A Numerical Study of Mesoscale-Topography Influence on the Heavy Rainband of Typhoon Hato (2017)

During the movement of Typhoon Hato (2017) over land, heavy rainfall occurred when the spiral rainband which was about 100 km distance away from the center of the typhoon passed the Dayao Mountain (with an elevation of 1.2 km). In this study, the structures and forming mechanism of the heavy rainband along the mountain range are investigated by using high-resolution model simulations. The results show the importance of topography in causing the heavy rainband. Upslope of the steep terrain lifts the cyclonic flow to produce strong upward motion when the rainband passes across with high wind speed. At the same time, the warm and humid air is lifted to the steep slope, causing unstable energy to accumulate over the windward slope, which is conducive to the occurrence of rainfall. In particular, the convective cells generated upstream of rainband will further strengthen and develop due to the uplift when they move close to the mountain foot. Some precipitation particles in the convective cells fall to the ground while others move downstream with the intense updrafts, forming heavy rainfall near the summit. As a result, the largest accumulative rainfall coincides well with the orientation of the mountain ridge.     

A Lagrangian approach to modelling stable isotopes in precipitation over mountainous terrain

A Lagrangian (Rayleigh) distillation model is used to track the evolution of stable isotopes in precipitation over mountainous terrain from the Pacific Coast of Canada to two alpine field sites in the Canadian Rocky Mountains. Precipitation δ¹⁸O at Vancouver constrains the model and air–mass back trajectories provide the water vapour pathway for 10 winter storm events. Isotopic values along storm pathways are modelled with a classical Rayleigh model that prescribes a linear decrease in temperature and pressure from initial to final conditions, and two models that account directly for orographic precipitation processes by: (i) applying an orographic rainfall model and (ii) using North American Regional Reanalysis data to calculate the change in vapour content along storm pathways. All models are significant predictors of snowpack δ¹⁸O, but the orographic model provides the best fit to precipitation‐weighted δ¹⁸O for each storm. The improvement in modelled δ¹⁸O by accounting for terrain along storm trajectories illustrates the need to account for orographically driven moisture loss when modelling vapour transport to ice core sites with mountainous upwind terrain. This finding is also applicable to isotopic studies of paleoaltimetry and source areas of groundwater recharge. Copyright © 2011 John Wiley & Sons, Ltd.     

气溶胶对秦岭山脉地形云降水的影响

以华山站为影响站,其周围华阴、渭南、西安为对比站,通过对影响站与对比站降水之比--地形强化因子(Ro)的变化趋势以及Ro与能见度变化关系的分析,研究了气溶胶对秦岭地形云降水的影响.结果表明:有观测以来Ro逐年递减,1980年后Ro递减更快,减幅达20%;Ro的减小趋势与能见度递减、气溶胶递增相吻合,说明气溶胶的增加抑制了地形云降水;华山1980年后的年平均雨量比1980年前减少了15%,达132mm,而平原地区的减少量不超过3%(16mm).分析气溶胶抑制地形云降水的物理过程发现,Ro的递减主要是减少了中小雨(日雨量小于30mm)的天数,而对大于30mm的降水影响较小,说明气溶胶对浅薄的生命期较短的地形云降水的抑制作用更明显;在以动力强迫抬升为主的春秋季,气溶胶对地形云降水的抑制作用明显强于平原地区,1980--2004年间降水减少了20%～30%;在热力作用下,气溶胶对地形云降水的抑制作用与平原地区相当.     

地形雨估算初探

阐述了在弧状山体有水汽输入的流域或地区，通过地形参数，控空资料及其有关气象因子、来估算地形雨的方法和步骤，从而使地形雨这一分量，可自大气环流，天气系统的降水量中分离出来。     

Prediction of monsoon rainfall with a nested grid mesoscale limited area model

At the India Meteorological Department (IMD), New Delhi, a 12-level limited area model with 100 km horizontal resolution has been in use for weather forecasting. The present study uses this model together with a higher horizontal resolution (50 km) and vertical resolution (16-levels) model to examine the impact of increased resolution to simulate mesoscale features of rainfall during monsoon disturbances. The model was run for 22 days in the month of August 1997 and one week in September 1997 during three monsoon depressions and one cyclonic storm in the Bay of Bengal. The model results are compared with observations. The study shows that the model can capture mesoscale convective organization associated with monsoon depression.     

ANALYSES OF THE DYNAMIC EFFECTS ON WINTER CIRCULATION OF THE TWO MAIN MOUNTAINS IN THE NORTHERN HEMISPHERE--Ⅱ.VERTICAL PROPAGATION OF PLANETARY WAVES

A linear,hemispheric and stationary spectral model with multilayers in the vertical was employed to simulate the vertical propagation of waves triggered by mountains.Results show that,in cooperation with the East Asia zonal mean flow,Tibetan Plateau can excite a strong wavenumber 1 perturbation in the stratosphere with its ridge and trough located over the Pacific and Atlantic Oceans respectively.On the other hand,the stratospheric wavenumber 1 perturbation caused by the mechanical forcing of the Rocky Mountains in cooperation with the North America zonal mean flow is very weak.Calculations from observational data of the vertical profile of critical wavenumber for vertically propagating waves imply that the tropospheric wavenumber 1 perturbation can hardly penetrate the North America tropopause upwards,whereas it can freely propagate through the East Asia tropopause into the stratosphere.Two-dimensional E-Pcross-sections obtained from both observational data and simulated results also demonstrate that waves excited by the Rocky Mountains are refracted towards low latitudes in the troposphere during their upward propagation:whereas,in addition to the above mentioned equatorward leaning branch,the wavenumber 1 and 2 planetary waves excited by the Tibetan Plateau possess another branch which is refracted to high latitudes during upward propagation and penetrates the tropopause into the stratosphere.It is therefore concluded that the difference in the horizontal and vertical wave propagations in the two hemispheres is a result of the different dynamical forcing induced by the two main mountains in the Northern Hemisphere.     

地形强迫的流场非地转性与广西旱涝的空间分布

本文分析了地面平均流场，从地转偏差的角度讨论了广西多雨区与少雨区空间分布的动力学原因，着重指出“弧形山”结构在形成广西气候特点上的特殊作用。     

Evolution and Frontogenesis of an Imbalanced Flow —the Influence of Vapor Distribution and Orographic Forcing

1.InttoductionIftheinitialfieldsarenotingeostrophicbalance,theadjustmentandevolutionwilloccurinthestratifiedfluid,andthefrontogenesiswilloccurundersuitableconditions.ThisaspectwasfirstinvestigatedbyRossby(1938),followedbymanyscientists(Blumen,1972;Gill,1976,1982;VanHeijst,1985;BossandThompson,1985;On,1984,1986;McWilliams,1988;Middleton,1987;Glendening,1993;BlumenandWu,1995;WuandBlumen,1995;Grimshaw1998;Blumen,1998;etc.).Intheseresearches,theenergyconversionratioy=AEk/BE,isaninterestin…     

Measuring the imprint of orographic rainfall gradients on the morphology of steady‐state numerical fluvial landscapes

We explore the imprint of spatial rainfall patterns on steady‐state landscapes with uniform rock uplift rate. A two‐dimensional (2D) orographic precipitation module is incorporated into the CHILD numerical landscape evolution model to provide a quantitative tool for exploring the co‐evolution of rainfall patterns and fluvial topography. Our results suggest that network organization and planform morphology are strongly impacted by rainfall patterns. Rainfall gradients that are perpendicular to a mountain range front produce narrower watersheds because channels show a tendency to flow along the rainfall gradient, rather than across it. The change in watershed shape is evidenced by smaller values of the exponent on distance in Hack's law and a less peaked width function. Narrower watersheds also lead to an increase in the valley spacing ratio and constrain trunk channels to follow a more direct path to the mountain front. Rainfall gradients also influence the distribution of topography across a watershed. Channel profiles record rainfall patterns in both the channel concavity and the channel steepness index (k_sn). Across short tributaries along which rainfall rate changes little, k_sn decreases systematically with tributary‐averaged rainfall rate. The hypsometric integral (HI), which increases with the amount of topography that is at relatively high elevations within a watershed, is negatively correlated with the profile concavity of the trunk channel. High rainfall rates at the ridge top lead to mainstem channels that have relatively low concavity, and watersheds with relatively higher HI in comparison with landscapes that have uniform rainfall. Finally, we contrast the impacts of rainfall patterns on landscape morphology with those resulting from a linear rock uplift gradient and uniform rainfall. Uplift patterns may have a similar impact on landscape morphology as rainfall gradients, making it challenging to decipher the relative roles of climate and tectonics on landscape evolution without a quantitative assessment of morphologic parameters. Copyright © 2015 John Wiley & Sons, Ltd.     

顾及地形特征的不规则三角网二次插值方法

在讨论不规则三角网插值方法特点和地形基本特征的基础上,提出了顾及地形特征的不规则三角网二次插值方法.这种插值方法同时兼顾地形特征、插值精度和计算速度,还根据地形特征对不规则三角网进行了编码分类,就各类型的不规则的插值计算提出了解决方案.