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- Author or Editor: F. Chen x
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Abstract
It is shown in the limit of small Ekman number that the preferred mode of the symmetric instability exhibits a slight angle of inclination with the direction of the mean flow. The sign of the angle depends an the sign of P − 1, where P is the Prandtl number. It is likely that owing to this effect the range of Richardson numbers for which the instability occurs is increased significantly beyond the limits derived by Kuo (1956) and by McIntyre (1970). Numerical computations are needed to establish this property quantitatively.
Abstract
It is shown in the limit of small Ekman number that the preferred mode of the symmetric instability exhibits a slight angle of inclination with the direction of the mean flow. The sign of the angle depends an the sign of P − 1, where P is the Prandtl number. It is likely that owing to this effect the range of Richardson numbers for which the instability occurs is increased significantly beyond the limits derived by Kuo (1956) and by McIntyre (1970). Numerical computations are needed to establish this property quantitatively.
Abstract
A composite quasi-Lagrangian kinetic energy budget is constructed from four synoptically similar cases of polar air penetration into the Caribbean from off the North American continent. Computations were carried out for both the upstream anticyclone and downstream cyclone accompanying the polar outbreak.
Use of the residual technique suggests an average upscale energy exchange of 45.0 W m−2 over the anticyclone volume with a corresponding downscale energy transfer of 59.0 W m−2 over the cyclone volume for the 24 h period centered on the time of furthest southward cold air thrust as defined by the 1000–500 mb thickness patterns. The results also indicate that the vertical flux of kinetic energy ranges from 50 to 100% of the horizontal flux of kinetic energy and is of opposite sign below 400 mb in the cyclone volume. Further-more, during incipient surface cyclogenesis the horizontal boundary flux of 17.7 m m−2 is a signification of the local kinetic energy generation of 24.5 W m−2 whereas in the following 12 h time period these numbers become 32.2 and 64.6 W m−2, respectively. The corresponding figures for the anticyclone region include a horizontal export of kinetic energy of 37.2 and 55.0 W m−2 and local kinetic energy destruction of 9.5 and 6.0 W m−2 respectively, for the same 12 h time periods.
Abstract
A composite quasi-Lagrangian kinetic energy budget is constructed from four synoptically similar cases of polar air penetration into the Caribbean from off the North American continent. Computations were carried out for both the upstream anticyclone and downstream cyclone accompanying the polar outbreak.
Use of the residual technique suggests an average upscale energy exchange of 45.0 W m−2 over the anticyclone volume with a corresponding downscale energy transfer of 59.0 W m−2 over the cyclone volume for the 24 h period centered on the time of furthest southward cold air thrust as defined by the 1000–500 mb thickness patterns. The results also indicate that the vertical flux of kinetic energy ranges from 50 to 100% of the horizontal flux of kinetic energy and is of opposite sign below 400 mb in the cyclone volume. Further-more, during incipient surface cyclogenesis the horizontal boundary flux of 17.7 m m−2 is a signification of the local kinetic energy generation of 24.5 W m−2 whereas in the following 12 h time period these numbers become 32.2 and 64.6 W m−2, respectively. The corresponding figures for the anticyclone region include a horizontal export of kinetic energy of 37.2 and 55.0 W m−2 and local kinetic energy destruction of 9.5 and 6.0 W m−2 respectively, for the same 12 h time periods.
Abstract
A composite cyclone-anticyclone couplet is constructed from four synoptically similar cases of polar air outbreaks into the Caribbean from off the North American continent. A quasi-Lagrangian vorticity budget is then computed from these data for two consecutive 12 h time periods.
The results show that the divergence and twisting terms in the lower troposphere, the horizontal advection term in the middle troposphere, and the horizontal, vertical and system advection in the upper troposphere are of primary importance in generating negative vorticity tendencies in the area toward (from) which the surface anticyclone (cyclone) is moving. In contrast, only the divergence term in the lower troposphere and horizontal advection term in the mid and upper troposphere are primarily responsible for the intensification and movement of the downstream cyclone.
Computations suggest an apparent anticyclonic vorticity source in the mid and upper troposphere and sink in the lower troposphere for the large-scale motions over the anticyclone region with the reverse true for the downstream cyclone region due to subgrid-scale processes.
Abstract
A composite cyclone-anticyclone couplet is constructed from four synoptically similar cases of polar air outbreaks into the Caribbean from off the North American continent. A quasi-Lagrangian vorticity budget is then computed from these data for two consecutive 12 h time periods.
The results show that the divergence and twisting terms in the lower troposphere, the horizontal advection term in the middle troposphere, and the horizontal, vertical and system advection in the upper troposphere are of primary importance in generating negative vorticity tendencies in the area toward (from) which the surface anticyclone (cyclone) is moving. In contrast, only the divergence term in the lower troposphere and horizontal advection term in the mid and upper troposphere are primarily responsible for the intensification and movement of the downstream cyclone.
Computations suggest an apparent anticyclonic vorticity source in the mid and upper troposphere and sink in the lower troposphere for the large-scale motions over the anticyclone region with the reverse true for the downstream cyclone region due to subgrid-scale processes.
Abstract
The direct radiative effects of Saharan mineral dust (SMD) aerosols on the nonlinear evolution of the African easterly jet–African easterly wave (AEJ–AEW) system is examined using the Weather Research and Forecasting Model coupled to an online dust model. The SMD-modified AEW life cycles are characterized by four stages: enhanced linear growth, weakened nonlinear stabilization, larger peak amplitude, and smaller long-time amplitude. During the linear growth and nonlinear stabilization stages, the SMD increases the generation of eddy available potential energy (APE); this occurs where the maximum in the mean meridional SMD gradient is coincident with the critical surface. As the AEWs evolve beyond the nonlinear stabilization stage, the discrimination between SMD particle sizes due to sedimentation becomes more pronounced; the finer particles meridionally expand, while the coarser particles settle to the surface. The result is a reduction in the eddy APE at the base and the top of the plume.
The SMD enhances the Eliassen–Palm (EP) flux divergence and residual-mean meridional circulation, which generally oppose each other throughout the AEW life cycle. The SMD-modified residual-mean meridional circulation initially dominates to accelerate the flow but quickly surrenders to the EP flux divergence, which causes an SMD-enhanced deceleration of the AEJ during the linear growth and nonlinear stabilization stages. Throughout the AEW life cycle, the SMD-modified AEJ is elevated and the peak winds are larger than without SMD. During the first (second) half of the AEW life cycle, the SMD-modified wave fluxes shift the AEJ axis farther equatorward (poleward) of its original SMD-free position.
Abstract
The direct radiative effects of Saharan mineral dust (SMD) aerosols on the nonlinear evolution of the African easterly jet–African easterly wave (AEJ–AEW) system is examined using the Weather Research and Forecasting Model coupled to an online dust model. The SMD-modified AEW life cycles are characterized by four stages: enhanced linear growth, weakened nonlinear stabilization, larger peak amplitude, and smaller long-time amplitude. During the linear growth and nonlinear stabilization stages, the SMD increases the generation of eddy available potential energy (APE); this occurs where the maximum in the mean meridional SMD gradient is coincident with the critical surface. As the AEWs evolve beyond the nonlinear stabilization stage, the discrimination between SMD particle sizes due to sedimentation becomes more pronounced; the finer particles meridionally expand, while the coarser particles settle to the surface. The result is a reduction in the eddy APE at the base and the top of the plume.
The SMD enhances the Eliassen–Palm (EP) flux divergence and residual-mean meridional circulation, which generally oppose each other throughout the AEW life cycle. The SMD-modified residual-mean meridional circulation initially dominates to accelerate the flow but quickly surrenders to the EP flux divergence, which causes an SMD-enhanced deceleration of the AEJ during the linear growth and nonlinear stabilization stages. Throughout the AEW life cycle, the SMD-modified AEJ is elevated and the peak winds are larger than without SMD. During the first (second) half of the AEW life cycle, the SMD-modified wave fluxes shift the AEJ axis farther equatorward (poleward) of its original SMD-free position.
Abstract
The direct radiative effects of Saharan mineral dust aerosols on the linear dynamics of African easterly waves (AEWs) are examined analytically and numerically. The analytical analysis combines the thermodynamic equation with a dust continuity equation to form an expression for the dust-modified generation of eddy available potential energy 
Abstract
The direct radiative effects of Saharan mineral dust aerosols on the linear dynamics of African easterly waves (AEWs) are examined analytically and numerically. The analytical analysis combines the thermodynamic equation with a dust continuity equation to form an expression for the dust-modified generation of eddy available potential energy
Abstract
Analytical and numerical analyses are used to examine how structural changes to the African easterly jet (AEJ) mediate the effects of Saharan mineral dust aerosols on the linear dynamics of African easterly waves (AEWs). An analytical expression for the generation of eddy available potential energy (APE) is derived that exposes how the AEJ and dust combine to affect the energetics of the AEWs. The expression is also used to interpret the numerical results, which are obtained by radiatively coupling a simplified version of the Weather Research and Forecasting Model to a conservation equation for dust. The WRF-Dust model is used to conduct linear simulations based on five observationally consistent zonal-mean AEJs: a reference AEJ and four other AEJs that are obtained by perturbing the maximum meridional and vertical shear. For a dust distribution consistent with summertime observations over North Africa, the numerical simulations show the following: (i) Irrespective of the AEJ structure or the zonal scale of the AEWs, the dust increases the growth rates of the AEWs. (ii) The growth rates of the AEWs are optimized when the ratio of baroclinic to barotropic energy conversions is largest. (iii) When the energy conversions are sufficiently large, the zonal scale of the fastest-growing AEW shortens. The numerical results confirm the analytical analysis, which shows that the dust effects, which are modulated by the Doppler-shifted frequency, are strongest north of the AEJ axis, a region where the dust augments the preexisting meridional temperature gradient.
Abstract
Analytical and numerical analyses are used to examine how structural changes to the African easterly jet (AEJ) mediate the effects of Saharan mineral dust aerosols on the linear dynamics of African easterly waves (AEWs). An analytical expression for the generation of eddy available potential energy (APE) is derived that exposes how the AEJ and dust combine to affect the energetics of the AEWs. The expression is also used to interpret the numerical results, which are obtained by radiatively coupling a simplified version of the Weather Research and Forecasting Model to a conservation equation for dust. The WRF-Dust model is used to conduct linear simulations based on five observationally consistent zonal-mean AEJs: a reference AEJ and four other AEJs that are obtained by perturbing the maximum meridional and vertical shear. For a dust distribution consistent with summertime observations over North Africa, the numerical simulations show the following: (i) Irrespective of the AEJ structure or the zonal scale of the AEWs, the dust increases the growth rates of the AEWs. (ii) The growth rates of the AEWs are optimized when the ratio of baroclinic to barotropic energy conversions is largest. (iii) When the energy conversions are sufficiently large, the zonal scale of the fastest-growing AEW shortens. The numerical results confirm the analytical analysis, which shows that the dust effects, which are modulated by the Doppler-shifted frequency, are strongest north of the AEJ axis, a region where the dust augments the preexisting meridional temperature gradient.
Abstract
A theoretical framework is presented that exposes the radiative–dynamical relationships that govern the subcritical destabilization of African easterly waves (AEWs) by Saharan mineral dust (SMD) aerosols. The framework is built on coupled equations for quasigeostrophic potential vorticity (PV), temperature, and SMD mixing ratio. A perturbation analysis yields, for a subcritical, but otherwise arbitrary, zonal-mean background state, analytical expressions for the growth rate and frequency of the AEWs. The expressions are functions of the domain-averaged wave activity, which is generated by the direct radiative effects of the SMD. The wave activity is primarily modulated by the Doppler-shifted phase speed and the background gradients in PV and SMD.
Using an idealized version of the Weather Research and Forecasting (WRF) Model coupled to an interactive dust model, a linear analysis shows that, for a subcritical African easterly jet (AEJ) and a background SMD distribution that are consistent with observations, the SMD destabilizes the AEWs and slows their westward propagation, in agreement with the theoretical prediction. The SMD-induced growth rates are commensurate with, and can sometimes exceed, those obtained in previous dust-free studies in which the AEWs grow on AEJs that are supercritical with respect to the threshold for barotropic–baroclinic instability. The clarity of the theoretical framework can serve as a tool for understanding and predicting the effects of SMD aerosols on the linear instability of AEWs in subcritical, zonal-mean AEJs.
Abstract
A theoretical framework is presented that exposes the radiative–dynamical relationships that govern the subcritical destabilization of African easterly waves (AEWs) by Saharan mineral dust (SMD) aerosols. The framework is built on coupled equations for quasigeostrophic potential vorticity (PV), temperature, and SMD mixing ratio. A perturbation analysis yields, for a subcritical, but otherwise arbitrary, zonal-mean background state, analytical expressions for the growth rate and frequency of the AEWs. The expressions are functions of the domain-averaged wave activity, which is generated by the direct radiative effects of the SMD. The wave activity is primarily modulated by the Doppler-shifted phase speed and the background gradients in PV and SMD.
Using an idealized version of the Weather Research and Forecasting (WRF) Model coupled to an interactive dust model, a linear analysis shows that, for a subcritical African easterly jet (AEJ) and a background SMD distribution that are consistent with observations, the SMD destabilizes the AEWs and slows their westward propagation, in agreement with the theoretical prediction. The SMD-induced growth rates are commensurate with, and can sometimes exceed, those obtained in previous dust-free studies in which the AEWs grow on AEJs that are supercritical with respect to the threshold for barotropic–baroclinic instability. The clarity of the theoretical framework can serve as a tool for understanding and predicting the effects of SMD aerosols on the linear instability of AEWs in subcritical, zonal-mean AEJs.
Abstract
Optically thin cirrus play a key role in the earth’s radiation budget and global climate change. Their radiative effects depend critically on the thin cirrus optical and microphysical properties. In this paper, inhomogeneous hexagonal monocrystals (IHMs), which consist of a pure hexagon with spherical air bubble or aerosol inclusions, are applied to calculate the single-scattering properties of individual ice crystals. The multiangular polarized characteristics of optically thin cirrus for the 0.865- and 1.38-μm spectral bands are simulated on the basis of an adding–doubling radiative transfer program. The sensitivity of total and polarized reflectance at the top of the atmosphere (TOA) to different aerosol, cirrus, and surface parameters is studied. A new sensitivity index is introduced to further quantify the sensitivity study. The TOA polarized reflectance measured by the Polarization and Directionality of the Earth’s Reflectance (POLDER) instruments is compared to simulated TOA total and polarized reflectance. The test results are reasonable, although small deviations caused by the change of aerosol properties and thin cirrus optical thickness do exist. Finally, on the basis of the sensitivity study, a conceptual approach is suggested to simultaneously retrieve thin cirrus clouds’ optical thickness, ice particle shape, and the underlying aerosol optical thickness using the TOA total and polarized reflectance of the 0.865- and 1.38-μm spectral bands measured at multiple viewing angles.
Abstract
Optically thin cirrus play a key role in the earth’s radiation budget and global climate change. Their radiative effects depend critically on the thin cirrus optical and microphysical properties. In this paper, inhomogeneous hexagonal monocrystals (IHMs), which consist of a pure hexagon with spherical air bubble or aerosol inclusions, are applied to calculate the single-scattering properties of individual ice crystals. The multiangular polarized characteristics of optically thin cirrus for the 0.865- and 1.38-μm spectral bands are simulated on the basis of an adding–doubling radiative transfer program. The sensitivity of total and polarized reflectance at the top of the atmosphere (TOA) to different aerosol, cirrus, and surface parameters is studied. A new sensitivity index is introduced to further quantify the sensitivity study. The TOA polarized reflectance measured by the Polarization and Directionality of the Earth’s Reflectance (POLDER) instruments is compared to simulated TOA total and polarized reflectance. The test results are reasonable, although small deviations caused by the change of aerosol properties and thin cirrus optical thickness do exist. Finally, on the basis of the sensitivity study, a conceptual approach is suggested to simultaneously retrieve thin cirrus clouds’ optical thickness, ice particle shape, and the underlying aerosol optical thickness using the TOA total and polarized reflectance of the 0.865- and 1.38-μm spectral bands measured at multiple viewing angles.
Abstract
This study presents a framework for comparing hydrometeor and vertical velocity fields from mesoscale model simulations of tropical cyclones with observations of these fields from a variety of platforms. The framework is based on the Yuter and Houze constant frequency by altitude diagram (CFAD) technique, along with a new hurricane partitioning technique, to compare the statistics of vertical motion and reflectivity fields and hydrometeor concentrations from two datasets: one consisting of airborne radar retrievals and microphysical probe measurements collected from tropical cyclone aircraft flights over many years, and another consisting of cloud-scale (1.67-km grid length) tropical cyclone simulations using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Such comparisons of the microphysics fields can identify biases in the simulations that may lead to an identification of deficiencies in the modeling system, such as the formulation of various physical parameterization schemes used in the model. Improvements in these schemes may potentially lead to better forecasts of tropical cyclone intensity and rainfall.
In Part I of this study, the evaluation framework is demonstrated by comparing the radar retrievals and probe measurements to MM5 simulations of Hurricanes Bonnie (1998) and Floyd (1999). Comparisons of the statistics from the two datasets show that the model reproduces many of the gross features seen in the observations, though notable differences are evident. The general distribution of vertical motion is similar between the observations and simulations, with the strongest up- and downdrafts making up a small percentage of the overall population in both datasets, but the magnitudes of vertical motion are weaker in the simulations. The model-derived reflectivities are much higher than observed, and correlations between vertical motion and hydrometeor concentration and reflectivity show a much stronger relationship in the model than what is observed. Possible errors in the data processing are discussed as potential sources of differences between the observed and simulated datasets in Part I. In Part II, attention will be focused on using the evaluation framework to investigate the role that different model configurations (i.e., different resolutions and physical parameterizations) play in producing different microphysics fields in the simulation of Hurricane Bonnie. The microphysical and planetary boundary layer parameterization schemes, as well as higher horizontal and vertical resolutions, will be tested in the simulation to identify the extent to which changes in these schemes are reflected in improvements of the statistical comparisons with the observations.
Abstract
This study presents a framework for comparing hydrometeor and vertical velocity fields from mesoscale model simulations of tropical cyclones with observations of these fields from a variety of platforms. The framework is based on the Yuter and Houze constant frequency by altitude diagram (CFAD) technique, along with a new hurricane partitioning technique, to compare the statistics of vertical motion and reflectivity fields and hydrometeor concentrations from two datasets: one consisting of airborne radar retrievals and microphysical probe measurements collected from tropical cyclone aircraft flights over many years, and another consisting of cloud-scale (1.67-km grid length) tropical cyclone simulations using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Such comparisons of the microphysics fields can identify biases in the simulations that may lead to an identification of deficiencies in the modeling system, such as the formulation of various physical parameterization schemes used in the model. Improvements in these schemes may potentially lead to better forecasts of tropical cyclone intensity and rainfall.
In Part I of this study, the evaluation framework is demonstrated by comparing the radar retrievals and probe measurements to MM5 simulations of Hurricanes Bonnie (1998) and Floyd (1999). Comparisons of the statistics from the two datasets show that the model reproduces many of the gross features seen in the observations, though notable differences are evident. The general distribution of vertical motion is similar between the observations and simulations, with the strongest up- and downdrafts making up a small percentage of the overall population in both datasets, but the magnitudes of vertical motion are weaker in the simulations. The model-derived reflectivities are much higher than observed, and correlations between vertical motion and hydrometeor concentration and reflectivity show a much stronger relationship in the model than what is observed. Possible errors in the data processing are discussed as potential sources of differences between the observed and simulated datasets in Part I. In Part II, attention will be focused on using the evaluation framework to investigate the role that different model configurations (i.e., different resolutions and physical parameterizations) play in producing different microphysics fields in the simulation of Hurricane Bonnie. The microphysical and planetary boundary layer parameterization schemes, as well as higher horizontal and vertical resolutions, will be tested in the simulation to identify the extent to which changes in these schemes are reflected in improvements of the statistical comparisons with the observations.
Abstract
An analysis is performed on a microburst line-producing cloud that occurred near Denver, Colorado on 13 July 1982. The cloud line developed in an environment conducive to the production of low-reflectivity microbursts. Doppler radar analysis revealed strong convergence above cloud base into the region of downdraft 3.5 to 4.5 km above ground. Aircraft measurements detected light rain with graupel aloft in microburst downdrafts. A two-dimensional cloud model simulation captured many of the observed features of the cloud line structure and wind fields. In particular, both the development of multiple microbursts and the convergence aloft were well simulated. The formation of graupel/hail was important to the precipitation process in the model. The loading of rain and graupel and the cooling effect of rain evaporation and graupel melting were all important in microburst production—the graupel in the formative stages of the downdraft, and the rain in the further intensification of the downdraft and enhancement of the microburst outflow.
Abstract
An analysis is performed on a microburst line-producing cloud that occurred near Denver, Colorado on 13 July 1982. The cloud line developed in an environment conducive to the production of low-reflectivity microbursts. Doppler radar analysis revealed strong convergence above cloud base into the region of downdraft 3.5 to 4.5 km above ground. Aircraft measurements detected light rain with graupel aloft in microburst downdrafts. A two-dimensional cloud model simulation captured many of the observed features of the cloud line structure and wind fields. In particular, both the development of multiple microbursts and the convergence aloft were well simulated. The formation of graupel/hail was important to the precipitation process in the model. The loading of rain and graupel and the cooling effect of rain evaporation and graupel melting were all important in microburst production—the graupel in the formative stages of the downdraft, and the rain in the further intensification of the downdraft and enhancement of the microburst outflow.
