【作者】 崔洋；

【导师】 王澄海；

【作者基本信息】 兰州大学 ， 气象学， 2010， 博士

【摘要】 青藏高原作为亚洲夏季风系统的主要成员之一,其热力强迫在亚洲夏季风系统中具有重要作用。由于其特殊的地理位置和海拔高度,青藏高原成为中、低纬度地带多年冻土面积最广、厚度最深、温度最低的地区,其中积雪和冻土是青藏高原地表过程中最重要的因子。以积雪和冻土为主要内容的高原陆面过程必然影响青藏高原的热状况和热性质,进而对青藏高原与其上大气之间的相互作用产生影响。对青藏高原陆面过程与亚洲夏季风之间相互联系的研究,不仅对加深理解青藏高原地区的非绝热加热在亚洲夏季风系统中的作用,深入认识春季青藏高原陆面过程特征及其与其上大气之间的相互作用具有重要意义,也有助于我们深入认识青藏高原热力强迫在亚洲夏季风系统中的贡献,有利于提高我国夏季季风降水的预测。在用感、潜热通量,积雪,冻土等对青藏高原热力作用对亚洲季风影响研究的基础上,本文主要关注的问题是,在季风建立前期的春季,青藏高原陆面过程与亚洲夏季风之间存在着什么样的联系。因此,本文主要点集中在青藏高原春季的主要过程——融雪和融冻引起的陆面特征的变化与亚洲夏季风之间的联系展开研究。首先,利用高原东北部地区玛曲观测站的资料,分析了青藏高原陆面过程的水热变化特征,重点分析了夏季风建立前的春季,高原融冻过程中土壤水热状况和热性质随季节的变化特征。在此基础上,利用GAME/Tibet试验中,高原西部地区改则和狮泉河两站近地面边界层观测资料和NCEP、ERA40等再分析资料,进一步深入研究了季风建立前期的春季,青藏高原融雪和融冻过程对高原地表与其上大气水热交换过程及高原地表非绝热加热计算的影响。在上述工作的基础上,利用台站观测的地温、气温资料,采用EOF和REOF方法,分析了季风建立前期的春季,青藏高原地表非绝热加热的时空变化及其与高原积雪融雪和冻土融冻过程之间的联系。其次,基于再分析资料和观测资料,研究了青藏高原非绝热加热与东亚夏季风强度之间的联系；通过检测春季青藏高原非绝热加热异常变化中的低频信号,分析研究了典型强弱季风年东亚夏季风建立期间青藏高原非绝热加热与纬向风相互作用及其传播特征；揭示了春季高原非绝热加热变化与东亚夏季风之间的联系机制。最后,通过RegCM3.0区域气候模式的数值模拟试验,研究探讨了春季青藏高原陆面过程中土壤湿度变化对季风建立过程中高原地表热状况,大气环流,及中国夏季降水的影响。主要的研究内容和结论如下：(1)季风建立前期青藏高原地区水热变化特征的分析。结果表明：在亚洲夏季风建立前期的春季,青藏高原土壤融冻过程发生期间,随着土壤温度上升,土壤湿度有一段快速升高的阶段。不同土壤状态下(完全冻结,冻融过渡、完全融化),土壤温度和湿度之间存在不同的关系。春季土壤冻融过程对高原浅层土壤热通量的分布具有重要影响。(2)青藏高原陆面过程对非绝热加热计算影响的分析。春季,高原土壤冻融的日周期循环会引起土壤水分的变化,而土壤水分的变化会进一步引起感、潜热的变化。因此,春季冻融日周期循环引起的土壤水分的剧烈变化会在计算高原地区的地表感、潜热通量中产生显著影响。但高原融雪和土壤冻融过程引起的土壤湿度与三种再分析资料的土壤湿度之间存在的差异,也是造成再分析感、潜热资料计算误差的主要原因。因此,NCEP-Ⅰ、AR-Ⅱ和ERA40感、潜热再分析资料在青藏高原地区存在着差异,在诊断高原春季非绝热加热异常状况中需审慎对待,也必须对其进行修正。(3)季风建立前期青藏高原地表非绝热加热变化特征的分析研究。夏季风建立前期,受高原积雪融雪和冻土融冻过程的影响,青藏高原地表非绝热加热的年际异常变化敏感区从3月高原中部的河谷地带移到4月的东南部地区,5月异常敏感区在面积扩大之后稳定在高原东南部地区。季风建立前期的春季4月份,由于积雪的反照率引起的辐射冷却作用,高原地表非绝热加热在20世纪60年代到70年代中期呈显著减小趋势,之后呈增大的趋势。在5月份,从20世纪60年代到90年代,受地表融雪和融冻过程的影响,高原地表非绝热加热呈现出减小趋势。(4)高原非绝热加热与东亚夏季风强度联系的研究。EP通量的分析结果显示,在强、弱季风年春季3-5月,青藏高原地表非绝热加热强迫激发的大气热力波均向对流层中上层传播。强季风年春季,高原北侧40-500N的区域对流层上层的大气定常波同时向中高纬度地区和对流层低层传播并增强；而在弱季风年,其仅向中高纬度地区传播。青藏高原及其北侧40～60°N区域是强、弱季风年EP通量散度差异最大的区域。春季青藏高原地表非绝热加热异常对亚洲夏季风建立前期中高纬度西风气流的变化具有重要影响。春季青藏高原非绝热加热异常和东亚夏季风强度之间存在显著的反相关关系。(5)青藏高原非绝热加热与东亚夏季风建立关系的研究。春季,东亚夏季风建立期间,青藏高原地区的高度场和纬向风场存在30～60天大气低频振荡、准双周和5-7天的大气振荡。在典型强季风年份,高原在北部形成低频振荡并向北传播；而在弱季风年份,高原地区的低频振荡具有原地振荡的显著特征。在强季风年,高原的非绝热加热削弱高原地区低频波,非绝热加热在高原东西两侧以外的地区再现出来。在弱季风年份,高原地区的非绝热加热起着加强高原地区低频波的作用；形成了以高原为中心的准南北方向上的振荡特征。在强季风年,高原非绝热加热与大气低频振荡之间的相互作用不仅显著且会持续到南海季风稳定建立。相反在弱季风年,两者之间的相互作用在南海季风建立之后迅速消失。高原地区的非绝热加热和季风强度之间在时空上都存在着密切联系。(6)青藏高原非绝热加热异常在亚洲季风中作用的研究。为了验证与高原春季陆面过程相联系的非绝热加热异常在亚洲夏季风中的作用,设计了一组土壤湿度变化的数值试验。模拟结果表明：高原春季土壤湿度的增加(减少),会引起4月份对流层中上层东亚急流区附近西风气流的减弱(增强),高原南侧的南支急流增强(减弱)；夏季风建立前期的4、5月份东亚中高纬度地区的槽脊系统出现异常变化；6月份我国东部季风区低层的南风加强(减弱)；我国长江和黄河中下游地区、华南东南部地区初夏降水的增加(减少),华南其他地区和淮河流域初夏降水的减少(增加)。表明春季青藏高原冻融过程中土壤湿度的变化,通过对亚洲夏季风建立前期4、5月份的高原南支气流、中高纬度的西风气流和槽脊系统演变的影响,对我国东部的夏季降水产生影响。更多还原

【Abstract】 As one of the critical members of Asian summer monsoon(ASM) system, the diabatic heating of Qinghai-Xizang plateau(QXP) plays an important role in the Asian summer monsoon. Because of its special geographical location and altitude, the snow cover and frozen ground are the main surface features of the QXP. The changes of land surface processes will inevitably affect on the thermal status property of the QXP, furthermore affect the interaction between the land surface and the air over the QXP. Therefore, studying on the relationships between the land surface processes of the QXP and AMS, not only benefits us to understand the interaction between land and the air over the QXP, but also to expand our comprehension the role of the QXP diabatic heating in the ASM system. Certainly, it is also beneficial to enhance the predicting ability of the summer monsoon rainfall of China.By using sensible and latent heat fluxes, snow cover and frozen ground over the QXP, a lot of researcher have investigated the thermal effects of the QXP on the ASM, and got much meaningful achievements. The relationships between the land surface process of Qinghai-Xizang plateau in spring and the Asian summer monsoon are concerned in this paper. First, by using of observational data of Maqu weather station, we comprehensively analyzed the soil thermal property variation of Qinghai-Xizang plateau, especially in the early stage of Asian summer monsoon onset. Basing on the above results, using the observational data of GAME/Tibet in Shiquan River and Gaize stations, and the reanalysis dataset (NCEP-I, AR-II, ERA40), the effects of the soil moisture and temperature variation, which caused by snow melting and soil thawing processes, on computing the surface diabatic heating of the QXP are further investigated. Then, using the the observational surface and air temperature data, the temporal-spatial changing characteristics of the diabatic heating over the QXP in spring of the early stage of ASM establishment are studyed through the empirical orthogonal function(EOF) and rotated empirical orthogonal function(REOF) methods. The relationships between the diabatic heating of the QXP and the snow cover, soil freezing processes are also discussed. Second, we investigate the relationships between the diabatic heating of the QXP in spring and the intensity of East Asian summer monsoon. Furthermore, the interaction between the diabatic heating and the zonal wind and its propagation characteristics are analyzed. Finally, the influences of the soil moisture variation in spring on the QXP surface thermal regime, atmospheric circulation during the ASM building-up, and eastern China early summer precipitation are also investigated through the RegCM3.0 regional climate model simulation. The major contents and results as follows:(1) The soil hydrothermal features of the QXP in spring are studied by using the station observational data. The results show that the time soil freezing processes occurred is the early stage of ASM onset. In the soil freezing processes, with the soil temperature rises up, the soil moisture sharply increases in a short time. As the soil under the different condition(frozen, frozen-thawing transition, melt), there arethe different relationships between the soil temperature and soil moisture. In spring the soil freezing processes siginificantly influences the soil heat flux gradient distribution in the shallow layer over the QXP.(2) The land surface process impacts on the diabatic heating over the QXP are investigated. In spring, the diurnal freezing-thawing cycle of the soil over the QXP leads to the soil moisture changes, furthermore causes the sensible and latent heat fluxes changes. Therefore, the change of soil moisture, which is caused by the soil diurnal freezing-thawing cycle, has a significant influence on the calculation of surface heating fluxes over the QXP. The soil moisture differences among the observation NCEP-Ⅰ, AR-Ⅱand ERA40 reanalysis data, which are caused by the snow melting and soilthawing processes, are the major reason for the computing error of reanalysis sensible and latent heat fluxes. In spring, due to the impacts of the snow melting and soil thawing processes, there are remarkable differences among the three kinds of reanalysis sensible and latent heat fluxes. It needs to be careful and make correction, while the sensible and latent heat fluxes in reanalysis data are used to diagnosed the diabatic heating anomaly of the QXP in spring.(3) The temporal-spatial characteristics of the diabatic heating of the QXP in spring are analyzed. During the early stage of ASM onset, due to the soil thawing and snow melting process impacts, the interannual anomaly center of diabatic heating of the QXP moves from the valley areas of the central in March to the southeastern the QXP in April. Coming into May, after its range expands, it steadily locates on the southeastern the QXP. Due to the influence of the cooling effect of the snow albedo, the diabatic heating of the QXP in April remarkably decreased from 1960s to 1970s, then increased until to the end of 1990s. The diabatic heating of the QXP in May took on a decrease trend from 20th century 60s to 90s, which closely related to the snow melting and soil thawing processes over the QXP under the global warming background.(4) The relationships between the diabatic heating of the QXP and the intensity of East Asian summer monsoon are studied through EP flux and other methods. In spring of the strong/weak monsoon years, the atmospheric waves that were excitated by the heat forcing of the QXP propagated to the upper layer of troposphere and strengthened. In the spring of strong monsoon years, the atmospheric stationary waves, which located in upper layer of troposphere on the north side region of the QXP at 40-50°N, spread to the high latitude areas and the lower troposphere, and gradually strengthened; but in the weak monsoon years, they only spread to the high latitude areas. The QXP and 40-60°N areas on the north side of the QXP are the remarkable difference regions of EP flux divergence between the strong and weak monsoon years. In the early stage of ASM onset, the diabatic heating anomaly of the QXP in spring has the important influence on the west flow in the middle-high latitude. There is a signicifant negative correlation between the diabatic heating of the QXP and strength of East Asian summer monsoon.(5) The relationships between the diabatic heating of the QXP and the East Asian summer monsoon onset are investigated. The results indicate that, in the spring, the early stage of the East Asian summer monsoon onset, there are 30～60 days, quasi-bi-weekly, and 5-7 days atmospheric oscillation over the QXP. The locations and strength and transmission of the MJO are different in strong and weak monsoon years. The MJO propagates northward in strong monsoon years, but remains in situ in weak monsoon years. In strong monsoon years, the diabatic heating of the QXP prevents low-frequency oscillation and reproduces in the east and west sides of the QXP. But in weak monsoon years, the diabatic heating of the QXP strengthens the MJO over the QXP, and forms the quasi-south-north oscillation centered on thethe QXP. In the strong years, the interactions between the diabatic heating of the QXP and zonal wind MJO are remarkable, and persistent to the South China Sea summer monsoon building up. On the contrary, the interactions between diabatic heating and zonal wind MJO rapidly disappear, while South China Sea summer monsoon breaks out in the weak years.(6) The role of diabatic heating of the QXP in the Asian summer monsoon is investigated through numerical simulation experiments of RegCM3.0, The numerical simulation results show that:the soil moisture increase (decrease) over the QXP in the spring, may lead to weaken(strengthen) the upper tropospheric westerly flow near the East Asian jet region in April, and the south branch airflow of the QXP strengthen (weakened). It also may cause the anomaly of the trough-ridge systems in middle-high latitude areas in the early stage of ASM onset. In addition, the soil moisture increase (decrease) over QXP in the spring may lead to the change of the early summer precipitation over eastern China, such as an increase (decrease) in the amount of the monsoon precipitation over the lower-middle reaches of Yangtze River, Yellow River and the southeast areas of South China, and an decrease (increase) over other areas of South China and Huaihe River areas. All of the above indicating that, through influencing on the adjustment of south branch airflow of the QXP, westerly flow and trough-ridge systems during the early stage of ASM onset, the soil moisture change of the QXP in spring has the significant impact on the early summer precipitation over eastern China.更多还原