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1.
M. Janga Reddy S. Adarsh 《Stochastic Environmental Research and Risk Assessment (SERRA)》2016,30(4):1063-1085
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
2.
The relationship between the monsoon rainfall throughout all India, northwest India and peninsular India as well as the onset dates of the monsoon and two indices of southern oscillation (SOI), namely Isla de Pascua minus Darwin (I-D) and Tahiti minus Darwin (T-D) pressure anomaly have been studied for different periods. The study indicates that the monsoon rainfall shows a strong and significant direct relationship with SOI for the concurrent, succeeding autumn and succeeding winter seasons. The magnitude of the direct correlation coefficient for the SOI using (I-D) is enhanced over all India and peninsular India if the above seasons happen to be associated with an easterly phase of the QBO (Quasi-Biennial Oscillation) at 50 mb. The result indicates that the strength of the monsoon plays an important role in the following southern oscillation events in the Pacific Ocean. The premonsoon tendency of the SOI anomaly spring minus winter SOI shows a significant positive correlation with monsoon rainfall over all India, northwest India and peninsular India. The absolute value of the positive correlation coefficient becomes highly enhanced over all India, northwest India as well as peninsular India if the 6-month period from December to March is associated with the westerly phase of the QBO. Hence, the premonsoon SOI tendency parameter can be a useful predictor of Indian monsoon rainfall especially if it happens to be associated with the westerly QBO. Significant negative association is also found between the anomaly of monsoon onset dates and SOI of the previous spring season, the absolute value being higher for SOI (T-D) than for SOI (I-D). The negative correlation coefficient becomes enhanced if the previous springs are associated with a westerly phase of the QBO. It shows that the previous spring SOI has some predictive value for the onset date of Indian monsoon, a positive SOI followed by an early onset of monsoon, andvice versa, especially if it is associated with a westerly phase of the QBO. 相似文献
3.
Ramesh V. Godbole 《Pure and Applied Geophysics》1977,115(5-6):1373-1381
A diagnostie method of cumulus parameterization is suggested in which vertical transport of horizontal momentum by cumulus-scale is derived by making use of large-scale vorticity as well as divergence budget equations. Data for composite monsoon depression over India available from our earlier studies used to test the method. As a first approximation, the results are obtained using only the vorticity budget equation.The results show that in the southwest sector of the monsoon depression, which is characterized by maximum cloudiness and precipitation, there is an excess of cyclonic vorticity in the lower troposphere and anticyclonic vorticity in the upper troposphere associated with the large-scale motion. The distribution of eddy vertical transport of horizontal momentum is such that anticyclonic vorticity is generated in the lower troposphere and cyclonic vorticity aloft. Cumulus-scale eddies thus work against the large-scale system and tend to off-set the large-scale imbalance in vorticity. 相似文献
4.
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
5.
This study addresses an understanding of the possible mutual interactions of sub-seasonal variability of the two neighboring
regional monsoon systems through data analysis. The NCEP/NCAR re-analysis and OLR data for three years was used to reveal
the large-scale organization of convective episodes on synoptic (~5 days) and low frequency (15–50 day) scales. It is found
that synoptic scale organization over both the sectors is influenced by the eastward migration of large-scale convective episodes
associated with the Madden Julian Oscillation (MJO) on the low frequency scale. The organization of convection associated
with the African monsoon on the synoptic scale is influenced by the pulsatory character of lower mid-troposphere and upper
troposphere wind regimes moving westward over the African sector. Over the Indian region formation of low pressure areas and
depressions in the monsoon trough occur in an overlapping manner under an envelope of low frequency seasonal oscillation.
We have also found some correspondence between the summer monsoon rainfall over tropical North Africa and India on a decadal
basis, which would suggest a common mode of multi-decadal variability in the two monsoon systems. The study points out the
need to organize simultaneous field campaigns over the Indian and the African monsoon regions so as to bring out observational
features of possible interactions between the two neighboring systems, which could then be validated through modeling studies. 相似文献
6.
—The thermodynamic characteristics of the Asian summer monsoon are examined with a global analysis-forecast system. In this study, we investigated the large-scale balances of heat and moisture by making use of operational analyses as well as forecast fields for June, July and August (JJA), 1994. Apart from elucidating systematic errors in the temperature and moisture fields, the study expounds the influence of these errors on the large-scale budgets of heat and moisture over the monsoon region. The temperature forecasts of the model delineate predominant cooling in the middle and lower tropospheres over the monsoon region. Similarly, the moisture forecasts evince a drying tendency in the lower troposphere. However, certain sectors of moderate moistening exist over the peninsular India and adjoining oceanic sectors of the Arabian Sea and Bay of Bengal.¶The broad features of the large-scale heat and moisture budgets represented by the analysis/forecast fields indicate good agreement with the observed aspects of the summer monsoon circulation. The model forecasts fail to retain the analyzed atmospheric variability in terms of the mean circulation, which is indicated by underestimation of various terms of heat and moisture budgets with an increase in the forecast period. Further, the forecasts depict an anomalous diabatic cooling layer in the lower middle troposphere of the monsoon region which inhibits vertical transfer of heat and moisture from the mixed layer of the atmospheric boundary layer to the middle troposphere. In effect, the monsoon circulation is considerably weakened with an increase in the forecast period. The treatment of shallow convection and the use of interactive clouds in the model can reduce the cooling bias considerably. 相似文献
7.
Archana Nair Nachiketa Acharya Ankita Singh U. C. Mohanty T. C. Panda 《Pure and Applied Geophysics》2013,170(11):1945-1967
In this study the predictability of northeast monsoon (Oct–Nov–Dec) rainfall over peninsular India by eight general circulation model (GCM) outputs was analyzed. These GCM outputs (forecasts for the whole season issued in September) were compared with high-resolution observed gridded rainfall data obtained from the India Meteorological Department for the period 1982–2010. Rainfall, interannual variability (IAV), correlation coefficients, and index of agreement were examined for the outputs of eight GCMs and compared with observation. It was found that the models are able to reproduce rainfall and IAV to different extents. The predictive power of GCMs was also judged by determining the signal-to-noise ratio and the external error variance; it was noted that the predictive power of the models was usually very low. To examine dominant modes of interannual variability, empirical orthogonal function (EOF) analysis was also conducted. EOF analysis of the models revealed they were capable of representing the observed precipitation variability to some extent. The teleconnection between the sea surface temperature (SST) and northeast monsoon rainfall was also investigated and results suggest that during OND the SST over the equatorial Indian Ocean, the Bay of Bengal, the central Pacific Ocean (over Nino3 region), and the north and south Atlantic Ocean enhances northeast monsoon rainfall. This observed phenomenon is only predicted by the CCM3v6 model. 相似文献
8.
Summary The changes in circulation patterns over Eurasia during break monsoon condition over India are studied in comparison to the active monsoon condition. Break monsoon condition seems to set in over the India Sub-Continent in association with eastward movement of middle latitude westerly trough at 500 mb, having large amplitude extending into west Pakistan and northern India. Simultaneously the subtropical anticyclonic ridge over Arabia protrudes into central and Peninsular India. The high latitude blocking high over East Siberia retrogrades and the East Asiatic trough deepens and moves eastwards. The west Pacific subtropical ridge recedes eastward from the China continent. During this period the monsoon trough shifts, from its normal position over Gangetic plains, northwards to the foot of the Himalayas and the monsoon westerlies in the lower troposphere extends right upto the rim of the Tibetan Plateau. The sub-tropical ridge line in the upper troposphere shifts northwards during break and lies approximately above the lower monsoon trough. This seems to provide an effective process of removing ascending air in the lower monsoon trough causing exceptionally heavy rainfall over Assam and along the foot of the Himalayas. 相似文献
9.
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
10.
Wave data collected off Goa along the west coast of India during February 1996-May 1997 has been subjected to spectral analysis, and swell and wind sea parameters have been estimated by separation frequency method. Dominance of swells and wind seas on monthly and seasonal basis has been estimated, and the analysis shows that swells dominate Goa coastal region not only during southwest monsoon (93%), but also during the post-monsoon (67%) season. Wind seas are dominant during the pre-monsoon season (51%). The mean wave periods (Tm) during southwest monsoon are generally above 5 s, whereas Tm is below 5 s during other seasons. Co-existence of multiple peaks (from NW and NE) was observed in the locally generated part of the wave spectrum, especially during the post-monsoon season. NCEP reanalysis winds have been used to analyse active fetch available in the Indian Ocean, from where the predominant swells propagate to the west coast of India. A numerical model was set up to simulate waves in the Indian Ocean using flexible mesh bathymetry. The correlation coefficients between measured and modelled significant wave heights and mean wave periods are 0.96 and 0.85, respectively. Numerical simulations reproduced the swell characteristics in the Indian Ocean, and from the model results potential swell generation areas are identified. The characteristics of swells associated with tropical storms that prevail off Goa during 1996 have also been analysed. 相似文献
11.
D. R. Pattanaik 《Pure and Applied Geophysics》2007,164(8-9):1527-1545
The present study is an attempt to examine the variability of convective activity over the north Indian Ocean (Bay of Bengal
and Arabian Sea) on interannual and longer time scale and its association with the rainfall activity over the four different
homogeneous regions of India (viz., northeast India, northwest India, central India and south peninsular India) during the
monsoon season from June to September (JJAS) for the 26 year period (1979 to 2004). The monthly mean Outgoing Long-wave Radiation
(OLR) data obtained from National Oceanic and Atmospheric Administration (NOAA) polar orbiting spacecraft are used in this
study and the 26-year period has been divided into two periods of 13 years each with period-i from 1979 to 1991 and period
-ii from 1992 to 2004. It is ascertained that the convective activity increases over the Arabian Sea and the Bay of Bengal
in the recent period (period -ii; 1992 to 2004) compared to that of the former period (period -i; 1979 to 1991) during JJAS
and is associated with a significantly increasing trend (at 95% level) of convective activity over the north Bay of Bengal
(NBAY). On a monthly scale, July and August also show increase in convective activity over the Arabian Sea and the Bay of
Bengal during the recent period and this is associated with slight changes in the monsoon activity cycle over India. The increase
in convective activity particularly over the Arabian Sea during the recent period of June is basically associated with about
three days early onset of the monsoon over Delhi and relatively faster progress of the monsoon northward from the southern
tip of India. Over the homogeneous regions of India the correlation coefficient (CC) of OLR anomalies over the south Arabian
Sea (SARA) is highly significant with the rainfall over central India, south peninsular India and northwest India, and for
the north Arabian Sea (NARA), it is significant with northwest India rainfall and south peninsular rainfall. Similarly, the
OLR anomalies over the south Bay of Bengal (SBAY) have significant CC with northwest India and south peninsular rainfall,
whereas the most active convective region of the NBAY is not significantly correlated with rainfall over India. It is also
found that the region over northeastern parts of India and its surroundings has a negative correlation with the OLR anomalies
over the NARA and is associated with an anomalous sinking (rising) motion over the northeastern parts of India during the
years of increase (decrease) of convective activity over the NARA. 相似文献
12.
Kusuma G. Rao 《Pure and Applied Geophysics》1988,127(4):669-693
An attempt to diagnose the dominant forcings which drive the large-scale vertical velocities over the monsoon region has been made by computing the forcings like diabatic heating fields,etc. and the large-scale vertical velocities driven by these forcings for the contrasting periods of active and break monsoon situations; in order to understand the rainfall variability associated with them. Computation of diabatic heating fields show us that among different components of diabatic heating it is the convective heating that dominates at mid-tropospheric levels during an active monsoon period; whereas it is the sensible heating at the surface that is important during a break period. From vertical velocity calculations we infer that the prime differences in the large-scale vertical velocities seen throughout the depth of the atmosphere are due to the differences in the orders of convective heating; the maximum rate of latent heating being more than 10 degrees Kelvin per day during an active monsoon period; whereas during a break monsoon period it is of the order of 2 degrees Kelvin per day at mid-tropospheric levels. At low levels of the atmosphere, computations show that there is large-scale ascent occurring over a large spatial region, driven only by the dynamic forcing associated with vorticity and temperature advection during an active monsoon period. However, during a break monsoon period such large-scale spatial organization in rising motion is not seen. It is speculated that these differences in the low-level large-scale ascent might be causing differences in convective heating because the weaker the low level ascent, the lesser the convective instability which produces deep cumulus clouds and hence lesser the associated latent heat release. The forcings due to other components of diabatic heating, namely, the sensible heating and long wave radiative cooling do not influence the large-scale vertical velocities significantly. 相似文献
13.
Some aspects of the monsoon circulation and monsoon rainfall 总被引：1，自引：0，他引：1
R. Ananthakrishnan 《Pure and Applied Geophysics》1977,115(5-6):1209-1249
Summary The south Asian summer monsoon from June to September accounts for the greater part of the annual rainfall over most of India and southeast Asia. The evolution of the summer and winter monsoon circulations over India is examined on the basis of the surface and upper air data of stations across India. The salient features of the seasonal reversals of temperature and pressure gradients and winds and the seasonal and synoptic fluctuations of atmospheric humidity are discussed. The space-time variations of rainfall are considered with the help of climatic pentad rainfall charts and diagrams. The rainfall of several north and central Indian stations shows a minimum around mid-August and a maximum around mid-February which seem to be connected with the extreme summer and winter positions of the ITCZ and the associated north-south shifts in the seasonal circulation patterns. Attention is drawn to the characteristic features of the monsoon rainfall that emerge from a study of daily and hourly rainfall of selected stations. Diurnal variations of temperature, pressure, wind and rainfall over the monsoon belt are briefly treated. 相似文献
14.
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
15.
A Study on Climatological Features of the Asian Summer Monsoon: Dynamics, Energetics and Variability 总被引：1，自引：0，他引：1
A continuing goal in the diagnostic studies of the atmospheric general circulation is to estimate various quantities that cannot be directly observed. Evaluation of all the dynamical terms in the budget equations for kinetic energy, vorticity, heat and moisture provide estimates of kinetic energy and vorticity generation, diabatic heating and source/sinks of moisture. All these are important forcing factors to the climate system. In this paper, diagnostic aspects of the dynamics and energetics of the Asian summer monsoon and its spatial variability in terms of contrasting features of surplus and deficient summer monsoon seasons over India are studied with reanalysis data sets. The daily reanalysis data sets from the National Centre for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) are used for a fifty-two year (1948–1999) period to investigate the large-scale budget of kinetic energy, vorticity, heat and moisture. The primary objectives of the study are to comprehend the climate diagnostics of the Asian summer monsoon and the role of equatorial convection of the summer monsoon activity over India.It is observed that the entrance/exit regions of the Tropical Easterly Jet (TEJ) are characterized by the production/destruction of the kinetic energy, which is essential to maintain outflow/inflow prevailing at the respective location of the TEJ. Both zonal and meridional components contribute to the production of kinetic energy over the monsoon domain, though the significant contribution to the adiabatic generation of kinetic energy originates from the meridional component over the Bay of Bengal in the upper level and over the Somali Coast in the low level. The results indicate that the entire Indian peninsula including the Bay of Bengal is quite unstable during the summer monsoon associated with the production of vorticity within the domain itself and maintain the circulation. The summer monsoon evinces strong convergence of heat and moisture over the monsoon domain. Also, considerable heat energy is generated through the action of the adiabatic process. The combined effect of these processes leads to the formation of a strong diabatic heat source in the region to maintain the monsoon circulation. The interesting aspect noted in this study is that the large-scale budgets of heat and moisture indicate excess magnitudes over the Arabian Sea and the western equatorial Indian Ocean during surplus monsoon. On the other hand, the east equatorial Indian Ocean and the Bay of Bengal region show stronger activity during deficient monsoon. This is reflected in various budget terms considered in this study. 相似文献
16.
During the summer monsoon season over India a range of intraseasonal modulations of the monsoon rains occur due to genesis of weather disturbances over the Bay of Bengal (BOB) and the east Arabian Sea. The amplitudes of the fluctuations in the surface state of the ocean (sea-surface temperature and salinity) and atmosphere are quite large due to these monsoonal modulations on the intraseasonal scale as shown by the data collected during the field programs under Bay of Bengal Monsoon Experiment (BOBMEX) and Arabian Sea Monsoon Experiments (ARMEX). The focus of BOBMEX was to understand the role of ocean-atmospheric processes in organizing convection over the BOB on intra-seasonal scale. ARMEX-I was aimed at understanding the coupled processes in the development of deep convection off the West Coast of India. ARMEX-II was focused on the formation of the mini-warm pool across the southeast Arabian Sea in April-May and its role in the abrupt onset of the monsoon along the Southwest Coast of India and its further progress along the West Coast of India. The paper attempts to integrate the results of the observational studies and brings out an important finding that atmospheric instability is prominently responsible for convective organization whereas the upper ocean parameters regulate the episodes of the intraseasonal oscillations. 相似文献
17.
P. L. S. Rao 《Pure and Applied Geophysics》2003,160(12):2447-2462
In this paper, the maintenance of Asian summer monsoon circulation is compared in the National Centres for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalysis and National Centre for Medium Range Weather Forecasting (NCMRWF), India operational analysis. The time mean summer monsoon circulation is bifurcated into stable mean and transient eddy components. The mean component of the summer monsoon circulation is elucidated through the study of large-scale balances of kinetic energy, heat and moisture. Although the balance requirements are fairly satisfied by both NCEP/NCAR and NCMRWF fields, a major difference is noticed. Strong monsoon circulation is evinced by NCEP/NCAR over the Oceanic regions and NCMRWF over the landmass. The various mean budgets under consideration indicated this feature invariably. 相似文献
18.
Flooding due to excessive rainfall in a short period of time is a frequent hazard in the flood plains of monsoon Asia. In late September 2000, a devastating flood stuck Gangetic West Bengal, India. This particular event has been selected for this study. Instead of following the conventional approach of flooded area delineation and overall damage estimation, this paper seeks to identify the rural settlements that are vulnerable to floods of a given magnitude. Vulnerability of a rural settlement is perceived as a function of two factors: the presence of deep flood water in and around the settlement and its proximity to an elevated area for temporary shelter during an extreme hydrological event. Landsat ETM+ images acquired on 30 September 2000 have been used to identify the non‐flooded areas within the flooded zone. Particular effort has been made to differentiate land from water under cloud shadow. ASTER digital elevation data have been used to assess accuracy and rectify the classified image. The presence of large numbers of trees around rural settlements made it particularly difficult to extract the flooded areas from their spectral signatures in the visible and infrared bands. ERS‐1 synthetic aperture radar data are found particularly useful for extracting the settlement areas surrounded by trees. Finally, all information extracted from satellite imageries are imported into ArcGIS, and spatial analysis is carried out to identify the settlements vulnerable to river inundation. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
19.
Arabian Peninsula-North Pacific Oscillation and its association with the Asian summer monsoon 总被引：2，自引：0，他引：2
Using correlation and EOF analyses on sea level pressure from 57-year NCEP-NCAR reanalysis data, the Arabian Peninsula-North Pacific Oscillation (APNPO) is identified. The APNPO reflects the co-variability between the North Pacific high and South Asian summer monsoon low. This teleconnec- tion pattern is closely related to the Asian summer monsoon. On interannual timescale, it co-varies with both the East Asian summer monsoon (EASM) and South Asian summer monsoon (SASM); on decadal timescale, it co-varies with the EASM: both exhibit two abrupt climate changes in the middle 1960s and the late 1970s respectively. The possible physical process for the connections between the APNPO and Asian summer monsoon is then explored by analyzing the APNPO-related atmospheric circulations. The results show that with a strong APNPO, the Somali Jet, SASM flow, EASM flow, and South Asian high are all enhanced, and an anomalous anticyclone is produced at the upper level over northeast China via a zonal wave train. Meanwhile, the moisture transportation to the Asian monsoon regions is also strengthened in a strong APNPO year, leading to a strong moisture convergence over India and northern China. All these changes of circulations and moisture conditions finally result in an anoma- lous Asian summer monsoon and monsoon rainfall over India and northern China. In addition, the APNPO has a good persistence from spring to summer. The spring APNPO is also significantly corre- lated with Asian summer monsoon variability. The spring APNPO might therefore provide valuable in- formation for the prediction of Asian summer monsoon. 相似文献
