Heat Flux in the Strong-Wind Nocturnal Boundary Layer

Mahrt, L.

doi:10.1007/s10546-016-0219-9

Cited by 14 publications

(9 citation statements)

References 34 publications

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“…One can notice that at high wind speed, there is a quasi balance between heating due to horizontal advection and turbulent cooling due to mixing with the underneath colder surface. Following Mahrt (), the role of horizontal advection of heat in strong wind conditions explains why in our simulations, for

U_{9 m} > 7 m {normals}^{- 1}

, the near‐surface inversion no longer decreases with increasing wind speed—as it would do in a turbulent SBL in horizontally homogeneous conditions—but it stabilizes at values around 3–4 K (see Figures e–7h). Moreover, Figure b shows that for near‐surface inversions greater than 10 K, the enthalpy budget tends to be dominated by the heating associated to the subsidence (adiabatic heating + vertical advection) and by the longwave radiative cooling.…”

Section: Representing the Boundary‐layer Dynamics At Dome Csupporting

confidence: 84%

Modeling the Dynamics of the Atmospheric Boundary Layer Over the Antarctic Plateau With a General Circulation Model

Vignon

Hourdin

Genthon

et al. 2018

J Adv Model Earth Syst

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Observations evidence extremely stable boundary layers (SBL) over the Antarctic Plateau and sharp regime transitions between weakly and very stable conditions. Representing such features is a challenge for climate models. This study assesses the modeling of the dynamics of the boundary layer over the Antarctic Plateau in the LMDZ general circulation model. It uses 1 year simulations with a stretched‐grid over Dome C. The model is nudged with reanalyses outside of the Dome C region such as simulations can be directly compared to in situ observations. We underline the critical role of the downward longwave radiation for modeling the surface temperature. LMDZ reasonably represents the near‐surface seasonal profiles of wind and temperature but strong temperature inversions are degraded by enhanced turbulent mixing formulations. Unlike ERA‐Interim reanalyses, LMDZ reproduces two SBL regimes and the regime transition, with a sudden increase in the near‐surface inversion with decreasing wind speed. The sharpness of the transition depends on the stability function used for calculating the surface drag coefficient. Moreover, using a refined vertical grid leads to a better reversed “S‐shaped” relationship between the inversion and the wind. Sudden warming events associated to synoptic advections of warm and moist air are also well reproduced. Near‐surface supersaturation with respect to ice is not allowed in LMDZ but the impact on the SBL structure is moderate. Finally, climate simulations with the free model show that the recommended configuration leads to stronger inversions and winds over the ice‐sheet. However, the near‐surface wind remains underestimated over the slopes of East‐Antarctica.

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U_{9 m} > 7 m {normals}^{- 1}

Section: Representing the Boundary‐layer Dynamics At Dome Csupporting

confidence: 84%

Modeling the Dynamics of the Atmospheric Boundary Layer Over the Antarctic Plateau With a General Circulation Model

Vignon

Hourdin

Genthon

et al. 2018

J Adv Model Earth Syst

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“…Variations of kinematic heat flux with stability show the occurrences of maximum downward heat flux under moderately stable conditions (the turning point ). This stability turning point, which marks the transition from weakly to strongly stable regimes (Mahrt, ; Van De Wiel et al, ), increases with height. This increase of the turning point with height implies that a stronger stratification is required to offset the greater shear‐generated mixing at higher levels with stronger wind speeds.…”

Section: Resultsmentioning

confidence: 99%

Distinct Turbulence Structures in Stably Stratified Boundary Layers With Weak and Strong Surface Shear

Lan

et al. 2018

JGR Atmospheres

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Turbulence structures and exchange of momentum and heat in the nocturnal stable boundary layer (SBL) show distinct features under different stability conditions prompting interest in their connection.Here eddy covariance data collected at four different heights on a 62-m meteorological tower over a large open flat terrain are used to characterize different SBL states and associated turbulence structures. In a SBL characterized by strong near-surface winds, turbulent eddy sizes scale with their observational heights and the SBL experiences enhanced turbulent mixing of momentum and heat throughout (a state hereafter referred to as a "coupled" state). Conversely, in a decoupled SBL, weak winds occur near the surface and turbulent eddies are depressed and detached from the boundary leading to suppressed vertical mixing and layered SBL profiles. Because the transport of momentum and heat to the surface across SBL layers is determined by turbulent eddies, cross-layer correlation and the aforementioned SBL coupling states can be delineated by distinct turbulence structures.

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“…Clear skies and significant radiative cooling induce drainage/down-valley flows and thus prevent very low wind speeds. In addition, the relationship between the wind and stratification at the SCP site depends partly on the wind direction through horizontal temperature advection (Mahrt 2017a).…”

Section: Dependence On Wind Speedmentioning

confidence: 99%

Small-Scale Variability in the Nocturnal Boundary Layer

Mahrt

Pfister

Thomas

2019

Boundary-Layer Meteorol

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Nocturnal variations of temperature and wind are examined at three contrasting sites. After the early evening period of rapid cooling, the magnitude of the variations of temperature on a time scale of 10 min to an hour often become larger than the corresponding temperature change due to the nocturnal trend. These shorter-term temperature variations are forced by wave-like motions and more complex modes. Observations from a network of stations across a shallow valley at one of the sites are analyzed in more detail. Typically, decreasing wind speed corresponds to less mixing and lower temperature at the surface followed by increasing wind speed, increased mixing, and higher temperatures. The flow may continue to switch back and forth between these two states for much of the night. These non-stationary motions interact with motions induced by the gentle local topography, leading to intermittent local drainage flows, transient cold pools, and both propagating and semi-stationary microfronts.

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Heat Flux in the Strong-Wind Nocturnal Boundary Layer

Cited by 14 publications

References 34 publications

Modeling the Dynamics of the Atmospheric Boundary Layer Over the Antarctic Plateau With a General Circulation Model

Modeling the Dynamics of the Atmospheric Boundary Layer Over the Antarctic Plateau With a General Circulation Model

Distinct Turbulence Structures in Stably Stratified Boundary Layers With Weak and Strong Surface Shear

Small-Scale Variability in the Nocturnal Boundary Layer

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