William R. Cotton's research while affiliated with Colorado State University and other places
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Publications (242)
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...
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 mo...
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 snowfal...
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...
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 a...
In this study we explore the role that cloud base height (or cloud base temperature) plays in the response of hailstorms to increased concentrations of cloud condensation nuclei (CCN). A rather large number of cloud-resolving simulations were performed which explored the parameter space varying both potential CCN concentration and cloud base height...
The desire to improve the forecasting skill of the intensity and size of tropical cyclones has prompted the investigation into numerous physical processes that can impact these quantities. The modification of cloud properties via aerosols injected into a tropical cyclone can initiate interactions between cloud microphysics and storm dynamics that u...
The impacts of enhanced CCN concentrations on various cloud and precipitation systems are potentially significant both to the large-scale climate system and local precipitation patterns. Precipitating stable orographic cloud systems are particularly susceptible to increases in CCN as parcel lifetimes within these clouds are typically short compared...
Observations from multiple satellites and large-eddy simulations (LESs) from the Regional Atmospheric Modeling System (RAMS) are used to determine the extent to which free-tropospheric clouds (FTCs) affect the properties of stratocumulus. Overlying FTCs decrease the cloud-top radiative cooling in stratocumulus by an amount that depends on the upper...
The southwest United States is a region with huge demands on water resources. But the water resources in the Colorado River Basin (CRB) are potentially impacted by aerosol pollution and dust acting as cloud-nucleating aerosol as well as dust affecting snowpack albedo.
The aerosol pollution can act as cloud condensation nuclei (CCN) and lead to the...
1] The role of aerosols' effect on two tropical cyclones over the Bay of Bengal is investigated using a convection-permitting model with a two-moment mixed-phase bulk cloud microphysics scheme. The simulation results show the role of aerosol on the microphysical and dynamical properties of the cloud and bring out the change in efficiency of the clo...
In this study we examine the cumulative effect of dust acting as cloud
nucleating aerosol (cloud condensation nuclei (CCN), giant cloud
condensation nuclei (GCCN), and ice nuclei (IN)) over the entire
Colorado Rocky Mountains for the months of mid-February to mid-April in
2005, 2006, 2007 and 2008. These four consecutive seasons provide a
range of...
The Regional Atmospheric Modeling System was used to simulate four winter snowfall events over the Park Range of Colorado. For each event, three hygroscopic aerosol sensitivity simulations were performed with initial aerosol profiles representing clean, moderately polluted, and highly polluted scenarios. Previous work demonstrates that the addition...
The US Department of Homeland Security (DHS) organized a workshop in February 2008 named the Hurricane Aerosol Microphysics Program (HAMP)to develop a program to study the potential for tropical cyclones (TC) mitigation in the wake of the disaster inflicted on the United States by Hurricane Katrina. Penetration of continental aerosols to the TC per...
Four three-dimensional, nested-grid numerical simulations were performed using the Regional Atmospheric Modeling System (RAMS) to compare the effects of aerosols acting as cloud condensation nuclei (CCN) to those of low-level moisture [and thus convective available potential energy (CAPE)] on cold-pool evolution and tornadogenesis within an idealiz...
This chapter presents equations that govern kinematic and thermodynamic processes in clouds and cloud systems. First a set of state properties are presented, and then equations for conservation of mass, momentum, and energy are provided. Some predictive equations that are commonly used in numerical weather prediction models are also presented and a...
Chapter 9 discusses the dynamics of precipitating mesoscale systems, including tropical and mid-latitude squall lines, tropical and mid-latitude cloud clusters including mesoscale convective complexes, mesoscale convective vortices, and genesis of tropical cyclones.
Chapter 6 provides an overview of the dynamics of fogs and stratocumulus clouds. Included are fog formation mechanisms and various forcing factors that influence fog occurrence and intensity. Likewise factors influencing boundary layer cloud occurrence and intensity are reviewed along with discussions of models used to describe and predict boundary...
The first half of Chapter 10 begins with an introduction to the synoptic features of extratropical cyclones, after which the focus is on the properties and processes of various mesoscale features associated with extratropical cyclones. These include rainbands, snowbands, orographic influences, precipitation cores and gaps, split fronts, cold fronts...
This chapter addresses the small-scale, random motions (i.e., turbulence) in clouds. Because the smallest scales of motion in clouds are many orders of magnitude smaller than the characteristic length scale of clouds, then the governing equations must be filtered (i.e., averaged) to make simulation possible. The Reynolds averaging approach is exami...
Chapter 7 focuses on cumulus clouds including boundary layer, fair weather cumuli, and towering cumuli or cumulus congestus.
Chapter 11 focuses on wintertime clouds and cloud systems that are forced at least in part by orography, but excluding deep convective clouds. It includes examining air motions over mountainous terrain that are conducive to the formation and special distribution of precipitation. Also considered is the interaction between flow over mountains and la...
Chapter 4 provides an overview of the general theory of cloud precipitation processes. We then review cloud microphysics parameterization methodologies. We conclude but summarizing how cloud microphysics impacts cloud dynamics.
In Chapter 12 we discuss the role of clouds on the global radiation budget, on the energetics of the tropical atmosphere, their impacts on the global hydrological cycle, and cloud transports of pollutants out of the boundary layer and into the upper troposphere.
Cloud microphysical processes can be seen as a swarm or ensemble of particles that contribute collectively, and in an integrated way, to the macroscale dynamics and thermodynamics of the cloud. This chapter presents a similar perspective of small-scale air motions in clouds and examines the collective behavior or statistical contributions of the sm...
Chapter 8 focuses on cumulonimbus clouds including those that produce severe weather like flash floods, tornadoes, lightning, and hail.
Chapter 5 presents and overview of radiative transfer in a cloudy atmosphere and its parameterization. Included in this chapter is a discussion of how radiation influences cloud droplet growth, and how aerosols effect the radiative properties of clouds.
Hygroscopic pollution aerosols have the potential to alter winter orographic snowfall totals and spatial distributions by modification of high-elevation supercooled orographic clouds and the riming process. The authors investigate the cumulative effect of varying the concentrations of hygroscopic aerosols during January-February for four recent win...
My main research areas are in modeling and observation of clouds and storms and how aerosols can influence clouds and storms, and as a result climate. I have recently co-authored two books and a chapter in another book that are relevant to the topic of this chapter. They are “Human Impacts on Weather and Climate, 2nd Edition” (Cotton and Pielke 200...
Model investigations of aerosol–cloud interactions across spatial scales are necessary to advance basic understanding of aerosol impacts on climate and the hydrological cycle. Yet these interactions are complex, involving numerous physical and chemical processes. Models capable of combining aerosol dynamics and chemistry with detailed cloud microph...
Oxygen isotopic ratios (δ18O) and sulfate concentrations were measured in cloud water and snow collected at Storm Peak Laboratory (SPL) during winter, 2007. The rimed mass fraction (RMF) was estimated as the ratio of sulfate concentration in snow to that in cloud water. A sharp increase in the RMF at mean droplet diameters above 10 μm confirmed the...
In spite of its short period of existence, a great deal was accomplished during the Hurricane-Aerosol-Microphysics Program (HAMP). The research supports the original hypotheses that seeding with high concentrations of pollution sized aerosols in the outer rainbands of hurricanes can lead to significant weakening of a storm. This was demonstrated by...
The North American monsoon system is known to produce significant summertime precipitation on the west coast of Mexico and the southwestern United States, with some areas receiving greater than 50% of their yearly rainfall between the months of July and September. The onset of the monsoon is attributed to a shift in the large-scale upper-level anti...
Pollution aerosols acting as cloud condensation nuclei (CCN) have the potential to alter warm rain clouds via the aerosol first and second indirect effects in which they modify the cloud droplet population, cloud lifetime and size, rainfall efficiency, and radiation balance from increased albedo. For constant liquid water content, an increase in CC...
[1] To investigate the effects of both cloud condensational nuclei (CCN) and giant CCN (GCCN), the Regional Atmospheric Modeling System was used to investigate the effects of various CCN and GCCN concentrations on the development of precipitating trade wind cumuli in a large eddy simulation (LES) framework. The sounding to initialize the LES was ta...
[1] The mechanisms by which Saharan dust acting as cloud condensation nuclei (CCN) impact tropical cyclone (TC) evolution were examined by conducting numerical simulations of a mature TC with CCN added from lateral boundaries. CCN can affect eyewall development directly through release of latent heat when activated and subsequent growth of cloud dr...
Experts consider the many roles that clouds play in the the changing climate—one of the least understood and most puzzling aspects of atmospheric science.
More than half the globe is covered by visible clouds. Clouds control major parts of the Earth's energy balance, influencing both incoming shortwave solar radiation and outgoing longwave thermal...
Aerosols impact global climate in a number of ways. First they directly affect the Earth’s radiation budget by absorbing and reflecting solar radiation and to a lesser extent altering the profile of IR absorption in the atmosphere. Second, by serving as cloud condensation nuclei (CCN) and ice nuclei (IN), they determine cloud microphysics, the form...
The history of numerical modeling of the effect of aerosols on clouds dates back at least 50 years to the work of Howell (1949) and Mordy (1959), who considered the growth of a population of aerosol particles in a rising parcel of air. Models such as these addressed the effects of both aerosol and dynamical parameters (i.e. updraft velocity) on the...
Deliberate cloud seeding, with the goal of increasing precipitation by the injection of specific types of particles into clouds, has been pursued for over 50 years. Efforts to understand the processes involved have led to a significant body of knowledge about clouds and about the effects of the seeding aerosol. A number of projects focused on the s...
Life on Earth is critically dependent upon the continuous cycling of water between oceans, continents and the atmosphere. Precipitation (including rain, snow, and hail) is the primary mechanism for transporting water from the atmosphere back to the Earth's surface. It is also the key physical process that links aspects of climate, weather, and the...
In this chapter we provide an overview of the basic physical processes responsible for the formation of clouds and precipitation. A number of important concepts are discussed, and terms defined, which will be used in later chapters. For more detail on these topics the reader is referred to textbooks by Pruppacher and Klett (1997), Rogers and Yau (1...
Numerical simulations of an idealized supercell thunderstorm were performed to assess effects of increased aerosol concentrations acting as cloud condensation nuclei (CCN) and giant CCN (GCCN) on tornadogenesis. Initial background profiles of CCN and GCCN concentrations were set to represent “clean” continental and aerosol-polluted environments, re...
This paper presents the development and application of a binned approach to cloud-droplet riming within a bulk microphysics model. This approach provides a more realistic representation of collision-coalescence that occurs between ice and cloud particles of various sizes. The binned approach allows the application of specific collection efficiencie...
It is recommended that a series of international projects targeted toward unraveling the complex interactions among aerosols, clouds, and precipitation be implemented. A series of international workshops and field studies are needed to address the impacts on clouds and precipitation of aerosols from a range of sources including biomass burning, dus...
1] The impact of dust in the Saharan Air Layer (SAL) acting as cloud condensation nuclei (CCN) on the evolution of a tropical cyclone (TC) was examined by conducting simulations initialized with an idealized pre-TC mesoscale convective vortex (MCV) using the Regional Atmospheric Modeling System (RAMS). Increasing the background CCN concentration fr...
The impacts of urban-enhanced aerosol concentrations on convective storm development and precipita- tion over and downwind of St. Louis, Missouri, are investigated. This is achieved through the use of a cloud-resolving mesoscale model, in which sophisticated land use processes and aerosol microphysics are both incorporated. The results indicate tha...
The Town Energy Budget (TEB) model, a detailed urban parameterisation using a generalised canyon geometry, coupled with the
Regional Atmospheric Modelling System (RAMS) is used to simulate the wintertime local circulation in the megacity environment
of the metropolitan area of Sao Paulo (MASP) in Brazil. Model simulations are performed using actual...
The Regional Atmospheric Modeling System (RAMS) at the Colorado State University has been adapted to produce meteorological predictions for the Analysen and Konzepte TopTask Competition (TTC) soaring flight planning and analysis algorithm. The predictions were for regions surrounding the major gliderports in Colorado USA. The TTC algorithm requires...
The Colorado State University Regional Atmospheric Modeling System (RAMS) has been used to emulate cloud seeding operations in the Colorado Rocky Mountains for the winter of 2003-2004 in a previous study (Cotton et al. 2006). This paper documents new developments in RAMS since that study using a winter storm simulation that occurred in Colorado fro...
An overview of simulations of hurricane response to African dust is presented. Those simulations suggest that under some conditions storm intensity might be reduced if large concentrations of small hygroscopic particles are present at the time the storms develop. Based on those results, it is proposed that seeding hurricanes with small hygroscopic...
In this talk we summarize the major findings and conclusions made in the WMO International Aerosol Precipitation Science Assessment Group (IAPSAG) report on aerosol impacts on precipitation. At the time of writing this abstract external reviews of the report have been received and by the time of the fall AGU meeting the final draft should be comple...
Toward the end of the Cirrus Regional Study of Tropical Anvils and Cirrus Layer Florida Area Cirrus Experiment (CRYSTAL FACE) field campaign held during July 2002, high concentrations of Saharan dust, which can serve as cloud condensation nuclei (CCN), giant CCN (GCCN), and ice-forming nuclei (IFN) were observed over the peninsula of Florida. To in...
An idealized supercell simulation using the Regional Atmospheric Modeling System (RAMS) produced an elongated low-level mesocyclone that subsequently collapsed into a concentrated vortex. Though vorticity continually increased in the mesocyclone due to horizontal convergence, the collapse phase was additionally characterized by rapidly decreasing p...
An idealized simulation of a supercell using the Regional Atmospheric Modeling System (RAMS) was able to produce a low-level mesocyclone near the intersection of the forward- and rear-flank downdrafts. The creation of the low-level mesocyclone is similar to previous studies. After 3600 s, the low-level mesocyclone underwent a period of rapid intens...
Air pollution generated in industrial and urban areas can act to
suppress precipitation by creating a narrow cloud droplet spectrum,
which inhibits the collision and coalescence process. In fact,
precipitation ratios of elevated sites to upwind coastal urban areas
have decreased during the twentieth century for locations in California
and Israel wh...
This paper is the second in a two-part series describing recent additions to the microphysics module of the Regional Atmospheric Modeling System (RAMS) at Colorado State University. These changes include the addition of a large-cloud-droplet mode (40–80 m in diameter) into the liquid-droplet spectrum and the parameterization of cloud-droplet nuclea...
One problem in computing cloud microphysical processes in coarse-resolution numerical models is that many microphysical processes are nonlinear and small in scale. Consequently, there are inaccuracies if microphysics parameterizations are forced with grid box averages of model fields, such as liquid water content. Rather, the model needs to determi...
1] Using large-eddy simulations (LESs) of six observed boundary layer cases as benchmark data sets, we diagnosed vertical velocity as the sum of the grid-mean (w) and the subgrid-scale w using an empirical method we have developed. We have found that the subgrid-scale w is best characterized by the root-mean square of w 02 . The vertical velocity i...
Research and operational approaches to weather modification expressed in
the National Research Council's 2003 report on “Critical Issues in
Weather Modification Research” and in the Weather Modification
Association's response to that report form the basis for this
discussion. There is agreement that advances in the past few decades
over a broad fro...
An artificial neural network (ANN) based algorithm is implemented and tested for soil moisture estimation. The ANN model is calibrated (trained) and validated (tested) with data including National Centers for Environmental Protection (NCEP) daily precipitation; normalized difference vegetation index (NDVI) data processed by the US Geological Survey...
Short-term forecasting of precipitation often relies on meteorological radar coverage to provide information on the intensity, extent, and motion of approaching mesoscale features. However, in significant portions of mountainous regions, radar coverage is lacking because of topographic blocking, and the absence of radar signatures in sections of th...
This study examines the sensitivity of varying the horizontal heterogeneities of the soil moisture initialization (SMI) in the cloud-resolving grid of a real-data simulation of a midlatitude mesoscale convective system (MCS) during its genesis phase. The quasi-stationary MCS of this study formed in the Texas/Oklahoma panhandle with a lifetime of 9...
The Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) was initiated to sample MCs capable of producing damaging surface winds and capable of producing long-lived MCVs, as well as to observe mesoscale convective surfaces (MCSs) in their mature state relatively devoid of convection. New procedures were implemented regarding the deployment o...
Variations in storm microstructure due to updraft strength, liquid water content, and the presence of dry layers, wind shear, and cloud nucleating aerosol concentrations are likely to lead to changes in hail sizes within deep convective storms. The focus of this paper is to determine how the overall dynamics and microphysical structure of deep conv...
One of th€ highest wave flighb ever achiered over th€ Catskill Mountains of south€rn N€w York Stat€ USA occurred surprisingly on 5 July 2002 (17,990 ft, 5,l{]5 n MSL). Using obs€rved net€orolosical conditions with numerical-simulation mod€ls, most festures of the wave were reproduc€d. Using the simulations, it appears th€ flight could hav€ gone muc...
The microphysics module of the version of the Regional Atmospheric Modeling System (RAMS) maintained at Colorado State University has undergone a series of improvements, including the addition of a large-cloud- droplet mode from 40 to 80 mm in diameter and the prognostic number concentration of cloud droplets through activation of cloud condensatio...
An investigation of several hundred mesoscale convective systems (MCSs) during the warm seasons (April-August) of 1996-98 is presented. Circular and elongated MCSs on both the large and small scales were classified and analyzed in this study using satellite and radar data. The satellite classification scheme used for this study includes two previou...
A storm-resolving version of the Regional Atmospheric Modeling System is executed over St. Louis, Missouri, on 8 June 1999, along with sophisticated boundary conditions, to simulate the urban atmosphere and its role in deep, moist convection. In particular, surface-driven low-level convergence mechanisms are investigated. Sensitivity experiments sh...
A review of convective cloud modeling spanning the period from the days of the NOAA Experimental. Meteorology Laboratory (EML) in the late 1960s to 2000 is presented. The intent is to illustrate the evolution of cloud models from the one-dimensional parcel-type models to the current generation of three-dimensional convective storm models and cloud...
A new single-column model for the cloudy boundary layer, described in a
companion paper, is tested for a variety of regimes. To represent the
subgrid-scale variability, the model uses a joint probability density
function (PDF) of vertical velocity, temperature, and moisture content.
Results from four different cases are presented and contrasted wit...
