Assimilation of radar dual-polarization observations in the AROME model

Augros, Clotilde ; Caumont, Olivier ; Ducrocq, Véronique ; Gaussiat, Nicolas

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This article describes the results of assimilation experiments of dual-polarization (DPOL) radar observations in the AROME model with its 3D-Var assimilation system. The assimilation of dual-polarization observations is derived from the current operational assimilation method for radar reflectivities. It has two steps (1D+3D-Var). Relative humidity pseudo-observations are first retrieved from observed profiles of reflectivity (Zhh), with or without attenuation correction, from differential reflectivity (Zdr), from specific differential phase (Kdp), or from a combination of Zhh, Zdr and Kdp, through a unidimensional (1D) Bayesian inversion. The retrievals are then assimilated in the AROME 3D-Var system. In the case of attenuation due to precipitation, the values of the relative humidity profiles retrieved with uncorrected reflectivity were found lower on average than with attenuation-corrected reflectivity or with Kdp. In regions with large values of Kdp, both methods using Kdp produced the profiles with the largest values of relative humidity. Assimilation experiments were conducted for two convective cases. They demonstrated the benefit of the assimilation of Kdp and of attenuation-corrected Zhh in the model analysis. The humidity increments were found higher in convective regions when assimilating either Kdp or attenuation-corrected Zhh. Furthermore, the impact of DPOL observations on precipitation forecasts was found slightly positive for the periods of intense convection.</p>

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