Seasonal predictability of onset and cessation of the east African rains
Seasonal predictability of onset and cessation of the east African rains
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东非降雨开始和停止的季节可预测性
DOI：
10.1016/j.wace.2018.05.003
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发表时间：
2018
影响因子：
8
通讯作者：
MacLeod D
中科院分区：
文献类型：
--
作者：
MacLeod D
雨季推迟开始或提前结束的预警对东非农民来说是极其宝贵的信息，也是区域利益攸关方的共同要求。这些警告已经开始提供，但这些指标的预测技能尚未得到证明。在这里，ECMWF的季节性后报的预测技能进行评估的开始和停止在东非的预测。预测和观察到的发病和停止异常的相关性仅在95%以上的统计显著性水平的一小部分的域在长降雨，而它们是显着的短降雨发病和停止在该地区的大部分地区。评估了使用扩展范围46天预测更新预测展望的附加值，这给出了一个小的改进。对于短期降雨的早期发病的检测是更好的海岸附近，晚发病的检测是更好的肯尼亚西北部。在异常干旱的年份，检测发病和停止的方法失败。使用这个作为一个失败的季节的定义，该模型显示出显着的技能，在预测长雨季失败在肯尼亚西北部，和短降雨失败在索马里和肯尼亚东北部。此外，长期降雨停止和季节性总的相关性之间的强度被证明是特别弱的观测，但过于强大的后报。两个季节的发病和停止的可预测性似乎主要来自与季节性总的联系，目前还不清楚该模型代表的发病和停止的变化超出了这一点。这对业务预报有重要影响：任何与季节性总降雨量（如早发病，但总降雨量低）“不一致”的季节性时间预测必须谨慎对待。晚发病与东风（西风）异常在长（短）雨，虽然强度和空间格局的关系没有很好地代表在模型中。早停止与东风异常在两个季节的大部分地区的观测和后报。然而，对于长降雨的相关性的符号是反向的沿着海岸的观察，但不是在后报。这些动力学上的不一致性可能对预报技能产生负面影响，并有可能为该区域基于过程的气候建模发展提供信息。
Advanced warning of delayed onset or early cessation of the rainy seasons would be extremely valuable information for farmers in east Africa and is a common request from regional stakeholders. Such warnings are beginning to be provided, however forecast skill for these metrics has not been demonstrated. Here the forecast skill of the ECMWF seasonal hindcasts is evaluated for onset and cessation forecasts over east Africa. Correlations of forecasted and observed onset and cessation anomalies are only above a 95% statistical significance level for a small part of the domain during the long rains, whilst they are significant for short rains onset and cessation over a large part of the region. The added value of updating the forecast outlook with the extended range 46 day forecast is assessed and this gives a small improvement. For the short rains detection of early onset is better near the coast, and late onset detection is better over northwestern Kenya.During exceptionally dry years the method to detect onset and cessation fails. Using this as a definition of a failed season, the model shows significant skill at anticipating long rains season failure in the northwest of Kenya, and short rains failure in Somalia and northeast Kenya.In addition the strength of the correlation between long rains cessation and seasonal total is shown to be particularly weak in observations but too strong in the hindcasts. Predictability of onset and cessation for both seasons appears to arise primarily from the link with seasonal total and it is unclear that the model represents variability in onset and cessation beyond this. This has important implications for operational forecasting: any forecast of season timing which is ‘inconsistent’ with seasonal total (e.g. an early onset but low total rainfall) must be treated with caution.Finally links with zonal winds are investigated. Late onset is correlated with easterly (westerly) anomalies during the long (short) rains, though the strength and spatial pattern of the relationship is not well represented in the model. Early cessation is correlated with easterly anomalies in both seasons for most of the region in both observations and hindcasts. However for the long rains the sign of the correlation is reversed along the coast in observations but not in the hindcasts. These dynamical inconsistencies may have a negative impact on forecast skill and have the potential to inform process-based development of climate modelling in the region.
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