Collaborative Research: Four-Dimensional (4D) Investigation of Tropical Waves Using High-Resolution GNSS Radio Occultation from Strateole2 Balloons

Northwest Research Associates, Incorporated, Seattle WA

Investigators

Abstract

This award supports the continued participation of the Principal Investigators (PIs) in the Strateole-2 field campaign, organized by the French space agency (CNES, for Centre National d'Etudes Spatiales) and the Dynamic Meteorology Laboratory at the University of Paris-Saclay. The campaign makes observations of the tropical tropopause layer (TTL), the layer of the atmosphere from roughly 14km to 18km between the tropical troposphere and stratosphere, using balloons designed to float at a constant altitude for flights of up to 3 months. The balloons are launched from the Seychelles and float around the equator at the top of the TTL (18km) or in the lower stratosphere (20km). Strateole-2 was planned as a set of three deployments, a preliminary engineering deployment with 8 balloon flights followed by two science deployments with 20 flights each. The first two deployments took place in 2019 and 2021 and the PIs participated in these deployments using funds from AGS-1642650 and AGS-1642644. The PIs' participation in the third deployment, scheduled to begin in October 2025, is supported here. The PIs' role in Strateole-2 is to build and fly a Radio OCcultation receiver called ROC, which detects the refraction of radio waves transmitted by satellites from the Global Navigation Satellite System (GNSS, which includes the GPS satellites launched by the US). The strength of the refraction can be used to infer atmospheric temperature along the line of sight between ROC and a transmitter satellite, thus ROC can create temperature profiles by tracking a GNSS satellite as it descends to the horizon or rises from below it. Funds from this award are used to build six ROC receivers, manage their field deployment, and collect and analyze the data they generate. The temperature profiles from ROC are of interest because they show temperature fluctuations associated with wave motions in the TTL generated by large areas of tropical convection. One reason these waves are of interest is that they drive the quasi-biennial oscillation (QBO), an alternation between eastward and westward winds in the equatorial stratosphere which begins in the upper stratosphere and descends to the tropopause over the course of roughly two years. The QBO is confined to the tropics but it affects weather and climate around the world. It is well known that the QBO is driven by vertical momentum flux from waves that propagate upward from the TTL, but it is not clear what types of waves, particularly in terms of wavelengths and frequencies, are most important for driving the QBO. Another reason the waves are of interest is that their up-and-down motions are associated with cooling and warming of the ambient air, and cooling induced by rising motions can cause water vapor to freeze into ice particles (a process called deposition). Ice formation matters because it dehydrates air as it enters the stratosphere, thereby regulating the humidity of the stratosphere, and because ice particles form cirrus clouds which affect Earth's climate by trapping outgoing infrared radiation. Work on the wave driving of the QBO focuses on waves with periods of three or four days which were found to be prominent in the previous deployments. The PIs seek to determine the three-dimensional structure of the waves and their intrinsic frequencies, factors which together determine their wave momentum flux and thus their potential importance for QBO driving. The PIs have developed techniques for probing wave structure using the fact that the RO profiles are side-looking from the balloon and measure temperature at successively lower heights with distance from the balloon gondola. The three-dimensional structure of the waves can thus be reconstructed by combining consecutive RO profiles along the balloon flight path. As for cirrus cloud formation, four of the six ROC receivers will be flown with a downward-pointing lidar called BeCOOL, the Balloon-borne Cloud Overshoot Observation Lidar (BeCOOL), developed by a French team. BeCOOL observations of cirrus clouds can be combined with ROC observations of wave-induced temperature fluctuations to determine the extent to which cirrus clouds occur in the cold phases of waves in the TTL. The work has societal value through its connections to weather forecasting. Radio occultation receivers on satellites are an important source of observations used in operational weather prediction and work performed here includes an effort to assimilate ROC observations into weather models and test their value for prediction. The data assimilation and prediction effort involves collaborations with two operational centers. In addition, weather models have difficulty simulating the QBO and its global impacts, thus better understanding of the wave driving of the QBO can contribute to better forecast models. All data from the campaign are made freely available to the global research community and can be used in a variety of ways that go beyond the goals of the campaign. The project also builds the scientific workforce by supporting two graduate students and providing internship opportunities for undergraduates including two students from the Seychelles. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.