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1

Vertical structure of the stable boundary layer detected by RASS-SODAR and in-situ measurements in SABLES 2006 field campaign

Viana S., Yague C., Maqueda G.

Acta Geophysica

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2012

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Vol. 60, no. 5

1261-1286

EN

Data from the SABLES 2006 field campaign are used in order to analyse some of the main processes present along the nocturnal periods: surface-based inversions, low level jets, katabatic winds, wave-like motions, pressure perturbations, etc. These processes have an important influence on the vertical structure (both thermal and dynamical) of the atmospheric boundary layer, and can be better described with the synergetic combination of RASS-SODAR data and in-situ measurements (such as sonic anemometer data and high-resolution pressure series from microbarometers). It is shown how the different air masses and their evolution are easily identified when pressure and RASS-SODAR wind and temperature data are presented together. Likewise, periodic pressure fluctuations observed in the surface array of microbarometers reveal the existence of gravity wave motions whose propagation is better understood after locating the wave ducting layers with the help of RASS-SODAR average wind ant temperature profiles.

2

Estimation of orographically induced wave drag in the stable boundary layer during the CASES-99 experimental campaign

Steeneveld G.J., Nappo C.J., Holtslag A.A.M.

Acta Geophysica

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2009

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Vol. 57, no. 4

857-881

EN

This paper addresses the quantification of gravity wave drag due to small hills in the stable boundary layer. A single column atmospheric model is used to forecast wind and temperature profiles in the boundary layer. Next, these profiles are used to calculate vertical profiles of gravity wave drag. Climatology of wave drag magnitude and "wave drag events" is presented for the CASES-99 experimental campaign. It is found that gravity wave drag events occur for several relatively calm nights, and that the wave drag is then of equivalent magnitude as the turbulent drag. We also illustrate that wave drag events modify the wind speed sufficiently to substantially change the surface sensible heat flux.

3

Assessment of the hydraulic slope flow approach using a mesoscale model

Martínez D., Cuxart J.

Acta Geophysica

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2009

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Vol. 57, no. 4

882-903

EN

The simplified hydraulic two-layer model for a katabatic flow is analysed using the outputs from a high-resolution mesoscale simulation. A stably stratified night is simulated for the Duero basin, a complex terrain area located in the northern Spanish plateau, with large vertical and horizontal spatial resolution. Well-defined katabatic flows on the basin slopes are generated by the simulation, that are relatively stationary and quasi-bidimensional for some areas in the central part of the night. The bulk quantities used in the two-layer approach as well as the different terms in the equations are computed from the three-dimensional information provided by the mesoscale simulation. This method allows to inspect how well the simplified approach represents the katabatic flow generated by the mesoscale model. The study shows that the hydraulic model allows for a comprehensive analysis of the basic mechanisms of the slope flows but is not able to close the budget equations, since the residuals are large.

4

Turbulent measurements in the stable atmospheric boundary layer during SHEBA: ten years after

Grachev A., Landreas E., Fairall C., Guest P., Persson P.

Acta Geophysica

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2008

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Vol. 56, no. 1

142-166

EN

This paper surveys results of the comprehensive turbulent measurements in the stable boundary layer (SBL) made over the Arctic pack ice during the Surface Heat Budget of the Arctic Ocean experiment (SHEBA) in the Beaufort Gyre from October 1997 through September 1998. Turbulent fluxes and mean meteorological data were continuously measured and reported hourly at five levels on a 20-m main SHEBA tower. Eleven months of measurements during SHEBA cover a wide range of stability conditions, from the weakly unstable regime to very stable stratification, and allow studying the SBL in detail. A brief overview of the SBL regimes, the fluxprofile relationships, the turbulent Prandtl number, and other parameters obtained during SHEBA is given. The traditional Monin-Obukhov approach, z-less scaling, and gradient-based scaling are evaluated and discussed based on the data from SHEBA.

5

The CIRES Tethered Lifting System: a survey of the system, past results and future capabilities

Balsley B.

Acta Geophysica

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2008

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Vol. 56, no. 1

21-57

EN

This paper presents a survey of the Tethered Lifting System (TLS) being developed for atmospheric research by the Cooperative Institute for Research in Environmental Sciences (CIRES) at University of Colorado. Basic elements of the system are described, and examples of both published results and work in progress are presented.

6

Stable-boundary-layer regimes from the perspective of the low-level jet

Banta R.

Acta Geophysica

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2008

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Vol. 56, no. 1

58-87

EN

This paper reviews results from two field studies of the nocturnal stable atmospheric boundary layer (SBL) over the Great Plains of the United States. Data from a scanning remote-sensing system, a High-Resolution Doppler Lidar (HRDL), provided measurements of mean and turbulent wind components at high spatial and temporal resolution through the lowest 500-1000 m of the atmosphere. This data set has allowed the characteristics of the low-level jet (LLJ) maximum (speed, height, direction) to be documented through entire nights. LLJs form after sunset and pro-duce strong shear in the layer below the LLJ maximum or nose, which is a source of turbulence and mixing in the SBL. Simultaneous HRDL measurements of turbulence quantities related to turbulence kinetic energy (TKE) has allowed the turbulence in the subjet layer to be related to LLJ properties. Turbulence structure was found to be a function of the bulk stability of the subjet layer. For the strong-LLJ (> 15 m s-1), weakly stable cases the strength of the turbulence is proportional to the strength of the LLJ. For these cases with nearly continuous turbulence in the subjet layer, low-level jet scaling, in which lengths are scaled by the LLJ height and velocity variables are scaled by the LLJ speed, was found to be appropriate. For the weak-wind (< 5 m s-1 in the lowest 200 m), very stable boundary layer (vSBL), the boundary layer was found to be very shallow (sometimes < 10 m deep), and turbulent fluxes between the earth's surface and the atmosphere were found to be essentially shut down. For more intermediate wind speeds and stabilities, the SBL shows varying degrees of intermittency due to various mechanisms, including shear-instability and other gravity waves, density currents, and other mesoscale disturbances.

7

An inconvenient “truth” about using sensible heat flux as a surface boundary condition in models under stably stratified regimes

Basu S., Van De Wiel B., Moene A., Steeneveld G.

Acta Geophysica

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2008

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Vol. 56, no. 1

88-99

EN

In single column and large-eddy simulation studies of the atmospheric bound-ary layer, surface sensible heat flux is often used as a boundary condition. In this paper, we delineate the fundamental shortcomings of such a boundary condition in the context of stable boundary layer modelling and simulation. Using an analytical approach, we are able to show that for reliable model results of the stable boundary layer accurate surface temperature prescription or prediction is needed. As such, the use of surface heat flux as a boundary condition should be avoided in stable condi-tions.