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“98.7”特大暴雨中尺度系统发展的热量和水汽收支诊断
DIAGNOSES OF THE MESOSCALE HEAT AND MOISTURE BUDGETS DURING THE "98.7" EXTRAORDINARY HEAVY RAINFALL
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摘要: 1998年7月20~23日 (“98.7”) 发生在鄂东和鄂西南地区的特大暴雨过程, 不仅与700 hPa上低涡切变线的生成和持续发展密切相关, 而且与沿低涡切变线相继生成和强烈发展的MαCS与MβCS直接关联。利用非静力模式MM5.V2.12成功模拟提供的高分辨输出资料对这次特大暴雨中尺度系统发展的热量和水汽收支进行了诊断。结果发现:当有强对流发生并伴有强降水时, 就会有强的视热源Q1和视水汽汇Q2出现, 而强的Q1与Q2和强降水区基本是对应的; Q1随高度增高而增大, 最大加热位面基本上都在486.1 hPa (σ=0.54) 附近; 在对流层深厚的中空加热层是积云对流活跃和强暴雨持续发生、发展的一种重要热力机制; 在对流层上半部的相对冷层为暴雨区上空积云对流提供了极为有利的热力不稳定条件, 积云对流在中、低空的凝结潜热不仅加热对流层中层大气, 而且向高层输送, 加热高层的环境大气; 在暴雨初期, Q2的双峰结构与低空层积云及中空积云对流凝结变干有关; Q2的中空峰值大体与Q1的峰值相应, Q2的深厚变干层与Q1的深厚加热层非常一致。 诊断结果表明, 用非静力中尺度模式成功模拟的高分辨输出资料对Q1和Q2进行数值诊断是可行的。通过对强暴雨过程Q1和Q2的诊断, 可为改进积云对流参数化中加热和增湿廓线提供可靠的物理依据。Abstract: During the period of 20-23 July 1998 ("98.7"), an extraordinary heavy rainfall event occurred in the eastern and southwest parts of Hubei Province, which was in close relationship with the low vortex with shear line on 700 hPa as well as with the successively generated and intensively developing MαCS and MβCS along the shear line. The numerical simulation for this event is conducted successfully using nonhydrostatic version MM5 (V2.12). The mesoscale heat and moisture budgets are diag nosed using the output data of MM5. The finding sare as follows: strong apparent heat sources Q1 and apparent moisture sink Q2 would appear while the strong convection occurs with heavy rainfall; Q1 and Q2 are basically correspondent with the regions of heavy rains; Q1 increases with height while the maximum heating level is located at 486.1 hPa (σ=0.54); the deep and thick heating layer in the middle troposphere is the main thermody namic mechanism for the both cumulus convective activities and the successive generation and development of severe rainstorms; the relatively cool layer in the upper troposphere provides a favorable thermody namic condition for intensive cumulus convection over the heavy rain areas; the condensation latent heating of cumulus convection in the lower and middle troposphere is not only heating the middle troposphere atmosphere, but also heating the upper environment atmosphere through transporting heat upward; the double-peak structure of Q2 at the early stage of heavy rainfall is relevant to the drying associated with convective condensation of both stratocumulus at the lower levels and cumulus at the middle levels; the peak value of Q2 in the middle level is basically corresponding with that of Q1; the deep and thick drying layer is consistent with the deep and thick condensation-heating layer of Q1. The diagnostic results show that it is reasonable to diagnose Q1 and Q2 using successfully stimulated output data with high resolution. The reliable physical proof for improving heating and moisturizing profiles in the cumulus convective parameterization can be obtained through diagnosing Q1 and Q2 of the rainstorm event.
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图 1 1998年7月20日08:00至22日20:00湖北省24 h (a)、36 h (b)、48 h (c) 和60 h (d) 的累积降水量 (mm) 分布
(图中+和●是间隔1°的经、纬线, 粗虚线是长江, 图 (a) 中 ■、▲和●分别代表黄石、武汉和来凤)
图 3 1998年7月21日08:00至22日20:00 700 hPa位势场 (细实线)、温度场 (虚线) 及风场 (每长杆4 m/s),
(粗黑线为切变线) (a) 21日08:00, (b) 21日20:00, (c) 22日08:00, (d) 22日20:00
图 10 7月21日08:00的Q1 (a) 和Q2 (b) 诸分量平均垂直廓线
(a) (压缩功率项: —○—; 温度局地变化项: —●—; 温度水平平流项: —□— ; 温度垂直平流项: —■—) (b) (比湿垂直平流项: —○— ; 比湿局地变化项: —●— ; 比湿水平平流项: —■—)
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