Exploiting the benefits of convective-scale ensemble forecasts

业务中心和研究界现在广泛使用对流尺度集合预报，以估计高度影响天气的位置和严重程度的不确定性。这些预报在网格间距约为2公里的区域范围内运行，使用更粗分辨率的全球预报（约10- 20公里）来提供横向边界条件;单个集合成员在初始条件、边界条件和模式物理学方面有所不同。该项目的目的是研究在多个空间尺度和不同流态下控制对流尺度集合传播的因素，从而提高短期天气预报的技能。虽然它们的制作现在是常规的，但用于初始化多重预测的技术仍然没有牢固建立。特别是英国气象局正在探索创建扰动预报的新策略，例如他们最近将其对流允许集合预报系统（MOGREPS-UK）从六小时循环改为具有多个滞后时间的小时循环集合。其目的是通过缩短同化周期和预报可用性之间的时间来提供更及时的预报，并通过考虑来自连续数据同化（DA）周期的额外扰动来增加集合的传播。类似于大多数业务中心产生的对流尺度集合，MOGREPS-UK分布不足，例如，与许多事件的预报误差相比，计算出的降水概率过高。然而，不同类型的扰动也可能导致不必要的传播，而对流尺度的集合行为可能取决于天气状况，即分散的对流阵雨将与天气锋内有组织的对流行为不同。在这个项目中，将应用最近开发的用于评估集合传播和空间技巧尺度的指标，包括基于邻域的“分数技能分数”（FSS），色散FSS（系综色散）、系综一致尺度和对应比。将确定从不同度量（也包括展布/技能比和频谱分析的标准客观验证度量）和不同输出字段以及针对不同天气状况解释的展布的一致性。另外，功率谱分析可以用于进一步告知系综扩展对空间尺度的灵敏度。该验证分析可以支持基于新的DA发展和扰动类型以及系综配置的良好分布系综的设计。在今后几年中，MOGREPS-UK将成为DA系统若干发展的中心，例如全球集合MOGREPS-G的DA方案的变化（从ETKF到En-En 4DVar）。在与大尺度相关的扰动之上，对流尺度DA也将发生新的发展，包括一个新的DA系统，混合UKV。这些发展对流规模合奏设计为我们提供了机会，调查合奏传播和技能的反应。长期的目标是优化设计对流尺度合奏扰动类型，合奏规模和验证指标，重点是在小尺度上的传播评估。

Convection-scale ensemble forecasts are now widely used in operational centres and the research community to provide an estimation of uncertainty in the location and the severity of high-impact weather. These forecasts are run in regional domains with grid spacings of ~2km using much coarser resolution global forecasts (~10-20km) to provide lateral boundary conditions; the individual ensemble members differ in their initial conditions, boundary conditions and model physics. The project aim is to investigate the factors controlling the spread of convective-scale ensembles at multiple spatial scales and under different flow regimes, and so improve the skill of short-range weather forecasts. While their production is now routine, the techniques for initialising the multiple forecasts are still not firmly established. The Met Office, in particular, is exploring new strategies for creating perturbed forecasts e.g. they have recently changed their convection-permitting ensemble forecasting system (MOGREPS-UK) from a six-hourly-cycling to an hourly-cycling ensemble with multiple lag times. Their aim is to both provide more timely forecasts by reducing the time between assimilation cycles and forecast availability, and to increase the spread of the ensemble by taking into account additional perturbations coming from successive data assimilation (DA) cycles.Similar to convective-scale ensembles produced by most operational centres, MOGREPS-UK is under-spread e.g. the calculated probabilities of precipitation are too high when compared to the forecast error across many events. However, different types of perturbations may also lead to unwanted spread and convective-scale ensemble behaviour is likely to depend on the weather regime i.e. scattered convective showers will behave differently to organised convection within a weather front.In this project recently-developed metrics for evaluating ensemble spread and spatially-skilful scales will be applied including the neighbourhood-based "Fractions Skill Score" (FSS), dispersion FSS (for ensemble dispersion), ensemble agreement scale and correspondence ratio. The consistency of spread interpreted from different metrics (including also standard objective verification metrics of spread/skill ratio and spectral analysis) and different output fields, and for different weather regimes, will be determined. Additionally, power spectrum analysis may be used to further inform the sensitivity of the ensemble spread to spatial scales. This verification analysis could support the design of a well-spread ensemble based on new DA developments and perturbation types, as well as ensemble configuration. In the next few years MOGREPS-UK will be at the centre of several developments in the DA system, such as the change in the DA scheme for the global ensemble MOGREPS-G (from ETKF to En-En4DVar). On top of the perturbations associated with the large scales, new developments will also be taking place in convective-scale DA including a new DA system, hybrid-UKV. These developments in convective-scale ensemble design provide us with the opportunity to investigate the response of the ensemble spread and skill. The long-term goal is to optimally design convective-scale ensembles in terms of perturbation type, ensemble size and verification metrics with emphasis on the assessment of spread at small scales.