From model to radar variables: a new forward polarimetric radar operator for COSMO

Atmos. Meas. Tech., 11, 3883–3916, 2018 https://doi.org/10.5194/amt-11-3883-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. From model to radar variables: a new forward polarimetric radar operator for COSMO Daniel Wolfensberger and Alexis Berne LTE, Ecole polytechnique fédérale de Lausanne (EPFL), Lausanne, Switzerland Correspondence: Alexis Berne () Received: 27 November 2017 – Discussion started: 21 December 2017 Revised: 12 May 2018 – Accepted: 7 June 2018 – Published: 4 July 2018 Abstract. In this work, a new forward polarimetric radar operator for the COSMO numerical weather prediction (NWP) model is proposed. This operator is able to simulate measurements of radar reflectivity at horizontal polarization, differential reflectivity as well as specific differential phase shift and Doppler variables for ground based or spaceborne radar scans from atmospheric conditions simulated by COSMO. The operator includes a new Doppler scheme, which allows estimation of the full Doppler spectrum, as well a melting scheme which allows representing the very specific polarimetric signature of melting hydrometeors. In addition, the operator is adapted to both the operational onemoment microphysical scheme of COSMO and its more advanced two-moment scheme. The parameters of the relationships between the microphysical and scattering properties of the various hydrometeors are derived either from the literature or, in the case of graupel and aggregates, from observations collected in Switzerland. The operator is evaluated by comparing the simulated fields of radar observables with observations from the Swiss operational radar network, from a high resolution X-band research radar and from the dual-frequency precipitation radar of the Global Precipitation Measurement satellite (GPM-DPR). This evaluation shows that the operator is able to simulate an accurate Doppler spectrum and accurate radial velocities as well as realistic distributions of polarimetric variables in the liquid phase. In the solid phase, the simulated reflectivities agree relatively well with radar observations, but the simulated differential reflectivity and specific differential phase shift upon propagation tend to be underestimated. This radar operator makes it possible to compare directly radar observations from various sources with COSMO simulations and as such is a valuable tool to evaluate and test the microphysical parameterizations of the model. 1 Introduction Weather radars deliver areal measurements of precipitation at a high temporal and spatial resolution. Most recent operational weather radar systems have dual-polarization and Doppler capabilities (called polarimetric below), which provide not only information about the intensity of precipitation, but also about the type of precipitation (e.g., phase, homogeneity and shape of hydrometeors). Additionally, the Doppler capability of weather radars allows monitoring the radial velocity of hydrometeors. In view of their capacities, weather radars offer great opportunities for validation of and assimilation in numerical weather prediction (NWP) models. This is unfortunately far from being a trivial task since radar observables that are derived from the backscattered power and phase from precipitation cannot be simply put into relation with the state of the atmosphere as simulated by the model. There is thus the need for a conversion tool, able to simulate synthetic radar observations from simulated model variables: a so-called forward radar operator. Over the past few years, several forward radar operators have been developed. One of the first efforts was made by Pfeifer et al. (2008) who designed a polarimetric operator for the COSMO model, able to simulate horizontal reflectivity ZH , differential reflectivity ZDR , and linear depolarization ratio (LDR) observations. The operator relies on the T-matrix method (Mishchenko et al., 1996) to estimate scattering properties of individual hydrometeors. Assumptions about shape, density, and canting angles, which cannot be ob- Published by Copernicus Publications on behalf of the European Geosciences Union. 3884 D. Wolfensberger and A. Berne: A forward polarimetric radar operator for the COSMO NWP model tained from the NWP model were obtained from a sensitivity study. A limitation of this operator is that it does not perform any integration over the antenna power density pattern and thus neglects the beam broadening effect which can be quite significant at longer distances from the radar (Ryzhkov, 2007). Cheong et al. (2008) developed a three-dimensional stochastic radar simulator able to simulate raw time series of weather radar data. Doppler characteristics are retrieved by moving discrete scatterers with the three-dimensional model wind field, which allows producing sample-to-sample time series data, instead of theoretical moments as with conventional radar simulators. Thanks to this, the radar simulator is able to generate the full Doppler spectrum; however, this is at the expense of a high computation cost and without taking attenuation into account. Jung et al. (2008) developed a polarimetric radar operator able to simulate ZH , ZDR as well as the specific differential phase on propagation Kdp and adapted it for two different microphysical schemes: one single-moment scheme and one two-moment scheme. The authors also proposed a method to simulate the effect of the melting layer with a weather model that does not explicitly simulate wet hydrometeors. They used this operator to simulate realistic polarimetric radar signatures of a supercell storm from simulations obtained with the Advanced Regional Prediction System (ARPS; Xue et al., 2000). However, the validation of the operator was limited to idealized cases at S-band only. Ryzhkov et al. (2011) developed an advanced forward radar operator for a research cloud model with spectral microphysics able to simulate ZH , ZDR , LDR, and Kdp . Scattering amplitudes of smaller particles are estimated with the Rayleigh approximation whereas the T-matrix method is used for larger hydrometeors. However, note that this cloud model is computationally expensive and is not used for operational weather prediction. Augros et al. (2016) elaborated a polarimetric forward radar operator for the French non-hydrostatic mesoscale research NWP model Meso-NH (Lafore et al., 1998) based on the forward conventional radar operator of Caumont et al. (2006) which simulates all operational polarimetric radar observables: ZH , ZDR , the differential phase shift upon propagation φDP , the co-polar correlation coefficient ρhv and Kdp . The operator uses the T-matrix method for rain, snow, and graupel particles and Mie scattering for pristine ice particles. Beam-broadening is taken into account by approximating the integration over the antenna normalized power density pattern with a Gauss–Hermite quadrature scheme. Finally, Zeng et al. (2 (...truncated)