Quasi-optic approximation for orographic waves

PULIDO M. AND C. RODAS

Montreal Canada

Otro; MOCA Joint Assembly; 2009

International Association of Meteorology and Atmospheric Sciences (IAMAS), IAPSO and IACS

Ray techniques are a promising tool for developing orographic gravity wave drag schemes. However, the modeling of the propagation of orographic waves using ray techniques in realistic background wind conditions usually encounters several regions, called caustics, where the ray approximation breaks down. In this work we develop  a higher order approximation than ray theory, known as quasi-optics, for orographic waves in realistic background wind conditions. Our analytical results show that this formulation is free of the singularities that arise in ray theory. We apply the quasi-optics approximation to waves that are propagating in a background wind that is turning with height, same conditions as in Shutts' work. We show that the evolution of the amplitude is well defined in this approximation for all times and locations including the initial time. We compare our results with a numerical model, there is good agreement of the amplitude and wavenumber evolution for both short and long times. In particular the convective instability threshold is precisely determined with quasi-optic approximation.