This is a 3-year observation and modeling project to be undertaken as part of the Coupling, Energetics and Dynamics of Atmospheric Regions (CEDAR) program. Atmospheric gravity waves are believed to play a vital role in the transport of energy and momentum between atmospheric layers. This project concerns the ubiquitous wave field in the thermosphere at the low latitude Arecibo Observatory (AO) site. Observing campaigns with the Arecibo incoherent scatter radar that employ a newly developed and tested innovative measurement technique will be used together with ray tracing, and first principles models. The goal is to self-consistently determine all of the parameters needed for reverse ray tracing for the gravity waves observed at the AO, to identify the most likely lower atmospheric and thermospheric sources for dozens of these gravity wave events, to determine the temporal evolution of the F region neutral wind, and to determine the neutral wind acceleration caused by select sources.
Gravity waves are a significant forcing source on mesosphere and lower thermosphere dynamics and their effects strongly impact general circulation and climate models. In addition, numerical weather prediction models, despite their tropospheric focus, have been shown to make better predictions when the upper-atmospheric dynamics are more accurately modeled. The project is collaboration between a female PI at North West Research Associates and an early career scientist at SRI International and also involves scientists at the NSF AO facility.
Atmospheric gravity waves (GWs) are created by many different processes, including thunderstorms, ocean waves, winds flowing over mountains, geostrophic adjustment processes, etc. For this project, we observed and studied the properties of GWs moving in the Earth's atmosphere 100 to 300 kilometers above the Earth's surface. We used data from the Arecibo Observatory along with models of GW propagation and dissipation to study these waves. The Arecibo Observatory, located in Puerto Rico and sponsored by the National Science Foundation, is the most sensitive ground-based instrument for studying properties of the ionosphere and thermosphere, with the ability to sense ionospheric perturbations (such as those induced by GWs) to better than 1 part in 1000. As part of the project, we designed and implemented a novel rotating dual-beam experiment, which was able to observe the waves and their periodicities, direction of propagation, and amplitudes. It was found that the GWs propagated southward, eastward, or northward, but generally not westward, and exhibited considerable day-to-day variability. In addition to wave properties, we also measured the lower thermospheric neutral, horizontal, mean winds, which control the wave populations that reach the thermosphere through filtering. The winds are large and rotate with altitude, indicating that their primary effect limits the wave populations in the thermosphere to those with the largest amplitudes and highest phase speeds. Preferential propagation directions may be caused by several factors, including wind and/or dissipative filtering, source location anisotropy, and other factors. Possible sources are thunderstorms and ocean waves. The observation of high-altitude, high phase-speed wave activity with long-lasting preferential propagation directions suggests that location anisotropy may be a factor. However, conclusions concerning source regions are only speculative at this point and further understanding requires detailed investigation of the wind environment throughout the atmosphere. To understand the properties of the GWs from ocean waves in the Earth's atmosphere, we developed a new model that couples ocean waves to atmospheric waves. We found that GWs with many different frequencies are excited by an ocean wave packet, and that they propagate at different speeds into the upper atmosphere. More precise modeling in the future will allow researchers to better understand the influence of ocean waves on the Earth's atmosphere above the Arecibo Observatory. Finding that ocean waves are a source of thermospheric GWs would lead to important knowledge about a source of global ionospheric and thermospheric variability. Thus, this project provides a better understanding of disturbances in the Earth's atmosphere and ionosphere.
