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

Abstract

Abstract. Temperature profiles based on radio occultation (RO) measurements with the operational European METOP satellites are used to derive monthly mean global distributions of stratospheric (20-40 km) gravity wave (GW) potential energy densities (E P ) for the period July 2014-December 2016. In order to test whether the sampling and data quality of this data set is sufficient for scientific analysis, we investigate to what degree the METOP observations agree quantitatively with ECMWF operational analysis (IFS data) and reanalysis (ERA-Interim) data. A systematic comparison between corresponding monthly mean temperature fields determined for a latitude-longitude-altitude grid of 5 • by 10 • by 1 km is carried out. This yields very low systematic differences between RO and model data below 30 km (i.e., median temperature differences is between −0.2 and +0.3 K), which increases with height to yield median differences of +1.0 K at 34 km and +2.2 K at 40 km. Comparing E P values for three selected locations at which also ground-based lidar measurements are available yields excellent agreement between RO and IFS data below 35 km. ERA-Interim underestimates E P under conditions of strong local mountain wave forcing over northern Scandinavia which is apparently not resolved by the model. Above 35 km, RO values are consistently much larger than model values, which is likely caused by the model sponge layer, which damps small-scale fluctuations above ∼ 32 km altitude. Another reason is the wellknown significant increase of noise in RO measurements above 35 km. The comparison between RO and lidar data reveals very good qualitative agreement in terms of the seasonal variation of E P , but RO values are consistently smaller than lidar values by about a factor of 2. This discrepancy is likely caused by the very different sampling characteristics of RO and lidar observations. Direct comparison of the global data set of RO and model E P fields shows large correlation coefficients (0.4-1.0) with a general degradation with increasing altitude. Concerning absolute differences between observed and modeled E P values, the median difference is relatively small at all altitudes (but increasing with altitude) with an exception between 20 and 25 km, where the median difference between RO and model data is increased and the corresponding variability is also found to be very large. The reason for this is identified as an artifact of the E P algorithm: this erroneously interprets the pronounced climatological feature of the tropical tropopause inversion layer (TTIL) as GW activity, hence yielding very large E P values in this area and also large differences between model and observations. This is because the RO data show a more pronounced TTIL than IFS and ERA-Interim. We suggest a correction for this effect based on an estimate of this "artificial" E P using monthly mean zonal mean temperature profiles. This correction may be recommended for application to data sets that can only be analyzed using a vertical background determination me...

How does knowledge of atmospheric gravity waves guide their parameterizations?

“…The amplitudes are often communicated by estimating the momentum fluxes, although the derivation and necessary assumptions imply significant uncertainties. Other measures of gravity wave amplitudes, such as potential energy, have also been reported (e.g., de la Torre et al ., ; Rapp et al ., ).…”

Removing spurious inertial instability signals from gravity wave temperature perturbations using spectral filtering methods

“…Inertial instabilities have not been considered in these discussions. Rapp et al (2018b) further showed in their study that subtracting a large-scale background estimated from ERA-Interim reanalysis temperatures can eliminate spurious inertial instability signals from the gravity wave temperature perturbations and leaves gravity wave activity around the polar vortex. In a comment on this study, Harvey and Knox (2019) pointed out that it is 85 not clear how often inertial instabilities induced variations have been misinterpreted as gravity wave signals in the past.…”

“…Recently, Rapp et al (2018a) derived a global, gravity wave temperature perturbation climatology from MetOp GPS radio occultation (GPS-RO) data and found unexpectedly high potential energy densities in the tropics that could not be linked to gravity wave sources. In further investigations, they were able to tie the unexpectedly high gravity wave activity to inertial instability signals remaining in their gravity wave perturbations (Rapp et al, 2018b). They showed the existence of a perturbation structure in their measurements with large horizontal and small vertical extent in the region of interest and demonstrated with 35 a vorticity argument that inertial instability induced those additional perturbations.…”

“…For a first access to the topic, we start by analysing artificial inertial instability perturbations. The structure was modeled according to the conditions found for a strong inertial instability event in December 2015, as previously discussed in Rapp et al (2018b). Then, we focus on a vertical and horizontal filtering approach on ERA5 temperatures.…”

“…To stabilize the imbalance, horizontal circulation generates transport of angular momentum in meridional direction. This results in shallow vertical motions accompanying the meridional flow (Rapp et al, 2018b). This flow causes layered temperature 40 perturbations observed around the equator in the stratosphere (Hitchman et al, 1987;Hayashi et al, 1998;Smith and Riese, 1999).…”