Meteorological Source Variability in Atmospheric Gravity Wave Parameters Derived From a Tropical Infrasound Station

Abstract

Gravity waves are an important part of the momentum budget of the atmosphere. Despite this, parameterizations of gravity wave spectra in atmospheric models are poorly constrained. Gravity waves are formed by jet streams, flow over topography, and convection, all of which produce pressure perturbations as they propagate over the Earth's surface, detectable by microbarometer arrays used for sensing infrasound. In this study, observations of gravity waves between 2007 and 2011 at an infrasound station in the Ivory Coast, West Africa, are combined with meteorological data to calculate parameters such as intrinsic phase speed and wavenumber. Through spectral analysis, the seasonal and daily variations in all gravity wave parameters are examined. The gravity wave back azimuth varies with the migration of the Intertropical Convergence Zone, a region of intense convection, supporting previous studies. Daily variations in gravity wave arrivals at the station can be linked to two distinct convective cycles over the land and ocean. This was achieved by combining the gravity wave parameters with lightning strikes detected by the Met Office's Arrival Time Difference lightning detection system. Noise generated by turbulence in the middle of the day was found to attenuate smaller pressure amplitude gravity waves, artificially amplifying the daily variations in some gravity wave parameters. Detection of daily and seasonal variations in gravity wave parameters has the potential be used to improve the representation of gravity wave spectra in atmospheric models.

Infrasound Thunder Detections across 15 Years over Ivory Coast: Localization, Propagation, and Link with the Stratospheric Semi-Annual Oscillation

“…Furthermore, the monthly distribution (not shown here) is also similar to that observed for WWLLN flashes, namely between March and June and between September and November (Figure 3d). A westward tropospheric waveguide forms due to the presence of a westward low level jet (LLJ) at about 4 km height in spring (MAM) and fall (SON), as shown, for instance, in Figure 1 of Marlton et al [31]. The LLJ and its seasonal impact on the westward tropospheric infrasound guiding was confirmed by looking at vertical effective sound speed profiles obtained using ECMWF analysis products (not shown), as in Section 6, where the seasonal pattern of the stratospheric detections is discussed.…”

“…They suggested these disturbances could be due to cold pool propagation. Marlton et al [31] utilized these gravity wave observations to derive gravity wave parameters such as the horizontal wave number, which exhibits a similar seasonal cycle.…”