Derivation of vertical wavelengths of gravity waves in the MLT-region from multispectral airglow observations

Abstract

We present a new method for the derivation of gravity wave vertical wavelengths from OH airglow observations of different vibrational transitions. It utilizes small phase shifts regularly observed between the OH(3-1) and OH(4-2) intensities in the spectra of the GRIPS (GRoundbased Infrared P-branch Spectrometer) instruments, which record the OH airglow emissions in the wavelength range from 1.5µm to 1.6µm simultaneously. These phase shifts are interpreted as being due to gravity waves passing through the OH airglow layer and affecting individual vibrational transitions at slightly different times due to small differences in their emission heights. The results are compared with co-located observations of the OH(6-2) and O 2 b(0-1) transitions by means of spectrometer observations (TANGOO instrument, Tilting-filter spectrometer for Atmospheric Nocturnal Ground-based Oxygen & hydrOxyl emission measurements) performed from 2013 until 2016 at Oberpfaffenhofen (48.08° N, 11.27° E), Germany, and with Na-Lidar measurements acquired between 2010 and 2014 at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR, 69.28° N, 16.01° E), Norway. The latter comparison shows best agreement if the mean height difference of the OH(3-1) and OH(4-2) emission is assumed to be 540 m (1σ=160 m), confirming the result of von Savigny et al. (2012), who derived a height difference of approximately 500 m between each vibrational level. For approximately 40 % of all wave events observed with GRIPS, a quantitative estimate of the phase relationship between the OH(3-1) and OH(4-2) intensities can be retrieved from the spectra allowing derivation of vertical wavelengths. The retrieval performs best for wave periods below two hours (80 % success rate) and worse for periods above ten hours (successful in less than 10 % of the cases). The average wavelength determined from 102 events amounts to 22.9 km (1σ: 9.0 km).

Intra-annual variations of spectrally resolved gravity wave activity in the UMLT region

“…These are found to be in good agreement with the kinetic mesospheric temperatures (see e.g. Bittner et al, 2010;Noll et al, 2016) and can be derived from the line intensities of the OH* airglow radiation, which are measured with spectrometers (Mulligan et al, 1995;Espy & Stegman, 2002;Espy et al, 2003;French & Burns, 2004;Schmidt et al, 2013Schmidt et al, , 2018.…”

“…This makes gravity waves and other dynamical features visible in the images of infrared cameras (see e.g. Taylor, 1997;Pautet et al, 2014;Hannawald et al, 2016;Sedlak et al, 2016;Hannawald et al, 2019;Wüst et al, 2019) but also in the time series of OH* rotational temperatures (Hines & Tarasick, 1987;Simkhada et al, 2009;Reisin & Scheer, 2001;Offermann et al, 2009;Wachter et al, 2015;Wüst et al, 2016Wüst et al, , 2017aWüst et al, , 2018Silber et al, 2017).…”

Can VHF radars at polar latitudes measure mean vertical winds in the presence of PMSE?

“…The recorded raw voltages were analysed with respect to the radial velocity, signal-to-noise ratio (SNR) and spectral width using the truncated Gaussian fitting (Kudeki et al, 1999;Sheth et al, 2006;Chau and Kudeki, 2006;Stober et al, 2018b). The fitting routine also includes an estimate of the statistical uncertainties for the radial velocities and spectral width.…”

Are mean vertical velocities from PMSE a good representation of mean vertical winds?

“…Hence, the horizontal transport of tracer is also essential to describe the observed vertical fluctuations. The large-scale GWs generated and transported from the lower altitudes have a characteristic vertical wavelength of 15-20 km at upper mesospheric altitudes (Mitchell and Howells, 1998;Schmidt et al, 2018). Up-or downwelling phase of such motion exceeds the width of PMSE layer and are capable to transport air masses from lower or upper heights where the ice clouds and PMSE are not present.…”