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H. F. Vugts and F. Cannemeijer

Abstract

During unstable conditions the drag coefficient of the surf zone and the roughness length of a sand beach have been measured. It is concluded from ∼100 half-hour profiles that the drag coefficient under unstable conditions over the surf zone is the same as over the ocean. For wind speeds <6 m s−1 the roughness length of a sand beach is of the same order of magnitude as for the surf zone. For wind speeds >8 m s−1 the roughness length of a sand beach is ∼10 times higher than the value measured below 6 m s−1. This jump is related to aeolian sand transport at high wind speeds.

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J. Oerlemans and H. F. Vugts

Preliminary results are described from a glaciometeorological experiment carried out in the margin (melting zone) of the Greenland ice sheet in the summers of 1990 and 1991. This work was initiated within the framework of a Dutch research program on land ice and sea level change. Seven meteostations were operated along a transect running from the tundra well onto the ice sheet. At the ice edge, humidity, temperature, and wind profiles were obtained with a tethered balloon. On the ice sheet, 90 km from the edge, a boundary-layer research unit, including a sound detecting and ranging system (SODAR) and a radio acoustic sounding system (RASS), was established.

Although focusing on the relation between surface energy balance, glacier mass balance, and ice flow, the experiment has also delivered a unique dataset on the dynamics of the atmospheric boundary layer around the warm tundra–cold ice sheet transition.

Unexpected behavior was found for the surface albedo during the melt season. Lowest values are not found close to the ice edge, which is usual for glaciers, but higher on the ice sheet. Meltwater accumulation due to inefficient surface drainage was found to be the cause for this.

The wind regime is dominated by katabatic flow from the ice sheet. The katabatic layer is typically 100–200 m thick. Close to the ice edge, the flow exhibits a very regular daily rhythm, with maximum wind speed in the afternoon. Farther on the ice sheet, the regime changes, and wind speed reaches maximum values in late night/early morning.

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E. P. Weijers, H. F. Vugts, A. G. C. A. Meesters, and A. van Delden

Abstract

The horizontal perturbation wind field within thermal structures encountered in the atmospheric surface layer was investigated. A field experiment with four sonic anemometers on the vertices and one in the centroid of a square (with sides of 80 m) was performed to obtain the necessary dataset. Structures were selected on a typical ramplike appearance in the temperature time series. Ultimately, a set of 47 “ramps” was obtained. Conditional sampling and block averaging followed by a compositing technique were applied to construct ensemble averages of turbulent temperature and horizontal and vertical velocity. Properties of the horizontal velocity field were expressed in terms of ensemble averages of horizontal divergence, vertical vorticity, and deformation.

The ensemble-averaged behavior at the five masts during passage of thermal activity was consistent. The convergent wind field within a ramp attains its maximum simultaneously with the maximum in vertical velocity. Both precede the temperature extreme. The air in the ramp is clearly decelerated. while it is accelerated in die succeeding downdraft. In the frame of reference moving with the ramp the average orientation of the wind vector in the accompanying downdrafts always directed toward the position of the ramp. Within the ramp, direction of air is measured in the direction of the mean wind. Near the microfront, contraction of air occurs with a maximum in the succeeding downdraft. Vertical vorticity (of opposite sign) is measured in the right and left half of the ramps. Phenomena involved in the generation of this vorticity are discussed. The strength of the background wind might play a role in the generation of these rotations.

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A. A. Van de Griend, M. Owe, H. F. Vugts, and S. D. Prince

From January–March 1989 an intensive field campaign was held in the eastern savanna of Botswana, Africa, to collect ground data to parameterize models which can be used to determine the physical status and water-balance terms of the earth's surface by satellite remote sensing. This paper describes the objectives of this international research activity and gives an outline of the data collected. Some preliminary measuring results also are presented.

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E. P. Weijers, A. Van Delden, H. F. Vugts, and A. G. C. A. Meesters

Abstract

Principal component analysis (PCA) was applied to 182 half-hour runs containing time series of turbulent wind velocity and temperature measured in the convective atmospheric surface layer. A field experiment with four sonic anemometers on the vertices and one in the centroid of a square (with sides of 80 m) was performed to obtain the necessary dataset. Physical explanations of the most important eigenvectors are presented. Two of the major principal components (PCs) identify the variance in wind speed along and across the background wind direction. Always, one major PC accounts for the presence of large-scale thermal activity: periods with higher (lower) temperatures coincide with lower (higher) wind speeds, convergence (divergence) in the wind fields, and upward (downward) movements. As an application, variance in the velocity fields was expressed in terms of horizontal divergence and vertical vorticity. These can be derived directly from the eigenvectors when PCA is combined with a planimetric method. Using the PC that identifies thermal activity, it is found that the magnitude of divergence increases and the magnitude of vorticity decreases when atmospheric conditions become more unstable. It is found that the (absolute) ratio between vorticity and divergence scales with a function of the friction velocity divided by the convective vertical scaling velocity. Both kinematic parameters are larger for updrafts than for downdrafts. It is concluded that PCA can be a useful tool to distinguish variance of thermal and nonthermal origin and in the estimation of the kinematics of dominant flow fields.

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