为何“雾霾”最后一天的矮马预报在北京地区出现较大偏差?

很多矮马预报用户注意到,最近发生在京津冀地区的连续三次重污染时段期间,矮马对北京城区污染结束日的预报与实际情况有较大出入。具体表现为明显低估污染的严重程度。譬如今天,在冷空气到来前,北京地区的实际污染态势较前两天继续一路走高,出现严重污染等级直至几欲爆表的状况,然而矮马预报仅做出中度污染的预报。这是为什么?

首先,注意到矮马预报并没有对京津冀所有地区在污染时段结束前都出现低报现象,比如今天对天津的预报维持了重度污染的预测,与实际观测结果是基本吻合的。那么北京地区有什么特殊性呢?这就是我们在前面“马拉松日北京“雾霾”:为何不同时次的矮马预报有差别”一文里提到的,气象预报对冷空气强弱以及来临的时间点预测的不确定性造成的影响。北京城的地理位置决定了所在地区的空气质量预测对这个气象预报上的不确定性非常敏感。冷空气来临的路径上游对北京来说是基本没有污染的,这一方面造就了北京居民“等北风来”的普遍心态。因为大风过后,北京必然重现蓝天。但是同时,也造成了空气质量数值预报的困难。空气质量评估的数值是以日为单位的,对冷空气到来的时间点以及强弱的预判误差,即使较小的失误,比如两个小时的提前影响,都会造成日均预测数值较大的误差。

另外一个同样重要的原因是,污染时段结束前往往是污染物浓度累积到高点的时候,而气象预报模式对局地的污染物(尤其是颗粒物)浓度累积对小尺度气象状况的改变多有考虑不周的地方。局地颗粒物浓度经过累积升高后,在大大降低能见度的同时也显著改变了该地的辐射状况。气象和空气质量耦合模式数值模拟研究表明,出现持续200微克每立方米PM2.5浓度的情况下,地面气温往往会有2-3度左右的冷却效应。而这种地面辐射和温度结构的改变,会进一步导致局地混合层高度的降低和混合层内垂直扩散系数的减小。后两者除了会直接导致近地面污染物浓度攀升以外,也会间接地加快污染物化学生成速率,从而起到进一步助推污染物浓度攀升的作用。简言之,这时就会进入一个不断恶化的“正”反馈循环。目前所有的天气预报模式在预报天气状况时都没有考虑这种局地污染带来的反馈机制。因此国际上先进的空气质量数值预报系统对这一现象的模拟也都存在先天的缺陷。科学家们利用在线的气象和空气质量耦合模式来试图解决这一问题,这方面的研究仍在探索之中。

从数值模拟的角度看,这两个问题的解决都需要时日。前者需要更大规模和高密度的气象观测资料和实时同化,这涉及到国家层面的观测网络建设和配套的数值同化技术。后者则需要科学家们进一步理论探索和模拟实践。

那么现阶段,对于这种情况下的污染等级误判就没有办法了吗?有,答案是在数值预报基础之上的人工预报(见“人工预报和数值预报”一文)。经验表明,冷空气来临之前,空气污染往往会进一步加重,我们称之为“黎明前的黑暗”。如果你在关注矮马预报时,兼听则明,也同时追踪地方气象、环保机构的人工预报,比如北京气象台对25日北京地区的基于数值天气预报的空气污染人工预报,你会发现,首席预报员准确地预测了在前半夜以前重度到严重污染的发生。这是一个很好的例子,凸显了基于模式预报结果的人工预报的重要性。借此,矮马预报再次呼吁志愿者加入矮马人工预报的行列,请有志者与我们联系。我们同时也在加紧研发矮马智能人工预报。

文章分类 灰霾预警, 预报札记
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