Observations and Numerical Model Simulation of a Partialy Trapped Lee Wave over the Welsh Mountains

Abstract

A large-amplitude lee-wave event detected in radiosonde ascents during a field experiment in the Welsh mountains is described and wave characteristics are deduced. The synoptic-flow pattern accompanying this wave field is used to initialize the U.K. Meteorological Office's nonhydrostatic mesoscale model (with a 3-km horizontal grid length) in an attempt to simulate the gravity-wave response over the Welsh mountains. After 4 h of integration time the resulting flow held is quasi-study and exhibits a fairly regular wave field with a dominant wavelength at about 22 km and nearly vertical phase lines. Verification is achieved by comparing rate-of-ascent variations with model vertical velocity variations along the trajectory of a radiosonde: the agreement is very good. The vertical structure equation for linear gravity waves is solved using the smoothed wind and temperature fields obtained from the sondes. A ?leaky? resonance is identified at the dominant wavelength of the model simulation. Limited confirmation of the horizontal wavelength comes from satellite imagery. One implication of this study is that nonhydrostatic, operational weather forecasting models are likely to exhibit useful predictability even at the scale of orographically forced trapped lee waves and could assist aviation forecasters in predicting hazardous wave-induced turbulence. They can also be used to diagnose wave momentum transfer and pressure drag for the purposes of refining gravity-wave-drag parameterization schemes.