Radiosonde observations of partially trapped lee waves over Tasmania, Australia

Abstract

A well‐defined family of lee waves over southern Tasmania, Australia, is analyzed using a routine radiosonde sounding released from Hobart at 1100 UTC June 18, 1991. Pronounced fluctuations in the ascent rate of the radiosonde are analyzed using a new technique called the horizontal projection method. The dominant horizontal wavelength is found to be 8249 m. An advanced very high resolution radiometer (AVHRR) infrared satellite image at 1554 UTC is used to check this calculation and gives a horizontal wavelength of 8.9±1.4 km. The horizontal projection method takes account of the vertical variation of background flow and the trajectory of the radiosonde. When the Scorer parameter is a strong function of height, as is the case with the 1100 UTC sounding, it is inappropriate to assume that the vertical wavenumber is constant with height. In particular, the vertical wavelength at low levels is about 4.5 km, the waves become evanescent between the heights of 2.5 km and 9 km, and above that the vertical wavelength is typically 10–15 km. As a consistency check, linear wave theory is used to calculate the resonant wave modes supported by the background profiles of wind and potential temperature. These are the wave modes that are most likely responsible for the lee‐wave clouds seen in the AVHRR satellite image. The horizontal wavelength for the resonant wave mode is 8188 m, in agreement with the estimates based on the AVHRR satellite image and from the horizontal projection method. The paper highlights the dangers of interpreting radiosonde soundings as vertical “snapshots” of the atmosphere and assuming that the vertical wavenumber can be characterized by a single value. It is shown that treating the radiosonde sounding as a vertical profile and analysing wave‐like structures in the ascent rate using standard spectral methods gives an incorrect vertical wavelength of 1906±238 m. The paper concludes that operational radiosonde soundings can be used to make accurate, quantitative measurements of orographically generated waves, provided that the vertical variation in the background flow and the motion of the radiosonde through the wave field are taken into account.