On mountain wave drag over complex terrain

Summary Mountain wave drag is calculated for rotating, stratified, nonhydrostatic Boussinesq flow over a mountain ridge using linear theory for a variety of mountain profiles representing complex/irregular terrain. The inclusion of a sinusoidal corrugation to the familiar witch-of-Agnesi profile creates a stegosaurus profile. The associated drag is greatly enhanced for mesoscale mountains when the corrugation wave-number matches that for the dominant inertia-gravity wave contribution to the cross-mountain surface pressure gradient. Similarly, increasing the jaggedness (by decreasing the exponentb) increases the drag for mesoscale mountains whose topographic spectral intensity,M(k), has the form of a power law:M(k)=mk ^–b wherek is the zonal wavenumber.Spectral analysis of one-kilometer resolution topographic data for the Appalachian Mountains suggests that a power law profile withb=1.7 accurately represents the topographic spectral intensity and that it yields good estimates of the drag.The application of these results to the parameterization of mountain wave drag in general circulation models is discussed.With 7 Figures     

Modifications of the structure of cold fronts over the foreland and in a mountain valley

Summary The investigations during the German Frontal Experiment 1987 show the modification of structure and movement of cold fronts in the vicinity of the Alps and in an Alpine valley. The two cold fronts that moved nearly parallel to the mountains were modified at the mountain barrier by the channelling effect. In those cases the postfrontal air flowed into the Inn valley from the valley's exit up-valley. The upper and lower parts of the fronts became detached, and the structure of the lower part changes on its way from the foreland to the mountain barrier.In contrast the fourth cold front, which moved from north-west to south-east, crossed the mountains without appreciable deformation although it suffered some degree of blocking and retardation.With 7 Figures     

Lidar and Sun photometer observations of atmospheric boundary-layer characteristics over an urban area in a mountain valley

We present measurements of the vertical aerosol structure and the aerosol optical depth in the lower troposphere performed above the city of Sofia (an urban area situated in a mountain valley), western Bulgaria by means of a ground-based aerosol lidar operating continuously for a number of years. The lidar measurements were accompanied by measurements of the aerosol optical depth (AOD) in the visible and near infrared regions of the spectrum performed in October 2004 using Microtops II radiometers. The maximum values of the AOD were found to occur 1–2 h before the complete development of the atmospheric boundary layer, i.e. during the residual layer destruction, which confirms our hypothesis concerning the slope circulation effect on the processes taking place in the atmospheric boundary layer. The AOD values obtained by the lidar are lower than those taken by the sun photometer. Further, the AOD exhibits two different types of behaviour. In the case of a ‘clear atmosphere’ (i.e. in the absence of volcanic eruptions and/or dust transport from the Sahara) most of the aerosol accumulated within the atmospheric boundary layer over the urban area considered. The combined use of the two instruments allows the comparison between the optical characteristics of the atmospheric aerosol (e.g. aerosol extinction coefficient, etc.) obtained by the lidar and through an independent method (sun photometer).     

江淮流域持续性暴雨过程前期的欧亚波列特征及其模拟研究

利用50余年的NCAR/NCEP再分析资料,对江淮流域持续性暴雨过程前期的环流特征进行了相关与合成分析。发现在过程前2—1周的对流层中高层西风带上,中西亚、中国东部沿海和白令海南侧相继有低压槽发展,呈现显著的罗斯贝波列信号特征。分析表明,正是该欧亚波列的东传和消失伴随的环流调整,导致了有利于持续性暴雨产生的稳定环流的形成。进一步采用区域数值模式检验了该波列特征对后期持续性暴雨的作用和影响过程,以1991年一次典型的江淮流域持续性暴雨过程为例,根据欧亚波列信号特征设计一组初始流场扰动,进行初值集合模拟试验。结果表明,过程强雨带及其环流背景的模拟对于初始场上欧亚波列信号区的扰动特征是相当敏感的,该初始扰动对模拟环流的后继影响,与持续性暴雨期间稳定环流特征的建立和维持确有密切关系。     

Results from the MERKUR experiment: Mass budget and vertical motions in a large valley during mountain and valley wind

Summary Based on measurements made in March 1982 in the Inn valley during the MERKUR experiment, an attempt was made to compute the mass budget of a large alpine valley during periods of mountain and valley wind.The computations come from measurements of the alongvalley mass flux and assumptions on the fluxes in the slope layer and tributaries.Vertical motions in the valley's atmosphere have been evaluated from the mass budget computations. These motions, including the subsidence which compensates for daytime upslope winds and the subsidence which compensates for the valley wind flowing into tributaries during the day, are of great importance for the understanding of the thermal circulation.The results allow better estimation of vertical advection, which contributes to the budgets of momentum and energy.With 8 Figures     

The Surface Friction and the Flow over Mountain

The flow over mountain is quite complicated. There are a lot of papers on this problem and a lot of progresses have been made. However, in the most of these papers, just the dynamics contributions of mountain have been analysed; the effect of the friction is often neglected. Since the frictional effect is always associated with flow, especially when it flows over the mountain. The study shows that the friction is small in the magnitude but it is not a negligible effect because it changes the features of the flow. In the case of super-or sub-critical flow, there are two extremes: one maximum, one minimum of the fluid surface on the lee-side of the mountain, while in the inviscid fluid, there is just one extreme. The frictional effect should neither be too strong nor too weak to make the situation happened according to the investigation of this paper.     

Airflow over a mountain and the convective boundary layer

In this paper, the analytical model coupling the convective boundary layer (CBL) with the free atmosphere developed by Qi and Fu (1992) is improved. And by this improved model, the interaction between airflow over a mountain and the CBL is further discussed. The conclusions demonstrate: (1) The perturbation potential temperatures in the free atmosphere can counteract the effect of orographic thermal forcing through entraining and mixing in the CBL. If _M > _F , the feedback of the perturbation potential temperatures in the free atmosphere is more important than orographic thermal forcing, which promotes the effect of interfacial waves. If _M < _F , orographic thermal forcing is more important, which makes the interfacial height and the topographic height identical in phase, and the horizontal speeds are a maximum at the top of the mountain. (2) The internal gravity waves propagating vertically in the free atmosphere cause a strong downslope wind to become established above the lee slope in the CBL and result in the hydraulic jump at the top of the CBL. (3) With the CBL deepening, the interfacial gravity waves induced by the potential temperature jump at the top of the CBL cause the airflow in the CBL to be subcritical.     

夏季江淮气旋的结构

本文用1980年6月23—25日和8月23—25日的常规观测资料,分析两次江淮气旋的运动场和热力场的平均结构.分析中使用了活动坐标网格,对风、温度、湿度和降水等气象要素进行合成.研究指出,这类气旋的波长为2000—2500公里,水平闭合环流的尺度为100—1500公里,垂直伸展高度达7—8公里左右,气旋轴线随高度显著地向北倾斜.气旋性环流和气旋性涡度在对流层中层的700毫巴附近最强.锋区在对流层上部明显,在对流层低层反而较弱;在气旋中心附近及其北侧,在700毫巴附近及以下仅有一狭窄的冷带.强降雨区出现在低     

青藏高原臭氧谷极端事件的定义及其特征

利用1979—2016年ERA-interim逐日再分析资料,定义了青藏高原臭氧谷(Ozone Valley over the Tibetan Plateau,OVTP)极端和普通强(弱)事件,并讨论了其特征。结果表明:1) OVTP极端强事件在夏秋季节多发,10月最多,频率达2. 0%; OVTP普通强事件在春夏季多发,7月最多,频率达1. 7%。OVTP极端弱事件在秋冬季多发,12月最多,频率达3. 8%; OVTP普通弱事件在冬季多发,1月最多,频率达2. 0%。2) OVTP极端强事件出现频率显著增加(0. 004%·a~(-1)),极端弱事件出现频率显著减少(-0. 015%·a~(-1))。OVTP普通事件的变化均不显著。3) OVTP极端强事件的面积和强度均在秋季最大,10月达到最大值,面积为4. 3×10~5km~2,强度为1. 5×10~5t; OVTP普通强事件的面积和强度均在夏季最大,7月达到峰值,面积为1. 7×105km~2,强度为4. 1×10~3t。OVTP极端弱事件的面积和强度在春夏较小,4月达到最小值,面积为3. 2×10~4km~2,强度为1. 1×10~2t; OVTP普通弱事件的面积和强度在春夏秋均较小,4月和10月达到极小值,4月面积为2. 5×10~4km~2,强度为68 t,10月面积为2. 2×10~4km~2,强度为97t。4) OVTP极端和普通强事件的面积(强度)均呈显著增大(增强)趋势,极端强事件的面积达2. 5×10~2km~2·a~(-1),强度达2. 5×10~2t·a~(-1),普通强事件的面积达4. 5×10~2km~2·a~(-1),强度达4. 5 t·a~(-1)。极端和普通弱事件的面积(强度)均呈显著减小(减弱)趋势,极端弱事件的面积达-1. 7×10~4km~2·a~(-1),强度达-7. 0×10~3t·a~(-1),普通弱事件的面积达-2. 3×10~3km~2·a~(-1),强度达-2. 7×102t·a~(-1)。     

Characteristics and evolution of atmospheric circulation patterns during Meiyu over the Jianghuai valley

This study focuses on the evolution of large-scale circulations before and after the beginning of the Meiyu and analyzes the formations of the typical vertical circulation pattern associated with Meiyu and its relationship with the Meiyu rainband. Results show that the typical vertical circulation pattern during the Meiyu season is characterized by the ??two-leg??-type pattern in the vorticity field, a typhoon eye-like structure of the equivalent potential temperature field, as well as the sharp gradient of the equivalent potential temperature zone, i.e., the Meiyu front are typically presented in the Meiyu season. Tracking its evolution process, we find that the typical vertical circulation pattern is built at late March and early April along with the rainband locating at the area south to the Yangtze River. This typical pattern and the rainband both advance northward affecting Jianghuai valley since the beginning of Meiyu. Moreover, the typical vertical circulation pattern derived from Meiyu season has been formed in April and corresponds to the reverse of the land-sea thermal contrast between the Eastern Asia and western Pacific Ocean, demonstrating the close relationship of the movement between the rainband and the march of the East Asian subtropical summer monsoon.     

淮河流域持续性强降水过程的环流变化特征

利用淮河流域4省20：00-次日20：00逐日降水量资料,界定了1961-2006年淮河流域持续性强降水过程,并利用NCEP资料,应用合成分析方法对强降水过程和前期环流特征进行了研究。结果表明,从持续性强降水开始之前到强降水过程中乌拉尔山附近阻高减弱,贝加尔湖以北到鄂霍次克海附近的阻高偏强;西太平洋副热带高压在500hPa上表现为西伸北抬,但在850hPa上西伸明显,基本无北抬;南亚高压范围明显扩大,尤其是淮河流域以北地区200hPa高度明显增大,在中国东部到日本上空200hPa急流中心也有明显变化。850hPa西南急流的建立与西太平洋副热带高压的西伸有关,同时乌拉尔山附近高压减弱,冷空气南下,东西气压梯度增大也有利于西南急流的建立。     

南亚高压对青藏高原臭氧谷的动力作用

利用臭氧观测光谱仪/太阳紫外线后向散射仪(TOMS/SBUV)的臭氧总量资料和SAGEⅡ臭氧廓线资料计算了青藏高原区纬向偏差(一个量减去该量的纬圈平均值,定义为该量的纬向偏差)臭氧总量的逐月变化和高原区150-50 hPa高度纬向偏差臭氧量的变化,二者相关显著,相关系数为0.977.由于在150-50 hPa高度,夏季青藏高原臭氧谷最强,南亚高压最活跃,因此,青藏高原臭氧谷与南亚高压可能存在联系.在运行WACCM3模式时,将青藏高原地形高度削减至1500 m,在150-50 hPa高度南亚高压和青藏高原臭氧谷仍存在;该高度上南亚高压强度变小,青藏高原臭氧谷也减弱;南亚高压季节移动发生改变,青藏高原臭氧谷季节变化也随之改变.因此,推测南亚高压可能对青藏高原臭氧谷有重要作用.接着分析了模式输出的青藏高原区经向、纬向和垂直方向的臭氧输送.在南亚高压季节变化的不同阶段和不同方向上,环流对青藏高原臭氧谷的作用明显不同.150-50 hPa,南亚高压上高原时,纬(经)向输送使青藏高原臭氧谷加深(变浅),垂直输送在低(高)层使青藏高原臭氧谷加深(变浅),总的动力作用使青藏高原臭氧谷减弱;南亚高压稳定在高原上空时,纬(经)向输送使青藏高原臭氧谷变浅(加深),垂直输送在中(底和顶)层使青藏高原臭氧谷加深(变浅),总的动力作用使青藏高原臭氧谷加深;在南亚高压从高原撤退时,纬(经)向作用使青藏高原臭氧谷加深(变浅),垂直作用使青藏高原臭氧谷变浅,总的动力作用使青藏高原臭氧谷中(底和顶)层加深;当南亚高压移至热带太平洋时,南亚高压对高原区臭氧影响较弱.     

伊犁河谷和天山北坡暴雨过程水汽特征分析

选取4次伊犁河谷、天山北坡暴雨天气过程,利用地面逐时降水、常规、NCEP/NCAR 1°×1°再分析及地基GPS遥感的大气水汽总量资料(GPS-PWV),通过合成分析方法得到暴雨期间大气环流的基本配置,阐明了伊犁河谷、天山北坡地区强降水期间环流形势及水汽输送的异同,结果表明:(1)强降水过程中暴雨区上空200 h Pa强辐散气流、500 h Pa槽前正涡度平流、西南气流利于垂直运动的发展,低层偏西、偏东和偏北气流为暴雨区提供水汽和不稳定能量,低层辐合、高层辐散,配合地形辐合抬升,上升运动进一步增强,造成强降水发生;(2)深厚的西西伯利亚低涡低槽系统移速缓慢,停滞时间长,造成强降水前暴雨站增湿时间更长,比较发现强降水发生前暴雨站GPS-PWV均存在1~3 d的增湿过程,暴雨期间测站GPS出现明显跃变,峰值可达到气候平均值的2倍左右;(3)GPS大气可降水量的演变与大尺度的水汽输送、聚集有较好的对应关系,但GPS高值区并不代表降水大值区,还应和动力热力等条件综合判断降水的强弱。     

非均相化学过程在青藏高原臭氧低谷形成中的作用

分析SAGE II资料发现:青藏高原对流层顶最高不超过17.6 km,夏季平均在17 km以下:来自对流层中低层的物质很少能被输送到17.5 km以上,来自对流层中低层的输送不会长时间持续地影响17.5 km以上的区域。青藏高原以及同纬度地区在15 20 km高度温度低于210 K,非均相化学反应可能在此起着重要作用。夏季青藏高原臭氧低谷的形成高度主要是15—20km,而且是长时间持续性的。分析结果显示:15—20km非均相化学过程在夏季青藏高原臭氧低谷的形成中可能起重要作用,特别在17.5—20km高度其所起的作用可能是主要的;而在17.5 km以下,从以往的研究可以知道青藏高原的动力和热力作用对夏季青藏高原臭氧低谷的形成起着主要作用。在春季和秋季,青藏高原上15—20km的臭氧"亏损"也可能是由于非均相化学过程造成的,使得青藏高原臭氧低谷每年从4月持续到10月。并且,影响非均相化学过程的主导因素可能是温度。     

欧亚大陆春季融雪与长江流域夏季降水的可能联系

利用1979—2007年全球月平均的积雪水当量资料,定义了春季积雪水当量增量指数,该指数可以较为直观地反映春季融雪的情况。通过这一纽带,分析了欧亚大陆春季融雪与长江流域夏季降水之间的联系。研究表明:欧亚大陆春季融雪与中国长江流域夏季降水是负相关关系,这与高原积雪的影响是不一致的。春季融雪量的减少,使得欧亚大陆北部夏季剩余积雪偏多,夏季融雪增多。融雪的局地效应使得土壤湿度增加,加大了欧亚大陆南北热力差异。从而,夏季中纬度的纬向风切变增大,对流层上层的副热带西风急流增强,副热带高压增强西伸,但是北抬受到抑制。长江流域位于异常西南暖湿气流与冷空气的辐合带上,上升运动活跃,有利于降水偏多。     

第二松花江流域夏季降水年际增量预测模型研究

利用1980—2016年第二松花江流域(SSR)夏季(6—8月)平均降水量资料、NCEP/NCAR再分析月平均环流场资料、NOAA的月平均海温场资料,采用年际增量预测方法,通过分析与SSR夏季降水年际增量相关的环流及海温,确定了超前12个月内的6个预测因子,包括:11月东亚200 hPa纬向风、12月西藏高原-2指数、12月赤道中东太平洋200 hPa纬向风、2月印度洋海温、10月西太平洋暖池海温、4月东亚100 hPa经向风。在此基础上利用这6个预测因子,利用1980—2010资料建立SSR夏季降水年际增量的统计预测模型,最后根据年际增量给出SSR夏季降水的预测结果。经检验,1981—2010年SSR夏季降水年际增量的预测拟合系数是0.83,SSR夏季降水预测结果拟合系数为0.67,SSR夏季降水预测结果相对均方根误差为15%,均通过了显著性检验;对2011—2016年进行试报实验,该模型也很好地预测出降水的年际增量变化趋势,除2014年以外,SSR夏季降水预测结果相对均方根误差绝对值都控制在23%以内,2016年仅为-9.9%。因此,通过预测降水的年际增量,进而再预测降水的方法,具...     

海陀山冬奥气象综合观测平台及研究进展

气象保障是历届冬季奥林匹克运动会（冬奥会）成功举办的最重要条件之一。延庆赛区（位于延庆海陀山区）地形复杂、山高坡陡、垂直落差大,因而观测难度大、气象数据稀少,成为影响赛区精细化预报的核心因素,对冬奥气象服务提出了严峻挑战。为此,北京市气象局组织开展了冬奥海陀山综合气象观测,针对该区复杂地形和多尺度气象影响系统构建了包含中、小、微尺度的三维立体实时综合观测平台,观测要素涵盖了三维风场、温湿度场、云和能见度、近地面湍流与辐射等。综合观测将提高对复杂山区高影响天气特征和机理的认知,为改进和提高预报模式提供数据支撑。文中系统介绍了海陀山冬奥气象综合观测平台的科学目标、观测布局、观测内容以及观测平台在降雪过程中的初步应用,最后阐述了基于该观测进一步开展的研究计划。     

分层气流条件下地形降水的二维理想数值试验

利用WRF v3.5中尺度数值模式,在条件不稳定层结下,针对分层气流(基本气流风速和大气湿浮力频率呈二层均匀分布)过山时,地形对降水的影响进行了多组二维理想数值试验,以研究不同高度、尺度山脉和不同方向基本气流对降水形态和分布的影响。模拟结果表明,地形重力波触发对流是地形降水的主要机制之一,地形波的特征(波长、振幅)和传播均受到地形和基本气流的影响,其中,强基本气流流经高而陡峭的山脉时,更容易在其背风坡捕捉到重力波,地形降水呈现多种模态,反之亦然;当改变基本气流与山脉交角时,主要通过影响地形强迫抬升速度、基流对波动稳定性发展来进一步影响地形降水的强度和分布。     

21世纪前期长江中下游流域极端降水预估及不确定性分析

在全球变暖背景下,极端降水的频率、强度以及持续时间均在显著增加,尤其是对于气候变化敏感的长江中下游流域。由于模式本身、温室气体排放情景以及自然变率存在较大的不确定性,因此未来预估变化的不确定性一直备受关注。为了能够得到对于未来极端降水更为准确的预估结果,使用NEX-GDDP（NASA Earth Exchange Global Daily Downscaled Projections）提供的19个CMIP5降尺度高分辨率数据（0.25°×0.25°）,给出21世纪前期（2016—2035年）长江中下游流域极端降水的可能变化。根据长江中下游流域178个气象站1981—2005年的逐日降水量数据,计算了能够代表极端降水不同特征的指数,在评估模拟能力的基础上给出了21世纪前期RCP4.5情景下极端降水的变化。结果表明,降尺度结果对长江中下游流域极端降水有很好的模拟能力,除R90N外,所有模式模拟其余指数的空间结构与观测的相关系数均超过了0.6。其中所有模式模拟PRCPTOT和R10的相关系数均超过0.95。21世纪前期,长江中下游地区降水趋于极端化,尤其是在流域的西部地区。极端降水日数的变化在减少,表明对于极端降水的贡献主要来自于极端降水日的较大日降水量,而非极端降水日数。未来预估不确定性的大值区主要位于流域的南部地区,流域的西部地区不确定性较低,西部地区极端降水的增加应该受到更多的重视。