Satellite Observations and a Numerical Study of Recurring Atmospheric Gravity Waves Along the South China Sea Coast

Abstract

AbstractSeveral groups of diurnal recurring atmospheric gravity waves (AGWs) were observed on an Environmental Satellite advanced synthetic aperture radar image and six Moderate Resolution Imaging Spectroradiometer (MODIS) images acquired along the South China Sea coast for 4 days from 3 to 6 March 2010. The unique observations are that the previously observed AGWs seldom recurred in a region for multiple days. Each cycle of these observed AGWs had undergone three stages: generation, propagation, and dissipation daily. Both SAR‐imaged sea surface roughness patterns and MODIS‐observed cloud imprints show that the AGWs were a group of transverse waves along the coast. We implemented the Weather Research and Forecasting (WRF) model to understand the physical characteristics and dynamical mechanisms of these AGWs. The WRF simulated results indicated that the AGWs were upstream waves triggered by the interaction of the southerly wind and topography and dissipated by the diurnal warming. We applied the spectral analysis to the model‐simulated vertical wind field and explored that the leading wave crest length and crest‐to‐crest wavelength are 156.25 and 7.14 km, respectively. The wave period and phase speed of the upstream AGWs within the vital observation of SAR are 1.81 hr and 1.26 m/s, respectively. The fifth‐generation European Centre for Medium‐Range Weather Forecasts reanalysis hourly data was used to explore the rarity of the recurring AGWs and proved that the stable air‐sea temperature difference was the main reason for the rarity of this event. And sufficient cloud content (>50%) was necessary for the MODIS to capture the AGWs' imprints.