Where to see climate change best in radio occultation variables – study using GCMs and ECMWF reanalyses

Abstract

Abstract. Radio occultation (RO) is a new technique to observe the upper troposphere and lower stratosphere (UTLS), a region that reacts particularly sensitive to climate change. Featuring characteristics such as long-term stability, SI traceability, all-weather capability, global coverage, and high accuracy and vertical resolution, RO data fulfill the requirements for climate monitoring in the UTLS. However, while a range of studies has shown the climate utility of RO it has not yet been explored sytematically where to see climate change best in RO variables. Therefore we perform here a systematic trend study for the RO variables refractivity, pressure, and temperature (bending angle, not depending on height but impact parameter, is left for separate study). The trends, given at geopotential height levels and for layer gradients, are explored to determine seasons, geographic regions, and height domains, which show a significant trend signal. Because continuous RO data are available since 2001 only, reanalyses (ERA-40 and ERA-Interim) and global circulation model simulations of the Intergovernmental Panel on Climate Change Assessment Report 4 (CCSM3, ECHAM5, HadCM3) are used as proxy data for RO. It is shown that RO data are sensitive at different height ranges and that thus several indicators of climate change can be retrieved. Refractivity emerges as indicator in the lower stratosphere (LS) and tropopause region at about 14 km to 24 km, pressure over the whole UTLS, and both in all large-scale regions except the polar caps. Temperature qualifies as indicator in the upper troposphere below about 16 km and in the LS above about 21 km. Overall, refractivity and pressure alone are adequate indicators for the UTLS, but temperature as commonly used variable facilitates easy interpretation of results. Layer gradients were found to be further sensitive indicators providing additional information. Besides large-scale global and hemiCorrespondence to: B. C. Lackner (bettina.lackner@uni-graz.at) spheric means the tropics and the mid-latitudes appear as regions suitable to track climate change with RO data. The results also point to the value of utilizing in addition to annual means specific seasons, such as northern hemispheric fall and summer, for early climate signal detection. Since RO data feature much better vertical resolution than the proxy data of this study, more detailed insights can be expected when a longer RO record will be available.

Tropical Temperature Variability in the UTLS: New Insights from GPS Radio Occultation Observations

“…The zonal-mean influence of positive ENSO is warming in the tropical upper troposphere and cooling in the lower stratosphere (Fig. 10b) with maximum zonal-mean response approximately 3 months after ENSO forcing (Randel et al 2009;Lackner et al 2011b;Scherllin-Pirscher et al 2012). The most pronounced zonal-mean tropospheric warming (8-15 km) reaches up to 2 K. Longitudinal UTLS response to ENSO effects occur more rapidly (within 1 month) and feature maximum amplitude in the upper troposphere (near 11 km) and with opposite polarity in a shallow layer near the tropopause (Scherllin- Pirscher et al 2012).…”

“…Vedel and Stendel (2003), Stendel (2006), and Leroy et al (2006a) investigated the value of RO geopotential height and refractivity for climate studies. The complementary sensitivity for monitoring the UTLS was demonstrated with observing system simulation experiments for a medium emission scenario (Steiner et al 2001;Foelsche et al 2008b) and climate change indicators (Lackner et al 2011a).…”

Influence of changes in humidity on dry temperature in GPS RO climatologies

“…Due to the high consistency of RO data from different satellites (Hajj et al, 2004;Schreiner et al, 2007;Foelsche et al, 2011) and a comparatively small structural uncertainty (Ho et al, 2012;Steiner et al, 2013), trend studies could already be successfully carried out based on real data (Steiner et al, 2009;Schmidt et al, 2010;Lackner et al, 2011). In contrast to applications in numerical weather prediction, where parameters close to the observed quantities, such as bending angles, are used in the assimilation, in climate applications all atmospheric parameters need to be considered, since there are regions in the atmosphere, which will, e.g., not show trends in the bending angle, but in temperature (Foelsche et al, 2008b).…”

Influence of changes in humidity on dry temperature in GPS RO climatologies

“…The climate monitoring capability has first been demonstrated in simulation studies (e.g., Leroy et al, 2006;Ringer and Healy, 2008). Due to the high consistency of RO data from different satellites (Hajj et al, 2004;Schreiner et al, 2007;Foelsche et al, 2011) and a comparatively small structural uncertainty (Ho et al, 2012;Steiner et al, 2013), trend studies could already be successfully carried out based on real data (Steiner et al, 2009;Schmidt et al, 2010;Lackner et al, 2011). In contrast to applications in numerical weather prediction, where parameters close to the observed quantities, such as bending angles, are used in the assimilation, in climate applications all atmospheric parameters need to be considered, since there are regions in the atmosphere, which will, e.g., not show trends in the bending angle, but in temperature (Foelsche et al, 2008b).…”