The Koorin nocturnal low-level jet

It has been long known by Australian meteorologists that a nocturnal jet is a frequent feature of the low-level wind profile at Daly Waters in the Northern Territory of Australia. The Koorin Expedition during the southern winter of 1974 obtained data on wind and temperature profiles in the boundary layer which form an ideal base for the study of this phenomenon. In this paper, a simple model of the jet is described and the results of the model are compared with observations. The model is essentially a development of the pioneering work of Blackadar, who proposed that a low-level jet could develop as an inertial oscillation. The observations do show some of the features of the model.     

The decay of the ALPEX low-level jet: Fine structure observation by aircraft data

Summary This case study describes the decay of a low-level jet in the alpine foreland as a planetary boundary layer phenomenon. Few measurements are known, which document this transition period of the boundary layer from night to day. Analysis of 1 and 20 Hz data of temperature and the three wind components of the ELECTRA aircraft mission on April 16, 1982 between 5 and 11 GMT allow an assessment of the temporal and spatial fine structure during the decay phase of the jet in the morning. Using the flight technique of horizontal and vertical zigzagging, the coupling of thermal stability, turbulence intensity, topography and behavior of the jet is shown.Vertical profiles and cross-sectional analysis of mean and turbulent parameters document three separate jet decay phases: decoupling, transition and erosion. During the first phase, the jet maximum is situated between surface and synoptic inversions, decoupling from surface friction and prohibiting momentum transport towards the free atmosphere. During the transition phase, the jet maximum increases in altitude. In the erosion phase, the jet covers the top of a developing well mixed layer. Turbulent mixing and entrainment on top of the layer are responsible for the decay of the jet. This is confirmed by calculating the decrease of the wind speed maximum from the turbulent momentum flux and the growth rate of the mixing layer by means of a mixed layer model in comparison to the measured wind speed jump and to other observations.With 7 Figures     

一次黔北暴雨过程中低空急流与强降水的关系

利用分辨率为1°×1°的NCEP/NCAR每日4次的再分析资料以及常规资料和卫星资料,对2009年6月7-8日发生在贵州北部大暴雨的成因进行分析。结果表明:西南低涡切变与低空急流发展是本次大暴雨的主要影响系统。高层辐散是维持和加强低层辐合上升运动中不可缺少的条件。低空急流的加强为暴雨提供了高温高湿的大气,并为中尺度系统的发展、移动提供了有利的动力条件。低空急流对对流云团的发展和移动路径起着重要的预示作用。     

苏州城区大气边界层低空急流特征分析

利用2012年苏州城区风廓线雷达的观测资料,从低空急流个例分析入手,选取1、4、7、10月四个典型月份,分析该地区边界层低空急流的时空分布及强度变化特征。结果表明:冬春两季低空急流发生频率最高,夏季出现频率最低。在4个典型月份里低空急流均表现出日落后出现频率升高,夜间保持稳定,日出后出现频率降低的特征。全年有80%的低空急流分布在900 m以下高度上,冬、夏季平均高度最低。全年低空急流风速70%以上集中在4~12 m·s~(-1),小于4 m·s~(-1)和大于20 m·s~(-1)的低空急流出现频率较低。     

An orographically split mixed-layer and the alpine low-level jet

Summary A study is undertaken of the semi-geostrophic flow of shallow water incident tangentially upon a sharp leading edged, slender, orographic feature. The nature of the response is shown to depend upon the Froude number of the upstream flow and upon the orographic shape. In particular it is deduced that the orographically split flow has a laterally asymmetric response, and that for sub-critical upstream settings a jet can develop on one particular side of the orography. Features of an easterly low-level jet observed on the Alpine northside during the ALPEX project are compared with the theory.With 5 FiguresJosef Horn died in an Alpine accident in September 1987.     

次天气尺度低空急流的形成

本文指出次天气尺度低空急流的超地转特征主要是由变压风叠加在地转风场上所致。并且高层辐散引起低层辐合,上下层质量调整是造成低层出现变压风的一个重要原因。明确了暴雨对急流反馈的直接效果就是增加其变压风.通过一个实例的诊断分析,对上述观点进行了验证。     

低空急流的研究进展与展望

低空急流与强降水事件、空气污染、风能利用、航空安全、沙尘暴、森林火灾及其他诸多天气、气候现象存在着密切的联系,因此自发现之日起便受到人们的广泛关注。在过去的半个多世纪中,中外气象学家围绕着低空急流事件开展了大量的研究并取得了丰硕的研究成果。对这些成果进行了综合回顾,重点对低空急流的分类和定义、分布状况及结构特征,低空急流形成演变机制,低空急流与降水及其他领域的关系等三个方面的研究工作进行了总结。最后通过对比分析中外针对低空急流事件的研究历史和现状,指出了中国现有研究工作中所存在的诸如低空急流选取标准宽松、观测数据缺乏且质量控制不完善、低空急流自身特征与形成演变机制研究数量和深度有限、交叉领域研究工作匮乏等方面的问题,并对未来的研究方向进行了展望。     

A down-valley low-level jet event during T-REX 2006

A prolonged down-valley flow and low-level jet were observed throughout the Enhanced Observing Period 4 (April 28–29) of the 2006 Terrain-induced Rotor Experiment, held in the Owens Valley of California near the town of Independence. The low-level jet was strongest during the nocturnal hours, and special field observations captured important details of the event lifecycle. High-resolution simulations using the Weather Research and Forecasting numerical weather prediction model were generated, with underlying assumptions being that model resolution, boundary layer physics, and nesting configuration would be dominant controlling factors in reproducing the jet. The large-scale conditions were dry throughout the event, so moist physics were not a significant forcing consideration. For the control simulation, a two-nest (4.5 and 1.5 km grid spacing) configuration with 90 vertical levels was applied. Additionally, the Quasi-Normal Scale Elimination planetary boundary and surface layer option were selected due to its published performance under conditions of stable stratification. Three other sensitivity simulations were run for comparison, differing from the control just in the choice of vertical resolution (60 versus 90 levels with Quasi-Normal Scale Elimination) and planetary boundary/surface layer physics (90 levels/Mellor-Yamada-Jancic; 90 levels/Yonsei State University). Although the gross evolution (location, height, and timing) of the low-level jet is captured by all model runs (with the 1.5 km inner nest providing the more accurate details), there were at times large underestimations of the nocturnal jet speed max in each simulation (approaching 100 % error, or up to almost 10 m s^?1). Overall, the variations of vertical resolution and planetary boundary/surface physics against the control seemed to (1) yield little overall improvement to statistical or subjective evaluations; (2) do little to improve deficiencies in reproducing the magnitude strength of the nocturnal down-valley low-level jet. Since the cold-start simulations spanned 36 h (including a 12-h spin-up period), it was suspected that the lateral boundary conditions imposed on the outermost 4.5 km nest might negatively impact the interior model solutions in the Owens Valley. To investigate this possibility, an additional simulation was executed by adding two extra nests to the control configuration: an outer 13.5 km and an inner 500 m. This simulation produced a better evolution of the nocturnal low-level jet and especially the speed max. The addition of the larger 13.5 km nest appears more critical to this improvement than that of the extra spatial resolution provided by the inner 500 m nest, which supports the idea that accurate capturing of the large-scale synoptic condition was critical in reproducing important details of this down-valley low-level jet event. The extra 500-m resolution did seem to improve the morning valley cold pool forecast.     

基于多普勒测风激光雷达的低空急流观测研究

基于相干多普勒测风激光雷达于2018年8月在山东德州获取的为期一个月的风廓线观测数据,进行了低空急流的判定、识别与统计分析。参考BONNER对低空急流的判定标准,对1 500 m高度以下的每10 min平均风廓线数据进行低空急流识别与统计,急流发生频率仅为3.6%。参考张世丰对低空急流的判定标准,统计了350 m高度以下10 min平均风廓线的低空急流风速、高度、风向及风切变等结构特征。急流发生频率为24.9%,急流速度主要介于6～10 m·s-1之间,急流高度出现3个峰值,分别位于110 m、160 m和220 m左右,急流风向主要为偏东风和偏南风。结果表明,多普勒激光雷达可以获取高时空分辨率的风廓线数据,进而可以有效检测低空急流结构的存在及其特征。     

低空急流对黄土高原大暴雨的作用

利用天气图、物理量和多普勒雷达资料对2005年7月2日凌晨发生在黄土高原延川县的突发性大暴雨天气进行综合分析。结果表明:大暴雨发生在700 hPa低涡附近;700 hPa西南急流为大暴雨提供了充沛的水汽和能量;多普勒速度图证实了700 hPa低空急流的形成是大暴雨产生的主要原因,大暴雨夜发性是低空急流的日变化造成的。     

涡度收支与潜热释放对西南低涡形成的作用

利用中尺度WRF模式对2008年6月30日—7月1日生成于川东南地区的一个西南低涡的发生发展过程进行了数值模拟研究。模拟结果显示低涡首先出现在850 h Pa上,几个小时后700h Pa上才有低涡生成,850 h Pa低涡的形成与西南低空急流有着密切的联系。通过ω方程的诊断分析表明,涡度的水平平流项和辐散项对850 h Pa低涡的形成起主要作用,而潜热释放对850 h Pa低涡的形成作用不大;潜热加热是700 h Pa气流不断辐合从而形成低涡的主要因子。干敏感性试验研究进一步证实了潜热释放对850 h Pa低涡的影响不明显,但是会导致700 h Pa上气旋性的切变加强辐合从而形成低涡。     

用轴对称的平衡模式对热带气旋发展过程中积云动量垂直混合作用的模拟

经过新中国成立以来多次中尺度天气试验研究,江淮以南地区暴雨和强降水的中尺度天气动力学研究取得长足进展。其进展主要体现在两个方面:一是用较高分辨率的观测资料（包括地面和高空探测的加密观测以及卫星和雷达的观测）对形成暴雨和强降水的中尺度天气系统的结构特征和生命史过程建立了天气学概念模型,特别是对暴雨和强降水有重要影响的α中尺度和β中尺度对流系统,建立了观测个例的中尺度物理模型;二是探索了暴雨和强降水的动力学和热力学的物理机制,为精细化数值模式的设计提供了理论支撑,为灾害性天气预测预警提供了技术支持。本文在以往暴雨和强降水研究的基础上,着重对暴雨和强降水的中尺度天气科学试验、江淮以南地区暴雨和强降水发生主要区域的中尺度天气动力学研究进行了概述。

     

The interface effect and the formation of a low-level jet along the east side of the rocky mountains

Based upon the analysis of several different causes for the low-level jet along the east side of the Rocky Moun-tains, the concept of “Interface Effect” is established. The basic mechanism for the formation of the low-level jet in North America has been found to be the compression and divergence, under the driving of ageostrophic winds, of the air columns between two surfaces-the ground and the bottom of inversion-which slope with different patterns in a cross-section normal to the jet stream. As a result, the air parcel is accelerated along the stream-line and the anticyclonic shear of the current increased. Also, the diurnal variation of the jet is determined by the interface effect.     

低空急流在重庆2010年“6.7”低能暴雨中的作用

利用常规观测、地面逐时降水、NCEP再分析资料和卫星雷达资料,从影响系统,低空急流强度和位置演变,低空急流对暴雨区动力、水汽和热量输送等方面,对2010 年6 月7 日重庆地区低能暴雨天气过程进行分析。结果表明,西南低空急流加强北抬,引起局地涡度增加,使西南低涡得以生成发展及维持,暴雨产生在西南涡闭合环流形成阶段,同时也是西南低空急流推动θse 锋区经过重庆上空的时段,使暴雨区获得充沛的热力和水汽供给,其低层辐合、高层辐散形势明显;强降水在低空急流到达后产生,与急流带上风速加大且向下扩展相对应,降水量随急流上西南风增大而增加。     

Nonlinear interaction of a zonal jet and barotropic Rossby-wave turbulence: the problem of turbulent friction

Interaction of a zonal jet and small-amplitude Rossby-wave turbulence is studied within the framework of the barotropic β-plane model. It is demonstrated that turbulent-laminar interaction in this case transfers energy from the wave turbulence to the laminar flow (the effect of negative friction). We derive a conclusion that, as the geophysical turbulence is determined partly by wave turbulence and none of the traditional heuristic models can adequately describe the effect of negative friction associated with wave turbulence, the application of these models to the ‘real’ ocean and atmosphere is unreliable.It is also demonstrated that, as they are affected by the turbulence, all westward jets slowly expand without strengthening. Each jet has a core, within the limits of which the velocity of the fluid is constant. In some cases, the core expands faster than the jet periphery, resulting in jumps on the profile of the flow. All eastward jets are steady irrespective of their profiles.     

An aircraft study of turbulence dissipation rate and temperature structure function in the unstable marine atmospheric boundary layer

The dissipation rate of turbulent kinetic energy, , and the temperature structure function parameter, C _T ², have been measured over water from the near surface (Z = 3 m) to the top of the boundary layer. The near surface values of and C _T ² were used to calculate the velocity and temperature Monin-Obukhov scaling parameters u _* and T _*. The data collected during unstable lapse rates were used to evaluate the feasibility of extrapolating the values of and C _T ² as a function of height with empirical scaling formulae. The dissipation rate scaling formula of Wyngaard et al. (l971 a) gave a good fit to an average of the data for Z < 0.8 Z _i. In the surface layer the scaling formula of Wyngaard et al. (1971b) disagreed with the C _T ² values by as much as 50%. This disagreement is due to an unexpected reduction in the measured values of C _T ² forZ < 30 m. At this point it is not clear if the discrepancy is a unique property of the marine boundary layer or if it is simply some unknown instrumental or analytical problem. The mixed layer scaling results were similar to the overland results of Kaimal et al. (1976).     

暴雨过程中低空急流形成的诊断分析

一、前言 分析预报经验指出,较大范围暴雨的发生同在1—4公里间低空出现强风区有密切联系,一般认为在850mb面上出现16m/s或以上的强风区可作为预报大到暴雨的一个很好指标。这种低空强风区即称为低空急流,在我国大多数为西南风急流,也有少数为东南风急流。低空急流不但能把低层水汽、热量集中往下游输送,使下游地区造成或增大     

Nocturnal CO2 exchange over a tall forest canopy associated with intermittent low-level jet activity

Summary Nocturnal eddy-covariance carbon dioxide fluxes have uncertainties arising from non-stationary atmospheric processes. Low-level jets (LLJ) are one of the prominent nocturnal boundary-layer phenomena observed over non-mountainous terrain, and are capable of generating shear and turbulence close to the ground. The influence of intermittent LLJ activity on nocturnal carbon dioxide exchange measurements is investigated using wind profile observations and eddy-covariance flux measurements over a tall forest canopy. Results suggest that the buildup and venting of CO₂ are closely associated with LLJ activity during the night. Of significance in quantifying nocturnal fluxes, this paper demonstrates how low-level jet activity introduces sporadic coupling between the canopy and the atmosphere.     

Climatic simulations of the eastern Andes low-level jet and its dependency on convective parameterizations

The South American low level jet (SALLJ) of the Eastern Andes is investigated with Regional Climate Model version 3 (RegCM3) simulations during the 2002–2003 austral summer using two convective parameterizations (Grell and Emanuel). The simulated SALLJ is compared with the special observations of SALLJEX (SALLJ Experiment). Both the Grell and Emanuel schemes adequately simulate the low level flow over South America. However, there are some intensity differences. Due to the larger (smaller) convective activity, the Emanuel (Grell) scheme simulates more intense (weaker) low level wind than analysis in the tropics and subtropics. The objectives criteria of Sugahara (SJ) and Bonner (BJ) were used for LLJ identification. When applied to the observations, both criteria suggest a larger frequency of the SALLJ in Santa Cruz, followed by Mariscal, Trinidad and Asunción. In Mariscal and Asunción, the diurnal cycle indicates that SJ occurs mainly at 12 UTCs (morning), while the BJ criterion presents the SALLJ as more homogenously distributed. The concentration into two of the four-times-a-day observations does not allow conclusions about the diurnal cycle in Santa Cruz and Trinidad. The simulated wind profiles result in a lower than observed frequency of SALLJ using both the SJ and BJ criteria, with fewer events obtained with the BJ. Due to the stronger simulated winds, the Emanuel scheme produces an equal or greater relative frequency of SALLJ than the Grell scheme. However, the Grell scheme using the SJ criterion simulates the SALLJ diurnal cycle closer to the observed one. Although some discrepancies between observed and simulated mean vertical profiles of the horizontal wind are noted, there is large agreement between the composites of the vertical structure of the SALLJ, especially when the SJ criterion is used with the Grell scheme. On an intraseasonal scale, a larger southward displacement of SALLJ in February and December when compared with January has been noted. The Grell and Emanuel schemes simulated this observed oscillation in the low-level flow. However, the spatial pattern and intensity of rainfall and circulation anomalies simulated by the Grell scheme are closer to the analyses than those obtained with the Emanuel scheme.