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1.
The monsoon seasons of 2010 and 2011, with almost identical seasonal total rainfall over India from June to September, are associated with slightly different patterns of intraseasonal rainfall fluctuations. Similarly, the year 2012, with relatively less rainfall compared to 2010 and 2011, also witnessed different intraseasonal rainfall fluctuations, leading to drought-like situations over some parts of the country. The present article discusses the forecasting aspect of monsoon activity over India during these 3 years on an extended range time scale (up to 3 weeks) by using the multimodel ensemble (MME), based on operational coupled model outputs from the ECMWF monthly forecasting system and the NCEP’s Climate Forecast System (CFS). The average correlation coefficient (CC) of weekly observed all-India rainfall (AIR) and the corresponding MME forecast AIR is found to be significant, above the 98 % level up to 2 weeks (up to 18 days) with a slight positive CC for the week 3 (days 19–25) forecast. However, like the variation of observed intraseasonal rainfall fluctuations during 2010, 2011 and 2012 monsoon seasons, the MME forecast skills of weekly AIR are also found to be different from one another, with the 2012 monsoon season indicating significant CC (above 99 % level) up to week 2 (12–18 days), and also a comparatively higher CC (0.45) during the week 3 forecast (days 19–25). The average CC between observed and forecasted weekly AIR rainfall over four homogeneous regions of India is found to be the lowest over the southern peninsula of India (SPI), and northeast India (NEI) is found to be significant only for the week 1 (days 5–11) forecast. However, the CC is found to be significant over northwest India (NWI) and central India (CEI), at least above the 90 % level up to 18 days, with NWI having slightly better skill compared to the CEI. For the individual monsoon seasons of 2010, 2011 and 2012, there is some variation in CC and other skill scores over the four homogeneous regions. Thus the slight variations in the characteristics of intraseasonal monsoon rainfall over India is associated with variations in predictive skill of the coupled models and the MME-based predictions of intraseasonal monsoon fluctuations for 2–3 weeks, providing encouraging results. The MME forecast in 2010 is also able to provide useful guidance, well in advance, about an active September associated with a delayed withdrawal of the monsoon and also the heavy rainfall over north Pakistan.  相似文献   

2.
The study focuses on the spatial and temporal variations of intense/extreme rainfall events over Gujarat State (India) during the period 1970–2014. Average monsoon rainfall for the state shows a significant increasing trend, with an increase of 48 mm/decade. Some of the stations in the Saurashtra region show a statistically significant increasing trend but none of the stations in the state show a decreasing trend. The increasing trend in monsoon rainfall is very significant for the past three decades, with an increase of 167 mm/decade. Instead of fixed absolute threshold values, relative threshold values of rainfall corresponding to the 95th, 98th, 99th and 99.5th percentiles for each station have been proposed to represent heavy, very heavy, intense and extreme rainfall, which varied between 70–120, 105–160, 130–210 and 165–280 mm, respectively. Significant increasing trends are observed for the frequency of heavy and very heavy rainfall events over the state.  相似文献   

3.
Asian summer monsoon sets in over India after the Intertropical Convergence Zone moves across the equator to the northern hemisphere over the Indian Ocean. Sea surface temperature (SST) anomalies on either side of the equator in Indian and Pacific oceans are found related to the date of monsoon onset over Kerala (India). Droughts in the June to September monsoon rainfall of India are followed by warm SST anomalies over tropical Indian Ocean and cold SST anomalies over west Pacific Ocean. These anomalies persist till the following monsoon which gives normal or excess rainfall (tropospheric biennial oscillation). Thus, we do not get in India many successive drought years as in sub-Saharan Africa, thanks to the ocean. Monsoon rainfall of India has a decadal variability in the form of 30-year epochs of frequent (infrequent) drought monsoons occurring alternately. Decadal oscillations of monsoon rainfall and the well-known decadal oscillation in SST of the Atlantic Ocean (also of the Pacific Ocean) are found to run parallel with about the same period close to 60 years and the same phase. In the active–break cycle of the Asian summer monsoon, the ocean and the atmosphere are found to interact on the time scale of 30–60 days. Net heat flux at the ocean surface, monsoon low-level jetstream (LLJ) and the seasonally persisting shallow mixed layer of the ocean north of the LLJ axis play important roles in this interaction. In an El Niño year, the LLJ extends eastwards up to the date line creating an area of shallow ocean mixed layer there, which is hypothesised to lengthen the active–break (AB) cycle typically from 1 month in a La Niña to 2 months in an El Niño year. Indian monsoon droughts are known to be associated with El Niños, and long break monsoon spells are found to be a major cause of monsoon droughts. In the global warming scenario, the observed rapid warming of the equatorial Indian ocean SST has caused the weakening of both the monsoon Hadley circulation and the monsoon LLJ which has been related to the observed rapid decreasing trend in the seasonal number of monsoon depressions.  相似文献   

4.
—The atmospheric surface layer over land may behave differently in the tropics, particu larly during the monsoon. A preliminary attempt is made to observe the behavior of surface layer characteristics such as fluxes of momentum, sensible heat and latent heat, friction velocity, friction temperature, M-O length scale, Richardson number and Bowen’s ratio over Kharagpur (22°20′N, 87°18′E), a typical moist tropical station. The diurnal and day-to-day variations have been studied. It is observed that during the active phase of the monsoon the sensible heat flux and Bowen’s ratio are low. The diurnal variation is apparent for most parameters. Mostly near neutral conditions are observed.  相似文献   

5.
The 2010 boreal summer marked a worldwide abnormal climate. An unprecedented heat wave struck East Asia in July and August 2010. In addition to this, the tropical Indian Ocean was abnormally warm during the summer of 2010. Several heavy rainfall events and associated floods were also reported in the Indian monsoon region. During the season, the monsoon trough (an east–west elongated area of low pressure) was mostly located south of its normal position and monsoon low pressure systems moved south of their normal tracks. This resulted in an uneven spatial distribution with above-normal rainfall over peninsular and Northwest India, and deficient rainfall over central and northeastern parts of India, thus prediction (and simulation) of such anomalous climatic summer season is important. In this context, evolution of vertical moist thermodynamic structure associated with Indian summer monsoon 2010 is studied using regional climate model, reanalysis and satellite observations. This synergised approach is the first of its kind to the best of our knowledge. The model-simulated fields (pressure, temperature, winds and precipitation) are comparable with the respective in situ and reanalysis fields, both in intensity and geographical distribution. The correlation coefficient between model and observed precipitation is 0.5 and the root-mean-square error (RMSE) is 4.8 mm day?1. Inter-comparison of model-simulated fields with satellite observations reveals that the midtropospheric temperature [Water vapour mixing ratio (WVMR)] has RMSE of 0.5 K (1.6 g kg?1), whereas the surface temperature (WVMR) has RMSE of 3.4 K (2.2 g kg?1). Similarly, temporal evolution of vertical structure of temperature with rainfall over central Indian region reveals that the baroclinic nature of monsoon is simulated by the model. The midtropospheric warming associated with rainfall is captured by the model, whereas the model failed to capture the surface response to high and low rainfall events. The model has strong water vapour loading in the whole troposphere, but weaker coherent response with rainfall compared to observations. Thus, strong water vapour loading and overestimation of rainfall are reported in the model. This study put forward that the discrepancy in the model-simulated structure may be reduced by assimilation of satellite observations.  相似文献   

6.
The water vapor is one of the important constituents of the atmosphere that affects the thermodynamics of the atmosphere and has direct impact on the weather conditions. The total column atmospheric water vapor, obtained from Global Positioning System (GPS) and Moderate Resolution Imaging Spectroradiometer (MODIS), is found to be very dynamic over the Indo-Gangetic (IG) plains. In this paper, we present an analysis of GPS data recently deployed (as of May 2007) on the campus of Banaras Hindu University, Varanasi (latitude 25°15′N, longitude 82°59′E). Further, we have compared the variability of water vapor from Kanpur GPS, AERONET and MODIS water vapor data for the year 2007. The monthly variability of water vapor shows characteristic features and dynamics of water vapor between two closely spaced GPS stations, found to be controlled by monsoon dynamics and wind pattern.  相似文献   

7.
《Water Policy》2001,3(1):101-107
An attempt has been made to study the occurrence of floods in the two important river systems of north India, viz. Brahmaputra and Ganga. Both these river systems are located north of Lat. 22°N and lie in the longitudinal belt of north India from Long. 73°E to 97°E. Both these river systems are affected by floods during monsoon months of June–September. It has been seen after examining rainfall and floods of the period 1986–1999 that although variation of monsoon rainfall magnitudes received by these two basins differ considerably in each monsoon season, but by and large, the frequency of floods at their terminal gauge/discharge (G/D) sites at Dhubri and Farakka do not differ very much from each other.  相似文献   

8.
--A large part of the rainfall over India during the summer monsoon season (June-September) is contributed by synoptic scale disturbances such as monsoon depressions. To study the evolution of such disturbances in Atmospheric General Circulation Models (AGCM), the Hadley Centre AGCM (HadAM2b) has been integrated for 15 summer monsoons (1979-1993) and the output was examined for the presence of synoptic scale disturbances such as monsoon depressions, low pressure areas, land lows and land depressions over the Indian summer monsoon region. The atmospheric initial condition for each of these integrations was of 23rd May and observed Sea Surface Temperatures (SST) were described as a boundary condition.¶Although the horizontal resolution of the AGCM used in this study is only 2.5° 2 3.75° lat. long., the model is able to simulate a few monsoon disturbances. The important features of these simulated disturbances are presented. The features of the simulated disturbances are realistic. The morphologies of a well simulated monsoon depression and a simulated low pressure area are presented as examples. The frequency of the simulated monsoon depressions is less than the climatological frequency of the depressions during all four monsoon months.  相似文献   

9.
Time–frequency characterization is useful in understanding the nonlinear and non-stationary signals of the hydro-climatic time series. The traditional Fourier transform, and wavelet transform approaches have certain limitations in analyzing non-linear and non-stationary hydro-climatic series. This paper presents an effective approach based on the Hilbert–Huang transform to investigate time–frequency characteristics, and the changing patterns of sub-divisional rainfall series in India, and explored the possible association of monsoon seasonal rainfall with different global climate oscillations. The proposed approach integrates the complete ensemble empirical mode decomposition with adaptive noise algorithm and normalized Hilbert transform method for analyzing the spectral characteristics of two principal seasonal rainfall series over four meteorological subdivisions namely Assam-Meghalaya, Kerala, Orissa and Telangana subdivisions in India. The Hilbert spectral analysis revealed the dynamic nature of dominant time scales for two principal seasonal rainfall time series. From the trend analysis of instantaneous amplitudes of multiscale components called intrinsic mode functions (IMFs), it is found that both intra and inter decadal modes are responsible for the changes in seasonal rainfall series of different subdivisions and significant changes are noticed in the amplitudes of inter decadal modes of two seasonal rainfalls in the four subdivisions since 1970s. Further, the study investigated the links between monsoon rainfall with the global climate oscillations such as Quasi Bienniel Oscillation (QBO), El Nino Southern Oscillation (ENSO), Sunspot Number (SN), Atlantic Multidecadal Oscillation (AMO) etc. The study noticed that the multiscale components of rainfall series IMF1, IMF2, IMF3, IMF4 and IMF5 have similar periodic structure of QBO, ENSO, SN, tidal forcing and AMO respectively. As per the seasonal rainfall patterns is concerned, the results of the study indicated that for Assam-Meghalaya subdivision, there is a likelihood of extreme rare events at ~0.2 cycles per year, and both monsoon and pre-monsoon rainfall series have decreasing trends; for Kerala subdivision, extreme events can be expected during monsoon season with shorter periodicity (~2.5 years), and monsoon rainfall has statistically significant decreasing trend and post-monsoon rainfall has a statistically significant increasing trend; and for Orissa subdivision, there are chances of extremes rainfall events in monsoon season and a relatively stable rainfall pattern during post-monsoon period, but both monsoon and post-monsoon rainfall series showed an overall decreasing trend; for Telangana subdivision, there is a likelihood of extreme events during monsoon season with a periodicity of ~4 years, but both monsoon and post-monsoon rainfall series showed increasing trends. The results of correlation analysis of IMF components of monsoon rainfall and five climate indices indicated that the association is expressed well only for low frequency modes with similar evolution of trend components.  相似文献   

10.
A relationship between summer monsoon rainfall and sea surface temperature anomalies was investigated with the aim of predicting the monthly scale rainfall during the summer monsoon period over a section (80°–90°E, 14°–24°N) of eastern India that depends heavily upon the rainfall during the summer monsoon months for its agricultural practices. The association between area-averaged rainfall of June over the study zone and global sea surface temperature (SST) anomalies for the period 1982–2008 was examined and the variability of rainfall in monthly scale was calculated. With a view to significant variability in the rainfall in the monthly scale, it was decided to implement the artificial neural network (ANN) for forecasting the monthly scale rainfall using the SST anomalies as a predictor. Finally, the potential of ANN in this prediction has been assessed.  相似文献   

11.
In this study, sensitivity of the Indian summer monsoon simulation to the Himalayan orography representation in a regional climate model (RegCM) is examined. The prescribed height of the Himalayan orography is less in the RegCM model than the actual height of the Himalayas. Therefore, in order to understand the impact of the Himalayan orography representation on the Indian summer monsoon, the height of the Himalayan orography is increased (decreased) by 10 % from its control height in the RegCM model. Three distinct monsoon years such as deficit (1987), excess (1988) and normal rainfall years are considered for this study. The performance of the RegCM model is tested with the use of a driving force from the reanalysis data and a global model output. IMD gridded rainfall and the reanalysis-2 data are used as verification analysis to validate the model results. The RegCM model has the potential to represent mean rainfall distribution over India as well as the upper air circulation patterns and some of the semi-permanent features during the Indian summer monsoon season. The skill of RegCM is reasonable in representing the variation in circulation and precipitation pattern and intensity during two contrasting rainfall years. The simulated seasonal mean rainfall over many parts of India especially, the foothills of the Himalaya, west coast of India and over the north east India along with the whole of India are more when the orography height is increased. The low level southwesterly wind including the Somali jet stream as well as upper air circulation associated with the tropical easterly jet stream become stronger with the enhancement of the Himalayan orography. Statistical analysis suggests that the distribution and intensity of rainfall is represented better with the increased orography of RegCM by 10 % from its control height. Thus, representation of the Himalayan orography in the model is close to actual and may enhance the skill in seasonal scale simulation of the Indian summer monsoon.  相似文献   

12.
—The study presents the results of the statistical relationship between seasonal northeast monsoon rainfall over Tamil Nadu state of India (TNR) and southeast India (SER) and mid-latitude circulation indices viz., zonal index (ZON) meridional index (MER) and the ratio of meridional to zonal index (M/Z) between the geographical area 35°N to 70°N at 500 hPa level over three sectors and hemisphere, based on 19 years (1971–1989) of data. The results indicate that northeast monsoon rainfall over India shows a strong antecedent relationship with the strength of ZON over all the sectors and hemisphere. The best association is observed during antecedent March over sector I (45°W–90°E) where direct and strong correlation coefficients of 0.69 and 0.64 are obtained with TNR and SER, respectively. Antecedent MAM (spring) season over sector I also shows a significant positive correlation with TNR/SER. Thus, the mid-latitude zonal circulation index may have possible use for the long-range forecasting of northeast monsoon rainfall over India.  相似文献   

13.
Sediment transport from mountainous to lowland areas is considered one of the most important geomorphological processes. In the present study, variations in transported sediment loads and dissolved loads have been studied over 3 years (2008–2011) for two forested catchments located in the Lesser Himalayan region of India. Seasonal and annual suspended sediment flux was strongly influenced by amounts of rainfall and streamflow. On average, 93% of annual load was produced during the monsoon, of which 62–78% occurred in only five peak events. Sediment production by the degraded forest catchment (Bansigad) was 1.9-fold (suspended sediment load) to 5.9-fold (bedload) higher than the densely forested catchment (Arnigad). The dissolved organic matter potentially influences total dissolved solids in the stream. Heavy rainfall triggers both stream discharge and landslides, which lead to higher bedload transport. Total denudation rates for Arnigad and Bansigad were estimated at 0.68 and 1.02 mm?year?1, respectively.  相似文献   

14.
Regional climate models are important tools to examine the spatial and temporal characteristics of rainfall and temperature at high resolutions. Such information has potential applications in sectors like agriculture and health. In this study, the Regional Climate Model Version 3 (RegCM3) has been integrated in the ensemble mode at 55 km resolution over India for the summer monsoon season during the years 1982–2009. Emphasis has been given on the validation of the model simulation at the regional level. In Central India, both rainfall and temperature show the best correlations with respective observed values. The model gives rise to large wet biases over Northwest and Peninsular India. RegCM3 slightly underestimates the summer monsoon precipitation over the Central and Northeast India. Nevertheless, over these regions, RegCM3 simulated rainfall is closer to the observations when compared to the other regions where rainfall is overestimated. The position of the monsoon trough simulated by the model lies to the north of its original observed position. This is similar to the usual monsoon break conditions leading to less rainfall over Central India. RegCM3 simulated surface maximum temperature shows a large negative bias over the country while the surface minimum temperature is close to the observation. Nevertheless, there is a strong correlation between the all India weighted average surface temperature simulated by RegCM3 and IMD observed values. While examining the extreme weather conditions in Central India, it is found that RegCM3 simulated frequencies of occurrence of very wet days, extremely wet days, warm days and warm nights more often as compared to those in IMD observed values. However, these are systematic biases. The model biases in the frequencies of distribution of rainfall extremes explain the wet and dry biases in different regions in the country. Overall, the inter-annual characteristics of both the rainfall and temperature extremes simulated by RegCM3 in Central India are well in phase with those found in the observed data.  相似文献   

15.
A high-resolution (～1 km horizontal grid and 21 vertical layers) numerical model based on the Princeton Ocean Model (POM) has been used to study the 3D dynamics of the Upper Gulf of Thailand (UGOT). While influenced by tides and rivers like other estuarine systems, the UGOT is unique because it is wide (～100 km?×?100 km), it is shallow (average depth of only ～15 m), it is located in low latitudes (～12.5°N–13.5°N), and it is influenced by the seasonal monsoon. Sensitivity studies were thus conducted to evaluate the impact that surface heat fluxes, monsoonal winds, river runoffs, and the low latitude may have on the dynamics; the latter has been evaluated by modifying the Coriolis parameter and comparing simulations representing low and mid latitudes. The circulation in the UGOT changes seasonally from counter-clockwise during the northeast monsoon (dry season) to clockwise during the southwest monsoon (wet season). River discharges generate coastal jets, whereas river plumes tend to be more symmetric near the river mouth and remain closer to the coast in low latitudes, compared with mid-latitude simulations. River plumes are also dispersed along the coast in different directions during different stages of the monsoonal winds. The model results are compared favorably with a simple wind-driven analytical estuarine model. Comparisons between an El Niño year (1998) and a La Niña year (2000) suggest that water temperatures, warmer by as much as 2 °C in 1998 relative to 2000, are largely driven by decrease cloudiness during the El Niño year. The developed model of the UGOT could be used in the future to address various environmental problems affecting the region.  相似文献   

16.
Indian summer monsoon and El Nino   总被引：1，自引：0，他引：1  
The associations between strong to moderate El Nino events and the all-India and subdivisional summer monsoon rainfall is examined for the period 1871 to 1978. The significance of the association is assessed by applying the Chi-square test to the contingency table. The analysis indicates that during 22 El Nino years the Indian monsoon rainfall was mostly below normal over most parts of the country. However, the association between El Nino and deficient rainfall or drought is statistically significant over the subdivisions west of longitude 80°E and north of 12°N. During the five strong El Nino years—1877, 1899, 1911, 1918, and 1972—many areas of India suffered large rainfall deficiencies and severe droughts. There are four moderate El Nino years—1887, 1914, 1953, and 1976—when the suffering was marginal. The relationship between El Nino and the Indian monsoon rainfall is expected to be useful in forecasting large-scale anomalies in the monsoon over India.  相似文献   

17.
Advances in studying interactions between aerosols and monsoon in China   总被引：1，自引：0，他引：1  
Scientific issues relevant to interactions between aerosols and the Asian monsoon climate were discussed and evaluated at the 33rd “Forum of Science and Technology Frontiers” sponsored by the Department of Earth Sciences at the Chinese Academy of Sciences. Major results are summarized in this paper. The East Asian monsoon directly affects aerosol transport and provides a favorable background circulation for the occurrence and development of persistent fog-haze weather. Spatial features of aerosol transport and distribution are also influenced by the East Asian monsoon on seasonal, inter-annual, and decadal scales. High moisture levels in monsoon regions also affect aerosol optical and radiative properties. Observation analyses indicate that cloud physical properties and precipitation are significantly affected by aerosols in China with aerosols likely suppressing local light and moderate rainfall, and intensifying heavy rainfall in southeast coastal regions. However, the detailed mechanisms behind this pattern still need further exploration. The decadal variation in the East Asian monsoon strongly affects aerosol concentrations and their spatial patterns. The weakening monsoon circulation in recent decades has likely helped to increase regional aerosol concentrations. The substantial increase in Chinese air pollutants has likely decreased the temperature difference between land and sea, which favors intensification of the weakening monsoon circulation. Constructive suggestions regarding future studies on aerosols and monsoons were proposed in this forum and key uncertain issues were also discussed.  相似文献   

18.
Seasonal and annual trends of changes in rainfall, rainy days, heaviest rain and relative humidity have been studied over the last century for nine different river basins in northwest and central India. The majority of river basins have shown increasing trends both in annual rainfall and relative humidity. The magnitude of increased rainfall for considered river basins varied from 2–19% of mean per 100 years. The maximum increase in rainfall is observed in the Indus (lower) followed by the Tapi river basin. Seasonal analysis shows maximum increase in rainfall in the post‐monsoon season followed by the pre‐monsoon season. There were least variations in the monsoon rainfall during the last century and winter rainfall has shown a decreasing trend. Most of the river basins have experienced decreasing trends in annual rainy days with a maximum decrease in the Mahanadi basin. The heaviest rain of the year has increased from 9–27 mm per 100 years over different river basins with a maximum of 27 mm for the Brahamani and Subaranrekha river basins. A combination of increase in heaviest rainfall and reduction in the number of rainy days suggest the possibility of increasing severity of floods. Such information is useful in the planning, development and management of water resources in the study area. Further, the majority of river basins have also experienced an increasing trend in relative humidity both on seasonal and annual scales. An increase in annual mean relative humidity for six river basins has been found in the range of 1–18% of mean per 100 years, while a decrease for three river basins from ? 1 to ? 13% of mean per 100 years was observed, providing a net increase in the study area by 2·4% of mean per 100 years. It is understood that an increase in areal extent of vegetation cover as well as rainfall over the last century has increased the moisture in the atmosphere through enhanced evapotranspiration, which in turn has increased the relative humidity. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

19.
Monsoon water cycle is the lifeline to over 60 per cent of the world’s population. Throughout history, the monsoon-related calamities of droughts and floods have determined the life pattern of people. The association of Green House Gases (GHGs) particularly Carbon dioxide (CO2) with monsoon has been greatly debated amongst the scientific community in the past. The effect of CO2 on the monsoon rainfall over the Indian–Indonesian region (8–30°N, 65°–100°E) is being investigated using satellite data. The correlation coefficient (Rxy) between CO2 and monsoon is analysed. The Rxy is not significantly positive over a greater part of the study region, except a few regions. The inter-annual anomalies of CO2 is identified for playing a secondary role to influencing monsoon while other phenomenon like ENSO might be exerting a much greater influence.  相似文献   

20.
The present paper presents a diagnostic study of two recent monsoon years, of which one is dry monsoon year (2009) and the other is wet monsoon year (2010). The study utilized the IMD gridded rainfall data set in addition to the Reynolds SST, NCEP-NCAR reanalysis wind and temperature products, and NOAA OLR. The study revealed that the months July and August are the most crucial months to decide whether the ISMR is wet or dry. However, during July 2009, most of the Indian subcontinent received more than 60 % in the central and western coastal regions. In a wet monsoon year, about 35–45 % of rainfall is contributed during June and July in most parts of India. During these years, the influence of features in the Pacific Ocean played vital role on the Indian summer monsoon rainfall. During 2009, Pacific SST was above normal in nino regions, characteristic of the El Nino structure; however, during 2010, the nino regions were clearly below normal temperature, indicating the La Nina pattern. The associated atmospheric general circulation through equatorial Walker and regional Hadley circulation modulates the tropospheric temperature, and hence the organized convective cloud bands. These cloud bands show different characteristics in northward propagation during dry and wet years of ISMR. During a dry year, the propagation speed and magnitudes are considerably higher than during a wet monsoon year.  相似文献   

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