Limited angle tomography of mesoscale gravity waves by the infrared limb-sounder GLORIA
Abstract. Three-dimensional measurements of gravity waves are required in order to
quantify their direction-resolved momentum fluxes and obtain a better
understanding of their propagation characteristics. Such 3-D measurements of
gravity waves in the lowermost stratosphere have been provided by the
airborne Gimballed Limb Observer for Radiance Imaging of the Atmosphere
(GLORIA) using full angle tomography. Closed flight patterns of sufficient
size are needed to acquire the full set of angular measurements for full
angle tomography. These take about 2 h and are not feasible everywhere due
to scientific reasons or air traffic control restrictions. Hence, this paper
investigates the usability of limited angle tomography for gravity wave
research based on synthetic observations. Limited angle tomography uses only
a limited set of angles for tomographic reconstruction and can be applied to
linear flight patterns. A synthetic end-to-end simulation has been performed
to investigate the sensitivity of limited angle tomography to gravity waves
with different wavelengths and orientations with respect to the flight path.
For waves with wavefronts roughly perpendicular to the flight path, limited
angle tomography and full angle tomography can derive wave parameters like
wavelength, amplitude, and wave orientation with similar accuracy. For waves
with a horizontal wavelength above 200  km and vertical wavelength
above 3  km , the wavelengths can be retrieved with less than
10  % error, the amplitude with less than 20  % error, and
the horizontal wave direction with an error below 10 ∘ . This is
confirmed by a comparison of results obtained from full angle tomography and
limited angle tomography for real measurements taken on 25 January 2016 over
Iceland. The reproduction quality of gravity wave parameters with limited
angle tomography, however, depends strongly on the orientation of the waves
with respect to the flight path. Thus, full angle tomography might be
preferable in cases in which the orientation of the wave cannot be predicted
or waves with different orientations exist in the same volume and thus the
flight path cannot be adjusted accordingly. Also, for low-amplitude waves and
short-scale waves full angle tomography has advantages due to its slightly
higher resolution and accuracy.