Prospects of clear air monitoring with the MPAR

The anticipated deployment of a network of Multimission Phased-Array Radars (MPARs) ensures an improvement in the detection and monitoring of severe weather. The ability to monitor clear air can provide insight into the state of the planetary boundary layer (PBL), such as prior to severe weather, and how it might develop over time. This capability is explored for 10-cm, dual-polarized radars such as the MPAR, and a scanning strategy complementing this purpose is considered.

Weather radars operating in clear air, also known as hydrometer-free environments, often detect echoes at the juxtaposition of air masses that have different thermodynamic properties (these echoes are frequently called Bragg scatter). Moisture and temperature differences between such parcels are largely responsible for creating a refractive index change, which is the principal cause of reflecting radar energy, albeit weakly when compared with traditional 'targets.' Turbulent mixing can intensify these gradients to the threshold of detectability. Theorists including Hardy et al (1966) have posited that the strength of reflectivity is directly proportional to the magnitude of the structure function parameter C_n² describing the intensity of this turbulence. An adequately sensitive radar can thus locate the extent of convective activity, and even measure the height of the planetary boundary layer.

High frequency instruments like the NOAA Profiler Network have been effectively making localized measurements of these boundaries for years. This functionality could readily be extended to S-band 88-D weather radars, providing vastly more expansive spatial datasets of convection. Structures capable of producing Bragg scatter at S-band have dimensions close to the limiting inner scale of turbulence, but using specialized methods developed for polarimetric radar has shown that Bragg scatter can be detected using a 10-cm radar. This study evaluates data collected using KOUN (a polarimetric WSR-88D), along with observations from a NOAA UHF wind profiler and the Oklahoma Mesonet to further qualify the effectiveness of a 10-cm radar in detecting Bragg scatter. Research suggests that reflectivity factors as low as -28.5 dB to a 10 km range could be detected with KOUN, with potential for further range using a longer pulse. In addition, polarimetric variables can be used to make the critical distinction between Bragg scatter and atmospheric biota. As routine identification of Bragg scatter becomes common, a benefit to the calibration of dual-polarimetric radars may also arise, dependent on using the characteristic isotropy of Bragg scatter as a benchmark for calibrating ZDR to zero.

While impractical for traditional weather radars with less dynamic scan settings to use the mode presented here, it is likely that MPAR could incorporate a routine scan of the PBL, with great benefit to the monitoring of convection, to PBL parameterization in atmospheric models, and to radar calibration.