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- Author or Editor: Man Kong Yau x
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Abstract
A two-cylinder model suitable for computing vertical transports in cumulus cells is formulated. The model includes explicit computation of perturbation pressure and allows the study of the evolution of raindrop size spectra. Sensitivity tests were performed to determine the response of the model to different assumptions. Analysis of the moisture and heat budgets yields information on the effect of cumulus convection on the larger scale environment. Results of applying the model to a computation of heat and mass transports by cumulus cells are presented. Comparison of the present results for Boston in the month of July with those obtained by Houze (1973) indicates that the two models give comparable total heat transports in the region of the cloud column. The level of maximum heat transport is noticeably lower in the present computation.
Abstract
A two-cylinder model suitable for computing vertical transports in cumulus cells is formulated. The model includes explicit computation of perturbation pressure and allows the study of the evolution of raindrop size spectra. Sensitivity tests were performed to determine the response of the model to different assumptions. Analysis of the moisture and heat budgets yields information on the effect of cumulus convection on the larger scale environment. Results of applying the model to a computation of heat and mass transports by cumulus cells are presented. Comparison of the present results for Boston in the month of July with those obtained by Houze (1973) indicates that the two models give comparable total heat transports in the region of the cloud column. The level of maximum heat transport is noticeably lower in the present computation.
Abstract
Steiner's (1973) three-dimensional cloud model has been updated to incorporate the deep anelastic system of equations and the microphysical parameterization scheme of Kessler (1969) for precipitation growth. The interactions between cloud microphysics and dynamics for an isolated cumulus cloud in a unidirectional shearing environment has been examined in four comparative experiments. The results confirm that 1) the intensity of convection is suppressed by shear except during the initial stage, and 2) evaporative cooling can have a more dominant effect than water loading in limiting the vertical development of a moderate size cumulus.
Abstract
Steiner's (1973) three-dimensional cloud model has been updated to incorporate the deep anelastic system of equations and the microphysical parameterization scheme of Kessler (1969) for precipitation growth. The interactions between cloud microphysics and dynamics for an isolated cumulus cloud in a unidirectional shearing environment has been examined in four comparative experiments. The results confirm that 1) the intensity of convection is suppressed by shear except during the initial stage, and 2) evaporative cooling can have a more dominant effect than water loading in limiting the vertical development of a moderate size cumulus.
Abstract
A simple model of perturbation pressure in cumulus convection is presented. The results show that the buoyancy- and drag-induced perturbation pressures act against the prescribed forcing. The dynamic pressure is found to be a consequence of the Bernoulli effect and a balance for the centrifugal force which arises from the curved motion of the air. Quantitative calculations reveal that the pressure force is of the same order of magnitude as the buoyancy and drag and offers a plausible explanation for the acceleration of negatively buoyant air near the base of convective updrafts.
Abstract
A simple model of perturbation pressure in cumulus convection is presented. The results show that the buoyancy- and drag-induced perturbation pressures act against the prescribed forcing. The dynamic pressure is found to be a consequence of the Bernoulli effect and a balance for the centrifugal force which arises from the curved motion of the air. Quantitative calculations reveal that the pressure force is of the same order of magnitude as the buoyancy and drag and offers a plausible explanation for the acceleration of negatively buoyant air near the base of convective updrafts.
Abstract
An analysis of 5 min resolution Quadra data on day 261 of GATE, (0953–1451 GMT) is made to yield statistics of maximum area, echo top, lifetime and maximum reflectivity factor in medium-sized convective cells. The results, obtained by tracking 140 echoes throughout their lifetime, indicate that the maximum area is log-normally distributed, 90% of the echoes being smaller than 40 km2 and existing less than 60 min. The modes of the maximum echo top and maximum reflectivity factor distributions are around 2.5 km and 30 dBZ, respectively. Further stratification of the data according to minimum edge to edge separation (d) reveals that merging cells (d = 0) have an average lifetime three times longer and a maximum area five times larger than isolated ones (d > 7 km). For a fixed maximum area, however, echo parameters generally decrease with decreasing d.
A fully three-dimensional cloud model including precipitation processes is used to simulate the development of an isolated and two adjacent cells. Comparison of modeled and observed echo parameters indicates a fair degree of realism in the simulations. The computed maximum reflectivity factor, however, is considerably higher than that of the observations because of the unrealistic drop-size distribution assumed in the model. Results of two cloud simulations suggest that the alignment of the clouds in relation to the wind-shear vector is an important factor in addition to d in determining the intensity of cloud development. The upshear cell of the parallel clouds, even with a small d value, behaves similarly to an isolated one. The suppression experienced by adjacent cells is attributed to the reduced low-level moisture convergence.
Abstract
An analysis of 5 min resolution Quadra data on day 261 of GATE, (0953–1451 GMT) is made to yield statistics of maximum area, echo top, lifetime and maximum reflectivity factor in medium-sized convective cells. The results, obtained by tracking 140 echoes throughout their lifetime, indicate that the maximum area is log-normally distributed, 90% of the echoes being smaller than 40 km2 and existing less than 60 min. The modes of the maximum echo top and maximum reflectivity factor distributions are around 2.5 km and 30 dBZ, respectively. Further stratification of the data according to minimum edge to edge separation (d) reveals that merging cells (d = 0) have an average lifetime three times longer and a maximum area five times larger than isolated ones (d > 7 km). For a fixed maximum area, however, echo parameters generally decrease with decreasing d.
A fully three-dimensional cloud model including precipitation processes is used to simulate the development of an isolated and two adjacent cells. Comparison of modeled and observed echo parameters indicates a fair degree of realism in the simulations. The computed maximum reflectivity factor, however, is considerably higher than that of the observations because of the unrealistic drop-size distribution assumed in the model. Results of two cloud simulations suggest that the alignment of the clouds in relation to the wind-shear vector is an important factor in addition to d in determining the intensity of cloud development. The upshear cell of the parallel clouds, even with a small d value, behaves similarly to an isolated one. The suppression experienced by adjacent cells is attributed to the reduced low-level moisture convergence.
Abstract
Hill's (1974) method of random surface heating as an initiation mechanism for convection is incorporated into a three-dimensional cumulus cloud model. Results are presented on the development of a cumulus ensemble, the interactions of clouds in the population, and the mechanism for the successive generation of clouds in a shearing environment. Comparison of the characteristics of the simulated echoes with statistics of radar observations indicates fair agreement for echoes with an area less than 60 km2. A particularly good correspondence is demonstrated between the observed and simulated echo top distributions.
Abstract
Hill's (1974) method of random surface heating as an initiation mechanism for convection is incorporated into a three-dimensional cumulus cloud model. Results are presented on the development of a cumulus ensemble, the interactions of clouds in the population, and the mechanism for the successive generation of clouds in a shearing environment. Comparison of the characteristics of the simulated echoes with statistics of radar observations indicates fair agreement for echoes with an area less than 60 km2. A particularly good correspondence is demonstrated between the observed and simulated echo top distributions.
Abstract
A relatively simple model of warm and cold rain microphysics in cumulus cells is developed. The proposed model avoids the large amounts of computation required in a non-parameterized treatment and yet removes some of the restrictions imposed by Kessler's microphysical parameterization. The growth of cloud droplets is bypassed, but evolution of particle size spectra for larger hydrometeors and the effects of differential fallspeeds are allowed by the growth of rain and graupel particles in a total of 25 size categories. The processes included are condensation, evaporation, accretion, collection, breakup, freezing, deposition, riming and melting. Experiments in the context of a kinematic updraft indicate results comparable to those of a stochastic model in warm rain development. It is found that a counterbalancing mechanism between auto- conversion and accretion causes the results to be relatively insensitive to assumptions about the auto- conversion process. Further sensitivity tests point out the important contributions of rain-rain interactions in the evolution of drop-size spectra, the essential role of impaction breakup as a limiting mechanism for drop growth, and the modes in which the presence of graupel affects the particle-size distributions.
Abstract
A relatively simple model of warm and cold rain microphysics in cumulus cells is developed. The proposed model avoids the large amounts of computation required in a non-parameterized treatment and yet removes some of the restrictions imposed by Kessler's microphysical parameterization. The growth of cloud droplets is bypassed, but evolution of particle size spectra for larger hydrometeors and the effects of differential fallspeeds are allowed by the growth of rain and graupel particles in a total of 25 size categories. The processes included are condensation, evaporation, accretion, collection, breakup, freezing, deposition, riming and melting. Experiments in the context of a kinematic updraft indicate results comparable to those of a stochastic model in warm rain development. It is found that a counterbalancing mechanism between auto- conversion and accretion causes the results to be relatively insensitive to assumptions about the auto- conversion process. Further sensitivity tests point out the important contributions of rain-rain interactions in the evolution of drop-size spectra, the essential role of impaction breakup as a limiting mechanism for drop growth, and the modes in which the presence of graupel affects the particle-size distributions.
Abstract
This study performs cloud-resolving simulations of cumulus convection over an idealized surface-based convergence zone to investigate the mechanisms and sensitivities of deep convection initiation forced by mesoscale ascent. The surface convergence forms in response to a localized diurnal heating anomaly over an otherwise homogeneous and unheated surface, producing a strong boundary layer updraft over the center of the heat source. This updraft gives rise to a line of cumuli that gradually deepen and, in some cases, transition into deep convection. To statistically investigate the factors controlling this transition, a new thermal-tracking algorithm is developed to follow incipient cumulus cores as they ascend through the troposphere. This tool is used to isolate the impacts of key environmental parameters (cloud-layer lapse rate, midlevel humidity, etc.) and initial core parameters near cloud base (horizontal area, vertical velocity, etc.) on the ultimate cloud-top height. In general, the initial core size determines which thermals in a given cloud field will undergo the deepest ascent, and the sensitivity of cloud depth to initial core parameters increases in environments that are more hostile to deep convection. Diurnal midlevel moistening from detraining cumuli above the convergence line produces a small but robust enhancement in cloud-top height, particularly for smaller cores.
Abstract
This study performs cloud-resolving simulations of cumulus convection over an idealized surface-based convergence zone to investigate the mechanisms and sensitivities of deep convection initiation forced by mesoscale ascent. The surface convergence forms in response to a localized diurnal heating anomaly over an otherwise homogeneous and unheated surface, producing a strong boundary layer updraft over the center of the heat source. This updraft gives rise to a line of cumuli that gradually deepen and, in some cases, transition into deep convection. To statistically investigate the factors controlling this transition, a new thermal-tracking algorithm is developed to follow incipient cumulus cores as they ascend through the troposphere. This tool is used to isolate the impacts of key environmental parameters (cloud-layer lapse rate, midlevel humidity, etc.) and initial core parameters near cloud base (horizontal area, vertical velocity, etc.) on the ultimate cloud-top height. In general, the initial core size determines which thermals in a given cloud field will undergo the deepest ascent, and the sensitivity of cloud depth to initial core parameters increases in environments that are more hostile to deep convection. Diurnal midlevel moistening from detraining cumuli above the convergence line produces a small but robust enhancement in cloud-top height, particularly for smaller cores.
Abstract
In this study, we introduce a parameterization scheme for slantwise convection (SC) to be considered for models that are too coarse to resolve slantwise convection explicitly (with a horizontal grid spacing coarser than 15 km or less). This SC scheme operates in a locally defined 2D cross section perpendicular to the deep-layer-averaged thermal wind. It applies momentum tendency to adjust the environment toward slantwise neutrality with a prescribed adjustment time scale. Condensational heating and the associated moisture loss are also considered. To evaluate the added value of the SC scheme, we implement it in the Weather Research and Forecasting (WRF) Model to supplement the existing cumulus parameterization schemes for upright convection and test for two different numerical setups: a 2D idealized, unforced release of conditional symmetric instability (CSI) in an initially conditionally stable environment, and a 3D real-data precipitation event containing both CSI and conditional instability along the cold front of a cyclonic storm near the United Kingdom. Both test cases show significant improvements for the coarse-gridded (40-km) simulations when parameterizing slantwise convection. Compared to the 40-km simulations with only the upright convection scheme, the counterparts with the additional SC scheme exhibit a larger extent of CSI neutralization, generate a stronger grid-resolved slantwise circulation, and produce greater amounts of precipitation, all in better agreement with the corresponding fine-gridded reference simulations. Given the importance of slantwise convection in midlatitude weather systems, our results suggest that there exist potential benefits of parameterizing slantwise convection in general circulation models.
Abstract
In this study, we introduce a parameterization scheme for slantwise convection (SC) to be considered for models that are too coarse to resolve slantwise convection explicitly (with a horizontal grid spacing coarser than 15 km or less). This SC scheme operates in a locally defined 2D cross section perpendicular to the deep-layer-averaged thermal wind. It applies momentum tendency to adjust the environment toward slantwise neutrality with a prescribed adjustment time scale. Condensational heating and the associated moisture loss are also considered. To evaluate the added value of the SC scheme, we implement it in the Weather Research and Forecasting (WRF) Model to supplement the existing cumulus parameterization schemes for upright convection and test for two different numerical setups: a 2D idealized, unforced release of conditional symmetric instability (CSI) in an initially conditionally stable environment, and a 3D real-data precipitation event containing both CSI and conditional instability along the cold front of a cyclonic storm near the United Kingdom. Both test cases show significant improvements for the coarse-gridded (40-km) simulations when parameterizing slantwise convection. Compared to the 40-km simulations with only the upright convection scheme, the counterparts with the additional SC scheme exhibit a larger extent of CSI neutralization, generate a stronger grid-resolved slantwise circulation, and produce greater amounts of precipitation, all in better agreement with the corresponding fine-gridded reference simulations. Given the importance of slantwise convection in midlatitude weather systems, our results suggest that there exist potential benefits of parameterizing slantwise convection in general circulation models.
Abstract
Observations of large concentrations of ice particles in the dissipating stage of warm-based precipitating shallow cumulus clouds point to the limitations of scientists’ understanding of the physics of such clouds and the possible role of cloud dynamics. The most commonly accepted mechanisms of ice splinter production in the riming process have limitations to properly explain the rapid production of ice bursts. A more detailed description of the temporal and spatial evolution of hydrometeors and their interaction with cloud condensation nuclei and ice nuclei is needed to understand this phenomenon. A cloud model with bin-resolved microphysics can describe the time-dependent evolution of liquid droplets and ice particles and provide insights into how the physics and dynamics and their interaction may result in ice initiation and ice multiplication. The authors developed a 1.5-dimensional nonhydrostatic convective cloud and aerosol interaction model with spectral (bin) microphysics. The number and mass concentrations of aerosols, including ice nuclei and cloud condensation nuclei, were explicitly followed. Since both in situ observations of bioaerosols and laboratory experiments pointed to efficient nucleation capabilities at relative warm temperatures, it was assumed that ice-nucleating bioaerosols are involved in primary ice particle formation in condensation and immersion modes. Results show that bioaerosols can be the source of primary ice pellets, which in turn lead to high ice concentrations.
Abstract
Observations of large concentrations of ice particles in the dissipating stage of warm-based precipitating shallow cumulus clouds point to the limitations of scientists’ understanding of the physics of such clouds and the possible role of cloud dynamics. The most commonly accepted mechanisms of ice splinter production in the riming process have limitations to properly explain the rapid production of ice bursts. A more detailed description of the temporal and spatial evolution of hydrometeors and their interaction with cloud condensation nuclei and ice nuclei is needed to understand this phenomenon. A cloud model with bin-resolved microphysics can describe the time-dependent evolution of liquid droplets and ice particles and provide insights into how the physics and dynamics and their interaction may result in ice initiation and ice multiplication. The authors developed a 1.5-dimensional nonhydrostatic convective cloud and aerosol interaction model with spectral (bin) microphysics. The number and mass concentrations of aerosols, including ice nuclei and cloud condensation nuclei, were explicitly followed. Since both in situ observations of bioaerosols and laboratory experiments pointed to efficient nucleation capabilities at relative warm temperatures, it was assumed that ice-nucleating bioaerosols are involved in primary ice particle formation in condensation and immersion modes. Results show that bioaerosols can be the source of primary ice pellets, which in turn lead to high ice concentrations.
