4384 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 62 Large-Scale Waves in the Mesosphere and Lower Thermosphere Observed by SABER ROLANDO R. GARCIA National Center for Atmospheric Research,* Boulder, Colorado RUTH LIEBERMAN Colorado Research Associates, Boulder, Colorado JAMES M. RUSSELL III Hampton University, Hampton, Virginia MARTIN G. MLYNCZAK NASA Langley Research Center, Hampton, Virginia (Manuscript received 31 December 2004, in final form 10 May 2005) ABSTRACT Observations made by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board NASA’s Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) satellite have been processed using Salby’s fast Fourier synoptic mapping (FFSM) algorithm. The mapped data provide a first synoptic look at the mean structure and traveling waves of the mesosphere and lower thermosphere (MLT) since the launch of the TIMED satellite in December 2001. The results show the presence of various wave modes in the MLT, which reach largest amplitude above the mesopause and include Kelvin and Rossby–gravity waves, eastward-propagating diurnal oscillations (“non-sun- synchronous tides”), and a set of quasi-normal modes associated with the so-called 2-day wave. The latter exhibits marked seasonal variability, attaining large amplitudes during the solstices and all but disappearing at the equinoxes. SABER data also show a strong quasi-stationary Rossby wave signal throughout the middle atmosphere of the winter hemisphere; the signal extends into the Tropics and even into the summer hemisphere in the MLT, suggesting ducting by westerly background zonal winds. At certain times of the year, the 5-day Rossby normal mode and the 4-day wave associated with instability of the polar night jet are also prominent in SABER data. 1. Introduction 2004) in addition to the quasi-stationary waves common at lower altitudes. The increasing prominence of fast A number of studies over the last couple of decades waves at high altitudes is expected because, as a spec- Յ have documented the presence of fast (period 5 days) trum of waves propagates from the lower atmosphere, traveling planetary-scale waves in the mesosphere and damping and absorption at critical levels will remove lower thermosphere (MLT) (e.g., Hirota, 1978, 1980; lower frequency components most effectively (see, e.g., Salby et al. 1984; Canziani et al. 1994; Lawrence and Garcia and Salby 1987). Although high-frequency, Randel 1996; Limpasuvan and Wu 2003; Pancheva et al. planetary-scale waves have been detected in the MLT by both ground-based instrumentation and polar- orbiting satellites, the latter have the advantage that * The National Center for Atmospheric Research is sponsored they can provide a global view of the phenomena. by the National Science Foundation. We report here on preliminary studies of large-scale waves observed by the Sounding of the Atmosphere Corresponding author address: Dr. Rolando R. Garcia, NCAR, using Broadband Emission Radiometry (SABER) in- 1850 Table Mesa Dr., Boulder, CO 80307-3000. strument on board the National Aeronautics and Space E-mail: [email protected] Administration (NASA) Thermosphere–Iono- © 2005 American Meteorological Society Unauthenticated | Downloaded 09/26/21 06:21 PM UTC JAS3612 DECEMBER 2005 GARCIA ET AL. 4385 sphere–Mesosphere Energetics and Dynamics • 15 June–14 July 2002, (TIMED) spacecraft, which was launched on 7 Decem- • 14 June–14 July 2003, and ber 2001. We use SABER observations from several • 27 January–22 February 2004. continuous data segments currently available (typically Each of these periods comprises approximately one about 30 days long), which are particularly well suited month, so their synoptic spectra have approximately for processing using Salby’s (1982a,b) fast Fourier syn- the same bandwidth and thus are directly comparable. optic mapping (FFSM) technique (see section 3). Furthermore, they cover four solstice seasons (two bo- The SABER data reveal the presence of a rich spec- real and two austral summers) plus one equinox, which trum of large-scale motions in the MLT. In addition to helps illustrate the interhemispheric and interannual stationary Rossby waves, several planetary-scale waves variability of the 2-day wave. For each period we use with period Յ5 days are identified. These include SABER observations spanning the altitude range ϳ15– Kelvin and Rossby–gravity waves, eastward- 120 km. propagating diurnal oscillations, the 4- and 5-day waves, and several spectral components associated with the “2-day wave,” all of which are discussed in detail in 3. Fast Fourier synoptic mapping section 4. Westward-propagating diurnal oscillations, including the sun-synchronous tides, are not resolved As with any polar-orbiting satellite, observations synoptically by the SABER sampling pattern and are made by TIMED are asynoptic; that is, global coverage not discussed here.1 is not instantaneous because measurements at different locations are made at different universal times. How- 2. SABER data ever, Salby (1982a,b) has proven an “asynoptic sam- pling theorem” that guarantees that the information SABER uses limb-scanning, broadband infrared ra- content of combined-node asynoptic observations is diometry to measure temperature, ozone, water vapor, equivalent to that of twice-daily synoptic sampling carbon dioxide, nitric oxide, and airglow emissions over within the Nyquist limits common to both. In particular, a broad range of altitude from near-tropopause levels the theorem implies that, if an observed field contains to the lower thermosphere (Mlynczak 1997; Russell et negligible variability beyond the Nyquist limits of asyn- al. 1999). The method used for retrieving SABER tem- optic sampling, it is possible to reconstruct from the peratures is described by Mertens et al. (2001). Obser- latter the full, instantaneous synoptic evolution of the vations are made during both ascending and descending field. This property of asynoptic sampling is desirable portions of the orbit. In this study we use “level 2A” because it allows a straightforward but rigorous assess- retrievals of temperature, which are publicly available ment of the information content of observations made as version 1.04 (v1.04) from late January 2002 through from polar orbit, and their limitations due to aliasing. August 2004 (more information available online at Salby (1982a,b) has used these results to develop a http://saber.larc.nasa.gov/). mapping algorithm that can be used to obtain synoptic The v1.04 data consist of vertical profiles registered spectra from asynoptic observations. The utility of in pressure as functions of longitude, latitude, and uni- FFSM has been demonstrated by a number of recent versal time. Aside from yaw maneuvers and instrument applications (e.g., Lieberman 1991; Lait and Stanford downtime, SABER has made almost continuous obser- 1988; Canziani et al. 1994; Manney et al. 1998; Sassi and vations since launch. However, processed v1.04 data Salby 1999). In spite of its power, FFSM has not been are currently available as continuous sequences only for used very widely because it requires sampling at regular certain periods of varying length, ranging from a few intervals in space and time for extended periods, some- days to over one month. From January 2002 to Febru- thing that is not often available from satellite observa- ary 2004 there are several data sequences of between 25 tions. During the design of SABER, emphasis was and 40 days suitable for synoptic mapping via Salby’s placed from the outset on regularity and continuity of FFSM method. Observations taken during the follow- the sampling pattern; as a result, archived SABER ing periods are used in the analyses presented here: products provide observations that are particularly well • 25 January–24 February 2002, suited for analysis via FFSM. • 31 March–10 May 2002, Most of the planetary-scale waves documented in this study have wavenumbers and frequencies that fall well 1 Sun-synchronous, or “migrating,” tides may be estimated by within the Nyquist limits of asynoptic sampling. In compositing data taken over the precession cycle of the satellite. zonal wavenumber, these limits range from 0 (the zonal Such estimates will be presented elsewhere. mean) to 7. In frequency, they range from slightly less Unauthenticated | Downloaded 09/26/21 06:21 PM UTC 4386 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 62 2002 to illustrate the fact that SABER is able to ob- serve accurately all the principal features of the zonal- mean temperature structure from the tropopause [2 scale heights (sh), or about 14 km assuming a uniform scale height of 7 km] to the lower thermosphere (17 sh, or about 120 km). In particular, the figure shows a cold tropical tropopause (ϳ195 K near 2.4 sh, or ϳ17 km), a warm summer stratopause ranging between 280 K at the summer pole and 250–260 K in the Tropics, a very cold summer mesopause (Ͻ130 K poleward of 75°N), and a rapid temperature rise in the thermosphere above 15 sh. The quality of SABER temperature data has been demonstrated in detail by Remsberg et al. (2003), who compared an earlier version of SABER tempera- FIG. 1. Zonal-mean temperature field observed by SABER dur- ture against the Met Office analyses. More recently, ing Jun–Jul 2002. The vertical scale is pressure-scale height, sh ϭ Mertens et al. (2004) have carried out comparisons of ϭ ln(p0/p), with p0 1000 mb. Approximate altitudes can be ob- SABER temperatures at polar latitudes with falling- tained by multiplying s times a typical scale height for the middle h sphere measurements. Retrieval of nonlocal thermody- atmosphere, H ϭ 7 km. The contour interval is 10 K. namic equilibrium (NLTE) temperatures in the cold summer mesopause is perhaps the most stringent test of than 1 cycle per day (cpd) to slightly more (i.e., more the SABER retrieval algorithm; the comparisons with negative) than Ϫ1 cpd, where positive (negative) fre- falling-sphere data yield rather good agreement and in- quencies denote westward (eastward) propagating os- crease our confidence in the quality of SABER v1.04 cillations.