COSMIC-2 RO Profile Ending at PBL Top with Strong Vertical Gradient of Refractivity

Abstract

The Formosa Satellite-7/Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (Satellite-7/COSMIC-2), which was successfully launched on 25 June 2019, provides dense radio occultation (RO) observations over the tropics and subtropics. This study examines the RO-observed lowest altitude and its possible relationship to refractivity gradients and planetary boundary layer (PBL) heights. COSMIC-2 RO data over the Southeast Pacific region (SEP) and the South-Central Pacific (SCP) from August 2020 are employed to determine their RO-observed lowest altitudes, and the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis data are used to obtain the gradients of refractivity. Results show that there are no ray perigees below the PBL top when the vertical gradient of N − N(r) is strong (<−65 N-unit km−1), where N(r) represents the vertical profile of the spherically symmetric refractivity. Significantly strong local vertical gradients due to atmospheric ducting occur more frequently over the SEP than the SCP areas. For some cases, a strong local horizontal gradient of refractivity in the tangent direction of a ray near its perigee point can also limit the RO profile from going further below even when the vertical gradient of N − N(r) is relatively weak. Fortunately, only about 0.6% COSMIC-2 RO profiles are unaffected by the above factors but cannot observe below 2-km altitude.

Tropical Cyclone Planetary Boundary Layer Heights Derived from GPS Radio Occultation over the Western Pacific Ocean

“…In Xu and Zou [26], spherically symmetric refractivity was introduced to describe the fundamental variation of refractivity with altitude due to the different densities of upper and lower atmospheres. The spherically symmetric refractivity is defined as the global average of the refractivity at the same r, where r is the distance from the Earth's center at each profile point, r = z + r 0, where r 0 is the radius of the earth.…”

“…The atmospheric refractivity, therefore, has a fundamental change with altitude. Spherically symmetric refractivity was developed to depict a reference change of refractivity with altitude [26]. Compared with N, N i representing the residual from the fundamental vertical variations of refractivity is more conducive to highlighting the local variation characteristics of the atmosphere and the influences of different weather conditions on the vertical gradient of refractivity and is more suitable for determining the PBLH.…”