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- Author or Editor: Brad Schoenberg Ferrier x
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Abstract
A detailed ice-phase bulk microphysical scheme has been developed for simulating the hydrometeor distributions of convective and stratiform precipitation in different large-scale environmental conditions. The proposed scheme involves 90 distinct microphysical processes, which predict the mixing ratios and the number concentrations of small ice crystals, snow, graupel, and frozen drops/hail, as well as the mixing ratios of liquid water on wet precipitation ice (snow, graupel, frozen drops). The number of adjustable coefficients has been significantly reduced in comparison with other bulk schemes. Additional improvements have been made to the parameterization in the following areas: 1) representing small ice crystals with nonzero terminal fall velocities and dispersive size distributions, 2) accurate and computationally efficient calculations of precipitation collection processes, 3) reformulating the collection equation to prevent unrealistically large accretion rates, 4) more realistic conversion by riming between different classes of precipitation ice, 5) preventing unrealistically large rates of raindrop freezing and freezing of liquid water on ice, 6) detailed treatment of various rime-splintering ice multiplication mechanisms, 7) a simple representation of the Hobbs-Rangno ice enhancement process, 8) aggregation of small ice crystals and snow, and 9) allowing explicit competition between cloud water condensation and ice deposition rates rather than using saturation adjustment techniques. For the purposes of conserving the higher moments of the particle distributions, preserving the spectral widths (or slopes) of the particle spectra is shown to be more important than strict conservation of particle number concentration when parameterizing changes in ice-particle number concentrations due to melting, vapor transfer processes (sublimation of dry ice, evaporation from wet ice), and conversion between different hydrometeor species.
The microphysical scheme is incorporated into a nonhydrostatic cloud model in Part II of this study. The model performed well in simulating the radar and microphysical structures of a midlatitude–continental squall line and a tropical–maritime squall system with minimal tuning of the parameterization, even though the vertical profiles of radar reflectivity differed substantially between these storms.
Abstract
A detailed ice-phase bulk microphysical scheme has been developed for simulating the hydrometeor distributions of convective and stratiform precipitation in different large-scale environmental conditions. The proposed scheme involves 90 distinct microphysical processes, which predict the mixing ratios and the number concentrations of small ice crystals, snow, graupel, and frozen drops/hail, as well as the mixing ratios of liquid water on wet precipitation ice (snow, graupel, frozen drops). The number of adjustable coefficients has been significantly reduced in comparison with other bulk schemes. Additional improvements have been made to the parameterization in the following areas: 1) representing small ice crystals with nonzero terminal fall velocities and dispersive size distributions, 2) accurate and computationally efficient calculations of precipitation collection processes, 3) reformulating the collection equation to prevent unrealistically large accretion rates, 4) more realistic conversion by riming between different classes of precipitation ice, 5) preventing unrealistically large rates of raindrop freezing and freezing of liquid water on ice, 6) detailed treatment of various rime-splintering ice multiplication mechanisms, 7) a simple representation of the Hobbs-Rangno ice enhancement process, 8) aggregation of small ice crystals and snow, and 9) allowing explicit competition between cloud water condensation and ice deposition rates rather than using saturation adjustment techniques. For the purposes of conserving the higher moments of the particle distributions, preserving the spectral widths (or slopes) of the particle spectra is shown to be more important than strict conservation of particle number concentration when parameterizing changes in ice-particle number concentrations due to melting, vapor transfer processes (sublimation of dry ice, evaporation from wet ice), and conversion between different hydrometeor species.
The microphysical scheme is incorporated into a nonhydrostatic cloud model in Part II of this study. The model performed well in simulating the radar and microphysical structures of a midlatitude–continental squall line and a tropical–maritime squall system with minimal tuning of the parameterization, even though the vertical profiles of radar reflectivity differed substantially between these storms.
Abstract
A one-dimensional time-dependent cumulonimbus model is designed that, unlike in previous one-dimensional models, simulates cloud-top heights, vertical velocities, and water contents that are reasonably consistent with those observed in real convective cores. The model successfully simulates deep tropical oceanic cumulonimbus with results that are in agreement with aircraft observations of vertical velocity, observations of radar reflectivity, and three-dimensional model simulations. These results are achieved by improving the parameterizations of the following physical processes: vertical mixing through the inclusion of an overturning thermal circulation near cloud top, lateral entrainment by modifying the assumed shape of the cloud, initiating convection with sustained boundary-layer forcing that resembles the lifting by gust fronts associated with tropical oceanic cumulonimbus, and making the pressure perturbation internally consistent with the horizontal distribution of vertical velocity in the cloud. The effect of a tilted updraft on precipitation fallout and enhanced cloud growth are also examined.
Abstract
A one-dimensional time-dependent cumulonimbus model is designed that, unlike in previous one-dimensional models, simulates cloud-top heights, vertical velocities, and water contents that are reasonably consistent with those observed in real convective cores. The model successfully simulates deep tropical oceanic cumulonimbus with results that are in agreement with aircraft observations of vertical velocity, observations of radar reflectivity, and three-dimensional model simulations. These results are achieved by improving the parameterizations of the following physical processes: vertical mixing through the inclusion of an overturning thermal circulation near cloud top, lateral entrainment by modifying the assumed shape of the cloud, initiating convection with sustained boundary-layer forcing that resembles the lifting by gust fronts associated with tropical oceanic cumulonimbus, and making the pressure perturbation internally consistent with the horizontal distribution of vertical velocity in the cloud. The effect of a tilted updraft on precipitation fallout and enhanced cloud growth are also examined.
Abstract
Different definitions of storm precipitation efficiency were investigated from numerical simulators of convective systems in widely varying ambient conditions using a two-dimensional cloud model with sophisticated ice microphysics. The model results indicate that the vertical orientation of the updrafts, which is controlled by the vertical wind shear, and the ambient moisture content are important in determining storm efficiency.
In terms of rainfall divided by condensation, simulated efficiencies ranged from 20%–35% for convective systems that tilted strongly against the low-level shear (upshear), to 40%–50% for erect storms. Changes in environmental moisture produced smaller variations in efficiency that were less than 10%. Upright convection allows for effective collection of cloud condensate by precipitation, whereas lower efficiencies in upshear storms are due to greater evaporation of cloud at middle levels and evaporation of rain at lower levels. Development of trailing stratiform precipitation is promoted by the rearward transport of moisture and condensate in upshear-tilted updrafts with evaporation moistening the ambient air as it passes through the convection. The fraction of rainfall from stratiform processes increases with upshear tilt of the convection and is inefficient. Rainfall from convection tilting downshear is efficient in terms of the total condensation, but is inefficient in terms of the flux of vapor into the storm because the gust fronts are too weak to completely block the low-level inflow.
Different closure assumptions in cumulus parameterization schemes that use functional relationships for precipitation efficiency were evaluated. None of them showed consistent agreement with the efficiency parameters diagnosed from the simulations.
Detailed diagnostics over various temporal and spatial scales indicate that storm efficiency determined by total condensation varied much less than that obtained from moisture convergence. The former definition should be more useful in cumulus parameterizations. Spatial variations in moisture convergence were dominated by changes in net condensation within the area of the storm, while variability at larger scales resulted from the advection of dry air in downdraft wakes.
Abstract
Different definitions of storm precipitation efficiency were investigated from numerical simulators of convective systems in widely varying ambient conditions using a two-dimensional cloud model with sophisticated ice microphysics. The model results indicate that the vertical orientation of the updrafts, which is controlled by the vertical wind shear, and the ambient moisture content are important in determining storm efficiency.
In terms of rainfall divided by condensation, simulated efficiencies ranged from 20%–35% for convective systems that tilted strongly against the low-level shear (upshear), to 40%–50% for erect storms. Changes in environmental moisture produced smaller variations in efficiency that were less than 10%. Upright convection allows for effective collection of cloud condensate by precipitation, whereas lower efficiencies in upshear storms are due to greater evaporation of cloud at middle levels and evaporation of rain at lower levels. Development of trailing stratiform precipitation is promoted by the rearward transport of moisture and condensate in upshear-tilted updrafts with evaporation moistening the ambient air as it passes through the convection. The fraction of rainfall from stratiform processes increases with upshear tilt of the convection and is inefficient. Rainfall from convection tilting downshear is efficient in terms of the total condensation, but is inefficient in terms of the flux of vapor into the storm because the gust fronts are too weak to completely block the low-level inflow.
Different closure assumptions in cumulus parameterization schemes that use functional relationships for precipitation efficiency were evaluated. None of them showed consistent agreement with the efficiency parameters diagnosed from the simulations.
Detailed diagnostics over various temporal and spatial scales indicate that storm efficiency determined by total condensation varied much less than that obtained from moisture convergence. The former definition should be more useful in cumulus parameterizations. Spatial variations in moisture convergence were dominated by changes in net condensation within the area of the storm, while variability at larger scales resulted from the advection of dry air in downdraft wakes.
Abstract
Part I of this study described a detailed four-class bulk ice scheme (4ICE) developed to simulate the hydro-meteor profiles of convective and stratiform precipitation associated with mesoscale convective systems. In Part II, the 4ICE scheme is incorporated into the Goddard Cumulus Ensemble (GCE) model and applied without any “tuning” to two squall lines occurring in widely different environments, namely, one over the “Pica) ocean in the Global Atmospheric Research Program's (GARP) Atlantic Tropical Experiment (GATE) and the other over a midlatitude continent in the Cooperative Huntsville Meteorological Experiment (COHMEX). Comparisons were made both with earlier three-class ice formulations and with observations. In both cases, the 4ICE scheme interacted with the dynamics so as to resemble the observations much more closely than did the model runs with either of the three-class ice parameterizations. The following features were well simulated in the COHMEX case: a lack of stratiform rain at the surface ahead of the storm, reflectivity maxima near 60 dBZ in the vicinity of the melting level, and intense radar echoes up to near the tropopause. These features were in strong contrast with the GATE simulation, which showed extensive trailing stratiform precipitation containing a horizontally oriented radar bright band. Peak reflectivities were below the melting level, rarely exceeding 50 dBz, with a steady decrease in reflectivity with height above. With the other bulk formulations, the large stratiform rain areas were not reproduced in the GATE conditions.
The microphysical structure of the model clouds in both environments were more realistic than that of earlier modeling efforts. Number concentrations of ice of O(100 L−1) occurred above 6 km in the GATE model clouds as a result of ice enhancement and rime splintering in the 4ICE runs. These processes were more effective in the GATE simulation, because near the freezing level the weaker updrafts were comparable in magnitude to the fall speeds of newly frozen drops. Many of the ice crystals initiated at relatively warm temperatures (above −15°C) grew rapidly by deposition into sizes large enough to be converted to snow. In contrast, in the more intense COHMEX updrafts, very large numbers of small ice crystals were initiated at colder temperatures (below −15°C) by nucleation and stochastic freezing of droplets, such that relatively few ice crystals grew by deposition to sizes large enough to be converted to snow. In addition, the large number of frozen drops of O(5 L−1) in the 4ICE run am consistent with airborne microphysical data in intense COHMEX updrafts.
Numerous sensitivity experiments were made with the four-class and three-class ice schemes, varying fall speed relationships, particle characteristics, and ice collection efficiencies. These tests provide strong support to the conclusion that the 4ICE scheme gives improved resemblance to observations despite present uncertainties in a number of important microphysical parameters.
Abstract
Part I of this study described a detailed four-class bulk ice scheme (4ICE) developed to simulate the hydro-meteor profiles of convective and stratiform precipitation associated with mesoscale convective systems. In Part II, the 4ICE scheme is incorporated into the Goddard Cumulus Ensemble (GCE) model and applied without any “tuning” to two squall lines occurring in widely different environments, namely, one over the “Pica) ocean in the Global Atmospheric Research Program's (GARP) Atlantic Tropical Experiment (GATE) and the other over a midlatitude continent in the Cooperative Huntsville Meteorological Experiment (COHMEX). Comparisons were made both with earlier three-class ice formulations and with observations. In both cases, the 4ICE scheme interacted with the dynamics so as to resemble the observations much more closely than did the model runs with either of the three-class ice parameterizations. The following features were well simulated in the COHMEX case: a lack of stratiform rain at the surface ahead of the storm, reflectivity maxima near 60 dBZ in the vicinity of the melting level, and intense radar echoes up to near the tropopause. These features were in strong contrast with the GATE simulation, which showed extensive trailing stratiform precipitation containing a horizontally oriented radar bright band. Peak reflectivities were below the melting level, rarely exceeding 50 dBz, with a steady decrease in reflectivity with height above. With the other bulk formulations, the large stratiform rain areas were not reproduced in the GATE conditions.
The microphysical structure of the model clouds in both environments were more realistic than that of earlier modeling efforts. Number concentrations of ice of O(100 L−1) occurred above 6 km in the GATE model clouds as a result of ice enhancement and rime splintering in the 4ICE runs. These processes were more effective in the GATE simulation, because near the freezing level the weaker updrafts were comparable in magnitude to the fall speeds of newly frozen drops. Many of the ice crystals initiated at relatively warm temperatures (above −15°C) grew rapidly by deposition into sizes large enough to be converted to snow. In contrast, in the more intense COHMEX updrafts, very large numbers of small ice crystals were initiated at colder temperatures (below −15°C) by nucleation and stochastic freezing of droplets, such that relatively few ice crystals grew by deposition to sizes large enough to be converted to snow. In addition, the large number of frozen drops of O(5 L−1) in the 4ICE run am consistent with airborne microphysical data in intense COHMEX updrafts.
Numerous sensitivity experiments were made with the four-class and three-class ice schemes, varying fall speed relationships, particle characteristics, and ice collection efficiencies. These tests provide strong support to the conclusion that the 4ICE scheme gives improved resemblance to observations despite present uncertainties in a number of important microphysical parameters.
