A modelling investigation into lake-breeze development and convection triggering in the Nam Co Lake basin,Tibetan Plateau

This paper uses the cloud resolving Active Tracer High-resolution Atmospheric Model coupled to the interactive surface model Hybrid in order to investigate the diurnal development of a lake-breeze system at the Nam Co Lake on the Tibetan Plateau. Simulations with several background wind speeds are conducted, and the interaction of the lake breeze with topography and background wind in triggering moist and deep convection is studied. The model is able to adequately simulate the systems most important dynamical features such as turbulent surface fluxes and the development of a lake breeze for the different wind conditions. We identify two different mechanisms for convection triggering that are dependent on the direction of the background wind: triggering over topography, when the background wind and the lake breeze have the same flow direction, and triggering due to convergence between the lake-breeze front and the background wind. Our research also suggests that precipitation measurements at the centre of the basins on the Tibetan Plateau are not representative for the basin as a whole as precipitation is expected to occur mainly in the vicinity of the topography.     

Summertime Thermally-Induced Circulations over the Lake Nam Co Region of the Tibetan Plateau

Performance of the fifth-generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model(MM5) over the Lake Nam Co region of the Tibetan Plateau was evaluated based on the data from five surface observation sites in 2006. The interaction between two thermally-induced circulations(lake breezes and mountain-valley winds) was also investigated. The results show that MM5 could be used to simulate 2-m air temperature; however, MM5 needs improvement in wind field simulation.Two numerical simulations were conducted to study the effect of the lake on the local weather and wind system. The original land cover of the model was used in the control experiment, and the lake was replaced with grassland resembling the area surrounding the lake in the sensitive experiment. The results of the simulations indicate that the lake enhanced the north slope mountain-valley wind and the mountain changed the offshore flow direction at the north shore. During the day, a clear convergent zone and a strong upflow were observed over the north slope of the Nyainq?entanglha Range, which may cause frequent precipitation over the north slope. During the night, the entire area was controlled by a south flow.     

Major ionic composition of precipitation in the Nam Co region, Central Tibetan Plateau

A total of 48 precipitation samples have been collected from individual precipitation events at the Nam Co Monitoring and Research Station for Multisphere Interactions (Nam Co Station, 30°47′N, 90°58′E; 4730 m a.s.l) located in the central Tibetan Plateau from August 2005 to August 2006. All samples were analyzed for major cations (NH₄⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺) and anions (Cl⁻, NO₃⁻ and SO₄²⁻), conductivity and pH. Precipitation pH values ranged from 6.03 to 7.38 with an average value of 6.59. The high pH is due to large inputs of crustal aerosols in the atmosphere, which contain a large fraction of carbonate. Ca²⁺ is the dominant cation in precipitation with an average value of 65.58 μeq L^− 1 (4.91–301.41 μeq L^− 1), accounting for 54% of the total cations in precipitation. HCO₃⁻ is the predominant anion, accounting for 62% of the total anions. When compared with data from a snow pit in the Zhadang Glacier 50 km away (5800 m a.s.l), major ion concentration in precipitation at the Nam Co Station is much higher due to local aerosol inputs. Correlation and empirical orthogonal function (EOF) analysis indicate that regional crustal aerosols and species from combustion emissions of residents are the major sources for these ions, lake salt aerosols from the Nam Co nearby and regional mineral aerosols from dry lake sediments are secondary sources, and sea salt contribution is the least due to the long distance transport.     

Improvements of the coupled WRF-Lake model over Lake Nam Co,Central Tibetan Plateau

A series of model sensitivity simulations are carried out to calibrate and improve the Weather Research and Forecasting Model coupled with a one-dimensional lake model (WRF-Lake) based on observations over Lake Nam Co. Using the default lake model parameters, the solution of WRF-Lake exhibits significant biases in both the lake thermodynamics and regional climatology, i.e., higher lake surface temperature (LST), earlier onset of summer thermal stratification, and overestimated near-surface air temperature and precipitation induced by the lake’s excessive warming and moistening impacts. The performance of WRF-Lake is improved through adjusting the initial lake temperature profile, the temperature of maximum water density (Tdmax), the surface roughness length, and the light extinction coefficient. Results show that initializing the water temperature with spring observation mitigates the LST overestimation and reduces the timing error of the onset of thermal stratification. By further adjusting Tdmax from 4 °C to the observed value of 3.5 °C, the LST increase from June to mid-July is enhanced and the buildup of thermal stratification is more accurately predicted. Through incorporating the parameterized surface roughness length and decreasing the light extinction coefficient, the model better reproduces the observed daily evolution of LST and vertical lake temperature profile. The calibrated WRF-Lake effectively mitigates the overestimation of over-lake air temperature at 2 m height and precipitation over regions downwind the lake. This suggests that an improved lake scheme within the coupled WRF-Lake is essential for realistically simulating the lake–air interactions and the regional climate over the lake-rich Tibetan Plateau.

     

2006—2011年西藏纳木错湖冰状况及其影响因素分析

湖冰是气候变化的指示器,为分析纳木错地区气候对湖冰冰情的影响,利用2006—2011年西藏纳木错(面积2000km2)和白马纳木错(面积1.45 km2)湖冰的观测资料,结合MODIS遥感影像资料分析了两个湖泊完全冻结日期、完全解冻日期、封冻期、湖冰厚度的状况及其与气温和风速的关系。纳木错湖湖冰冰情主要受气温的影响,同时也受风速的影响。纳木错湖的完全冻结日期集中在2月,完全解冻日期在5月中旬,封冻期平均天数为90 d,封冻期与冬季负积温具有较好的对应关系。面积较小的白马纳木错冰情的年际波动较大,其平均封冻期为124 d。纳木错湖的最大冰厚一般出现在3月,其厚度为58~65 cm。     

西藏羊八井和纳木错太阳辐射特征分析

为了研究青藏高原地区太阳短波辐射,特别是其与大气因子、天空状况定量关系对高原辐射收支及其对东亚天气气候的作用,利用羊八井和纳木错2个高原腹地站一年的高时间分辨率观测资料并结合NCEP/DOE再分析资料,分析了太阳总辐射的变化特征,并通过短波通量透过率分析了各因素对短波辐射的作用.结果表明,羊八井和纳木错晴天通量透过率均较高,分别达0.807和0.817,且变动性很小,与两站地形高度和纬度有明显关系;云的存在使两站接收到的短波辐射的差异减小,羊八井和纳木错的年平均通量透过率分别为0.674和0.675,高原云的存在与变化使通量透过率产生强烈的时间变化.与地面观测的太阳短波辐射资料相比,在完全无云的晴天,NCEP同化太阳短波辐射资料对高原地区的模拟效果较好,但存在偏小的误差,羊八井和纳木错分别平均偏小5.74％和8.49％;考虑云天后,NCEP再分析值与实测值对比关系的离散度大,并总体偏高,在应用时需特别注意.     

Some characteristics of cumulus convection over the Tibetan Plateau

The diagnostic model of the cumulus convection proposed by Yanai et al. (1973) was applied to the atmosphere over the Tibetan Plateau, and used to estimate the vertical mass flux, entrainment and detrainment, excess temperature and moisture, liquid water content, and condensation and precipitation rates of highland cloud clusters. The re-sults illustrated that in clouds over the Tibetan Plateau, the water vapor condensation rate, liquid water content, and efficiency of the rain generation process are less than those in the tropics (represented by the Marshall Islands region). Therefore, the condensational latent heat released over the Tibetan Plateau, overall, is much smaller than that in the tropics. The water vapor and liquid water detrainment from shallow nonprecipitating cumulus clouds, and their entrainment into deep cumulus clouds, serve as a growing mechanism for the deep precipitating cumulus towers over the Tibetan Plateau. It should be noted that there is a stronger detrainment of liquid water from cumulus clouds and a stronger re-evaporation rate in environment. The process of the condensation-detrainment-re-evaporation-entrainment is repeatedly in progress. It would play an important role in maintaining of cumulus convection on the condition that the supply of moisture is not plentiful over the Tibetan Plateau.The analyses also showed that the cloud mass flux Mc over the Tibetan Plateau is less, and the large-scale av-erage upward motion is much less than those over the Marshall Islands. Stronger compensating downward motion in the cloud environment over the Tibetan Plateau, responsible for the area’s strong environmental heating rate was re-vealed, and would link to the stability of the South Asian High in summer.     

Turbulent flux observations and modelling over a shallow lake and a wet grassland in the Nam Co basin,Tibetan Plateau

The Tibetan Plateau plays an important role in the global water cycle and is strongly influenced by climate change. While energy and matter fluxes have been more intensely studied over land surfaces, a large proportion of lakes have either been neglected or parameterised with simple bulk approaches. Therefore, turbulent fluxes were measured over wet grassland and a shallow lake with a single eddy-covariance complex at the shoreline in the Nam Co basin in summer 2009. Footprint analysis was used to split observations according to the underlying surface, and two sophisticated surface models were utilised to derive gap-free time series. Results were then compared with observations and simulations from a nearby eddy-covariance station over dry grassland, yielding pronounced differences. Observations and footprint integrated simulations compared well, even for situations with flux contributions including grassland and lake. The accessibility problem for EC measurements on lakes can be overcome by combining standard meteorological measurements at the shoreline with model simulations, only requiring representative estimates of lake surface temperature.     

基于静止卫星的青藏高原及周边地区夏季对流的气候特征分析

利用2010-2014年静止气象卫星FY-2E的红外TBB资料,分析了夏季青藏高原（高原）及周围地区对流的气候特征。分析表明,5月,高原最主要的对流发生在东部边缘。6月,随着亚洲夏季风爆发,最强的对流（强对流）发生在高原的东南侧。7-8月,强盛的西南风给高原中东部部分地区带来丰沛的水汽,高原的东南部形成一条对流（强对流）活跃带。在高原西部,对流发生频率大于6%的区域出现在西部南麓的时间约为37候,并于7月底-8月初到达最北。在高原中部,对流（强对流）开始活跃的时间为6月上旬（中旬）,维持整个盛夏,并分别经历3次向北推进,最北约到达34°N。在高原东部,5月底开始对流都处于相对活跃期,有3次（两次）对流（强对流）的北进。高原对流（强对流）发生频率存在两个季节内变率大值区,分别位于高原中南部雅鲁藏布江中段和高原东南部西藏、青海、四川三省交界处。对流发生频率的第一模态主要是高原东南部和南部的印度季风区对流的反向模态,第二模态则体现了高原西部和印度大陆80°E以西地区与南亚大陆80°E以东地区的对流发生频率的三极型变化。      

青藏高原东部地区降水的定量数值预报初探

运用LASGη坐标有限区域数值预报模式对青藏高原东部的川、渝两省市1988年的18次强降水过程进行了不同方式下的定量降水预报的数值模拟,并根据中国气象局制定的有关规定对模拟预报结果进行了评估.该区域地形复杂,天气系统多变,对数值预报模式有较高的要求,LASGη坐标模式以其比较完善的动力框架和物理过程处理方法获得了较好的预报结果,其对小雨、中雨、大雨预报的TS评分可超过主观预报的水平.     

卷云对青藏高原的射出长波辐射影响的研究

本文叙述了一种求解卷云射出长波辐射的方法。通过建立卷云示迹模式,用累加法进行分析,并通过分析卷云的一些微物理特征,假设卷云含水量垂直分布模式,由三层近似化筒求得射出长波辐射通量密度。根据模式计算得到:1.与卫星观测青藏高原高云地区对应射出长波辐射通量密度基本一致的结果。2.卷云底高度及卷云厚度与射出长波辐射通量密度相关的一些有意义的结果,并讨论了青藏高原上卷云影响的特殊情况。     

青藏高原那曲地区夏季对流云结构及雨滴谱分布日变化特征

利用青藏高原东侧甘孜站Ka波段云雷达2019年6—8月观测资料,对该地区非降水云垂直结构特征进行了分析。结果表明：（1）甘孜非降水云中单层云的出现率为78.3%,高于两层云的出现率18.3%和多层云的出现率3.4%。分不同高度云来看,低云的出现率为46%,中云和高云各占27%,当云层数增加时,中云和高云的出现率增加。（2）云的出现率具有白天小、夜间大的日变化特征,云层数增多后,上层云出现率的日变化特征减弱;地形对云出现率的日变化有一定影响。（3）云底高和云顶高的垂直分布结构多为双峰形态,当出现三层云时,下层云的垂直结构为单峰形态。（4）甘孜云厚呈现出云压缩现象：单层云的平均云厚约为3.8 km;两层云的下层云平均云厚约为2.5 km,上层云平均云厚约为1.5 km;三层云的下层云平均云厚减小至约1.8 km,上层云平均云厚减小至约1.2 km,中层云平均云厚最小,约为1 km;云压缩现象随云层数增加而愈发明显。地基云雷达展示了局地云探测的优势,有益于高原云探测和研究。

     

夏季青藏高原地面热源和高原低涡生成频数的日变化

通过1981—2010年NCEP/NCAR再分析资料,分析出夏季青藏高原地面热源具有强烈的日变化,白天高原是强热源,夜间高原地面转变为弱热汇,日较差可达420 W·m~(-2),呈由西向东递减分布。其中地面感热和潜热加热的日变化均十分明显,日较差分别可达300 W·m~(-2)和200 W·m~(-2);感热加热的日变幅由西北向东南递减,而潜热加热由南向北递减。同时,利用人工识别的高原低涡数据集初步分析了夏季高原低涡生成频数的日变化,发现夜间生成的高原低涡频数略高于白天,其中00 UTC的低涡源地主要在西藏那曲和林芝(工布江达),12 UTC低涡源地主要在西藏那曲和青海玉树。     

青藏高原那曲地区一次对流云降水的数值模拟

利用NCEP FNL再分析资料为初始场,通过WRF中尺度数值模式(V3.9.1版本)对2015年8月26~27日青藏高原那曲地区一次对流云降水过程进行了模拟,分析了不同积云对流参数化方案和云微物理参数化方案组合对本次降水过程中降水量、环流场、雷达反射率以及云微物理特征模拟效果的影响。结果表明:WRF模式能较好地模拟出本次降水的时空变化特征,但不同参数化方案组合各有优势,总体而言,Grell-Devenyi+SUB_YLIN和Grell-Freitas+SUB_YLIN组合模拟性能最优。本次对流云降水以冰相过程为主,雪粒子贡献最大,暖云粒子对降水的影响并不明显。从云微物理过程的时间演变可看出,性能最好的SUB_YLIN方案能合理模拟降水过程中雪粒子与冰晶粒子间的转换过程,雪粒子可在凝结过程中释放潜热促使对流运动发展,也可通过融化过程促进降水发生,对流层高层冰晶粒子凝华产生的潜热释放亦为深对流的发展创造了有利条件。      

青藏高原水循环中高原低涡及多季风交汇的作用：进展与展望

由于青藏高原特殊而强大的动力和热力效应，东亚季风、印度季风和高原季风多季风交汇于此。受多季风交汇影响，青藏高原低涡影响水循环的过程极为复杂，而高原水循环的异常往往造成高原及周边乃至我国中东部地区频发灾害性天气过程，因此一直是国内外学者关注的热点和难点问题。本文系统回顾了国内外关于高原低涡活动特征、结构特征、生成发展机制等方面研究进展，并从低涡的水汽输送、降水和云-降水物理等角度概括了高原低涡参与水循环过程研究成果，特别在总结东亚季风、印度季风和高原季风相互作用研究的基础上分析了多季风交汇对高原水循环的影响。最后对当前高原低涡在多季风作用下影响水循环过程方面存在的问题进行了讨论，并展望了未来研究方向。     

Dynamic and Numerical Study of Waves in the Tibetan Plateau Vortex简

In terms of its dynamics, The Tibetan Plateau Vortex （TPV） is assumed to be a vortex in the botmdary layer forced by diabatic heating and friction. In order to analyze the basic characteristics of waves in the vortex, the governing equations for the vortex were established in column coordinates with the balance of gradient wind. Based on this, the type of mixed waves and their dispersion characteristics were deduced by solving the linear model. Two numerical simulations with triple-nested domains--one idealized large-eddy simulation and one of a TPV that took place on 14 August 2006---were also carried out. The aim of the simulations was to validate the mixed wave deduced from the governing equations. The high-resolution model output data were analyzed and the results showed that the tangential flow field of the TPV in the form of center heating was cyclonic and convergent in the lower levels and anticyclonic and divergent in the upper levels. The simulations also showed that the vorticity of the vortex is uneven and might have shear flow along the radial direction. The changing vorticity causes the formation and spreading of vortex Rossby （VR） waves, and divergence will cause changes to the n~otion of the excitation and evolution of inertial gravity （IG） waves. Therefore, the vortex may contain what we call mixed ：inertial gravity-vortex Rossby （IG-VR） waves. It is suggested that some strongly developed TPVs should be studied in the future, because of their effects on weather in downstream areas.     

应用NASA MERRA再分析资料对一次高原切变线的诊断分析

在引发中国东部夏季降水的天气系统中,高原低值系统扮演着十分重要的角色,其中高原低涡与高原切变线对强降水的协同作用是高原天气影响的一种常见样式,预报员将其称为低涡切变暴雨。本文回顾了高原低涡、切变线及其暴雨的研究历史和当前研究所取得的最新成果,重点探讨了人工智能应用、诊断计算、动力理论以及数值模式等多方法研究高原切变线与高原低涡的关系、相互作用过程以及诱发暴雨机理等科学问题,并基于低涡、切变线暴雨的最新研究成果和相关理论方法、技术手段的发展应用趋势,提出这一研究领域值得关注的一些新方向。由于目前对这两类几何形状迥异但物理属性相近的高原低值天气系统关系的认知仍存较大分歧,两者相互作用进而引发高影响天气过程的物理机理尚不十分清楚,高原低涡、切变线气候学统计结果的差异还相当明显。因此,对这一研究领域的深入探索与交叉拓展,不仅对推动青藏高原天气、气候影响的理论发展有重要科学意义,对高原及下游灾害性天气、气候业务能力的提升亦有较大应用推广价值。

     

冷空气对高原低涡移出青藏高原的影响

在对1998—2004年冷空气影响高原低涡移出青藏高原 (以下简称高原) 观测事实分析的基础上, 利用NCEP再分析资料对2002年8月12—14日托勒低涡移出高原的位涡进行诊断分析, 并通过数值试验揭示了托勒低涡移出高原的冷空气侵入特征和影响机理。结果表明:这次托勒低涡是受我国东北冷空气影响, 有高位涡空气伸入低涡区, 使冷空气迫近暖湿空气, 低涡处在斜压不稳定增强情况下移出高原的。在低涡区域没有冷空气或我国东北不存在冷温度槽情况下, 将会使伸向高原东北部的冷空气主力偏东、减弱, 使低涡受到我国东北冷空气影响减弱, 斜压不稳定减弱, 从而使高原低涡移出高原的速度减慢, 低涡强度减弱, 尤其是我国东北冷温度槽的影响更为明显, 在我国东北没有冷温度槽存在的情况下, 低涡24 h内西退, 在高原边缘徘徊。     

青藏高原加热对东亚地区夏季降水的影响

东亚地区降水主要集中在夏季,是亚洲夏季风系统的重要特征.本文利用NCEP再分析资料和CRU的降水资料,分析了青藏高原非绝热加热对东亚夏季降水的影响.结果表明,东亚地区夏季降水的分布形势与青藏高原非绝热加热变化有很好的相关关系.由于高原非绝热加热可在亚洲东部沿海地区强迫出类似Rossby波列的大气环流低频振荡结构,而此低频波可以影响到西太平洋副热带高压的形态和位置变化,从而使得东亚夏季降水的形势发生变化.而青藏高原非绝热加热的形态从春季到夏季有很好的持续性,春季高原加热与夏季东亚的降水形势分布也有很好的相关.本研究中采用的青藏高原非绝热加热指数可作为东亚夏季降水预测的一个指标,亚洲季风降水不仅受赤道太平洋海温的影响,青藏高原地区的非绝热加热对其也有显著的影响作用.     

Study on Ozone Change over the Tibetan Plateau

This paper reviewed the main results with respect to the discovery of low center of total column ozone (TCO) over the Tibetan Plateau (TP) in summer, and its formation mechanism. Some important advances are summarized as follows: The fact is discovered that there is a TCO low center over the TP in summer, and the features of the background circulation over the TP are analyzed; it is confirmed that the TP is a pathway of mass exchange between the troposphere and stratosphere, and it influences the TCO low center over the TP in summer; models reproduce the TCO low center over the TP in summer, and the formation mechanism is explored; in addition, the analyses and diagnoses of the observation data indicate that not only there is the TCO low center over the TP in summer, but also TCO decrease trend over the TP is one of the strong centers of TCO decrease trend in the same latitude; finally, the model predicts the future TCO change over the TP.