S. Monserrat et Aj. Thorpe, USE OF DUCTING THEORY IN AN OBSERVED CASE OF GRAVITY-WAVES, Journal of the atmospheric sciences, 53(12), 1996, pp. 1724-1736
This paper attempts to explain the properties of a gravity wave event
observed on Mallorca (Balearic islands) using an array of microbarogra
phs. The waves propagated coherently for a relatively long distance wi
th a speed of about 29 m s(-1) and were nondispersive. In the absence
of a continuous forcing, long-lived atmospheric gravity waves can only
exist if some mechanism prevents the vertical leakage of energy throu
gh wave propagation. trapping the gravity wave in a duct layer near th
e surface. Lindzen and Tung showed the necessary conditions for trappi
ng and discussed the properties of neutral modes for constant stabilit
y and wind in the duct. The role of shear in the wind profile near the
ground is examined here by first finding three distinct neutral mode
types using a matrix eigenvalue method. applying the wave theory given
by Booker and Bretherton, those neutral modes in the sheared duct tha
t have critical levels within the stable duct will be mostly absorbed
at their critical level. Therefore, they will be negligible in surface
pressure measurements. Another mode type has no critical level and pr
opagates energy away from the duct. On the other hand, neutral modes w
ith a critical level above the top of the duct will be reflected and s
o will constitute the main signal in the surface observations. This ap
pears to explain observations in Mallorca and shows the significant ro
le of wind shear in selecting the phase speed of the ducted waves.