基于Cloudsat探测的一次非典型东北冷涡结构及其降水

采用ERA-Interim气象分析资料、云顶亮温TBB资料、Cloudsat云雷达资料、降雨量资料等,对2009年6月10日至12日我国东北地区的一次冷涡天气过程进行研究,重现了该冷涡的精细三维结构和演变过程.分析表明冷涡发生前,东北亚地区处于南北双槽结构之间,随后北槽向赤道发展切断后形成东北冷涡.南槽背景的冷涡热力结构特殊,强冷空气集中在涡内西北象限,暖湿空气在东北象限,南部为相对中性空气,该配置导致北部暖锋强盛,西部冷锋仅在发展初期较强,冷涡过程没有经典挪威学派的气旋锢囚锋出现.冷涡发展初期,狭长冷舌快速入侵南下,冷舌前冷锋对流降水较强,冷舌后部左侧还有暖锋降水;冷涡发展后期,冷锋减弱,冷锋上的高层云停止降水,系统内主要为冷涡北部的暖锋雨层云降水;冷涡成熟后,中心辐合加强,有较强的对流性降水.     

中国南部对流层中上层臭氧增加的气象场判识及臭氧变化的多尺度特征

副热带急流对中国南部地区对流层中上层臭氧浓度的影响程度及地理范围目前还研究较少,且缺乏综合使用常规气象资料及卫星资料来判识对流层中上层臭氧浓度增高的方法.本文利用NCEP再分析与最终分析资料、日本GMS-5地球静止卫星水汽云图资料,以2001年3月27~29日中国南部的临安、昆明、香港臭氧探测个例为基础,结合1996年3月29日香港与2001年4月13日临安对流层中上层高浓度臭氧分布个例对副热带急流对中国南部对流层中上层臭氧浓度的影响进行了详细分析,提出根据气象要素场判识春季中国南部对流层中上层臭氧浓度增高的充分条件为根据卫星水汽图像上的暗区、高空急流入口区的左侧辐合区、高空锋区、对流层中上层≥1 PVU的向下伸展的舌状高位涡区来综合判断.本文的分析结果表明,本文个例中对流层中上层高浓度臭氧来自平流层;香港对流层中上层低浓度臭氧来自热带海洋地区.不仅臭氧垂直廓线的多个极小与极大值表明臭氧垂直分布的多尺度变化特征,而且对流层中上层PV分布以及卫星水汽图像分析也表明大气中的多尺度运动对臭氧垂直分布特征有显著影响.本文的结果表明与副热带高空急流相联系的平流层空气侵入不仅发生在中国大陆的较高纬度地区,较低纬度的昆明与香港地区也有平流层空气侵入导致对流层中上层臭氧浓度升高.     

The effects of asymmetric potential vorticity forcing on the instability of South Asia High and Indian summer monsoon onset

Based on the theory of potential vorticity(PV),the unstable development of the South Asia High(SAH)due to diabatic heating and its impacts on the Indian Summer Monsoon(ISM)onset are studied via a case diagnosis of 1998.The Indian Summer Monsoon onset in 1998 is related to the rapidly strengthening and northward moving of a tropical cyclone originally located in the south of Arabian Sea.It is demonstrated that the rapid enhancement of the cyclone is a consequence of a baroclinic development characterized by the phase-lock of high PV systems in the upper and lower troposphere.Both the intensification of the SAH and the development of the zonal asymmetric PV forcing are forced by the rapidly increasing latent heat released from the heavy rainfall in East Asia and South East Asia after the onsets of the Bay of Bengal(BOB)monsoon and the South China Sea(SCS)monsoon.High PV moves southwards along the intensified northerlies on the eastern side of the SAH and travels westwards on its south side,which can reach its northwest.Such a series of high PV eddies are transported to the west of the SAH continuously,which is the main source of PV anomalies in the upper troposphere over the Arabian Sea from late spring to early summer.A cyclonic curvature on the southwest of the SAH associated with increasing divergence,which forms a strong upper tropospheric pumping,is generated by the anomalous positive PV over the Arabian Sea on 355 K.The cyclone in the lower troposphere moves northwards from low latitudes of the Arabian Sea,and the upper-layer high PV extends downwards and southwards.Baroclinic development thus occurs and the tropical low-pressure system develops into an explosive vortex of the ISM,which leads to the onset of the ISM.In addition,evolution of subtropical anticyclone over the Arabian Peninsula is another important factor contributing to the onset of the ISM.Before the onset,the surface sensible heating on the Arabian Peninsula is very strong.Consequently the subtropical anticyclone which dominated the Arabian Sea in spring retreats westwards to the Arabian Peninsula and intensifies rapidly.The zonal asymmetric PV forcing develops gradually with high PV eddies moving southwards along northerlies on the eastern side of the anticyclone,and a high PV trough is formed in the middle troposphere over the Arabian Sea,which is favorable to the explosive barotropic development of the tropical cyclone into the vortex.Results from this study demonstrate that the ISM onset,which is different from the BOB and the SCS monsoon onset,is a special dynamical as well as thermodynamic process occurring under the condition of fully coupling of the upper,middle,and lower tropospheric circulations.     

Teleconnection between the Indian summer monsoon onset and the Meiyu over the Yangtze River Valley

Based on the Indian and Chinese precipitation data and the NCEP-NCAR reanalysis circulation data, the relationship between the Indian summer monsoon (ISM) onset and the Meiyu over the Yangtze River Valley has been discussed by the methods of correlation analysis and composite analysis. The results show that the date of ISM onset over Kerala in the southwestern coast of the Indian Peninsula is about two weeks earlier than the beginning of the Meiyu over the Yangtze River Valley. After the outbreak of ISM, the teleconnection mode sets up from the western coast of India via the Bay of Bengal (BOB) to the Yangtze River Valley and southern Japan. It is different both in time and space from the telecon- nection mode which is from the northwest of India via the Tibetan Plateau to northern China. The for- mer mode is defined as the "south" teleconnection of the Asian summer monsoon, forming in the pe- riod of ISM onset; while the latter mode is called the "north" teleconnection, mainly occurring in the Asian monsoon culminant period. During the process of the "south" teleconnection’s formation, the Asian monsoon circulation has experienced a series of important changes: ISM onset, the northward movement of the south Asia high (SAH), the onset vortex occurrence, the eastward extension of the stronger tropical westerly belt, and the northeastward jump of the western Pacific subtropical high (WPSH), etc. Consequently, since ISM sets up over Kerala, the whole Asian continent is covered by the upper SAH after about two weeks, while in the mid- and lower troposphere, a strong wind belt forms from the Arabian Sea via the southern India, BOB and the South China Sea (SCS), then along the western flank of WPSH, to the Yangtze River Valley and southern Japan. With the northward moving of the subtropical jet streams, the upper westerly jet stream and the low level jet have been coupled ver- tically over east Asia, while the Yangtze River Valley happens to locate in the ascending motion area between the upper jet stream and the low level jet, i.e. right of the entrance of the upper jet stream and left of the low level jet. Such a structure of the vertical circulation can trigger the Meiyu onset over the Yangtze River Valley.     

基于CloudSat资料的北上江淮气旋暴雪云系结构特征

2007年3月3-5日和2013年11月24-25日,受江淮气旋北上影响,我国北方大部地区遭遇罕见暴风雪天气,2次暴雪过程有很多相似之处.利用常规观测、CloudSat卫星云廓线雷达的探测资料和NECP/NCAR再分析资料,分析了这2次暴雪过程江淮气旋云系结构和微物理特征.结果表明：（1）北上江淮气旋的冷锋云系较窄,以深厚对流云为主,回波核心在2~7 km,其结构在气旋发展的不同阶段变化不大;（2）逗点头云系范围宽广,在气旋的不同发展阶段,结构和强度有显著差异.气旋初始锋面波动和锋面断裂阶段,逗点头云系有两个降水区：北部为由多个单体组成的大范围层状云区,强回波从地表向上伸展,上空有高空对流泡,建立了播撒云-供水云机制,有利于下部冰晶粒子长大;南部有对流云柱发展.逗点头西部的冷输送带云系主要集中在6 km以下,强度弱,冰粒子含量少;（3）气旋暖锋后弯阶段,干侵入加强,冷锋后部的无云区或少云区范围扩大,逗点头云系南北范围收缩、变窄,云系的高度、强度和含水量减弱,冷锋云系也减弱;（4）气旋冷锋云系和逗点头南部的对流云柱以降雨为主,位于高纬度陆地上的逗点头云系以降雪为主,当逗点头云系处于海上有对流不稳定发展,以降雨为主.冷锋云系北部和逗点头云系南部均有由层积云或高积云组成的低云,以毛毛雨为主.冷锋云系和逗点头云系北部100-200 km的范围为随高度和距离逐渐变薄的高层云,无降水对应.     

东北冷涡引起的对流输送过程个例分析和数值模拟

平流层-对流层物质交换是影响全球大气成分收支的重要过程.过去的研究认为大尺度的交换过程在平流层-对流层物质交换中最为重要,但是近些年的研究表明,中小尺度过程对平流层-对流层物质交换也有重要贡献.本文利用OMI和MLS数据、ERA-Interim再分析资料,结合中尺度WRF模式综合分析了东北地区发生在冷涡前部和冷涡后部的两次强对流天气过程.结果表明：发生在冷涡前部暖锋云系中的强对流持续时间长,对流垂直尺度小,下平流层静力稳定度高;发生在冷涡后部的孤立强对流持续时间短,水平尺度较小,且在对流层顶附近,静力稳定度小,对流可穿出热力学对流层顶.从示踪物分布情况来看,两次强对流都可将示踪物输送到对流层顶附近,但是冷涡前部对流可将示踪物从边界层输送到整个对流层,而孤立对流是把示踪物输送到对流层顶,而不与自由对流层空气发生混合.     

African and Atlantic short-term climatic variations described from Meteosat water vapor channel

Pluriannual series of Meteosat-2 water vapor (WV) images are used to build average maps of decadal and monthly brightness temperatures in the 6.3 μm channel. This processing is applied to all the 3-hourly scenes, clear or cloudy, for July 1983 to July 1987. The ISCCP cloudiness analyses confirm that the warmest spots in the monthly WV images correspond to scenes either clear or covered with low clouds, whereas the coldest areas correspond to scenes where cloud tops above 440 hPa frequently occur. The WV statistics are then used to characterize seasonal and interannual variations of both the ITCZ (InterTropical Convergence Zone) and the warm (dry) areas, corresponding to subtropical subsidence. Thanks mainly to the seasonal variations, relationships between the variations in the ITCZ and in dry subtropical areas can be studied. It is shown that, for the Meteosat sector, a wetter subtropical high troposphere is associated with an enhanced activity of the ITCZ, and vice versa. For this area where the north-south assymetry is large, the negative water vapor feedback previously proposed seems not to occur.     

一次切断低压诱发的暖区深对流与异常副热带锋及其平流层-对流层交换

采用Cloudsat/CPR云雷达,FY2C/TBB亮温,Aura/MLS大气成分等卫星遥感资料,结合ECMWF气象分析资料和HYSPLIT4轨迹模式,研究了2009年6月一次东亚切断低压的暖区深对流和异常副热带锋面的结构和演变.分析表明,由于低压切断前的旧槽背景,在低涡的近成熟期,内部冷、暖锋降水偏弱,边沿的高空副热带锋面异常发展到对流层底部,低空西南暖湿水汽在副热带锋前聚集,形成千公里长的暖区深对流降水带.随着该锋面的快速东移,副热带锋区进入原暖区雨带,锋区热力间接次级环流的强上升支,加强了锋下冷侧（原暖湿区）的深对流,但该锋面阻挡了来自暖侧的水汽补充,降水结束.该异常副热带锋区还发生了强烈的平流层-对流层相互交换,在高空急流出口区的下方,平流层1.5PVU等位涡线向下入侵可达5.5 km（约500 hPa）处,锋下向上的深对流注入可达10 km,在入侵-注入混合区,臭氧和水汽的散点图上出现了二者浓度双高和双低的特殊气团.     

A Survey of Precipitation-Induced Atmospheric Cold Pools over Oceans and Their Interactions with the Larger-Scale Environment

Pools of air cooled by partial rain evaporation span up to several hundreds of kilometers in nature and typically last less than 1 day, ultimately losing their identity to the large-scale flow. These fundamentally differ in character from the radiatively-driven dry pools defining convective aggregation. Advancement in remote sensing and in computer capabilities has promoted exploration of how precipitation-induced cold pool processes modify the convective spectrum and life cycle. This contribution surveys current understanding of such cold pools over the tropical and subtropical oceans. In shallow convection with low rain rates, the cold pools moisten, preserving the near-surface equivalent potential temperature or increasing it if the surface moisture fluxes cannot ventilate beyond the new surface layer; both conditions indicate downdraft origin air from within the boundary layer. When rain rates exceed \(\sim\) 2 mm h\(^{-1}\), convective-scale downdrafts can bring down drier air of lower equivalent potential temperature from above the boundary layer. The resulting density currents facilitate the lifting of locally thermodynamically favorable air and can impose an arc-shaped mesoscale cloud organization. This organization allows clouds capable of reaching 4–5 km within otherwise dry environments. These are more commonly observed in the northern hemisphere trade wind regime, where the flow to the intertropical convergence zone is unimpeded by the equator. Their near-surface air properties share much with those shown from cold pools sampled in the equatorial Indian Ocean. Cold pools are most effective at influencing the mesoscale organization when the atmosphere is moist in the lower free troposphere and dry above, suggesting an optimal range of water vapor paths. Outstanding questions on the relationship between cold pools, their accompanying moisture distribution and cloud cover are detailed further. Near-surface water vapor rings are documented in one model inside but near the cold pool edge; these are not consistent with observations, but do improve with smaller horizontal grid spacings.     

Airborne Lidar Observations of Water Vapor Variability in Tropical Shallow Convective Environment

An airborne downward-pointing water vapor lidar provides two-dimensional, simultaneous curtains of atmospheric backscatter and humidity along the flight track with high accuracy and spatial resolution. In order to improve the knowledge on the coupling between clouds, circulation and climate in the trade wind region, the DLR (Deutsches Zentrum für Luft- und Raumfahrt) water vapor lidar was operated on board the German research aircraft HALO during the NARVAL (Next Generation Aircraft Remote Sensing for Validation Studies) field experiment in December 2013. Out of the wealth of about 30 flight hours or 25,000 km of data over the Tropical Atlantic Ocean east of Barbados, three ~ 2-h-long, representative segments from different flights were selected. Analyses of Meteosat Second Generation images and dropsondes complement this case study. All observations indicate a high heterogeneity of the humidity in the lowest 4 km of the tropical troposphere, as well as of the depth of the cloud (1–2 km thick) and sub-cloud layer (~ 1 km thick). At the winter trade inversion with its strong humidity jump of up to 9 g/kg in water vapor mixing ratio, the mixing ratio variance can attain 9 (g/kg)², while below it typically ranges between 1 and 3 (g/kg)². Layer depths and partial water vapor columns within the layers vary by up to a factor of 2. This affects the total tropospheric water vapor column, amounting on average to 28 kg/m², by up to 10 kg/m² or 36%. The dominant scale of the variability is given by the extent of regions with higher-than-average humidity and lies between 300 and 600 km. The variability mainly stems from the alternation between dry regions and moisture lifted by convection. Occasionally, up to 100-km large dry regions are observed. In between, convection pushes the trade inversion upward, sharpening the vertical moisture gradient that is colocated with the trade inversion. In most of the water vapor profiles, this gradient is stronger than the one located at the top of the sub-cloud layer. Lidar observations in concert with models accurately reproducing the observed variability are expected to help evaluate the role these findings play for climate.     

有限区域分解分析方法在2006年一次东北冷涡暴雨分析中的应用

2006年7月19-24日,东北地区出现一次明显的冷涡发展导致强降水的过程.对这次东北冷涡过程的天气形势分析表明,该东北冷涡的维持和发展与冷涡东部阻塞高压的建立与消亡有关.本文根据500 hPa环流形势演变特征,将东北冷涡发生发展过程分为4个阶段,并借助调和-余弦谱展开方法,对东北冷涡各阶段850 hPa水平风和水汽通量进行无辐散和无旋转分量分解,分析各阶段无旋转风动能和无辐散风动能之间的能量转化.研究结果表明,分解得到的无辐散风及其水汽通量清楚地展现出了东北冷涡的大尺度环流和水汽输送通道及水汽来源,而从无旋转风及其水汽通量上则可以直观地看到冷涡低层的中小尺度风场及水汽辐合辐散区,为分析东北冷涡内部对流提供帮助.东北冷涡发展的不同阶段其水汽来源有所不同,初始阶段的水汽主要来自黄海和渤海地区,发展阶段水汽主要来自日本海,而到成熟阶段和减弱阶段,水汽输送通道被破坏,冷涡的水汽供应大大减少,与同时期暴雨减弱一致.同时,无旋转风辐合强值区和无旋转风水汽通量大值区的重合区域有利于强对流的发生发展,表现为重合区与TBB(Temperature of Black Body,黑体辐射温度)强对流云带的形状和位置对应良好,与降水落区也较为一致,可为预报东北冷涡引发的强降水落区这一预报难点问题提供参考.从动能转化上看,无旋转风和无辐散风的动能转化项能很好地反映东北冷涡整个生命史过程中各阶段强度的变化特点,对冷涡强度预报具有一定的指示意义.     

Simultaneous measurement of humidity and temperature in the lower troposphere over Chung-Li,Taiwan

A combined Raman–Rayleigh lidar has been designed at Chung-Li, Taiwan for the simultaneous measurement of water-vapor mixing ratio, temperature and extinction-to-backscatter ratio of aerosol in the lower troposphere. The technique of Raman–Rayleigh lidar can retrieve correct temperature profile in the lower troposphere where the measurements are underestimated due to the aerosol loading. Two typical cases are discussed under different humidity (dry/wet) conditions. The water vapor and temperature profile have shown a good agreement with radiosonde. Simultaneous measurement of Raman–Rayleigh lidar also illustrates the physical nature of the aerosol and is useful in understanding the effects of humidity on aerosol swelling.     

An Observational View of Relationships Between Moisture Aggregation,Cloud, and Radiative Heating Profiles

Data from several coincident satellite sensors are analyzed to determine the dependence of cloud and precipitation characteristics of tropical regions on the variance in the water vapor field. Increased vapor variance is associated with decreased high cloud fraction and an enhancement of low-level radiative cooling in dry regions of the domain. The result is found across a range of sea surface temperatures and rain rates. This suggests the possibility of an enhanced low-level circulation feeding the moist convecting areas when vapor variance is large. These findings are consistent with idealized models of self-aggregation, in which the aggregation of convection is maintained by a combination of low-level radiative cooling in dry regions and mid-to-upper-level radiative warming in cloudy regions.     

太湖流域梅雨的划分及其典型年异常成因分析

针对目前尚缺乏客观的流域梅雨划分指标的现状,本文依据最新的梅雨监测国家标准与NCEP/NCAR再分析资料,利用19862016年太湖流域水文年鉴逐日雨量整编资料,重新划分了太湖流域入/出梅日期,计算了梅雨特征量,构建了梅雨洪水指数(RFI),并对梅雨期超设计、超警戒洪水年的环流异常成因进行分析.研究表明:(1)新标准下太湖流域多年平均在6月17日入梅,7月11日出梅,梅雨期长度24 d,梅雨量266.8 mm;与历史序列相比,新标准确定的梅雨量一致率较高,其次是入梅时间和出梅时间.(2)雨日数和副高脊线北跳时间是影响入/出梅确定的两个重要因素,新标准将1992、2013年定为空梅,1986、1987、1989、1996、2005年历史入/出梅日期向后调整,1988、2007年出梅日期向前调整,更为合理地反映了梅雨的高温高湿气候特征,客观性较强.(3)梅雨量越大,雨强越大,太湖水位越高,流域越易涝;以梅雨洪水指数作为参考因子,考虑到影响太湖洪水形成的两个关键因子(梅雨期起涨水位、最大7 d降水量占梅雨量的比例),对入梅起涨水位异常偏高、因集中强降雨引起太湖洪水的指示意义较强.(4)高低纬环流配置关系密切,来自西太平洋经南海的偏南气流、印度洋经孟加拉湾的西南暖湿气流汇合后与来自北方的冷空气在太湖流域交汇,太湖流域垂直上升运动异常强烈,触发降雨层结不稳定能量释放,导致暴雨持续形成洪水.     

The role of the stratosphere in Iberian Peninsula rainfall: A preliminary approach in February

This paper attempts to establish a connection between stratospheric anomalies in the North Pole and rainfall on the Iberian Peninsula through the occurrence of major midwinter warmings (MMWs) and cold events (CEs), taking February as a preliminary approach. We define the MMWs as the warmings which break down the polar vortex, whereas the CEs are the episodes in which the polar vortex remains cold and undisturbed. Both anomalies lead to a wind anomaly around the north polar stratosphere, which is connected with a shortly lagged tropospheric anomaly through a stratosphere–troposphere coupling in winter. A T-mode principal component analysis (PCA) was used as an objective pattern classification method for identifying the main daily surface-level pressure (SLP) patterns for February for the 1961–1990 reference period. Subsequently, those February months with an MMW or a CE influence in the troposphere are identified in the whole study period (1958–2000) by means of the Arctic Oscillation Index (AOI). Thus, performing the same analysis for the selected February months, new principal patterns for detecting changes in surface circulation structure and morphology are obtained. The results show a significant decrease in the westerlies and a southward shift of the storm tracks in Western Europe some weeks after an MMW occurrence, leading to an increase in precipitation in western Iberia and a slight decrease on the eastern Mediterranean fringe. The results are quite the opposite under a CE influence: the westerlies are strengthened and shifted northwards due to the displacement of the Atlantic anticyclone towards Central Europe; dry conditions are established throughout Iberia, except for the Mediterranean fringe, where precipitation shows a considerable increase due to the greater frequency of the northeasterly winds. Finally, an 11-year sunspot cycle–quasi-biennial oscillation (QBO) modulation might be demonstrated in Iberian rainfall in February through the occurrence of these stratospheric anomalies.     

IMF-associated cloudiness above near-pole station Vostok: Impact on wind regime in winter Antarctica

Changes in the cloudiness above the Antarctic station Vostok during the winter season were examined in relation to strong disturbances in the interplanetary magnetic field (IMF). A reliable relationship between cloud formation and IMF has been found: cloudiness increased under the influence of a strong southward IMF and decreased under the northward IMF. The surface temperature at Vostok station, which is derivative of the constant radiation cooling of air situated at the ice sheet and adiabatic warming of the air masses, incoming into the central Antarctica from the middle and upper troposphere, is enhanced or reduced. Quite opposite regularity in the temperature changes is typical of altitudes higher than the suggested cloud layer position (5–8 km). The processes occurring on the Antarctic ridge leads to anomalous winds at the ice dome and decay of the circumpolar vortex at the periphery of the Antarctic continent. As a result, the surface easterlies at the coast stations are replaced by southerlies, and the cold air masses flow from Antarctica out over the Southern Ocean.     

Effects of surface drag on low-level frontogenesis within baroclinic waves

Using a three-dimensional nonhydrostatic mesoscale numerical model (MM5), the evolution and structures of baroclinic waves with and without surface drag in case of dry and moist atmosphere are simulated, with special emphases on the effects of surface drag on the low-level frontal structure and frontogenesis. There are two different effects of surface drag on the low-level frontogenesis in the dry case. On one hand, the surface drag weakens the low-level frontogenesis and less inclined to develop the baroclinic wave due to the dissipation. But on the other hand, the surface drag induces a strong ageostrophic flow, which prolongs the low-level frontogenesis and finally leads to the enhancement of cold front. Compared with the no surface drag case, the surface drag increases the frontal slope espe- cially in the boundary layer, where the front is almost vertical to the surface, and then enhances the prefrontal vertical motion. All these conclusions expanded the analytical theory of Tan and Wu (1990). In the moist atmosphere, the influence of surface drag on frontal rainbands is also obvious. The surface drag weakens the convection, and reduces the energy dissipation near the surface when the initial relative humidity is relatively weak. At this time, the confluence induced post-frontal updrafts moves across the cold front and reinforces the prefrontal convection, which is beneficial to the maintenance of the rainband in cold sector. Given the enhancement of relative humidity, the moist convection domi- nates the low-level frontogenesis while the retardation of surface drag on energy dissipation is not obvious, therefore the effects of surface drag on the low-level frontogenesis and precipitation are re- duced.     

A distribution law for relative humidity in the upper troposphere and lower stratosphere derived from three years of MOZAIC measurements

Data from three years of MOZAIC measurements made it possible to determine a distribution law for the relative humidity in the upper troposphere and lower stratosphere. Data amounting to 13.5% of the total were obtained in regions with ice supersaturation. Troposphere and stratosphere are distinguished by an ozone concentration of 130 ppbv as threshold. The probability of measuring a certain amount of ice supersaturation in the troposphere decreases exponentially with the degree of ice supersaturation. The probability of measuring a certain relative humidity in the stratosphere (both with respect to water and ice) decreases exponentially with the relative humidity. A stochastic model that naturally leads to the exponential distribution is provided. Mean supersaturation in the troposphere is about 15%, whereas ice nucleation requires 30% supersaturation on the average. This explains the frequency of regions in which aircraft induce persistent contrails but which are otherwise free of clouds. Ice supersaturated regions are 3-4 K colder and contain more than 50% more vapour than other regions in the upper troposphere. The stratospheric air masses sampled are dry, as expected, having mean relative humidity over water of 12% and over ice of 23%, respectively. However, 2% of the stratospheric data indicate ice supersaturation. As the MOZAIC measurements have been obtained on commercial flights mainly between Europe and North America, the data do not provide a complete global picture, but the exponential character of the distribution laws found is probably valid globally. Since water vapour is the most important greenhouse gas and since it might enhance the anthropogenic greenhouse effects via positive feedback mechanisms, it is important to represent its distribution correctly in climate models. The discovery of the distribution law of the relative humidity makes possible simple tests to show whether the hydrological cycle in climate models is represented in an adequate way or not.     

A comparative study of the atmospheric circulations associated with rainy-season floods between the Yangtze and Huaihe River Basins

Here we present the results from the composite analyses of the atmospheric circulations and physical quantity fields associated with rainy-season for the selected floods cases over the Yangtze and Huaihe River basins for the 21 years(1990–2010),using the daily rain gauge measurements taken in the 756 stations throughout China and the NCEP/reanalysis data for the rainyseasons(June–July)from 1990 to 2010.The major differences in the atmospheric circulations and physical quantity fields between the Yangtze and Huaihe River basins are as follows:for flooding years of the Yangtze River Basin,the South Asia high center is located further east than normal,the blocking high over the Urals and the Sea of Okhotsk maintains,and the Meiyu front is situated near 30°N whereas for flooding years of the Huaihe River Basin,the South Asia high center is further west than normal,the atmospheric circulations over the mid and high latitudes in the Northern Hemisphere are of meridional distribution,and the Meiyu front is situated near 33°N.In addition,there are distinct differences in water vapor sources and associated transports between the Yangtze and Huaihe River basins.The water vapor is transported by southwesterly flows from the Bay of Bengal and monsoon flows over the South China Sea for flooding years of the Yangtze River Basin whereas by southeast monsoons from the eastern and southern seas off China and monsoon flows over the South China Sea for flooding years of the Huaihe River Basin.     

Air–Sea Interaction Processes Influencing the Development of A Shapiro–Keyser Type Cyclone over the Subtropical South Atlantic Ocean

This study analyzes the impacts of latent and sensible heat exchanges between the atmosphere and the ocean in a non-explosive Shapiro–Keyser type cyclogenesis event that occurred over the southwestern South Atlantic Ocean. The synoptic evolution shows a relatively strong warm front and a cold frontal fracture during the system’s development and a warm seclusion in its mature stage, characterizing a Shapiro–Keyser type cyclone. Numerical experiments with the ARW-WRF Model version 3.3 were used to investigate the influences of sensible and latent fluxes on the track of the surface low, intensity of the fronts and coupling of the lower and upper troposphere. The simulations indicate that in the presence of these fluxes the cyclone underwent greater intensification, had a longer life time and longer trajectory, and presented a typical southeastward movement. In the absence of these fluxes, the cyclone developed a weaker warm front with consequent reduction of diabatic heating due to grid scale precipitation along it. This reduced the negative pressure tendency southeast of the cyclone center and the surface cyclone moved northeastward, showing a decoupling of the lower- and upper-level waves. A consequence of this anomalous tracking is the location of the surface cyclone beneath the upper-level trough axis, where there is no upper-level divergence associated with cyclonic vorticity advection contributing to the further system intensification. Numerical experiments suggest that for this Shapiro–Keyser type cyclone the air–sea interaction processes are crucial to obtain a cyclone with features similar to the observations.