高空西风急流和低空南风急流中的冷锋环流

文中利用原始方程模式研究了不同背景风场中的冷锋环流，计算结果显示，对越锋非地转环流和锋区垂直运动起主要作用的是垂直于锋面的背景风场，沿锋的背景风场也有影响，但比前者小得多，后者产生的越锋非地转环流和上升速度也比前者弱得多；具有强垂直切变的西风急流更有利于在锋区产生强的越锋环流和大的上升速度；此外，越锋非地转环流还与Richardson数的小值区相联系。     

欧亚大陆雪盖月际变化特征及其与我国气候异常的关系初探

利用ＮＯＡＡ提供的逐月欧亚大陆雪盖面积指数资料，分析了欧亚大陆雪盖月际动态演变特征，结果表明其月际动态演变特征非常复杂，且具有明显的年代际和年际变化，此外，还初步探讨了其与后期区域气候的关系。     

Advances in Low-Level Jet Research and Future Prospects

The low-level jet （LLJ） is closely related to severe rainfall events, air pollution, wind energy utilization, aviation safety, sandstorms, forest fire, and other weather and climate phenomena. Therefore, it has attracted considerable attention since its discovery. Scientists have carried out many studies on LLJs and made significant achievements during the past five or six decades. This article summarizes and assesses the current knowledge on this subject, and focuses in particular on three aspects： 1） LLJ classification, definition, distribution, and structure; 2） LLJ formation and evolutionary mechanisms; and 3） relationships between LLJ and rainfall, as well as other interdisciplinary fields. After comparing the status of LLJ research at home （China） and abroad, we then discuss the shortcomings of LLJ research in China. We suggest that this includes： coarse definitions of the LLJ, lack of observations and inadequate quality control, few thorough explorations of LLJ characteristics and formation mechanisms, and limited studies in interdisciplinary fields. The future prospects for several LLJ research avenues are also speculated.     

Variability of modeled runoff over China and its links to climate change

Runoff is a key component of the water cycle over land, with direct impact on regional ecosystems and water resources. This study investigates historical runoff variability and change over China in 1951–2008 using the Community Land Model and in situ observations of atmospheric forcing fields. Model simulations are first evaluated against in situ observations of streamflow for four major rivers, as well as soil moisture and water table depths, before further analysis is conducted. Then, quantile regression is used to analyze runoff variability and its relation to precipitation and temperature. The spatial pattern of monthly climatological runoff over China is characterized by maxima in the humid south and a gradual decrease toward the arid northwest. Runoff increases in the humid south, slightly decreases in the transition zone, and shows nonsignificant trends in the arid northwest. The footprint of decadal variability can be seen from 1951 to 2008. The annual precipitation advances the spatiotemporal variability of runoff despite locally distinct runoff–precipitation responses. The runoff-temperature relationship shows complex spatiotemporal characteristics that depend on the feedback from precipitation.     

低空急流在一次持续性暴雨过程中的作用

本文利用常规高空观测、地面加密自动站和NCEP再分析资料,对2012年8月30日～9月1日发生在四川盆地东部的一次持续性暴雨过程进行了分析.结果表明,过程期间低层西南风风速和雨强具有明显的日变化特征,低层西南风风速变化表明西南低空急流具有日变化特征;低空急流的日变化使得其对暴雨区的水汽、热力和动力条件的输送也具有日变化,从而造成暴雨过程期间雨强在夜间增强白天减弱;低空急流及其输送的水汽、热力和动力条件主要位于700hPa层附近.     

低空急流与山西大暴雨的统计关系及流型配置

利用山西省气象信息中心归档的109个测站1957—2008年的暴雨观测资料、高低空常规气象观测资料及山西省水文站的部分暴雨监测资料,研究了低空急流与山西省区域性暴雨、大暴雨以及特大暴雨的特征。结果表明:1957—2008年的192个暴雨日(区域性暴雨)、118个大暴雨日、10个特大暴雨日,有偏南风(或偏东风)最大风轴相伴出现的分别有165,110和10次,分别占其总次数的86%,93.2%和100%,表明其预示性极强;当山西大暴雨中有台风介入时,台风一般沿25°N以南西行,在其北侧与副热带高压之间形成强的东到东南风带,经鄂西北、豫西一带受西风槽或高原低值系统前部的偏南气流阻挡折向西北使副热带高压西北侧的西南气流加强,形成中尺度低空西南急流;当无台风介入时,热带辐合区一般在17°N以南,高原低值系统活跃,副热带高压与高原低值系统之间形成的天气尺度西南低空急流常常伸向黄河中游,大暴雨的落区主要在太原以南地区;根据高、低空急流的位置以及有、无台风介入,归纳出的6种大暴雨预报模型基本涵盖了山西大暴雨的落区与中、低空急流的关系,但中、低空急流的类别、位置不同以及经、纬向副热带高压、500hPa急流、西风槽、高原槽和台风等的不同配置,可使大暴雨的落区和强度不同。     

On the Turbulence in the Upper Part of the Low-Level Jet: An Experimental and Numerical Study

The characteristics of low-level jets (LLJ) observed at the “Centro de Investigacion de la Baja Atmósfera” (CIBA) site in Spain are analysed, focussing on the turbulence generated in the upper part of the jet, a feature that is still to be thoroughly understood. During the Stable Boundary Layer Experiment in Spain (SABLES) 1998, captive balloon soundings were taken intensively, and their analyses have highlighted the main characteristics of the jet’s wind and temperature structure, leading to a composite profile. There are indications that the turbulence has a minimum at the level of the wind maximum, with elevated turbulence in a layer at a height between two and three times that of the LLJ maximum, but no direct measurements of turbulence were available at these heights. In September 2001, a 100-m tower at the same site was re-instrumented to give turbulence measurements up to 96.6 m above ground level. All occurrences of LLJ below this height between September 2002 and June 2003 have been selected and significant turbulence above the LLJ has been found. Simulations with a single-column turbulence kinetic energy model have been made in order to further investigate the generation of elevated turbulence. The results correlate well with the measurements, showing that in the layer above the LLJ, where there is significant shear and weakly stable stratification, conditions are conducive to the development of turbulence.     

Variability of extreme events in the Colombian Pacific and Caribbean catchment basins

This paper analyses the behavior of extreme events of surface precipitation and temperature inside the Pacific and Caribbean Catchment Basins in Colombia using several datasets such as observations, reconstructed data, NCEP-NCAR and ERA-40 reanalyses and data from the regional model REMO. We use an extreme value method that selects the time series excesses over a nonstationary threshold and adjusts them to a generalized Pareto distribution. The goodness of fit is evaluated through a test that includes the Cramer–von Mises, Kolmogorov–Smirnov and Anderson–Darling statistics and the p values generated by parametric bootstrap resampling. The test not only evaluates the goodness of fit but also the threshold choice. The parameters are presented in maps that allow recognition of the features of the extreme behaviour inside the catchment basins, and differences and similarities between them. Maps of return periods for the maximum extreme events are also presented. A strong influence of the El Niño–Southern oscillation on the extreme events of both temperature and precipitation is found in the two catchment basins.     

茂名一次暴雨低空急流脉动与强降水的关系

利用茂名博贺观测站提供的高时空分辨率的LQ-7风廓线雷达资料对茂名2008年6月5—6日暴雨过程的强降雨时段进行分析,结果表明:超低空急流风速中心的出现和较强降水的出现在时间上相吻合;低空急流的脉动及向地面扩展程度与中小尺度强降水之间有着密切关系,低空急流指数对强降水的出现和雨强的大小有一定的预示作用,对短时强降水的临近预报也有着指示作用。     

Projected changes in the physical climate of the Gulf Coast and Caribbean

As the global climate warms due to increasing greenhouse gases, the regional climate of the Gulf of Mexico and Caribbean region will also change. This study presents the latest estimates of the expected changes in temperature, precipitation, tropical cyclone activity, and sea level. Changes in temperature and precipitation are derived from climate model simulations produced for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4), by comparing projections for the mid- and late-21st century to the late 20th century and assuming a “middle-of-the-road” scenario for future greenhouse gas emissions. Regional simulations from the North America Regional Climate Change Program (NARCCAP) are used to corroborate the IPCC AR4 rainfall projections over the US portion of the domain. Changes in tropical cyclones and sea level are more uncertain, and our understanding of these variables has changed more since IPCC AR4 than in the case of temperature and precipitation. For these quantities, the current state of knowledge is described based on the recent peer-reviewed literature.     

Impact of the Nocturnal Low-Level Jet and Orographic Waves on Turbulent Motions and Energy Fluxes in the Lower Atmospheric Boundary Layer

The nocturnal low-level jet (LLJ) and orographic (gravity) waves play an important role in the generation of turbulence and pollutant dispersion and can affect the energy production by wind turbines. Additionally, gravity waves have an influence on the local mixing and turbulence within the surface layer and the vertical flux of mass into the lower atmosphere. On 25 September 2017, during a field campaign, a persistent easterly LLJ and gravity waves were observed simultaneously in a coastal area in the north of France. We explore the variability of the wind speed, turbulent eddies, and turbulence kinetic energy in the time–frequency and space domain using an ultrasonic anemometer and a scanning wind lidar. The results reveal a significant enhancement of the turbulence-kinetic-energy dissipation (by?50%) due to gravity waves in the LLJ shear layer (below the jet core) during the period of wave propagation. Large magnitudes of zonal and vertical components of the shear stress (approximately 0.4 and 1.5 m² s^?2, respectively) are found during that period. Large eddies (scales of 110 to 280 m) matching the high-wind-speed regime are found to propagate the momentum downwards, which enhances the mass transport from the LLJ shear layer to the roughness layer. Furthermore, these large-scale eddies are associated with the crests while comparatively small-scale eddies are associated with the troughs of the gravity wave.

     

Future climate of the Caribbean from a super-high-resolution atmospheric general circulation model

Present-day (1979–2003) and future (2075–2099) simulations of mean and extreme rainfall and temperature are examined using data from the Meteorological Research Institute super-high-resolution atmospheric general circulation model. Analyses are performed over the 20-km model grid for (1) a main Caribbean basin, (2) sub-regional zones, and (3) specific Caribbean islands. Though the model’s topography underestimates heights over the eastern Caribbean, it captures well the present-day spatial and temporal variations of seasonal and annual climates. Temperature underestimations range from 0.1 °C to 2 °C with respect to the Japanese Reanalysis and the Climatic Research Unit datasets. The model also captures fairly well sub-regional scale variations in the rainfall climatology. End-of-century projections under the Intergovernmental Panel on Climate Change SRES A1B scenario indicate declines in rainfall amounts by 10–20 % for most of the Caribbean during the early (May–July) and late (August–October) rainy seasons relative to the 1979–2003 baselines. The early dry season (November–January) is also projected to get wetter in the far north and south Caribbean by approximately 10 %. The model also projects a warming of 2–3 °C over the Caribbean region. Analysis of future climate extremes indicate a 5–10 % decrease in the simple daily precipitation intensity but no significant change in the number of consecutive dry days for Cuba, Jamaica, southern Bahamas, and Haiti. There is also indication that the number of hot days and nights will significantly increase over the main Caribbean basin.     

On the Formation of an Elevated Nocturnal Inversion Layer in the Presence of a Low-Level Jet: A Case Study

We report on observed nocturnal profiles, in which an inversion layer is located at the core of a low-level jet, bounded between two well-mixed layers. High-resolution vertical profiles were collected during a field campaign in a small plain in the Israeli desert (Negev), distant 100 km from the eastern shore of the Mediterranean Sea. During the evening hours, the synoptic flow, superposed on the late sea breeze, forms a low-level jet characterized by a maximum wind speed of 12 m s ⁻¹ at an altitude of 150 m above the ground. The strong wind shear at the jet maximum generates downward heat fluxes that act against the nocturnal ground cooling. As a result, the typical ground-based nocturnal inversion is “elevated” towards the jet centre, hence a typical early morning thermal profile is observed a few hours after sunset. Since the jet is advected into the region, its formation does not depend on the presence of a surface nocturnal inversion layer to decouple the jet from surface friction. On the contrary, here the advected low-level jet acts to hinder the formation of such an inversion. These unusual temperature and wind profiles are expected to affect near-ground dispersion processes.     

Investigating the Summertime Low-Level Jet Over the East Coast of the U.S.A.: A Case Study

The vertical structure of the marine atmospheric boundary layer (MABL) and the summertime low-level jet (LLJ) along the east coast of the U.S.A. was studied in the framework of a joint expedition (CBLAST-Low), carried out during the summer of 2003 (31 July–27 August) at Nantucket island, Massachusetts, U.S.A. Analyzing measurements from radiosondes, in-situ and remote sensing systems, it was found that within the lower part of the stable MABL, intermittent and persistent LLJ events were frequently observed. Evidence is presented implying that the analyzed jet case was induced by the interaction of a slow-moving cyclone over north-eastern U.S.A. and the stationary high pressure system over the Atlantic Ocean, as well as by the sloping inversion of the MABL. Focused on a 5-day period of persistent south-westerly (marine) flow, the analysis of sodar and radiosonde data revealed the presence of a stable layer associated with increased static stability just before the emergence of low-level flow acceleration. As indicated by the Richardson number profiles, the increased stability of the lower MABL suppressed turbulence, allowed the decoupling of LLJ from friction, providing a favourable environment for the development of inertial oscillations. Significant amplitudes of inertial motions, which were confirmed by the application of a Hilbert–Huang transform, are associated with the acceleration at the LLJ’s core, due to the frontal events and the subsequent frictional decoupling, both leading to a modification of the large-scale flow structure.     

夏季东亚高空急流的变化及其对东亚季风的影响

东亚高空急流是东亚夏季风系统的一个重要组成部分,对东亚地区的天气和气候有着重要影响.近十年来,对东亚高空的变异规律及其相关机理、急流对东亚气候的影响等方面做了大量的观测资料分析和数值模拟研究.本文从夏季东亚高空急流的变化特征及其与东亚气候的关联、东亚高空急流的变异机理以及当前的气候模式对东亚高空急流的模拟能力评估和未来预估等几个方面,对这些研究结果进行了综述.     

Variability of frost depth in the Volga River basin and its impact on runoff formation in winter and spring under climate change

The results of the analysis of spatiotemporal variability of frost depth in the Volga River basin are presented. It is demonstrated that in the geographic zones of the basin the mean frost depth decreased in wide limits: from 10-20 to 90-100 cm and more. It was revealed that the frost depth decrease in the Volga River basin leads to the average increase in winter runoff by 17% (the variation range is from 3 to 42%).     

机器学习的原理及其在气候预测中的潜在应用

经历了两次"人工智能寒冬"之后,机器学习于近十年再次进入大众视野,且有腾飞发展之势,已在图像识别和语音识别系统等实际应用方面取得了巨大成功.从已知数据集中总结关键信息和主要特征,从而对新数据做出准确的识别和预测,分别是机器学习的主要任务和主要目标之一.从这个角度看,将机器学习整合到气候预测的思路切实可行.本文,首先以线...     

Assessing the effect of domain size over the Caribbean region using the PRECIS regional climate model

This study investigates the sensitivity of the one-way nested PRECIS regional climate model (RCM) to domain size for the Caribbean region. Simulated regional rainfall patterns from experiments using three domains with horizontal resolution of 50 km are compared with ERA reanalysis and observed datasets to determine if there is an optimal RCM configuration with respect to domain size and the ability to reproduce important observed climate features in the Caribbean. Results are presented for the early wet season (May–July) and late wet season (August–October). There is a relative insensitivity to domain size for simulating some important features of the regional circulation and key rainfall characteristics e.g. the Caribbean low level jet and the mid summer drought (MSD). The downscaled precipitation has a systematically negative precipitation bias, even when the domain was extended to the African coast to better represent circulation associated with easterly waves and tropical cyclones. The implications for optimizing modelling efforts within resource-limited regions like the Caribbean are discussed especially in the context of the region’s participation in global initiatives such as CORDEX.     

帕米尔高原东部径流量变化及其对气候变化的响应

水资源是制约中国西北干旱区社会经济可持续发展和生态安全的关键因素.以发源于帕米尔高原东部的喀什噶尔河和叶尔羌河流域为研究区,基于该区6个气象站月平均气温和降水量观测资料,以及5条代表性河流的出山口水文站1950年代晚期以来的月径流量观测数据,分析了该区域气候和水文年际变化特征,以及气候变化背景下径流量的响应特征.结果发...