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- Author or Editor: Douglas W. Johnson x
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Abstract
It has been recognized that progress toward understanding the mesoscale structure of the ocean requires more knowledge of the small-scale structure of the marine atmosphere and the processes that couple the air and the sea. The coastal ocean is characterized by large variations in sea surface temperature and surface roughness that affect the structure of the atmosphere. Stable layers are frequent features of the coastal marine atmospheric boundary layer. Their main effect is the formation of a discontinuity between the sea surface and the upper part of the boundary layer that supports gravity waves, wind speed jets, and large wind shear. The general structure of a stable internal boundary layer (IBL) that forms over the sea, downstream of a warm landmass, is discussed. Aircraft data are presented from the Internal Boundary Layer Experiment (IBLEX) conducted over the Irish Sea in 1990. With the airflow from the land to the sea, thermodynamic profiles were obtained perpendicular to the coast to investigate the modification of the boundary layer. Horizontal legs were flown parallel to the coast to obtain information about the turbulence structure at the upwind and downwind ends of the research area.
Despite the large horizontal inhomogeneity in the IBL, local similarity scaling applies throughout the IBL below LL , where LL , is the local similarity length scale. The turbulence parameters, which are nondimensionalized with the local scales, are generally constant with respect to height. The IBL is characterized by large temperature and moisture gradients and a large wind shear that maintains a Richardson number close to its critical value. Turbulence appears to be continuous, maintained by the strong wind speed shear against the stabilizing effect of the downward-directed heat flux.
The turbulence fluxes indicate generally cogradient heat fluxes in the IBL and large momentum fluxes due to the strong shear and high winds. Small-scale countergradient beat fluxes are observed that may be the result of the breakup of large, cogradient turbulent eddies near the sea surface and the return of the air to buoyant equilibrium at smaller spatial scales.
Abstract
It has been recognized that progress toward understanding the mesoscale structure of the ocean requires more knowledge of the small-scale structure of the marine atmosphere and the processes that couple the air and the sea. The coastal ocean is characterized by large variations in sea surface temperature and surface roughness that affect the structure of the atmosphere. Stable layers are frequent features of the coastal marine atmospheric boundary layer. Their main effect is the formation of a discontinuity between the sea surface and the upper part of the boundary layer that supports gravity waves, wind speed jets, and large wind shear. The general structure of a stable internal boundary layer (IBL) that forms over the sea, downstream of a warm landmass, is discussed. Aircraft data are presented from the Internal Boundary Layer Experiment (IBLEX) conducted over the Irish Sea in 1990. With the airflow from the land to the sea, thermodynamic profiles were obtained perpendicular to the coast to investigate the modification of the boundary layer. Horizontal legs were flown parallel to the coast to obtain information about the turbulence structure at the upwind and downwind ends of the research area.
Despite the large horizontal inhomogeneity in the IBL, local similarity scaling applies throughout the IBL below LL , where LL , is the local similarity length scale. The turbulence parameters, which are nondimensionalized with the local scales, are generally constant with respect to height. The IBL is characterized by large temperature and moisture gradients and a large wind shear that maintains a Richardson number close to its critical value. Turbulence appears to be continuous, maintained by the strong wind speed shear against the stabilizing effect of the downward-directed heat flux.
The turbulence fluxes indicate generally cogradient heat fluxes in the IBL and large momentum fluxes due to the strong shear and high winds. Small-scale countergradient beat fluxes are observed that may be the result of the breakup of large, cogradient turbulent eddies near the sea surface and the return of the air to buoyant equilibrium at smaller spatial scales.
Abstract
Observations of the mean and turbulent structure of the marine atmospheric boundary layer (MABL). obtained using the U.K. Meteorological Research Flight C-130 Hercules aircraft are used to investigate the momentum balance over the Irish Sea when warm air is advected offshore. The marine boundary layer is made up of two layers: a strongly stable internal boundary layer (IBL). and a stable residual layer located between the top of the IBL and the base of the planetary boundary layer inversion.
Measurements obtained near the upwind coast indicate that the flow is highly ageostrophic. Downwind of the Irish coast, there is a transition toward equilibrium between the geostrophic, Coriolis. and friction components of the flow along part of the flight track. However, another segment of the flight track indicates an imbalance between the pressure gradient and the other measured terms, which may be attributable to gravity waves affecting the adjustment process. This is more apparent in the leg perpendicular to the coast where the pressure gradient is balanced by the observed acceleration with negligible contributions from the Coriolis and friction terms.
Gravity waves associated with mountain lee waves propagate along the direction of the mean wind shear in the IBL, which is directed to the right of the wind measured along the flight track perpendicular to the coast at 30-m altitude. The dominant wavelength is about 19 km, which corresponds with the buoyancy frequency of the MABL new the Irish coast and is supported by satellite images of the cloud structure. Farther downstream the buoyancy frequency increases, but the longer wavelength signal remains dominant. An important result of the gravity waves is the modification of the wind field and wind stress within the IBL. The largest effect is observed in the stress direction, but large changes in magnitude are also observed. The results indicate that the direction of the wind stress corresponds to a large degree with the direction of the mean horizontal wind sheer.
Abstract
Observations of the mean and turbulent structure of the marine atmospheric boundary layer (MABL). obtained using the U.K. Meteorological Research Flight C-130 Hercules aircraft are used to investigate the momentum balance over the Irish Sea when warm air is advected offshore. The marine boundary layer is made up of two layers: a strongly stable internal boundary layer (IBL). and a stable residual layer located between the top of the IBL and the base of the planetary boundary layer inversion.
Measurements obtained near the upwind coast indicate that the flow is highly ageostrophic. Downwind of the Irish coast, there is a transition toward equilibrium between the geostrophic, Coriolis. and friction components of the flow along part of the flight track. However, another segment of the flight track indicates an imbalance between the pressure gradient and the other measured terms, which may be attributable to gravity waves affecting the adjustment process. This is more apparent in the leg perpendicular to the coast where the pressure gradient is balanced by the observed acceleration with negligible contributions from the Coriolis and friction terms.
Gravity waves associated with mountain lee waves propagate along the direction of the mean wind shear in the IBL, which is directed to the right of the wind measured along the flight track perpendicular to the coast at 30-m altitude. The dominant wavelength is about 19 km, which corresponds with the buoyancy frequency of the MABL new the Irish coast and is supported by satellite images of the cloud structure. Farther downstream the buoyancy frequency increases, but the longer wavelength signal remains dominant. An important result of the gravity waves is the modification of the wind field and wind stress within the IBL. The largest effect is observed in the stress direction, but large changes in magnitude are also observed. The results indicate that the direction of the wind stress corresponds to a large degree with the direction of the mean horizontal wind sheer.
Abstract
The LES model is applied for studying ship track formation under various boundary layer conditions observed during the Monterey Area Ship Track experiment. Simulations in well-mixed and decoupled boundary layers show that ship effluents are easily advected into the cloud layer in the well-mixed convective boundary layer, whereas their transport may be suppressed by the subcloud transitional layer in the decoupled case. The clear difference between the well-mixed and decoupled cases suggests the important role of diurnal variation of solar radiation and consequent changes in the boundary layer stability for ship track formation. The authors hypothesize that, all other conditions equal, ship track formation may be facilitated during the morning and evening hours when the effects of solar heating are minimal.
In a series of experiments, the authors also studied the effects of additional buoyancy caused by the heat from the ship engine exhaust, the strength of the subcloud transitional layer, and the subcloud layer saturation conditions. The authors conclude that additional heat from ship engine and the increase in ship plume buoyancy may indeed increase the amount of the ship effluent penetrating into the cloud layer. The result, however, depends on the strength of the stable subcloud transitional layer. Another factor in the ship effluent transport is the temperature of the subcloud layer. Its decrease will result in lowering the lifting condensation level and increased ship plume buoyancy. However, the more buoyant plumes in this case have to overcome a larger potential barrier. The relation between all these parameters may be behind the fact that ship tracks sometimes do, and sometimes do not, form in seemingly similar boundary layer conditions.
Abstract
The LES model is applied for studying ship track formation under various boundary layer conditions observed during the Monterey Area Ship Track experiment. Simulations in well-mixed and decoupled boundary layers show that ship effluents are easily advected into the cloud layer in the well-mixed convective boundary layer, whereas their transport may be suppressed by the subcloud transitional layer in the decoupled case. The clear difference between the well-mixed and decoupled cases suggests the important role of diurnal variation of solar radiation and consequent changes in the boundary layer stability for ship track formation. The authors hypothesize that, all other conditions equal, ship track formation may be facilitated during the morning and evening hours when the effects of solar heating are minimal.
In a series of experiments, the authors also studied the effects of additional buoyancy caused by the heat from the ship engine exhaust, the strength of the subcloud transitional layer, and the subcloud layer saturation conditions. The authors conclude that additional heat from ship engine and the increase in ship plume buoyancy may indeed increase the amount of the ship effluent penetrating into the cloud layer. The result, however, depends on the strength of the stable subcloud transitional layer. Another factor in the ship effluent transport is the temperature of the subcloud layer. Its decrease will result in lowering the lifting condensation level and increased ship plume buoyancy. However, the more buoyant plumes in this case have to overcome a larger potential barrier. The relation between all these parameters may be behind the fact that ship tracks sometimes do, and sometimes do not, form in seemingly similar boundary layer conditions.
Abstract
The structure and evolution of the extratropical marine atmosphere boundary layer (MABL) depend largely on the variability of stratus and stratocumulus clouds. Stratus clouds are generally associated with a well-mixed MABL, whereas daytime observations of stratocumulus-topped boundary layers generally indicate that the cloud and subcloud layers are decoupled. In the Atlantic Stratocumulus Transition Experiment, aircraft measurements show a surface-based mixed layer separated from the base of the stratocumulus by a layer that is stable to dry turbulent mixing. This layer forms due to shortwave heating of the stratocumulus clouds. Cumulus clouds often develop in this transition layer and they play a fundamental role in the redistribution of heat in the decoupled stratcumulus-capped boundary layer. They are, however, very sensitive to small changes in the heat and moisture in the boundary layer and are generally transient features that depend directly on the surface sensible and latent heat fluxes. The cumulus contribute a bimodal drop-size distribution to the stratocumulus layer skewed to the smallest sizes but may contain many large drops. Clouds increase at night in response to the combined effect of convection, which can transport drops to the top of the MABL, and outgoing longwave radiation, which cools the boundary layer. The relationship between the cumulus clouds and the latent heat flux is complex. Small cumulus may enhance the flux, but as more water vapor is redistributed vertically by an increase in convective activity the latent heat flux decreases.
This study illustrates the need for boundary-layer models to properly handle the occurrence of intermittent cumulus to predict the diurnal evolution of the stratocumulus-capped MABL.
Abstract
The structure and evolution of the extratropical marine atmosphere boundary layer (MABL) depend largely on the variability of stratus and stratocumulus clouds. Stratus clouds are generally associated with a well-mixed MABL, whereas daytime observations of stratocumulus-topped boundary layers generally indicate that the cloud and subcloud layers are decoupled. In the Atlantic Stratocumulus Transition Experiment, aircraft measurements show a surface-based mixed layer separated from the base of the stratocumulus by a layer that is stable to dry turbulent mixing. This layer forms due to shortwave heating of the stratocumulus clouds. Cumulus clouds often develop in this transition layer and they play a fundamental role in the redistribution of heat in the decoupled stratcumulus-capped boundary layer. They are, however, very sensitive to small changes in the heat and moisture in the boundary layer and are generally transient features that depend directly on the surface sensible and latent heat fluxes. The cumulus contribute a bimodal drop-size distribution to the stratocumulus layer skewed to the smallest sizes but may contain many large drops. Clouds increase at night in response to the combined effect of convection, which can transport drops to the top of the MABL, and outgoing longwave radiation, which cools the boundary layer. The relationship between the cumulus clouds and the latent heat flux is complex. Small cumulus may enhance the flux, but as more water vapor is redistributed vertically by an increase in convective activity the latent heat flux decreases.
This study illustrates the need for boundary-layer models to properly handle the occurrence of intermittent cumulus to predict the diurnal evolution of the stratocumulus-capped MABL.
Abstract
Emissions of particles, gases, heat, and water vapor from ships are discussed with respect to their potential for changing the microstructure of marine stratiform clouds and producing the phenomenon known as “ship tracks.” Airborne measurements are used to derive emission factors of SO2 and NO from diesel-powered and steam turbine-powered ships, burning low-grade marine fuel oil (MFO); they were ∼15–89 and ∼2–25 g kg−1 of fuel burned, respectively. By contrast a steam turbine–powered ship burning high-grade navy distillate fuel had an SO2 emission factor of ∼6 g kg−1.
Various types of ships, burning both MFO and navy distillate fuel, emitted from ∼4 × 1015 to 2 × 1016 total particles per kilogram of fuel burned (∼4 × 1015–1.5 × 1016 particles per second). However, diesel-powered ships burning MFO emitted particles with a larger mode radius (∼0.03–0.05 μm) and larger maximum sizes than those powered by steam turbines burning navy distillate fuel (mode radius ∼0.02 μm). Consequently, if the particles have similar chemical compositions, those emitted by diesel ships burning MFO will serve as cloud condensation nuclei (CCN) at lower supersaturations (and will therefore be more likely to produce ship tracks) than the particles emitted by steam turbine ships burning distillate fuel. Since steam turbine–powered ships fueled by MFO emit particles with a mode radius similar to that of diesel-powered ships fueled by MFO, it appears that, for given ambient conditions, the type of fuel burned by a ship is more important than the type of ship engine in determining whether or not a ship will produce a ship track. However, more measurements are needed to test this hypothesis.
The particles emitted from ships appear to be primarily organics, possibly combined with sulfuric acid produced by gas-to-particle conversion of SO2. Comparison of model results with measurements in ship tracks suggests that the particles from ships contain only about 10% water-soluble materials. Measurements of the total particles entering marine stratiform clouds from diesel-powered ships fueled by MFO, and increases in droplet concentrations produced by these particles, show that only about 12% of the particles serve as CCN.
The fluxes of heat and water vapor from ships are estimated to be ∼2–22 MW and ∼0.5–1.5 kg s−1, respectively. These emissions rarely produced measurable temperature perturbations, and never produced detectable perturbations in water vapor, in the plumes from ships. Nuclear-powered ships, which emit heat but negligible particles, do not produce ship tracks. Therefore, it is concluded that heat and water vapor emissions do not play a significant role in ship track formation and that particle emissions, particularly from those burning low-grade fuel oil, are responsible for ship track formation. Subsequent papers in this special issue discuss and test these hypotheses.
Abstract
Emissions of particles, gases, heat, and water vapor from ships are discussed with respect to their potential for changing the microstructure of marine stratiform clouds and producing the phenomenon known as “ship tracks.” Airborne measurements are used to derive emission factors of SO2 and NO from diesel-powered and steam turbine-powered ships, burning low-grade marine fuel oil (MFO); they were ∼15–89 and ∼2–25 g kg−1 of fuel burned, respectively. By contrast a steam turbine–powered ship burning high-grade navy distillate fuel had an SO2 emission factor of ∼6 g kg−1.
Various types of ships, burning both MFO and navy distillate fuel, emitted from ∼4 × 1015 to 2 × 1016 total particles per kilogram of fuel burned (∼4 × 1015–1.5 × 1016 particles per second). However, diesel-powered ships burning MFO emitted particles with a larger mode radius (∼0.03–0.05 μm) and larger maximum sizes than those powered by steam turbines burning navy distillate fuel (mode radius ∼0.02 μm). Consequently, if the particles have similar chemical compositions, those emitted by diesel ships burning MFO will serve as cloud condensation nuclei (CCN) at lower supersaturations (and will therefore be more likely to produce ship tracks) than the particles emitted by steam turbine ships burning distillate fuel. Since steam turbine–powered ships fueled by MFO emit particles with a mode radius similar to that of diesel-powered ships fueled by MFO, it appears that, for given ambient conditions, the type of fuel burned by a ship is more important than the type of ship engine in determining whether or not a ship will produce a ship track. However, more measurements are needed to test this hypothesis.
The particles emitted from ships appear to be primarily organics, possibly combined with sulfuric acid produced by gas-to-particle conversion of SO2. Comparison of model results with measurements in ship tracks suggests that the particles from ships contain only about 10% water-soluble materials. Measurements of the total particles entering marine stratiform clouds from diesel-powered ships fueled by MFO, and increases in droplet concentrations produced by these particles, show that only about 12% of the particles serve as CCN.
The fluxes of heat and water vapor from ships are estimated to be ∼2–22 MW and ∼0.5–1.5 kg s−1, respectively. These emissions rarely produced measurable temperature perturbations, and never produced detectable perturbations in water vapor, in the plumes from ships. Nuclear-powered ships, which emit heat but negligible particles, do not produce ship tracks. Therefore, it is concluded that heat and water vapor emissions do not play a significant role in ship track formation and that particle emissions, particularly from those burning low-grade fuel oil, are responsible for ship track formation. Subsequent papers in this special issue discuss and test these hypotheses.
