Using Satellite Observations to Constrain Parameterizations of Gravity Wave Effects for Global Models

Abstract

Small-scale gravity waves are common features in atmospheric temperature observations. In satellite observations, these waves have been traditionally difficult to resolve because the footprint or resolution of the measurements precluded their detection or clear identification. Recent advances in satellite instrument resolution coupled to innovative analysis techniques have led in the last decade to some new global datasets describing the temperature variance associated with these waves. Such satellite observations have been considered the best hope for quantifying the global properties of gravity waves needed to constrain parameterizations of their effects for global models. Although global maps of averaged gravity wave temperature variance have now been published from a variety of different instruments on Earth-orbiting platforms these maps have not provided the needed constraints. The present paper first summarizes what has been learned from traditional temporally and spatially averaged analyses of satellite gravity wave observations and why new analysis methods are needed. Then an alternative is offered to these traditional analyses that recognizes the fact that the waves occur in large-amplitude events, or wave packets, that can be analyzed individually and in a statistical sense with probability density functions. For this purpose the authors present some examples of the occurrence of short-horizontal-scale waves appearing in Atmospheric Infrared Sounder (AIRS) radiance measurements and present statistics on the wave properties compiled over 1 month of data for a geographic region over Patagonia, South America.

A decadal satellite record of gravity wave activity in the lower stratosphere to study polar stratospheric cloud formation

“…This difference between the hemispheres is related to differences in the typical gravity wave spectra in both hemispheres. For the 21 peak events analyzed here, a 2-D spectral analysis of the AIRS data using the S-transform method (Stockwell et al, 1996;Alexander and Barnet, 2007) yielded mean horizontal wavelengths of 116 ± 52 km in the Northern Hemisphere and 133 ± 72 km in the Southern Hemisphere. These differences might be due to the gravity wave sources, such as orography, convection, and jets, as well as differences in the background flow between the hemispheres.…”

“…Conversely, weak background winds are associated with gravity waves with short vertical wavelengths, which are generally not detectable by AIRS. This combined effect of the stratospheric background winds and the vertical wavelength sensitivity of AIRS is referred to as an "observational filter" (Alexander and Barnet, 2007;Hoffmann et al, 2014Hoffmann et al, , 2016a. Figures 7 and 8 also show the contours of ERA-Interim monthly mean zonal winds at 30 hPa.…”

“…Radiative transfer calculations have been performed for polar summer and polar winter conditions, the nadir direction and the outermost scan angles (referred to as "sub-limb"), and a 1 km altitude grid. (Alexander and Barnet, 2007;Hoffmann and Alexander, 2009;Gong et al, 2012;Hoffmann et al, 2014). For this study, we identified a set of 21 AIRS channels in the 15 µm CO 2 waveband to study gravity wave activity in the polar lower stratosphere.…”

“…Background signals are caused not only by large-scale temperature gradients and planetary waves, but also by the limbbrightening effect. A standard detrending technique for AIRS is to remove the background defined by means of a polynomial fit to the brightness temperature measurements of each across-track scan (Wu, 2004;Eckermann et al, 2006;Alexander and Barnet, 2007;Hoffmann et al, 2014). Usually, a fourth-order polynomial is applied for this purpose.…”

Upper atmospheric gravity wave details revealed in nightglow satellite imagery

“…Since the 1960s, satellites have characterized gravity wave distributions in the middle atmosphere (21)(22)(23). Whereas nadir-viewing infrared sounders, which offer much higher horizontal spatial resolution (∼10 km) than limb sounders and GPS radio occultation systems (∼100 km), have provided a global climatology of stratospheric gravity waves, corresponding high-resolution observations of waves occurring at mesospheric levels and above have not been available on a regular basis.…”