What is the Planetary Boundary Layer Height in a Global Perspective?

The planetary boundary layer (PBL) height is a fundamental parameter characterizing the vertical extent of atmospheric mixing near the surface. It is critical for understanding the PBL process and low cloud evolution and its feedback on the climate system, which remains a key uncertainty in climate modeling. The PBL height is generally defined as the altitude of a transition layer where air temperature or humidity gradient are significant within the lowest 1-5 kilometers above the surface. Numerous thermodynamic parameters, including temperature, humidity (specific/relative humidity) and their derivatives (e.g., potential/virtual potential temperature etc.) have been widely used to define the PBL height. Advances in satellite remote sensing technique allow novel ways to detect the PBL heights from space. Many new parameters are proposed for PBL height detection including GPS radio occultation (RO) measurements (e.g., refractivity, bending angle and dry-temperature) and CALIPSO lidar backscattering measurements (e.g., cloud-top-height).

Large discrepancy among various PBL height definitions was revealed from radiosonde analyses, which however are restricted over lands and represent limited horizontal scales of atmospheric conditions. In this talk, we investigate the definition difference in a global perspective by using multi-year high-resolution ERA-interim (1 degree grid with 60 vertical layers) global analysis. Automatic algorithms are applied to compute the PBL heights with various physical parameters (both conventional and GPS RO) at each model grid. The global PBL height seasonal climatology and the difference among the climatologies are derived. Large discrepancy between the thermal-based and humidity-based PBL height definitions is most prominent over tropical and polar regions. Humidity-based PBL heights become problematic over dry regions, especially over high-latitude in winter season. The cloud-top height from CALIPSO is consistent with most physical parameter based PBL heights over subtropical eastern oceans. But much larger differences are seen elsewhere. The physical meaning of various PBL heights and their implication for evaluating weather and climate models with satellite observations will be discussed.