杭州萧山机场两次中雪过程的对比分析

A Comparative Analysis of Two Moderate Snow Events at Hangzhou Xiaoshan Airport

杭州萧山机场在2018年1月和2016年1月出现的两次中雪天气。通过机场观测资料发现18年降雪过程中有冰粒出现,承接降水到降雪互相间的转变,16年降雪过程中雨夹雪或小雨相态出现较多,两次过程中的中雪都在液态降水向固态降水转变并且气温降低后出现。ECMWF ER5数据显示两次降雪过程浙北地区都受寒潮影响,500 hPa都为平直西风带控制,700 hPa都为西南偏西急流影响。不同是18年过程850 hPa冷空气先影响浙北地区,其后强的偏南暖湿气流输送至浙北。16年过程冷空气以横槽转竖的形式缓慢扩散南下,与西南暖湿气流相遇。18年过程中干冷空气长时间影响浙北,强迫从南海及西太平洋北上的暖湿空气抬升,形成中雪。该过程中水汽通量散度大,垂直运动强,云团中冰水粒子混合使得雷达回波图像类似混合性降水,对应实况出现冰粒可能是这一类型降雪过程的标志,中雪对应的雷达回波信号达50 dBz。而16年过程则是干冷气团向暖湿气团移动,为冷流降雪,水汽通量散度小,雷达回波信号20~30 dBz且分布均匀,中雪对应回波信号为25~30 dBz。

Hangzhou Xiaoshan Airport experienced two moderate snow events in January 2018 and January 2016. Through airport observation data, it was found that there were ice pellets in the snowfall process in 2018, which bridged the transition between precipitation and snowfall. In the snowfall process in 2016, there were more occurrences of sleet or light rain. The moderate snow in both processes appeared after the liquid precipitation changed to solid precipitation and the temperature dropped. ECMWF ER5 data showed that the northern Zhejiang area was affected by cold waves in both snow events, and 500 hPa was controlled by a zonal westerly belt, and 700 hPa was influenced by a southwest-to-west jet stream. The difference was that in the 2018 process, the cold air at 850 hPa affected the northern Zhejiang area first, and then a strong southward warm and humid air flow was transported to the northern Zhejiang. In the 2016 process, the cold air spread southward slowly in the form of a horizontal trough turning vertical, and met with the southwest warm and humid air flow. In the 2018 process, the cold dry air affected the northern Zhejiang for a long time, lifting the warm and humid air which from the South China Sea and the North Pacific, then forming the moderate snow. In this process, the water vapor flux divergence was large, the vertical motion was strong, and the mixed ice-water particles in the cloud cluster made the radar echo image simi-lar to mixed precipitation, which corresponded to the observation of ice pellets. This may be a sign of this type of snowfall process. The radar echo signal corresponding to moderate snow reached 50 dBz. In the 2016 process, it was cold flow snowfall, where the dry cold air mass moved toward the warm and humid air mass. The water vapor flux divergence was small, and the radar echo signal was 20~30 dBz and evenly distributed. The radar echo signal corresponding to moderate snow was 25~30 dBz.