GOALS模式对热带太平洋ENSO年际变化特征的模拟评估

文中重点分析了中国科学院大气物理研究所LASG最新发展的全球大气环流谱模式(R42L9)与一全球海洋环流模式(T63L30)耦合形成的全球海洋-大气-陆面气候系统模式(GOALS/LASG)新版本已积分30 a的模拟结果,通过与多种观测资料的对比分析,讨论了赤道太平洋海表温度(SST)的年际变化及其纬向传播、赤道东太平洋SST异常与其他洋面SST变化之间的遥相关关系、赤道太平洋浅表层海温的年际变化特征等研究内容.结果表明,COALS模式模拟出了赤道太平洋SST异常出现不规则的年际变化特点;赤道东太平洋SST异常的向西传播过程;赤道太平洋混合层海温变化由西向东、由深层向浅层的传播过程;同时也模拟出了赤道东太平洋SST变化与赤道西太平洋以及与西南太平洋海温之间的反相关关系,与南印度洋和副热带大西洋SST之间的正遥相关关系等实际观测现象.但COALS模式也存在明显的不足,如对赤道东、中太平洋SST异常的年际变化幅度明显偏小,没能模拟出赤道东太平洋的SST变化比赤道中太平洋强的特点;赤道太平洋SST从东向西的传播速度明显比实际观测慢得多,但混合层海温极值变化由西向东的传播速度明显比实际情况快得多;没能模拟出赤道东太平洋SST变化同西北太平洋SST的负相关和北印度洋海温变化的正相关现象,因此也影响了对南亚、东南亚降水年际变化的模拟能力.     

海洋风暴形成的一种动力学机制

文中从观测统计学、瞬变涡动能量学和 MM5中尺度数值模拟角度 ,研究了海洋风暴 (爆发性气旋 )形成的气候特征及其可能的动力学机制 ,揭示了一幅爆发性发展的物理图像。结果表明 ,在冷季大气特别是日本以东洋面上大气特有的热力气候背景下 ,通过同海洋风暴过程相联系的涡动热通量 vθ的向极地输送 (- vθ· θm>0 ) ,将季节尺度的时间平均有效位能向瞬变涡旋时间尺度的涡动有效位能转换 ,是海洋风暴形成的主要动力机制。在该过程中转换来的具有最大贡献的涡动有效位能 ,连同具有次大贡献的积云加热制造的涡动有效位能(q3 )一起 ,通过暖异常区 (α >0 )暖湿空气上升运动 (-ω >0 )的斜压转换 (-ωα) ,促使涡动动能增长。同时 ,补充的涡动有效位能又加强了暖异常区的暖湿空气上升运动 ,进而产生积云对流活动及其潜热释放的正反馈过程 ,最终导致涡动动能急剧增长和海洋风暴的形成。海-气潜热输送的作用是在风暴形成初期提供后来积云尺度对流活动及潜热释放的水汽潜力。研究还表明 ,海洋风暴主要发生在冷季月份 1 3 0°E以东的中高纬洋面上 ,这种对特定季节和特定海域的依赖性是大气和海洋气候背景的动力 /热力共同作用的结果     

北半球高纬地区年际尺度循环过程中的气-海-冰相互作用关系

基于一个全球气-海-冰耦合模式数值模拟结果,对北半球高纬度地区年际尺度的气-海-冰相互作用进行了分析。在所使用的全球气-海-冰耦合模式中,大气环流模式和陆面过程模式来自国家气候中心,海洋环流模式和海冰模式来自中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室。采用一种逐日通量距平耦合方案实现次网格尺度海冰非均匀条件下大气环流模式和海洋环流模式在高纬地区的耦合。只对50 a模拟结果中的后30 a结果进行了分析。在分析中,首先对滤波后的北半球高纬度地区海平面气压、表面大气温度、海表面温度、海冰密集度及海表面感热通量的标准化距平做联合复经验正交函数分解,取第一模进行重建,然后讨论了在一个循环周期(约4 a)中北半球高纬度地区气-海-冰的作用关系。结果表明:(1)当北大西洋涛动处于正位相时,格陵兰海出现南风异常,使表面大气温度升高,海洋失去感热通量减少,海洋表面温度升高,海冰密集度减小;当北大西洋涛动处于负位相时,格陵兰海出现北风异常,使表面大气温度降低,海洋失去感热通量增多,海洋表面温度降低,海冰密集度增加。巴伦支海变化特点与格陵兰海相似,但在时间上并不完全一致。(2)多年平均而言,北冰洋内部靠近极点区域为冷中心。当北冰洋内部为低压异常时,因异常中心偏向太平洋一侧,使北冰洋内部靠近太平洋部分为暖平流异常,靠近大西洋一侧为冷平流异常。伴随着暖、冷平流异常,这两侧分别出现暖异常和冷异常,海表面给大气的感热通量分别偏少和偏多,上述海区海表面温度分别偏高和偏低,海冰密集度分别偏小和偏大。当北冰洋内部为高压异常时特点正好与上述相反。由上述分析结果可知,在海洋、大气年际循环中,大尺度大气环流变率起主导作用,海洋表面温度和海冰密集度变化主要是对大气环流变化的响应。     

Fine-resolution simulation of surface current and sea ice in the Arctic Mediterranean Seas

A fine-resolution model is developed for ocean circulation simulation in the National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Chinese Academy of Sciences, and is applied to simulate surface current and sea ice variations in the Arctic Mediterranean Seas. A dynamic sea ice model in elastic-viscous-plastic rheology and a thermodynamic sea ice model are employed. A 200-year simulation is performed and a dimatological average of a 10-year period (141st-150th) is presented with focus on sea ice concentration and surface current variations in the Arctic Mediterranean Seas. The model is able to simulate well the East Greenland Current, Beaufort Gyre and the Transpolar Drift, but the simulated West Spitsbergen Current is small and weak. In the March climatology, the sea ice coverage can be simulated well except for a bit more ice in east of Spitsbergen Island. The result is also good for the September scenario except for less ice concentration east of Greenland and greater ice concentration near the ice margin. The extra ice east of Spitsbergen Island is caused by sea ice current convergence forced by atmospheric wind stress.     

Progress in Studies of the Precipitation Diurnal Variation over Contiguous China

This paper summarizes the recent progress in studies of the diurnal variation of precipitation over con- tiguous China. The main results are as follows. （1） The rainfall diurnal variation over contiguous China presents distinct regional features. In summer, precipitation peaks in the late afternoon over the south- ern inland China and northeastern China, while it peaks around midnight over southwestern China. In the upper and middle reaches of Yangtze River valley, precipitation occurs mostly in the early morning. Summer precipitation over the central eastern China （most regions of the Tibetan Plateau） has two diurnal peaks, i.e., one in the early morning （midnight） and the other in the late afternoon. （2） The rainfall diurnal variation experiences obvious seasonal and sub-seasonal evolutions. In cold seasons, the regional contrast of rainfall diurnal peaks decreases, with an early morning maximum over most of the southern China. Over the central eastern China, diurnal monsoon rainfall shows sub-seasonal variations with the movement of summer monsoon systems. The rainfall peak mainly occurs in the early morning （late afternoon） during the active （break） monsoon period. （3） Cloud properties and occurrence time of rainfall diurnal peaks are different for long- and short-duration rainfall events. Long-duration rainfall events are dominated by strat- iform precipitation, with the maximum surface rain rate and the highest profile occurring in the late night to early morning, while short-duration rainfall events are more related to convective precipitation, with the maximum surface rain rate and the highest profile occurring between the late afternoon and early night. （4） The rainfall diurnal variation is influenced by multi-scale mountain-valley and land-sea breezes as well as large-scale atmospheric circulation, and involves complicated formation and evolution of cloud and rainfall systems. The diurnal cycle of winds in the lower troposphere also contributes to the regional differences     

两步保形平流方案在高分辨率球面经纬网格下的跳点差分试验

尝试用一个基于有限差分的两步保形平流方案（TSPAS）替代美国国家大气研究中心第5版公用大气模式（NCAR-CAM5）欧拉-谱动力框架中的半拉格朗日传输（SLT）平流方案,并针对不同分辨率版本,计算水汽等物质的传输过程。通过设计保证守恒性的跳点差分法,解决了基于经纬网格的模式在高分辨率下由于极区纬向格距过小而造成的差分方案积分时间步长过短的问题,使得TSPAS在高分辨率下仍然可以使用较大的时间步长。理想试验表明：（1）跳点TSPAS在守恒性、数值精度（量级上）、保形性等方面均较好地保持了原TSPAS的特点,而积分步长可增大到与CAM5-SLT相同,比原TSPAS提高超过一个数量级;（2）较半拉格朗日传输方案,尽管TSPAS的耗散更大,但是在相同时间步长下更省时,且改进了半拉格朗日传输求解平流方程造成的不守恒问题。CAM5的比较试验表明：（1）采用跳点TSPAS的模式结果与非跳点计算结果相当,在高纬度跳点区域也并未出现模拟异常;（2）在高分辨率下,采用跳点TSPAS方案的CAM5模拟结果与原模式结果相近,并初步显示对东亚青藏高原南侧的虚假降水有所抑制。该工作确保了基于欧拉通量差分型的TSPAS在高分辨率模式下的大时间步长稳定积分,为解决有限差分方法在高分辨率模式发展中的相关计算问题提供了启示,为后续针对东亚地区的模式性能改进奠定了基础。     

A Case Study on the Triggering of Thermal Convective Precipitation

Thermal convective precipitation （TCP） often occurs over mainland China in summer when the area is dominated by the western Pacific subtropical high （WPSH）. It is well known that the WPSH often brings about large scale subsidence, then why could deep moist convection occur and where does the water vapor come from？ In this paper, a deep convective precipitation case that happened on 2 August 2003 is studied in order to address these two questions. First, the characteristics of the TCP event are analyzed using the Tropical Rainfall Measuring Mission （TRMM） satellite data, automatic weather station observations, and the data from the US National Centers for Environmental Prediction （NCEP）. Second, water vapor sources are identified through examining surface evaporation, water vapor advection, and water vapor flux divergence calculated by using a regionally averaged water vapor budget equation. Furthermore, using an Advanced Regional Eta-coordinate Model （AREM）, contributions of sensible and latent heat fluxes to the TCP are compared through four sensitivity experiments. The results show that in the regions controlled by the WPSH, surface temperature rises rapidly after sunrise. Upon receiving enough sensible heat, the air goes up and leads to convergence in the lower atmosphere. Then the water vapor assembled from the surroundings and the ground surface is transported to the upper levels, and a favorable environment for the TCP forms. A model data diagnosis indicates that about half of precipitable water comes from the convergence of horizontal fluxes of water vapor, and the other half from surface evaporation, while little is from advection. Additional sensitivity experiments prove that both sensible and latent heating are essential for the onset of the TCP. The sensible heat flux triggers thermodynamic ascending motion, and the latent heat flux provides water vapor, but the contribution to TCP from the latter is a little smaller than that from the former.     

热通量和风应力影响北太平洋SST年际和年代际变率的数值模拟

利用一个大洋环流模式LICOM, 通过1958～2001年风应力 (ERA40) 和热通量驱动下的两组模拟试验, 检验了二者在北太平洋年际和年代际变率形成中的作用。结果表明, 尽管在年际尺度上热带太平洋变率主要受风应力影响, 但合理考虑热通量异常的强迫作用能够显著改进模式对El Niño的模拟效果, 包括对El Niño周期非规则性的成功模拟; 北太平洋SST的年际和年代际异常主要受热通量异常的影响, 合理考虑热通量强迫的年代际变化能够改善模式对北太平洋年代际变率的模拟效果。在北太平洋海盆的不同区域, 导致SST变率异常的因子不同: 在加利福尼亚沿岸, 冬季平均海温的变率异常主要由热通量的异常决定; 在北太平洋中部, 温度趋势异常主要受热通量和水平平流的作用影响; 在黑潮及其延伸体区域, 对温度趋势异常起主导作用的是热通量和海洋非线性作用, 与此同时, 水平平流和扩散的作用亦不容忽视。     

四川盆地西部暴雨对初始水汽条件敏感性的模拟研究

基于区域暴雨数值预报模式AREM, 针对2003年8月发生在四川盆地西部的多次局地强暴雨过程, 通过数值试验, 讨论了川西暴雨对初始水汽条件的高度敏感性, 揭示了川西暴雨过程独特的局地水汽循环特征。初始水汽条件不仅决定着暴雨的强度, 还对最大降水发生时间产生明显影响, 从而决定模式对降水日变化的模拟效果。局地初始水汽偏大, 不仅导致24小时降水总量的迅速增加, 甚至空报暴雨, 还使得主要降水时段明显提前, 使夜雨变为昼雨。局地初始水汽减少不仅会显著减小降水量, 甚至会致使暴雨消失,还会使主要降水时段滞后。     

Increased Tibetan Plateau Snow Depth：An Indicator of the Connection between Enhanced Winter NAO and Late-Spring Tropospheric Cooling over East Asia

The authors present evidence to suggest that variations in the snow depth over the Tibetan Plateau (TP) are connected with changes of North Atlantic Oscillation (NAO) in winter (JFM). During the positive phase of NAO, the Asian subtropical westerly jet intensifies and the India-Myanmar trough deepens. Both of these processes enhance ascending motion over the TP. The intensified upward motion, together with strengthened southerlies upstream of the India-Myanmar trough, favors stronger snowfall over the TP, which is associated with East Asian tropospheric cooling in the subsequent late spring (April--May). Hence, the decadal increase of winter snow depth over the TP after the late 1970s is proposed to be an indicator of the connection between the enhanced winter NAO and late spring tropospheric cooling over East Asia.