VHF radar measurements and model simulations of mountain waves over Wales

Abstract

AbstractVery high frequency (VHF) radar measurements of mountain waves over Aberystwyth, Wales, during 29 November 1997 are presented and compared with the predictions made by a three‐dimensional numerical model which is based on the linearized equations of motion. The radar height–time plot of vertical velocity reveals a wave field in the troposphere and lower stratosphere which changes significantly throughout the day. Four separate radiosonde soundings taken during the day are used to represent the steady background flow in the numerical model. In this case the steady‐state model wave fields are shown to compare qualitatively well with the radar measurements and the three‐dimensional information provided by the model is used to help explain the changes in the wave field observed by the radar throughout the day.The radiosonde profiles are interpolated in time to provide approximate basic‐state data at two‐hour intervals. Model steady‐state vertical‐velocity fields, based on each of these 13 approximate basic‐state profiles are used to generate a model time–height vertical‐velocity plot which is qualitatively similar to that obtained from the radar measurements, indicating that, to a reasonable approximation, the evolving wave field can be regarded as a sequence of independent steady states. This is generally true if the basic‐state flow remains steady over the time taken for wave packets to propagate over several wavelengths. Group‐velocity arguments are used to show that, for typical mountain waves in the troposphere, such time‐scales are generally less than one hour.Model simulations in which the basic‐state flow is allowed to vary in time continually (based on either the interpolated radiosonde soundings or the radar horizontal‐wind measurements) are conducted. In this case the simulated wave fields are always unsteady; the waves show a tendency to drift in response to changes in the basic‐state wind field. For example, during instances when the wind speed at a given level decreases (whilst the wind direction remains the same) the waves show an upwind phase propagation with a typical phase speed of the order of 1 m s−1. The consequence of this unsteadiness for the propagation of the waves is discussed. Copyright © 2002 Royal Meteorological Society.