Calibrating Ground-Based Radars Using TRMM and GPM

Calibration error represents a significant source of uncertainty in quantitative applications of ground-based radar (GR) reflectivity data. To correct it we need to quantify it and this requires knowledge of the true reflectivity at well-defined locations and times during a volume scan. Previous work has demonstrated that observations from certain spaceborne radar (SR) platforms may be suitable for this purpose. Specifically, the Ku-band precipitation radars on board the Tropical Rainfall Measurement Mission (TRMM) satellite and its successor, the Global Precipitation Measurement (GPM) mission satellite, together provide nearly two decades of well-calibrated reflectivity measurements over low-latitude regions (±35°). However, when comparing SR and GR reflectivities great care must be taken to account for differences in sample volume, measurement frequency, and instrument sensitivity, as well as to ensure a precise spatial and temporal matchup. Here, the volume-matching algorithm (VMA) of Schwaller and Morris (2011) is adapted and used to quantify historical calibration errors for three S-band GRs in the vicinity of Sydney, Australia. In the first part of this presentation, the VMA will be introduced and its sensitivities discussed, with recommendations provided for future applications. The second part will then describe how GR bias estimates provided by the VMA can be combined with radar maintenance records to derive optimal estimates of calibration error through time. The efficacy of this approach is confirmed through comparisons between GR reflectivities in regions of overlapping coverage, with a significant increase in agreement when the estimated errors are removed.

References:

Schwaller, M. R., and K. R. Morris, 2011: A ground validation network for the Global Precipitation Measurement mission. J. Atmos. Oceanic Technol., 28, 301–319.