A Case Study of Low-level Internal Gravity Waves using Doppler Radar and ACARS

Abstract

An observational analysis of the low-level internal gravity waves over the Kanto Plain in Japan on 15 January 1998 was performed. The data from the Doppler radar for Airport Weather (DRAW), and aircraft soundings of wind and temperature (ACARS), provided unique observations with high spatial and temporal resolutions, and were used to analyze detailed wave structures and environmental conditions, especially wave duct structures.
According to the Doppler data analysis, the horizontal wavelength was approximately 6.5km, the ground relative phase speed of the waves was approximately 4m/s, and the direction of propagation was toward 50° (NE). The wave region spread 80km-100km horizontally. Doppler data analysis revealed that the waves maintained their coherent structures, lasted for more than one hour, and propagated horizontally for long distances as an isolated wave packet. The profiles from ACARS revealed the existence of a strong stable layer with vertical wind shear. Above the stable layer, there was a near-neutral layer with a critical level, around which the Richardson number was close to 0.25. These results indicate that the waves were trapped in the duct and propagated horizontally without a large amount of energy loss in the absence of a forcing mechanism.
The observed horizontal wavelength, profile of horizontal velocity amplitudes, and surface pressure perturbations are in agreement with our estimations derived from a linear theory on neutral modes.
Power spectrum and bandpass filtering analyses conducted on surface pressure show that the frequency and the phase of pressure perturbations are consistent with theoretical relations with internal gravity waves, suggesting that they were caused by the passage of the internal gravity waves.