Intra-annual variations of spectrally resolved gravity wave activity in the upper mesosphere/lower thermosphere (UMLT) region

Abstract

Abstract. The period range between 6 and 480 min is known to represent the major part of the gravity wave spectrum driving mesospheric dynamics. We present a method using wavelet analysis to calculate gravity wave activity with a high period resolution and apply it to temperature data acquired with the OH* airglow spectrometers called GRIPS (GRound-based Infrared P-branch Spectrometer) within the framework of the NDMC (Network for the Detection of Mesospheric Change; https://ndmc.dlr.de, last access: 22 September 2020). We analyse data measured at the NDMC sites Abastumani in Georgia (ABA; 41.75∘ N, 42.82∘ E), ALOMAR (Arctic Lidar Observatory for Middle Atmosphere Research) in Norway (ALR; 69.28∘ N, 16.01∘ E), Neumayer Station III in the Antarctic (NEU; 70.67∘ S, 8.27∘ W), Observatoire de Haute-Provence in France (OHP; 43.93∘ N, 5.71∘ E), Oberpfaffenhofen in Germany (OPN; 48.09∘ N, 11.28∘ E), Sonnblick in Austria (SBO; 47.05∘ N, 12.95∘ E), Tel Aviv in Israel (TAV; 32.11∘ N, 34.80∘ E), and the Environmental Research Station Schneefernerhaus on top of Zugspitze mountain in Germany (UFS; 47.42∘ N, 10.98∘ E). All eight instruments are identical in construction and deliver consistent and comparable data sets. For periods shorter than 60 min, gravity wave activity is found to be relatively low and hardly shows any seasonal variability on the timescale of months. We find a semi-annual cycle with maxima during winter and summer for gravity waves with periods longer than 60 min, which gradually develops into an annual cycle with a winter maximum for longer periods. The transition from a semi-annual pattern to a primarily annual pattern starts around a gravity wave period of 200 min. Although there are indications of enhanced gravity wave sources above mountainous terrain, the overall pattern of gravity wave activity does not differ significantly for the abovementioned observation sites. Thus, large-scale mechanisms such as stratospheric wind filtering seem to dominate the evolution of mesospheric gravity wave activity.

Gravity wave instability structures and turbulence from more than 1.5 years of OH* airglow imager observations in Slovenia

“…In 2016 we put into operation another FAIM instrument (FAIM 3) which still has a high temporal resolution of 2.8 s but also a high spatial resolution of up to 17 m per pixel (measurements in zenith direction utilizing a 100 mm SWIR objective lens). We were not only able to observe wave patterns on extraordinary small scales (smallest horizontal wavelength 550 m) but also the formation of a vortex which we interpret as the turbulent breakdown of a wave front (Sedlak et al, 2016).…”

“…8 km (Baker and Stair, 1988;Liu and Shepherd, 2006;Wüst et al, 2017b). Remote sensing techniques include spectroscopic measurements of strong emission lines and the analysis of temperature time series derived from these (Hines and Tarasick, 1987;Mulligan et al, 1995;Bittner et al, 2000;Reisin and Scheer, 2001;Espy and Stegman, 2002;Espy et al, 2003;French and Burns, 2004;Offermann et al, 2009;Schmidt et al, 2013Schmidt et al, , 2018Wachter et al, 2015;Silber et al, 2016;Wüst et al, 2016Wüst et al, , 2017aWüst et al, , 2018 but also two-dimensional imaging in the short-wave infrared (SWIR) range (see, for example, Peterson and Kieffaber, 1973;Hecht et al, 1997;Taylor, 1997;Moreels et al, 2008;Li et al, 2011;Pautet et al, 2014;Hannawald et al, 2016Hannawald et al, , 2019Sedlak et al, 2016;Wüst et al, 2019, and many more).…”

“…4-5 min in the upper mesosphere/lower thermosphere (UMLT) region (Wüst et al, 2017b) and represents the smallest possible period of gravity waves. They show diverse behaviour depending strongly on wave properties like their periodicity (Fritts and Alexander, 2003;Beldon and Mitchell, 2009;Hoffmann et al, 2010;Wüst et al, 2016;Sedlak et al, 2020), which makes it even harder to fully account for them by means of parameterization. Furthermore, gravity wave generation is not restricted to the troposphere but can also take place at higher altitudes, such as secondary wave excitation due to breaking gravity waves (see, for example, Holton and Alexander, 1999;Satomura and Sato, 1999;Vadas and Fritts, 2001;Becker and Vadas, 2018).…”

Gravity wave instability structures and turbulence from more than one and a half years of OH* airglow imager observations in Slovenia

“…instruments deliver time series of OH* rotational temperatures derived from the OH(3-1) P-branch (1.5 µm-1.6 µm) at an initial temporal resolution of 15 s. Unlike the general instrument details discussed by Schmidt et al (2013), the GRIPS 9 at OTL has a reduced aperture angle of 6.2 FWHM increasing its responsivity to smaller structures. As described in Sedlak et al (2020), gravity wave activity -the so-called significant wavelet intensity (SWI) -for the periods 6 -480 min (period resolution 1 min) can be calculated by applying a wavelet analysis to these temperature time series. The FOV of GRIPS 9 is also located above the Gulf of Trieste and in ca.…”

“…4 -5 min in the upper mesosphere / lower thermosphere (UMLT) region (Wüst et al, 2017b) and represents the smallest possible period of gravity waves. They show diverse behaviour depending strongly on wave properties like their periodicity (Fritts & Alexander, 2003;Beldon & Mitchell, 2009;Hoffmann et al, 2010;Wüst et al, 2016;Sedlak et al, 2020), which makes it even harder to fully account for them by means of parameterization. Furthermore, gravity wave generation is not restricted to the ground but can also take place at higher altitudes, such as secondary wave excitation due to breaking gravity waves (see e.g.…”