Changes in the characteristics of rain events in India

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114,, doi:10.1029/2008jd010572, 2009 Changes in the characteristics of rain events in India S. K. Dash, 1 Makarand A. Kulkarni, 1 U. C. Mohanty, 1 and K. Prasad 1 Received 10 June 2008; revised 23 January 2009; accepted 23 February 2009; published 29 May 2009. [1] Daily gridded (1 1 ) rainfall data prepared by the India Meteorological Department for the period 1951 2004 have been used in this study to examine possible changes in the frequency of rain events in India in terms of their duration and intensity per day. So far as the duration is concerned, a rain event is classified as short, long, dry, or prolonged dry spell. Similarly in terms of intensity, a rainy day is considered as low, moderate, or heavy. Changes in the frequency of these events have great relevance from the point of view of climate change. Threshold and limiting values for defining the heavy and moderate rain days are calculated in accordance with the gamma probability distribution. Results show that the frequencies of moderate and low rain days considered over the entire country have significantly decreased in the last half century. On the basis of the duration of rain events it is inferred that long spells show a significant decreasing trend over India as a whole while short and dry spells indicate an increasing tendency with 5% significance. The characteristics of rain events are also examined over six homogenous rainfall zones separately since the spatial distribution of rainfall over India shows large variability. In this study, the changes in the frequencies of different categories of rain events suggest weakening of the summer monsoon circulation over India. This hypothesis of a weakening of monsoon circulation is supported by significant reduction in the 850 hpa wind fields in the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalyzed data set. Citation: Dash, S. K., M. A. Kulkarni, U. C. Mohanty, and K. Prasad (2009), Changes in the characteristics of rain events in India, J. Geophys. Res., 114,, doi:10.1029/2008jd010572. 1. Introduction [2] The economy of India is largely based on agriculture which in turn depends on the temporal and spatial variations of rainfall, especially during the four months of summer monsoon from June to September. Therefore, the study of changes in the spatial and temporal distributions of rainfall in India has great relevance in the context of planning and policy formulation especially, in the context of global warming. Recent studies by Dash et al. [2007] and Dash and Hunt [2007] have emphasized on the decrease in the mean monsoon rainfall over India although the decreasing trend is small. Also earlier results of Srivastava et al. [1992] and Goswami et al. [2006] indicate that the change in the summer monsoon rainfall is not statistically significant. However, examination of extreme rainfall events has revealed that at several locations across India there are significant trends in the occurrences of heavy rain events during the summer monsoon season [Rakhecha and Soman, 1994; Sinha Ray and Srivastava, 2000; Goswami et al., 2006]. Sen Roy and Balling [2004] studied 903 different time series of 7 extreme rainfall indices of 129 stations for the period 1910 2000 in India and brought out that 61% of 1 Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, New Delhi, India. Copyright 2009 by the American Geophysical Union. 0148-0227/09/2008JD010572 these time series show significant increasing trend. It is important to note that socioeconomic impacts of extreme weather events in India is very large as examined by De et al. [2005] on the basis of the information collected in the past century. [3] In a recent study based on the outputs of coupled climate models, Kripalani et al. [2007] have projected weakening of the monsoon circulation over the South Asia. In an earlier study, Dash et al. [2004] had indicated that because of weakening of monsoon circulation in terms of the decrease in its horizontal and vertical wind shears, most of the low-pressure cyclonic systems do not get adequate energy for their intensification into monsoon depressions and cyclonic storms. As a consequence of this nonavailability of perturbation energy, although there is an increase in the number of low-pressure areas during the last 30 50 yrs, the number of monsoon depressions and cyclonic storms show a decreasing tendency during the same period. [4] It is well known that the summer monsoon season from June to September is the principal rainy season in India in which period about 80% of the total annual precipitation is received over a large part of the country except in Tamilnadu and Jammu and Kashmir. Tamilnadu receives maximum rain during the postmonsoon months of October December, usually known as the northeast monsoon season. On the other hand Jammu and Kashmir receive significant precipitation during the winter months January and February. Earlier studies [e.g., Parthasarthy et al., 1of12

1995; Dash et al., 2002] have shown that the spatial distribution of rainfall over the country is characterized by a large variability. The following rainfall statistics explain the spatial variability very clearly. For the entire India the mean summer monsoon rainfall is 852.4 mm [Parthasarthy et al., 1995] with a standard deviation of 84.7 mm and coefficient of variation 9.9%. As calculated by Dash et al. [2002] these values differ from region to region. The extreme values are observed in cases of northwest and northeast regions. For northwest India the mean monsoon rainfall, standard deviation and coefficient of variation are 490 mm, 132.4 mm, and 27%, respectively, whereas for northeast India the corresponding values are 1419.2 mm, 121.3 mm, and 8.5%. Thus it may be noted that the rainfall analysis based on the entire country does not reveal the true regional characteristics. It is necessary to examine the characteristics of rainfall for the entire country as well as for its several zones separately. [5] Goswami et al. [2006] analyzed long time series of gridded daily rainfall data over central India and concluded that there is a significant increase in the occurrences of extreme rainfall events over the area during the past 50 years. However, their study is confined to a selected box over central India and hence does not include the other important as well as widely divided regions such as western arid and northern hilly regions. It may be noted that any rain event has two aspects to be looked into viz. its intensity and duration. Further, these characteristics of rain events are equally important from the point of view of their impact on Indian agriculture, since it is mostly rain fed. Not much research has been conducted on the changes in the frequency of occurrence of rain events which supply precipitation for a number of consecutive days. The present study analyzes the gridded rain data covering the entire Indian landmass to examine rain events in terms of their duration such as short, long, dry, and prolonged dry spells and also for various intensities viz. heavy, moderate and low. In order to take care of the temporal variation of rainfall this study covers all the 365 days of a year. The importance of spatial correlation in regional trend analysis is also addressed in this study. The trends in the characteristics of monsoon rain events are discussed in the context of the weakening of monsoon circulation which has been supported by the analysis of wind field at 850 hpa level. Section 2 deals with the data source and methodology. Low, moderate, and heavy rain events are discussed in section 3.1 whereas short, long, dry, and prolonged dry spells are examined in section 3.2. The weakening of summer monsoon circulation is examined in section 3.3 Results of this study are summarized in section 4 along with the conclusions. 2. Data and Methodology [6] The basic data used in the present study are the daily gridded rainfall values (version 2) prepared by the India Meteorological Department (IMD) [Rajeevan et al. 2006] based on the measurements at 2140 stations in India for the period 1951 2004. The data are available at grid resolution of 1 1 latitude/longitude for all the 12 months of the year. Rajeevan et al. [2005] have discussed the method of gridded data preparation in detail. They have interpolated the station data to the specific grid points using Objective Analysis. For this purpose the methodology proposed by Shepard [1968] has been adopted; wherein in addition to a distance factor a direction factor has also been introduced while defining the weights for interpolation. On the basis of the temporal and spatial variations of precipitation, in the present study the entire country has been divided into six homogeneous rainfall zones as defined by the Indian Institute of Tropical Meteorology (IITM), Pune (www.tropmet.res.in). The locator map consisting of the six homogenous rainfall zones such as peninsular, west central, northwest, northeast, central northeast India and hilly region is given in Figure 1a. The corresponding regions which cover the grid points of IMD rain data are given in Figure 1b. This study has been conducted separately for each of the six zones and also for the entire country during each of the four dominant seasons in India. [7] In this study short rain events are defined as those having continuous rainfall with intensity 2.5 mm/d for less than 4 consecutive days. Similarly long spell rain events are those when rainfall occurs for more than or equal to 4 consecutive days. On the other hand if rainfall <2.5 mm/d occurs for a day or for more than 1 day consecutively then it is considered as a dry event. In case of rainfall <2.5 mm/d for more than or equal to 4 days continuously the event is defined as a prolonged dry spell. We have examined these spells of rain events only for the summer monsoon season during the months of June September because in this main rainy season all the four categories of rain spells are usually observed. As per the prevalent practice, IMD uses pentad rainfall values by taking the average of precipitation over 5 consecutive days. In this study we have used the minimum duration of a long rain event as 4 as well as 5 consecutive days and found marginal difference in the results. Hence, for the convenience of defining short spells, we have taken 4 as the dividing line instead of 5 days. It may be mentioned here that such categorization is made for every grid box of 1 1 size over the entire Indian landmass as well as for every grid box of each homogeneous zone. Also the total number of a particular type of event for a region is considered as the sum total of the number of such events occurring at all the grid boxes lying in that region. Time series of the individual events are examined for the entire 54 years of study and subjected to trend analysis. The calculation of trend statistics has been done using both linear and Mann-Kendall schemes and the significance of linear trend is tested using t score. In this study, the changes noticed in the rainfall characteristics are associated with the weakening of the summer monsoon circulation. Further, the wind fields in the National Centers for Environmental Prediction (NCEP)/ National Center for Atmospheric Research (NCAR) Reanalysis-1 data set have been used to examine the weakening of the monsoon circulation. [8] For categorizing rainy days on the basis of the amount of precipitation per day, one needs to define threshold values. In some of the earlier studies heavy and extreme rain days were categorized on the basis of different threshold values. For example, Dhar and Nandargi [1995] defined 25 mm/d as the lower limit for extreme rain while Goswami et al. [2006] considered 100 mm/d. On the other hand, Sen Roy and Balling [2004] carried out trend analysis of the 90th, 95th, and 97.5th percentiles of the daily rainfall 2of12

Figure 1. (a) Map of homogeneous rainfall zones (source is www.tropmet.res.in) and (b) corresponding regions covering the grids of rain data. 3of12

Table 1. Moderate Rainfall Values Defined as Inverse of the Gamma Cumulative Distribution for Probability 0.4 to <0.99 a Seasons All India Peninsular West Central Northwest Northeast Hilly Region Central Northeast Winter 7.8 45.8 7.7 48.4 6.5 32.1 6.3 31.1 6.4 31.5 8.8 53.9 7.0 36.9 Premonsoon 8.3 50.8 8.3 53.6 6.2 32.7 6.3 35.2 9.2 55.1 8.7 54.1 7.0 36.6 Monsoon 10.7 78.1 10.7 78.0 10.7 78.8 9.9 73.1 11.4 80.6 10.5 83.3 10.5 72.8 Postmonsoon 9.2 65.1 10.0 70.6 8.5 53.6 8.2 59.2 9.3 67.1 8.6 59.4 9.3 72.0 Annual 9.9 71.3 10.1 72.4 10.1 73.8 9.4 69.3 10.4 72.4 9.6 69.5 9.8 68.3 a Rainfall values given in mm/d. values. Mooley [1973] and Stephenson et al. [1999] in their study of extreme rain days suggested gamma distribution as a good fit to the daily rainfall distribution. Following their methodology we compared gamma distribution and the actual PDFs qualitatively and found the use of gamma distribution appropriate here. In this study the precipitation values corresponding to parameters of gamma distribution <0.4 and 0.99 probabilities are considered as low and heavy rain days, respectively. The limiting values for a moderate rain day lie within 0.4 and <0.99. [9] While analyzing the rainfall data at regional levels it is very important to consider the spatial correlation since it is known that significance tests need independent observations. Vogel et al. [2001] have point out that the inclusion of spatial correlation in the studies of extreme flood events carried out using station data might change the results at regional level. Some earlier studies of Cannarozzo et al. [2006], Bradley [1998], and Hosking and Wallis [1988] have explained various techniques those can be used in regional flood frequency analysis. In the present study, we have used the formula mentioned by Hosking and Wallis [1988] to calculate the effective number of independent heavy and moderate rain days and then subjected those to trend analysis. This analysis, where spatial correlation is taken into account, does not alter the main conclusion of our present study regarding the trends in the number of heavy and moderate days events although their significance have reduced marginally. In the true sense finding out the number of independent rain bearing systems is very difficult unless one goes into the details of the weather events of different scales such as monsoon lows, depressions, deep depressions, cyclonic storms, thunderstorms and western disturbances for the entire country. In spite of such rigorous analysis some interdependence of systems will always remain. For example, because of a particular synoptic system, say monsoon depression, all the grid points under its influence will not have similar rainfall. Normally the southwest sector experiences heavy precipitation and also the peripheral points get reduced rainfall. It depends on the intensity of clouds and other in situ meteorological conditions. 3. Results and Discussion [10] As mentioned in the preceding section, the limiting values for heavy rain days are defined as the inverse of the gamma cumulative distribution function corresponding to probability 0.99. These threshold values for six homogenous zones as well as for entire India in the four dominant seasons and in a year are calculated. On the basis of the most dominant weather system, i.e., the southwest summer monsoon, we have divided the year into premonsoon (March May), monsoon (June September), postmonsoon (October December), and winter (January and February). The threshold values for each zone and season are given in Table 1. On the basis of these values the numbers of heavy rain days are computed for different zones and trend analysis has been conducted. Similar analyses have been conducted for trends in the moderate and low rainfall categories. As mentioned earlier, the limiting values for low rain days are obtained as inverse of the gamma cumulative distribution for probability <0.4. Thus the values lying in the range 0.4 and <0.99 are termed as moderate rain days. 3.1. Low, Moderate, and Heavy Rain Days 3.1.1. Premonsoon [11] In the premonsoon months of March May, the threshold value of rainfall in a heavy rain day is the maximum for the northeast at 55.1 mm/d. In order of magnitude of the threshold rainfall, the regions close to the northeast are the hilly region and the peninsular India. Next comes the threshold value of 50.8 mm/d for the whole of India. This value is not very much away from those in the northeast, hilly, and peninsular India. However, the threshold values of heavy rain per day are less in the central northeast, northwest, and west central India. The threshold values in low rain days in different regions of India follow the same increasing/decreasing order as those of heavy rain days. The highest threshold value of rainfall in a low rain day is 9.2 mm/d in the northeast. 3.1.2. Summer Monsoon [12] In India maximum precipitation occurs during the southwest monsoon season covering the months June September and hence the threshold values for heavy rainfall per day are expected to be the highest during this period. As shown in Table 1 precipitation amount lying between 10.7 and 78 mm/d is termed as moderate rain for peninsular as well as for the entire India. For northwest, the moderate rain amounts are small lying between 10 and 73 mm/d. For central northeast the moderate rain events have also small magnitudes lying between 10.5 and 73 mm/d. For west central, northeast, and hilly regions the corresponding lower and upper bounds are 11 and 79, 12 and 81, and 11 and 83 mm/d, respectively. Magnitudes of low and heavy rain per day can easily be worked out from the lower and upper bounds, respectively, given in Table 1. On the basis of these values the numbers of low and heavy rain days are computed for all India as well as for each homogenous zone. [13] Left and right panels in Figure 2 show the time series of heavy (rainfall/day is greater than or equal to the inverse of gamma cumulative distribution for probability 0.99) and moderate (rainfall per day for inverse of gamma cumulative distribution for probability between 0.4 and <0.99) rain days, respectively, in summer monsoon season for six 4of12

Figure 2. The time series of the numbers of heavy and moderate rain days shown on left and right, respectively, in the monsoon season. (a and b) Peninsular, (c and d) west central, (e and f) northwest, (g and h) northeast, (i and j) hilly region, (k and l) central northeast, and (m and n) all India. The dashed line in each subplot is a linear trend line. homogenous zones and all India. These results are also summarized schematically in Figure 3 using the map of India. The linear trend analysis and Mann-Kendall trend statistics show 5% level of significance in the increasing trend in the heavy rain days in the northeast and also in the country as a whole. Increasing trends in the numbers of heavy rain days in the northwest and central northeast are not statistically significant. Remaining three regions show decreasing trends which are also not statistically significant. On the other hand the right hand panels in Figure 2 in general show decreasing trends in the moderate rain days in all the regions and for the country as a whole. It may be noted that decreasing trends in the context of all India and the west central are significant at 5% level. These results agree well with those reported by Goswami et al. [2006] where they considered the limiting values for moderate rain 5of12

Figure 3. Summary of the trends in heavy and moderate rain days occurring during the summer monsoon season in different regions. Asterisks denote a significant trend at the 5% level. day as 5 to 100 mm/d. The frequencies of low rain days identically decrease as those of moderate rain days and hence the corresponding figures are not shown here. 3.1.3. Postmonsoon [14] In postmonsoon months of October December, the limiting values for moderate rain days are defined as those with precipitations lying between 8.5 and 53 mm/d for west central, and about 8 60 mm/d for both the northwest and hilly regions. For the northeast, central northeast, and peninsular zones the corresponding limiting values are 9 and 67, 9 and 72, and 10 and 70 mm/d, respectively. While the country averaged limiting values for moderate rain days lie between 9 and 65 mm/d. [15] The numbers of heavy and moderate rain days have been further examined only for the three zones where the threshold value for heavy rain is around 70 mm/d. Figure 4 indicates that central northeast shows decreasing trend while peninsular and northeast have increasing trend in the numbers of heavy rain days. But significant trend score is noticed only in the northeast. Time series of moderate rain days are also shown in the right hand panels of Figure 4. It is evident that the numbers of moderate rain days decrease in the central northeast and northeast zones and increase in the peninsular India. But these trends are not statistically significant. 3.1.4. Winter [16] In the winter season, the western disturbances moving across the Himalayas and the north India deliver significant amount of precipitation in the hilly region. Though January and February are usually dry in most parts of the country, in the peninsular India, occasional rainfall (sometimes heavy) occurs in association with westward propagating waves in the easterlies. At the same time other parts of north and central India receive small amounts of precipitation due to induced low-pressure systems associated with the western disturbances. For winter the threshold values for heavy rain in the northwest, northeast, and west central regions are about 31 mm/d. For central northeast the corresponding value is 37 mm/d. But for peninsular and hilly regions these values are higher at 48 and 54 mm/d, respectively. While the country averaged threshold value of winter heavy rain is 46 mm/d. [17] Characteristics of rainy days in the winter season are discussed only for the hilly region, since this region has the 6of12

Figure 4. The time series of numbers of heavy and moderate rain days shown on left and right, respectively, in the postmonsoon season. (a and b) Peninsular, (c and d) northeast, and (e and f) central northeast. The dashed line in each subplot is a linear trend line. Figure 5. The time series of the numbers of (a) heavy and (b) moderate rain days in the winter season over the hilly region. Dashed lines show linear trends. 7of12

Figure 6. Numbers of long and short spells of rainfall plotted on left and right, respectively, for the monsoon season. (a and b) Peninsular, (c and d) west central, (e and f) northwest, (g and h) northeast, (i and j) hilly region, (k and l) central northeast, and (m and n) all India The dashed line in each subplot is a linear trend line. 8of12

Figure 7. The time series of the numbers of dry and prolonged dry spells considered over all India in the monsoon season. Dashed lines show linear trends. Figure 8. Summary of trends in long, short, dry, and prolonged dry spells of rainfall in different Indian regions for the monsoon season. Asterisks denote a significant trend at the 5% level. 9of12

Figure 9. Difference between the 850 hpa mean monsoonal wind speeds in the two decades 1991 2000 and 1951 1960. The shaded regions indicate significant change at the 5% level. highest threshold value. Figure 5 shows the time series of the numbers of heavy and moderate rain days in the hilly region for the winter season. The decreasing trend of moderate rain days is significant while the increasing trend of heavy rain days is not statistically significant. 3.2. Short, Long, Dry, and Prolonged Dry Spells [18] Figure 6 shows the variations in the numbers of short spell rain events (continuous rainfall for <4 days) averaged over the six homogeneous zones individually and over the entire India in the monsoon months. The numbers of short spell events over the peninsular, west central, and hilly regions and entire country show enhancement at 5% level of significance, in both the linear and Mann-Kendall trend analysis. On the other hand the numbers of long spell events (Figure 6) show decreasing trend in summer monsoon months in all the regions except in the northeast. Also the trend scores are significant at 5% level in case of the entire country and the west central region. [19] Similar statistical study has been conducted for the dry events (<2.5 mm/d) for the monsoon season. Figure 7 depicts the time series of the country averaged dry spells and prolonged dry spells. Both the numbers of dry and prolonged dry spells (dry days for more than or equal to 4 consecutively) show significant increasing trend at 5% level at the all India level. The characteristics of trends in short, long, dry, and prolonged dry events for the whole of India as well as for its different zones are summarized in Figure 8. 3.3. Weakening of Monsoonal Winds [20] Over the peninsular India the westerly jet stream is prominent during the summer monsoon months. Since the strength of wind field at lower atmosphere is one of the main components in the development and growth of convective processes over the tropical regions, changes in its strength will suggest to the variation in the monsoon circulation. Thus in the context of the possible changes in the strength of summer monsoon circulation in the warming 10 of 12

atmosphere, it is most appropriate to examine the wind fields at 850 hpa level. NCEP/NCAR reanalyzed data have been used by several scientists since it is one of the best sources of data. Hence, in this study NCEP/NCAR wind fields at 850 hpa have been analyzed to examine changes in the strength of monsoon circulation. For this purpose, the difference in the seasonal mean winds for the two decades (1991 2000) and (1951 1960) is shown in Figure 9. It can be observed that the strength of wind field has significantly (5% level) reduced over peninsular India and the adjoining southern parts of Bay of Bengal and the Arabian Sea. Figure 9 also shows regions of wind enhancement, but spatial coverage of significant results is small. 4. Conclusions [21] This study emphasizes the changes in the characteristics of rain events in terms of their duration and intensity per day based on the gridded rainfall data of IMD. Results show that in the last half century, the numbers of moderate rain days averaged over the whole of India have significantly decreased during the summer monsoon season. Similar significant decrease is noticed in the number of low rain days. On the other hand, the number of heavy rain days considered over the entire country shows some indications of increase, though the trend is not statistically significant. Since the summer monsoon rainfall has large temporal and spatial variations, this study has also been conducted for the six homogeneous zones into which India has been divided. Significant decrease in the numbers of moderate and low rain days have been observed in the hilly region and the west central India. On the other hand the numbers of heavy rain days have increased significantly only in the northeast. [22] So far as the trend in the rain spells are concerned, all India analysis shows significant decreasing trend in the occurrences of long rain spells and increasing trend in the rain spells of other categories such as short, dry, and prolonged dry. While examining the regional trends it is found that the numbers of long rain spells significantly decrease only in the west central. However, the occurrences of short, dry, and prolonged dry spells significantly increase in the west central, hilly region, and the peninsular India. It is interesting to note that in general, the numbers of short rain events and the dry spells show increasing tendency for the country as a whole and also for the six homogeneous zones, although these are not statistically significant in case of three zones such as the northwest, central northwest, and northeast. [23] The short spell rain events usually occur because of local convection, thunderstorms and other such mesoscale phenomena which are not necessarily due to monsoon circulation and organized convection. The increase in the number of short rain spells may be taken as an indication of increase in the number of intensified mesoscale conventions which are caused because of western disturbances and westward waves on the easterlies. This aspect needs to be further examined on the basis of the history of weather phenomena which are nonmonsoonal in character. On the other hand long spell rain events are usually associated with synoptic-scale or planetary-scale phenomena. During the Indian summer monsoon season, because of a number of cyclonic systems such as depressions, deep depressions and cyclonic storms, there are incidents of continuous rainfall for more than 4 days at several places in India. 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