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Status of the SPARC Radiosonde Initiative

R. Vincent, Univ. of Adelaide, Australia (robert.vincent@adelaide.edu.au)

The SPARC radiosonde initiative was established to exploit the unique resource of high-resolution radiosonde observations made by weather agencies. These observations, made regularly once or twice per day, provide one of the few ways of characterising the gravity wave field up to heights in the lower to middle stratosphere (up to ~30 km). The results complement measurements made with ground-based radars and lidars and emerging techniques using spaceborne instrumentation, such as the GPS/MET observations (e.g. Tsuda et al., 2000). The goal is to produce a climatology of GW parameters in the lower stratosphere in order to help constrain GW parameterization schemes used in numerical climate models (Hamilton and Vincent, 1995).

The basic analysis of the observations is based on the original work of Allen and Vincent (1995), who analysed temperature-only measurements made in the Australian sector of the Southern Hemisphere. Using wind, as well as temperature observations, the analysis techniques were extended to include directions of wave propagation (e.g. Vincent et al., 1997).

Scientists from over a dozen countries are involved in the analysis of the radiosonde data for this initiative. Figure 1 shows the distribution of sounding sites currently used in the project. While radiosondes are routinely launched over a wide part of the globe, it is unfortunate that more national meteorological services do not archive their observations at high resolution, despite strong encouragement from SPARC. It is apparent that large gaps exist in the network, especially in central Asia and large parts of the Southern Hemisphere. Observations taken with more specialised networks, such as the SOWER campaigns (Shiotani et al., 2000) and the SHADOWZ ozone sondes (Thompson et al, 2002; see also Thompson et al. in this Newsletter), have been especially helpful in filling in gaps in tropical regions. Despite the gaps, there are regions, such as the USA, where large numbers of sites may make possible the identification of wave sources, such as jet streams, fronts etc., and quantify their relative importance.

Figure 1. Location of radiosonde stations

A workshop held in Abingdon, Oxford, in July 1999 considered a number of issues associated with the analysis and interpretation of data. One of the first decisions concerned the height coverage of the analysis. Allen and Vincent (1995) analysed data in 7 km height ranges, covering 2-9 km in the troposphere and 17-24 km in the stratosphere. However, it was decided for the global stratospheric analysis to use a 7 km height range just above the tropopause. The shaded region in Figure 2 shows the height coverage used and is based on a climatology of the tropopause described in Hoinka (1998). At the poles the height range is 12-19 km, corresponding to a mean pressure of ~100 hPa, while in the tropics the height coverage is from 18-25 km (~40 hPa).

Figure 2. The solid line shows a mean tropopause based on Hoinka (1998). The shaded region indicates the 7 km height range used in the stratospheric analysis of gravity waves.

The workshop also decided that the basic analysis for the SPARC project should concentrate on producing climatologies of wave kinetic (KE) and potential energy (PE) and directions of propagation. Other potentially important parameters included in this analysis are parameters of the vertical wavenumber spectra, such as spectral slope, axial ratios of the perturbation wind hodographs and the mean vertical wavelength. Other potentially important, but more controversial quantities, such as momentum fluxes derived from combining the wind and temperature measurements (e.g. Vincent and Alexander, 2000), were left for individual researchers to pursue as research topics.

Examples of some of the products coming from this study are shown in Figures 3 and 4. Kinetic energies (KE) are about 1.5-2 times larger than potential energies (PE), consistent with domination of the short vertical wavelength component of the wave field by inertia gravity waves. RMS wave amplitudes range from ~6 ms-1at the equator to 2.5 ms-1at the pole. The steady decrease of wave energy with increasing latitude may be due to observational selection, related to the latitudinal increase in the inertial frequency and the slow vertical group velocity of inertia gravity waves (Alexander et al., 2002). An apparent 'spike' in energy at the equator, which is particularly strong at the November-February period, is probably associated with short vertical wavelength Kelvin waves (Holton et al., 2001).

Figure 3. Annual average values of gravity wave kinetic (red) and potential energy (blue) in the stratosphere in 10-degree latitude bands.

Figure 4. Annual mean values of the propagation direction of waves observed in the stratosphere.

Horizontal directions of propagation show a systematic shift as a function of latitude. In tropical regions waves tend to propagate eastward, while at latitudes greater than ~25° the waves propagate westward, with a poleward component (northwestward in the Northern Hemisphere and southwestward in the Southern Hemisphere). In general, the waves are observed to propagate against the prevailing winds. The sense of rotation of the perturbation wind hodographs shows predominantly anticyclonic rotation in both hemispheres, consistent with upward energy propagation.

Some global general circulation models (GCMs) are now being run at sufficiently high vertical and horizontal resolution that comparisons of statistical behaviour of simulated wind and temperature fluctuations with the high-resolution radiosonde observations may be appropriate. For example, the GFDL "SKYHI" troposphere-stratosphere-mesosphere GCM has been run with 160 levels in the vertical, corresponding to a level spacing of about 300 m in the lower stratosphere (Hamilton et al., 1999). Wave characteristics derived from the radiosonde data set are now being compared with wind and temperature fluctuations sampled over the same geographical network in the high-resolution SKYHI simulation. Preliminary results suggest that the explicitly resolved waves in the lower stratosphere of the GCM are similar in many respects to those seen in the radiosonde observations.

References:

Alexander, M.J., T. Tsuda, and R.A. Vincent, Latitudinal variations observed in gravity waves with short vertical wavelengths, J. Atmos. Sci., 59, 1394-1404, 2002.

Allen, S.J., and R.A. Vincent, Gravity-wave activity in the lower atmosphere: Seasonal and latitudinal variations, J. Geophys. Res., 100, 1327-1350, 1995.

Hamilton, K., and R.A. Vincent, High-resolution radiosonde data offer new prospects for research, Eos Trans. AGU, 497, 1995.

Hamilton, K., R.J. Wilson and R.S. Hemler, Middle atmosphere simulated with high vertical and horizontal resolution versions of a GCM: Improvement in the cold pole bias and generation of a QBO-like oscillation in the tropics. J. Atmos. Sci., 56, 3829-3846, 1999.

Hoinka, K.P., Temperature, humidity, and wind at the global tropopause, Mon. Wea. Rev., 127, 2248-2265, 1999.

Holton, J.R., M.J. Alexander, and M.T. Boehm, Evidence for short vertical wavelength Kelvin waves in the Department of Energy-Atmospheric Radiation Measurement Nauru99 radiosonde data, J. Geophys. Res., 106, 20,125-20,129, 2001.

Shiotani, M. et al., SOWER/Pacific the Shoyo-maru Pacific ocean-atmosphere survey, SPARC Newsletter, No 14, January 2000.

Thompson, A.M. et al., The 1998-2000 SHADOZ (Southern Hemisphere Additional Ozonesondes) Tropical ozone climatology: Comparisons with TOMS and ground-based measurements, J. Geophys. Res., (in press), 2002.

Tsuda, T., M. Nishida, C. Rocken, and R. Ware, Global morphology of gravity wave activity in the stratosphere revealed by the GPS occultation data (GPS/MET), J. Geophys. Res., 105, 7257-7274, 2000.

Vincent, R.A., and M.J. Alexander, Gravity waves in the tropical lower stratosphere: An observational study of seasonal and interannual variability, J. Geophys. Res., 105, 17,971-17,982, 2000.

Vincent, R.A., S.J. Allen, and S.D. Eckermann, Gravity-wave parameters in the lower stratosphere, in Gravity wave Processes: Their Parameterization in Global Climate Models, (ed. K. Hamilton), Springer-Verlag, Berlin, 7-25, 1997.