School of Meteorology (Defense)

Prospects of Clear Air Monitoring with the Multimission Phased Array Radar

Eric Jacobsen
OU School of Meteorology

23 January 2014, 10:30 AM

National Weather Center, Room 3620
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

Radars operating at 10 cm wavelengths can observe echoes in the clear atmosphere, which are due to irregularities in refractive index, generated by turbulent motion and gradients of temperature and humidity. The process associated with generating these echoes is known as Bragg scatter. The turbulent eddies responsible for Bragg scattering have useful properties for remote sensing, particularly when their scales are within the Inertial Subrange (ISR). Reflectivity maxima, in regions of greatest turbulence, may be associated with boundary layer depth. Using polarimetric radar, the characteristic isotropy and homogeneity of ISR turbulence lead to expected correlation coefficients and differential reflectivity approximately equal to one and zero, respectively (Melnikov et al., 2011). The ability to characterize clear air turbulence with polarimetric radars leads to potential insight into the state of the convective boundary layer (CBL). This capability is explored for 10 cm (S-band), dual-polarized radars such as the Multi-mission Phased Array Radars (MPAR) being developed along with corresponding scanning strategies.

Range height intensity scans obtained using KOUN (a polarimetric WSR-88D) while operating with high angular resolution observed spring and summertime cases of cloudless CBL in Oklahoma. Additional data were collected from a NOAA UHF wind profiler (PRCO2), soundings, and large eddy simulations. The techniques of Melnikov et al. (2011) were used to achieve greater sensitivity and range resolution with the S-band radar. Horizontal mean and variance calculations on profiles of power, correlation coefficient, and ZDR yielded bulk characteristics wherein the heights of local extrema could be readily correlated with mixing depths (or zi). These features were benchmarked against time series of wind profiler zi estimates following the technique of Angevine et al. (1994). While the traditional method of identifying turbulent layers through power maxima proved useful in zi estimation, polarimetric properties were shown to be very useful in identifying this height as well. For overall tracking of the convective boundary layer life cycle on June 9, 2013, and in particular during its early development, correlation coefficients proved to be an exceptional marker of mixing depth. Values of zi obtained using local maxima in correlation coefficient data produced a root-mean-square error of 57 m relative to the wind-profiler derived zi over a three-and-a-half hour development period. From these data it was also possible to compute an entrainment rate of 217 m h-1. In addition, a technique for studying the instantaneous morphology of the entrainment interface is proposed, which may ultimately yield insight into particular entrainment dynamics.

The anticipated deployment of a network of MPARs promises greater flexibility in the monitoring of conditions during and prior to severe weather. A schedule of CBL scans such as this could add considerable value to the platform during clear sky periods, and provide observations useful for initializing models, which better prepare forecasters.

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