Regional shear stress of broken forest from radiosonde wind profiles in the unstable surface layer

The determination of the sensible heat flux over urban terrain is challenging due to irregular surface geometry and surface types. To address this, in 2006-07, a major field campaign (LUCE) took place at the École Polytechnique Fédérale de Lausanne campus, a moderately occupied urban site. A distributed network of 92 wireless weather stations was combined with routine atmospheric profiling, offering high temporal and spatial resolution meteorological measurements. The objective of this study is to estimate the sensible heat flux over the built environment under convective conditions. Calculations were based on Monin-Obukhov similarity for temperature in the surface layer. The results illustrate a good agreement between the sensible heat flux inferred from the thermal roughness length approach and independent calibrated measurements from a scintillometer located inside the urban canopy. It also shows that using only one well-selected station can provide a good estimate of the sensible heat flux over the campus for convective conditions. Overall, this

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“…This is in good agreement with Parlange and Brutsaert [38] who found u * /U = 0.131 over a forested area for the same reference height.…”

Section: Determination Of the Thermal Roughness Lengthsupporting

confidence: 92%

Estimation of urban sensible heat flux using a dense wireless network of observations

et al. 2009

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“…One of them is the wind profile equation for surface layer of the ABL, derived on the basis of Monin-Obukhov similarity theory. Even though it was obtained and has been extensively tested over uniform sites, recent findings appear to support the validity of the equation over a complex region as well [e.g., Kustas and Brutsaert, 1986;Parlange and Brutsaert, 1993; Hiyama et al, 1996], provided that certain conditions are satisfied. More specifically, the measurement height, the horizontal scale of surface variability, and their relative magnitude play an important role in whether or not the profile equation is valid over the region.…”

Section: Introductionmentioning

confidence: 97%

Regional surface momentum flux derived from atmospheric boundary layer bulk similarity approach

Sugita¹,

Endo²,

Hiyama³

1999

J. Geophys. Res.

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Abstract. Wind speed data in the atmospheric boundary layer (ABL) observed during two recent experiments over a wide stability range and over different experimental areas allowed an extensive test of the ABL bulk similarity equation with the similarity functions B w previously proposed and calibrated with the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) data set in order to obtain regional surface momentum fluxes. It was found that FIFE-calibrated Bw functions produced similarly good results with the two new data sets; furthermore, one similarity function B w based on a simple dual-layer ABL model which worked best with the FIFE data set was also found to perform best with the new data sets. Recalibration of the constant of this B• function with all three combined data sets resulted in the same value as that calibrated with the FIFE data set. IntroductionThe knowledge of surface momentum fluxes of a region is needed in many Earth science disciplines such as hydrology and meteorology. Because of its importance, there has been steady progress in the method of determining the regional fluxes. One possible approach is to determine the regional surface roughness parameters and to use them together with wind data in the atmospheric boundary layer ( Another formulation one can employ on is the ABL bulk similarity approach. It can be derived from two governing equations for the mixed layer and for the surface layer by assuming an overlap region of the two, and by "patching" these two equations [e.g., Brutsaert, 1982]. For this equation to be valid over a complex region, the same conditions that need to be satisfied for the application of the profile equation over such a region must also be satisfied. However, since the height of measurements for the ABL bulk similarity equation is higher than that for the surface layer profile equation, the restrictions for use of the ABL bulk similarity equation over a complex region are expected to be relaxed to some extent.The ABL bulk similarity equations have been known for a long time, but only recently have they come to be recognized as a practical tool for determining surface momentum fluxes. Brutsaert and and Sugita and Brutsaert [1992]

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“…There is a natural gap in scale between the mesoscale patches (studied as GCM subgrid processes) and the local patches (scale O 2 km) addressed here, for while the ABL adjusts fully (i.e., organizes) to each of the mesoscale patches [e.g., Raupach, 1991], the flux from the local patches is integrated or mixed through the ABL with coupling between patches affected by the boundary layer turbulence [Brutsaert and Parlange, 1992; Parlange and Brutsaert, 1993]. The ABL thus represents the composite effect of the local patches, including the nonlinear interaction between the patches.…”

Section: Introductionmentioning

confidence: 99%

Surface length scales and shear stress: Implications for land‐atmosphere interaction over complex terrain

Albertson¹,

Parlange²

1999

Water Resources Research

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239

167

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Abstract. A large eddy simulation (LES) code of the atmospheric boundary layer (ABL)has been developed and applied to study the effect of spatially variable surface properties on the areally averaged surface shear stress at the land-atmosphere interface. The LES code simulates the space and time evolution of the large-scale turbulent eddies and their transport effects in the ABL. We report here on simulations of flow over spatially variable roughness fields. The dynamics are simulated, and the resulting space-time fields are averaged to explore the effects of the surface variability length scales on the average surface shear stress, as used in large-scale models to estimate scalar fluxes, such as evaporation. We observe asymmetrical response of the smooth-to-rough and rough-tosmooth transitions, such that the effects of the transitions accumulate rather than cancel.It is shown that the presence of abrupt changes in surface roughness and the atmosphere's response to these patches create a marked dependence of the statistical structure of surface shear stress on the length scale of the surface patches. An increase in regionally averaged surface stress for decreasing horizontal patch length scale is found. IntroductionSuccessful modeling of surface hydrologic and atmospheric processes hinges on the ability to describe the exchange of water, heat, and momentum across the land-atmosphere interface. A major concern is how to account for the effect of the spatial variability of surface conditions on scales smaller than the model grid cell. The actual total grid cell exchange is an integration of small-scale exchange processes over the area of the cell, where the physical integration is provided by the turbulent mixing in the atmospheric boundary layer (ABL).

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Regional shear stress of broken forest from radiosonde wind profiles in the unstable surface layer

Cited by 51 publications

References 21 publications

Estimation of urban sensible heat flux using a dense wireless network of observations

Estimation of urban sensible heat flux using a dense wireless network of observations

Regional surface momentum flux derived from atmospheric boundary layer bulk similarity approach

Surface length scales and shear stress: Implications for land‐atmosphere interaction over complex terrain

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