Analysis of profiles of meteorological measurements from a 160 m high mast at the National Test Site for wind turbines at Høvsøre (Denmark) and at a 250 m high TV tower at Hamburg (Germany) shows that the wind profile based on surface-layer theory and Monin-Obukhov scaling is valid up to a height of 50-80 m. At higher levels deviations from the measurements progressively occur. For applied use an extension to the wind profile in the surface layer is formulated for the entire boundary layer, with emphasis on the lowest 200-300 m and considering only wind speeds above 3 m s −1 at 10 m height. The friction velocity is taken to decrease linearly through the boundary layer. The wind profile length scale is composed of three component length scales. In the surface layer the first length scale is taken to increase linearly with height with a stability correction following Monin-Obukhov similarity. Above the surface layer the second length scale (L MBL ) becomes independent of height but not of stability, and at the top of the boundary layer the third length scale is assumed to be negligible. A simple model for the combined length scale that controls the wind profile and its stability dependence is formulated by inverse summation. Based on these assumptions the wind profile for the entire boundary layer is derived. A parameterization of L MBL is formulated using the geostrophic drag law, which relates friction velocity and geostrophic wind. The empirical parameterization of the resistance law functions A and B in the geostrophic drag law is uncertain, making it impractical. Therefore an expression for the length scale, L MBL , for applied use is suggested, based on measurements from the two sites.
Two synthetic aperture radar (SAR) images acquired by the European Remote Sensing Satellite ERS-1 over the Jade-Weser estuary in the German Bight of the North Sea on January 2 and 20, 1992, are analyzed. The images show sea surface manifestations of atmospheric boundary layer rolls. This is inferred from the orientation of the quasi-periodic sea surface patterns which are aligned approximately with the wind direction, from the ratio of the wavelength of the patterns to the height of the boundary layer, and from the conditions encountered in the atmospheric boundary layer as measured quasi-simultaneously by radiosondes. The atmospheric boundary layer rolls were generated by a dynamic instability on January 2 and by a thermal instability on January 20. For the first time, quantitative estimates of variations of the wind velocity at the sea surface associated with the atmospheric rolls are extracted from a spaceborne radar SAR image. It is shown that wind velocities derived from SAR image intensity variations are in agreement with theoretical estimates. tion (i.e. VV polarization), this relationship is given by the so-called CMOD4 model function (see section 5). In this paper we analyze two ERS-1 SAR images which were acquired on January 2 and 20, 1992, over the J ade-Weser estuary in the German Bight of the North Sea. They show quasi-periodic patterns on the sea surface aligned approximately with the wind which are interpreted as sea surface manifestations of atmospheric boundary layer rolls. Atmospheric boundary layer rolls are helical circulation patterns in the atmospheric boundary layer which are superimposed on the mean wind field, i.e., the primary flow (Figure 1). They can be generated either by thermal instability (Rayleigh-Bdnard instability) when the layer is heated from below or cooled from above or by dynamic instability (inflection point instability) when the wind velocity changes with height in such a way that an inflection point occurs in the wind component normal to the roll axis, as, e.g., in the Ekman wind profile. Observations [e.g., LeMone, 1973; Briimmer, 1985], theoretical analyses [e.g., Brown, 1970] and numerical model simulations [Deardorff, 1972; Mason, 1983] give a fairly good view of the roll-scale flow or secondary flow pattern. The roll axes are oriented between the directions of the mean surface wind and the geostrophic wind above the boundary layer. Usually the boundary layer is capped by an inversion so that the depths of the boundary layer and the roll layer coincide. In the case of a thermal instability, the aspect ratio, i.e., the horizontal wavelength of the roll pattern A divided by the roll height h, is 2.8 according to the linear P•ayleigh-Bdnard convection. The most frequently observed values range between 2 and 4 [LeMone, 1973; Kelly, 1984; Kuettner, 1971]. In the case of an inflection point instability, the aspect ratio is about 2 ac-12,613 12,614 I scatterometer calibration and validation activities at ECMWF, B, From radar backscatter characteristics to wind vector soluti...
During the field experiment ARKTIS 1993 ten cases of boundary-layer modification in wintertime cold-air outbreaks from the Arctic sea ice in the Spitsbergen region were observed by aircraft over a distance ranging from about 50 km over the ice to about 300 km over the water. The modification depends decisively on the initial conditions over the ice, the boundary conditions at the bottom and top of the boundary layer and on the conditions of the large-scale flow. The modification of the bulk boundary-layer characteristics in relation to these conditions is presented.Besides the air-sea temperature contrast, the most important role for the boundary-layer modification is played by the stability on top of the boundary layer and by the divergence of the large-scale flow. According to the high variability of these conditions the observed boundary-layer modifications were very variable ranging from 100 to 300 m thick boundary layers with air temperatures between -32 and -22 'C over the ice to thicknesses between 900 and 2200 m and air temperatures between -15 and -5 'C after 300 km fetch over the open water. In most cases the large-scale flow was anticyclonic and divergent over the ice and changed to cyclonic and convergent over the water and an ice-sea breeze was superimposed on it.The sensible and latent heat fluxes are the dominant terms in the surface energy budget over the open water and ranged between 200 and 700 W m -' whereas the net longwave radiation is the dominating term over the ice with the heat fluxes only about 10 W me2.
Wind-speed observations from tall towers are used in combination with observations up to 600 m in altitude from a Doppler wind lidar to study the long-term conditions over suburban (Hamburg), rural coastal (Høvsøre) and marine (FINO3) sites. The variability in the wind field among the sites is expressed in terms of mean wind speed and Weibull distribution shape-parameter profiles. The consequences of the carrier-to-noise-ratio (CNR) threshold-value choice on the wind-lidar observations are revealed as follows. When the wind-lidar CNR is lower than a prescribed threshold value, the observations are often filtered out as the uncertainty in the wind-speed measurements increases. For a pulsed heterodyne Doppler lidar, use of the traditional -22 dB CNR threshold value at all measuring levels up to 600 m results in a ≈7 % overestimation in the long-term mean wind speed over land, and a ≈12 % overestimation in coastal and marine environments. In addition, the height of the profile maximum of the shape parameter of the Weibull distribution (so-called reversal height) is found to depend on the applied CNR threshold; it is found to be lower at small CNR threshold values. The reversal height is greater in the suburban (high roughness) than in the rural (low roughness) area. In coastal areas the reversal height is lower than that over land and relates to the internal boundary layer that develops downwind from the coastline. Over the sea the shape parameter increases towards the sea surface. A parametrization of the vertical profile of the shape parameter fits well with observations over land, coastal regions and over the sea. An applied model for the dependence of the reversal height on the surface roughness is in good agreement with the observations over land.
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