Characteristics of preferential flow and groundwater discharge to Shingobee Lake,Minnesota, USA

Small‐scale heterogeneities and large changes in hydraulic gradient over short distances can create preferential groundwater flow paths that discharge to lakes. A 170 m² grid within an area of springs and seeps along the shore of Shingobee Lake, Minnesota, was intensively instrumented to characterize groundwater‐lake interaction within underlying organic‐rich soil and sandy glacial sediments. Seepage meters in the lake and piezometer nests, installed at depths of 0·5 and 1·0 m below the ground surface and lakebed, were used to estimate groundwater flow. Statistical analysis of hydraulic conductivity estimated from slug tests indicated a range from 21 to 4·8 × 10^?3 m day^?1 and small spatial correlation. Although hydraulic gradients are overall upward and toward the lake, surface water that flows onto an area about 2 m onshore results in downward flow and localized recharge. Most flow occurred within 3 m of the shore through more permeable pathways. Seepage meter and Darcy law estimates of groundwater discharge agreed well within error limits. In the small area examined, discharge decreases irregularly with distance into the lake, indicating that sediment heterogeneity plays an important role in the distribution of groundwater discharge. Temperature gradients showed some relationship to discharge, but neither temperature profiles nor specific electrical conductance could provide a more convenient method to map groundwater–lake interaction. These results suggest that site‐specific data may be needed to evaluate local water budget and to protect the water quality and quantity of discharge‐dominated lakes. Copyright © 2002 John Wiley & Sons, Ltd.     

Energy budget above a temperate mixed forest in northeastern China

Components of the energy budget were measured continuously above a 300‐year‐old temperate mixed forest at the Changbaishan site, northeastern China, from 1 January to 31 December 2003, as a part of the ChinaFlux programme. The albedo values above the canopy were lower than most temperate forests, and the values for snow‐covered canopy were over 50% higher than for the snow‐free canopy. In winter, net radiation R_n was generally less than 5% of the summer value due to high albedo and low incoming solar radiation. The annual mean latent heat LE was 37·5 W m^?2, accounting for 52% of R_n. The maximum daily evaporation was about 4·6 mm day^?1 in summer. Over the year, the accumulated precipitation was 578 mm; this compares with 493 mm of evapotranspiration, which shows that more than 85% of water was returned to the atmosphere through evapotranspiration. The LE was strongly affected by the transpiration activity and increased quickly as the broadleaved trees began to foliate. The sensible heat H dropped at that time, although R_n increased. Consequently, the seasonal variation in the Bowen ratio β was clearly U‐shaped, and the minimum value (0·1) occurred on a sunny day just after rain, when most of the available energy was used for evapotranspiration. Negative β values occurred occasionally in the non‐growing season as a result of intensive radiative cooling and the presence of water on the surface. The β was very high (up to 13·0) in snow‐covered winter, when evapotranspiration was small due to low surface temperature and available soil water. Vegetation phenology and soil moisture were the key variables controlling the available energy partitioning between H and LE. Energy budget closure averaged better than 86% on a half‐hourly basis, with slightly greater closure on a daily basis. The degree of closure showed a dependence on friction velocity u*. Copyright © 2006 John Wiley & Sons, Ltd.     

宁夏水汽输送气候特征

利用NCEP／NCAR和欧洲中心提供的逐日、逐月再分析资料（包括风场、湿度场资料,分别为17层等压面和23层等压面）和地面气压场资料,计算了宁夏区域的水汽汇和水汽通量。结果表明：宁夏区域水汽汇空间分布在春季为南北正东西负型,夏季为中间正、东西侧负型,秋季为南正北负型,冬季为东正西负型。夏季整个区域的水汽汇为正值。各季的多年时间变化具有相似的特点,为线性减少趋势,3次曲线拟合为先增后减再增的趋势。宁夏地区的水汽来自3个方向：西向、西南向和东南向,其中西南向的值较大,是宁夏的主要水汽通道。多年水汽汇的19a周期通过了0．1信度的红噪声检验,多年水汽汇序列变化较为剧烈,发生了多次突变。     

Water Budgets of Tropical Cyclones: Three Case Studies

In this study, three tropical cyclones (TCs) that passed through the Taiwan Strait were analyzed; our results show that precipitation is not directly related to the intensity of TCs. From the perspective of water budget, moisture flux convergence was dominant and contributed ～70% of the moisture for TC precipitation over the ocean and almost all over the land, especially inside the TC circulation. Their spatial distributions were also similar. Evaporation contributed ～30% of the moisture for precipitation over the ocean but changed little with the time. Moisture flux convergence can be divided into two parts: wind convergence and moisture advection. Moisture flux convergence was mostly due to wind convergence, which was dominant in the southwestern quadrants of the TCs. Moisture advection was located in the northern area, and becomes relatively important when the TCs approached the land. The moisture flux convergence and its two parts varied during TC movement, with strengthening and contraction of moisture convergence present near landfall. The vertical structure of the three TC cases all indicated that the moisture convergence was mainly confined to the lower atmosphere under 800 hPa and a weak divergence region was present in the middle troposphere around 550 hPa.     

Numerical simulation of the sudden rainstorm associated with the remnants of Typhoon Meranti (2010)

The Advanced Research Weather Forecasting (ARW) model was used to simulate the sudden heavy rainstorm associated with the remnants of Typhoon Meranti in September 2010. The results showed that the heavy rainfall was produced when the remnant clouds redeveloped suddenly, and the redevelopment was caused by rapid growth of micro/mesoscale convective systems (MCSs). As cold air intruded into the warm remnant clouds, the atmosphere became convectively unstable and frontogenesis happened due to strong wind shear between weak northerly flow and strong southwesterly flow in the lower levels. Under frontogenesis-forcing and warm-air advection stimulation in updrafts, vertical convection developed intensely inside the remnant clouds, with MCSs forming and maturing along the front. The genesis and development of MCSs was due to the great progress vertical vorticity made. The moist isentropic surface became slantwise as atmospheric baroclinity intensified when cold air intruded, which reduced the convective instability of the air.Meanwhile, vertical wind shear increased because the north cold air caused the wind direction to turn from south to north with height. In accordance with slantwise vorticity development (SVD), vertical vorticity would develop vigorously and contribute greatly to MCSs. Buoyancy, the pressure gradient, and the lifting of cold air were collectively the source of kinetic energy for rainfall. The low-level southwesterly jet from the western margin of the Western Pacific Subtropical High transported water and heat to remnant clouds. Energy bursts and continuous water vapor transportation played a major role in producing intense rainfall in a very short period of time.     

广东前汛期持续性暴雨的变化特征及其环流形势

利用1961-2011年广东省86个测站地面观测逐日降水资料及1979-2011年NCEP-DOE第2套分析资料,提出了广东暴雨日的定义指标,分析了近51a广东前汛期及其各月持续性暴雨的变化特征,进一步诊断了持续性暴雨过程的环流特征和水汽输送来源.结果表明,广东暴雨新指标避免了原指标定义存在的不合理情况和需要人工操作等问题,并且得到的广东暴雨日数及暴雨累积雨量的年际变化更能反映广东的旱涝情况.51a平均前汛期持续性暴雨过程在4、5、6月分别占10%、29%和61%,20世纪60年代后期至80年代持续性暴雨多发生在5月,而60年代前中期、90年代至2011年持续性暴雨则主要在6月出现,这种年代际变化是由东亚地区中低纬度大气环流系统的变化所造成的.除了过去所认识的“三脊两槽”和“两脊一槽”两种类型天气形势外,广东前汛期持续性暴雨过程的500hPa中高纬度环流还有一种“高纬阻塞-中纬平缓型”(称为Ⅲ型),Ⅲ型主要出现于6月,占6月持续性暴雨过程的22%,因此,这种环流类型的补充提出,将减少6月持续性暴雨过程的漏报现象.由于气候背景场各月有所不同,无论是三脊二槽型还是二脊一槽型,各区域槽(或脊)的相对强弱和形态也随月份有所不同,例如二脊一槽型,东北亚地区高压脊4、5、6月逐月加强向北扩展,贝加尔湖槽区逐月变宽,中低纬度阿拉伯海以东槽逐月向西移动,孟加拉湾槽、西太平洋副热带高压逐月加强.前汛期持续性暴雨过程的主要水汽来源随月份发生变化,4月热带西太平洋地区水汽来源贡献最大,5月孟加拉湾、西太平洋和中国南海的水汽都有贡献,而6月则主要来源于孟加拉湾的水汽输送.所有类型持续性暴雨过程,广东大部分地区都处于水汽通量辐合区,说明动力辐合作用和水汽来源是暴雨的必备条件,持续性暴雨是各类中高纬度环流型的稳定维持与来源于热带的水汽输送共同作用的结果.     

陕西中南部一次秋季连阴雨中区域性暴雨的成因分析

利用常规高低空气象观测资料、探空资料和FY-2C红外云图,对2011年9月16-18日陕西中南部一次秋季连阴雨中区域性暴雨过程进行了分析,结果表明,暴雨发生在500hPa稳定的东高西低形势下,西西伯利亚阻塞高压和副热带高压强度偏强,持续强劲的东路冷空气和西南暖湿气流为区域性暴雨天气提供了有利的环流背景;高、低空急流是暴雨产生的主要影响系统。暴雨系统的层结特征与夏季暴雨相比有明显的差异,即暴雨发生在大气层结稳定的状态下;卫星云图显示,暴雨云系主要以中低云为主,云顶亮温一般最低为-43℃;低层东路的冷空气对暴雨的作用远远超过中空冷空气;低层锋区附近的强水汽辐合是秋季区域性暴雨产生的主要机制。     

新疆中天山一次城市暴雪过程诊断分析

采用NCEP逐日4次1°×1°再分析资料和Micaps常规观测等资料,对2011年3月发生在新疆中天山城市暴雪过程进行天气学诊断分析。诊断计算包括：中尺度分析、水汽通量、水汽通量散度、水平散度、垂直速度、高低空风场、螺旋度、假相当位温等。结果表明：暴雪是南北两支锋区在中亚地区交汇后东移发展造成的,降雪前乌鲁木齐城区出现东南大风,地面强烈减压升温为暴雪天气触发不稳定能量提供了热力条件,500 hPa有＞30 m·s^-1的西南急流,700 hPa存在低空切变,散度和垂直速度表现为明显的高层辐散、低层辐合的对称结构。降雪强盛期整层呈现上负下正的垂直螺旋度对,θ_se低能舌伸至200 hPa,700 hPa至400 hPa维持θ_se高能舌区,湿层厚度高达300 hPa。这种物理量场的配置有利低层湿空气聚合及向上的抬升运动,为暴雪的产生提供了必要条件。此次冷空气以偏西路径影响城区,在冷空气明显的条件下,受城市热岛效应影响,强降雪容易发生在温度较高的城区,同时降水中心倾向于出现在锋区的位置。     

Modulation of Madden-Julian Oscillation Activity by the Tropical Pacific-Indian Ocean Associated Mode

In this study, the impacts of the tropical Pacific–Indian Ocean associated mode (PIOAM) on Madden–Julian Oscillation (MJO) activity were investigated using reanalysis data. In the positive (negative) phase of the PIOAM, the amplitudes of MJO zonal wind and outgoing longwave radiation are significantly weakened (enhanced) over the Indian Ocean, while they are enhanced (weakened) over the central and eastern Pacific. The eastward propagation of the MJO can extend to the central Pacific in the positive phase of the PIOAM, whereas it is mainly confined to west of 160°E in the negative phase. The PIOAM impacts MJO activity by modifying the atmospheric circulation and moisture budget. Anomalous ascending (descending) motion and positive (negative) moisture anomalies occur over the western Indian Ocean and central-eastern Pacific (Maritime Continent and western Pacific) during the positive phase of the PIOAM. The anomalous circulation is almost the opposite in the negative phases of the PIOAM. This anomalous circulation and moisture can modulate the activity of the MJO. The stronger moistening over the Indian Ocean induced by zonal and vertical moisture advection leads to the stronger MJO activity over the Indian Ocean in the negative phase of the PIOAM. During the positive phase of the PIOAM, the MJO propagates farther east over the central Pacific owing to the stronger moistening there, which is mainly attributable to the meridional and vertical moisture advection, especially low-frequency background state moisture advection by the MJO’s meridional and vertical velocities.     

夏季青藏高原及周边上对流层水汽质量及其向平流层传输年际异常. I：水汽质量异常主导型

本文利用逐年7~8月平均的ERA-Interim再分析资料并结合SWOOSH(Stratospheric water and ozone satellite homogenized)水汽数据,分析了青藏高原及周边地区330~360 K层次水汽质量分布的年际异常特征及其成因。结果表明,水汽质量分布异常表现为整体异常型、东西偶极异常型和南北偶极异常型三个主导分布型。整体异常型在水汽质量整体偏多时,青藏高原地区对流和垂直向上的水汽质量非绝热传输偏强,上对流层为异常偏强的水汽质量非绝热辐合;此时对应南亚高压偏强,青藏高原地区上对流层的水汽质量绝热辐散和高原以西地区的水汽质量绝热辐合都异常偏强,水汽质量整体偏少时则相反。东西偶极异常型水汽质量呈西多/东少分布时,青藏高原西部(中东部)对流和垂直向上的水汽质量非绝热传输异常偏强(弱),上对流层的水汽质量非绝热辐合和水汽质量绝热辐散也异常偏强(偏弱);同时对应南亚高压偏西,青藏高原以西到伊朗高原的上对流层有异常的自东向西的水汽质量绝热输送和水汽质量绝热辐合。水汽质量呈西少/东多分布时则有相反的结果。南北偶极异常型水汽质量呈北多/南少分布时,对应南亚高压偏北,青藏高原北部的上对流层有异常自南向北的水汽质量绝热输送所造成的水汽质量辐合,同时该地区低层异常偏强的自下向上的水汽质量非绝热输送也加强水汽质量辐合,而青藏高原南侧上对流层则为异常偏弱的水汽质量绝热辐散和水汽质量非绝热辐合,水汽质量呈北少/南多分布时相反。