基于ERA5再分析资料的余姚地区阵风预报模型探究

利用2007—2017年余姚地区44个气象自动站观测数据和欧洲中心ERA5再分析资料,对余姚地区日极大风分布进行了统计分析,并提出一种本地化的经验阵风预报模型(雷暴日除外)。研究发现:余姚地区日极大风呈正态分布,风力峰值为4～5级,平均每年大风日占比达7.03%,累计大风时次占比3.32%,年平均大风日26 d。风力越小,阵风与平均风之间的线性拟合效果越好。影响阵风预报的因子主要有10 m平均风速、925 hPa风速、地面粗糙度、摩擦速度和3 h变压。6级及以下阵风预报中,业务中的阵风经验系数1.4容易造成余姚本地阵风预报偏低,适用于余姚本地阵风预报经验系数为1.775;6级以上阵风预报经验方程考虑了平均风速、垂直动量下传、水平动量传输、地面摩擦和海拔高度订正,其与925 hPa风速的平方呈正相关,与摩擦速度呈自然指数相关,经验方程相对于经验阵风系数的预报拟合优度提升了59.61%;经验阵风预报方程通过了2018—2019年的数据检验,该方程对1～2级阵风预报偏高,3～5级效果最好,6级阵风的预报偏低;6级以上的阵风等级预报准确率达55.2%。地形摩擦作用在冷空气大风与台风大风过程中尤为重要,这两类过程阵风系数分布类似,但台风带来的动量下传比冷空气更为明显。

Analysis of wind gust forecast model in Yuyao based on the reanalysis data ERA5

Based on the observation data from 44 Automatic Weather Stations(AWS) in Yuyao and the reanalysis data ERA5 from ECMWF during 2007 to 2017, a statistical analysis on daily maximum wind gust of each AWS was conducted and a Localized Empirical Wind Gust Prediction Model(LEWGPM) without considering thunderstorm day was proposed. Results show that daily maximum wind gust is normally distributed with the peak of wind scale 4-5. The averaged gale day per year is 26, which account for 7.03% yearly and 3.32% hourly. It also shows the weaker the wind, the better the liner fitting between wind gust and mean winds. Some surveys on the factors that may influence the wind gust prediction were made and 10 meter mean winds, 925 hPa mean winds, roughness, friction velocity and 3 hours pressure change appear good reactions were found out. In the LEWGPM below or equal to wind scale 6, traditional empirical coefficient 1.4 has the risk of weakening the wind gust forecast and the new empirical coefficient 1.775 will lead to better forecast. When it comes to LEWGPM above wind scale 6, the mean winds, downward momentum transportation, horizontal momentum transfer, roughness and latitude are took into consideration, which found out it liner fitting to squared 925 hPa mean winds and logarithmic correlation to friction velocity. Compared to traditional empirical coefficient forecast, LEWGPM above wind scale 6 promoted 59.61%. Finally, the LEWGPM with data from 2018 to 2019 was tested, which figure out that its prediction strengthens wind scale 1-2, weakens wind scale 6, fits well with wind scale 3-5, and the accuracy of wind scale over 6 is 55.2%. The topographic friction is particularly important during typhoons and cold air events. Both the two gale processes share the similar distribution of gust factor but the different downward momentum, which means the typhoons always transfer more momentum downward.