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Abstract
The authors compare the mixing depths in the daytime convective boundary layers that were observed remotely by wind profilers and an airborne lidar during the 1995 Southern Oxidants Study. The comparison is used to determine whether the mixing depths deduced from radar reflectivity profiles measured by the wind profilers are the appropriate mixing depths to use in air pollution applications. The profiler mixing depths are based on evidence that the profile of the refractive index structure function parameter exhibits a peak at the boundary layer capping inversion. The lidar mixing depths are determined from the gradient in aerosol backscatter at the top of an aerosol mixing layer. The results of linear regression analysis show that the mixing depths measured by the wind profiler and lidar are in good agreement, particularly in the absence of scattered clouds forming at the top of the convective boundary layer. When significant cumulus convection occurs, the definition of mixing depth from both experimental and theoretical points of view is ill defined.
Abstract
The authors compare the mixing depths in the daytime convective boundary layers that were observed remotely by wind profilers and an airborne lidar during the 1995 Southern Oxidants Study. The comparison is used to determine whether the mixing depths deduced from radar reflectivity profiles measured by the wind profilers are the appropriate mixing depths to use in air pollution applications. The profiler mixing depths are based on evidence that the profile of the refractive index structure function parameter exhibits a peak at the boundary layer capping inversion. The lidar mixing depths are determined from the gradient in aerosol backscatter at the top of an aerosol mixing layer. The results of linear regression analysis show that the mixing depths measured by the wind profiler and lidar are in good agreement, particularly in the absence of scattered clouds forming at the top of the convective boundary layer. When significant cumulus convection occurs, the definition of mixing depth from both experimental and theoretical points of view is ill defined.
Abstract
The impact of global orography on Northern Hemisphere wintertime climate is revisited using the Whole Atmosphere Community Climate Model, version 6 (WACCM6). A suite of experiments explores the roles of both resolved orography and the parameterized effects of unresolved orographic drag (hereafter parameterized orography), including gravity waves and boundary layer turbulence. Including orography reduces the extratropical tropospheric and stratospheric zonal mean zonal wind
Abstract
The impact of global orography on Northern Hemisphere wintertime climate is revisited using the Whole Atmosphere Community Climate Model, version 6 (WACCM6). A suite of experiments explores the roles of both resolved orography and the parameterized effects of unresolved orographic drag (hereafter parameterized orography), including gravity waves and boundary layer turbulence. Including orography reduces the extratropical tropospheric and stratospheric zonal mean zonal wind
Abstract
The design of efficient linear statistical operators for the 24-hour prediction of the sea-level pressure distribution over the United States is considered. Factor analysis techniques for reduction and selection of independent variables in regression analysis are used as a means of obtaining efficient statistical forecasting equations. The effects of the variations in data density in time and space, and the extent of geographical coverage upon the explained variance of the sea-level pressure are examined.
Abstract
The design of efficient linear statistical operators for the 24-hour prediction of the sea-level pressure distribution over the United States is considered. Factor analysis techniques for reduction and selection of independent variables in regression analysis are used as a means of obtaining efficient statistical forecasting equations. The effects of the variations in data density in time and space, and the extent of geographical coverage upon the explained variance of the sea-level pressure are examined.
Abstract
Forty-three eastern North Pacific tropical cyclone remnants with varying impact on the southwestern United States during the period 1992–2005 are investigated. Of these, 35 remnants (81%) brought precipitation to some part of the southwestern United States and the remaining 8 remnants (19%) had precipitation that was almost entirely restricted to Mexico, although cloud cover did advect over the southwestern United States in some of these cases. Although the tropical cyclone–strength winds rapidly diminish upon making landfall, these systems still carry a large quantity of tropical moisture and, upon interaction with mountainous topography, are found to drop up to 30% of the local annual precipitation.
Based on common rainfall patterns and large-scale circulation features, the tropical cyclones are grouped into five categories. These include a northern recurving pattern that is more likely to bring rainfall to the southwestern United States; a southern recurving pattern that brings rainfall across northern Mexico and the Gulf Coast region; a largely north and/or northwestward movement pattern that brings rainfall to the west coast of the United States; a group that is blocked from the southwest by a ridge, which limits rainfall to Mexico; and a small group of cases that are not clearly any of the previous four types. Composites of the first four groups are shown and forecasting strategies for each are described.
Abstract
Forty-three eastern North Pacific tropical cyclone remnants with varying impact on the southwestern United States during the period 1992–2005 are investigated. Of these, 35 remnants (81%) brought precipitation to some part of the southwestern United States and the remaining 8 remnants (19%) had precipitation that was almost entirely restricted to Mexico, although cloud cover did advect over the southwestern United States in some of these cases. Although the tropical cyclone–strength winds rapidly diminish upon making landfall, these systems still carry a large quantity of tropical moisture and, upon interaction with mountainous topography, are found to drop up to 30% of the local annual precipitation.
Based on common rainfall patterns and large-scale circulation features, the tropical cyclones are grouped into five categories. These include a northern recurving pattern that is more likely to bring rainfall to the southwestern United States; a southern recurving pattern that brings rainfall across northern Mexico and the Gulf Coast region; a largely north and/or northwestward movement pattern that brings rainfall to the west coast of the United States; a group that is blocked from the southwest by a ridge, which limits rainfall to Mexico; and a small group of cases that are not clearly any of the previous four types. Composites of the first four groups are shown and forecasting strategies for each are described.
Abstract
Nucleation experiments with monodisperse NaCl aerosols showed good agreement with the Köhler theory relating the critical super-saturation Sc to the dry size. Aerosols produced by condensing NaCl showed the same Sc as those produced by evaporating aqueous NaCl solution droplets. This indicates that if there is an energy barrier in going from a dry NaCl particle to a solution droplet, this energy barrier is small. The fact that the evaporation aerosol particles are cubical crystals and the condensation aerosols are amorphous spheres is shown to make no difference in the nucleation threshold.
The investigation also gives insights into the performance of the equipment used, especially the commercial electrostatic aerosol classifier and the vertical flow thermal diffusion chamber developed in this laboratory. When operating this chamber in the isothermal mode, a 36% upper limit was found on the uncertainty in Sc due to index of refraction sensitivity in sizing the water drops. Within this range of uncertainty, the isothermal mode data agreed with the Köhler theory.
Abstract
Nucleation experiments with monodisperse NaCl aerosols showed good agreement with the Köhler theory relating the critical super-saturation Sc to the dry size. Aerosols produced by condensing NaCl showed the same Sc as those produced by evaporating aqueous NaCl solution droplets. This indicates that if there is an energy barrier in going from a dry NaCl particle to a solution droplet, this energy barrier is small. The fact that the evaporation aerosol particles are cubical crystals and the condensation aerosols are amorphous spheres is shown to make no difference in the nucleation threshold.
The investigation also gives insights into the performance of the equipment used, especially the commercial electrostatic aerosol classifier and the vertical flow thermal diffusion chamber developed in this laboratory. When operating this chamber in the isothermal mode, a 36% upper limit was found on the uncertainty in Sc due to index of refraction sensitivity in sizing the water drops. Within this range of uncertainty, the isothermal mode data agreed with the Köhler theory.
Abstract
A systematic series of condensation coefficient measurements of water have been made using the University of Missouri—Rolla cooled-wall expansion chamber which simulates the thermodynamics of cloud. This coefficient is seen to decrease from a value near unity, at the outset of simulation, to a value in the neighborhood of 0.01 toward the end of a simulation. Final values of this coefficient are sufficiently low as to contribute significantly to the broadening of the drop-size distribution in cloud.
Abstract
A systematic series of condensation coefficient measurements of water have been made using the University of Missouri—Rolla cooled-wall expansion chamber which simulates the thermodynamics of cloud. This coefficient is seen to decrease from a value near unity, at the outset of simulation, to a value in the neighborhood of 0.01 toward the end of a simulation. Final values of this coefficient are sufficiently low as to contribute significantly to the broadening of the drop-size distribution in cloud.
Abstract
The mixing depth of the boundary layer is an input to most atmospheric transport and dispersion (ATD) models, which obtain mixing depths in one of four ways: 1) observations by radiosondes, sodars, or other devices; 2) simulations by regional or mesoscale meteorological models; 3) parameterizations based on boundary layer similarity theory; or 4) climatological averages. This paper describes a situation during a field experiment when exceptionally low mixing depths persisted in the morning and led to relatively high observed tracer concentrations. The low mixing depths were caused by synoptic effects associated with a nearby stationary front and the outflow from a mesoscale thunderstorm complex located 20–50 km away. For the same time period, the ATD model-parameterized mixing depth was a factor of 5–10 higher, leading to predicted concentrations that were less than the observations by a factor of 5–10. The synoptic situation is described and local radiosonde and radar observations of mixing depth are presented, including comparisons with other more typical days. Time series of local observations of near-surface sensible heat fluxes are also plotted to demonstrate the suppression of turbulence by negative sensible heat fluxes during the period in question.
Abstract
The mixing depth of the boundary layer is an input to most atmospheric transport and dispersion (ATD) models, which obtain mixing depths in one of four ways: 1) observations by radiosondes, sodars, or other devices; 2) simulations by regional or mesoscale meteorological models; 3) parameterizations based on boundary layer similarity theory; or 4) climatological averages. This paper describes a situation during a field experiment when exceptionally low mixing depths persisted in the morning and led to relatively high observed tracer concentrations. The low mixing depths were caused by synoptic effects associated with a nearby stationary front and the outflow from a mesoscale thunderstorm complex located 20–50 km away. For the same time period, the ATD model-parameterized mixing depth was a factor of 5–10 higher, leading to predicted concentrations that were less than the observations by a factor of 5–10. The synoptic situation is described and local radiosonde and radar observations of mixing depth are presented, including comparisons with other more typical days. Time series of local observations of near-surface sensible heat fluxes are also plotted to demonstrate the suppression of turbulence by negative sensible heat fluxes during the period in question.
Abstract
Ocean–atmosphere interaction above warm western boundary currents such as the Gulf Stream, the Kuroshio Current, and the Agulhas Current often leads to very high evaporation rates. In the case of the Agulhas Current, which flows just off the southeast coast of Africa, such high latent heat fluxes may lead to increased low-level advection of moisture onshore and local intensification of storm systems. Observational evidence for the significant latent heat fluxes in the Agulhas Current area was obtained during the Agulhas Current Air Sea Exchange Experiment, which showed that about 5 times as much water vapor is transferred to the atmosphere above the 80–100-km-wide core of the current than from the neighboring waters. Using NCEP reanalyses, Meteosat, and Tropical Rainfall Measuring Mission (TRMM) satellite data and local station and radiosonde observations, this study investigates the evolution of a severe storm and flood event that occurred over the southern coastal regions of South Africa on 14–15 December 1998. Heavy rainfall occurred in two widely separated locations, and tornadoes were reported. Moisture flux transects through the storm region and backward trajectories of air parcels suggest that low-level onshore flow of moisture from the Agulhas Current region played a significant role in the storm evolution. However, because the NCEP data on which these moisture fluxes are based are known to significantly underestimate the surface latent heat flux when compared with ship observations, it is suggested that the actual contribution of the Agulhas Current moisture source to the storm may have been even greater than is documented in this paper.
Abstract
Ocean–atmosphere interaction above warm western boundary currents such as the Gulf Stream, the Kuroshio Current, and the Agulhas Current often leads to very high evaporation rates. In the case of the Agulhas Current, which flows just off the southeast coast of Africa, such high latent heat fluxes may lead to increased low-level advection of moisture onshore and local intensification of storm systems. Observational evidence for the significant latent heat fluxes in the Agulhas Current area was obtained during the Agulhas Current Air Sea Exchange Experiment, which showed that about 5 times as much water vapor is transferred to the atmosphere above the 80–100-km-wide core of the current than from the neighboring waters. Using NCEP reanalyses, Meteosat, and Tropical Rainfall Measuring Mission (TRMM) satellite data and local station and radiosonde observations, this study investigates the evolution of a severe storm and flood event that occurred over the southern coastal regions of South Africa on 14–15 December 1998. Heavy rainfall occurred in two widely separated locations, and tornadoes were reported. Moisture flux transects through the storm region and backward trajectories of air parcels suggest that low-level onshore flow of moisture from the Agulhas Current region played a significant role in the storm evolution. However, because the NCEP data on which these moisture fluxes are based are known to significantly underestimate the surface latent heat flux when compared with ship observations, it is suggested that the actual contribution of the Agulhas Current moisture source to the storm may have been even greater than is documented in this paper.
