Convective Boundary Layer Depth and Morphology, Observations with an S-band Weather Radar

S-band radars can observe echoes in the clear atmosphere that arise from irregularities in the refractive index, generated by turbulent motion and gradients of temperature and humidity.  This process 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).  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 S-band, dual-polarized radars such as the Multimission Phased-Array Radar.

Range height indicator (RHI) scans obtained using KOUN (a polarimetric WSR-88D) observed spring and summer cases of cloudless convective boundary layers (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.  Bulk characteristics of power, correlation coefficient, and ZDR data contained local extrema whose heights could be readily correlated with CBL mixing depths (or zi).  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, 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 3.5 hour development period.  An entrainment rate of 217 m h^-1 was also computed.  In addition, a technique for studying the instantaneous morphology of the entrainment interface is proposed, which may yield insight into particular entrainment dynamics.