerosol over the entire Colorado Rocky Mountains from mid- February to mid-April has been examined.