Marine Atmospheric Boundary Layer Observations from GPS Occultation and CALIPSO over Subtropical Eastern Oceans

The highly reflective low clouds that are generally trapped below the shallow marine atmospheric boundary layer (MABL) inversion produce profound radiative cooling effects in the climate system. The low cloud feedback remains a primary cause of uncertainty in global climate model projections. Understanding how climate sensitivity is controlled by these low clouds remains one of the key challenges partly due to the lack of global observation of the low cloud morphology and MABL thermodynamic structures, such as the cloud height, thickness, cloud fraction and cloud top inversion strength, which are difficult to model and integrate into global climate/forecast simulations. High-resolution, self-calibrated Global Positioning System (GPS) radio occultation (RO) soundings provide a unique capability for MABL sensing in all-weather conditions. The thermal inversion along with a large moisture decrease across the MABL top leads to a large bending angle and a sharp refractivity gradient that can be precisely detected by the GPS RO measurements. On the other hand, the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar offers cloud-top-height (CTH) measurements with superior resolution (60 m). In this study, we derive the MABL height climatology from Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) GPS RO observations over five selected regions in subtropical eastern oceans, which include the northeast and southeast Pacific and Atlantic as well as the southeast Indian Ocean off the coast of west Australia. The CTH climatology is also derived from CALIPSO lidar measurements. The CALIPSO CTH shows consistent features with the GPS thermal inversion height over stratocumulus region. However, a large discrepancy is found over the trade-cumulus region. A further comparison of the MABL height observations from GPS/CALIPSO with state-of-the-art global reanalyses, such as the ECMWF-ERA-interim, the NOAA Climate Forecast System Reanalysis (CFSR), and the GMAO Modern Era Retrospective-Analysis for Research and Applications (MERRA) will also be presented. The discrepancy will be explored, and the implication to the MABL parameterization in weather and climate models will be discussed.