Observations and numerical modelling of mountain waves over the Southern Alps of New Zealand

A detailed case study of mountain waves observed over the South Island of New Zealand during the Southern Alps Experiment is presented. The purpose of the study is to document the waves generated over this region, to explore the extent to which high-resolution radiosonde data can be used to determine gravity-wave parameters, and to estimate the vertical flux of horizontal momentum produced by flow over the Southern Alps. Organized wave clouds were observed in a visible satellite image on 20 October 1996 and are thought to have been produced by partially trapped lee waves. Using background flow fields derived from a radiosonde sounding, the theoretical horizontal wavelength of the partially trapped mode is calculated and agrees well with that measured from the satellite image. A technique, called the horizontal projection method, is developed to analyse radiosonde soundings for mountain waves. Specifically, the technique determines the dominant horizontal wavelength over the entire depth of a radiosonde sounding. This method uses a coordinate transformation to take account of vertical changes in the background flow and, hence, vertical variations in the vertical wave number, as well as changes in the path of the radiosonde. This method is applied to a radiosonde sounding to identify the dominant wave mode, and emphasizes that radiosonde soundings should not, in general, be treated as instantaneous vertical profiles when analysing mountain waves. The mountain waves observed are simulated using a high-resolution numerical model and the results are used to check the consistency of the analysis. The numerical model produces a horizontally averaged value of momentum flux in the lower stratosphere of −0.02 N m−2 although this value is probably underestimated by at least a factor of four.