目前西北四省区春季第一场透雨标准的确定主要来自业务人员长期的工作经验积累,针对此,利用陕西、甘肃、青海、宁夏249个气象站1960—2016年的日资料,通过分析西北地区相对湿润度指数,结合潜在蒸散特征,确定了春季第一场透雨阈值。与已有研究相比,本方法充分考虑了研究区各地相对湿润度指数的气候特征与各地的能量及水热平衡,理论依据明确,选用的指标能充分反映各气象要素对气候干湿状况的综合影响,因此确定的透雨阈值稳定,月差异小。随后按照这一指标分析西北地区春季第一场透雨特征,结果表明：该地区春季第一场透雨自东南向西北逐步推进,陕南南部出现最早,主要在3月,关中、陕南北部出现在3、4月的概率之和>80%;甘肃河东大部、宁夏大部、青海东部出现在4、5月的概率之和>70%;青海西部是出现最晚的区域,基本上出现在6月。从出现日期的年代际变化看,甘肃河东出现时间最稳定,宁夏、陕西出现日期变化最大,青海西部与甘肃河西较一致,介于二者之间;同时,甘肃河西、陕西关中、陕南春季第一场透雨出现时间变晚趋势明显,其余地区变化趋势不明显。研究结果可供在透雨监测、预测中使用,也可供在制定西北地区春季第一场透雨标准时参考,以便于区域气候评估使用。

Precipitation in spring has an important impact on agricultural production in the northwest of China, including Shaanxi, Gansu, Qinghai, and Ningxia, where winter and spring wheat are the major crops, and it is crucial for winter wheat turning green and jointing and for spring wheat and autumn crop sowing. Therefore, the first saturating rain of spring that can relieve drought is one of the important concerns of scientific and technical researchers. At present, the identification of the first spring saturating rain in the four provinces of northwest China is mainly empirical. Although there is some exploration of this problem based on the spring drought index, there is a lack of the first spring saturating rain being determined based on theory. Using daily data of 249 meteorological stations in Qinghai, Gansu, Ningxia, and Shaanxi provinces from 1960—2016, the first spring saturating rainfall in northwest China, shown by theoretical calculation of potential evapotranspiration and the relative humidity index, which is consistent with the previous studies, is required to be 22 mm. Such results also represent the amount of precipitation that is required when the relative humidity index turns to “no drought” from “drought”. Furthermore, it can be defined as the penetration standard of the first spring saturating rain in northwest China that the 48-h precipitation is greater than or equal to 22 mm or the 24-h precipitation is greater than or equal to 15 mm. The method presented in this paper takes into consideration the characteristics of the relative humidity index, energy balance, and water-heat balance in each region of northwest China, and its theoretical basis is clear. The indices selected here are able to reflect the combined influence of the meteorological factors on the dry and wet conditions. Therefore, the threshold of the first saturating rain determined by this method is stable, with a small monthly difference. Meanwhile, the difference from the first spring rain penetration threshold is determined using the spring drought index, and this avoids the influence of the rapid increase in spring precipitation and temperature. These results compensate for the lack of empirical identification of the first rain penetration in spring in northwest China. At the same time, they can be used in monitoring and forecasting and are easily available for regional climate assessment.