暴雨过程中中尺度地形作用的数值试验

本文利用美国PSU/NCAR中尺度模式（MM4）对1991年7月4—5日浙西皖南山区的大暴雨过程进行了地形作用的数值模拟对比试验。结果表明，中尺度地形对强降水区的分布和强度有重要影响，强降水中心位于山地附近；地形的动力及屏障作用对气流有明显影响；山区的水汽和热量对静力不稳定亦具有相当作用。     

次网格坡地辐射参数化对RegCM4.1模式模拟东亚夏季气候的影响

本研究将次网格坡地辐射参数化方案引入到区域气候模式(RegCM4. 1)中,并研究次网格坡地辐射参数化对RegCM4. 1模拟东亚夏季气候性能的影响。结果表明:RegCM4. 1高估了夏季青藏高原的热源作用,模拟低层偏强的西南季风导致了模拟的中国夏季降水量总体偏大。引入次网格坡地辐射参数化方案后,模式模拟的青藏高原夏季地表太阳辐射通量和长波辐射通量减小分别可达5%和12%以上,夏季减弱的青藏高原热源作用使得低层西南季风减弱,从而改善了模式对中国夏季降水的模拟;而且改善程度自东南向西北递减,在东南地区模拟夏季降水的相对均方根误差减小9%,空间相关系数和Taylor评分分别提高0. 14和0. 08。     

Analysis of the Characteristics of Inertia-Gravity Waves during an Orographic Precipitation Event

A numerical experiment was performed using the Weather Research and Forecasting(WRF) model to analyze the generation and propagation of inertia-gravity waves during an orographic rainstorm that occurred in the Sichuan area on 17 August 2014. To examine the spatial and temporal structures of the inertia-gravity waves and identify the wave types, three wavenumber-frequency spectral analysis methods(Fourier analysis, cross-spectral analysis, and wavelet cross-spectrum analysis)were applied. During the storm, inertia-gravity waves appeared at heights of 10–14 km, with periods of 80–100 min and wavelengths of 40–50 km. These waves were generated over a mountain and propagated eastward at an average speed of 15–20 m s~(-1). Meanwhile, comparison between the reconstructed inertia-gravity waves and accumulated precipitation showed there was a mutual promotion process between them. The Richardson number and Scorer parameter were used to demonstrate that the eastward-moving inertia-gravity waves were trapped in an effective atmospheric ducting zone with favorable reflector and critical level conditions, which were the primary causes of the long lives of the waves. Finally, numerical experiments to test the sensitivity to terrain and diabatic heating were conducted, and the results suggested a cooperative effect of terrain and diabatic heating contributed to the propagation and enhancement of the waves.     

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。     

Effect of Aerosol Particles on Orographic Clouds:Sensitivity to Autoconversion Schemes

Aerosol particles can serve as cloud condensation nuclei(CCN) to influence orographic clouds. Autoconversion,which describes the initial formation of raindrops from the collision of cloud droplets, is an important process for aerosol-cloud-precipitation systems. In this study, seven autoconversion schemes are used to investigate the impact of CCN on orographic warm-phase clouds. As the initial cloud droplet concentration is increased from 100 cm^-3 to 1000 cm^-3(to represent ...     

Implications for groundwater recharge from stable isotopic composition of precipitation in Hawai'i during the 2017–2018 La Niña

The project captured a subset of the hydrological cycle for the tropical island of O'ahu, linking precipitation to groundwater recharge and aquifer storage. We determined seasonal storm events contributed more to aquifer recharge than year-round baseline orographic trade wind rainfall. Hydrogen and oxygen isotope values from an island-wide rain collector network with 20 locations deployed for 16 months and sampled at 3-month intervals were used to create the first local meteoric water line for O'ahu. Isotopic measurements were influenced by the amount effect, seasonality, storm type, and La Niña, though little elevation control was noted. Certain groundwater compositions from legacy data showed a strong similarity with collected precipitation from our stations. The majority of these significant relationships were between wet season precipitation and groundwater. A high number of moderate and heavy rainfall days during the dry season, large percentage of event-based rainfall, and wind directions outside of the typical NE trade wind direction were characteristics of the 2017–2018 wet season. This indicates that the majority of wet season precipitation is from event-based storms rather than typical trade wind weather. The deuterium-excess values provided the strongest evidence of a relationship between groundwater and different precipitation sources, indicating that this may be a useful metric for determining the extent of recharge from different rain events and systems.     

人为气溶胶对地形云降水的影响:以黄山地区为例

选取黄山站为高山站,周围黄山区、绩溪、黄山市三个低海拔高度站为对比站,比较高山站与对比站1960—2009年降水量差值,即地形影响因子R0的变化趋势,以及同期能见度的变化,分析了人为气溶胶对黄山地形云降水的可能影响。结果表明,1960—1979年能见度下降,气溶胶含量增大,R0升高;1980—1989年,能见度升高,气溶胶含量有下降趋势,不同对比站R0变化趋势不同;1990—2009年,能见度下降,气溶胶含量升高,R0显著下降。气溶胶对降水的影响作用与背景气溶胶浓度有关,背景气溶胶浓度较低时,增加气溶胶浓度可促进降水;背景气溶胶浓度较高时,增加气溶胶含量对降水抑制作用显著,对应的能见度阈值为10km。当气溶胶对降水起抑制作用时,抑制作用与风速成反比,与风频和各风向平均降水量呈显著正相关。     

The Rose Spit Eddy in Dixon Entrance: Evidence for its existence and underlying dynamics

Abstract

The existence and dynamics of the so‐called “Rose Spit Eddy” in Dixon Entrance, British Columbia, are investigated by (i) analysing published observations of low‐frequency Eulerian and Lagrangian currents in the region; (ii) interpreting tidal residuals produced by the Hecate Model (a non‐rotating hydraulic model of Hecate Strait and Dixon Entrance); and (iii) running a barotropic, non‐linear numerical tidal model over simplified topography to investigate residuals produced over the Rose Spit sill.

Observations have consistently revealed persistent basin‐wide, surface‐intensified cyclonic shears within central and eastern Dixon Entrance. The Hecate hydraulic tidal model also produced a tidal residual cyclonic gyre in central Dixon Entrance, but with velocities considerably larger than those observed. Barotropic numerical simulations of tidal streams flowing over a representation of the Rose Spit sill produced residual flows along the sill in reasonable agreement with observations and theory. A southward‐directed jet flow was produced off Cape Chacon. Elsewhere, tidal rectification was weak. Run without the Coriolis force, organized flow along the sill broke down, although the headland jet off Cape Chacon persisted.

We submit that the observed Rose Spit eddy results from interactions between buoyancy‐driven coastal currents and tidally rectified flows generated over the Rose Spit sill, and near Cape Chacon, and perhaps indirectly, over the western flank of Learmonth Bank (which although west of the Rose Spit eddy, contributes to the cross‐channel flow across the Entrance). These regions of localized tidal stress will each favour recirculation of a portion of the coastal current within the Entrance, helping to form the eddy.

We believe that the Hecate hydraulic model eddy was generated to a significant degree by phase errors introduced at the northern open boundary, where a rocking barrier was used to force currents. A second rocking barrier also produced a large cyclonic gyre, not supported by observations, near the model's southern boundary.     

数值预报模式动力框架发展的若干问题综述

作者就大气数值模式动力框架发展的几个问题做了回顾和展望.关于模式地形的处理,讨论了"地形追随"(terrain-following)坐标和"台阶地形"(step-mountain)η坐标的优点、问题和对策.关于物理量守恒格式的构造,回顾了从"瞬时"守恒到隐式、显示和半隐式完全(包括时间离散)守恒格式的发展和近况,介绍了加速非线性全隐式问题迭代收敛的途径.还讨论了半拉格朗日守恒格式的构造问题.最后,对目前广泛使用的全球谱模式,讨论了其长处、局限和发展前景,并简单介绍了谱元方法.     

0907号热带风暴“天鹅”暴雨的数值模拟

利用新一代中尺度数值模式WRF,对登陆后滞留粤西地区近50 h并带来了暴雨以上强降水的0907号热带风暴"天鹅"进行数值模拟,取得了较好的效果。该模式成功模拟出"天鹅"的移动路径、强度和强降水分布,暴雨的中心强度与实况基本一致。利用模式输出的高分辨率结果分析研究"天鹅"的暴雨降水原因,通过改变地形高度的敏感性试验表明,沿海地区地形的抬升作用对降雨有显著的增幅作用,并且使降水分布更加不均匀。