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Abstract
A pole-to-pole study of density deviations near the 80th meridian west is presented from the surface to 31 km altitude. Density deviations are greatest at the surface, and under extreme conditions may range from 1.0 to 2.0 kg m−2. Density decreases almost exponentially with altitude and occasionally falls below 0.01 kg m−3 at 31 km. Density deviations decrease from the surface to an isopycnic layer, which varies in height from 6 km in polar regions to 12 km at the equator. Above this isopycnic layer, density variations increase with altitude to a maximum density deviation layer. This maximum density deviation layer occurs along the base of the summer tropopause and is approximately the center of the tropospheric wind maximum. The maximum density deviation layer is parallel to, and 50 per cent higher in altitude than the lower isopycnic layer. A weaker, second isopycnic layer is shown above and parallel to the maximum density deviation layer; this second isopycnic layer is found in tropical regions and near the south pole. Because of large seasonal and latitudinal variations in atmospheric density, no single standard atmosphere can present density data adequate for high speed vehicle operations on a global basis.
Abstract
A pole-to-pole study of density deviations near the 80th meridian west is presented from the surface to 31 km altitude. Density deviations are greatest at the surface, and under extreme conditions may range from 1.0 to 2.0 kg m−2. Density decreases almost exponentially with altitude and occasionally falls below 0.01 kg m−3 at 31 km. Density deviations decrease from the surface to an isopycnic layer, which varies in height from 6 km in polar regions to 12 km at the equator. Above this isopycnic layer, density variations increase with altitude to a maximum density deviation layer. This maximum density deviation layer occurs along the base of the summer tropopause and is approximately the center of the tropospheric wind maximum. The maximum density deviation layer is parallel to, and 50 per cent higher in altitude than the lower isopycnic layer. A weaker, second isopycnic layer is shown above and parallel to the maximum density deviation layer; this second isopycnic layer is found in tropical regions and near the south pole. Because of large seasonal and latitudinal variations in atmospheric density, no single standard atmosphere can present density data adequate for high speed vehicle operations on a global basis.
Abstract
Seven years of temperature observations from Cape Canaveral, Florida, are used for a detailed examination of the vertical temperature structure. In the troposphere, temperature variations increase with time; in the stratosphere 12-hour temperature changes are greatest and 24-hour changes are least, showing the diurnal temperature control. In the troposphere, temperature variations are greatest in winter, least in summer; the reverse is true in the stratosphere. The smallest annual median temperature range, 3.6C, occurs at 13 km altitude. Stratospheric temperatures over Cape Canaveral are warmest in early spring, coinciding with the maximum ozone concentration of early spring, with only 2–3 months elapsing between the warmest and coldest temperatures of the year. Cape Canaveral summer temperatures compare closely to the mean summer temperatures of other tropical maritime areas, as shown by earlier studies.
Three years of temperature data, at 23 stations from Eureka to Amundsen-Scott, are used to establish a global profile of summer and winter tropopause heights. In the equatorial belt, temperatures in the lower stratosphere are coldest in January and warmest in July, both north and south of the equator. The temperature distribution indicates that the tropopause is generally higher in winter than in summer in all latitudes. The lowering of the summer tropopause occurs with an increase of water vapor in the lower troposphere. From Buffalo to Albrook, somewhere in the 12–18 km region, a temperature belt is found that is warmer than the annual average at that attitude in winter, and colder than the annual average in summer. Another region with seasonal temperature reversals is indicated above 32 km.
Abstract
Seven years of temperature observations from Cape Canaveral, Florida, are used for a detailed examination of the vertical temperature structure. In the troposphere, temperature variations increase with time; in the stratosphere 12-hour temperature changes are greatest and 24-hour changes are least, showing the diurnal temperature control. In the troposphere, temperature variations are greatest in winter, least in summer; the reverse is true in the stratosphere. The smallest annual median temperature range, 3.6C, occurs at 13 km altitude. Stratospheric temperatures over Cape Canaveral are warmest in early spring, coinciding with the maximum ozone concentration of early spring, with only 2–3 months elapsing between the warmest and coldest temperatures of the year. Cape Canaveral summer temperatures compare closely to the mean summer temperatures of other tropical maritime areas, as shown by earlier studies.
Three years of temperature data, at 23 stations from Eureka to Amundsen-Scott, are used to establish a global profile of summer and winter tropopause heights. In the equatorial belt, temperatures in the lower stratosphere are coldest in January and warmest in July, both north and south of the equator. The temperature distribution indicates that the tropopause is generally higher in winter than in summer in all latitudes. The lowering of the summer tropopause occurs with an increase of water vapor in the lower troposphere. From Buffalo to Albrook, somewhere in the 12–18 km region, a temperature belt is found that is warmer than the annual average at that attitude in winter, and colder than the annual average in summer. Another region with seasonal temperature reversals is indicated above 32 km.
Abstract
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Abstract
Twelve acoustic grenade experiments were conducted during the period September 1968–February 1969 from Barrow, Alaska (71N). These measurements were intended to monitor the transition in the thermal structure of the mesosphere from the persistent summertime case to the dynamic and highly variable winter-time case. The disturbed features typical of winter appeared in the high mesosphere in September, and at successively lower altitudes until December, at which time the full winter structure had been established. In early January, a warming at the stratopause began a chain of events which eventually would restore the summertime structure and thus complete the cycle.
Abstract
Twelve acoustic grenade experiments were conducted during the period September 1968–February 1969 from Barrow, Alaska (71N). These measurements were intended to monitor the transition in the thermal structure of the mesosphere from the persistent summertime case to the dynamic and highly variable winter-time case. The disturbed features typical of winter appeared in the high mesosphere in September, and at successively lower altitudes until December, at which time the full winter structure had been established. In early January, a warming at the stratopause began a chain of events which eventually would restore the summertime structure and thus complete the cycle.
Abstract
Shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary. A field program involving high-resolution velocity and density measurements, as well as high-frequency acoustic imagery, allowed direct comparison of instantaneous Richardson number distributions to the occurrence of shear instability. The theoretical stability threshold of 0.25 is consistent with the measurements, based on estimates of gradients that contain the mean as well as fluctuations due to internal waves. An effective stability threshold based on mean gradients is found to be approximately one-third, reflecting a significant contribution of internal wave shear. The integral effect of the mixing process is to homogenize the gradients of velocity and density, producing linear profiles of these quantities across the pycnocline. A turbulent Prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixing. Based on these observations, a simple model for mixing in stratified shear flows is proposed, which is applicable to estuaries and other environments with a dominant mean shear.
Abstract
Shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary. A field program involving high-resolution velocity and density measurements, as well as high-frequency acoustic imagery, allowed direct comparison of instantaneous Richardson number distributions to the occurrence of shear instability. The theoretical stability threshold of 0.25 is consistent with the measurements, based on estimates of gradients that contain the mean as well as fluctuations due to internal waves. An effective stability threshold based on mean gradients is found to be approximately one-third, reflecting a significant contribution of internal wave shear. The integral effect of the mixing process is to homogenize the gradients of velocity and density, producing linear profiles of these quantities across the pycnocline. A turbulent Prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixing. Based on these observations, a simple model for mixing in stratified shear flows is proposed, which is applicable to estuaries and other environments with a dominant mean shear.
Abstract
No Abstract Available.
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No Abstract Available.
Abstract
A high-resolution one-dimensional version of a second-order turbulence radiative–convective model, developed at Los Alamos National Laboratory, is used to simulate the diurnal cycle of the marine stratocumulus cloud-capped boundary layer. The fidelity of the model to the underlying physics is assessed by comparing the model simulation to data taken at San Nicolas Island during the intensive field observation (IFO) of the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE), conducted during June and July 1987. The model is able to reproduce the observed diurnal cycle of the liquid water content, cloud-base height, radiative heating or cooling rates, and the mean and turbulence variables fairly well. The mechanisms that cause the diurnal variation and the decoupling of the boundary layer are examined.
The possible role of an imposed diurnal cycle for the subsidence in inducing the cloud-top diurnal cycle observed during the FIRE IFO is also addressed. Three regimes of subsidence influence are identified for the stratocumulus-capped boundary layer. Regimes I and III are characterized by vertical propagation of the inversion height and erratic fluctuation of turbulence in the region of the inversion. Regime II is characterized by a continuum of quasi-equilibrium states that can exist for a range of subsidence values. In this regime, the boundary layer height is fairly insensitive to changes in the subsidence. The boundary layer behavior implied for these regimes is used to explore the effect of a diurnally varying subsidence rate on the diurnal cycle for the cloud-top height.
Abstract
A high-resolution one-dimensional version of a second-order turbulence radiative–convective model, developed at Los Alamos National Laboratory, is used to simulate the diurnal cycle of the marine stratocumulus cloud-capped boundary layer. The fidelity of the model to the underlying physics is assessed by comparing the model simulation to data taken at San Nicolas Island during the intensive field observation (IFO) of the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE), conducted during June and July 1987. The model is able to reproduce the observed diurnal cycle of the liquid water content, cloud-base height, radiative heating or cooling rates, and the mean and turbulence variables fairly well. The mechanisms that cause the diurnal variation and the decoupling of the boundary layer are examined.
The possible role of an imposed diurnal cycle for the subsidence in inducing the cloud-top diurnal cycle observed during the FIRE IFO is also addressed. Three regimes of subsidence influence are identified for the stratocumulus-capped boundary layer. Regimes I and III are characterized by vertical propagation of the inversion height and erratic fluctuation of turbulence in the region of the inversion. Regime II is characterized by a continuum of quasi-equilibrium states that can exist for a range of subsidence values. In this regime, the boundary layer height is fairly insensitive to changes in the subsidence. The boundary layer behavior implied for these regimes is used to explore the effect of a diurnally varying subsidence rate on the diurnal cycle for the cloud-top height.
Abstract
A high-resolution one-dimensional version of a second-order turbulence closure radiative-convective model, developed at Los Alamos National Laboratory, is used to simulate the interactions among turbulence, radiation, and bulk cloud parameters in stratiform clouds observed during the Arctic Stratus Experiment conducted during June 1980 over the Beaufort Sea. The fidelity of the model to the underlying physics is assessed by comparing the modeled evolution of the cloud-capped boundary layer against data reported for two particular days of observations. Over the period encompassed by these observations, the boundary layer evolved from a well-mixed cloud-capped boundary layer overlying a stable cloudy surface layer to a shallower well-mixed boundary layer with a single upper cloud deck and a clear, diminished, stable surface layer. The model was able to reproduce the observed profiles of the liquid water content, cloud-base height, radiative heating rates, and the mean and turbulence variables over the period of observation fairly well. The formation and eventual dissipation of the surface cloud feature over the period of the simulation was found to be caused by the formation of a stable surface layer as the modeled air mass moved over the relatively cold Beaufort Sea region. Condensation occurred as heat in the surface layer was transported downward toward the sea surface. Eventual dissipation of the surface cloud layer resulted from the transport of moisture in the surface layer downward toward the sea surface. The results show that the subsidence was the major influence on the evolution of the cloud-top height but was not a major factor for dissipation of either cloud layer during the simulation.
Abstract
A high-resolution one-dimensional version of a second-order turbulence closure radiative-convective model, developed at Los Alamos National Laboratory, is used to simulate the interactions among turbulence, radiation, and bulk cloud parameters in stratiform clouds observed during the Arctic Stratus Experiment conducted during June 1980 over the Beaufort Sea. The fidelity of the model to the underlying physics is assessed by comparing the modeled evolution of the cloud-capped boundary layer against data reported for two particular days of observations. Over the period encompassed by these observations, the boundary layer evolved from a well-mixed cloud-capped boundary layer overlying a stable cloudy surface layer to a shallower well-mixed boundary layer with a single upper cloud deck and a clear, diminished, stable surface layer. The model was able to reproduce the observed profiles of the liquid water content, cloud-base height, radiative heating rates, and the mean and turbulence variables over the period of observation fairly well. The formation and eventual dissipation of the surface cloud feature over the period of the simulation was found to be caused by the formation of a stable surface layer as the modeled air mass moved over the relatively cold Beaufort Sea region. Condensation occurred as heat in the surface layer was transported downward toward the sea surface. Eventual dissipation of the surface cloud layer resulted from the transport of moisture in the surface layer downward toward the sea surface. The results show that the subsidence was the major influence on the evolution of the cloud-top height but was not a major factor for dissipation of either cloud layer during the simulation.
Abstract
Effects of rotation on finite-length line plumes are studied with a three-dimensional nonhydrostatic numerical model. Geophysical convection with this source geometry occurs, for example, as the result of fissure releases of hot hydrothermal fluids at the seafloor from terrestrial release of hot gases and ash during volcanic activity along fissures and in the descent from the sea surface of brines formed during freezing of ice leads at high latitudes. Here the model treats the case of a starting plume of dense fluid descending into a rotating environment. Results are compared with laboratory experiments so that the validity of the model, particularly the nonlinear subgrid-scale mixing formulation, might first be established. Differences in plumes caused by varying rotation rate, &ohm, and buoyancy flux, B 0, are the primary focus, with experiments in fluid of depth h spanning a convective Rossby number [B 0 1/3/(2Ωh)] of 0.01−1.0. Rotation initiates spiraling of the descending plumes but it has little effect on the speed of plume descent; the latter depends on the strength of turbulent mixing. Low rotation rates allow the descending plume cap to be broad and the stem to be narrow. Higher rotation rates retard the lateral spread of the plume cap and widen the plume stem. Updraft at the stem edge is very much larger at higher rotation rates, and that appears to be instrumental in determining stem and cap width. Values of turbulent mixing coefficients within the plume are dependent on B 0 but not on Ω. Thus rotational effects on turbulence are not needed to account for differences in plume structure arising solely from Ω variation. Agreement between model and laboratory results did not occur without a nonlinear time- and space-dependent subgrid-scale mixing parameterization, suggesting that model applications to convective geophysical problems identified above require the same.
Abstract
Effects of rotation on finite-length line plumes are studied with a three-dimensional nonhydrostatic numerical model. Geophysical convection with this source geometry occurs, for example, as the result of fissure releases of hot hydrothermal fluids at the seafloor from terrestrial release of hot gases and ash during volcanic activity along fissures and in the descent from the sea surface of brines formed during freezing of ice leads at high latitudes. Here the model treats the case of a starting plume of dense fluid descending into a rotating environment. Results are compared with laboratory experiments so that the validity of the model, particularly the nonlinear subgrid-scale mixing formulation, might first be established. Differences in plumes caused by varying rotation rate, &ohm, and buoyancy flux, B 0, are the primary focus, with experiments in fluid of depth h spanning a convective Rossby number [B 0 1/3/(2Ωh)] of 0.01−1.0. Rotation initiates spiraling of the descending plumes but it has little effect on the speed of plume descent; the latter depends on the strength of turbulent mixing. Low rotation rates allow the descending plume cap to be broad and the stem to be narrow. Higher rotation rates retard the lateral spread of the plume cap and widen the plume stem. Updraft at the stem edge is very much larger at higher rotation rates, and that appears to be instrumental in determining stem and cap width. Values of turbulent mixing coefficients within the plume are dependent on B 0 but not on Ω. Thus rotational effects on turbulence are not needed to account for differences in plume structure arising solely from Ω variation. Agreement between model and laboratory results did not occur without a nonlinear time- and space-dependent subgrid-scale mixing parameterization, suggesting that model applications to convective geophysical problems identified above require the same.
