NUMERICAL STUDY OF OROGRAPHIC INFLUENCE ON LOW FREQUENCY OSCILLATION

Orographic effects on monthly-and seasonal-scale low frequency oscillation are investiged in terms of a five-level global spectral model with a triangle truncation at wavenumber 10 that contains relatively full physical parameterization, followed by analysis and comparison of the lowpass filtered data separately obtained from models with and without orography. Results show that remarkable seasonal characteristics are displayed in the orographic forcing-generated low frequency wavetrain on monthly and seasonal scales. It is found that the Northern Hemisphere summer orography-produced tropical heating acts as source of the low-frequency wavetrain for both hemispheres. Besides, the simulations indicate that the orographic wavetrain perturbation can give rise to the anomaly in the equatorial zonal flow, whose transient forcing will cause a new wavetrain in the Southern Hemisphere, thus completing the cross-equatorial propagation of the northern wave in interhemispheric action.     

Influence of the orographic roughness of glacier valleys across the Transantarctic Mountains in an atmospheric regional model

Glacier valleys across the Transantarctic Mountains are not properly taken into account in climate models, because of their coarse resolution. Nonetheless, glacier valleys control katabatic winds in this region, and the latter are thought to affect the climate of the Ross Sea sector, frsater formation to snow mass balance. The purpose of this paper is to investigate the role of the production of turbulent kinetic energy by the subgrid-scale orography in the Transantarctic Mountains using a 20-km atmospheric regional model. A classical orographic roughness length parametrization is modified to produce either smooth or rough valleys. A one-year simulation shows that katabatic winds in the Transantarctic Mountains are strongly improved using smooth valleys rather than rough valleys. Pressure and temperature fields are affected by the representation of the orographic roughness, specifically in the Transantarctic Mountains and over the Ross Ice Shelf. A smooth representation of escarpment regions shows better agreement with automatic weather station observations than a rough representation. This work stresses the need to improve the representation of subgrid-scale orography to simulate realistic katabatic flows. This paper also provides a way of improving surface winds in an atmospheric model without increasing its resolution.     

THE SENSITIVITY OF THE NUMERICAL SIMULATION TO OROGRAPHY SPECIFICATION IN THE LOWRESOLUTION SPECTRAL MODEL－PART II: IMPACT OF THE SMOOTHED OROGRAPHY AND RIPPLES ON SIMULATIONS

In order to investigate the impact of the smoothed orography and the spurious orographic ripples on simu-lations in the low-resolution spectral model, three different numerical tests, that is, the unsmoothed orography scheme, the smoothed orography scheme and non-ripples scheme are performed. In this paper, the model used by us is the same as Part I except for orographic specification.The results from simulations indicate that, as far as the climatic simulation is concerned, some aspects of the simulated stationary disturbances, zonal and meridional wind, temperature and precipitation in the low-resolu-tion spectral model with properly smoothed mountains are significantly improved, especially in winter hemis-phere.The deep ripples in the model with the unsmoothed orography produce spurious high pressure regions at the surface with subsidence, and suppress rainfall, causing an unrealistic splitting of the precipitation area in northern winter and summer. Removal of tbe deep ripples by using the special procedure for smoothing topog-raphy allows a strong upward motion in the ripple area with heavy rainfall, eliminating the unrelistic split in the precipitation area.     

Mapping near-surface air temperature,pressure, relative humidity and wind speed over Mainland China with high spatiotemporal resolution

As part of a joint effort to construct an atmospheric forcing dataset for mainland China with high spatiotemporal reso- lution, a new approach is proposed to construct gridded near-surface temperature, relative humidity, wind speed and surface pressure with a resolution of 1 km× 1 km. The approach comprises two steps： （1） fit a partial thin-plate smoothing spline with orography and reanalysis data as explanatory variables to ground-based observations for estimating a trend surface; （2） apply a simple kriging procedure to the residual for trend surface correction. The proposed approach is applied to observations collected at approximately 700 stations over mainland China. The generated forcing fields are compared with the corresponding components of the National Centers for Environmental Predic- tion （NCEP） Climate Forecast System Reanalysis dataset and the Princeton meteorological forcing dataset. The comparison shows that, both within the station network and within the resolutions of the two gridded datasets, the interpolation errors of the proposed approach are markedly smaller than the two gridded datasets.     

Resolution of the atmospheric model matters for the Northern Hemisphere Mid-Holocene climate

This study evaluates the dependence of simulated surface air temperatures on model resolution and orography for the mid-Holocene. Sensitivity experiments with the atmospheric general circulation model ECHAM5 are performed with low (∼3.75°, 19 vertical levels) and high (∼1.1°, 31 vertical levels) resolution. Results are compared to the respective preindustrial runs. It is found that the large-scale temperature anomalies for the mid-Holocene (compared to preindustrial) are significantly different in the low- and high-resolution versions. For boreal winter, differences are mainly related to circulation changes caused by the response to thermal forcing in conjunction with orographic resolution. For summer, shortwave cloud radiative forcing emerges as an important factor. The anomaly differences (low minus high resolution version) in the Northern Hemisphere are regionally as large as the anomalous mid-Holocene temperature signals. Furthermore, they depend on the applied surface boundary conditions. We conclude that the resolution matters for the Northern Hemisphere response in mid-Holocene simulations, which should be taken into account in model-model and data-model comparisons.     

台湾岛地形对台风移动路径的作用

本文用一个β平面准地转正压模式实施了九组试验，研究台湾岛地形对台风移动路径的作用。初始场系根据SPECTRUM和TCM-90期间加强观测资料确定。结果表明:台湾岛地形的引进会使台风路径向右偏移；地形使台风环流的最大风速区向台风中心方向推移，造成台风环流空间尺度减小，这个结构的变化是路径右偏的一个可能原因。     

A Case of Convection Development over the Western Mediterranean Sea: A Study through Numerical Simulations

Summary  A convective case producing heavy precipitation in the western Mediterranean region, characterized by pronounced upper level forcing and main rainfall over the sea, is studied. On the day of the event (September 28^th, 1994), more than 140 mm of precipitation were recorded in coastal lands of eastern Spain, and 180 mm were estimated over the sea with radar data. Synoptically, the case appears to combine warm and moist easterly advection at low levels, typically observed in torrential rainfall events of the region, with a less common strong upper level dynamical forcing. A set of mesoscale numerical simulations using the Hirlam model is performed to investigate the mechanisms responsible for the convection development, and to assess the influence of the orography on the rainfall field. Model output diagnosis indicates that in addition to the lower level forcing, a two-jets interaction is decisive for the triggering and driving of the convection during the event. Moreover, a non-topographic simulation reveals a relatively weak influence of the orography on this event when compared with other similar heavy precipitation cases in eastern Spain. Previous studies have shown an orographic influence of more than 90% on the rainfall whereas in this case about 50% of the precipitation over the area is attributed to the orographic forcing. The study is extended with an analysis of the individual effects of the Atlas and Iberian Peninsula, by means of a factor separation technique. It is shown that the Atlas range induces a redistribution of the precipitation over the Mediterranean, whereas local enhancements can be attributed to the Iberian topography. Received March 2, 1999     

Planetary stationary waves in the atmosphere Part I: Orographic stationary waves

It is proposed that the orographic stationary waves are required by long-term balance of momentum in the at-mosphere with zonally asymmetric orographic forcing, This hypothesis may be confirmed successfully with the theo-retical model of geostrophic waves. In the Part I, we will explain the observed phase distributions of orographic sta-tionary waves at middle and high latitudes of the Northern Hemisphere, according to the long-term balance of zonal momentum over the stationary orographic forcing. It is revealed that the geographic distribution of stationary waves depends not only on local topgraphy but also on mean circulation fields and angular momentum flux in the atmos-phere. So these waves cannot be simulated by the models in a restricted area.     

Assessment of the WRF model in reproducing a flash-flood heavy rainfall event over Korea

This study examines the ability of the cloud-resolving weather research and forecasting (WRF) model to reproduce the convective cells associated with the flash-flooding heavy rainfall near Seoul, South Korea, on 12 July 2006. A triply nested WRF model with the highest resolution of 3-km horizontal grid spacing was integrated with conventional analysis data. The WRF model simulated the initiation of isolated thunderstorms, and the formation of a convective band, cloud cluster, and squall line at nearly the right time. The corresponding precipitation simulation was also reasonably reproduced in its distribution, although the amount was underestimated. A sensitivity experiment that excludes the orography over the peninsula revealed that orographic forcing over the peninsula is responsible for about 20% increase in precipitation over the heavy rainfall region. It was identified that in addition to the up-lifting local orographic forcing to the west of the mountain range in South Korea, anticyclonic circulation due to the presence of the Gaema Heights in North Korea contribute to the confinement of convective activities in the heavy rainfall region.     

Different Asian Monsoon Rainfall Responses to Idealized Orography Sensitivity Experiments in the HadGEM3-GA6 and FGOALS-FAMIL Global Climate Models

Recent work has shown the dominance of the Himalaya in supporting the Indian summer monsoon(ISM),perhaps by surface sensible heating along its southern slope and by mechanical blocking acting to separate moist tropical flow from drier midlatitude air.Previous studies have also shown that Indian summer rainfall is largely unaffected in sensitivity experiments that remove only the Tibetan Plateau.However,given the large biases in simulating the monsoon in CMIP5 models,such results may be model dependent.This study investigates the impact of orographic forcing from the Tibetan Plateau,Himalaya and Iranian Plateau on the ISM and East Asian summer monsoon(EASM) in the UK Met Office's Had GEM3-GA6 and China's Institute of Atmospheric Physics FGOALS-FAMIL global climate models.The models chosen feature oppositesigned biases in their simulation of the ISM rainfall and circulation climatology.The changes to ISM and EASM circulation across the sensitivity experiments are similar in both models and consistent with previous studies.However,considerable differences exist in the rainfall responses over India and China,and in the detailed aspects such as onset and retreat dates.In particular,the models show opposing changes in Indian monsoon rainfall when the Himalaya and Tibetan Plateau orography are removed.Our results show that a multi-model approach,as suggested in the forthcoming Global Monsoon Model Intercomparison Project(GMMIP) associated with CMIP6,is needed to clarify the impact of orographic forcing on the Asian monsoon and to fully understand the implications of model systematic error.     

The effects of mechanical forcing on the mean meridional circulation and transfer properties of the atmosphere

Experiments using a quasi-geostrophic model and the ECMWF T21 spectral model with and without orography are performed to investigate the effects of mechanical forcing on the mean meridional circulation. Results show that mechanical forcing intensifies the horizontal poleward heat flux and redistributes the eddy angular momentum in the vertical, and that this changes significantly the intensity and location of the mean me-ridional circulation centres.It is shown how the mean meridional circulation is set up in such a way to satisfy both the dynamical and thermodynamical transport requirements of the model atmosphere. Whenever external forcing changes the eddy fluxes, the Coriolis torques from the upper horizontal branches of the mean meridional circulations change to balance the extra divergence of eddy momentum flux, and additional adiabatic heating is produced by the vertical branches of the toroids to balance the extra divergence of eddy heat flux. The mean meridional circula-tion is, therefore, confirmed to be very sensitive to mechanical forcing, and can be used as an efficient tool to quantitatively diagnose the adequacy of the orographic representation of numerical forecasting and general cir-culation models.     

地形湍流拖曳力参数化及在GRAPES中的应用

分辨率的限制使得不能被模式识别的地形称为次网格尺度地形,次网格尺度地形在热力和动力方面对实际大气有着不可忽略的作用,其效应只能通过参数化的形式回馈给模式。分辨率的提高使得与较小尺度地形相联系的地形湍流拖曳力凸显其重要性。数值模式中地形湍流拖曳力的参数化对完善模式物理过程和改善模式近地层预报效果具有积极意义,其方法包括有效粗糙度法和直接参数化法,而GRAPES模式中并未以任何方法考虑次网格尺度地形的影响。该文通过单柱模式比较了有效粗糙度法和直接参数化法的优劣, 发现后者在有些方面优于前者。最后,将应用于实际的一个直接参数化方案接入GRAPES中尺度模式中,进行个例模拟,并与NCEP再分析资料进行对比,结果表明:考虑地形湍流拖曳力方案对模式预报具有改进作用,尤其对局地低层风场具有积极影响。     

Diagnosis and numerical simulation of a torrential precipitation event in Catalonia (Spain)

Summary A torrential precipitation event occurred in Catalonia (northeastern part of Spain) during 9 and 10 October 1994. More than 400mm were registered in the south of Catalonia. A diagnostic study shows that most of the ingredients to produce heavy rain (large scale upward vertical motion, instability, high moisture content in all the troposphere) were present over the Spanish coast and western Mediterranean. Mesoscale triggering mechanisms have been associated with the orographic forcing, not only through physical lifting of moist air by the coastal mountains, but also by the redistribution of the surface pressure field induced by the Atlas and Pyrenees ranges. A numerical simulation of the event using a meso- model has been performed. The model forecasts qualitatively well the rainfall distribution but underestimates the maximum rainfalls. The effects of the orography and the evaporation from the sea have been also studied. The simulations have shown that the action of the orography is decisive for the rainfall, pressure and wind distrbutions over the Spanish coast and the western Mediterranean. The isolated action of the evaporation turns to be much less important. However the combined effect of orography and evaporation is the most important factor in the areas where the greatest amount of rainfall occurred.With 26 Figures     

Objective synoptic classification combined with high resolution meteorological models for wind mesoscale studies

Summary A methodology developed for automatic classification of Objective Synoptic Processes (OSP) and its application to the study of the mesoscale atmospheric circulatory patterns associated with them is described. The classification was based on the analysis of the evolution of surface pressure and geopotencial height at 500 hPa during three days. An iterative procedure results in an objective grouping of the main configurations describing different large-scale situations. This routine has been applied over an appropriate domain covering the Iberian Peninsula to obtain OSPs in the region for the two month period July–August, using daily synoptic maps for years 1990 to 1999 (a total of 7304 surface and 500 hPa synoptic maps have been used in the analysis). Finally, for a characteristic day for two of the OSPs obtained, a mesoscale meteorological model (TAPM) has been run at high resolution for the region of Catalonia, Northeastern Spain, in order to describe the local atmospheric circulatory patterns associated with a given large-scale situation. Results show that the complex orography modifies the large-scale forcings resulting in wind fields with a very important horizontal variability, significant daily cycle, and specific local features related to orographic elements, which the model was able to incorporate due to the highly-resolved orography used.     

What shapes mesoscale wind anomalies in coastal upwelling zones?

Observational studies have shown that mesoscale variations in sea surface temperature may induce mesoscale variations in wind. In eastern subtropical upwelling regions such as the California coast, this mechanism could be of great importance for the mean state and variability of the climate system. In coastal regions orography also creates mesoscale variations in wind, and the orographic effect may extend more than 100?km offshore. The respective roles of SST/wind links and coastal orography in shaping mesoscale wind variations in nearshore regions is not clear. We address this question in the context of the California Upwelling System, using a high-resolution regional numerical modeling system coupling the WRF atmospheric model to the ROMS oceanic model, as well as additional uncoupled experiments to quantify and separate the effects of SST/wind links and coastal orography on mesoscale wind variations. After taking into account potential biases in the representation of the strength of SST/wind links by the model, our results suggest that the magnitude of mesoscale wind variations arising from the orographic effects is roughly twice that of wind variations associated with mesoscale SST anomalies. This indicates that even in this region where coastal orography is complex and leaves a strong imprint on coastal winds, the role of SST/winds links in shaping coastal circulation and climate cannot be neglected.     

On the vertical structure of composite surface cyclones in the Mediterranean region

Summary The average pressure distribution at mean sea level and the vertical structure of synoptic scale surface cyclones (with central pressure less than 1000 hPa) that occur in the Mediterranean region is studied for a 40 year period (1958–1997) on a seasonal and daily basis. The cyclonic occurrences are studied in three regions of enhanced cyclonic activity: gulf of Genoa, Southern Italy and Cyprus. The cyclones are identified with the aid of an objective method based on grid point values, available every 6 hours. The analysis revealed different characteristics of the cyclones that occur in the three regions, reflecting the different mechanisms that are responsible for their occurrence in each region. For the Genoa region the cyclone pressure minimum is located over the gulf, associated with orographic forcing, while surface dynamics occur further south. Over Southern Italy, the pressure minimum covers a wide area, whilst the surface dynamics are found to act in the same region, becoming more important in winter and spring. The pressure minimum of cyclones over Cyprus is located over the land during winter and spring and is influenced by surface dynamics and orography. Received November 7, 2000 Revised July 14, 2001     

我国地形与暴雨的若干气候统计分析

本文以详实的资料，通过对我国不同的地区，不同地形特征的暴雨分析后，得出了地形对暴雨影响的若干气候统计特征。     

Mountain–valley precipitation differences in the northern Alps: an exemplary high-resolution modeling study

This paper investigates the dependence on environmental conditions of altitudinal precipitation differences in the northern Alps, based on high-resolution numerical simulations with the MM5 model for a selected region in the Bavarian Alps (Zugspitze mountain and surrounding valley stations). Three exemplary precipitation events representing climatological regimes with different orographic enhancement characteristics are selected. After validating the MM5 precipitation fields against the available surface observations, the model results are used to analyse the interactions of atmospheric dynamics and cloud microphysics with the local orography. The first two cases (19–22 March 1997, 05–09 February 1999) are characterized by a strong northwesterly or northerly flow, associated with large precipitation differences between the mountain and the surrounding valley stations. For these cases, the model results indicate a dominance of the classical seeder–feeder mechanism, with strong orographic lifting generating dense orographic clouds over each individual mountain ridge, which in turn intensify precipitation. The related surface precipitation maxima can be found near the mountain peaks or somewhat in the lee due to hydrometeor drifting. The third case (05–07 December 1992) represents conditions with relatively small (i.e. below climatological average) precipitation differences between the Zugspitze and the surrounding valley stations. For this event, the model results indicate that relatively weak ambient winds at and below Alpine crest level (700 hPa) were primarily responsible for the lack of substantial precipitation enhancement. Precipitation was nevertheless moderately intense because of strong frontal lifting at higher levels. In all three cases, the agreement between simulated and observed precipitation patterns is so high that there is good reason to expect that mountain–valley precipitation differences will be quantitatively predictable for nonconvective events once a sufficiently high model resolution is computationally affordable.     

台风Rananim登陆期间地形对其降水和结构影响的数值模拟试验

应用非静力平衡中尺度模式MM5(V3.6),对0414号台风Rananim在登陆期间移动路径和所产生的降水进行了数值模拟研究,模式较好地再现了台风Rananim的移动路径和所产生的降水,但模拟的过程降雨量与实况值还有所偏差。多普勒雷达探测资料表明,台风Rananim登陆期间,强回波带出现在台风移动的右前方,螺旋云带中镶嵌着大量的对流云团;垂直液态水含量的高值区出现在台风中心的西北侧。作者通过在浙江、福建东部沿海一带进行有无地形的数值对比试验,着重讨论了台风登陆期间地形对台风降水、台风结构特征变化的影响。结果表明:（1）台风登陆期间, 地形的影响对台风降雨量有明显的增幅作用。由地形强迫产生的降雨量和地形走向相一致,迎风坡降雨量增加,背风坡降雨量减少,地形影响使浙江东部一带增加的平均降雨量约占该地区模拟平均总降雨量的40%左右。（2）台风登陆期间,地形的强迫作用有利于在低层台风眼的西北侧形成明显的辐合带,高层为明显的辐散区;在中尺度环流场上,地形的影响有利于台风中心西北侧低层中尺度气旋性涡旋系统的发生发展,从而激发中尺度对流云团,形成中尺度雨团,造成了台风中心南北雨区和雨量的不对称分布。（3）地形的强迫作用,可以使台风流场局部发生改变。当地形强迫产生与台风环流同向的中尺度扰动时,将使台风环流局部明显增强;当地形强迫产生与台风环流反向的中尺度扰动时,将使台风环流局部明显减弱。（4）台风登陆期间,地形的影响可以使台风靠近陆地一侧眼壁内的垂直上升速度增大,位涡明显增强,从而造成台风涡旋的增强。     

斜压大气中的地形扰动

本文主要讨论地形对天气系统的影响,研究了斜压大气中地形背风气旋的问题。用小参数法简化基本运动方程,利用青藏高原和落基山脉的实际地形,采用FFT的数值方法,考虑了不同地形、风切变、大气层结等诸因素对地形扰动的影响。结果表明:青藏高原和落基山脉东侧的地形扰动与统计结果较一致;层结对地形扰动的影响较敏感。