Evaluate the impact of power-law scattering amplitude fitting on dual-polarization radar data assimilation – summertime cases study

<p>Different configurations within the observation operator cause dual-polarization radar parameters to exhibit various characteristics, which affect the structure of background error covariance as well as the results of data assimilation. Through real case data assimilation experiments, this study evaluates the raindrop-contributed term in the simulated reflectivity (<span class="inline-formula"><i>Z</i>_HH</span>) and differential reflectivity (<span class="inline-formula"><i>Z</i>_DR</span>) to describe the effect of different calculation methods within the operator: the fitting and direct integration methods. In the fitting method, dual-polarization variables are calculated using an analytic function, which assumes a gamma-shaped drop size distribution and fits the relationship between the scattering amplitude (SA) and drop size. In the direct integration method, the quantities of the hydrometeor species and SA are integrated with respect to drop size during the calculation. The results indicate that the fitting method effectively simulates the <span class="inline-formula"><i>Z</i>_HH</span>. However, the limitations of the fitting function may impact the accuracy when represents the structure of <span class="inline-formula"><i>Z</i>_DR</span>. By contrast, the direct integration method effectively simulates polarimetric variables. Validation of the raindrop mass-weighted mean diameter (<span class="inline-formula"><i>D</i>_mr</span>) indicates that assimilation of dual-polarization radar data into the model results in adjustment of the raindrop size distribution regardless of which configuration is used. However, the <span class="inline-formula"><i>D</i>_mr</span>–<span class="inline-formula"><i>Z</i>_DR</span> structure is closer to the observed structure, and the <span class="inline-formula"><i>Z</i>_DR</span> structure is more reasonable when the direct integration method is employed. In summary, different configurations within the operator directly affect the results of data assimilation, and the direct integration method has more reasonable performance with respect to simulating dual-polarization radar variables.</p>