Climatology and modeling of quasi‐monochromatic atmospheric gravity waves observed over Urbana Illinois

Abstract

Abstract. From analyzing nine months of airglow imaging observations of atmospheric gravity waves (AGWs) over Adelaide, Australia (35øS) [Walterscheid et al., 1999] have proposed that many of the quasi-monochromatic waves seen in the images were primarily thermally ducted. Here are presented 15 months of observations, from February 1996 to May 1997, for AGW frequency and propagation direction from a northern latitude site, Urbana Illinois (40øN). As Adelaide, Urbana is geographically distant from large orographic features. Similar to what was found in Adelaide, the AGWs seem to originate from a preferred location during the time period around summer solstice. In conjunction with these airglow data there exists MF radar data to provide winds in the 90 km region and near-simultaneous lidar data which provide a temperature climatology. The temperature data have previously been analyzed by States and Gardner [2000]. The temperature and wind data are used here in a full wave model analysis to determine the characteristics of the wave ducting and wave reflection during the 15 month observation period. This model analysis is applied to this and another existing data set recently described by Nakamura et al. [1999]. It is shown that the existence of a thermal duct around summer solstice can plausibly account for our observations. However, the characteristics of the thermal duct and the ability of waves to be ducted is also greatly dependent on the characteristics of the background wind. A simple model is constructed to simulate the trapping of these waves by such a duct. It is suggested that the waves seen over Urbana originate no more than a few thousand kilometers from the observation site. IntroductionThe ducting or trapping of atmospheric gravity waves (AGWs) has long been a subject of interest [e.g., Pitteway and

Reflection of a long‐period gravity wave observed in the nightglow over Arecibo on May 8–9, 1989?

“…Accordingly, we also use the horizontal wind model (HWM) [Hedin et al, 1996] to define the mean winds throughout the atmosphere for May 8/9. Tidal variations are quite strong in the lower thermosphere, and therefore the mean winds and the associated wave reflection may depend quite sensitively on local time [Hecht et al, 2001]. Therefore for the HWM winds we selected three different local times (0000, 0200, and 0400 LT), which corresponds to the time period analyzed in detail by W2000.…”

“…The full-wave model allows us to assess the strength of reflection while also providing values of 4)(•) for several airglow emissions. The model has been used extensively to simulate the ducting of very short period gravity waves in the lower thermospheric thermal duct while also including mean wind effects [Hecht et al, 2001 ]. Using this model, these studies have been able to explain the observed seasonal variation of the preferred azimuths of gravity wave propagation.…”

Recent progress in mesospheric gravity wave studies using nigthglow imaging system

“…Nakamura et al, 1999;Walterscheid et al, 1999;Hecht et al, 2001), some with strong meridional anisotropy. Walterscheid et al (1999) suggested this could be due to wave ducting rather than wind filtering effects.…”

Airglow variations associated with nonideal ducting of gravity waves in the lower thermosphere region

“…We have not performed simulations using either measured winds or using winds based on either numerical or empirical models. Climatological wind profiles have been used previously in wave-ducting simulations using our full-wave model by Hecht et al [2001 ]. These wind profiles combined empirical winds [Hedin et al, 1996] with a wind climatology for Urbana, Illinois, based on University of Illinois MF radar measurements (a good discussion of the MF radar winds is given by Franke and Thorsen [1993]).…”

“…wave may be internal in the region of local minimum of r B , and become evanescent at some distance either side of this where r• has increased [Hines, 1960;Wickersham, 1968;Hecht et al, 2001 ]. In this case, the wave would be thermally ducted.…”