Equatorial wave analysis from SABER and ECMWF temperatures

Abstract

Abstract. Equatorial planetary scale wave modes such as Kelvin waves or Rossby-gravity waves are excited by convective processes in the troposphere. In this paper an analysis for these and other equatorial wave modes is carried out with special focus on the stratosphere using temperature data from the SABER instrument as well as ECMWF temperatures. Space-time spectra of symmetric and antisymmetric spectral power are derived to separate the different equatorial wave types and the contribution of gravity waves is determined from the spectral background of the space-time spectra. Both gravity waves and equatorial planetary scale wave modes are main drivers of the quasi-biennial oscillation (QBO) in the stratosphere. Temperature variances attributed to the different wave types are calculated for the period from February 2002 until March 2006 and compared to previous findings. A comparison between SABER and ECMWF wave analyses shows that in the lower stratosphere SABER and ECMWF spectra and temperature variances agree remarkably well while in the upper stratosphere ECMWF tends to overestimate Kelvin wave components. Gravity wave variances are partly reproduced by ECMWF but have a significant low-bias. A case study for the time period of the SCOUT-O3 tropical aircraft measurement campaign in Darwin/Australia (in November and December 2005) is performed and we find that in the lower stratosphere also the longitude-time distribution of the Kelvin waves is correctly reproduced by ECMWF.

Planetary wave activity in the polar lower stratosphere

“…Unlike equatorial wave analysis, it is not necessary to separate the temperature data into symmetrical and anti-symmetrical components (Ern et al, 2009). The temperature spectra are not red so it is not necessary to divide the results by a background spectrum (Alexander et al, 2008b;Ern et al, 2008). Henceforth we use the notation E1 to represent any eastward s = 1 planetary wave, W1 to represent any westward s = 1 planetary wave, and so on.…”

“…Waves with ground based periods of 4 days (the Nyquist period) to 32 days and |s| < 9 are considered here. When considering ground-based frequencies, as measured by COSMIC and other satellites, the location of a wave in wavenumber-frequency space will not change with altitude under the assumption of a slowly varying background field despite changes in the background wind with altitude (Ern et al, 2008). The largest variances and thus largest wave activity are due to waves with |s| ≤ 2.…”

Properties of the average distribution of equatorial Kelvin waves investigated with the GROGRAT ray tracer

“…While the wave signal observed in the troposphere is located at relatively slow phase speeds and mostly directly coupled to the convective systems, Kelvin waves observed in the stratosphere are dominated by "free" wave modes, which are excited by deep convection in the troposphere but not longer linked with the space-time patterns of the convective forcing (Randel and Wu, 2005;Ern et al, 2008;Kiladis et al, 2009).…”

“…In addition, only little was known about the spectrum of Kelvin waves and its temporal variations. However, improved satellite sensors recently provide these information for the troposphere (e.g., Wheeler and Kiladis, 1999;Straub and Kiladis, 2003;Cho et al, 2004) as well as for the stratosphere (Ern et al, 2008;Alexander et al, 2008) and the mesosphere (Garcia et al, 2005;). In our work we additionally use European Centre for Medium-Range Weather Forecasts (ECMWF) analyses which assimilate tropospheric measurements of convection and rain rates and have been validated to well represent also the stratospheric Kelvin wave amplitudes (Ern et al, 2008) and also Kelvin wave fluxes and zonal wind forcing have been found to be in good agreement with measurements (Ern and Preusse, 2009).…”

The benefit of limb cloud imaging for infrared limb sounding of tropospheric trace gases

“…McIntyre and Palmer, 1984;Riese et al, 2002), tropical wave modes (e.g. Canziani, 1999;Smith et al, 2002;Ern et al, 2008) and tides (e.g. Ward et al, 1999;Zhang, 2008).…”