Abstract
Observations from radiosondes or from vertically pointing remote sensing profilers are often used to estimate the vertical flux of momentum due to gravity waves. For planar, monochromatic waves, these vertically integrated fluxes are equal to the phase averaged flux and equivalent to the horizontal averaging used to deduce momentum flux from aircraft data or in numerical models. Using a simple analytical solution for two-dimensional hydrostatic gravity waves over an isolated ridge, it is shown that this equivalence does not hold for mountain waves. For a vertical profile, the vertically integrated flux estimate is proportional to the horizontally integrated flux and decays with increasing distance of the profile location from the mountain. For tilted profiles, such as those obtained from radiosonde ascents, there is a further sampling error that increases as the trajectory extends beyond the localised wave field. The same sampling issues are seen when the effects of the Coriolis force on the gravity waves are taken into account. The conclusion of this work is that caution must be taken when using radiosondes or other vertical profiles to deduce mountain wave momentum fluxes.
Highlights
Atmospheric gravity waves play a key role in the atmosphere in transporting energy and momentum
The drag exerted by gravity waves acts as an important brake on atmospheric circulations, and failure to account for this drag properly in global weather and climate models can lead to significant biases
This paper aims to assess the potential impact of the sampling error from using radiosondes or other vertical profiles to determine momentum flux
Summary
Atmospheric gravity waves play a key role in the atmosphere in transporting energy and momentum. Some methods use velocity directly, while others instead use the gravity wave polarization relationship and temperature fluctuations, T 0 , to derive w0 and sometimes u0 [14] Radiosondes can measure both velocity and temperature perturbations directly; other remote sensing techniques often just measure temperature perturbations. The work in [24] compared observations of gravity wave momentum flux from a variety of methods, including radiosondes, with wave flux in several climate models. The focus of this paper is on direct calculation of momentum fluxes from the perturbations in the velocity field, the same sampling errors occur for retrievals that use the polarization equations where the velocity perturbations are assumed proportional to the temperature fluctuations, T 0
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