Potential Aerosol Impacts on Orographic Precipitation

William R CottonDept of Atmospheric Science

Colorado State University

• In the case of wintertime orographic clouds, we expect that increased concentrations of CCN will increase the cloud droplet number concentration, reduce cloud droplet diameters, reduce riming growth of ice, and thereby reduce precipitation.

Light riming polluted (left) Heavier riming clean(right)

• Saleeby et al. (2009) examined the Borys hypothesis by performing 3D simulations varying concentrations of CCN over the Park Range of Colorado. Higher CCN concentrations lead to the formation of smaller, more numerous droplets and reduced riming. Reduced riming lowered snow water equivalent precipitation amounts on the windward side of the mountain barrier and increased it on the lee slopes. Overall total precipitation was reduced only a small amount but in the case of the Park Range, the “spillover effect” led to a downstream shift of precipitation from the CRB to the Atlantic watershed. They also showed that this effect was only important for relatively wet storms where riming is important. Low supercooled liquid water content storms are less influenced by aerosol pollution.

Simulations of Aerosol Impact on 60-Day Cumulative Difference in SWE (mm)

An increase in aerosol number concentration resulted in a shift in the SWE (“spill-over effect”)

from the windward slope to the leeward slope of the major CO mountain ranges.

The San Juan Range is the overall highest and largest in expanse, and it typically boasts the greatest snowfall accumulations in the state. In the simulations it exhibited the greatest spatially continuous snowfall “spill-over effect” compared to the other major mountain ranges

Why is total water changed so little?

Wegener-Bergeron-Findeisen process

• A well known phenomena in cloud physics is the WBF process whereby ice crystals grow at the expense of cloud droplets

• This is a result of the fact that the saturation vapor pressure over ice is lower than over water

• Thus in a water saturated environment Ice crystals find themselves in a highly supersatureated environment and when they grow they deplete vapor causing droplets to evaporate

• But if the droplets are small, they expose more surface area, and evaporate more water than if they are large

Role of the Wegener-Bergeron-Findeisen process

• Near the crest and lee-side of the mountains the WBF process is active

• Other things being the same, more numerous small cloud droplets evaporate more quickly than fewer bigger droplets(owing to larger surface area)

• As a result the WBF process is enhanced in the polluted clouds thereby buffering the riming induced reduction in precipitation efficiency

• In summary the WBF process acts to buffer the impact of enhanced CCN concentrations on total precipitation

• Nonetheless the “spillover effect” leads to substantial redistribution of water substance with losses of precipitation of the windward side of barriers and gains on the leeward slopes

CCN Suppression of Drizzle• Lynn et al. (2007) suggested that increased aerosol concentrations can

actually increase the LWC available for accretion within the supercooled region of the orographic cloud by shutting off collision-coalescence at lower elevations within the cloud.

• Muhlbauer et al. (2010) suggest that the sensitivity of orographic snow to CCN was highly variable on a case-to-case basis.

• We argue that because wintertime orographic precipitation in Colorado is almost entirely derived from cold precipitation processes, these cloud responses to increased CCN is less of a factor within that region.

• Thus we explore this effect in the Sierra Nevada Mountains of Southern California, where maritime air-masses, rich in moisture, can generate clouds at altitudes low enough for warm-rain processes to occur.

The location of the 2-D domain used for the numerical experiments

• A total of 49 simulations were performed in which moisture content was systematically varied.

• Potential CCN concentrations were varied from 300 to 1500 cm-3

• All simulations used a uniform wind speed normal to the barrier of 15m/s, constant with height to 10km, and increases linearly above 10km to 40m/s at the model top.

Percent change in the integral mass of precipitation for the various humidity levels

• CCN effects on the integral mass of precipitation tend to be negative although small(lower than 1%) for the lowest humidity levels.

• For the highest humidity levels, increasing potential CCN concentrations results in a monotonic snow precipitation enhancement up to 4%.

• This is an example of buffering of CCN effects

Potential Dust Impacts on Orographic Precipitation

Vandana Jha’s PhD research

• The focus of this study is on the impacts of dust acting as cloud nucleating aerosol on wintertime orographic precipitation in the Colorado Mountains.

• Simulations have been run for the San Juan area in the Rocky Mountains for April 12th and 29th 2005.

The dust model

• A dust source and transport module is incorporated into RAMS based on that of Ginoux et al., 2001 , which advects lofted dust into two size bins: accumulation mode and coarse mode.

• Dust is lofted as a function of windspeed, terrain, and soil moisture amounts.

Dust activation

• Dust can be activated as CCN• Dust can be activated as GCCN• Dust can be activated as IN

Expected dust impacts

• Dust acting as CCN would be expected to decrease precipitation unless the dust depletes supersaturation owing to its large size thereby reducing the activation of normal CCN

• Dust acting as GCCN would increase precipitation

• Dust acting as IN would increase precipitation

DOMAIN OF STUDY

• The dust deposited in the San Juan Mountains migrate from the Colorado plateau deserts (NE Arizona, NW NM, and SE Utah).

EVENT 1SIMULATION RESULTS I

Fig.2: Dust concen Apr 12th

EVENT 1SIMULATION RESULTS II

Fig.6: Dust concen, Apr 29

Fig.3: Precip without emission Fig.4: Precip with emission

Fig.7: Diff in precip for Apr 29

CDNC HIGH WHEN KAPPA IS NUMBER WEIGHTED

Summary

• These preliminary results suggest that dust can enhance wintertime orographic precipitation

• What needs to be determined is which mode of nucleation dominates?

• Is it dust activation as IN, GCCN, or CCN(not likely for this case)

Conclusion

• The evidence so far is that for cold-based wintertime orographic clouds, aerosol pollution acting as CCN will produce a pronounced “spillover” effect but have a small impact on total precipitation

• There is a suggestion that dust may actually enhance wintertime orographic precipitation

• What is the net effect for an entire winter season where dust is often episodic and late in the season, and pollution events can be more frequent??

Implications to IN cloud seeding

• IN cloud seeding should be done with a knowledge of the natural variability of aerosol

• I speculate the seeding in an aerosol polluted environment would enhance the “spill-over” effect. In some water basins like the Colorado River Basin, this could lead to a major shift in water resources out of the basin.

• I also speculate that seeding in a dust rich environment will have little impact on surface precipitation