Towards a global climatology of the planetary boundary layer

Planetary Boundary Layer (PBL) processes control exchanges of energy, water, and trace substances between the surface and free troposphere. Realistic representation of the PBL in climate, weather, and air quality models is critical to accurately simulate these exchanges. But, although PBL characteristics have been studied in detail in field campaigns on local scales and for limited periods of time, no long-term global PBL height climatology exists for evaluation of model representations of the PBL.

Several options exist for developing a global climatology of the PBL. Traditional methods for determining the height of the PBL are based on in situ meteorological (e.g., radiosonde temperature and moisture) soundings, but new methods, based on GPS/RO (Global Positioning System/Radio Occultation) refractivity data and on remotely-sensed aerosol concentrations, are emerging. Comprehensive comparisons of these methods have not yet been performed.

We present PBL height estimates from the global radiosonde network based on ten years of data from more than 800 stations using seven different methods. These include four traditional methods (based on temperature, potential temperature, and virtual potential temperature profiles) and three methods proposed for use with GPS RO data (based on specific humidity, relative humidity, and refractivity). The resulting climatologies are compared using statistical tests that reveal significant differences among the methods, including biases and differences in seasonal and diurnal variations. The traditional “mixing height”, which is based on virtual potential temperature profiles and is sensitive to atmospheric stability, is systematically lower than other PBL height estimates and exhibits stronger seasonal and diurnal variability. Implications for developing a global climatology of PBL height will be discussed.