气候与环境研究  2016, Vol. 21 Issue (6): 687-699 PDF

1 中国科学院大气物理研究所, 北京 100029;
2 中国气象科学研究院, 北京 100081

Numerical Studies of Influences of Ice-Phase Change Induced Diabatic Heating on Mesoscale Convective Clouds and Precipitation over the South China Sea Monsoon Region
FU Danhong1, GUO Xueliang2
1 Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029;
2 Chinese Academy of Meteorological Sciences, Beijing 100081
Abstract: A cloud-resolving model (CRM) and sounding data observed in the South China Sea Monsoon Experiment (SCSMEX) from 15 May to 11 June 1998 are used in this study to explore the effects of diabiatic heating induced by the ice-phase change on mesoscale convective system (MCS), precipitation, radiation, and the large scale environment. The results show that the effects of the latent heat released by the ice-phase change on the net cloud radiation can be neglected, but the diabatic heating leads to obvious changes in heat fluxes at the ocean surface. The latent heat released during the sublimation and deposition processes has a heating effect on the large-scale environment, and the atmospheric stratification becomes more stable. As a result, latent and sensible heat fluxes at the ocean surface both decrease, convective activities become weak, and precipitation decreases by about 10.11% over the northern South China Sea (SCS). The freezing process can also result in a more stable atmosphere, which is not favorable for the development of mesoscale convective system and thereby leads to a decrease in the accumulated rainfall over the northern SCS by about 2.2%. The melting process can lead to an increase in accumulated rainfall over the northern SCS, which is mainly attributed to its cooling effect on large-scale environment below the melting level. This cooling effect produces an unstable atmosphere at the lower levels, and increases the sensible heat flux transfer from the ocean surface to the atmosphere. Precipitation increases subsequently. The melting process can increase the accumulated rainfall by about 4.1%. Therefore, the diabatic heating influences precipitation mainly by directly influences the atmospheric stability, which affects vertical transport of latent and sensible heat fluxes at the ocean surface and mesoscale convective system development. Precipitation over the northern SCS changes correspondingly.
Key words: South China Sea Monsoon Experiment (SCSMEX)     Mesoscale convective systems     Diabiatic heating due to the ice-phase change     Precipitation
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1 引言

1998年5~8月在我国南海季风区域进行了国际综合观测试验(South China Sea Monsoon Experiment，简称SCSMEX)(Lau et al., 2000Ding and Liu, 2001Ding et al., 2004)，国内外利用此次试验资料和数值模式分析研究了南海季风爆发的大尺度环流结构和演变和中尺度对流云系及降水特征，南海季风爆发期伴随的风速、云量、降水、湿度、太阳辐射和海面温度等物理量的迅速转化，而季风爆发后的季风环流等大尺度环境对中尺度对流系统的形成和发展则有着重要的影响(Chan et al., 2000柳艳菊和丁一汇, 2000, 2005Ding and Liu, 2001丁一汇等，2002Johnson and Ciesielski, 2002Tao et al., 2003Wang，2004柳艳菊等, 2005a, 2005bWang and Carey, 2005Wang et al., 2007李香淑等, 2008, 2011Fu et al., 2011Li et al., 2013)。随着南海季风区中尺度对流系统的形成和发展，对流云微物理过程对区域降水、辐射能量传输平衡以及中尺度对流系统发展演变过程也产生了重要的影响，其中冷云过程有利于南海季风区降水增加以及中尺度对流系统发展增强，而冷云过程与辐射过程的相互作用则导致区域降水减小和中尺度对流系统发展的减弱(Tao et al., 2003Gao et al., 2006Wang et al., 2007Fu et al., 2011)。为了更好的了解冷云过程中冰相相变潜热对中尺度对流系统和降水的影响，本文利用加入大尺度强迫项影响的可分辨云模式模拟研究了南海季风区中尺度对流系统冰相相变潜热过程对中尺度对流云和区域降水的影响和作用。

2 资料、模式设计及验证

1998年5月15日至6月11日在南海北部试验区的高层为强西风带，中低层也表现为相对较强的西风带(图 1a)。同时，在南海北部试验区的高层也存在较强北风带，而在中低层的南风气流相对较弱(图 1b)。

 图 1 南海季风北部试验区平均动力场的垂直分布随时间的变化：（a）纬向风速（单位：m s-1）；（b）经向风速（单位：m s-1） Fig. 1 Temporal evolutions of the averaged dynamic fields in the NESA (Northern Enhanced Sounding Array): (a) u wind (m s-1); (b) v wind (m s-1)

 ${\left( {\frac{{\partial u}}{{\partial t}}} \right)_{1{\rm{s}}}} = \frac{{\bar u - {{\bar u}_{{\rm{obs}}}}}}{\tau },$ (1)
 ${\left( {\frac{{\partial u}}{{\partial t}}} \right)_{1{\rm{s}}}} = \frac{{\bar v - {{\bar v}_{{\rm{obs}}}}}}{\tau },$ (2)
 ${\left( {\frac{{\partial \bar \theta }}{{\partial t}}} \right)_{1{\rm{s}}}} = - \bar V \cdot \nabla \bar \theta - \bar w\frac{{\partial \bar \theta }}{{\partial z}},$ (3)
 ${\left( {\frac{{\partial {{\bar q}_{\rm{v}}}}}{{\partial t}}} \right)_{1{\rm{s}}}} = - \bar V \cdot \nabla {{\bar q}_{\rm{v}}} - \bar w\frac{{\partial {{\bar q}_{\rm{v}}}}}{{\partial z}},$ (4)

1998年5月15日至6月11日期间，中国南海北部地区出现两次较强的降水过程，分别发生在5月15~24日的季风爆发期间和5月30日至6月8日的季风爆发后，可分辨云模式很好地再现了中国南海北部地区降水的变化(Fu et al., 2011)。

 图 2 二维数值模拟和TRMM TMI观测的区域-时间平均水成物的垂直分布：(a)云水；(b)雨水：(c)冰晶；(d)冰相降水粒子。图中水平点线表示0 ℃温度层 Fig. 2 Vertical profiles of domain-and time-averaged contents of hydrometeors: (a) Cloud water; (b) rain water; (c) ice crystals; (d) ice-phase precipitation particles simulated by the 2D model and from TRMM (Tropical Rainfall Measuring Mission) TMI (TRMM Microwave Imager), respectively. The short dashed line indicates the melting level simulated by the WRF (Weather Research and Forecasting) model

 图 3 二维与三维数值模拟的区域平均降水强度随时间的变化 Fig. 3 Temporal changes in the domain-averaged rainfall rate simulated by the 2D and 3D models

 图 4 二维与三维数值模拟的区域-时间平均水成物含量(单位：g kg-1)的垂直分布：(a)云水；(b)冰晶；(c)雪；(d)霰；(e)雨水 Fig. 4 Vertical profiles of domain-and time-averaged contents of hydrometeors (g kg-3): (a) Cloud water; (b) cloud ice; (c) snow; (d) graupel; (e) rain water simulated by 2D and 3D models, respectively

3 结果分析 3.1 冰相相变潜热对云和降水的影响

 图 5 南海季风北部试验区混合云过程与敏感性试验过程中(a)区域平均降水量差值和(b)最大降水强度随时间的变化 Fig. 5 Temporal evolutions of (a) the differences in domain-averaged accumulative rainfall between ice run and that based on the sensitivity experiments, and (b) the maximum rain rate in the WRF domain of simulation
3.2 冰相相变潜热对云强迫辐射效应的影响分析

 图 6 南海季风北部试验区混合云过程和敏感性试验过程中区域平均(a)云顶长波辐射强迫(LWCF)、(b)云顶短波辐射强迫(SWCF)、(c)云顶总辐射强迫随时间的演变 Fig. 6 Temporal evolutions of domain-averaged (a) longwave cloud forcing (LWCF), (b) shortwave cloud forcing (SWCF), and (c) total cloud forcing at the top of the atmosphere in the ice run and the sensitivity experiments

 图 7 南海季风北部试验区混合云过程与敏感性试验过程中平均(a)向下短波辐射通量和(b)向下长波辐射通量随时间的变化 Fig. 7 Temporal evolutions of domain-averaged (a) downward shortwave flux and (b) downward longwave flux at the ocean surface in the ice run and the sensitivity experiments
3.3 冰相相变潜热过程对大尺度环境场的影响

 ${C_{{\rm{APE}}}} = g\int\limits_{{L_{{\rm{FC}}}}}^{{E_{\rm{L}}}} {\frac{{{T_{{\rm{VP}}}} - {T_{{\rm{VE}}}}}}{{{T_{{\rm{VE}}}}}}} {\rm{d}}z,$ (5)

 图 8 南海北部地区混合云过程和敏感性试验过程中区域平均对流有效位能(CAPE)随时间的变化 Fig. 8 Temporal evolutions of the domain-averaged convective available potential energy (CAPE) in the ice run and the sensitivity experiments over the northern South China Sea
3.4 冰相相变潜热对降水的影响机制

 图 9 南海季风北部试验区混合云过程和敏感性试验过程中区域平均洋面热通量随时间的演变：(a)感热通量；(b)潜热通量 Fig. 9 Temporal evolutions of domain-averaged (a) sensible heat flux and (b) latent heat flux at the ocean surface in the ice run and the sensitivity experiments
4 结论