Characterization of Free Tropospheric Layers With Polar Radio Occultation Data

Abstract. Polarimetric Radio Occultation (PRO) concurrently detects heavy precipitation, ice, and the vertical thermodynamic structure inside clouds, enhancing traditional radio occultation measurements. We compare cloud top heights (CTOP) defined as the uppermost altitude at which the polarimetric phase difference between the horizontal and vertical components, Δϕ, exceeds 1 mm, for three years of PRO data from the Radio Occultation and Heavy Precipitation (ROHP) experiment, to the local tropical tropopause layer (TTL) base, a minimum stability level determined as the maximum lapse rate height (LRMAX). The TTL base coincides with the 80^th – 90^th percentiles of CTOP globally. We examine the skill of using the Tropopause Inversion Layer and the minimum vertical gradient of the lapse rate (∂LR/∂z)_min to characterize the tropopause compared to the lapse rate and the cold point tropopauses. The TTL thickness, defined as the height of the (∂LR/∂z)_min minus the LRMAX, is thinnest over the Tropical Warm Pool where LRMAX and CTOP are deepest. The steepest meridional gradient with latitude of the TTL top height is just equatorward of the subtropical maxima of the frequency of double tropopauses. For tropical raining clouds, when the maximum Δϕ, Δϕ_max exceeds 10 mm, the mean binned CTOP is 2.7 km below the mean LRMAX, with a slope of nearly one. Using 0.8 mm for the Δϕ CTOP threshold is optimal, while reducing below 0.8 mm decreases the CTOP and LRMAX spatial correlation. Globally, cloud tops associated with the largest 99^th-percentile Δϕ_max are 0.4 km above LRMAX.