Structure of a daytime convective boundary layer revealed by a virtual radar based on large eddy simulation

The daytime atmospheric convective boundary layer (CBL) is characterized by strong turbulence that is primarily caused by buoyancy forcing from the heated underlying surface. Turbulence in the dry CBL develops when this surface heating becomes absolutely unstable in the near-surface portion of the layer. The ensuing rising motions have the forms of convective plumes or thermals. In the middle of the CBL, strong vertical upward and downward motions effectively mix momentum and potential temperature fields. The resulting mixed (sub)layer is in most cases the thickest sublayer within the CBL. The CBL is topped by the entrainment zone, where stable stratification inhibits vertical mixing, and vertical gradients of averaged meteorological fields become comparatively large. In many CBL cases, the entrainment zone is collocated with the region of maximum gradients in the potential temperature profile - the so-called capping inversion layer (or simply capping inversion).

A widely used instrument for the study and monitoring of the lower atmosphere is the boundary layer radar (BLR). The term BLR is generally applied to a class of pulsed Doppler radar that transmits radio waves vertically or near vertically and receives Bragg backscattered signals from the refractive-index fluctuations of the optically clear atmosphere. The operating frequency of this type of radar is typically near 1 GHz. Therefore, the Bragg scale is such that BLRs are sensitive to turbulent structures, which have a spatial scales near 10 cm. Enhanced refractive index variations, associated with strong density variations within the entrainment zone at the CBL top, can be detected by clear-air radar. Profiles of the wind vector directly above the instrument are obtained using the Doppler beam swinging (DBS) method. BLRs are also sensitive to Rayleigh scatter from hydrometeors and are used to study clouds and precipitation. In this way, the BLR can be used to study the boundary layer under a wide variety of meteorological conditions and is has proven to be invaluable for such investigations.

Complementary to field observations of the CBL by in-situ and remote sensing measurement methods, numerical simulation approaches - specifically, the large eddy simulation (LES) technique - is widely employed to study physical processes in the atmospheric CBL. The present study considers a unique combination of radar and LES techniques to characterize the CBL. These techniques have been synthesized through the development and implementation of a new radar simulator. High-resolution three-dimensional wind and thermodynamic fields representing a clear CBL are generated using LES. These data are then virtually probed using a ultra high frequency (UHF) BLR similar to those conventionally used in numerous atmospheric studies and field campaigns.

The time-series data for the virtual BLR are generated considering the Eulerian frame approach. First, a field of refractive index C_n² is calculated from the LES output using computed values of pressure, potential temperature, and specific humidity. The Bragg scattering amplitude is then related to the calculated values of C_n², and the phases are computed using the numerically generated velocity fields at each LES time step using the frozen turbulence hypothesis. The amplitude and phases are then interpolated in time between the consecutive LES 1-second time steps. Radar time-series are compiled by summing the contribution from each point within the radar sampling volume which is determined by the radar pulse width and beam width. The virtual radar is capable of generating the multiple beams and frequencies.

Unfortunately, BLRs must operate within stringent frequency management constraints, which limit their range resolution. A typical range resolution for BLR measurements of the ABL is about 100 m, which is too coarse to adequately reproduce the spatial structure of turbulent flow within the entrainment zone. Multiple-frequency techniques, like Range Imaging (RIM), have been successfully used to study the ABL at UHF. RIM uses several closely spaced carrier frequencies for transmission and reception. A constrained optimization method is invoked to image the reflectivity (and velocity) structure within the resolution volume.

In the present work, a simple multi-radar experiment (MRE) was implemented in order to validate and test the virtual radar. In the MRE, five virtual radars pointing to the same resolution volume were deployed within the LES domain. After retrieving the three-dimensional wind fields, using the DBS techniques, they were compared with the “reference” winds form the LES. The same procedure was used after the implementation of RIM, in which the C_n² and velocity fields from the LES and virtual radar are compared with very good agreement. Finally, algorithms have been developed to characterize the CBL (i.e., BL depth, entrainment zone thickness, turbulence, and intensity) based on radar measurements and compared with the ground-truth from the LES fields.