2014年10月19日，北京在18日重度污染的前提下，空气质量进一步恶化，多站点一度飙升至严重污染等级。在多项国际赛事举行的今天，尤其是万众瞩目的室外项目北京马拉松赛在如此恶劣的空气条件下举行，无疑是对运动员的身体的伤害，也是对北京城市名声的损害。
有细心的矮马预报用户注意到，矮马预报从5天前开始一直预测10月19日为轻度污染，直到昨天的预报才预测重度污染。为什么矮马预报会“临阵变卦”？在这种情况下，如何“相信”矮马预报？或者说如何使用矮马预报的预测信息？
首先，不同时次的预报结果相差很大，主要是气象预报的差别引起的。理论上时间越近的预报，相对越准。这主要是由于初始化气象条件的不同造成的。越是临近的预报，初始化的误差影响越小，由此造成的“误判”也越小。同时，空气质量预报对预测的气象场，尤其是预测的大气稳定度的敏感性很高，这在秋、冬季节表现最为明显。在基本类似的天气形势下，比如冷空气来临前，预测的冷空气的强弱或者经过具体地区的时间点稍有差异，都会造成空气质量预测结果在该具体地区的较大不同。对北京10月19日的预报就是一个显著的例子。矮马预报一直预测19日是轻度污染，日均PM2.5浓度大致在100微克每立方米以下。但是最后一个时次即18日的预报，则预测了重度污染，日均PM2.5浓度在接近200微克每立方的水平。如果仔细观察可以发现，对19日的五次预报，前四次预测出的空气污染的强度和最后一次有很大差异。但是这五个时次的预报除第一次预报外，在京津冀地区的污染空间分布其实是非常相似的。这表明这几次预测的大的天气 形势是基本一致的。但是冷空气前沿经过具体地点的时间点则存在差异，同时预测的的大气稳定度强弱在具体地点也有显著不同。最后一次的预报结果显示北京及周边地区的大气静稳程度要比前几次高出很多，而与实际情况更吻合。其中主导风向的预测也有少许变化，比如最后时次的预测是北京地区以南风为主，而前几个时次的预报都是西南偏南风主导。但是这些相对“细微”的差异却造成对空气污染预测的等级差异。
第五时次的预报（即最后时次的预报）
第四时次的预报
第三时次的预报
第二时次的预报
第一时次的预报（即最初时次的预报）
那么在这种情况下，如何合理使用矮马预报的预测信息？
首先，兼听则明。如果您跟踪矮马预报的历史评估有些时日，您会注意到，总体来说，矮马预报能够准确预测出中国环境监测总站实测的全国重点城市AQI日报的污染水平及其空间分布状况，但是如果仅关注一城一地，预报的准确性会随地区不同有显著差异。这主要是空气质量预报，不仅要准确预报气象条件，还要准确预测污染物的排放情况，两方面误差的叠加会造成矮马预报结果准确性的地区差异以及季节差异。如果您所在城市政府有发布第二天的空气质量预报，那么请您以政府信息为准，以矮马预报为辅，兼听则明是明智选择。但是，目前各地的人工空气质量预报并不提供两天以上的提前预报，因此矮马预报是您的唯一选择。即使如此，我们仍要郑重地提醒您，兼听则明仍然是明智选择，比如多多查看我们的近期回顾，看看不同时次的矮马预报对同一天预报的重复性如何，重复性越高，准确程度越高。
其次，以保守为原则，合理分析和利用矮马预报信息。以这次18日和19日北京的情况为例，矮马预报在5天前就预测了北京城区的轻度PM2.5污染。而且连续时次的预报都持续预报了轻度污染的发生。在这种情况下，如果您属于敏感人群，就应该选择相信污染发生的可能性很大，应提前做好防护准备，并做好实际污染可能比预报更为严重的心理准备和相应的防护准备。
最后，国际预报经验表明，弥补数值模式预报困难的最好办法是在数值模式预报的基础之上增加人工预报，我们称之为专家预报（见“人工预报与数值预报”一文）。专家预报可以利用人工经验和其他实时信息，纠正模式预报的偏差，从而大大提高预报准确性。尤其是在国内目前空气质量数值预报因数据不公开、数据质量低下而整体水平较低的情况下，人工预报的迫切性尤其凸显。矮马预报正在筹备矮马人工预报以及矮马智能人工预报，为进一步提高预报准确率而努力。矮马人工预报期待您的参与！请与我们联系。
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