【作者】 何编；

【导师】 吴国雄； 孙照渤； 刘屹岷；

【作者基本信息】 南京信息工程大学 ， 气象学， 2013， 博士

【摘要】 亚洲夏季风的活动规律在海陆气相互作用的影响下表现的非常复杂,青藏高原的动力、热力强迫作用历来被科学家们认为对亚洲季风具有重要的调控作用,然而在夏季青藏高原如何影响南亚夏季风形成和维持的物理机制理解上科学家们仍旧存在着分歧,大多数科学研究表明青藏高原的地表感热加热和高空的潜热加热影响到周围大气环流,是南亚高压建立的主要因素,也对南亚夏季风的形成与维持占有重要作用；然而也有学者提出了不同观点,他们认为是高原的隔断作用使得暖湿空气在喜马拉雅山南侧进行堆积,地表高的比焓造成对流的产生,决定了对流层高层的温度廓线,而温度场暖中心的形成给南亚夏季风提供了驱动力。因此,有必要对青藏高原以及喜马拉雅山的动力、热力强迫作用进行一个再认识。本文利用多种观测资料,并结合数值试验研究了青藏高原和伊朗高原的动力强迫作用以及热力强迫作用对南亚夏季风形成与维持的影响,在此基础之上进一步讨论了青藏高原南侧斜坡对大气的加热效应以及喜马拉雅山热力强迫效应,此外,文章还从热成风平衡和地转平衡关系出发,从理论上讨论了南亚高压脊线附近大气非绝热加热如何影响高原西侧增暖的物理机制,并用数值试验证明了理论推论的结果,所得到的主要结论如下：(1)利用FGOALS-s2陆气耦合分量模式的地形敏感性试验结果表明,海陆热力差异是导致南亚夏季风南支环流形成与维持的主要原因,青藏高原和伊朗高原的动力、热力作用主要影响了印度大陆和高原南侧季风环流和降水的形成。而仅有青藏高原和伊朗高原的动力强迫存在时,由于没有感热加热,气流在经过高原时沿等熵面做绝热运动形成绕流,南亚夏季风降水无法北推至印度北部平原地区,南亚夏季风北支环流无法形成。青藏高原地表的感热加热使得环流场向加热场适应,气流有了穿越等熵线的向高原上爬坡运动,将低层的水汽带入到对流层高层,而大尺度的加热导致了低层气旋式环流异常,在仅有青藏高原地形的热力作用试验中,气旋式环流异常使得高原西侧降水减少而高原的南侧和东亚地区降水增加；仅有伊朗高原地形的热力试验中,在伊朗高原附近的气旋式环流异常使得青藏高原西侧的降水有所增加。两组试验的线性叠加结果基本与整体的热力强迫试验结果一致,所造成的降水分布与海陆热力对比试验的结果是互补的,说明青藏-伊朗高原的加热作用是南亚夏季风北支环流形成与维持的最主要原因,而不是隔断作用。(2)对青藏高原斜坡、平台以及印度北部平原的热力作用敏感性试验结果表明,高原斜坡的热力强迫是造成亚洲大陆低层气旋式环流形成的主要因素,而高原平台地区以及印度北部平原地区的热力作用对夏季风降水乃至环流的影响都非常小,可以忽略。由于斜坡的加热作用使得附近等熵面发生弯曲,表层出现了很强的上升运动,造成南亚夏季风北支季风降水的形成。而平台的加热导致了对流层高层暖中心的明显变化,但是由于4000m上水汽很少,其对季风降水的影响并不明显；此外,印度北部平原的热力强迫对局地的季风降水有一定影响,但不是南亚夏季风的主要驱动力。(3)喜马拉雅山地形的抬升试验表明,随着地形的不断升高,山脉南侧的感热加热增强使得斜坡上的垂直运动不断增强,同时导致了南亚夏季风北支降水的增加；此外,通过热力学方程和Sverdrup平衡关系,我们推导出低层的经向风对非绝热加热与位温分布的变化有指数上的响应关系,通过计算地形抬升试验中这种响应关系和非绝热加热场、垂直速度场、降水以及模拟的经向风的时间序列我们得出低层经向风对加热的响应存在一种线性增长的趋势,并且环流对加热的响应跟位温的空间分布以及垂直加热尺度密切相关。(4)通过热成风关系我们诊断出副热带地区夏季青藏高原西侧对流层上层暖中心的形成是准地转平衡下温度场和环流场相互适应的结果,而并不直接来自于湿对流活动的加热以及地表比焓分布的对应关系。而在热成风平衡关系下,温度场的极大值必然要出现在南亚高压的脊线附近,并且,高原东侧以及东亚地区的潜热释放对大气环流的影响是高原西侧暖中心形成的驱动力,温度场的变化正比于非绝热加热的纬向梯度。(5)根据上述关系,我们考虑在副高脊线上的加热如何影响西侧增暖的物理机制,由于副热带地区满足准地转平衡,科氏力与气压梯度力相当,当高原东侧有非绝热加热产生时,由于垂直方向的非均匀加热,导致经向风的变化,使得科氏力增强,气块向西移动增加气压梯度以平衡科氏力,使得高原西侧气柱增厚,南亚高压增强,厚度场加厚、温度场增暖；同时因为产生了向下的垂直速度场异常,使得对流活动减弱,降水减少。观测资料的环流和降水的年代际变化证明了这个纬向次级环流的可能存在性,我们进一步利用数值试验证实了这种物理机制的合理性,通过在高原东侧对流层高层副高脊线上给定固定热源强迫,在高原西侧的温度场暖中心明显增强,纬向的垂直剖面图异常场上表现出了高层东风异常以及在暖中心上空的下沉气流；此外,给定准60天振荡的热源强迫试验结果表明,热源强迫的振荡周期可以从高原东侧传播到高原西侧,温度场以及垂直运动场的时间序列和东侧热源的时间序列同位相。以上结果表明了,高原西侧暖中心的形成不是斜坡隔断作用造成湿对流发展的结果,而是温度场对高原东侧大尺度非绝热加热在环流场上的响应。结果表明,南亚夏季风主要是受到海陆热力对比和亚洲大地形热力作用的调控作用而形成的。更多还原

【Abstract】 The activities of Asian summer monsoon are very complicated under the interactions of sea-ocean-atmosphere. The scientists always consider the mechanical and thermal forcing of Tibetan Plateau (TP) as an important factor in regularing Asian monsoon. But, in boreal summer, there still exist academic difference on the physical mechanism of TP effect over the formation and maintenance of South Asian Summer Monsoon (SASM). Most of the studies indicate that the surface sensible heat over TP and the latern heating in upper troposphere could have an impaction on the surrounding circulations, and it is the mainly contributor to the buildup of the South Asian High (SAH) together with the formation and maintenance of SASM. On the contrary, some scholars have raised a different viewpoint, the insulation of Himalaya make warm entropy accumulated on the south foothill and the cold entropy on the north. The high entropy caused the moist convections and linked directly with the vertical profile of virtual temperature, so the temperature in high troposphere becomes warmer and drives the SASM circulation. Therefore, it is necessary to have a new acquaintance on the mechanical and thermal effect of TP and Himalaya.Based on various observations and numerical simulations, the mechanical and thermal effects of TP and Iran Plateau (IP) are studied on the topic of formation and maintenance of SASM. A further discussion has been taken to find out the heating effect over south slope of TP and thermal forcing of Himalaya. Besides, based on the thermal wind balance and geostrophic wind balance, we discuss the physical mechanism of diabatic heating near the SAH ridge line could warm the troposphere temperature on its west, and test this hypothesis by numerical simulations. The main conclusions are derived as follows:(1) The topographical sensitive tests by FGOALS-s2atmospheric component indicate that, the thermal contrast of large scale land sea distribution is the dominant control of the SASM south branch, the mechanical and thermal impact of TP and IP is the main cause of the precipitation and circulation over India continent and south slope of TP. While on the condition of mechanical forcing of TP and IP without sensible heating, the air flow move along isentropic surface around the plateau, the precipitation cannot shift to the north of India continent, and the north branch of SASM cannot buildup. While the surface sensible heat exists over TP, the circulation must adapt to the diabatic heating, the air flow near TP can penetrate the isentropic surface and tend to cross the plateau, bring the water vapor at low level to the high level of troposphere. The large scale diabatic heating cause the cyclonic low level circulation, in the thermal experiment of TP solo, the cyclonic circulation anomaly cause the precipitation over west of TP to decrease and south of TP with East Asia to increase; while in the thermal experiment of IP solo, the cyclonic anomaly cause the precipitation over west of TP to increase. The linear composition of the results of these two experiments are almost in accordance with the results of whole thermal experiment, the precipitation pattern is also very similar to the land-sea contrast experiment, it indicates that the heating effect of TP and IP is the dominant control of the formation and maintenance of SASM north branch, while the insulation effect is not important.(2) The results of slope, platform and north India thermal experiments show that the slope heating is the main cause of the Asia continental cyclonic circulation formation at low level, while the thermal effects of the platform and north India on monsoon precipitation and circulation are quite weak and can be neglected. The diabatic heating over the TP slope bend the isentropic surface on each level and cause the vertical motion at the slope surface, form the precipitation of SASM north branch. The heating on the platform cause the upper level temperature change greatly, but due to the little water vapor above4000m, the precipitation of SASM is not affected much by platform heating. In addition, the thermal forcing over India north continent has a limited effect on the local precipitation, but it is not the main driven force of SASM.(3) The sensitive test of Himalaya orographic uplift shows that, as the orography increasing, the sensible heating of Himalaya south slope is also intensified, it cause the SASM north branch precipitation to increase. Moreover, with the relationship of thermodynamic balance and Sverdrup balance, we come to the exponential relationship between the meridional wind change with the distribution of diabatic heating and potential temperature. The calculated wind response is well in coordinated with the increase of diabatic heating, vertical velocity, precipitation and the simulated wind, moreover, the circulation pattern is determined by the spatial pattern of potential temperature and vertical heating scale.(4) According to the thermal wind balance, we diagnose that the upper-level temperature maximum over west TP is a kind of quasi-geostropic adaption. It is not the result of moist convection heating or the surface high entropy, but is a response of temperature to the circulations. On the condition of thermal wind balance, the temperature must achieve its maximum on the SAH ridge line, and the diabatic heating in the mid-troposphere over East of TP and East Asia is the driven force of this warm center, the change of the temperature in the west is in proportion to the zonal gradient of diabatic heating.(5) According to the relationship we found, a new physical mechanism has been proposed on how the heating over SAH ridge line increasing the temperature on its west. Because in the subtropical region, the atmospheric motion should obey quasi-geostropic relationship, the Coriolis force must equal to pressure gradient force. While the diabatic heating generate on the east of TP, the vertical inhomogeneous diabatic heating cause the meridional wind to change on the upper level by Sverdrup balance, so the Coriolis force enhanced and the balance of the two force break up, the air particle must move to the west to intensify the pressure gradient, result in the air column to be thick over west of TP, the SAH intensified, and the thickness field increasing, cause the temperature to increase; meanwhile, because of the generation of the downward vertical velocity, the convections over west of TP are weaken, the precipitation tend to decrease. The observed decadal change of circulation and precipitation in Asian boreal summer support the possibility of the secondary circulation’s existence. We further test the rationality of this hypothesis by numerical simulations. By adding a fixed heating source on the ridge line over east of TP, the temperature maximum response over west is intensifed obviously, the zonal cross-section of the circulation indicate that there exist a east wind anomaly on the upper troposphere, and a descend motion anomaly under the temperature maximum; in addition, a quasi60days cycle heating source test has been preformed, and the results show that, the heating period can be propagate from east of TP to its west, the time serials of temperature maximum and vertical velocity on west of TP are in the same phase with the heating source over east. Then we come to the conclusion that the formation of the upper-level temperature maximum over west of TP is not driven from the moist convection caused by insulate effect of Himalaya, but is the response to the circulation by the large scale diabatic heating on east of TP.All the evidence shows that, the formation of SASM is dominant controlled by land sea thermal contrast and Asia continental heating effect.更多还原