Sources of Mesoscale Variability of Gravity Waves. Part I: Topographic Excitation

Abstract

Aircraft measurements of winds and temperatures collected during the GASP program are used to study the effects of topography as a source of mesoscale variability. Variances of fluctuations at the mesoscale over rough terrain are enhanced up to nearly two orders of magnitude compared to nonsource regions in some cases and are frequently enhanced by an order of magnitude. The implications of these episodic enhancements of variances for the vertical transports of energy and momentum are considered in the framework of gravity wave theory. The observed flight data are used to estimate the momentum flux u?w? on several flight segments. Results show that the flux is generally negative with mean value ?0.26 m2 s?2 and with magnitudes ranging up to ?1.5 m2 s?2. Spectral analysis shows that the largest contributions to the net flux come from horizontal scales of ?25 < ?x <60 km. Finally, the observed momentum fluxes are used to infer the anisotropy factor of gravity waves over rough terrain, which is found to be about 0.45.