Abstract. Numerical weather prediction systems still employ many simplifications when assimilating microwave radiances in all-sky conditions (clear sky, cloudy, and precipitation). For example, the orientation of ice hydrometeors is ignored, along with the polarization that this causes. We present a simple approach for approximating hydrometeor orientation, requiring minor adaption of software and no additional calculation burden. The approach is introduced in the RTTOV (Radiative Transfer for TOVS) forward operator and tested in the Integrated Forecast System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). For the first time within a data assimilation (DA) context, this represents the ice-induced brightness temperature differences between vertical (V) and horizontal (H) polarization, the polarization difference (PD). The discrepancies in PD between observations and simulations decrease by an order of magnitude at 166.5 GHz, with maximum reductions of 10–15 K. The error distributions, which were previously highly skewed and therefore problematic for DA, are now roughly symmetrical. The approach is based on rescaling the extinction in V- and H-channels, which is quantified by the polarization ratio ρ. Using dual polarization observations from Global Precipitation Mission microwave imager (GMI), suitable value for ρ was found to be 1.5 and 1.4 at 89.0 and 166.5 GHz, respectively. The scheme was used for all the conical scanners assimilated at ECMWF, with broadly neutral impact on the forecast, but with an increased physical consistency between instruments that employ different polarizations. This opens the way towards representing hydrometeor orientation for cross-track sounders, and at frequencies above 183.0 GHz where the polarization can be even stronger.

中文翻译：

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将水汽定向引入全天微波/亚毫米同化

摘要。当在全天空条件下（晴朗的天空，多云和降水）吸收微波辐射时，数字天气预报系统仍采用许多简化方法。例如，忽略了水凝冰的方向以及由此引起的极化。我们提出了一种简单的方法来近似水凝流向，只需要对软件进行较小的改动就不会增加计算负担。该方法在RTTOV（TOVS的辐射转移）前向算子中引入，并在欧洲中程天气预报中心（ECMWF）的综合预报系统（IFS）中进行了测试。这是数据同化（DA）上下文中的第一次，这代表冰在垂直（V）和水平（H）极化之间引起的亮度温度差异，即极化差异（PD）。观测和模拟之间的局部放电差异在166.5 GHz时减小了一个数量级，最大减小量为10-15K。以前非常偏斜的误差分布，因此对于DA来说是有问题的，现在大致对称。该方法基于重新缩放V通道和H通道的消光，该消光由极化率ρ量化。使用全球降水任务微波成像仪（GMI）的双极化观测结果，在89.0和166.5 GHz处，ρ的合适值分别为1.5和1.4。该方案用于ECMWF吸收的所有锥形扫描仪，对预报具有广泛的中性影响，但采用不同极化的仪器之间的物理一致性有所提高。