夏季黄河下游地区中尺度对流系统的气候特征分布

利用1996~2008年逐小时卫星资料、NCEP再分析资料及统计方法, 研究了位于黄河下游地区的中尺度对流系统(Mesoscale Convective System, 简称MCS)的气候特征, 其中包括中尺度对流复合体(Mesoscale Convective Complex, 简称MCC)、持续拉长状对流系统(Permanent Elongated Convective System, 简称PECS)、β中尺度对流复合体(Meso-β Scale MCC, 简称MβCCS>)、β中尺度持续拉长状对流系统(Meso-β Scale PECS, 简称MβECS)4类。结果表明:MCC和PECS是黄河下游地区影响夏季降水的主要MCS, 其中7月份MCC最多, 并且MCC的数量明显大于PECS;与发生在美国的MCS比较, 发生在黄河下游地区的MCC和PECS在成熟期的面积和平均偏心率较大、生命史较长, 但MβCCS和MβECS的生命史较短、平均偏心率变化不大;黄河下游地区PECS表现出成熟较快和消亡较慢的特征, 其最低相当A1] 黑体温度 (BlackBody Temperature, 缩写为TBB) 平均值为-72℃, 比MCC低1℃左右, 生命史比MCC长0.9 h;在MCC的形成、成熟及消亡期, 其日循环特征均表现为明显的双峰特征, 而PECS却呈现出单峰特征;黄河下游地区MCC的发生时间主要集中在2个时段, 一个是在下午形成, 傍晚成熟, 凌晨消亡, 另一个则在后半夜形成, 凌晨成熟, 上午甚至中午才消亡;MCS具有明显的年际变化特点, 在MCS较少的1999年, 500 hPa的副热带高压偏南, 华北地区位势高度较常年明显偏高, 而在MCS较多的2001年, 副高异常偏强, 华北地区位势高度较常年明显偏低, 850 hPa上为一低压槽, 黄河下游地区主要受副高边缘的西南气流影响。

Analysis of Climatic Characteristics of Mesoscale Convective System Over the Lower Reaches of the Yellow River during Summer

Mesoscale convective system (MCS), including mesoscale convective complexe (MCC), permanent elongated convective system (PECS), meso-β-scale convective complexe (MβCC), and meso-β-scale PECS (MβPECS), over the lower reaches of the Yellow River during summer from 1996 to 2008 were analyzed using hourly1996-2008 satellite date, NCEP data, and a statistical method. MCC and PECS were the main MCS in this region, and the number of MCC, most of which appear in July, is greater than that of PECS. The mean mature areas (≤-52℃ cold cloud shield), eccentricities (minor axis/major axis), and lifetimes of MCC and PECS that occur in this region are much larger than those of MCS that occur in the United States. However, the mean eccentricities of MβCC and MβPECS resemble those of systems in the United States, and their lifetimes are shorter than those of systems in the United States. PECS have a mean minimum temperature of black body of -72℃, 1℃ lower than that of MCC, and a mean lifetime 0.9 h longer than that of MCC, indicating rapid maturing and slow extinction. Two peaks corresponding to the diurnal cycle are clearly seen in the formation, maturation, and extinction period of MCC, but PECS exhibit only one peak. MCC over the lower reaches of the Yellow River occur mainly in two periods. One type forms in the afternoon, matures in the late afternoon, and is extinct in the early morning; the other forms in the late evening, matures in the early morning, and is extinct in the morning, or as late as noon. MCS show obvious yearly variations. For example, in 1999, when there were fewer MCS, the West Pacific subtropical high at 500 hPa leaned southward, and the geopotential at 500 hPa in North China was higher than in normal years. In 2001, however, when there were more MCS, the subtropical high was abnormally strong, the geopotential at 500 hPa in North China was lower than usual, and a trough of low pressure appeared at 850 hPa. Consequently, the lower reaches of the Yellow River region was influenced by the southwest airflow located at the edge of the subtropical anticyclone.