Gravity Wave Driving of the Stratospheric Transport Circulation

Abstract:

Waves of all types including synoptic waves, extratropical planetary waves, and gravity waves, combine to produce the equator-to-pole Brewer-Dobson circulation, the circulation describing chemical transport in the stratosphere. Small-scale gravity wave effects have been difficult to quantify with observations, since three-dimensional, global, high-resolution measurements are required. We describe a new method utilizing Aura observations to compute gravity wave driving of the circulation.

HIRDLS data are unique among high-resolution datasets available for wave studies. The vertical width of the field of view projected at the limb is 1.2 km giving vertical resolution ∼1 km, and rivaling that of GPS radio occultation temperature retrievals. This resolution is crucial for wave studies in the lower stratosphere, where the vertical wavenumber spectrum of gravity wave energy peaks at a vertical wavelength near 2km. HIRDLS horizontal resolution is also unique. The field of view projected at the limb is ∼20km wide, and individual HIRDLS profiles are spaced ∼100km apart. These properties combine to give HIRDLS uniquely fine characteristics for gravity wave studies. Like most high-inclination limb-scanning satellites, HIRDLS provides no cross-track measurements: The high resolution lies along a single measurement track line. For wave studies, the lack of cross-track sampling limits the global wavenumbers that can be studied to wavenumbers ∼0-7 and limits estimates of gravity wave horizontal wavenumber to only the apparent wavenumber along the measurement track. The lack of cross-track sampling leads to large uncertainties in gravity wave momentum fluxes due to the lack of information on wave propagation direction. The resulting momentum flux uncertainties are likely a factor of 2 or more, and the lack of directional information further limits the use of HIRDLS data for inferring wave effects on the circulation.

We augment HIRDLS measurements with Global Positioning System (GPS) radio occultation temperature profiles to provide missing cross-track information. We use these data to compute correction factors for errors in gravity wave horizontal wavelengths and momentum fluxes globally, and to estimate gravity wave momentum flux direction for improved estimates of their effects on the circulation.