Long-term lidar observations of wintertime gravity wave activity over northern Sweden

Abstract

Abstract. This paper presents an analysis of gravity wave activity over northern Sweden as deduced from 18 years of wintertime lidar measurements at Esrange (68 • N, 21 • E). Gravity wave potential energy density (GWPED) was used to characterize the strength of gravity waves in the altitude regions 30-40 km and 40-50 km. The obtained values exceed previous observations reported in the literature. This is suggested to be due to Esrange's location downwind of the Scandinavian mountain range and due to differences in the various methods that are currently used to retrieve gravity wave parameters. The analysis method restricted the identification of the dominating vertical wavelengths to a range from 2 to 13 km. No preference was found for any wavelength in this window. Monthly mean values of GW-PED show that most of the gravity waves' energy dissipates well below the stratopause and that higher altitude regions show only small dissipation rates of GWPED. Our analysis does not reproduce the previously reported negative trend in gravity wave activity over Esrange. The observed interannual variability of GWPED is connected to the occurrence of stratospheric warmings with generally lower wintertime mean GWPED during years with major stratospheric warmings. A bimodal GWPED occurrence frequency indicates that gravity wave activity at Esrange is affected by both ubiquitous wave sources and orographic forcing.

Evaluation of methods for gravity wave extraction from middle-atmospheric lidar temperature measurements

“…A widely applied method is the use of the nightly mean temperature profile as a background temperature profile (e.g., Gardner et al, 1989;Rauthe et al, 2008;Ehard et al, 2014). It is then assumed that the timescales of phenomena other than gravity waves affecting the temperature profile are considerably larger and the timescales of gravity waves are smaller than the measurement period, which is typically in the range of 3-12 h.…”

“…case of conservative wave propagation. For a more detailed description and physical interpretation of the GWPED see, e.g., Rauthe et al (2008) and Ehard et al (2014). From TELMA observations above New Zealand over the period 1 July to 30 September 2014 we determined the mean GWPED per mass using the four methods of gravity wave extraction discussed in this study (Fig.…”

“…Several methods have been developed and used over the last decades. For example Gardner et al (1989), Rauthe et al (2008) and Ehard et al (2014) calculate a nightly mean profile and subtract it from the (time-resolved) individual profiles. Yamashita et al (2009) remove a background profile determined by a temporal running mean (in addition to vertical filtering).…”

“…Thus, results from different lidar studies become hardly comparable because one cannot distinguish between variations that are caused by a different methodology and variations that are geophysically induced. Ehard et al (2014) compared values of gravity wave potential energy density (GWPED) from different studies to their results. Due to potential methodological biases it remained unclear whether the differences were in fact of geophysical origin.…”

On the Interpretation of Gravity Wave Measurements by Ground-Based Lidars

“…There are specific case studies, e.g., [1][2][3][4][5][6][7]; there are climatologies to determine the seasonal dependence of gravity wave activity or their relation to the atmospheric background conditions [8][9][10][11][12][13][14][15][16][17][18][19]; and there are studies developing the methodology to retrieve gravity wave signatures, e.g., [20]. Recently, the output of the high-resolution analyses of the Integrated Forecast System of the ECMWF was compared with lidar measurements in the middle atmosphere, e.g., [21].…”