GPS Radio Occultation Boundary Layer Depth Seasonal L3 V1 (GPSROZPBLS)

It also carries an IGOR receiver and has been collecting GPS RO measurements since 2011. The instrument tracks the L-band microwave signal broadcast by a GPS satellite in a limb-viewing geometry. The IGOR receivers on COSMIC and TSX are capable of tracking the GPS signals in open loop through the middle to lower troposphere, which is essential for obtaining data with high quality for PBL height estimation, especially at low latitudes.

The refractivity profiles from COSMIC and TSX form the basis for this PBL height product. For each occultation, the PBL height is calculated as the height where the vertical gradient of the refractivity (dN/dz) is minimum. This algorithm is designed to locate the height where a large vertical change in refractivity occurs, corresponding to the transition from the free troposphere to the PBL.

More details can be found in Ao et al. (2012). Each PBL height is associated with a time (starting time of the occultation) and location (latitude and longitude of the tangent point at the minimum altitude). The PBL height data are then binned into 2 degree x 2 degree latitude/longitude regions and averaged to produce the mean and standard deviation values in the climatology products.

The refractivity profile has a vertical resolution of about 200 m and represents an along path horizontal averaging of ~100 km. Thus, occultations with tangent points near the coast may represent averaging over both land and ocean and should be interpreted with care. The refractivity gradient method used here is not the only method that can be used to estimate the PBL height.

Other algorithms have been proposed, including looking at "breakpoint" instead of minimum gradient, wavelet covariance transform, and using variables like bending angles or specific humidity instead of refractivity. However, the basic principle is the same. The difference between the different algorithms is small where the PBL is well-defined, with a strong capping inversion.