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Abstract
A method is described for parameterizing thermodynamic forcing by the mesoscale updrafts and downdrafts of mesoscale convective systems (MCSs) in models with resolution too coarse to resolve these drafts. The parameterization contains improvements over previous schemes, including a more sophisticated convective driver and inclusion of the vertical distribution of various physical processes obtained through conditional sampling of two cloud-resolving MCS simulations. The mesoscale parameterization is tied to a version of the Arakawa–Schubert convective parameterization scheme that is modified to employ a prognostic closure. The parameterized Arakawa–Schubert cumulus convection provides condensed water, ice, and water vapor, which drives the parameterization for the large-scale effects of mesoscale circulations associated with the convection. In the mesoscale parameterization, determining thermodynamic forcing of the large scale depends on knowing the vertically integrated values and the vertical distributions of phase transformation rates and mesoscale eddy fluxes of entropy and water vapor in mesoscale updrafts and downdrafts. The relative magnitudes of these quantities are constrained by assumptions made about the relationships between various quantities in an MCS’s water budget deduced from the cloud-resolving MCS simulations. The MCS simulations include one of a tropical MCS observed during the 1987 Australian monsoon season (EMEX9) and one of a midlatitude MCS observed during a 1985 field experiment in the Central Plains of the United States (PRE-STORM 23–24 June).
Abstract
A method is described for parameterizing thermodynamic forcing by the mesoscale updrafts and downdrafts of mesoscale convective systems (MCSs) in models with resolution too coarse to resolve these drafts. The parameterization contains improvements over previous schemes, including a more sophisticated convective driver and inclusion of the vertical distribution of various physical processes obtained through conditional sampling of two cloud-resolving MCS simulations. The mesoscale parameterization is tied to a version of the Arakawa–Schubert convective parameterization scheme that is modified to employ a prognostic closure. The parameterized Arakawa–Schubert cumulus convection provides condensed water, ice, and water vapor, which drives the parameterization for the large-scale effects of mesoscale circulations associated with the convection. In the mesoscale parameterization, determining thermodynamic forcing of the large scale depends on knowing the vertically integrated values and the vertical distributions of phase transformation rates and mesoscale eddy fluxes of entropy and water vapor in mesoscale updrafts and downdrafts. The relative magnitudes of these quantities are constrained by assumptions made about the relationships between various quantities in an MCS’s water budget deduced from the cloud-resolving MCS simulations. The MCS simulations include one of a tropical MCS observed during the 1987 Australian monsoon season (EMEX9) and one of a midlatitude MCS observed during a 1985 field experiment in the Central Plains of the United States (PRE-STORM 23–24 June).
Abstract
A number of large eddy simulations with the Regional Atmospheric Modeling System have been made to study the sensitivity of shallow marine cumulus convection to different microphysics and radiation schemes. In particular, the sensitivity of shallow marine cumulus convection to drizzle and radiation effects, and how drizzle and radiation modify turbulent fluxes, are investigated.
It is shown that for the case of prescribed radiative heating, drizzle—albeit very slight—leads to reduced buoyancy fluxes and less turbulence. Consequently, drizzling boundary layers appear to entrain less than their nondrizzling counterpart. Heavy drizzle events are simulated in association with deeper clouds as high as 2 km, even though the majority of clouds are only a few hundred meters deep. A heavier and longer lasting drizzle episode associated with a deeper boundary layer is produced when a two-stream radiative parameterization replaces the prescribed radiative heating in the simulation. Simulated surface precipitation rates agree reasonably well with observations. The greatest alteration in boundary layer structure is obtained when radiative heating interacts explicitly with the broadened drop distribution associated with drizzle formation.
Abstract
A number of large eddy simulations with the Regional Atmospheric Modeling System have been made to study the sensitivity of shallow marine cumulus convection to different microphysics and radiation schemes. In particular, the sensitivity of shallow marine cumulus convection to drizzle and radiation effects, and how drizzle and radiation modify turbulent fluxes, are investigated.
It is shown that for the case of prescribed radiative heating, drizzle—albeit very slight—leads to reduced buoyancy fluxes and less turbulence. Consequently, drizzling boundary layers appear to entrain less than their nondrizzling counterpart. Heavy drizzle events are simulated in association with deeper clouds as high as 2 km, even though the majority of clouds are only a few hundred meters deep. A heavier and longer lasting drizzle episode associated with a deeper boundary layer is produced when a two-stream radiative parameterization replaces the prescribed radiative heating in the simulation. Simulated surface precipitation rates agree reasonably well with observations. The greatest alteration in boundary layer structure is obtained when radiative heating interacts explicitly with the broadened drop distribution associated with drizzle formation.
Abstract
A two-dimensional, nonhydrostatic version of the Colorado State University Regional Atmospheric Modeling System (RAMS) was applied to the simulation of a midlatitude, continental mesoscale convective system. A control simulation was established that exhibited many features in common with observed MCSs. The sensitivity experiments revealed that the mesoscale circulations in the stratiform region were quite sensitive to longwave radiative cooling at stratiform anvil cloud top and heating at cloud base, and the presence of ice-phase precipitation processes and thermodynamics. Surface precipitation was little affected by radiational heating, however.
Turning off melting of ice particles had little influence on the strength of simulated mesoscale and convective-scale downdrafts and the strength of the middle-level, rear-to-front flow. Melting had the greatest impact on the strength of the convective scale updrafts. These numerical experiments suggest that the strength of the middle-level, rear-to-front flow is primarily modulated by heating in the upper troposphere and by the strength of the low-level, convective scale “up-down” downdraft component.
Abstract
A two-dimensional, nonhydrostatic version of the Colorado State University Regional Atmospheric Modeling System (RAMS) was applied to the simulation of a midlatitude, continental mesoscale convective system. A control simulation was established that exhibited many features in common with observed MCSs. The sensitivity experiments revealed that the mesoscale circulations in the stratiform region were quite sensitive to longwave radiative cooling at stratiform anvil cloud top and heating at cloud base, and the presence of ice-phase precipitation processes and thermodynamics. Surface precipitation was little affected by radiational heating, however.
Turning off melting of ice particles had little influence on the strength of simulated mesoscale and convective-scale downdrafts and the strength of the middle-level, rear-to-front flow. Melting had the greatest impact on the strength of the convective scale updrafts. These numerical experiments suggest that the strength of the middle-level, rear-to-front flow is primarily modulated by heating in the upper troposphere and by the strength of the low-level, convective scale “up-down” downdraft component.
Abstract
A three-dimensional numerical simulation of an intense, quasi-steady left-moving thunderstorm observed over mountainous terrain is presented. The observational analysis of the evolution of convection leading to this storm is presented in Part I, and a detailed analysis of the Doppler radar-observed storm structure is presented in Parts II and III. This storm was particularly interesting because it initially grew in an environment characterized by terrain-induced boundary layer convergence before a massive mesoscale cold front passed underneath. The front cooled and moistened low levels while veering the surface winds to the north, creating a hodograph of winds strongly backing with height. After frontal passage the initial storm cell grew explosively and turned to the left.
The observed storm evolution after the frontal passage was reproduced well by the numerical simulation. An observed secondary updraft which was not simulated, was attributed to residual effects of the prefrontal environment, which was not considered. The overall success of this simulation led to the conclusion that the storm structure was largely governed by the environmental wind shear and was only weakly influenced by its triggering mechanism.
The microphysical structure was reproduced only moderately well. The model had the greatest difficulty in simulating the echo intensity. This is attributed to the characteristics of the assumed Marshall–Palmer graupel distribution. However, no apparent degrading effects on the dynamical structure were found as a result.
The dynamical structure compared well with that of right-moving cells described observationally and simulated numerically by a number of authors. In particular, it was found that the leftward movement was induced by pressure forces projected to low levels within an anticyclonically rotating updraft in approximate cyclostrophic balance. The rotation was produced by the tilting of horizontal voracity (associated with the wind shear) into the vertical and subsequent stretching.
Trajectory analysis of updraft and downdraft parcels revealed the existence of both entrainment and pressure-forced downdrafts. It is demonstrated that much of the vertical pressure gradient acceleration of parcels may be accounted for by pressure in approximate hydrostatic equilibrium with the mean density anomaly of the local environment surrounding the parcel.
Abstract
A three-dimensional numerical simulation of an intense, quasi-steady left-moving thunderstorm observed over mountainous terrain is presented. The observational analysis of the evolution of convection leading to this storm is presented in Part I, and a detailed analysis of the Doppler radar-observed storm structure is presented in Parts II and III. This storm was particularly interesting because it initially grew in an environment characterized by terrain-induced boundary layer convergence before a massive mesoscale cold front passed underneath. The front cooled and moistened low levels while veering the surface winds to the north, creating a hodograph of winds strongly backing with height. After frontal passage the initial storm cell grew explosively and turned to the left.
The observed storm evolution after the frontal passage was reproduced well by the numerical simulation. An observed secondary updraft which was not simulated, was attributed to residual effects of the prefrontal environment, which was not considered. The overall success of this simulation led to the conclusion that the storm structure was largely governed by the environmental wind shear and was only weakly influenced by its triggering mechanism.
The microphysical structure was reproduced only moderately well. The model had the greatest difficulty in simulating the echo intensity. This is attributed to the characteristics of the assumed Marshall–Palmer graupel distribution. However, no apparent degrading effects on the dynamical structure were found as a result.
The dynamical structure compared well with that of right-moving cells described observationally and simulated numerically by a number of authors. In particular, it was found that the leftward movement was induced by pressure forces projected to low levels within an anticyclonically rotating updraft in approximate cyclostrophic balance. The rotation was produced by the tilting of horizontal voracity (associated with the wind shear) into the vertical and subsequent stretching.
Trajectory analysis of updraft and downdraft parcels revealed the existence of both entrainment and pressure-forced downdrafts. It is demonstrated that much of the vertical pressure gradient acceleration of parcels may be accounted for by pressure in approximate hydrostatic equilibrium with the mean density anomaly of the local environment surrounding the parcel.
Abstract
In order to simulate the stratocumulus-capped mixed layer, a one-dimensional stratocumulus model is developed. This model consists of five major points: 1) a one-dimensional (1D) option of the CSU Cloud/Mesoscale Model, 2) a partially diagnostic higher-order turbulence model, 3) an atmospheric radiation model, 4) a partial condensation parameterization, and 5) the drizzle process.
This model is tested against the observed structure of the marine stratocumulus layer reported by Brost et al. In this paper we also investigate the interactions among the following physical processes: atmospheric radiation, cloud microphysics, vertical wind shear, turbulent mixing, large-scale divergence, the sea surface temperature and the presence of high-level clouds above the capping inversion.
The model simulated fields were found to be in generally good agreement with observations, although the amount of cloud liquid water predicted was too large. This may have been a result of employing a wind profile that exhibits somewhat weaker shear than observed, since the sensitivity experiment with an unbalanced wind similar to that observed produced liquid water contents similar to the observed values.
It is also found that drizzle precipitation greatly alters the liquid water content of the cloud and the rate of radiative cooling. This then feeds back into the turbulence structure of the cloud.
For the case with large-scale subsidence and the presence of high-level clouds above the capping inversion, the effect of cloud top radiative cooling is found to become less important.
Longer time integrations (up to 6 hours) revealed a 15 to 20 min periodicity in cloud top entrainment. The length of the period of oscillation was regulated by the magnitude of shear and the presence of drizzle. Complete removal of shear and drizzle processes resulted in the elimination of sporadic entrainment.
Finally, sensitivity experiments were also conducted to examine the role of shortwave radiation. It is found that the influence of shortwave radiation on the cloud layer varies with the intensity of overlying large-scale subsidence and the moisture content of the airmass overlying the capping inversion.
Abstract
In order to simulate the stratocumulus-capped mixed layer, a one-dimensional stratocumulus model is developed. This model consists of five major points: 1) a one-dimensional (1D) option of the CSU Cloud/Mesoscale Model, 2) a partially diagnostic higher-order turbulence model, 3) an atmospheric radiation model, 4) a partial condensation parameterization, and 5) the drizzle process.
This model is tested against the observed structure of the marine stratocumulus layer reported by Brost et al. In this paper we also investigate the interactions among the following physical processes: atmospheric radiation, cloud microphysics, vertical wind shear, turbulent mixing, large-scale divergence, the sea surface temperature and the presence of high-level clouds above the capping inversion.
The model simulated fields were found to be in generally good agreement with observations, although the amount of cloud liquid water predicted was too large. This may have been a result of employing a wind profile that exhibits somewhat weaker shear than observed, since the sensitivity experiment with an unbalanced wind similar to that observed produced liquid water contents similar to the observed values.
It is also found that drizzle precipitation greatly alters the liquid water content of the cloud and the rate of radiative cooling. This then feeds back into the turbulence structure of the cloud.
For the case with large-scale subsidence and the presence of high-level clouds above the capping inversion, the effect of cloud top radiative cooling is found to become less important.
Longer time integrations (up to 6 hours) revealed a 15 to 20 min periodicity in cloud top entrainment. The length of the period of oscillation was regulated by the magnitude of shear and the presence of drizzle. Complete removal of shear and drizzle processes resulted in the elimination of sporadic entrainment.
Finally, sensitivity experiments were also conducted to examine the role of shortwave radiation. It is found that the influence of shortwave radiation on the cloud layer varies with the intensity of overlying large-scale subsidence and the moisture content of the airmass overlying the capping inversion.
Abstract
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 idealized supercell storm. The innermost grid possessed horizontal grid spacing of 111 m. The initial background profiles of CCN concentration and water vapor mixing ratio varied among the simulations (clean versus dusty and higher-moisture versus lower-moisture simulations). A fifth simulation was performed to factor out the impact of CAPE. The higher-moisture simulations produced spatially larger storms with stronger peak updrafts and low-level downdrafts, heavier precipitation, greater evaporative cooling, and stronger cold pools within the forward and rear flank downdrafts. Each simulated supercell produced a tornado-like vortex. However, the lower-moisture simulations produced stronger, longer-lived vortices, as they were associated with weaker cold pools and less negative buoyancy within the rear flank downdraft. Raindrop and hailstone concentrations (sizes) were reduced (increased) in the dusty simulations, resulting in less evaporative cooling and weaker cold pools compared to the clean simulations. With greater terminal fall speeds, the larger hydrometeors in the dusty simulations fell nearer to the storm’s core, positioning the cold pool closer to the main updraft. Tornadogenesis was related to the size, strength, and location of the cold pools produced by the forward and rear flank downdrafts. Not surprisingly, while the aerosol effect was evident, the influences of low-level moisture and CAPE had markedly larger impacts on tornadogenesis.
Abstract
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 idealized supercell storm. The innermost grid possessed horizontal grid spacing of 111 m. The initial background profiles of CCN concentration and water vapor mixing ratio varied among the simulations (clean versus dusty and higher-moisture versus lower-moisture simulations). A fifth simulation was performed to factor out the impact of CAPE. The higher-moisture simulations produced spatially larger storms with stronger peak updrafts and low-level downdrafts, heavier precipitation, greater evaporative cooling, and stronger cold pools within the forward and rear flank downdrafts. Each simulated supercell produced a tornado-like vortex. However, the lower-moisture simulations produced stronger, longer-lived vortices, as they were associated with weaker cold pools and less negative buoyancy within the rear flank downdraft. Raindrop and hailstone concentrations (sizes) were reduced (increased) in the dusty simulations, resulting in less evaporative cooling and weaker cold pools compared to the clean simulations. With greater terminal fall speeds, the larger hydrometeors in the dusty simulations fell nearer to the storm’s core, positioning the cold pool closer to the main updraft. Tornadogenesis was related to the size, strength, and location of the cold pools produced by the forward and rear flank downdrafts. Not surprisingly, while the aerosol effect was evident, the influences of low-level moisture and CAPE had markedly larger impacts on tornadogenesis.
Abstract
A three-dimensional model of deep, moist convection is described. The model is fully compressible and utilizes a “time-splitting” method of integration in order to make the model economically feasible.
This study represents an extension of the numerical experiments reported by Cotton (1975). In that work the profiles of the ratio of average cloud water content to the moist-adiabatic water content (Q̄c /QA ) predicted by a one-dimensional Lagrangian (1DL) and a one-dimensional time-dependent (1DTD) model are compared with case study observed data and the average Q̄c /QA profiles reported by Warner (1970a). In this work, data predicted by a three-dimensional (3D) cloud simulation in a stagnant environment and a 3D cloud simulation in the observed shear flow are compared with observed data and the earlier model calculations. The results of this study demonstrated that all the cloud simulations in an initially stagnant environment, including the 1DL, 1DTD and 3D models, predicted profiles of Q̄c /QA which exhibited very high magnitudes near the top of the rising cloud. The predicted magnitudes of Q̄c /QA near the top of the rising cloud exceeded the observed magnitude by as much as a factor of 3. In contrast, the 3D simulation in the observed shear flow predicted profiles of Q̄c /QA and magnitudes of peak Q̄c /QA which were in good agreement with observations.
What is most surprising is that the improved prediction of cloud liquid water content was not at the expense of the prediction of cloud-top height. Instead the cloud-top heights predicted in both the no-motion and shear-flow simulations were identical and equal to the observed cloud-top height. This is in contrast to the earlier 1DL and 1DTD model numerical experiments reported by Cotton using the same sounding. In those calculations, predicted cloud-top height varied considerably (over several kilometers) with different entrainment rates and eddy exchange coefficients. As a further benefit, the prediction of cloud-scale averaged vertical velocity in the shear-flow simulation was also better than that predicted in the no-motion simulation.
It is thus concluded that the interaction of a cumulus cloud with an environment characterized by vertical shear of the horizontal wind is a major control on the prediction of cloud internal properties. Associated with the improved prediction of Q̄c /QA , the 3D simulation in shear flow also exhibited major changes in the structure of the cloud circulation. A particularly interesting feature was the formation of rotating cloud elements in several portions of the main cloud element.
Abstract
A three-dimensional model of deep, moist convection is described. The model is fully compressible and utilizes a “time-splitting” method of integration in order to make the model economically feasible.
This study represents an extension of the numerical experiments reported by Cotton (1975). In that work the profiles of the ratio of average cloud water content to the moist-adiabatic water content (Q̄c /QA ) predicted by a one-dimensional Lagrangian (1DL) and a one-dimensional time-dependent (1DTD) model are compared with case study observed data and the average Q̄c /QA profiles reported by Warner (1970a). In this work, data predicted by a three-dimensional (3D) cloud simulation in a stagnant environment and a 3D cloud simulation in the observed shear flow are compared with observed data and the earlier model calculations. The results of this study demonstrated that all the cloud simulations in an initially stagnant environment, including the 1DL, 1DTD and 3D models, predicted profiles of Q̄c /QA which exhibited very high magnitudes near the top of the rising cloud. The predicted magnitudes of Q̄c /QA near the top of the rising cloud exceeded the observed magnitude by as much as a factor of 3. In contrast, the 3D simulation in the observed shear flow predicted profiles of Q̄c /QA and magnitudes of peak Q̄c /QA which were in good agreement with observations.
What is most surprising is that the improved prediction of cloud liquid water content was not at the expense of the prediction of cloud-top height. Instead the cloud-top heights predicted in both the no-motion and shear-flow simulations were identical and equal to the observed cloud-top height. This is in contrast to the earlier 1DL and 1DTD model numerical experiments reported by Cotton using the same sounding. In those calculations, predicted cloud-top height varied considerably (over several kilometers) with different entrainment rates and eddy exchange coefficients. As a further benefit, the prediction of cloud-scale averaged vertical velocity in the shear-flow simulation was also better than that predicted in the no-motion simulation.
It is thus concluded that the interaction of a cumulus cloud with an environment characterized by vertical shear of the horizontal wind is a major control on the prediction of cloud internal properties. Associated with the improved prediction of Q̄c /QA , the 3D simulation in shear flow also exhibited major changes in the structure of the cloud circulation. A particularly interesting feature was the formation of rotating cloud elements in several portions of the main cloud element.
Abstract
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 intensification, during which its form changed from an elongated patch to a compact center. This transition was also accompanied by a sudden decrease in pressure (to 12 mb below that of the neighboring flow), and was found to occur even in the absence of nested grids.
It is shown that the stage of strong intensification does not begin aloft, as in the dynamic pipe effect, and then descend to the surface. Rather, the vortex is initiated near the surface, and then builds upward. The process is completed in 5 min, and the final vortex can be clearly distinguished from the larger-scale mesocyclone at the cloud base. The reduction of pressure can be explained as a consequence of the evacuation of mass in the horizontal convergence equation. This is in contrast to axisymmetric models of vortex intensification, which generally rely on the evacuation of mass in the vertical divergence equation. In the latter cases a positive horizontal convergence tendency is what initiates the concentrated vortex. However, nondivergent models prove that vorticity concentration can occur in the absence of any horizontal convergence. Here the concentration is associated with a negative horizontal convergence tendency.
Abstract
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 intensification, during which its form changed from an elongated patch to a compact center. This transition was also accompanied by a sudden decrease in pressure (to 12 mb below that of the neighboring flow), and was found to occur even in the absence of nested grids.
It is shown that the stage of strong intensification does not begin aloft, as in the dynamic pipe effect, and then descend to the surface. Rather, the vortex is initiated near the surface, and then builds upward. The process is completed in 5 min, and the final vortex can be clearly distinguished from the larger-scale mesocyclone at the cloud base. The reduction of pressure can be explained as a consequence of the evacuation of mass in the horizontal convergence equation. This is in contrast to axisymmetric models of vortex intensification, which generally rely on the evacuation of mass in the vertical divergence equation. In the latter cases a positive horizontal convergence tendency is what initiates the concentrated vortex. However, nondivergent models prove that vorticity concentration can occur in the absence of any horizontal convergence. Here the concentration is associated with a negative horizontal convergence tendency.
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.
Abstract
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 μm in diameter) into the liquid-droplet spectrum and the parameterization of cloud-droplet nucleation through activation of cloud condensation nuclei (CCN) and giant CCN (GCCN). The large-droplet mode was introduced to represent more precisely the natural dual mode of the cloud-droplet distribution. The parameterized droplet nucleation replaces the former estimation of cloud-droplet formation solely from supersaturation calculations. In Part I of this series, details of the improvements to the microphysics were presented, including the set of equations governing the development of cloud droplets in the Lagrangian parcel model that was employed to parameterize this complex process. Supercell simulations were examined with respect to the model sensitivity to the presence and concentration of large cloud droplets, CCN, and GCCN. Part II examines the sensitivity of the model microphysics to imposed aerosol variations in a wintertime snowfall event that occurred over Colorado on 28–29 February 2004. Model analyses and sensitivity are compared with the real-time forecast version 4.3 of RAMS as well as selected snowpack telemetry (SNOTEL) accumulated precipitation data and surface data from Storm Peak Laboratory in Steamboat Springs, Colorado.
Abstract
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 μm in diameter) into the liquid-droplet spectrum and the parameterization of cloud-droplet nucleation through activation of cloud condensation nuclei (CCN) and giant CCN (GCCN). The large-droplet mode was introduced to represent more precisely the natural dual mode of the cloud-droplet distribution. The parameterized droplet nucleation replaces the former estimation of cloud-droplet formation solely from supersaturation calculations. In Part I of this series, details of the improvements to the microphysics were presented, including the set of equations governing the development of cloud droplets in the Lagrangian parcel model that was employed to parameterize this complex process. Supercell simulations were examined with respect to the model sensitivity to the presence and concentration of large cloud droplets, CCN, and GCCN. Part II examines the sensitivity of the model microphysics to imposed aerosol variations in a wintertime snowfall event that occurred over Colorado on 28–29 February 2004. Model analyses and sensitivity are compared with the real-time forecast version 4.3 of RAMS as well as selected snowpack telemetry (SNOTEL) accumulated precipitation data and surface data from Storm Peak Laboratory in Steamboat Springs, Colorado.
