Mountain waves over the Hohe Tauern: Influence of upstream diabatic effects

Abstract

AbstractThe structure and intensity of large‐amplitude trapped lee waves over the Hohe Tauern range of the Alps on 20 September 1999 are investigated through the analysis of in situ aircraft observations, airborne lidar, Global Positioning System dropsondes, rapid‐scan satellite imagery, and a suite of linear and nonlinear model simulations. Observations indicate that the lee waves attained a maximum vertical velocity of more than 9 m s−1, potential‐temperature perturbations greater than 10 K, and a horizontal wavelength of approximately 12–15 km. High‐resolution nonlinear simulations accurately capture the trapped‐wave evolution and characteristics. Blocking of the southerly flow upstream of the Hohe Tauern decreases the depth of the layer that ascends the mountain crest, and modulates the wave response. Vertically propagating waves are partially ducted into a train of lee waves as a result of weak stability aloft, which forms due to diabatic processes associated with precipitation upstream of the Hohe Tauern crest. Linear analytic solutions confirm the importance of the weak‐stability layer in the upper‐troposphere for the development of the non‐hydrostatic evanescent waves. Results from idealized two‐dimensional nonlinear simulations suggest that the vertical depth of diabatic heating associated with the upstream precipitation is important. In this case, latent heating over a relatively shallow depth tunes the atmosphere for a nonlinear resonance that leads to strong descent in the lee and reinforcement of the lee waves. Copyright © 2003 Royal Meteorological Society.