Development and validation of a full polarimetric radar simulator

A flexible, fully-featured radar simulator for polarimetric radar variables has been developed within the research mesoscale nonhydrostatic (Meso-NH) atmospheric model. The objective is to enable direct comparisons of kilometric- and subkilometric-scale atmospheric simulations with all observed polarimetric variables, which are now produced operationally by 16 radars of Météo France network operating at S, C, and X bands. The overarching goal is to determine if and how the information obtained from the polarimetric quantities can be assimilated in a convective-scale model in order to improve short term forecasts.

The polarimetric radar simulator, which is an upgraded version of the radar simulator developed by Caumont et al (2006), is developed based on calculations of electromagnetic wave propagation and scattering at S, C, and X bands. Beam propagation effects are considered, including (differential) attenuation and phase shift, beam bending, and beam broadening.

The simulator is fully consistent with the microphysical parameterizations of the Meso-NH model that uses a one-moment bulk microphysical scheme governing the equations of the six following water species: vapor, cloud water, liquid water, graupel, snow, and pristine ice. It takes as input the output of model simulations such as hydrometeor contents, temperature, and ice concentration. All polarimetric variables that are produced by the operational radar network are simulated: reflectivity at horizontal polarization, differential reflectivity, differential phase and cross-correlation coefficient. Additionally, the simulator provides the differential backscattering phase, the specific differential phase, the specific attenuation and specific differential attenuation. Polarimetric variables are calculated for each hydrometeor species and for the combination of all species if different species are present within a given pixel.

Within the simulator, pristine ice particles are considered to be spherical whereas rain, snow and graupel particles are simulated as spheroids. Different scattering methods have been implemented: Rayleigh or Mie for spheres, Rayleigh for spheroids or T-matrix for spheroids. For the T-matrix method canting of particles is simulated.

The performance of the radar simulator as well as the capacity of the model to realistically describe the processes involved in the formation and interactions of the hydrometeors are evaluated for convective cases (bow echo, convective line) that were observed in fall 2012 in South-Eastern France by operational and research radars, during the HyMeX campaign (HYdrological cycle in the Mediterranean EXperiment). This includes sensitivity studies to the simulator parameters (scattering method, shape, canting behavior, etc.) and comparisons with hydrometeor types retrieved through the fuzzy-logic classification algorithm used operationally at Météo-France.