Wind profiles, momentum fluxes and roughness lengths at Cabauw revisited

Abstract

We describe the results of an experiment focusing on wind speed and momentum fluxes in the atmospheric boundary layer up to 200 m. The measurements were conducted in 1996 at the Cabauw site in the Netherlands. Momentum fluxes are measured using the K-Gill Propeller Vane. Estimates of the roughness length are derived using various techniques from the wind speed and flux measurements, and the observed differences are explained by considering the source area of the meteorological parameters. A clear rough-to-smooth transition is found in the wind speed profiles at Cabauw. The internal boundary layer reaches the lowest k-vane (20 m) only in the south-west direction where the obstacle-free fetch is about 2 km. The internal boundary layer is also reflected in the roughness lengths derived from the wind speed profiles. The lower part of the profile (<40 m) is not in equilibrium and no reliable roughness analysis can be given. The upper part of the profile can be linked to a large-scale roughness length. Roughness lengths derived from the horizontal wind speed variance and gustiness have large footprints and therefore represent a large-scale average roughness. The drag coefficient is more locally determined but still represents a large-scale roughness length when it is measured above the local internal boundary layer. The roughness length at inhomogeneous sites can therefore be determined best from drag coefficient measurements just above the local internal boundary layers directly, or indirectly from horizontal wind speed variance or gustiness. In addition, the momentum and heat fluxes along the tower are analysed and these show significant variation with height related to stability and possibly surface heterogeneity. It appears that the dimensionless wind speed gradients scale well with local fluxes for the variety of conditions considered, including the unstable cases.

Continuous Single-Column Model Evaluation at a Permanent Meteorological Supersite

“…This has been illustrated by a number of studies [see, e.g., Fig. 1 of Verkaik and Holtslag (2007)]. For example, in the case of spatial heterogeneity in the roughness length of the surface, the behavior of the low-level wind can become dependent on the prevailing wind direction.…”

Buoyancy effects on the scaling characteristics of atmospheric boundary-layer wind fields in the mesoscale range

“…FINO 1 is an offshore platform in the North Sea [25][26][27] Over the past four decades, observational data from the Cabauw (the Netherlands) meteorological tower have been used in various ABL studies [28][29][30][31] All the aforementioned datasets contain variable amount of missing data. This data-loss problem is more severe for the sodars.…”

Dificuldades de estimativa dos parâmetros de rugosidade aerodinâmica pelo perfil logarítmico do vento sobre vegetação esparsa em região semi-árida

“…Os valores inconsistentes de d e z 0 podem ter sido proporcionados pela influência da estabilidade na forma do perfil, por baixas velocidades do vento ou por medidas fora da SCI (Molion & Moore, 1983;Wieringa, 1993;de Bruin & Verhoef, 1997;Britter & Hanna, 2003;Tsai & Tsuang, 2005;Verkaik & Holtslag, 2007).…”

“…Nesses métodosé necessário que observações acuradas de U z se restrinjamà subcamada inercial da camada limite de fluxo constante ou de equilíbrio para satisfazer as condições de validade do perfil logarítmico (Monteith & Unsworth, 1990;Wieringa, 1993;de Bruin & Verhoef, 1997;Britter & Hanna, 2003;Tsai & Tsuang, 2005;Verkaik & Holtslag, 2007). Ou seja, o nível inferior de medida deve ser acima da subcamada rugosa (z > z * ) e a bordadura (fetch ) com extensão entre 50 e 100 vezes a maior altura de medida de U z (Wieringa, 1993;de Bruin & Verhoef, 1997).…”

“…O fluxo na SCR varia em três dimensões, em função da proximidade dos elementos rugosos, com transferência vertical de momentum denominada de "dissipativa", resultado do movimento não turbulento, dependente da localização individual dos elementos rugosos (Raupach & Thom, 1981;Tsai & Tsuang, 2005). Enquanto na camada limite de fluxo constante, os fluxos estão em equilíbrio dinâmico, com desvio padrão normalizado do vento (σ u /u * ) e coeficiente de arrasto normalizado independente da altura e coeficiente adimensional de transporte turbulento para momentum (φ m ) igual a unidade, em condições adiabáticas, ou seja, a teoria de similaridade de Monin-Obukhové válida (Verkaik & Holtslag, 2007). A espessura da camada limite de equilíbrio depende das condições de estabilidade e da rugosidade aerodinâmica da superfície (Wieringa, 1993;Tsai & Tsuang, 2005).…”