ECMWF降水相态诊断产品(PTYPE)偏差原因分析及改进

The reason analysis and improvement of the deviation of the ECMWF precipitation type diagnostic product (PTYPE)

摘要

为改进数值模式降水相态诊断方案，改善业务降水相态预报结果，基于地面自动站和探空实况资料、ERA5再分析资料的降水相态诊断产品(precipitation type，PTYPE)以及高空和地面的气温、露点温度数据，对比2022年和2023年冬季我国北方、西南和江南3个区域不同降水相态天气的诊断探空和实况探空差异，结合统计概念模型和降水相态物理过程对PTYPE偏差的原因进行了分析，并提出改进方法。结果表明：(1)北方和江南地区如将PTYPE的干湿雪统归为降雪，其与实测降雪数据更为吻合。西南地区PTYPE的降雪较实况偏多，可用性明显低于其他地区。(2)当前PTYPE对冻雨的诊断方案与冻雨形成的基本物理过程不符，未充分考量液滴的“过冷”属性，这可能是PTYPE“空报”冻雨的主要原因。(3)PTYPE诊断冰粒时要求2 m气温低于0 ℃，而实况的2 m气温高于0℃，这是导致PTYPE对冰粒存在较多漏判的主要原因；此外，PTYPE诊断方案中只要求到达地面附近的水成物中，固态比例≥50%，而非绝大部分保持固态甚或完全冻结，这会导致冰粒预报偏多。(4)基于对比分析，得到了对PTYPE产品的改进方法，即采用2 m湿球温度作为雨夹雪和降雪的辅助判据，增加近地面液滴的温度作为冻雨的判据，在冰粒方案中剔除2 m气温的影响，并适当降低“地面液态水含量”的阈值，可望提升降水相态的诊断准确率。

Abstract

To improve the diagnostic scheme and the forecast results of precipitation types in numerical models, the surface observational and sounding data, precipitation type diagnosis products (PTYPE) from ERA5 reanalysis data, and temperature and dew point of different pressure levels and the surface are used in this study. The PTYPE diagnostic and observational sounding data for different precipitation types in the northern, southwestern, and Jiangnan area of China from the winter of 2022 to 2023 are compared to analyze the causes of forecast biases in the PTYPE products from the aspects of statistical conceptual models and physical processes, and then the improvement methods are proposed. The results are as follows. (1) The combination of dry and wet snow incidents recognized by PTYPE is consistent with the observed snow events in the northern and Jiangnan area. However, in the southwestern area, the PTYPE tends to overestimate the number of snowfall events, suggesting significantly lower reliability in this area. (2) The PTYPE diagnostic scheme is not consistent with the fundamental physical process of freezing rain formation, overlooking the droplets' supercooled characteristics, which could be the primary reason for the “false alarm” of the freezing rain in the PTYPE product. (3) For diagnosing ice pellets, the PTYPE scheme requires a 2-m surface temperature below 0 °C, which is usually not the case for observations. This could be the main reason for the missed alarm of ice pellets. Additionally, the diagnostic scheme only requires that the proportion of the solid particles near the surface is no less than 50%, rather than a percentage of most or even all, which leads to “false alarm”. (4) Base on comprehensive analysis, an improved PTYPE scheme is proposed: using the surface wet-bulb temperature as a criterion for sleet and snow; adding near-surface droplet temperature for freezing rain; removing the surface temperature in the ice pellet scheme, and appropriately lowering the threshold of "near-surface liquid water content", which are expected to improve the accuracy of these precipitation types.

关键词

降水相态 / PTYPE 产品 / ERA5 / 偏差原因

Key words

precipitation type / PTYPE / ERA5 / deviation cause

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基金资助

中国气象局复盘总结专项(FPZJ2025-044)