A hydrometeorological analysis of the severe rainstorm of 20–22 September 1900 over the Gangetic West Bengal

A hydrometeorological study is made of the September, 1900 severe rainstorm which led up to the record rain-falls over Gangetic West Bengal with subsequent disastrous flooding in the Damodar and the Hooghly rivers. The spatial extent of the rainstorm for different durations has been examined by constructing the isohyetal patterns based on rainfall records of stations affected by the storm. Areal rainfalls for 1,2 and 3-day periods are calculated and the values have been compared with similar values from other major rainstorms of the region. The comparison revealed that the September, 1900 rainstorm was the heaviest for 1,2 and 3-day durations for all the areas. The storm contrib-uted rainfalls of 33.0 cm, 52.0 cm and 62.0 cm over an area of 10,000 km2 in 1,2 and 3 days respectively. This rainstorm could, therefore, be considered as an important input in flood and design storm studies in the Gangetic West Bengal region. A relationship between point to areal rainfall has also been developed with a view to evaluate areal PMP estimates.     

强对流天气预报的一些基本问题

利用常规观测资料、多普勒天气雷达资料以及NCEP/NCAR再分析资料等,对2017年5月11日和6月5日发生在湖南的两次飑线过程（以下分别简称\     

A composite study of early summer squall lines and their environment over West Africa

Summary Composites of synoptic reports and wind observations in a coordinate system moving with the squall lines over the WAMEX area in June 1979 allow us to show the average structure of the airflow, the moisture and temperature fields in the synoptic scale environment of West African continental squall lines.The monsoon front, the African Easterly Jet, the easterly waves, and the squall lines interact and produce the observed air flow patterns. The results of the composite enalysis reveal a northward surge of the monsoonal air masses in the rear of the squall lines thus providing favorable conditions for the generation of new squall lines.With 14 Figures     

华东飑线过程中的地面中尺度物理特征

本文利用华东中尺度天气试验资料和雷达回波,分析了9例强飑线过程中逐时地面物理量场的变化。提出中尺度散度场的配置及变化与中尺度强对流天气有十分密切的关系,而中尺度扰动辐合强度演变与锋前暖区内的中尺度扰动风场汇合线有关,这些关系往往能预示中尺度强对流天气的出现和发展。有时,在飑线发展的过程中具有中尺度重力波特征。     

A note on: Estimation of rainfall due to squall lines over West Africa using meteosat imagery

Summary The results of the first step of a project to develop a method to estimate precipitation over the Soudano-Sahelian belt of West Africa are reported.The study has been performed over the period from 10 June to 9 July 1986 using hourly METEOSAT infrared images. 122 individual cloud clusters associated with squall lines or tropical storms have been tracked. For each event, the time variations of a convection index giving the volume of cloud cooler than –40°C has been determined every hour. The convection index exhibits a strong diurnal cycle. From daily rainfall amounts obtained at about 300 stations, and assuming a time apportion of rainfall within a cloud cluster, the time variations of the hourly total rainfall produced by the cluster can be determined and represents the precipitation index. Because of insufficient rainfall, the precipitation index has been determined for only 17 events. For 2/3 of the 17 cases, there is a significant correlation between the two indices. For each of the 17 events, precipitation has been regressed on the associated convective index and relative time variations. In that case, the results indicate that a convective index representing the life history of the cloud cluster can be calibrated with corresponding raingage measurements provided raingage data are available. Then, estimation of rainfall due to that event over data void regions can be obtained. However, this study shows that no universal relationship exists between precipitation: no rainfall can be estimated if there is not enough raingage measurements to construct a precipitation index for a tracked cloud cluster. This represents a limitation to the method.With 9 Figures     

Numerical Simulation of Microphysics in Meso-β-Scale Convective Cloud System Associated with a Mesoscale Convective Complex

Numerical simulation of meso-β-scale convective cloud systems associated with a PRE-STORM MCC case has been carried out using a 2-D version of the CSU Regional Atmospheric Modeling System (RAMS) nonhydrostatic model with parameterized microphysics. It is found that the predicted meso-γ-scale convective phenomena are basically unsteady under the situation of strong shear at low-levels, white the meso-β-scale convective system is maintained up to 3 hours or more. The meso-β-scale cloud system exhibits characteristics of a multi-celled convective storm in which the meso-γ-scale convective cells have lifetime of about 30 min. Pressure perturbation depicts a meso-low after a half hour in the low levels. As the cloud system evolves, the meso-low inten-sifies and extends to the upshear side and covers the entire domain in the mid-lower levels with the peak values of 5-8 hPa. Temperature perturbation depicts a warm region in the middle levels through the entire simulation period. The meso-γ-scale warm cores with peak values of 4-8oC are associated with strong convective cells. The cloud top evapo-ration causes a stronger cold layer around the cloud top levels.Simulation of microphysics exhibits that graupel is primarily concentrated in the strong convective cells forming the main source of convective rainfall after one hour of simulation time. Aggregates are mainly located in the stratiform region and decaying convective cells which produce the stratiform rainfall. Riming of the ice crystals is the predominant precipitation formation mechanism in the convection region, whereas aggregation of ice crystals is the predominant one in the stratiform region, which is consistent with observations. Sensitivity experiments of ice-phase microphysical processes show that the microphysical structures of the convective cloud system can be simulated better with the diagnosed aggregation collection efficiencies.     

亚洲季风区不同形态中尺度对流系统的特征分析

利用16 a的TRMM卫星观测资料,分析了亚洲季风区准圆状、线状和拉长状中尺度对流系统(MCSs)的空间分布、对流属性及其区域变化特征。结果表明:拉长状MCSs的数量最多,准圆状的其次而线状的最少。自西向东,准圆状(线状)MCSs数量占各区域MCSs总数的比例逐渐减小(增加),线状MCSs在副热带和洋面地区的产生几率相比更大。MCSs的发生频次呈现以暖季(5—9月)为峰值的单峰分布,准圆状和拉长状MCSs的暖季峰值比线状MCSs的大。MCSs主要发生在下午—傍晚时段,但线状MCSs在午夜—凌晨出现的概率比其它两种大。3种类型MCSs的整体强度基本表现为副热带地区弱于近热带地区,但不同类型MCSs的强度差异在各区域不尽相同,如中国中东部地区和西北太平洋地区的准圆状MCSs强度最强,但东海及其以东洋面的线状MCSs强度最强。     

The effects of mesoscale convective cells on the surface wind field over the ocean

Near-surface wind velocities were measured on an array of anchored spar buoys in the East China Sea in February 1975 as a part of the Japanese AMTEX '75 (Air Mass Transformation Experiment), a subprogram of GARP. These data were used to determine the effects of atmospheric convection associated with mesoscale convective cells (MCC) on near-surface winds over the ocean. When MCC were present, a peak occurred in the near-surface wind spectrum in the so-called spectral gap.     

青藏高原上中尺度对流系统(MCSs)的个例分析及其比较

对1995年7月25—28日高原上连续数日出现MCSs的现象进行了红外云图特征及其演变、大尺度环境背景场和对流有效位能的分析。可以发现,所有这些MCSs有着相似的日变化演变过程;它们的初始对流在中午由于日射加热开始活跃,之后迅速发展,这些MCSs在后下午形成,在傍晚达到最强,之后逐渐减弱。其中26日MCS最为强大,它是在单一的强大的近于圆形的高原反气旋高压背景下受强的低层热力强迫和条件不稳定的驱动而发生的。这些发生条件都与高原本身的热力作用紧密相关,所以它的发生发展主要与高原特有的较为纯粹的热力因子相联系。28日MCS是另一个很强的MCS,它明显地受到中纬度西风槽的斜压区的影响,这二个很强的MCS有着不同的发展机制和显著不同的表现特征。     

The performance of two convective parameterization schemes in a mesoscale model over the Indian region

Summary A comparative study has been carried out using two cumulus parameterization schemes, namely the Kain-Fritsch and Betts-Miller-Janjic schemes within the Atmospheric Regional Prediction System model. The performances of these two schemes are examined for four weather disturbances over the Indian region. The results are evaluated using predicted parameters such as mean sea level pressure, wind, temperature/moisture fields and rainfall. Rainfall and other predicted parameters are discussed on the basis of known synoptic features associated with the disturbances. Rainfall prediction is subjectively assessed based on the amount and spatial distribution. Results indicate that out of the four cases examined, in three, the Betts-Miller-Janjic scheme produced better results whilst in the fourth, the Kain-Fritsch scheme performed better. The Betts-Miller-Janjic convection scheme produced smoother and more convective rainfall rates in all cases.     

西北区一次联合探测层状云系云物理特征分析

对2003-09-17我国西北地区东部层状云降水天气过程的层状云系进行了联合探测。经观测对比分析发现：对同一天气系统开展多省联合探测．对认识云系发展变化、结构特征及降水潜力很有帮助；同一条锋面云系由于流场结构的差异，其不同位置的云层结构、温度、湿度、水汽含量存在一定差异；陇东和陕北虽然同处于副高西侧的偏南气流里，但是，低层天水处于反气旋区，抑制了水汽辐合，而延安处于横切变右侧的气旋性辐合区，有利于低层水汽辐合上升，促使该地区云系发展加强，其过冷层厚度大于天水地区；PMS粒子测量系统测得在整个过冷层中，小云粒子的数浓度平均值天水比延安少1个量级，2D—P测出延安的粒子谱较宽，说明上述条件下天水地区人工增雨潜力较小．延安地区相对条件较好，人工增雨潜力大。     

Variability of the TRMM-PR total and convective and stratiform rain fractions over the Indian region during the summer monsoon

Level 3 (3A25) TRMM Precipitation Radar (PR) data are used for 13 years period (1998–2010) to prepare climatology of TRMM PR derived near surface rain (Total rain) and rain fractions for the 4-months duration of Indian Summer Monsoon season (June–September) as well as for individual months. It is found that the total rain is contributed mostly (99 %) by two rain fractions i.e. stratiform and convective rain fractions for the season as well as on the monthly basis. It is also found that total rain estimates by PR are about 65 % of the gauge measured rain over continental India as well as on sub-regional basis. Inter-annual variability of TRMM-PR rain estimates for India mainland and its sub-regions as well as over the neighboring oceanic regions, in terms of coefficient of variability (CV) is discussed. The heaviest rain region over north Bay of Bengal (BoB) is found to have the lowest CV. Another sub-region of low CV lies over the eastern equatorial Indian ocean (EEIO). The CVs of total rain as well as its two major constituents are found to be higher on monthly basis compared to seasonal basis. Existence of a well known dipole between the EEIO and the north BoB is well recognized in PR data also. Significant variation in PR rainfall is found over continental India between excess and deficit monsoon seasons as well as between excess and deficit rainfall months of July and August. Examination of rainfall fractions between the BoB and Central India on year to year basis shows that compensation in rainfall fractions exists on monthly scale on both the regions. Also on the seasonal and monthly scales, compensation is observed in extreme monsoon seasons between the two regions. However, much less compensation is observed between the north BoB and EEIO belts in extreme rain months. This leads to speculation that the deficit and excess seasons over India may result from slight shift of the rainfall from Central India to the neighboring oceanic regions of north BoB. Contribution of stratiform and convective rain fractions have been also examined and the two fractions are found to contribute almost equally to the total rain. Results are further discussed in terms of the possible impact of the two rain fractions on circulation based on possible difference is vertical profiles of latent heat of two types of rain. Substantial differences in the lower and upper tropospheric circulation regimes are noticed in both deficit and excess monsoon months/seasons, emphasizing the interaction between rainfall (latent heat) and circulation.     

Prediction of a mesoscale convective system over catalonia (Northeastern Spain) with a nested numerical model

Summary A mesoscale convective system that affected Northeastern Spain on October 10, 1994, with rainfall amounts up to 400 mm, is simulated reasonably well by a nested 3-dimensional hydrostatic mesoscale model. Previous studies carried out in this region had already portrayed the main synoptic patterns that give rise to these devastating episodes. The present contribution takes a further step since it goes down to the mesoscale by means of a numerical model providing a more detailed representation not otherwise achieved by earlier analysis methods. Although the model was unable to forecast accurately the precipitation fields, it captured satisfactorily the framework in which the convective system originated and evolved.With 16 Figures     

青藏高原上中尺度对流系统(MCS)的数值模拟

A mesoscale convective system (MCS) developing over the Qinghai-Xizang Plateau on 26 July 1995 issimulated using the fifth version of the Penn State-NCAR nonhydrostatic mesoscale model (MM5). Theresults obtained are inspiring and are as follows. (1) The model simulates well the largescale conditionsin which the MCS concerned is embedded, which are the well-known anticyclonic Qinghai-Xizang PlateauHigh in the upper layers and the strong thermal forcing in the lower layers. In particular, the modelcaptures the meso-α scale cyclonic vortex associated with the MCS, which can be analyzed in the 500 hPaobservational winds; and to some degree, the model reproduces even its meso-β scale substructure similarto satellite images, reflected in the model-simulated 400 hPa rainwater. On the other hand, there aresome distinct deficiencies in the simulation; for example, the simulated MCS occurs with a lag of 3 hoursand a westward deviation of 3-5° longitude. (2) The structure and evolution of the meso-α scale vortexassociated with the MCS are undescribable for upper-air sounding data. The vortex is confined to thelower troposphere under 450 hPa over the plateau and shrinks its extent with height, with a diameter of4° longitude at 500 hPa. It is within the updraft area, but with an upper-level anticyclone and downdraftover it. The vortex originates over the plateau, and does not form until the mature stage of the MCS. Itlasts for 3-6 hours. In its processes of both formation and decay, the change in geopotential height fieldis prior to that in the wind field. It follows that the vortex is closely associated with the thermal effectsover the plateau. (3) A series of sensitivity experiments are conducted to investigate the impact of varioussurface thermal forcings and other physical processes on the MCS over the plateau. The results indicatethat under the background conditions of the upper-level Qinghai-Xizang High, the MCS involved is mainlydominated by the low-level thermal forcing. The simulation described here is a good indication that itmay be possible to reproduce the MCS over the plateau under certain large-scale conditions and with theincorporation of proper thermal physics in the lower layers.     

我国西南-华南地区中尺度对流复合体(MCC)的研究

本文普查了1983—1986年4—9月我国西南-华南地区MCC活动的基本特征。并以一个典型的例子讨论它的形成和成熟期的结构、云顶黑体温度(T_(BB))演变过程及降水等。     

The occurrence of the typical mesoscale convective system with a flood-producing storm in the wet season over the Greater Jakarta area

A complex convective cloud with a horizontal scale area of more than 100 km, known as the mesoscale convective system (MCS), is important to the study as it brings heavy rainfall from its activity. The analysis of MCS with the flood-producing storm on the Indonesian Maritime Continent (IMC) receives less attention. The purpose of this study is to identify and evaluate the temporal variability of the MCS to the frequency of flood-producing storms in Greater Jakarta (GJ) during the 2013–2015 wet season. The image data of Multi-functional Transport Satellite (MTSAT) -1R, which represents an equivalent blackbody temperature (TBB), and the tracking algorithm “Grab ‘em Tag ‘em Graph ‘em” (GTG) were used to detect the events of MCS. We also used a rainfall graph in this analysis to measure rainfall threshold values in order to classify flood-producing storms. The results show that MCS around GJ has typical characteristics of tropical belt regions. There is a small TBB scale (maximum size ≥13,000 km²) distinguished by a deep cloud up to 14 km in height. Through the active effects of monsoon and ITCZ, the land-breeze and/or sea-breeze circulations that contribute to MCS growth are triggered. However, about 32 percent of the MCS contributed to the flood-producing storm around the GJ region.     

Numerical prediction of an intense convective system associated with the July 1987 montreal flood. part II: A trailing stratiform rainband

Abstract

In this study, the internal circulation structures of the 14 July 1987 intense mesoscale convective system (MCS) are investigated using an improved high‐resolution version of the Canadian regional finite‐element model. It is found that although the MCS is characterized by a leading convective line followed by a trailing stratiform rainband, the associated circulation structures differ substantially from those in the classical midlatitude squall system. These include the rapid propagation and separation of the leading convection from the trailing rainband, the development of a surface‐based instead of an elevated rear‐to‐front descending flow and a shallow front‐to‐rear ascending flow associated with the stratiform precipitation, the generation of low‐ and mid‐level rather than mid‐ to upper‐level stratiform cloudiness and the development of a strong anticyclonic vorticity band at the back edge of the stratiform region. It is shown that the trailing stratiform rainband is dynamically forced by frontogenetical processes, and aided by the release of conditional symmetric instability and local orographical lifting. The intense leading and trailing circulations result from latent heat released by the convective and explicit cloud schemes, respectively. Sensitivity experiments reveal that the proper coupling of these two cloud schemes is instrumental in obtaining a realistic prediction of the above‐mentioned various mesoscale components. Vorticity budget calculations show that tilting of horizontal vorticity contributes the most to the amplification of the anticyclonic vorticity band, particularly during the squall's incipient stage. The sensitivity of the simulated squall system to other model physical parameters is also examined.