Imaging gravity waves in lower stratospheric AMSU-A radiances, Part 2: Validation case study

Abstract

Abstract. Two-dimensional radiance maps from Channel 9 (∼60-90 hPa) of the Advanced Microwave Sounding Unit (AMSU-A), acquired over southern Scandinavia on 14 January 2003, show plane-wave-like oscillations with a wavelength λ h of ∼400-500 km and peak brightness temperature amplitudes of up to 0.9 K. The wave-like pattern is observed in AMSU-A radiances from 8 overpasses of this region by 4 different satellites, revealing a growth in the disturbance amplitude from 00:00 UTC to 12:00 UTC and a change in its horizontal structure between 12:00 UTC and 20:00 UTC. Forecast and hindcast runs for 14 January 2003 using highresolution global and regional numerical weather prediction (NWP) models generate a lower stratospheric mountain wave over southern Scandinavia with peak 90 hPa temperature amplitudes of ∼5-7 K at 12:00 UTC and a similar horizontal wavelength, packet width, phase structure and time evolution to the disturbance observed in AMSU-A radiances. The wave's vertical wavelength is ∼12 km. These NWP fields are validated against radiosonde wind and temperature profiles and airborne lidar profiles of temperature and aerosol backscatter ratios acquired from the NASA DC-8 during the second SAGE III Ozone Loss and Validation Experiment (SOLVE II). Both the amplitude and phase of the stratospheric mountain wave in the various NWP fields agree well with localized perturbation features in these suborbital measurements. In particular, we show that this wave formed the type II polar stratospheric clouds measured by the DC-8 lidar. To compare directly with the AMSU-A data, we convert these validated NWP temperature fields into swath-scanned brightness temperatures using three-dimensional Channel 9 weighting functions and the actual AMSU-A scan patterns from each of the 8 overpasses of this region. These NWPCorrespondence to: S. D. Eckermann (stephen.eckermann@nrl.navy.mil) based brightness temperatures contain two-dimensional oscillations due to this resolved stratospheric mountain wave that have an amplitude, wavelength, horizontal structure and time evolution that closely match those observed in the AMSU-A data. These comparisons not only verify gravity wave detection and horizontal imaging capabilities for AMSU-A Channel 9, but provide an absolute validation of the anticipated radiance signals for a given three-dimensional gravity wave, based on the modeling of Eckermann and Wu (2006).

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

“…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.…”

Intercomparison of AIRS and HIRDLS stratospheric gravity wave observations

“…Both methods are standard methods that have been optimized for each instrument. The removal of background signals in AIRS temperature measurements follows the detrending method described by Wu and Zhang (2004), Eckermann et al (2006), and Alexander and Teitelbaum (2007). A fourth-order polynomial fit in the across-track direction is used in this method for defining the background.…”

Stratospheric gravity waves at Southern Hemisphere orographic hotspots: 2003–2014 AIRS/Aqua observations

“…The observed brightness temperatures are mainly composed of three contributions: (i) gravity wave signals, (ii) slowly varying background signals, and (iii) measurement noise. Background signals associated with large-scale temperature gradients or planetary waves are removed with the detrending procedure of Wu (2004), Eckermann et al (2006), and Alexander and Barnet (2007), i.e., brightness temperature perturbations are calculated as dif- Table 1 for details).…”

“…The analysis is based on a 12-year record (January 2003-December 2014) of 4.3 µm radiance observations of AIRS/Aqua. Stratospheric gravity wave signals in terms of brightness temperature perturbations and variances are extracted by applying a number of standard techniques developed for nadir sounders (Wu, 2004;Eckermann et al, 2006;Alexander and Barnet, 2007;Hoffmann et al, 2014). We introduce a simple and effective new method to detect orographic gravity wave signals from infrared nadir sounder measurements.…”