Role of unusual moisture transport across Equatorial Indian Ocean on the extreme rainfall event during Kerala flood 2018

Southwestern Indian state, Kerala experienced extreme devastating statewide flood event of the century during 2018 monsoon season. In this study, an attempt has been made to bring out the salient dynamical factors contributed to the Kerala flood. There were 3 active spells over Kerala during 2018 Monsoon season. All the three spells were accustomed with the intrinsic factors of low frequency components of the active spells such as strength of monsoon Low Level Jet (LLJ), Monsoon depressions in the Bay of Bengal, favorable Madden-Julian oscillation (MJO) phases and Western Pacific systems. Though all the common ingredients remain same, the third spell is distinct with the less evaporation flux over Western and Central Arabian Sea and unusual moisture transport from maritime continent through South Equatorial Indian ocean (SEIO) towards the Kerala coast across Equator. Strong meridional pressure gradient force created by the combined effect of high pressure anomaly oriented Northwest-Southeast direction across tropical Indian ocean and anomalous low pressure along monsoon trough might have contributed to this unusual moisture transport across SEIO originating from west of Australia. The anomalous high pressure in South Indian ocean was greatly influenced by the position of the Mascarene high. Subtropical Indian ocean dipole (SIOD) also exhibits an influential role by altering tropical Southern Indian ocean dynamics in favor of the unusual moisture transport. The position of the monsoon depression and presence of typhoons in Western Pacific might have aided to this moisture transport. However, the normal moisture transport from Central Arabian sea towards Kerala coast by virtue of the strong LLJ along with additional moisture transport directly from South of maritime continent through SEIO across the Equator towards Kerala coast might have played a dominant role in the historical flood event over entire Kerala state.     

Effect of offshore troughs on the South India erratic summer monsoon rainfall in June 2017

The onset and advance of southwest monsoon are accompanied by the appearance of the offshore trough along the southwest coast of India. This offshore trough escorts a deluge of rainfall to the southwest coast, and sometimes rainfall band moves eastward further into south India. These broad observations were noticed during the summer monsoon of June 2017. Meteorological agencies and media had reported a huge amount of rainfall over the southwest coast of India during the month. But, in the far interior of south India, rainfall was less. Due to the less rainfall, water resources depleted, which affected local farmers and common man of south India. The confused views of the common man on southwest coast rainfall could be due to lack of understanding related to various factors affecting rainfall over the same region. This article is an endeavor to address the preliminary understanding of the southwest coast rainfall during June 2017, with more stress on offshore troughs. The study begins with area-averaged rainfall statistics over south, southwest, and southeast India by employing satellite and rain gauge merged rainfall datasets. Area averaged analysis revealed offshore trough contributed 80 % of rainfall over the South West India, 68 % over South East India, contributing to an overall 75 % over south India in 2017. To identify offshore trough position and strength in the reanalysis and model simulations, a new method called VSV (Vertical Shear of Vorticity) method was introduced. The computed offshore troughs were categorized into Active, Normal, and Feeble based on the strength of meridional gradient of mean sea level pressure and 850 hPa horizontal winds. The contribution due to each category of the offshore trough over different sub-regions was investigated to find out the effect of the offshore trough to total rainfall. Dynamic and thermodynamic features of these categories of the offshore trough were investigated by using proxies like equivalent potential temperature and moisture flux convergence. We found that during active offshore trough an eastward propagation of rain bands persists, which was explained by using moisture flux convergence and equivalent potential temperature at different levels of the atmosphere.     

人类活动排放的CO2及气溶胶对20世纪70年代末中国东部夏季降水年代际转折的影响

全球变暖背景下,中国东部夏季降水在20世纪70年代末开始较19世纪呈现东北及长江中、下游地区多雨,华北及华南少雨的特征。与此同时,人类活动排放的CO₂及气溶胶量也发生了明显的年代际变化。文中利用地球系统耦合模式（CESM）诊断了中国东部夏季的水分收支对人类活动排放的CO₂及气溶胶年代际变化的响应。发现CO₂排放量增加后,江淮流域的水汽辐合以及中国南方的水汽辐散主要是与质量辐散有关的动力项及与湿度梯度相关的热力项共同作用的结果,但动力作用更显著。气溶胶效应则主要通过动力作用使得江淮流域水汽辐合,而中国南方地区水汽辐散。虽然CO₂和气溶胶对辐射量及温度的影响差别很大,但通过改变温度梯度,热成风效应产生的动力作用都会导致江淮流域上升运动增强,降水增多;而中国南方下沉运动显著,降水减少,与观测结果一致,且CO₂相较于气溶胶的影响更为显著,证实了20世纪70年代末人类活动对中国东部夏季降水年代际转折的影响。      

西北西部夏季干湿年的水汽输送特征

利用1961—2007年NCEP/NCAR逐月再分析资料和中国地面气候资料国际交换站数据集台站降水距平百分率资料,分析了西北西部干、湿年夏季的水汽输送差异。结果表明,北疆偏湿润年,在对流层中、低层有一支源于阿拉伯海的异常水汽输送通道,它向西北方向流经波斯湾后折向东北方向流入北疆,这是热带海洋水汽输送进入北疆的最短路径。南疆—河西走廊西部偏湿润年,异常水汽通道主要是位于对流层中、低层的一支源于北方的水汽输送带,它作反气旋式运动后以偏东气流的形式流入河西走廊西部及南疆,由于该水汽输送源自北方,水汽含量小,这可能是造成该地域极度干旱的直接原因之一;另一支更弱的异常水汽通道位于对流层高层,它源自阿拉伯海,流经印度半岛后折向北,越过青藏高原后进入南疆。     

2008年河南黄淮地区暴雨过程个例分析

利用常规气象资料和NCEP资料对2008年7月22日河南黄淮地区的暴雨过程进行了分析.结果表明:这次过程是在500 hPa槽前西南气流引导下,高低空急流耦合区内西南涡沿切变线移出,弱冷空气侵入暖倒槽触发不稳定能量释放造成的.垂直螺旋度计算结果显示:中低层正垂直螺旋度中心与降水落区有很好的对应关系,大暴雨中心位于正垂直螺旋度中心附近.湿位涡演变分析发现,这次过程有"干侵入"发生,暴雨区中低层对流不稳定和对称不稳定共存,有利于降水增幅.水汽条件分析表明:这次过程的水汽源地在孟加拉湾和南海,主要是低层和近地层的水汽辐合.     

西北太平洋热带气旋快速加强过程中的水汽特征分析

利用NCEP的1°(纬度)×1°(经度)全球最终分析资料和JTWC(Joint Typhoon Warning Center)最佳路径资料,对2002~2011年西北太平洋热带气旋(TC)非减弱阶段快速加强(Rapid Intensification,RI)和缓慢加强及强度稳定(Non-RI)过程中,TC环境场及其内部各区域水汽分布和输送特征进行统计分析,揭示水汽因子对TC随后24 h强度变化的影响,为TC强度突变的趋势预报提供依据。结果表明:对流层低层900 h Pa层半径3~10纬距区域平均相对湿度(RH_3-10)能明显区分RI与Non-RI过程,说明西北太平洋TC强度变化对水汽的敏感高度较大西洋更接近洋面;RI初始时刻的RH_3-10显著大于Non-RI,而水平水汽通量(F_all)则弱于Non-RI,说明RI开始时刻TC环境表现为高水汽含量和较小的水汽输送,而随着RI过程TC内强对流发展对水汽的消耗,水汽含量明显减小故水汽通量则出现增强;RI和Non-RI过程水汽因子的分布和输送在TC内核区和外雨带差异明显,初始时刻RI过程净水汽获得区域大于Non-RI。相关性分析同样表明,适宜的相对湿度和水汽通量是非减弱阶段RI的有效潜势预报因子。     

Contribution of realistic soil moisture initial conditions to boreal summer climate predictability

Seasonal climate forecasts mainly rely on the atmospheric sensitivity to its lower boundary conditions and on their own predictability. Besides sea surface temperature (SST), soil moisture (SM) may be an additional source of climate predictability particularly during boreal summer in the mid-latitudes. In this work, we investigate the role of SM initial conditions on near-surface climate predictability during ten boreal summer seasons using three complementary ensembles of AMIP-type simulations performed with the Arpège-Climat atmospheric general circulation model. First we have conducted an assessment of the SM predictability itself through a comparison of simple empirical SM models with Arpège-Climat. The statistical and dynamical models reveal similar SM prediction skill patterns but the Arpège-Climat reaches higher scores suggesting that it is at least suitable to explore the influence of SM initialization on atmospheric predictability. Then we evaluate the relationships between SM predictability and some near surface atmospheric predictability. While SM initialization obviously improves the predictability of land surface evaporation, it has no systematic influence on the precipitation and near surface temperature skills. Nevertheless, the summer hindcast skill is clearly improved during specific years and over certain regions (mainly north America and eastern Europe in the Arpège-Climat model), when and where the SM forcing is sufficiently widespread and strong. In this case, a significant impact is also found on the occurrence of heat waves and heavy rains, whose predictability at the seasonal timescale is a crucial challenge for years to come.     

察尔汁盐湖抽卤过程中晶间卤水氚含量变化趋势的研究

本文将氚的测定方法应用于察尔汗盐湖抽卤过程中晶间卤水氖含量变化趋势的研究，以多次的测试数据研究和探讨了大气降水、周边水、地下水对察尔汗盐湖晶间卤水的影响，特别是开渠后湖水对晶间卤水的影响，从水平和垂直两个面讨论察尔汗首采区晶间卤水氚含量的变化规律。     

西北干旱地区大气中水汽的平均输送

本文利用探空资料对西北干旱地区的水汽输送进行了分析，结果表明：（１）平均而言，该地区最大水汽输送高度在６００－７００ｈＰａ，但由于局地的影响，有些地方在近地面；（２）该区域夏季水汽输送量是冬季的两倍；（３）该区域的损失水量是由流入该流域的地表径流蒸发补给的。     

An application of Ensemble Kalman Filter in integral-balance subsurface modeling

Data assimilation method provides a framework to decrease the uncertainty of hydrological modeling by sequentially incorporating observations into numerical model. Such a process involves estimating statistical moments of different order based on the evolution of conditional probability distribution function. Because of the nonlinearity of many hydrological dynamics, explicit and analytical solutions for moments of state distribution are often impossible. Evensen [J Geophys Res 99(c5): 10143–10162 (1994)] introduced Ensemble Kalman Filtering (EnKF) method to address such problems. We test and evaluate the performance of EnKF in fusing model predictions and observations for a saturated–unsaturated integral-balance subsurface model. We find EnKF improve the model predictions, and also we conclude a good estimate of state variance is essential for the success of EnKF.