Study on Sensitivity of Observation Error Statistics of Doppler Radars to the Radar forward Operator in Convective-Scale Data Assimilation

Abstract

In the present work, we investigate the impacts on the observation error (OE) statistics due to different types of errors in the forward operator (FE) for both radar reflectivity and radial wind data, in the context of convective-scale data assimilation in the summertime. A series of sensitivity experiments were conducted with the Efficient Modular VOlume RADar Operator (EMVORADO), using the operational data assimilation system of the Deutscher Wetterdienst (DWD, German Weather Service). The investigated FEs are versatile, including errors caused by neglecting the terminal fall speed of hydrometeor, the reflectivity weighting, and the beam broadening and attenuation effects, as well as errors caused by different scattering schemes and formulations for melting particles. For reflectivity, it is found that accounting for the beam broadening effect evidently reduces the standard deviations, especially at higher altitudes. However, it does not shorten the horizontal or along-beam correlation length scales. In comparison between the Rayleigh and the Mie schemes (with specific configurations), the former one results in much smaller standard deviations for heights up to 4 km, and aloft, slightly larger standard deviations. Imposing the attenuation to the Mie scheme slightly reduces the standard deviations at lower altitudes; however, it largely increases the standard deviations at higher altitudes and it also leads to longer correlation length scales. For radial wind, positive impacts of considering the beam broadening effect on standard deviations and neutral impacts on correlations are observed. For both reflectivity and radial wind, taking the terminal fall speed of hydrometeor and the reflectivity weighting into account does not make remarkable differences in the estimated OE statistics.

Evaluating Latent-Heat-Nudging Schemes and Radar forward Operator Settings for a Convective Summer Period over Germany Using the ICON-KENDA System

“…Because it is closer to the melting layer, this is likely caused by the beam interception of the melting layer, which leads to the pulse-volume averaging over inhomogeneous vertical profiles of reflectivity. Results are in line with the smaller FE of EMVORADO by accounting for the beam broadening effect in Zeng et al [5].…”

“…Smaller reflectivity values are produced at the melting layer by using the Rayleigh-Oguchi approximation in E_ray_lhn2_bb, which are much closer to observations. Results are consistent with the smaller FE of EMVORADO around the melting layer by using the Rayleigh-Oguchi approximation in [5]. However, in case of the Mie scheme, by using the new parameterization for the melting particles (see Section 3.1), the overestimation due the brightband effect is reduced in E_mie2_lhn2_bb, which results in a similar pattern as E_ray_lhn2_bb.…”

“…For the latter one, two EMA approaches, namely the Maxwell-Garnett and Bruggemann models [20,21], are implemented, which allows for the tuning for the melting layer, depending on the melting state and the particle morphology [9]. For detailed descriptions of the EMVORADO (e.g., equations for simulating reflectivity and radial wind), readers are referred to [5]. In order to validate the impacts of LHN schemes, VISOP and RTTOV are [22] additionally used to provide synthetic satellite data.…”

“…The other three hindcasts are used to evaluate the different configurations of the EMVORADO, by which it is straightforward to see the representation error caused by the EMVORADO, not influenced by the LETKF algorithm or by the settings of the KENDA system. For instance, to evaluate the impacts of the beam broadening effect, E_mie1_lhn2 is compared to E_mie1_lhn2_bb, while the latter one accounts for the beam broadening effect with five auxiliary rays in the vertical direction (the horizontal direction is omitted because its impacts are insignificant [5]). To evaluate impacts of different scattering schemes and EMA approaches, E_mie1_lhn2_bb, E_ray_lhn2_bb and E_mie2_lhn2_bb are compared.…”

“…First, the LHN schemes could be associated with the imbalance (also called "ramp-down" [4]) issue, i.e., the model is nudged toward observations too strongly and shocks could be introduced to model (thermo)dynamics, which therefore could cause a rapid loss of forecast skills. Secondly, Zeng et al [5] used the ICON-D2-KENDA with the efficient modular volume radar operator (EMVORADO [6][7][8][9][10]) to investigate observation error statistics of reflectivity and radial wind. It is found that representation error in the radar forward operator (hereafter denoted by "FE") caused by neglecting the effects of reflectivity weighting or terminal fall speed of hydrometeors is omittable by average, but the FE due to neglecting the beam broadening effect as well as due to the scattering schemes can be significant.…”