Characterization of Free Tropospheric Layers with Polar Radio Occultation Data

Polarimetric Radio Occultation (PRO) extends traditional radio occultation by measuring the differential phase shift between horizontally and vertically polarized signals (Δϕ) as they pass through ice clouds and precipitation. This enables detection of ice hydrometeors and the vertical structure of precipitating clouds, providing information not available from standard RO. Cloud top height (CTOP) is a key indicator of ice cloud depth and atmospheric stability, yet coincident global observations of CTOP and changes in thermal stability within precipitating systems remain sparse. Using three years of PRO data from the Radio Occultation and Heavy Precipitation (ROHP) experiment, we compare PRO-derived CTOP - defined as the highest altitude where Δϕ exceeds 1 mm – to the height of maximum lapse rate (LRMAX), the level of minimum static stability. LRMAX acts as a robust upper bound on upper-level (75th-90th percentile) cloud tops globally, with r = 0.80. The most heavily precipitating clouds extend from ~1 km below to slightly above LRMAX, consistent with deep clouds reaching the least stable layer. Above LRMAX, stability increases, and two lapse-rate-based tropopause diagnostics closely track the WMO tropopause outside the tropics, allowing us to quantify how far LRMAX lies below the tropopause locally and regionally. A threshold of 0.8 mm in Δϕ yields the strongest CTOP-LRMAX correspondence, while lower thresholds tend to weaken the relationship. These results show that PRO provides physically interpretable and globally robust CTOP estimates that directly relate to atmospheric stability, thereby resolving a critical gap in observing the vertical structure of precipitating clouds.