Initial Polarimetric Radio Occultation Results from Spire’s Nanosatellite Constellation: Vertical Moisture Structure Characteristics in and Near Precipitation from Closely-Spaced Observations

Abstract

The polarimetric radio occultation (PRO) measurement has been demonstrated on-orbit since the deployment of the Spanish-developed Radio Occultations through Heavy Precipitation (ROHP) instrument in 2018, followed in recent years by numerous small satellites deployed by commercial RO providers. In addition to the capabilities of traditional RO, PRO measures the cross-polarized signal induced by the presence of large aspherical hydrometeors such as rain, graupel, and aligned ice crystals that exist along the propagation path, from which the propagation differential phase shift (denoted by Δɸ) is derived. In early 2023, three PRO-capable nanosatellites were deployed by Spire Global. The orbital patterns provided many clusters of PRO in close spatial and temporal proximity, including many near precipitating clouds. Water vapor and temperature profile characteristics from the collection of these data are contrasted, conditioned upon the magnitude and the height of the Δɸ profile relative to the freezing level as proxies for heavy precipitation 2 . Increasing thresholds of the Δɸ magnitude reveal the increased relative humidity within mid-tropospheric (~400-600 hPa) pressure levels, and how this differs across land and ocean surface backgrounds. From the clustered PRO observations, the relative humidity inside the heavy precipitation was ~5-10% higher at similar mid-tropospheric levels, relative to the relative humidity in the surrounding no-precipitation environment. Similar trends are noted when the conventional RO bending angle is conditioned on Δɸ. These mid-level moisture characteristics revealed by analyzing the Δɸ profile simultaneously with conventional RO bending angle, may provide a means to better incorporate RO data when assimilated into numerical weather prediction models.