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- Author or Editor: R. A. Brost x
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Abstract
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Abstract
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Abstract
For representative tropospheric profiles of water vapor, CO2 and temperature we have calculated in situ longwave radiative flux divergence for use in a simplified second-order closure model of nocturnal boundary-layer evolution. The time evolution of “bulk” boundary-layer parameters is little affected by radiational cooling, as seems to be the cast for stress, velocity variance and diffusivity profiles within this layer. In contrast the
Thermodynamically the boundary layer develops a three-layer structure—in the lowest (0.1 h thick) and uppermost (0.2 h thick) layers radiative cooling dominates the total cooling, while in the central layer occupying most of the boundary layer (0.7 h thick) turbulent cooling dominates. At h itself radiative cooling is a significant fraction of the surface cooling rate.
Radiative effects are greatest above the boundary layer where large gradient Richardson numbers am generated. Consequently, turbulence in this region decays rapidly after transition, while in the absence of such effects a much slower decay occurs.
Abstract
For representative tropospheric profiles of water vapor, CO2 and temperature we have calculated in situ longwave radiative flux divergence for use in a simplified second-order closure model of nocturnal boundary-layer evolution. The time evolution of “bulk” boundary-layer parameters is little affected by radiational cooling, as seems to be the cast for stress, velocity variance and diffusivity profiles within this layer. In contrast the
Thermodynamically the boundary layer develops a three-layer structure—in the lowest (0.1 h thick) and uppermost (0.2 h thick) layers radiative cooling dominates the total cooling, while in the central layer occupying most of the boundary layer (0.7 h thick) turbulent cooling dominates. At h itself radiative cooling is a significant fraction of the surface cooling rate.
Radiative effects are greatest above the boundary layer where large gradient Richardson numbers am generated. Consequently, turbulence in this region decays rapidly after transition, while in the absence of such effects a much slower decay occurs.
Abstract
No abstract available.
Abstract
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Abstract
A second-order turbulence model is used to study the stable boundary layer (SBL). Over a horizontal surface, a constant surface cooling rate drives the SBL to a steady state within a few hours. Parameterizations are developed for eddy diffusivities, the kinetic energy dissipation rate and the geostrophic drag law in this idealized case. Over a sloped surface, a constant cooling rate produces a quasi-steady-state SBL in which some flow properties continue to vary but h(|f|/u * L)½ becomes constant; however, this constant is a function of the wind direction relative to the slope and the baroclinity, as measured by the cooling rate times the slope. Calculated eddy diffusivity profiles in the baroclinic (sloping terrain) case compare well with recent data from Antarctica. If a surface energy budget is used rather than a constant cooling rate, the SBL does not reach a steady state even over a horizontal surface; the nondimensional height slowly decays. We conclude that equilibrium models of the SBL are likely to be much less applicable to the real world than are their counterparts for the convective boundary layer.
Abstract
A second-order turbulence model is used to study the stable boundary layer (SBL). Over a horizontal surface, a constant surface cooling rate drives the SBL to a steady state within a few hours. Parameterizations are developed for eddy diffusivities, the kinetic energy dissipation rate and the geostrophic drag law in this idealized case. Over a sloped surface, a constant cooling rate produces a quasi-steady-state SBL in which some flow properties continue to vary but h(|f|/u * L)½ becomes constant; however, this constant is a function of the wind direction relative to the slope and the baroclinity, as measured by the cooling rate times the slope. Calculated eddy diffusivity profiles in the baroclinic (sloping terrain) case compare well with recent data from Antarctica. If a surface energy budget is used rather than a constant cooling rate, the SBL does not reach a steady state even over a horizontal surface; the nondimensional height slowly decays. We conclude that equilibrium models of the SBL are likely to be much less applicable to the real world than are their counterparts for the convective boundary layer.
Abstract
Using simulations with a large-eddy model we have studied the decay of convective turbulence in the atmospheric boundary layer when the upward surface sensible heat flux is suddenly stopped. The decay of turbulent kinetic energy and temperature variance scales with the dimensionless time tw */h. The temperature fluctuations start to decrease almost immediately after the forcing has been removed, whereas the turbulent kinetic energy stays constant for a time t ≈ h/w *. Vertical velocity fluctuations decay faster than horizontal fluctuations. Entrainment persists well into the decay process and may explain departures from similarity. Some evidence suggests a decoupling of large and small scales during the decay.
Abstract
Using simulations with a large-eddy model we have studied the decay of convective turbulence in the atmospheric boundary layer when the upward surface sensible heat flux is suddenly stopped. The decay of turbulent kinetic energy and temperature variance scales with the dimensionless time tw */h. The temperature fluctuations start to decrease almost immediately after the forcing has been removed, whereas the turbulent kinetic energy stays constant for a time t ≈ h/w *. Vertical velocity fluctuations decay faster than horizontal fluctuations. Entrainment persists well into the decay process and may explain departures from similarity. Some evidence suggests a decoupling of large and small scales during the decay.
Abstract
The mean radiational, dynamical and thermodynamical structure of the marine stratocumulus-topped mixed layers of the California coast is described for two days in June 1976 using data from the NCAR Electra aircraft. We suggest that the synoptic conditions found may be typical of about half of the shallow stratocumulus-topped boundary layers that occur in this region during summer. The inversion was low near the coast and increased in height to the west, consistent with the average westward increase in sea-surface temperature. North–south inversion height change was largely due to entrainment and mean mesoscale vertical motions. Below the inversion, strong winds (12–20 m s−1 from the north) and horizontal inhomogeneities resulted in large advection terms in mean field equations. The sloping inversion often produced large vertical shears of the actual and geostrophic wind velocities across the inversion. Because of low liquid-water contents (0.1 g kg−1), temperature and water vapor could be measured in cloud with in situ instrumentation without significant errors due to wetting.
The longwave radiative extinction length was found to be relatively short; 63% of the cloud-top jump in radiation flux occurred within 40 m. Radiative heat loss was largely balanced by shear-driven entrainment. Compositing vertical gradients provided by individual aircraft ascents and descents is shown to overestimate vertical gradients at the inversion.
Abstract
The mean radiational, dynamical and thermodynamical structure of the marine stratocumulus-topped mixed layers of the California coast is described for two days in June 1976 using data from the NCAR Electra aircraft. We suggest that the synoptic conditions found may be typical of about half of the shallow stratocumulus-topped boundary layers that occur in this region during summer. The inversion was low near the coast and increased in height to the west, consistent with the average westward increase in sea-surface temperature. North–south inversion height change was largely due to entrainment and mean mesoscale vertical motions. Below the inversion, strong winds (12–20 m s−1 from the north) and horizontal inhomogeneities resulted in large advection terms in mean field equations. The sloping inversion often produced large vertical shears of the actual and geostrophic wind velocities across the inversion. Because of low liquid-water contents (0.1 g kg−1), temperature and water vapor could be measured in cloud with in situ instrumentation without significant errors due to wetting.
The longwave radiative extinction length was found to be relatively short; 63% of the cloud-top jump in radiation flux occurred within 40 m. Radiative heat loss was largely balanced by shear-driven entrainment. Compositing vertical gradients provided by individual aircraft ascents and descents is shown to overestimate vertical gradients at the inversion.
Abstract
This paper discusses the turbulence profiles and budgets for two days of radiation, dynamical and thermodynamical observations by the NCAR Electra in shallow marine stratocumulus off the California coast in June 1976.
The boundary layer is characterized by relatively high wind speeds (12–20 m s−1) and low liquid water contents (0.1 g kg−1); the clouds are not very convective and seem to have little influence on the turbulence budgets. In cloud, drizzle has a significant impact on the liquid water budget and occasionally even on the total water budget even though no drizzle is observed at the surface. The stresses, velocity variances, and their budgets behave as in a neutral boundary layer, sometimes with an additional peak in the cross-wind variance at the inversion due to shear production.
There is scant evidence of direct production of vertical velocity variance at cloud top due to radiative cooling or latent heat release; it is maintained principally by the pressure-scrambling terms through redistribution of the shear-produced energy. We find, however, that while the Rotta parameterization for pressure scrambling in the stress budgets works well near the surface and sometimes throughout the layer, it is unsatisfactory in the variance budgets.
Fluctuations of temperature and moisture on a scale of several hundred meters in cloud satisfy the Clausius-Clapeyron equation. When the boundary layer is well mixed in equivalent potential temperature and total water substance, the vertical turbulent fluxes of these quantities are usually almost linear. The efficiency of cloud-top radiative cooling in producing mixed-layer convection is also discussed.
Abstract
This paper discusses the turbulence profiles and budgets for two days of radiation, dynamical and thermodynamical observations by the NCAR Electra in shallow marine stratocumulus off the California coast in June 1976.
The boundary layer is characterized by relatively high wind speeds (12–20 m s−1) and low liquid water contents (0.1 g kg−1); the clouds are not very convective and seem to have little influence on the turbulence budgets. In cloud, drizzle has a significant impact on the liquid water budget and occasionally even on the total water budget even though no drizzle is observed at the surface. The stresses, velocity variances, and their budgets behave as in a neutral boundary layer, sometimes with an additional peak in the cross-wind variance at the inversion due to shear production.
There is scant evidence of direct production of vertical velocity variance at cloud top due to radiative cooling or latent heat release; it is maintained principally by the pressure-scrambling terms through redistribution of the shear-produced energy. We find, however, that while the Rotta parameterization for pressure scrambling in the stress budgets works well near the surface and sometimes throughout the layer, it is unsatisfactory in the variance budgets.
Fluctuations of temperature and moisture on a scale of several hundred meters in cloud satisfy the Clausius-Clapeyron equation. When the boundary layer is well mixed in equivalent potential temperature and total water substance, the vertical turbulent fluxes of these quantities are usually almost linear. The efficiency of cloud-top radiative cooling in producing mixed-layer convection is also discussed.
