William R. Cotton’s research while affiliated with Colorado State University and other places

This paper is based on the keynote talk that I presented at the International Congress on Clouds and Precipitation (ICCP, 2021), wherein I was awarded a lifetime membership of ICCP. I focus on the invigoration of cumulus clouds by high concentrations of ice nuclei and hygroscopic aerosol. As far as ice nuclei are concerned, I discuss the hypothesized invigoration of cumulus clouds by seeding with high concentrations of ice nuclei or what has been called rainfall enhancement by means of “dynamic seeding”. As to the effects of enhanced concentrations of hygroscopic aerosol on cumulus dynamics and rainfall, I discuss two mechanisms, (1) “mixed-phase invigoration” and (2) “condensational invigoration”. I conclude that the concept of invigoration of convective clouds using high concentrations of hygroscopic aerosol by means of “condensational invigoration” is the dominant response of cumuli to enhanced concentrations of hygroscopic aerosol. Moreover, the invigorated cumulus clouds produce more rainfall.

We examine the potential role of aerosol pollution on the rainfall and intensity of hurricane Harvey. For this study, we use the global model, OLAM, with aerosol estimates from the global atmospheric chemistry model GEOS-Chem. Two sets of simulations of hurricane Harvey were performed. Simulations in the first set cover the intensification phase of Harvey until initial landfall in Texas and focus on the sensitivity of storm track and intensity, while simulations in the second set examine the sensitivity of storm track and precipitation during the period after initial landfall when record flooding occurred near Houston. During each period, simulations were performed with no anthropogenic sources of aerosol, with both natural and anthropogenic aerosol sources, and with both sources enhanced ten times. During the rapid intensification phase, the results indicate that aerosol amounts had very little impact on storm motion. Moreover, very little difference was found on the intensity of the simulated storm to aerosol amounts for the no-anthropogenic vs the GEOS-Chem estimated amounts with anthropogenic sources. However, when both natural and anthropogenic aerosol amounts were enhanced ten times, the simulated storm intensity was enhanced appreciably in terms of minimum sea-level pressure. During the second period of the simulation, through which Harvey remained a tropical storm, the main result was that very little sensitivity was found in precipitation or any other TC characteristic to aerosol concentrations. We cannot definitively state why the individual convective cells did not respond to high aerosol concentrations during this phase of the storm. However, the abundant precipitation in all three simulations scavenged the vast majority of aerosol as it flowed radially inward, and we speculate that this modulated the potential impact of aerosols on the inner TC and eyewall Overall, the simulated response of hurricane Harvey to aerosols was far less spectacular than what has been simulated in the past. We conclude that this is because hurricane Harvey was a strongly dynamically-driven storm system that as a result was relatively impervious to the effects of aerosols.

Idealized large eddy simulations (LES) are performed of deep convective clouds over south Florida to examine the relative role of aerosol-induced condensational versus mixed-phase invigoration to convective intensity and rainfall. Aerosol concentrations and chemistry are represented by using output from the GEOS-Chem global atmospheric chemistry model run with and without anthropogenic aerosol sources. The results clearly show that higher aerosol concentrations result in enhanced precipitation, larger amounts of cloud liquid water content, enhanced updraft velocities during the latter part of the simulation, and a modest enhancement of the latent heating of condensation. Overall our results are consistent with the concept that convective cloud invigoration is mainly due to condensational invigoration and not primarily to mixed-phase invigoration. Furthermore, our results suggest that condensational invigoration can result in appreciable precipitation enhancement of ordinary warm-based convective clouds such as are common in locations like south Florida.

The cumulative effect of anthropogenic aerosol pollution acting primarily as cloud condensation nuclei (CCN) and dust acting as CCN, giant CCN, and ice nuclei (IN) is examined in this study, which covers the entire Colorado Rocky Mountains from the months of October to April in the year 2004–2005. The ~6.5-month analysis provides a range of snowfall totals and variability in concentrations of anthropogenic aerosol pollution and dust. This research quantifies the impacts of both dust and pollution aerosols on wintertime precipitation in the Colorado River Basin (CRB) using the Regional Atmospheric Modeling System (RAMS). In general, anthropogenic aerosol pollution reduces water resources in the CRB via the “spill-over” effect by enhancing cloud droplet concentrations and reducing riming, while dust enhances precipitation primarily by acting as IN. Dust is more episodic and anthropogenic aerosol is more pervasive throughout the winter season. Combined response to dust and anthropogenic aerosol is a net reduction of water resources in the CRB. The question is by how much are those water resources affected? We estimate that total winter-season precipitation loss for CRB in 2004–2005 due to the combined influence of anthropogenic aerosol and dust was 5,380,00 acre-feet of water (66.36 x 10⁶ m³).

Over the last decade, there have been numerous modeling and observational studies which suggest that anthropogenic aerosol pollution such as emitted by many industries, automobile exhaust, and coal-fired power plants, are quite hygroscopic and as such can serve as particles on which cloud droplets form or what we call cloud-condensation-nuclei (CCN). In the case of wintertime orographic clouds such as in Colorado, too many CCN lead to numerous cloud droplets which are so small that they suppress rain formation by collision and coalescence of cloud droplets (not very important for those clouds), but more importantly suppress snow formation via cloud droplets being collected by ice particles or what we call riming. Previous modeling studies by our group as well as other researchers suggest that aerosol pollution will result in reduced precipitation in the Colorado mountains, particularly in relatively wet storms. Another aerosol source important to precipitation processes is wind-blown dust. While some dust particles can serve as CCN, their numbers are so few that they are not competitive with natural or pollution aerosols in rain formation or ice particle riming. But dust is also known to be the most important natural source of ice nuclei (IN). Thus, long-range transported dust and locally produced in say the four corners region flow into the Colorado mountains and essentially seed those clouds with IN, much like purposely-produced cloud seeding, and enhance precipitation from wintertime orographic clouds. Here we present the first attempt to examine the combined impact of both aerosol pollution and dust on precipitation over the Colorado mountains for an entire snow year, the months of October to April in the year 2004-2005, which is a relatively wet snow year. (

Three-dimensional numerical simulations were performed to evaluate potential southwestern U.S. dust indirect microphysical and direct radiative impacts on a real severe storms outbreak. Increased solar absorption within the dust plume led to modest increases in pre-storm atmospheric stability at low levels, resulting in weaker convective updrafts and less widespread precipitation. Dust microphysical impacts on convection were minor in comparison, due in part to the lofted dust concentrations being relatively few in number when compared to the background (non-dust) aerosol population. While dust preferentially serving as cloud condensation nuclei (CCN) versus giant CCN had opposing effects on warm-rain production, both scenarios resulted in ample supercooled water and subsequent glaciation aloft, yielding larger graupel and hail. Associated latent heating from condensation and freezing contributed little to overall updraft invigoration. With reduced rain production overall, the simulations that included dust effects experienced slightly reduced grid-cumulative precipitation and notably warmer and spatially smaller cold pools. Dust serving as ice nucleating particles did not appear to play a significant role. The presence of dust ultimately reduced the number of supercells produced but allowed for supercell evolution characterized by consistently higher values of relative vertical vorticity within simulated mesocyclones. Dust radiative and microphysical effects were relatively small in magnitude when compared to those from altering the background convective available potential energy and vertical wind shear. It is difficult to generalize such findings from a single event, however, due to a number of case-specific environmental factors. These include the nature of the low-level moisture advection and characteristics of the background aerosol distribution.