A global analysis of wave potential energy in the lower stratosphere derived from 5 years of GPS radio occultation data with CHAMP

Abstract

This paper presents the first results of the global long‐term potential energy and mean potential energy per unit mass associated to wave activity (WA) in the lower and middle stratosphere, obtained from Global Positioning System radio occultation (GPS‐RO) temperature profiles, retrieved during the last 5 years from the CHAMP (CHAllenging Minisatellite Payload) satellite. We excluded temperature variations corresponding to the wavelike character of the Quasi Biennial oscillation (QBO). Possible limitations and distortions expected from our analysis are pointed out. Systematic annual and interannual features, clearly evidenced through 5 years of observations as a function of height, latitude and time are shown. We confirm some previously reported characteristics, in particular interannual requiring a sufficiently long period of observation, in addition to others not reported yet. In particular, a general stronger (weaker) wave activity is observed associated to apparent vertical wavelengths longer (shorter) than 4 km. The tropical/extratropical signatures decrease/increase with increasing altitude. At equatorial latitudes, WA interannual enhancements, related to QBO, are observed just below zonal wind zero contours corresponding to westerly shears. A significant decrease of WA is seen where the zonal wind is minimum. Both at equatorial and middle latitudes, an increased WA appears close above the TP, following its annual height oscillation and above 30 km height. At higher latitudes, a systematic annual variation of WA is observed, exhibiting stronger enhancements in winter SH respect to NH, but in SH, taking place during late winter and early spring. This enhanced WA, associated during 2002 to the stratospheric warming observed in that year, appears here as a systematic annual stratospheric feature. Its intensity increases with altitude, from 25 to 35 km. Inertio‐gravity waves generated by geostrophic adjustment during the maximum of the southern polar vortex (polar night jet) between late August and mid‐ September, could constitute a main source of this WA enhancement.

Gravity wave variances and propagation derived from AIRS radiances

“…This finding is consistent with what has been observed by Aura MLS (Wu and Eckermann, 2006). Some of the previous studies using radiosonde and GPS also found suppression of GWs in the QBO westerly phase (e.g., Wang and Geller, 2003;Torre et al, 2006), but they suggest that this suppression occur right along the descending of the zero-wind line of the QBO westerly phase. This is mainly caused by Kelvin waves that are unavoidably blended in the measurements from radiosonde and GPS.…”

A method to improve the determination of wave perturbations close to the tropopause by using a digital filter

“…A modulation function finally matches the perturbations to observed climatologies. More details of the buildup of the synthetic profiles is given by de la Torre et al (2010). For each latitude there were 500 created profiles, which allowed to calculate mean (reference) values of average temperature variance (mean average variance over 500 profiles) and their uncertainty (the standard deviation of the average variance over 500 profiles).…”

“…This has been concluded after almost a decade of work on global wave climatologies obtained from GPS RO satellite data (see, e.g. Steiner and Kirchengast, 2000;Tsuda et al, 2000;Ratnam et al, 2004;de la Torre et al, 2006;Namboothiri et al, 2008), which allowed to analyze, for the first time, a large amount of temperature profiles including both the troposphere and stratosphere. A large difference close to the LRT (lapse rate tropopause) between the temperature profile measured by a GPS RO example and the corresponding background determined by a filter (Scavuzzo et al, 1998) may be seen in Fig.…”

“…The separate filtering method significantly reduced the usually observed artificial wave activity enhancement in the tropopause region. An assessment of the significant errors introduced by the tropopause artificial enhancement when using a digital filter for the "complete" and "separate" methods applied to GPS RO temperature profiles has been recently carried out by de la Torre et al (2010). We will compare below these two alternatives with the double filtering method.…”

A gravity waves study close to the Andes mountains in Patagonia and Antarctica with GPS radio occultation observations

“…The vertical resolution of the temperature profiles typically ranges from 0.5 km in the lower troposphere to 1.4 km in the stratosphere (Kursinski et al, 1997), therefore only waves with vertical wavelengths greater than 2.8 km can be detected in our study. Several kilometers long columns may be needed for calculating the average energy (de la Torre et al, 2006c) and here we will follow this procedure. Wave activity was quantified by calculating the mean potential energy per unit mass E p through the average relative temperature T variance between altitudes z 1 and z 2 in each profile (Tsuda et al, 2000):…”