PBL: The MPLNET Planetary Boundary Layer Product
The planetary boundary layer (PBL) height product is distributed, for the first time, in the V3 PBL product. The PBL product quality in earlier versions was deemed too low and, therefore, was not made available to the public. Lewis et al (2013) describes the V3 algorithm used to determine the height of the surface-attached mixed layer top. However, the polarized MPL had not been incorporated into the network at the time Lewis et al 2013 was written, and some changes have been made to the algorithm to make use of the volume depolarization ratio measurements.
In brief, the algorithm uses a combination of the wavelet covariance transform (WCT) with a first-derivative Gaussian wavelet and the Canny edge detection method to identify features from 5-min averages of the normalized relative backscatter (NRB) and volume depolarization ratio. While most feature heights are retrieved from the NRB, one feature from the volume depolarization ratio is included if it (1) is at least 250 m from an existing NRB feature and (2) is obtained from the maximum peak in the WCT for the volume depolarization ratio. The V3 PBL product contains up to three feature heights: the altitude of the lowest detected feature and the altitudes of the two largest peaks in the WCT. A fuzzy logic attribution scheme that considers the local time of day, variance in lidar backscatter profiles, and the most recent retrievals is used to determine the best-estimate surface-attached mixed layer top from the three feature heights. While the automated best-estimate is provided, users are still advised to manually inspect the data to determine if one of the other feature heights is better suited for their needs.
PBL Product Levels and QA
L1, L15, and L2 PBL products have identical file formats and content. The primary difference between L1 and L15 PBL products are that data failing to meet the L15 QA criteria are screened and replaced with NaN in the files. In addition, the PBL processing uses NRB products as input. NRB profiles containing clouds within 5 km of the surface are permitted in L1, but removed from higher-level processing. For L15 processing, L15 NRB data are used which in addition to applying the PBL QA criteria yields a net screened PBL product. As a result, simply screening a L1 PBL file based on its QA criteria will not yield identical results in a L15 PBL file. The primary difference between L2 and L1/15 files is that L2 may have additional post-calibrations applied to the input NRB product as well as corrections to instrument temperatures. In addition, any L2 NRB data that fail to meet L2 QA criteria would also be screened and replaced with NaN in the files.
PBL products have three potential QA confidence levels: high, moderate, and low. The CLD QA Confidence Levels are: