Satellite observations of gravity wave momentum flux in the mesosphere and lower thermosphere (MLT): feasibility and requirements

Abstract

Abstract. In the recent decade it became evident that we need to revise our picture of how gravity waves (GWs) reach the mesosphere and lower thermosphere (MLT). This has consequences for our understanding not just of the properties of the GWs themselves, but in particular of the global circulation in the MLT. Information on spectral distribution, direction, and zonal mean GW momentum flux is required to test the theoretical and modeling findings. In this study, we propose a constellation of two CubeSats for observing mesoscale GWs in the MLT region by means of temperature limb sounding in order to derive such constraints. Each CubeSat deploys a highly miniaturized spatial heterodyne interferometer (SHI) for the measurement of global oxygen atmospheric band emissions. From these emissions, the 3-D temperature structure can be inferred. We propose obtaining four independent observation tracks by splitting the interferograms in the center and thus gaining two observation tracks for each satellite. We present a feasibility study of this concept based on self-consistent, high-resolution global model data. This yields a full chain of end-to-end (E2E) simulations incorporating (1) orbit simulation, (2) airglow forward modeling, (3) tomographic temperature retrieval, (4) 3-D wave analysis, and (5) GW momentum flux (GWMF) calculation. The simulation performance is evaluated by comparing the retrieved zonal mean GWMF with that computed directly from the model wind data. A major question to be considered in our assessment is the minimum number of tracks required for the derivation of 3-D GW parameters. The main result from our simulations is that the GW polarization relations are still valid in the MLT region and can thus be employed for inferring GWMF from the 3-D temperature distributions. Based on the E2E simulations for gaining zonal mean climatologies of GW momentum flux, we demonstrate that our approach is robust and stable, given a four-track observation geometry and the expected instrument noise under nominal operation conditions. Using phase speed and direction spectra we show also that the properties of individual wave events are recovered when employing four tracks. Finally, we discuss the potential of the proposed observations to address current topics in the GW research. We outline for which investigations ancillary data are required to answer science questions.

Observation of horizontal temperature variations by a spatial heterodyne interferometer using single-sided interferograms

“…The mathematical derivation for an interferogram measured by an SHI is presented by Harlander et al (1992), Smith and Harlander (1999), and Cooke et al (1999). Chen et al (2022) give more details on the mathematics for the instrument described here.…”

“…This section introduces shortly the simulation of the O 2 A-band emission, which is described by Chen et al (2022) in detail. The O 2 A-band is a electronic transition from the excited state O 2 (b 1 Σ + g , v = 0) to the ground state O 2 (X 3 Σ − g , v = 0) centered at 762nm in the near-infrared.…”

“…Becker and Vadas (2020) point out that gravity waves can also have sources in higher altitudes which are not well understood, but can have a large effect on the MLT region. Chen et al (2022) underline the importance of gravity wave observations by multiple examples and outlines an observation method where gravity wave parameters can be extracted from temperature fields.…”

“…In the usual processing, the horizontal direction is used to extract spectral information and vertical direction gives spatial information. This allows to extract a finely resolved 1-D temperature profile from one image, which can be subsequently used to derive wave parameters as described by Ern (2004) and applied for our instrument by Chen et al (2022).…”

“…In our case, we use the single-sided interferogram to gain horizontal information of the measured atmospheric scene. Chen et al (2022) showed that medium-scale gravity waves can be resolved, if it it possible to obtain two temperature profiles from a single interferogram. The methodology to fulfill this requirement is described in this paper.…”