Tomographic observations of gravity waves with the infrared limb imager GLORIA

Abstract

Gravity waves drive global circulations in the mesosphere and stratosphere. Due to their small scales, they are usually not resolved in current global circulation models. Thus, their impact on the circulation is implemented in the form of simplified sub-models called parameterisation schemes. Several theoretical studies have highlighted that the assumptions on which these parameterisation schemes are based need to be reconsidered. However, the confirmation of these studies through measurements is still missing. A novel airborne remote sensing instrument, which can provide exactly such measurements, is the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA). GLORIA has two different measurement modes suitable for gravity waves: full angle tomography and limited angle tomography. Full angle tomography allows for the reconstruction of the atmospheric temperature structure with a spatial resolution of 20 km in both horizontal directions and 200 m in the vertical at an accuracy of 0.5 K. This spatial resolution is very high for remote sensing instruments. Three-dimensional volumes reconstructed with limited angle tomography have a resolution of 30 km in flight direction, 70 km across flight track, and 400 m in the vertical at an accuracy of 0.7 K. Full angle tomography is better suitable for small-scale gravity waves with unknown orientation and limited angle tomography for non-stationary waves. The first gravity wave field investigated in this thesis was measured above Iceland on 25 January 2016. Driven by the full wave characterisation achieved from the GLORIA measurements, the Gravity wave Regional Or Global RAy Tracer (GROGRAT) reveals a strong oblique propagation of this wave covering more than 2000 km horizontal distance. This strong oblique propagation happens mainly in a narrow altitude band between 15 km and 20 km. Even though many studies predicted oblique gravity wave propagation, it still surprises that it take place in such a narrow altitude band. Further, GROGRAT shows that in the case of solely vertical propagation, which is a common assumption used for gravity wave parameterisation schemes, the wave momentum is deposited not only at a completely wrong geographical location but also at a wrong altitude. The importance of non-linear processes for gravity wave propagation is investigated on a second case study using GLORIA measurements taken above southern Scandinavia on 28 January 2016. The results of the linear propagation model GROGRAT with satellite measurements from the Atmospheric Infrared Sounder (AIRS) agree very well if a detailed observational filter is considered for the satellite measurements. Thus, non-linear processes seem to be negligible for the propagation of the investigated gravity waves. A further result of this study is, that one needs to consider the detailed observational filter of a measurement technique to draw meaningful conclusions from comparisons between observations and models.