Abstract
AbstractThe tropospheric and lower stratospheric response to flow over complex terrain is investigated using observations of vertical and horizontal motions made by a clear‐air Doppler radar stationed at Toulon in southern France. Analysis of the data has disclosed that episodic enhancements in the variability of vertical and horizontal velocities are due to lee waves excited by obstacles located within 5 km of the radar. Three separate cases of lee‐wave activity are studied. Two‐dimensional linear perturbation theory and existing three‐dimensional simulation results, for appropriate parameter values, are used as guides to check against the vertical structure observed by radar. the magnitudes of vertical velocities observed during lee‐wave incidence are well within the range reported in previous observational and numerical simulation studies.From a detailed examination of one of the cases of trapped waves, the time variation, observed by the radar, of timescales of less than an hour, has emerged as the likely result of small directional changes in the upstream winds causing a bodily rotation of the downstream three‐dimensional wave pattern centred on the wave‐generating obstacle. the observed tendency of the vertical‐velocity perturbation to change sign with time above the radar location follows as a consequence of the presence of large horizontal gradients in the motion field, which are inherent in the structure of three‐dimensional lee waves.The temporal variability of the lee‐wave pattern has allowed the radar to sweep out a partial horizontal cross‐section, revealing vertical circulation features that are broadly consistent with gravity‐wave behaviour. However, analysis of the effects of finite antenna beam spacing on the sampled data indicates that a spurious phase‐shift might have been introduced between the vertical and horizontal velocity perturbations observed in the cross‐section. In contrast, the measured vertical velocities are not seriously affected by finite beam‐width effects.The lee waves examined in this study are quite moderate, nevertheless, this circumstance has provided a favourable opportunity to conduct a critical evaluation of the capabilities and limitations of this radar technique for observing lee‐wave phenomena in general. We have been able to show clearly how the incidence and vertical structure of the waves respond to the synoptic‐scale situation. In particular we have been able to distinguish between trapped and vertically propagating waves and to relate a transition between these wave types to synoptic‐scale changes in the static stability structure.
Published Version
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