Stratocumulus-topped atmospheric boundary layers: GPS RO observations vs. ECMWF analysis

Stratocumulus (Sc) clouds are typically trapped below a stable inversion layer and are mainly observed over eastern subtropical oceans. The persistent low cloud deck has similar temperature but much larger albedo compared to the ocean surface below, and thus produces an important negative climate feedback. The shallow Sc-topped atmospheric boundary layer (ABL), usually 0.5-2km deep, has been a great challenge for remote sensing from space. The detailed ABL observations remain limited to a number of ship and airplane measurements. GPS Radio Occultation (RO) technique emerges as a powerful sounding technique with potential to resolve ABL thermodynamic structures with superb vertical resolution (~200m). Launched in 2006, the COSMIC (Constellation Observing System for Meteorology, Ionosphere & Climate) RO offers over 2000 daily profiles and scans the ABL globally. However, thermodynamic structures of the ABL from RO remain largely unexplored due to a notorious, negative bias in the RO refractivity retrieval. The bias is likely caused by the presence of super-refraction (or ducting) condition, which is most commonly observed over the Sc-topped ABL. In this study, a multi-year bias-corrected ABL refractivity profiles are created. Spatial and temporal variations of the refractivity differences between COSMIC RO observations and the high-resolution ECMWF analysis (TL799L91) are investigated, with a goal to use the GPS RO for future ABL modeling and data assimilation. In particular, the ABL height differences between the two datasets are studied, and effects of vertical sampling resolution and some model parameterization assumptions are explored.