GPS radio occultation simulation experiments for the upcoming Strateole-2 superpressure balloon campaign investigating equatorial waves

Deep tropical convection influences the transport of mass and momentum from the equatorial upper troposphere into the lower stratosphere through the generation and interaction of waves at a broad range of scales. The France-US collaborative Stratéole-2 project will explore equatorial waves in the tropopause region with super-pressure balloons, designed to drift on quasi-Lagrangian trajectories in the lower stratosphere. The Stratéole-2 program will launch 5 balloons from the Seychelles in the Indian Ocean in 2018-2019, and 20 balloons in 2020-2021, each with a flight duration of about 80 days. Five balloons will carry the Radio OCcultation (ROC2) instrument at 20 km altitude to execute a continuous sequence of temperature profiles on either side of the balloon trajectory to sample the equatorial wave field in three dimensions. It will also carry a micro-lidar for detecting cirrus and convective cloud tops. The goals are to describe the horizontal and vertical structure of tropical waves and their impact on cirrus formation and to investigate the relationships of waves to convective clouds. The GPS measurements quantify wave activity by providing precise estimates of balloon velocity and height perturbations due to waves and by providing refractivity profiles that are sensitive to vertical temperature fluctuations caused by waves. We present ray-tracing simulations of the propagation of GPS signals through the Earth's atmosphere, where they will be bent and delayed due to the gradient of atmospheric refractive index. European Centre for Medium-Range Weather Forecasts (ECMWF) analyses are used to construct the refractive index of the equatorial atmosphere, in which abundant atmospheric waves are present. With the known GPS signal geometry, the excess phase/Doppler are simulated that reflect the wave signatures. The resulting refractivity retrievals provide guidance for interpreting the spectral range of waves that the ROC2 instruments are most likely to reveal.