Doppler ducting of atmospheric gravity waves

Abstract

Atmospheric gravity waves are subject to horizontal ducting in a stably stratified atmosphere that contains levels of wind maximum or minimum, quite apart from any superimposed consequences of vertical variation of stability and quite apart from the existence or otherwise of critical levels. The process, “Doppler ducting”, is examined here for two prototype cases (an isolated wind maximum and a wind minimum at ground level) and for two more realistic model cases (a strong tropospheric jet and a nocturnal boundary layer jet). The wave structure, modal behavior, and dispersion curves of all four are very similar. It is argued that Doppler ducting is available to a part of the gravity wave spectrum in all real circumstances and that it should be considered as a possible candidate whenever observations indicate that some ducting process is operative. One particular wave event is discussed in some detail, and it is shown that Doppler ducting in its simplest form provides a plausible alternative to a more complicated ducting process (which involved critical levels and energy leakage) previously inferred. Doppler ducting is available equally to gravity waves in the oceans when background currents vary with depth.

An investigation of long-distance propagation of gravity waves under CAWSES India Phase II Programme

“…The ducts due to the variations in the mean wind field are called Doppler ducts, while those associated with strong discontinuities in the temperature field are called thermal ducts. And gravity waves trapped in these ducts can travel (without being considerably attenuated) to large horizontal distances before dissipating their energy and momentum to the background atmosphere; hence, these ducted waves are more likely to be detectable at large distance from their origin (Chimonas and Hines, 1986;Fritts and Yuan, 1989;Isler et al, 1997;Ding et al, 2003;Snively et al, 2007;Bageston et al, 2011). At low latitudes, such waves are more common in occurrence compared to those freely propagating (Isler et al, 1997).…”

Vertical velocities associated with gravity waves measured in the mesosphere and lower thermosphere with the EISCAT VHF radar

“…In the case of Doppler ducts, Doppler shifting increases a wave's intrinsic frequency to a value above that of the Brunt-Vaisala frequency, N, outside of the duct. Chimonas and Hines (1986) described the behaviour of Doppler-ducted gravity wave and reported several examples of waves apparently so ducted at tropospheric heights. Similarly, thermal ducts may be formed when¯uctuations of N with height, perhaps caused by wave-induced perturbations of the background temperature gradient, trap a wave within a layer of atmosphere where at immediately greater and lesser heights the value of N is lower than the wave frequency.…”

Investigation of a mesospheric gravity wave ducting event using coordinated sodium lidar and Mesospheric Temperature Mapper measurements at ALOMAR, Norway (69°N)

“…Distinguishing between GWs that are vertically propagating and ducting events or mesospheric bores are challenging to characterize with confidence due to various remote and local potential sources and their expected sensitivity and responses to small-scale features in the environmental wind and temperature profiles [Chimonas and Hines, 1986;Fritts and Yuan, 1989;Pasko, 2003, 2008;Simkhada et al, 2009;Laughman et al, 2009Laughman et al, , 2011Walterscheid and Hickey, 2009;Snively et al, 2013].…”