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- Author or Editor: William R. Cotton x
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Abstract
A prolonged orographic precipitation event occurred over the Sierra Nevada in central California on 12–13 February 1986. This well-documented case was investigated via the nonhydrostatic version of the Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS). The two-dimensional, cross-barrier simulations produced flow fields and microphysical structure, which compared well with observations. The feasibility of producing quantitative precipitation forecasts (QPF) with an explicit cloud model was also demonstrated.
The experiments exhibited a profound sensitivity to the input sounding. Initializing with a sounding, which is representative of the upstream environment, was the most critical factor to the success of the simulation. The QPF was also quite sensitive to input graupel density. Decreasing the density of graupel led to increases in the overall precipitation. Sensitivities to other microphysical parameters as well as orography and dynamics were also examined.
Abstract
A prolonged orographic precipitation event occurred over the Sierra Nevada in central California on 12–13 February 1986. This well-documented case was investigated via the nonhydrostatic version of the Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS). The two-dimensional, cross-barrier simulations produced flow fields and microphysical structure, which compared well with observations. The feasibility of producing quantitative precipitation forecasts (QPF) with an explicit cloud model was also demonstrated.
The experiments exhibited a profound sensitivity to the input sounding. Initializing with a sounding, which is representative of the upstream environment, was the most critical factor to the success of the simulation. The QPF was also quite sensitive to input graupel density. Decreasing the density of graupel led to increases in the overall precipitation. Sensitivities to other microphysical parameters as well as orography and dynamics were also examined.
Abstract
Theoretical analysis shows that when water activity is larger than its threshold value and the dry radius of a particle is larger than 0.005 µm, the deviation of curvature correction from unity can be accurately represented by the product of two terms, with one term strongly depending upon water activity and the other depending upon dry radius. Moreover, experimental data show that the water-activity-dependent term can be approximated by linear and one-third power functions of water activity. According to the approximation made to curvature correction, water activity is solved as analytical functions of relative humidity (RH). The analytically solved water activity is then used to compute particle equilibrium sizes using a known (observed) relationship between water activity and water uptake by unit mass of dry material. The accuracy of equilibrium sizes calculated with this method is checked with seven typical classes of aerosols. Results show that when RH ≤ 99.99%, the equilibrium radius computed with this method is accurate to within 3% (6%) if the dry radius of a particle is larger (smaller) than 0.02 µm and that when RH > 99.99%, equilibrium sizes can be estimated with an accuracy higher than 84%. Analytical approximation formulas are also derived for calculating critical equilibrium radii and critical supersaturation. The maximum relative errors of these formulas range from 3% to 15%.
Abstract
Theoretical analysis shows that when water activity is larger than its threshold value and the dry radius of a particle is larger than 0.005 µm, the deviation of curvature correction from unity can be accurately represented by the product of two terms, with one term strongly depending upon water activity and the other depending upon dry radius. Moreover, experimental data show that the water-activity-dependent term can be approximated by linear and one-third power functions of water activity. According to the approximation made to curvature correction, water activity is solved as analytical functions of relative humidity (RH). The analytically solved water activity is then used to compute particle equilibrium sizes using a known (observed) relationship between water activity and water uptake by unit mass of dry material. The accuracy of equilibrium sizes calculated with this method is checked with seven typical classes of aerosols. Results show that when RH ≤ 99.99%, the equilibrium radius computed with this method is accurate to within 3% (6%) if the dry radius of a particle is larger (smaller) than 0.02 µm and that when RH > 99.99%, equilibrium sizes can be estimated with an accuracy higher than 84%. Analytical approximation formulas are also derived for calculating critical equilibrium radii and critical supersaturation. The maximum relative errors of these formulas range from 3% to 15%.
Abstract
This study employs a revised version of the Colorado State University three-dimensional numerical cloud scale model to investigate the observed behavior of deep convection over South Florida on 17 July 1973. A brief description of recent model improvements is made. A combined balance and dynamics initialization procedure designed to introduce variable magnitudes and distributions of low-level wind convergence to the initial fields is described.
Using radiosonde and PIBAL data collected by the NOAA/ERL Florida Area Cumulus Experiment (FACE) and the National Weather Service at Miami on 17 July 1973, composite wind, temperature, pressure and moisture profiles were constructed to depict the state of the atmosphere at the time of deep convection. Mesoscale convergence was estimated from results of a mesoscale model simulation of low-level sea breeze convergence made by Pielke (personal communication) for the same case study day. Several numerical simulations were performed using the sounding data as a basic state. The initial magnitude and distribution of low-level convergence was varied and the sensitivity of the model to some micro-physical parameters was examined.
The results of the numerical experiments show that (i) the magnitude of surface convergence over a finite area has a pronounced influence on the simulated storm circulation, the eddy kinetic energy of the storm and the total rainfall of the storm system; (ii) the horizontal distribution of convergence has a relatively large effect on the rates of entrainment into the updraft below 5 km MSL resulting in significant modulations in predicted precipitation, but only moderate changes in storm kinetic energy; (iii) variations in terminal velocity of precipitation associated with the introduction of the ice phase has only a minor effect on precipitation and total kinetic energy of the storm; and (iv) increased rain evaporation rates result in a moderate increase in the kinetic energy of the simulated storm, but at the expense of surface precipitation. Pressure forces are also shown to play an important role in initiating downdrafts and in biasing the direction of downdraft-associated outflow. Implications of these results to the modification of convective clouds are discussed.
Abstract
This study employs a revised version of the Colorado State University three-dimensional numerical cloud scale model to investigate the observed behavior of deep convection over South Florida on 17 July 1973. A brief description of recent model improvements is made. A combined balance and dynamics initialization procedure designed to introduce variable magnitudes and distributions of low-level wind convergence to the initial fields is described.
Using radiosonde and PIBAL data collected by the NOAA/ERL Florida Area Cumulus Experiment (FACE) and the National Weather Service at Miami on 17 July 1973, composite wind, temperature, pressure and moisture profiles were constructed to depict the state of the atmosphere at the time of deep convection. Mesoscale convergence was estimated from results of a mesoscale model simulation of low-level sea breeze convergence made by Pielke (personal communication) for the same case study day. Several numerical simulations were performed using the sounding data as a basic state. The initial magnitude and distribution of low-level convergence was varied and the sensitivity of the model to some micro-physical parameters was examined.
The results of the numerical experiments show that (i) the magnitude of surface convergence over a finite area has a pronounced influence on the simulated storm circulation, the eddy kinetic energy of the storm and the total rainfall of the storm system; (ii) the horizontal distribution of convergence has a relatively large effect on the rates of entrainment into the updraft below 5 km MSL resulting in significant modulations in predicted precipitation, but only moderate changes in storm kinetic energy; (iii) variations in terminal velocity of precipitation associated with the introduction of the ice phase has only a minor effect on precipitation and total kinetic energy of the storm; and (iv) increased rain evaporation rates result in a moderate increase in the kinetic energy of the simulated storm, but at the expense of surface precipitation. Pressure forces are also shown to play an important role in initiating downdrafts and in biasing the direction of downdraft-associated outflow. Implications of these results to the modification of convective clouds are discussed.
Abstract
Using the one-dimensional cumulus model developed by Cotton, predictions of the effects of seeding cumulus clouds were performed during the month of July 1973 as part of the Experimental Meteorology Laboratory's Florida Area Cumulus Experiment 1973 experiment. In Part I we compared seedability predictions with the Miami 1200 GMT soundings and soundings taken over the center of the experimental area (Central Site) at 1400 GMT. It was found that substantial differences between the two predictions occurred on a number of days in spite of the fact that the soundings are separated in time by only 2 h and in space by only 110 km.
In this paper we compare seedability predictions with the MIA 1200 GMT soundings and the CS 1800 GMT soundings. The CS 1800 GMT soundings were assumed to be representative of conditions over the experimental area during the period of operation of the experiment. We found that the predictions with the MIA 1200 GMT soundings were, on the average, more representative of conditions over the center of the experimental area during the period of operation of the experiment than were the predictions with the CS 1400 GMT soundings. The results of this study indicate that the choice of a sounding site and sounding time to be used for prediction of seeding effects over an experimental area must be carefully considered in the design of the experiment.
Abstract
Using the one-dimensional cumulus model developed by Cotton, predictions of the effects of seeding cumulus clouds were performed during the month of July 1973 as part of the Experimental Meteorology Laboratory's Florida Area Cumulus Experiment 1973 experiment. In Part I we compared seedability predictions with the Miami 1200 GMT soundings and soundings taken over the center of the experimental area (Central Site) at 1400 GMT. It was found that substantial differences between the two predictions occurred on a number of days in spite of the fact that the soundings are separated in time by only 2 h and in space by only 110 km.
In this paper we compare seedability predictions with the MIA 1200 GMT soundings and the CS 1800 GMT soundings. The CS 1800 GMT soundings were assumed to be representative of conditions over the experimental area during the period of operation of the experiment. We found that the predictions with the MIA 1200 GMT soundings were, on the average, more representative of conditions over the center of the experimental area during the period of operation of the experiment than were the predictions with the CS 1400 GMT soundings. The results of this study indicate that the choice of a sounding site and sounding time to be used for prediction of seeding effects over an experimental area must be carefully considered in the design of the experiment.
Abstract
Using the one-dimensional cumulus model developed by Cotton, predictions of the effects of seeding cumulus clouds were performed during the month of July 1973 as a part of the Experimental Meteorology Laboratory FACE 1973 Experiment. The calculations were performed with the Miami 1200 GMT soundings and soundings taken in the interior of Florida at 1400 GMT at the so-called Central Site (CS) location.A comparison of “seedability” predictions using the Miami 1200 GMT and CS 1400 GMT soundings have shown that substantial differences between the two seedability predictions occur on a number of days in spite of the fact that the soundings are separated in time by only 2 h and in space by only 110 km. The differences can be attributed to the frequent intrusion of dry air masses of varying height and thickness. The intensity of the dry layers is generally greatest over the higher-latitude CS location. The greatest differences between the two soundings, and hence the greatest difference between the predicted seeding effects, occurs during periods of transition from a disturbed, westerly flow regime to a well-defined, deep, easterly flow regime.
Abstract
Using the one-dimensional cumulus model developed by Cotton, predictions of the effects of seeding cumulus clouds were performed during the month of July 1973 as a part of the Experimental Meteorology Laboratory FACE 1973 Experiment. The calculations were performed with the Miami 1200 GMT soundings and soundings taken in the interior of Florida at 1400 GMT at the so-called Central Site (CS) location.A comparison of “seedability” predictions using the Miami 1200 GMT and CS 1400 GMT soundings have shown that substantial differences between the two seedability predictions occur on a number of days in spite of the fact that the soundings are separated in time by only 2 h and in space by only 110 km. The differences can be attributed to the frequent intrusion of dry air masses of varying height and thickness. The intensity of the dry layers is generally greatest over the higher-latitude CS location. The greatest differences between the two soundings, and hence the greatest difference between the predicted seeding effects, occurs during periods of transition from a disturbed, westerly flow regime to a well-defined, deep, easterly flow regime.
Abstract
Nine clouds are simulated by perturbing Florida Area Cumulus Experiment (FACE) field soundings employing the Colorado State University cloud model. After a cloud similar in size to the one observed is initiated, glaciation is simulated in experiments designed to study the mechanisms by which glaciation is communicated to the subcloud boundary layer. Numerical model results show that the vertical pressure mechanism consisting of hydrostatic and dynamic pressure gradient force and “pressure buoyancy” is present, as is the downdraft mechanism, but they are secondary to loading, temperature buoyancy, water vapor buoyancy and the horizontal dynamic forces on the scale of a single deep convective cloud. The communication mechanism that has the most sustained and coherent influence upon the subcloud layer is the settling and evaporation of precipitation. A clear implication of this study to weather modification is that for dynamic seeding to have a significant influence upon the upscale growth of a cloud system, artificially triggered explosive growth of relatively weak convective towers must also be aimed at a carefully designed increase in the rainfall from those clouds.
Abstract
Nine clouds are simulated by perturbing Florida Area Cumulus Experiment (FACE) field soundings employing the Colorado State University cloud model. After a cloud similar in size to the one observed is initiated, glaciation is simulated in experiments designed to study the mechanisms by which glaciation is communicated to the subcloud boundary layer. Numerical model results show that the vertical pressure mechanism consisting of hydrostatic and dynamic pressure gradient force and “pressure buoyancy” is present, as is the downdraft mechanism, but they are secondary to loading, temperature buoyancy, water vapor buoyancy and the horizontal dynamic forces on the scale of a single deep convective cloud. The communication mechanism that has the most sustained and coherent influence upon the subcloud layer is the settling and evaporation of precipitation. A clear implication of this study to weather modification is that for dynamic seeding to have a significant influence upon the upscale growth of a cloud system, artificially triggered explosive growth of relatively weak convective towers must also be aimed at a carefully designed increase in the rainfall from those clouds.
Abstract
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 efficiencies, within the stochastic collection equation, for individual size bins of droplets and ice particles; this is in sharp contrast to the bulk approach that uses a single collection efficiency to describe the growth of a distribution of an ice species by collecting cloud droplets. Simulations of a winter orographic cloud event reveal a reduction in riming when using the binned riming approach and, subsequently, larger amounts of supercooled liquid water within the orographic cloud.
Abstract
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 efficiencies, within the stochastic collection equation, for individual size bins of droplets and ice particles; this is in sharp contrast to the bulk approach that uses a single collection efficiency to describe the growth of a distribution of an ice species by collecting cloud droplets. Simulations of a winter orographic cloud event reveal a reduction in riming when using the binned riming approach and, subsequently, larger amounts of supercooled liquid water within the orographic cloud.
Abstract
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 while pollution emissions have increased. Precipitation suppression by pollution should also be evident in other areas of the world where shallow, orographic clouds produce precipitation. This study investigates the precipitation trends for sites along the Front Range of the Rocky Mountains to determine the effect of air pollution on precipitation in this region. The examination of precipitation trends reveals that the ratio of upslope precipitation for elevated sites west of Denver and Colorado Springs, Colorado, to upwind urban sites has decreased by approximately 30% over the past half-century. Similar precipitation trends were not found for more pristine sites in the region, providing evidence of precipitation suppression by air pollution.
Abstract
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 while pollution emissions have increased. Precipitation suppression by pollution should also be evident in other areas of the world where shallow, orographic clouds produce precipitation. This study investigates the precipitation trends for sites along the Front Range of the Rocky Mountains to determine the effect of air pollution on precipitation in this region. The examination of precipitation trends reveals that the ratio of upslope precipitation for elevated sites west of Denver and Colorado Springs, Colorado, to upwind urban sites has decreased by approximately 30% over the past half-century. Similar precipitation trends were not found for more pristine sites in the region, providing evidence of precipitation suppression by air pollution.
Abstract
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 μm in diameter and the prognostic number concentration of cloud droplets through activation of cloud condensation nuclei (CCN) and giant CCN (GCCN). The large-droplet mode was included to represent the dual modes of cloud droplets that often appear in nature. The activation of CCN is parameterized through the use of a Lagrangian parcel model that considers ambient cloud conditions for the nucleation of cloud droplets from aerosol. These new additions were tested in simulations of a supercell thunderstorm initiated from a warm, moist bubble. Model response was explored in regard to the microphysics sensitivity to the large-droplet mode, number concentrations of CCN and GCCN, size distributions of these nuclei, and the presence of nuclei sources and sinks.
Abstract
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 μm in diameter and the prognostic number concentration of cloud droplets through activation of cloud condensation nuclei (CCN) and giant CCN (GCCN). The large-droplet mode was included to represent the dual modes of cloud droplets that often appear in nature. The activation of CCN is parameterized through the use of a Lagrangian parcel model that considers ambient cloud conditions for the nucleation of cloud droplets from aerosol. These new additions were tested in simulations of a supercell thunderstorm initiated from a warm, moist bubble. Model response was explored in regard to the microphysics sensitivity to the large-droplet mode, number concentrations of CCN and GCCN, size distributions of these nuclei, and the presence of nuclei sources and sinks.
Abstract
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 to clouds of similar depth. As such, even small perturbations to the precipitation efficiency within these clouds can have substantial impacts. In the mountainous regions of the western United States, where water resources are derived primarily from orographic precipitation during the cold season, this effect is of particular interest. The aims of this study are twofold. The first part is focused on the implementation of a simplified aerosol emissions scheme into the Regional Atmospheric Modeling System (RAMS). This scheme uses aerosol output from the Weather Research and Forecast Chemistry model (WRF-Chem) to initialize aerosol sources in RAMS. The second part of this study uses this scheme in the simulation of an orographic snow case that occurred in northwest Colorado during February 2007. The result of this study suggests that atmospheric CCN concentrations can be reasonably simulated using a simplified parameterization of aerosol emissions, despite a lack of explicit secondary aerosol (SA) within the model. Furthermore, the spatial and temporal variations in CCN predicted by this scheme produced a complicated response in the surface distribution of precipitation from the orographic snowstorm, a result not seen in studies where CCN concentrations are set to be horizontally homogenous.
Abstract
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 to clouds of similar depth. As such, even small perturbations to the precipitation efficiency within these clouds can have substantial impacts. In the mountainous regions of the western United States, where water resources are derived primarily from orographic precipitation during the cold season, this effect is of particular interest. The aims of this study are twofold. The first part is focused on the implementation of a simplified aerosol emissions scheme into the Regional Atmospheric Modeling System (RAMS). This scheme uses aerosol output from the Weather Research and Forecast Chemistry model (WRF-Chem) to initialize aerosol sources in RAMS. The second part of this study uses this scheme in the simulation of an orographic snow case that occurred in northwest Colorado during February 2007. The result of this study suggests that atmospheric CCN concentrations can be reasonably simulated using a simplified parameterization of aerosol emissions, despite a lack of explicit secondary aerosol (SA) within the model. Furthermore, the spatial and temporal variations in CCN predicted by this scheme produced a complicated response in the surface distribution of precipitation from the orographic snowstorm, a result not seen in studies where CCN concentrations are set to be horizontally homogenous.
