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1.
The skill and efficiency of a numerical model mostly varies with the quality of initial values, accuracy on parameterization of physical processes and horizontal and vertical resolution of the model. Commonly used low-resolution reanalyses are hardly able to capture the prominent features associated with organized convective processes in a monsoon depression. The objective is to prepare improved high-resolution analysis by the use of MM5 modelling system developed by the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR). It requires the objective comparison of high and low-resolution analysis datasets in assessing the specific convective features of a monsoon depression. For this purpose, reanalysis datasets of NCAR/NCEP (National Center for Atmospheric Research/National Centers for Environmental Prediction) at a horizontal resolution of 2.5‡ (latitude/longitude) have been used as first guess in the objective analysis scheme. The additional asynoptic datasets obtained during BOBMEX-99 are utilized within the assimilation process. Cloud Motion Wind (CMW) data of METEOSAT satellite and SSM/I surface wind data are included for the improvement of derived analysis. The multiquadric (MQD) interpolation technique is selected and applied for meteorological objective analysis at a horizontal resolution of 30 km. After a successful inclusion of additional data, the resulting reanalysis is able to produce the structure of convective organization as well as prominent synoptic features associated with monsoon depression. Comparison and error verifications have been done with the help of available upper-air station data. The objective verification reveals the efficiency of the analysis scheme.  相似文献   

2.
This paper investigates the characteristic features of the coastal atmospheric boundary layer (CABL) along the west coast of India during the south-west monsoon (SWM) 2002. Extensive surface and upper-air findings were obtained during the same period from the Arabian Sea Monsoon Experiment (ARMEX; 15th June to 15th August 2002) 2002. The operational general circulation model (GCM) of the National Centre for Medium Range Weather Forecasting (NCMRWF) was used in this study to see the spatial variation of the CABL during two specific convective episodes that led to heavy rainfall along the west coast of India. The impact of a non-local closure (NLC) scheme employed in the NCMRWF GCM was carried out in simulating the CABL. The same episodes were also simulated using a similar parameterization scheme employed in the high resolution mesoscale modelling system (MM5). The diurnal variation of CABL is better represented from MM5 simulation. Comparing the MM5 simulation with that of the coarser grid NCMRWF GCM, we observed that the NCMRWF GCM underestimates the values of both latent heat flux (LHF) and the coastal atmospheric boundary layer height (CABLH). Results from MM5 therefore indicate that the best way to move forward in addressing the short-comings of coarse grid-scale GCMs is to provide a parameterization of the diurnal effects associated with convection processes.  相似文献   

3.
Objective analysis of daily rainfall at the resolution of 1° grid for the Indian monsoon region has been carried out merging dense land rainfall observations and INSAT derived precipitation estimates. This daily analysis, being based on high dense rain gauge observations was found to be very realistic and able to reproduce detailed features of Indian summer monsoon. The inter-comparison with the observations suggests that the new analysis could distinctly capture characteristic features of the summer monsoon such as north-south oriented belt of heavy rainfall along the Western Ghats with sharp gradient of rainfall between the west coast heavy rain region and the rain shadow region to the east, pockets of heavy rainfall along the location of monsoon trough/low, over the east central parts of the country, over north-east India, along the foothills of Himalayas and over the north Bay of Bengal. When this product was used to assess the quality of other available standard climate products (CMAP and ECMWF reanalysis) at the gird resolution of 2.5°, it was found that the orographic heavy rainfall along Western Ghats of India was poorly identified by them. However, the GPCC analysis (gauge only) at the resolution of 1° grid closely discerns the new analysis. This suggests that there is a need for a higher resolution analysis with adequate rain gauge observations to retain important aspects of the summer monsoon over India. The case studies illustrated show that the daily analysis is able to capture large-scale as well as mesoscale features of monsoon precipitation systems. This study with data of two seasons (2001 and 2003) has shown sufficiently promising results for operational application, particularly for the validation of NWP models.  相似文献   

4.
The present study is carried out to examine the performance of a regional atmospheric model in forecasting tropical cyclones over the Bay of Bengal and its sensitivity to horizontal resolution. Two cyclones, which formed over the Bay of Bengal during the years 1995 and 1997, are simulated using a regional weather prediction model with two horizontal resolutions of 165 km and 55 km. The model is found to perform reasonably well towards simulation of the storms. The structure, intensity and track of the cyclones are found to be better simulated by finer resolution of the model as compared to the coarse resolution. Rainfall amount and its distribution are also found to be sensitive to the model horizontal resolution. Other important fields, viz., vertical velocity, horizontal divergence and horizontal moisture flux are also found to be sensitive to model horizontal resolution and are better simulated by the model with finer horizontal grids.  相似文献   

5.
南亚季风降水的双极振荡*   总被引：1，自引：2，他引：1  
文章利用气象资料揭示在印度半岛南部和北部,南亚季风降水变化在10年尺度以上呈翘翘板变化形式;利用更长的季风降水资料,即300年的喜马拉雅山达索普冰芯降水记录和印度半岛南部石笋降水记录,发现印度南部和喜马拉雅山季风降水呈双极振荡行为。自1700年以来,喜马拉雅山,即印度北部(或印度半岛南部)季风降水经历了1700~1764年期间的减小(或增加)趋势,1764~1876年期间的增大(或减小)趋势,1876~2000年期间的减小(或增加)趋势。同时,发现印度半岛南部的季风降水同北半球温度变化具有相同的变化特征,而喜马拉雅山季风降水同北半球温度变化具有相反的变化特征。南亚季风降水的这种南北翘翘板变化形式,与跨赤道气流有密切的联系。  相似文献   

6.
To improve flood forecasting, the understanding of the atmospheric conditions associated with severe rainfall is crucial. We analysed the atmospheric conditions at Dhaka, Bangladesh, using upper-air soundings. We then compared these conditions with daily rainfall variations at Cherrapunjee, India, which is a main source of floodwater to Bangladesh, and a representative sample of exceptionally heavy rainfall events. The analysis focussed on June and July 2004. June and July are the heaviest rainfall months of the year at Cherrapunjee. July 2004 had the fourth-heaviest monthly rainfall of the past 31 years, and severe floods occurred in Bangladesh. Active rainfall periods at Cherrapunjee corresponded to “breaks” in the Indian monsoon. The monsoon trough was located over the Himalayan foothills, and strong westerly winds dominated up to 7 km at Dhaka. Near-surface wind below 1 km had southerly components, and the wind profile had an Ekman spiral structure. The results suggest that rainfall at Cherrapunjee strongly depends on the near-surface wind speed and wind direction at Dhaka. Lifting of the near-surface southerly airflow by the Meghalaya Plateau is considered to be the main contributor to severe rainfall at Cherrapunjee. High convective available potential energy (CAPE) also contributes to intense rainfall.  相似文献   

7.
Orissa is one of the most flood prone states of India. The floods in Orissa mostly occur during monsoon season due to very heavy rainfall caused by synoptic scale monsoon disturbances. Hence a study is undertaken to find out the characteristic features of very heavy rainfall (24 hours rainfall ≥125 mm) over Orissa during summer monsoon season (June–September) by analysing 20 years (1980–1999) daily rainfall data of different stations in Orissa. The principal objective of this study is to find out the role of synoptic scale monsoon disturbances in spatial and temporal variability of very heavy rainfall over Orissa. Most of the very heavy rainfall events occur in July and August. The region, extending from central part of coastal Orissa in the southeast towards Sambalpur district in the northwest, experiences higher frequency and higher intensity of very heavy rainfall with less interannual variability. It is due to the fact that most of the causative synoptic disturbances like low pressure systems (LPS) develop over northwest (NW) Bay of Bengal with minimum interannual variation and the monsoon trough extends in west-northwesterly direction from the centre of the system. The very heavy rainfall occurs more frequently with less interannual variability on the western side of Eastern Ghat during all the months and the season except September. It occurs more frequently with less interannual variability on the eastern side of Eastern Ghat during September. The NW Bay followed by Gangetic West Bengal/Orissa is the most favourable region of LPS to cause very heavy rainfall over different parts of Orissa except eastern side of Eastern Ghat. The NW Bay and west central (WC) Bay are equally favourable regions of LPS to cause very heavy rainfall over eastern side of Eastern Ghat. The frequency of very heavy rainfall does not show any significant trend in recent years over Orissa except some places in north-east Orissa which exhibit significant rising trend in all the monsoon months and the season as a whole.  相似文献   

8.
Homogeneous Indian Monsoon rainfall: Variability and prediction   总被引：1，自引：0，他引：1  
The Indian summer monsoon rainfall is known to have considerable spatial variability, which imposes some limitations on the all-India mean widely used at present. To prepare a spatially coherent monsoon rainfall series for the largest possible area, fourteen subdivisions covering the northwestern and central parts of India (about 55% of the total area of the country), having similar rainfall characteristics and associations with regional/global circulation parameters are merged and their area-weighted means computed, to form monthly and seasonal Homogeneous Indian Monsoon (HIM) rainfall series for the period 1871–1990. This paper includes a listing of monthly and seasonal rainfall of HIM region. HIM rainfall series has been statistically analysed to understand its characteristics, variability and teleconnections for long-range prediction. HIM rainfall series isfound to be homogeneous, Gaussian distributed and free from persistence. The mean (R) rainfall is 757 mm (87% of annual) and standard deviation (S) 119 mm, with a Coefficient of Variation (CV) of 16%. There were 21 dry (K, -<R S) and 19 wet (R i R + S) years during 1871–1990. There were clusters of frequent negative departures during 1899–1920 and 1965–1987 and positive departures during 1942–1961. The recent three decades show very high rainfall variability with 10 dry and 6 wet years. The decadal averages were alternatively positive and negative for three consecutive decades, viz., 1871–1900 (positive); 1901–1930 (negative); 1931–1960 (positive) and 1961–1990 (negative) respectively. Significant QBO and autocorrelation at 14th lag have been found in HIM rainfall series. To delineate the changes in the climatic regime of the Indian summer monsoon, sliding correlation coefficients (CCs) between HIM rainfall series and (i) Bombay msl pressure, (ii) Darwin msl pressure and (iii) Northern Hemisphere surface air temperature over the period 1871–1990 have been examined. The 31-year sliding CCs showed the systematic turning points of positive and negative CCs around the years, 1900 and 1940. In the light of other corroborative evidences, these turning points seem to delineate ‘meridional’ monsoon regime during 1871–1900 and 1940–1990 and ‘zonal’ monsoon regime during 1901–1940. The monsoon signal is particularly dominant in many regional and global circulation parameters, during 1951–1990. Using the teleconnections ofHIM series with 12 regional/global circulation parameters during the recent 36-year period 1951–86 regression models have been developed for long-range prediction. In the regression equations 3 to 4 parameters were entered, explaining upto 80% of the variance, depending upon the data period. The parameters that prominently enter the multiple regression equations are (i) Bombay msl pressure, (ii) April 500 mb Ridge at 75°E, (iii) NH temperature, (iv) Nouvelle minus Agalega msl pressure and (v) South American msl pressure. Eleven circulation parameters for the period 1951–80 were subjected to Principal Component Analysis (PCA) and the PC’s were used in the regression model to estimate HIM rainfall. The multiple regression with three PCs explain 72% of variance in HIM rainfall.  相似文献   

9.
In this paper an attempt has been made to search a new parameter for the prediction of the Indian summer monsoon rainfall. For this purpose the relationship of the global surface-air temperature of four standard seasons viz., Winter (December-January-February), Spring (March-April-May), Summer (June-July-August), Autumn (September-October-November) with the Indian summer monsoon rainfall has been carried out. The same analysis is also carried out with surface-air temperature anomalies within the tropical belt (30°S to 30°N) and Indian summer monsoon rainfall. For the present study data for 30 years period from 1958 to 1988 have been used. The analysis reveals that there is a strong inverse relationship between the monsoon activity and the tropical belt temperature.  相似文献   

10.
In this study, we elucidate the temporal characteristics of the onset and withdrawal of the Indian southwest monsoon, making use of the model integration and daily analyses of the National Centre for Medium Range Weather Forecasting, India. The onset of the Indian southwest monsoon over the Bay of Bengal is discernable by a gradual increase in the adiabatic generation of kinetic energy, while over the Arabian Sea it is first noticeable by a steep and abrupt increase of generation. The horizontal transport of heat indicates a convergence regime over the Bay of Bengal prior to onset, while over the Arabian Sea a convergence regime is indicated by a change from the divergence to the convergence regime. The withdrawal of the southwest monsoon is characterized by the horizontal transport of heat and moisture that evince a transition from the convergence to divergence regime; similarly, diabatic heating noticed during the active period changes to cooling. The withdrawal over the Arabian Sea is characterized by the divergence regime of the horizontal transport of moisture. This change precedes even the circulation changes over northwest India, which may be regarded as a precursor. The withdrawal is further supported by a monotonic decrease in the net tropospheric moisture over the Arabian Sea, followed by a similar change at land locations.  相似文献   

11.
Analysis of monthly momentum transport of zonal waves at 850 hPa for the period 1979 to 1993, between ‡S and ‡N for January to April, using zonal (u) and meridional (v) components of wind taken from the ECMWF reanalysis field, shows a positive correlation (.1% level of significance) between the Indian summer monsoon rainfall (June through September) and the momentum transport of wave zero TM(0) over latitudinal belt between 25‡S and 5‡N (LB) during March. Northward (Southward) TM(0) observed in March over LB subsequently leads to a good (drought) monsoon season over India which is found to be true even when the year is marked with the El-Nino event. Similarly a strong westerly zone in the Indian Ocean during March, indicates a good monsoon season for the country, even if the year is marked with El-Nino. The study thus suggests two predictors, TM(0) over LB and the strength of westerly zone in the Indian Ocean during March.  相似文献   

12.
通过对云南华坪葫芦洞FL4石笋进行高精度的ICP—MS—230Th/U测年和高分辨率的碳、氧同位素分析,建立了该地区6 060-4 185 a BP间高分辨率的西南季风气候变化时间序列,进而揭示了该时段发生的3次季风减弱事件。 这3次百年尺度（持续时间为90～240a）的干旱寒冷事件,分别发生在6 060－5 950 a BP、5 380－5 140 a BP、4 810-4 620 a BP,呈台阶状演变;而石笋的碳同位素记录揭示了2次强降水事件,分别发生在5 503－5 443 a BP和4 210-4 185 a BP,持续时间分别为25a和60a。石笋碳、氧同位素记录的西南季风减弱以及强降水事件明显受太阳辐射强度的控制。分辨率为3~10 a的碳、氧同位素记录表明,在百年尺度的西南季风气候变化上,叠加了一系列十年尺度的气候突变事件,呈锯齿状的高频波动。这些短时间尺度的季风气候波动事件与树轮14C 残差、冰芯记录极为相似,反映低纬度地区石笋记录的季风气候与高纬度及北极地区的气候具有极好的可比性,可能主要是受中低纬度太阳辐射强度以及北半球大气环流的影响,太阳辐射强度的变化是控制印度季风的快速推进或退出(萎缩)以及百年尺度上的气候波动的主要动因。   相似文献   

13.
西风与季风扰动对黑河流域降水影响的数值模拟   总被引：1，自引：1，他引：0  
利用中尺度天气模式WRF V2.2进行了两组风场的敏感性试验,分别模拟了西风与季风变化对黑河流域降水的影响.通过对大气环流、水汽输送、水汽辐合以及垂直上升运动的分析得出以下结论:西风与季风对黑河流域降水的影响方式不同,西风带直接作用于黑河流域,影响其降水,而季风则是通过对西风的调整间接影响黑河流域降水;西风与季风变化对黑河流域降水的影响范围不同,西风增强后,黑河流域南部山区降水落区西移,降水增加,最大值中心偏西北;季风增强后,黑河流域南部山区降水落区向东南移,降水增加,最大值中心偏东南.与其它量相比,黑河流域降水与垂直速度的对应关系最好.  相似文献   

14.
Skilful prediction of the monthly and seasonal summer monsoon rainfall over India at a smaller spatial scale is a major challenge for the scientific community. The present study is aimed at achieving this objective by hybridising two mathematical techniques, namely synthetic superensemble (SSE) and supervised principal component regression (SPCR) on six state-of-the art Global Climate Models (GCMs). The performance of the mathematical model is evaluated using correlation analysis, the root mean square error, and the Nash–Sutcliffe efficiency index. Results feature reasonable improvement over central India, which is a zone of maximum rainfall activity in the summer monsoon season. The study also highlights improvement in the monthly prediction of rainfall over raw GCMs (15–20% improvement) with exceptional improvement in July. The developed model is also examined for anomalous years of monsoon and it is found that the model is able to capture the signs of anomalies over different gridpoints of the Indian domain.  相似文献   

15.
Aerosols can affect the cloud-radiation feedback and the precipitation over the Indian monsoon region. In this paper, we propose that another pathway by which aerosols can modulate the multi-scale aspect of Indian monsoons is by altering the land–atmosphere interactions. The nonlinear feedbacks due to aerosol/diffuse radiation on coupled interactions over the Indian monsoon region are studied by: (1) reviewing recent field measurements and modeling studies, (2) analyzing the MODIS and AERONET aerosol optical depth datasets, and (3) diagnosing the results from sensitivity experiments using a mesoscale modeling system. The results of this study suggest that the large magnitude of aerosol loading and its impact on land–atmosphere interactions can significantly influence the mesoscale monsoonal characteristics in the Indo-Ganges Basin.  相似文献   

16.
A state-of-the-art regional climate modelling system, known as PRECIS (Providing REgional Climates for Impacts Studies) developed by the Hadley Centre for Climate Prediction and Research, UK is applied over the Indian domain to investigate the impact of global warming on the cyclonic disturbances such as depressions and storms. The PRECIS simulations at 50 × 50 km horizontal resolution are made for two time slices, present (1961–1990) and the future (2071–2100), for two socioeconomic scenarios A2 and B2. The model simulations under the scenarios of increasing greenhouse gas concentrations and sulphate aerosols are analysed to study the likely changes in the frequency, intensity and the tracks of cyclonic disturbances forming over north Indian Ocean (Bay of Bengal and Arabian Sea) and the Indian landmass during monsoon season. The model overestimates the frequency of cyclonic disturbances over the Indian subcontinent in baseline simulations (1961–1990). The change is evaluated towards the end of present century (2071–2100) with respect to the baseline climate. The present study indicates that the storm tracks simulated by the model are southwards as compared to the observed tracks during the monsoon season, especially for the two main monsoon months, viz., July and August. The analysis suggests that the frequency of cyclonic disturbances forming over north Indian Ocean is likely to reduce by 9% towards the end of the present century in response to the global warming. However, the intensity of cyclonic disturbances is likely to increase by about 11% compared to the present.  相似文献   

17.
郑伟鹏  俞永强 《第四纪研究》2009,29(6):1135-1145
本文分析了一个耦合模式FGOALS_g1.0对工业革命前气候(0ka)和中全新世时期(6ka)亚洲夏季风的模拟结果。在该研究中我们主要分析季风降水变率较大的区域,即东亚夏季风区(20°～45°N,110°～120°E)和印度夏季风区(10°～30°N,70°～80°E)。尽管耦合模式的普遍偏差依然存在,该模式反映出亚洲季风系统是海陆热力性质差异的结果,并较好地模拟出了0ka亚洲夏季风大尺度环流的特点和季节变化的特征。6ka和0ka比较分析的结果表明,6ka时期欧亚大陆增暖,海陆温度梯度加强; 印度夏季风降水从南亚大陆北移到 30°N 附近,位于青藏高原南侧的降水大值中心降水加强; 东亚季风区降水则表现为华北地区减少,长江流域和华南地区降水增加的特点。但合理地模拟季风爆发仍然是耦合气候系统模式的难点之一。
6ka时期亚洲夏季风变化是和大尺度季风环流的变化联系在一起的,而其根本原因是中全新世时期地球轨道参数变化所引起的太阳辐射变化,北半球季节循环的振幅加强。海陆热力性质的差异所导致海陆温差加大使得北半球的季风环流加强,印度夏季风高空东风在 20°～30°N 加强,低层赤道东风加强,跨赤道后的西南气流向北推移,从而使得印度夏季风降水雨带北移到 30°N 附近。东亚季风区的高低空温度场的配置使得副热带高空急流减弱,位置偏南,从而有利于华北地区的高空出现异常的辐合,中层为异常的辐散,抑制了季风降水的发展; 长江流域和华南地区则相反,季风降水降水加强。  相似文献   

18.
杨保  谭明 《第四纪研究》2009,29(5):880-887
集成中国季风区石笋氧同位素记录的共同变化特征,初步建立了近千年10年平均的东亚夏季风演变序列。在10年尺度以上分析了东亚夏季风演变与其他气候要素变化之间的关系。主要结论有： 1)近千年来东亚夏季风演变可划分为中世纪时期(11～13世纪初期)的季风稍弱阶段,13世纪中后期至14世纪前半叶的季风较强时期,14世纪后半叶至17世纪的季风较弱阶段,自18世纪开始持续约200年的季风再次增强时期,以及20世纪初开始的季风逐渐减弱阶段。2)近千年来东亚大陆或北半球温度的变化虽然对东亚夏季风变化具有一定的影响,但东亚夏季风强度的变化并不总是取决于陆地温度的变化。3)近千年来东亚夏季风的强弱变化与降水变化在低频趋势上有良好的对应关系,在东亚夏季风增强的时期,中国东部降水较多,而在夏季风减弱时,中国东部降水趋于偏少。  相似文献   

19.
The objective of this study was two-fold: the first to investigate the role of moist convection and nongeostrophic effects on the growth of the monsoon depressions using a linearized multi-level moist primitive equation (PE) model and quasi-geostrophic (QG) model with only vertical shear. The second was to study the nonlinear evolution, growth, movement and detailed energetics of the monsoon depressions using a nonlinear moist global spectral model. Our linear studies using both models revealed lower as well as upper tropospheric growing modes. For the lower tropospheric modes the shorter scales were found to grow faster. While the PE model showed faster growth rate for shorter scales, as compared to longer scales, the QG model showed less tendency for scale selection. The shorter scales in PE model had phase speeds ranging from 4 to — 1 ms−1 and in QG model from 8 to — 4 ms−1. The nongeostrophic effects were found to be, in general, important. One of the lower tropospheric modes with wavelength 2500 km was found to have many features similar to the observed monsoon depression of the Bay of Bengal. In the upper troposphere the PE model showed much faster growth rates compared to the QG model. Also the fastest growing mode with a doubling time of 2.5 days had a scale of 6000 km. This was shorter than the scale predicted in the QG model. This mode had many characteristics similar to the observed features of the monsoon upper tropospheric easterly waves. Using a nonlinear global spectral model, we simulated the monsoon depression around 21°N starting from an antisymmetric heating distribution (with respect to the equator) and with a specific vertical structure with and without basic flows. The model was integrated for a period of five days incorporating a simple form of cumulus heating. The simulated model disturbance showed a pronounced growth and a westward movement in the presence of cumulus heating. The detailed energetics calculations revealed that the baroclinic energy exchange is the primary energy exchange process and cumulus heating is the driving force for the generation of available potential energy.  相似文献   

20.
Numerical experiment with improved boundary layer physics has been performed to study the Planetary Boundary Layer (PBL) characteristics over the monsoon trough region. Details of the evolution and structure of the associated boundary layer processes in the monsoon trough and adjoining oceanic regions are examined by integrating the model up to a period of 48 hours. The model used for this study is a high resolution primitive equation, one with 0·5o latitude/longitude horizontal resolution and 16 levels in the vertical (7 levels in the PBL). The boundary layer treatment in the model is based on the Monin-Obukhov similarity theory for the surface layer and Turbulent Kinetic Energy (TKE) formulation based onE-ε approach for the mixed layer. The model is integrated using the FGGE level IIIb analysis of European Centre for Medium Range Weather Forecasts (ECMWF), U.K. The study shows that the diurnal variation of TKE over land is a dominant feature comparing with that over the ocean. Along the monsoon trough region, TKE increases from the eastern end to the western side which is mainly associated with the enhancement of sensible heat flux as we move from the eastern wet land to the western desert sector. It may be noted that the low level wind maximum, which is a characteristic feature over the monsoon region, is well simulated by this improved model physics.  相似文献   

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