Global gravity wave activity in the tropopause region from CHAMP radio occultation data

Abstract

We discuss the global gravity wave (GW) activity expressed by the specific potential energy in the altitude range from 5 km below to 10 km above the tropopause, derived from GPS radio occultation data from CHAMP (2001–2008). The GW analysis is based on vertical detrending of the individual measured temperature profiles by applying a Gaussian filter in two different ways: (i) filtering of the complete temperature profiles and (ii) separate filtering of the profiles for the tropospheric and lower stratospheric parts. The separate filtering method significantly reduces the usually observed wave activity enhancement in the tropopause region which highly depends on the performance of the complete filtering method to reproduce the change in the temperature gradient at the tropopause. We only consider vertical wavelengths less than 10 km. The global mean potential energy in the tropopause region deduced with these different background temperatures will be analyzed, differences will be emphasized and possible error sources of the new method will be considered.

An intercomparison of stratospheric gravity wave potential energy densities from METOP GPS radio occultation measurements and ECMWF model data

“…This is likely caused by the damping of small-scale model structures by the model's sponge layer. Also, it is well known that noise in RO data picks up substantially above 35 km such that several previous studies have recommended restricting the useful range of RO data for GW analysis to below 35 km (e.g., Schmidt et al, 2008). This previous recommendation is clearly supported by our analysis.…”

“…Here, neither the pronounced tropopause (at around 17 km) nor the inversion layer above (i.e., between 20 and 25 km) is well captured by the Butterworth filter, resulting in unrealistically large temperature perturbations which might not be confused with real gravitywave-induced temperature perturbations. This is a general problem with all techniques that analyze vertical temperature profiles, which has motivated many authors to exclude the tropopause region and the lowest altitudes above it from further analyses (see, e.g., Schmidt et al, 2008, for a detailed discussion and an approach to derive GW properties in the vicinity of the tropopause). For this reason, we will exclude altitudes below 20 km from our analysis and focus on the altitude range between 20 and 40 km only, knowing, of course, that the largest altitudes need to be treated with care since noise of RO data is known to pick up significantly above ∼ 35 km altitude (Marquardt and Healy, 2005).…”

“…However, global satellite observations are needed to determine dominant tropospheric source regions and processes as well as global propagation pathways and the resulting gravity wave drag imposed on the mean flow to constrain GW parameterizations for climate and weather prediction models (Alexander et al, 2010;Geller et al, 2013). Since the pioneering work by Fetzer and Gille (1994), Wu and Waters (1996), and Eckermann and Preusse (1999) there have been many attempts to characterize the global distribution of gravity wave activity using such different remote-sensing techniques as Limb (e.g., Ern et al, 2004Ern et al, , 2011Preusse et al, 2009;Zhang et al, 2012) and Nadir sounders (e.g., Hoffmann et al, 2016;Ern et al, 2017), as well as GPS-based radio occultation (RO) measurements (e.g., Tsuda et al, 2000;Hei et al, 2008;Schmidt et al, 2008Schmidt et al, , 2016Fröhlich et al, 2007;Hindley et al, 2015;Šácha et al, 2015;Khaykin et al, 2015;Khaykin, 2016). This paper focusses on the derivation of gravity wave potential energy densities (E P ) from GPS RO measurements on board the operational METOP-A and METOP-B satellites operated by EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites) and the subsequent systematic comparison of E P fields with ECMWF (European Centre for Medium-Range Weather Forecasts) operational forecast and reanalysis data.…”

Intercomparisons of HIRDLS, COSMIC and SABER for the detection of stratospheric gravity waves

“…However, in the tropics, the tropopause may potentially be as high as 20 km and give a kink in our smoothed profiles; whilst this would be seen at the same height in all three instruments, their differing resolutions may lead to the measured location not being observed in quite the same place in colocated measurements. Additionally, this kink may lead to an apparent enhancement in GW activity at the tropopause region (Schmidt et al, 2008). Consequently, a lower altitude bound of 60 hPa (∼20 km) will be used.…”

An intercomparison of stratospheric gravity wave potential energy densities from METOP GPS-radio occultation measurements and ECMWF model data

“…However, global satellite observations are needed to determine dominant tropospheric source regions and processes as well as global propagation pathways and the resulting gravity wave drag imposed on the mean flow to constrain GW parameterizations for climate and weather prediction models (Alexander et al, 2010;Geller et al, 20 2013). Since the pioneering work by Fetzer and Gille (1994), Wu and Waters (1996), and Eckermann and Preusse (1999) there have been many attempts to characterize the global distribution of gravity wave activity using such different remote-sensing techniques as Limb (e.g., Ern et al, 2004;Preusse et al, 2009;Ern et al, 2011;Zhang et al, 2012) and Nadir sounders (e.g., Hoffmann et al, 2016;Ern et al, 2016), as well as GPS-based radio occultation measurements (e.g., Tsuda et al, 2000;Hei et al, 2008;Schmidt et al, 2008;Fröhlich et al, 2007;Hindley et al, 2015;Šácha et al, 2015;Khaykin et al, 2015;Khaykin, 25 2016; Schmidt et al, 2016). This paper focusses on the derivation of gravity wave potential energy densities (E P ) from GPS radio occultation (RO) measurements onboard the operational METOP-A and METOP-B-satellites operated by EUMETSAT (=European Organisation for the Exploitation of Meteorological Satellites) and the subsequent systematic comparison of E P -fields with ECMWF (European Center for Medium range Weather Forecast) operational forecast and reanalysis data.…”