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Diurnal and spatial variation of Indian summer monsoon rainfall using tropical rainfall measuring mission rain rate
Journal of Hydrology 475 (2012) 248–258
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Diurnal and spatial variation of Indian summer monsoon rainfall using tropical rainfall measuring mission rain rate Hamza Varikoden ⇑, B. Preethi, J.V. Revadekar Indian Institute of Tropical Meteorology, Pashan, Pune 08, India
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Article history: Received 16 December 2011 Received in revised form 30 August 2012 Accepted 30 September 2012 Available online 13 October 2012 This manuscript was handled by Konstantine P. Georgakakos, Editor-in-Chief, with the assistance of David J. Gochis, Associate Editor Keywords: Summer monsoon rainfall Frequency Intensity Rainy hours
s u m m a r y This paper deals with the diurnal and spatial variation of summer monsoon (JJAS) rainfall, extremes and its frequency for a period of 12 years using the TRMM (Tropical Rainfall Measuring Mission) rain rate data set. Rainfall and frequency of rain events are more observed over the western coastal belts, northeastern regions, foothills of Himalayas and Bay of Bengal regions. Average seasonal rainfall is about 400 mm from 150 rainy hours. The rainfall amount and frequency of rain events show a peak at 1730 IST over Central India, 0230 IST over the foothills of Himalayas and 0830 IST over the north Bay of Bengal. Rainfall over the west coast is more evenly distributed throughout the day. Diurnal pattern of rain intensity is different than that of the rainfall, it is more over the Gujarat regions and peak is observed at 1730 IST and 0230 IST. The relative contribution of rainfall by each octet towards the seasonal total is different. Contribution of rainfall towards its seasonal is maximum at 1730 IST over the main land and is different over other regions. In addition to this, the diurnal pattern of high intensity (90th percentile and above) and low intensity (10th percentile and below) rainfall during the season were also analyzed to find out their spatial distribution. We found that the diurnal pattern is more prominent for the high intensity rainfall than that for the low rainfall intensity. Maximum rainfall hours show systematic southward propagation from early morning to late night. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction South Asian regions are dominated by seasonal climatic fluctuations and the major rainy season is the southwest monsoon season. About 70–90% of the annual rainfall is received during the four months (June–September) of southwest monsoon. In addition to the seasonal fluctuations, Indian summer monsoon is modulated by diurnal fluctuations (Pathan, 1994). Nature of diurnal variation of rainfall varies from place to place and depends upon the location, elevation or topography of the region. Also the time of occurrence of maximum and minimum rainfall differs substantially from region to region. Occasionally some regions are getting one-day precipitation higher than 40–100% or more than that of their mean annual rainfall (Dhar et al., 1982). Vaidya and Kulkarni (2007) reported that 94.4 cm of rainfall has recorded on 26 July 2005 in a Mumbai station (Santacruz) due to successive generation of thunder clouds. Diurnal variation of rainfall and other meteorological parameters have gained enough attention due to their vital role in regulating climate by many types of feed backs. Several scientists over the world have done many analysis of rainfall diurnal characteristic. Rainfall characteristics in terms of duration, intensity and depth ⇑ Corresponding author. E-mail address: [email protected] (H. Varikoden). 0022-1694/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2012.09.056
have been studied widely by several authors across the globe such as Burgueño et al. (2004) for Spain, Smith (2004) for Australia, Bidin and Chappell (2006) Varikoden et al. (2010; 2011) for Peninsular Malaysia. In addition to this, Oki and Musiake (1994) investigated the diurnal cycle of rainfall in the west coast, the inland and the east coast regimes of the Peninsular Malaysia. Shen et al. (2007) studied the anomalous precipitation events in China, Ashley et al. (2003) analyzed the mesoscale organization and their temporal distribution in USA. Fauchereau et al. (2003) studied the variation of rainfall over a century in the warming environment. Kousky (1980) examined the relationship between the orography and local wind systems and over Brazil and found significant relationship between the mountain wind circulation systems and orography. In case of Indian summer monsoon, several studies have investigated the variabilities of rainfall in different spatial and temporal scales. The variability of rain spells over the Indian sub-continent have been studied extensively by Rajeevan et al. (2008), Goswami et al. (2006), Deshpande et al. (2011). The variability and predictability of Indian summer monsoon rainfall are also reported (e.g. Rajeevan, 2001; Pai and Rajeevan, 2006; Goswami and Gouda, 2007; Raju et al., 2007). Studies explored that the Indian monsoon is more heterogeneous mainly due to its dependence on atmospheric (Dugam and Kakade, 2003) and oceanic (Pattanaik and Rajeevan, 2007) influences.
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In addition to the low frequency variabilities, high frequency variabilities such as diurnal cycle has also been a subject of research for several decades, the scarcity of observational data sets with high temporal and spatial resolution was the main reason for limited research on diurnal pattern of monsoon rainfall. Johnson (2008) presented a review on the diurnal pattern of convection over the Asian monsoon. The study observed that the peak in rainfall over land is during the afternoon/evening hours and is during the morning hours over the oceanic regions. Pathan (1994) inferred that maximum rainfall were observed between midnight to early morning hours over coastal and island stations, whereas an afternoon/evening maximum were observed over the inland stations. Krishnamurti and Kishtawal (2000) studied the diurnal mode of the Asian summer monsoon using satellite data (Meteosat-5 and TRMM) and they found that the diurnal divergent circulation has an ascending lobe over north central India during the afternoon hours, whereas the descending lobe reaches out radially toward central and southern parts of China, the equatorial Indian Ocean, and the western Arabian Sea. Sahany et al. (2010) studied the signatures of diurnal pattern of Indian monsoon rainfall using TRMM 3B42 data set and they found that the peak of rain events in terms of the time of occurrence and intensity of rainfall, are different in different locations. The characteristics of rainfall and associated convection varies from region to region and also depends on the topography and moisture availability. Houze et al. (2007) studied three dimensional behavior of deep convective monsoon systems over the mountain barrier in the complex terrains of Himalaya using the TRMM precipitation radar (PR) echoes and they found that deep and intense convective echoes are associated with the low level moist air from the Arabian Sea and broad stratiform echoes are associated with the convection in the Bay of Bengal. Nesbitt and Anders (2009) studied the rainfall characteristics and its interaction with the topography over the Indian continent using the TRMM 2A25 data set and they concluded that the heaviest precipitation observed follows the 500 m contour in the entire length of western ghat mountain range and also heavy precipitation was observed along the coastal planes of the west coastal stations. TRMM PR (2A25) data varies from year to year and it deteriorates in the morning hours for light rain over low elevations in the steep narrow valleys of Himalaya (Barros and Tao 2008). Climatological diurnal pattern of rainfall around the Himalayas using TRMM PR reflectivity was studied by Bhat and Nakamura (2005) and they noticed a diurnal peak during the afternoon hours over the Tibetan Plateau and that in the Karakoram valley is at early morning hours. Studies (Goswami et al. 2006; Ghosh et al. 2009) reveal that the occurrence of heavy to very heavy rainfall events are increasing, while low and moderate rainfall events are decreasing over the Indian subcontinent. Moreover, the occurrence of very heavy rainfall over a short period of time and continuous occurrence of dry hours produces adverse effect on crop, while the continuous occurrence of low rainfall intensity favors the crop growth (Revadekar and Preethi 2010). In the context of the recent changes in the distribution of rainfall, the understanding of diurnal variation of mean and extremes in rainfall is of great important. The high spatial and temporal coverage of the TRMM rain rate data provides an opportunity to study the diurnal characteristics of rainfall in detail. In the present study is therefore carried out for understanding of diurnal variation of rainfall over Indian subcontinent, using the 3-h climatology of rainfall, frequency of rain events and intensity of rainfall. Further the percentage contribution of rainfall at different octets to the seasonal summer monsoon seasonal rainfall have been studied to understand the hours which contributes more to the seasonal rainfall. In addition to this, hours at which maximum and minimum rainfall events occur were also analyzed.
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2. Data and method of analysis The study utilizes rain rate derived from the Tropical Rainfall Measuring Mission (TRMM) V6 3B42 data set for a 12 year period from 1999–2010 for 122 days of the Indian summer monsoon season (June–September). The data has a temporal resolution of 3 h and a spatial resolution of 0.25° 0.25° latitude-longitude grid (Kummerow et al., 1998). The 3-h averaged values are centered at the middle of each 3 h period; for instance, the rain rate corresponding to 0000 UTC represents the average value between 2230 and 0130 UTC. This product is an optimal combination of various high quality microwave rain rate estimates from all sensors (TMI, SSM/I, AMSR and AMSU) and microwave-calibrated infrared rain estimates and is also merged with gauge measurements. The TRMM multi-sensor precipitation analysis (3B42) which combines Precipitation Radar (PR), multi-satellite passive microwave and geostationary infrared rainfall estimates every 3 h, adjusted with monthly rain gauge totals over land (Huffman et al. 2007). While comparing the 3B42 data set with PR reflectivity (2A25) data set of TRMM, the 3B42 has relatively low sampling error and is useful for studying the rainfall estimates because it is gridded and not instantaneous observation, however, the 2A25 data set is instantaneous (Nesbitt and Anders, 2009). The rain rate product from TRMM was validated over the Peninsular Malaysia with rain gauge observations by Varikoden et al. (2010) and over the Indian region with previous studies by Sahany et al. (2010). High spatial and temporal coverage of the data makes the product suitable to study the diurnal characteristics as well as the extremes of rainfall in detail. Several researchers utilized the data set to explore the diurnal characteristics of rainfall over different locations of the world. Harikumar et al. (2008) compared the TRMM rain rate data with disdrometer data over a tropical station of India and has reported a correlation coefficient value of 0.60 in the continental region. The TRMM rain rate show good agreement with the rainfall measurement from the manual rain gauges situated at different locations of Peninsular Malaysia (Varikoden et al. 2010) and has been used to study the diurnal characteristics of rainfall at different intensity classes during different seasons of the country (Varikoden et al. 2011). TRMM rain rate and ocean rain gauge data from moored buoys of the TAO/TRTON are compared well with a correlation value of 0.97 (Bowman et al. 2003; Bowman 2005). A detailed evaluation of the TRMM rainfall estimates over a western state of India can be obtained from Nair et al. (2009). Their study points out that the satellite estimate of rainfall are most accurate over the regions of moderate rainfall and inaccurate mainly in regions of sharp rainfall gradient. Though the rainfall amount estimated were sometime under/over estimated, the timing of the rain events are generally coincident with the gauge observations over most part of the regions except the immediate rain shadow region of the state. Estimating the peak of diurnal cycle of rainfall from TRMM 2B42 product and buoy data over Bay of Bengal, Sahany et al. (2010), concluded that analysis of the product regarding the peak octet of diurnal cycle of rainfall appears to be robust. All these studies show the reliability of the satellite data to study the rainfall characteristics in different intensity classes over the tropical regions. In this study, the climatology of seasonal rainfall (cumulative daily rainfall of the summer monsoon season), frequency of rain events (number of rain events per season) and rainfall intensity (average seasonal rainfall due to rainy events), defined as ratio of total seasonal rainfall to the number of rainy events have been computed at different octets using the 3-h TRMM 3B42 data set and the climatological diurnal variation of these parameters have been discussed. Further the percentage contribution of rainfall at different octets towards the seasonal rainfall is examined. The topographic map of India and names of various places are given in Fig. 1.
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In addition to the climatological diurnal variation of rainfall, an understanding of the variation of extremes in rainfall are also been made. The rainfall received during the season and also the higher and lower extreme values of rainfall has large spatial variability over the Indian subcontinent. Hence, extreme events defined based on percentile threshold (which varies from region to region depending upon the rainfall climatology of the region) are being widely used for the regions having large spatial variations (Revadekar and Preethi, 2010). In this context, we selected 90th percentile and 10th percentile threshold to represent heavy and low rainfall events respectively and the rainfall corresponding these thresholds are computed for each octets at each grid point over the subcontinent. These thresholds are based on the rainfall corresponding to the 90% (10%) of observations while sorted in ascending order and is called 90th (10th) percentile. The number of events when rainfall is higher (lower) than 90th percentile (10th percentile) threshold and also the amount of rainfall received from such events have been computed for all years for all the octets. Further, the spatial distribution of climatology of the extremes and their diurnal variation are explored. A discussion on the spatial variation
in the hours of maximum and minimum precipitation is also been made on this study. 3. Climatology of seasonal rainfall Climatology of rainfall, frequency of rain events and intensity of rainfall are computed at each grid point based on the 3-h data of 122 days of the monsoon season of 12 years (1999–2010) and is presented in Fig. 2. The regions along the west coast and northeastern parts of India receive large amount of rainfall during summer monsoon season as revealed by the earlier study (Rao, 1976). Orography plays an important role in the spatial distribution of rainfall over the country. Due to the orientation of western ghats from north to south, southwesterly monsoon winds shed most of the moisture over west coast of the country located along the windward side of mountains. The mountain ranges over northeastern parts are responsible for the heavy rainfall activity over these regions. The extreme northern part of the country is shut off from the southwest monsoon current by orographic barriers of Himalayas, however the regions gets rainfall from western disturbances,
Fig. 1. The topographic map of India.
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(a)
(b)
(c)
Fig. 2. Seasonal climatology (1999–2010) of (a) rainfall (mm/h), (b) frequency of rain events (numbers/season) and (c) mean rainfall intensity (mm/rainy hours) as derived from TRMM 3 h rain rate data for summer monsoon season.
which originates from the Mediterranean Sea. Northwest India and southeastern parts of the Indian peninsula receives lowest amount of rain (Fig. 2a). It is observed from the Figure that the heavy rainfall regions of west coast, northeast India and the foothills of Himalaya posses higher frequency of rainfall events (Fig. 2b). These regions receive seasonal rainfall of 400 mm from 150 rainy hours. However, the intensity of rainfall (Fig. 2c) is high over western part of the country, over the Gujarat region (more that 4 mm/h) and also over the regions of heavy rainfall, where the intensity exceeds 3.5 mm/h. The amount of rainfall received during the season has large diurnal variability. The spatial distribution of seasonal rainfall and the corresponding frequency of rain events during the different octets of a day can be observed from Figs. 3 and 4 respectively. The figures show that the west coastal regions of India receives heavy rainfall throughout the day. High amount of rainfall is also received over the foothills of Himalayas and north Bay of Bengal. Frequency of rain events is also high in the regions of high rainfall, however, the pattern of rain intensity is different (Fig. 5) from the pattern of seasonal rainfall and number of events (Figs. 3 and 4). Over central India, the rainfall amount varies from 0.2 to 0.6 mm/h and has a large diurnal variation with maximum rainfall during evening hours around 1730 IST and minimum during morning hours around 0830 IST. Over the region, the frequency of rainfall events vary from 10–15 in the morning hours of 0830 IST to 20–30 in the evening hours of 1730 IST. Large diurnal variation (similar to central India) is also seen in the foot hills of Himalayan regions with maximum around early morning hours of 0230 IST (with more than 32 rain events) and minimum around 1130 IST (with around 16 events). The diurnal variation over the north Bay of Bengal is slightly different from that of the foot hills. The minimum rainfall occurs around 2030 h and maximum rainfall around 0830 h. The rain events also show similar spatial pattern as that of the rainfall. During the minimum rainfall hours, the frequency of rain events are less than 32 per season and during the maximum rainfall hours the rain events are more than 44 per season. Rainfall intensity is calculated from the total rainfall and total number of rain events. Average seasonal rain intensity over 12 year period for the octets are given in Fig. 5. On the spatial domain, the hourly intensity is high over the Gujarat region in all the time. It indicates that high intensity regions are receiving rainfall from very few events, viz these regions receive considerable amount of rainfall within a short period of time. The peak in intensity of rainfall is at 1730 IST in the west coastal regions and minimum during
predawn and dawn hours. Gujarat region recorded the maximum intensity (more than 5.5 mm/rainy hours) at almost all the hours of a day. An intensity of 3 mm/rainy hours is observed along the west coast of India throughout the day. Large diurnal variation in the intensity of rainfall is seen over the foothills of Himalayas with minimum around 1130 and 2330 IST, and maximum in the early morning hours from 0230 to 0530 IST. Diurnal variation is less over central India, where early morning hours receive intense rainfall activity compared to the evening hours. The rainfall intensity over Central India is between 3.5 and 4.5 mm/rainy hours, therefore, we can conclude that the high amount of rainfall within a short interval of time are very few over the region. Contribution of hourly rainfall to the seasonal rainfall is studied to understand the time in which the maximum contribution takes place in a spatial domain. The percentage contribution of each mode of the octet has been computed and is provided in Fig. 6. The figure reveals that over most part of the country, more than 20% of rainfall is obtained during the evening hour of 1730 IST, with more contribution along the Indo-Gangetic plain, except over the foothills of Himalayas. A minimum contribution (<10%) of rainfall over the country is observed during 0830 IST. Whereas in the foothills of Himalayas, maximum contribution comes from the rainfall during morning hours and minimum from the evening hours. Over the Indo-Gangetic planes, the percentage contribution shows higher from 1430 to 2030 IST, however, over the peninsular region, high contribution is observed from 1730 to 2330 IST. From 0230 to 1130 IST the rainfall contribution in continental areas of Indian region is comparatively much less and it is below 9%. Foothills of Himalayas receives it is 70% of seasonal rainfall from the hours of 0230 and 1130 IST and diurnal peak is observed at 0230 IST. In the western Bay of Bengal the high contribution of rainfall is seen from 0230 to 1130 IST and low contribution from 1430 to 2330 IST. 4. Seasonal rainfall extremes Goswami et al. (2006) reported that heavy rain events are increasing and low rain events are decreasing over Central India during the recent decades. Based on finer scale analysis of the ISMR, Ghosh et al. (2009) also reported, that some places are having increasing trends in heavy rain events and some other regions with decreasing trends in low rain events. Frequent occurrence of low (deficient) rainfall has large negative impact on the crop compared to heavy rainfall (Revadekar and Preethi, 2010). In this context, it is important to see the spatial variation in rainfall extremes
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Fig. 3. Hourly climatology (1999–2010) of summer monsoon rainfall (mm/h) corresponding to the octets, 0230 IST through 2330 IST at 3-h interval.
Fig. 4. Hourly climatology (1999–2010) of frequency of rain events (numbers/season) during summer monsoon season corresponding to the octets, 0230 IST through 2330 IST at 3-h interval.
during different hours of a day. As the amount of rainfall received over the country has large spatial variability (Ghosh et al. 2009), percentile threshold have been used to define an extreme rainfall activity. 90th percentile of rainfall have been used to define heavy rainfall threshold while a 10th percentile of rainfall is used to show the lower end of rainfall. Similar to the spatial variation of seasonal rainfall obtained at different hours, both the extremes in rainfall has large spatial and temporal variations. The spatial variation of rainfall corresponding to the heavy (lower) rainfall thresholds,
the frequency of events with rainfall greater (lesser) than the threshold value are analyzed to obtain the diurnal variation in the heavy and low intensity of rainfall. 4.1. Rainfall corresponds to 90th percentile The amount of rainfall corresponding to the 90th percentile has adverse effect on agriculture because showers may damage the crop system. Here, we present the hourly rainfall that corresponds
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Fig. 5. Hourly climatology (1999–2010) of rainfall intensity (mm/rainy hours) during summer monsoon season corresponding to the octets, 0230 IST through 2330 IST at 3-h interval.
Fig. 6. Percentage contribution of rainfall (%) at different octet modes, 0230 IST through 2330 IST at 3-h interval, to the summer monsoon seasonal rainfall.
to 90th percentile to understand its variability in space and time (Fig. 7). Over the space, high amount of rainfall are seen over the western coastal belt, foothills of Himalayas and northeastern regions. Out of these three prominent areas, maximum rainfall is found over the Gujarat region. In addition to the spatial variation, rainfall exhibits dominant diurnal variation. Over west coastal region, it has maximum value, above 11.5 mm, during 0830 IST. The foothills of Himalayas and northeastern parts of the country
has its largest value (above 8.5 mm) during early morning hours (0530 IST). The region of Gujarat posses largest value of the 90th percentile compared to other parts of the country during all the hours, with maximum above 15 mm during 0830 IST and minimum around 8 mm in the evening hours. Over central Indian region, the 90th percentile of rainfall varies from 4–8 mm, with maximum during 0230 and 0530 IST hours. Southeastern part shows high amount of rainfall corresponding to the 90th percentile
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Fig. 7. Rainfall (mm/h) corresponding to 90th percentile at different octets, 0230 IST through 2330 IST at 3-h interval, during summer monsoon season.
during evening hours with a maximum of 8 mm at 1730 IST, and a minimum of 2 mm at 1130 IST. The number of events exceeding the rainfall amount corresponding to the 90th percentile in the southwest monsoon season follows the spatial distribution of seasonal rainfall. Average number of rain events exceeding the 90th percentile at each location is given in Fig. 8. High number of events are found over the north Bay of Bengal and it exceeds more than 4 events per season. Over this region, dominant peak is observed at 1130 IST and minimum at 2030 IST. However, the diurnal variation over the west coastal belts are not so prominent and it has been noted that the number of events exceeding the 90th percentile is between 2.5 and 3.5 events per season. Even though the frequency of occurrence of rain events above the 90th percentile is low, the relative contribution from these events are high (Fig. 9). In the west coastal areas, the rainfall is more than 28 mm/h in all the octets. Similarly, over the north Bay of Bengal, the amount of rainfall from high rain events are more than 40 mm/h from 2330 to 1130 IST (from the average number of events 4.5 per season). In other periods the amount of rainfall is above 32 mm/h from the 3.5 events. Over Central India, the contribution from the high intensity rainfall is more during 1430 to 2030 with a peak at 1730 IST and the minimum is observed at 0830 IST and there the average rainfall amount is less than 8 mm/h from about 1 event per season. In general, more than 50% of rainfall is contributing from the high intensity rain events or from the rain spell above the 90th percentile. 4.2. Rainfall corresponds to 10th percentile It is observed that the diurnal variation of high intensity events (events above 90th percentile) has prominent diurnal pattern. In the similar way, the study of the low intensity rainfall is also imperative to understand its spatial diurnal pattern. Spatial variation of rainfall corresponding 10th percentile at different octets is presented in Fig. 10. During southwest monsoon period, the low rainfall has been spreaded in large part of the Indian subcontinent, especially over Indo-Gangetic plains and west coastal areas. The rainfall shows low amount over the southwestern regions. The low intensity rainfall shows a coherent diurnal pattern over the
Indo-Gangetic plain and west coastal regions. The peak in rainfall is seen at 1730 and minimum is found at 2330 IST. The rain amount over both the regions exceeded 0.55 mm/h during the peak time. The Bay of Bengal show high amount of rainfall at early morning hours around 0530 IST and minimum at 2330 IST. During the peak time, the rainfall amount over most of the areas in the Bay of Bengal is about 0.5 mm/h, however, in certain pockets, it exceeded more than 0.55 mm/h. In general the value of rainfall amount below 10th percentile ranges from 0.1 to 0.7 mm/h over the country. The lowest value for 10th percentile of rainfall, indicates that the mid-night hours (2330 IST) are comparatively dry hours during the Indian summer monsoon season. The frequency of rainfall events having rainfall less than the 10th percentile and the amount of rainfall received due to these events has shown a similar spatial variation as that of the rainfall exceeding 90th percentile (Figure not shown). Obviously, the contribution of the low rainfall towards the seasonal mean is less.
4.3. Heavy and low rainfall hours In addition to above studies, we describe the climatological peak of maximum rainfall and minimum rainfall and it is given in Fig. 11. This information is important to understand the hours at which heavy and low rainfall activity occurs over the country. This is also a manifestation of diurnal pattern showing the minimum and maximum rainfall at their octet modes. Large spatial variations in the time of occurrence of heavy rainfall activity has been observed over the country. Over most part of the country the heavy rainfall occurs in the afternoon hours, the regions of west coast, Indogangetic plain, the state of Orissa, reports a rainfall maxima around 1730 IST, however, the rain shadow regions of northern Tamil Nadu, Andhra Pradesh, Maharashtra, northern parts of Gujarat, show rainfall maxima during night hours, around 2030 to 2330 IST. The foot hills of Himalayas, the northeastern states like Assam and some regions in the east coast of Andhra Pradesh, experiences heavy rainfall activity during early morning hours of 0230 to 0830 IST. Just south of the foot hills of Himalaya, the heavy rainfall peaked at 1430 IST. In general, maximum rainfall
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Fig. 8. Frequency of rain events (number/season) corresponding to 90th percentile at different octets, 0230 IST through 2330 IST at 3-h interval, during summer monsoon season.
Fig. 9. Rainfall (mm/season) received from the rain events exceeding the value corresponding to 90th percentile at different octets, 0230 IST through 2330 IST at 3-h interval, during summer monsoon season.
hours show systematic southward propagation from early morning to late night. In the case of low rainfall, we also noticed spatial distribution of trough in rainfall during different octet modes (Fig. 11b). The lowest rainfall occurs over the Indian subcontinent in most of the regions are between 0830 and 1130 IST, except in certain areas like foot hills of Himalayas and right over the coastal belts. The foot
hills of Himalayas, the northern and northeastern parts of the country experiences minimum rainfall during evening hours from 1730 to 2330 IST. The state of Jammu and Kashmir recorded it minimum rainfall at 1430 IST. Some regions in the northern part of west and east coast reports minimum rainfall during night hours, 2330 IST. The windward and leeward sides of the mountain ranges of western ghats does not show any difference in the hours of
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Fig. 10. Rainfall (mm/h) corresponding to 10th percentile at different octets, 0230 IST through 2330 IST at 3-h interval, during summer monsoon season.
Fig. 11. Peak octets (IST) of the (a) maximum and (b) minimum rainfall.
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minimum rainfall activity, suggesting that the orography does not play an important role in the spatial distribution of minimum rainfall. However there is considerable difference in the hour of occurrence of maximum rainfall between the windward and leeward sides of the western ghats, with west coastal regions reporting maximum rainfall activity during evening hours (1730 IST) and the rain-shadow regions during night hours (2030 to 2330 IST and even the early morning hours of 0230 IST). Most parts of the Arabian Sea and the Bay of Bengal show minimum rainfall during night hours at 2330 IST.
5. Summary and discussion Spatial structure of diurnal variation of mean rainfall and extremes in rainfall during the Indian summer monsoon season have been discussed in the present study. The study used 3-h high resolution (0.25° 0.25° latitude-longitude grid) Tropical Rainfall Measuring Mission (TRMM) 3B42 V6 rain rate for the recent 12 year period from 1999 to 2010 for the southwest monsoon period. We explained the diurnal pattern of number of events in seasonal rainfall and also the number of events above 90th percentile and below the 10th percentile, representing the heavy and low rainfall activities respectively. The spatial variation in the hours of maximum and minimum rainfall activity over the country has also been explored. In general, the heavy rainfall regions are observed over the west coast, northeast, north Bay of Bengal regions and the foothills of Himalayas. Similarly the rain events are also registered more over the high rainfall areas. The maximum rain events are noticed over the north Bay of Bengal and it is more than 250 events/season. On an average, the regions of high rainfall receive a seasonal rainfall of 400 mm from about 150 rainy events. However, the intensity of rainfall is high over western part of the country, over Gujarat region (more that 4 mm/rainy hours) and also over the regions of heavy rainfall, where the intensity exceeds 3.5 mm/rainy hours. The rainfall shows its peak during evening hours over the west coastal stations, wee hours over the foot hills of Himalayas and early morning hours over the north Bay of Bengal regions. Variability of rain events are almost very similar to that of rainfall in both the spatial and diurnal domains. However, the rain intensity is different from the rainfall and rain events. Here, we observed two peaks, one in the early morning hours and other one at evening/ night hours. Relative contribution of the hourly rainfall to the seasonal rainfall in its octet modes is more during the hours between 1430 and 2030 IST over the continental regions except over the foot hills of Himalayas, where the contribution is high during the wee hours. Moreover, the extremes of rainfall and their spatial and temporal distribution is also studied by considering percentile threshold of 90th percentile. We observed that the events and contribution of rainfall above and corresponding to 90th percentile has also registered prominent diurnal variation. In addition, we studied the low intensity rainfall with a threshold of 10th percentile in the same way. However, the diurnal variation is more dominant in the high intensity rainfall than the low intensity rainfall. This study provides detailed understanding of the diurnal and spatial variation of Indian summer monsoon rainfall and can also provide an insight into possible impact on various sectors including agriculture.
Acknowledgment The authors are thankful to the Director, IITM, Pune for providing the necessary facilities to carry out this research work.
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Geophys. Res. Lett. 30, 1757. http://dx.doi.org/10.1029/2003GL017552. Burgueño, A., Serra, C., Lana, X., 2004. Monthly and annual statistical distributions of daily rainfall at the Fabra Observatory (Barcelona, NE Spain) for the years 1917– 1999. Theor. Appl. Climatol. 77, 57–75. http://dx.doi.org/10.1007/s00704-0030020-9. Deshpande, N.R., Kulkarni, A., Krishna Kumar, K., 2011. Characteristic features of hourly rainfall in India. Int. J. Climatol. http://dx.doi.org/10.1002/joc.2375. Dhar, O.N., Mandal, B.N., Kulkarni, A.K., 1982. A study of heavy rainfall over the Ganga basin upto Hardwar. In: Proceedings of the International Symposium on Hydrological Aspects of Mountainous Watersheds, 1, 70–78. Dugam, S.S., Kakade, S.B., 2003. Indian monsoon variability in relation to regional pressure index. Proc. Indian Acad. Sci. (Earth Planet. Sci.) 112, 521–527. Fauchereau, N., Trzaska, S., Rouault, M., Richard, Y., 2003. Rainfall variability and changes in southern Africa during the 20th century in the global warming context. Nat. Hazards 29, 139–154. Ghosh, S., Luniya, V., Gupta, A., 2009. Trend analysis of Indian summer monsoon rainfall at different spatial scales. Atmos. Sci. Lett. http://dx.doi.org/10.1002/ asl.235. Goswami, B.N., Venugopal, V., Sengupta, D., Madhusoodanan, M.S., Xavier, P.K., 2006. Increasing trend of extreme rain events over India in a warming environment. Science 314, 1442–1445. Goswami, P., Gouda, K.C., 2007. Objective determination of the date of onset of monsoon rainfall over India based on duration of persistence. CSIR Centre for Mathematical Modelling and Computer Simulation. Research Report RR CM 0711. Harikumar, R., Varikoden, H., Sampath, S., Kumar, G.M., Gairola, R.M., 2008. Comparison of TRMM precipitation data with Micro Rain Radar and Disdrometer data during different monsoon seasons. In: 37th COSPAR Scientific Assembly, Montréal, Canada, pp. 2535. Houze, R.A., Wilton, D.C., Smull, B.F., 2007. Monsoon convection in the Himalayan region as seen by the TRMM precipitation radara. Q. J. R. Meteotol. Soc. 133, 1389–1411. Huffman, G.J., Adler, R.F., Bolvin, D.T., Gu, G., Nelkin, E.J., Bowman, K.P., Stocker, E.F., Wolff, D.B., 2007. The TRMM multi-satellite precipitation analysis: Quasiglobal, multi-year, combined-sensor precipitation estimates at fine scale. J. Hydrometeoro. 8, 33–55. Johnson, R.H., 2008. Diurnal cycle of monsoon convection. In: Paper Presented at 4th International Workshop on Monsoons, World Meteorol. Organ., Beijing. Kousky, V., 1980. Diurnal rainfall variation in northeast Brazil. Mon. Wea. Rev. 108, 488–498. Krishnamurti, T.N., Kishtawal, C.M., 2000. A pronounced continental-scale diurnal mode of the Asian summer monsoon. Mon. Wea. Rev. 128, 462–473. Kummerow, C., Barnes, W., Kozu, T., Shiue, J., Simpson, J., 1998. The Tropical Rainfall Measuring Mission (TRMM) sensor package. J. Atmos. Ocean Technol. 15, 809– 817. Nair, S., Srinivasan, G., Nemani, R., 2009. Evaluation of multi-satellite TRMM derived rainfall estimates over a western state of India. J. Meteorol. Soc. Jpn. 87, 927– 939. Nesbitt, S.W., Anders, A.M., 2009. Very high resolution precipitation climatologies from the tropical rainfall measuring mission precipitation radar. Geophys. Res. Lett. 36, L15815. http://dx.doi.org/10.1029/2009GL038026. Oki, T., Musiake, K., 1994. Seasonal change of the diurnal cycle of precipitation over Japan and Malaysia. J. Appl. Meteorol. 33, 1445–1463. Pai, D.S., Rajeevan, M., 2006. Empirical prediction of Indian summer monsoon rainfall with different lead periods based on global SST anomalies. Meteorol. Atmos. Phys. 92, 33–43. Pathan, J.M., 1994. Diurnal variation of southwest monsoon rainfall at Indian stations. Adv. Atmos. Sci. 11, 111–120. Pattanaik, D.R., Rajeevan, M., 2007. Northwest pacific tropical cyclone activity and July rainfall over India. Meteorol. Atmos. Phys. 95, 63–72. Rajeevan, M., 2001. Prediction of Indian summer monsoon: status, problems and prospects. Curr. Sci. India 81, 1451–1457. Rajeevan, M., Bhate, J., Jaswal, A.K., 2008. Analysis of variability and trends of extreme rainfall events over India using 104 years of gridded daily rainfall data. Geophys. Res. Lett. 35, L18707. http://dx.doi.org/10.1029/2008GL035143. Raju, P.V., Raju, S., Mohanty, U.C., Bhatla, R., 2007. Interannual variability of onset of the summer monsoon over India and its prediction. Nat. Hazards 42, 287–300.
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Rao, Y.P., 1976. Southwest Monsoon; Meteorological Monograph, Synoptic Met. No. 1/1976, India Meteorological Department. Revadekar, J.V., Preethi, B., 2010. Statistical analysis of the relationship between summer monsoon precipitation extremes and food grain yield over India. Int. J. Climatol. http://dx.doi.org/10.1002/joc.2282. Sahany, S., Venugopal, V., Nanjundiah, R.S., 2010. Diurnal-scale signatures of monsoon rainfall over the Indian region from TRMM satellite observations. J. Geophys. Res. 115, D02103. http://dx.doi.org/10.1029/2009JD012644. Shen, C., Wang, W.C., Hao, Z., Gong, W., 2007. Characteristics of anomalous precipitation events over eastern China during the past five centuries. Clim. Dynam. http://dx.doi.org/10.1007/s00382-007-0323.
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Diurnal and spatial variation of Indian summer monsoon rainfall using tropical rainfall measuring mission rain rate Hamza Varikoden ⇑, B. Preethi, J.V. Revadekar Indian Institute of Tropical Meteorology, Pashan, Pune 08, India
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Article history: Received 16 December 2011 Received in revised form 30 August 2012 Accepted 30 September 2012 Available online 13 October 2012 This manuscript was handled by Konstantine P. Georgakakos, Editor-in-Chief, with the assistance of David J. Gochis, Associate Editor Keywords: Summer monsoon rainfall Frequency Intensity Rainy hours
s u m m a r y This paper deals with the diurnal and spatial variation of summer monsoon (JJAS) rainfall, extremes and its frequency for a period of 12 years using the TRMM (Tropical Rainfall Measuring Mission) rain rate data set. Rainfall and frequency of rain events are more observed over the western coastal belts, northeastern regions, foothills of Himalayas and Bay of Bengal regions. Average seasonal rainfall is about 400 mm from 150 rainy hours. The rainfall amount and frequency of rain events show a peak at 1730 IST over Central India, 0230 IST over the foothills of Himalayas and 0830 IST over the north Bay of Bengal. Rainfall over the west coast is more evenly distributed throughout the day. Diurnal pattern of rain intensity is different than that of the rainfall, it is more over the Gujarat regions and peak is observed at 1730 IST and 0230 IST. The relative contribution of rainfall by each octet towards the seasonal total is different. Contribution of rainfall towards its seasonal is maximum at 1730 IST over the main land and is different over other regions. In addition to this, the diurnal pattern of high intensity (90th percentile and above) and low intensity (10th percentile and below) rainfall during the season were also analyzed to find out their spatial distribution. We found that the diurnal pattern is more prominent for the high intensity rainfall than that for the low rainfall intensity. Maximum rainfall hours show systematic southward propagation from early morning to late night. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction South Asian regions are dominated by seasonal climatic fluctuations and the major rainy season is the southwest monsoon season. About 70–90% of the annual rainfall is received during the four months (June–September) of southwest monsoon. In addition to the seasonal fluctuations, Indian summer monsoon is modulated by diurnal fluctuations (Pathan, 1994). Nature of diurnal variation of rainfall varies from place to place and depends upon the location, elevation or topography of the region. Also the time of occurrence of maximum and minimum rainfall differs substantially from region to region. Occasionally some regions are getting one-day precipitation higher than 40–100% or more than that of their mean annual rainfall (Dhar et al., 1982). Vaidya and Kulkarni (2007) reported that 94.4 cm of rainfall has recorded on 26 July 2005 in a Mumbai station (Santacruz) due to successive generation of thunder clouds. Diurnal variation of rainfall and other meteorological parameters have gained enough attention due to their vital role in regulating climate by many types of feed backs. Several scientists over the world have done many analysis of rainfall diurnal characteristic. Rainfall characteristics in terms of duration, intensity and depth ⇑ Corresponding author. E-mail address: [email protected] (H. Varikoden). 0022-1694/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2012.09.056
have been studied widely by several authors across the globe such as Burgueño et al. (2004) for Spain, Smith (2004) for Australia, Bidin and Chappell (2006) Varikoden et al. (2010; 2011) for Peninsular Malaysia. In addition to this, Oki and Musiake (1994) investigated the diurnal cycle of rainfall in the west coast, the inland and the east coast regimes of the Peninsular Malaysia. Shen et al. (2007) studied the anomalous precipitation events in China, Ashley et al. (2003) analyzed the mesoscale organization and their temporal distribution in USA. Fauchereau et al. (2003) studied the variation of rainfall over a century in the warming environment. Kousky (1980) examined the relationship between the orography and local wind systems and over Brazil and found significant relationship between the mountain wind circulation systems and orography. In case of Indian summer monsoon, several studies have investigated the variabilities of rainfall in different spatial and temporal scales. The variability of rain spells over the Indian sub-continent have been studied extensively by Rajeevan et al. (2008), Goswami et al. (2006), Deshpande et al. (2011). The variability and predictability of Indian summer monsoon rainfall are also reported (e.g. Rajeevan, 2001; Pai and Rajeevan, 2006; Goswami and Gouda, 2007; Raju et al., 2007). Studies explored that the Indian monsoon is more heterogeneous mainly due to its dependence on atmospheric (Dugam and Kakade, 2003) and oceanic (Pattanaik and Rajeevan, 2007) influences.
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In addition to the low frequency variabilities, high frequency variabilities such as diurnal cycle has also been a subject of research for several decades, the scarcity of observational data sets with high temporal and spatial resolution was the main reason for limited research on diurnal pattern of monsoon rainfall. Johnson (2008) presented a review on the diurnal pattern of convection over the Asian monsoon. The study observed that the peak in rainfall over land is during the afternoon/evening hours and is during the morning hours over the oceanic regions. Pathan (1994) inferred that maximum rainfall were observed between midnight to early morning hours over coastal and island stations, whereas an afternoon/evening maximum were observed over the inland stations. Krishnamurti and Kishtawal (2000) studied the diurnal mode of the Asian summer monsoon using satellite data (Meteosat-5 and TRMM) and they found that the diurnal divergent circulation has an ascending lobe over north central India during the afternoon hours, whereas the descending lobe reaches out radially toward central and southern parts of China, the equatorial Indian Ocean, and the western Arabian Sea. Sahany et al. (2010) studied the signatures of diurnal pattern of Indian monsoon rainfall using TRMM 3B42 data set and they found that the peak of rain events in terms of the time of occurrence and intensity of rainfall, are different in different locations. The characteristics of rainfall and associated convection varies from region to region and also depends on the topography and moisture availability. Houze et al. (2007) studied three dimensional behavior of deep convective monsoon systems over the mountain barrier in the complex terrains of Himalaya using the TRMM precipitation radar (PR) echoes and they found that deep and intense convective echoes are associated with the low level moist air from the Arabian Sea and broad stratiform echoes are associated with the convection in the Bay of Bengal. Nesbitt and Anders (2009) studied the rainfall characteristics and its interaction with the topography over the Indian continent using the TRMM 2A25 data set and they concluded that the heaviest precipitation observed follows the 500 m contour in the entire length of western ghat mountain range and also heavy precipitation was observed along the coastal planes of the west coastal stations. TRMM PR (2A25) data varies from year to year and it deteriorates in the morning hours for light rain over low elevations in the steep narrow valleys of Himalaya (Barros and Tao 2008). Climatological diurnal pattern of rainfall around the Himalayas using TRMM PR reflectivity was studied by Bhat and Nakamura (2005) and they noticed a diurnal peak during the afternoon hours over the Tibetan Plateau and that in the Karakoram valley is at early morning hours. Studies (Goswami et al. 2006; Ghosh et al. 2009) reveal that the occurrence of heavy to very heavy rainfall events are increasing, while low and moderate rainfall events are decreasing over the Indian subcontinent. Moreover, the occurrence of very heavy rainfall over a short period of time and continuous occurrence of dry hours produces adverse effect on crop, while the continuous occurrence of low rainfall intensity favors the crop growth (Revadekar and Preethi 2010). In the context of the recent changes in the distribution of rainfall, the understanding of diurnal variation of mean and extremes in rainfall is of great important. The high spatial and temporal coverage of the TRMM rain rate data provides an opportunity to study the diurnal characteristics of rainfall in detail. In the present study is therefore carried out for understanding of diurnal variation of rainfall over Indian subcontinent, using the 3-h climatology of rainfall, frequency of rain events and intensity of rainfall. Further the percentage contribution of rainfall at different octets to the seasonal summer monsoon seasonal rainfall have been studied to understand the hours which contributes more to the seasonal rainfall. In addition to this, hours at which maximum and minimum rainfall events occur were also analyzed.
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2. Data and method of analysis The study utilizes rain rate derived from the Tropical Rainfall Measuring Mission (TRMM) V6 3B42 data set for a 12 year period from 1999–2010 for 122 days of the Indian summer monsoon season (June–September). The data has a temporal resolution of 3 h and a spatial resolution of 0.25° 0.25° latitude-longitude grid (Kummerow et al., 1998). The 3-h averaged values are centered at the middle of each 3 h period; for instance, the rain rate corresponding to 0000 UTC represents the average value between 2230 and 0130 UTC. This product is an optimal combination of various high quality microwave rain rate estimates from all sensors (TMI, SSM/I, AMSR and AMSU) and microwave-calibrated infrared rain estimates and is also merged with gauge measurements. The TRMM multi-sensor precipitation analysis (3B42) which combines Precipitation Radar (PR), multi-satellite passive microwave and geostationary infrared rainfall estimates every 3 h, adjusted with monthly rain gauge totals over land (Huffman et al. 2007). While comparing the 3B42 data set with PR reflectivity (2A25) data set of TRMM, the 3B42 has relatively low sampling error and is useful for studying the rainfall estimates because it is gridded and not instantaneous observation, however, the 2A25 data set is instantaneous (Nesbitt and Anders, 2009). The rain rate product from TRMM was validated over the Peninsular Malaysia with rain gauge observations by Varikoden et al. (2010) and over the Indian region with previous studies by Sahany et al. (2010). High spatial and temporal coverage of the data makes the product suitable to study the diurnal characteristics as well as the extremes of rainfall in detail. Several researchers utilized the data set to explore the diurnal characteristics of rainfall over different locations of the world. Harikumar et al. (2008) compared the TRMM rain rate data with disdrometer data over a tropical station of India and has reported a correlation coefficient value of 0.60 in the continental region. The TRMM rain rate show good agreement with the rainfall measurement from the manual rain gauges situated at different locations of Peninsular Malaysia (Varikoden et al. 2010) and has been used to study the diurnal characteristics of rainfall at different intensity classes during different seasons of the country (Varikoden et al. 2011). TRMM rain rate and ocean rain gauge data from moored buoys of the TAO/TRTON are compared well with a correlation value of 0.97 (Bowman et al. 2003; Bowman 2005). A detailed evaluation of the TRMM rainfall estimates over a western state of India can be obtained from Nair et al. (2009). Their study points out that the satellite estimate of rainfall are most accurate over the regions of moderate rainfall and inaccurate mainly in regions of sharp rainfall gradient. Though the rainfall amount estimated were sometime under/over estimated, the timing of the rain events are generally coincident with the gauge observations over most part of the regions except the immediate rain shadow region of the state. Estimating the peak of diurnal cycle of rainfall from TRMM 2B42 product and buoy data over Bay of Bengal, Sahany et al. (2010), concluded that analysis of the product regarding the peak octet of diurnal cycle of rainfall appears to be robust. All these studies show the reliability of the satellite data to study the rainfall characteristics in different intensity classes over the tropical regions. In this study, the climatology of seasonal rainfall (cumulative daily rainfall of the summer monsoon season), frequency of rain events (number of rain events per season) and rainfall intensity (average seasonal rainfall due to rainy events), defined as ratio of total seasonal rainfall to the number of rainy events have been computed at different octets using the 3-h TRMM 3B42 data set and the climatological diurnal variation of these parameters have been discussed. Further the percentage contribution of rainfall at different octets towards the seasonal rainfall is examined. The topographic map of India and names of various places are given in Fig. 1.
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In addition to the climatological diurnal variation of rainfall, an understanding of the variation of extremes in rainfall are also been made. The rainfall received during the season and also the higher and lower extreme values of rainfall has large spatial variability over the Indian subcontinent. Hence, extreme events defined based on percentile threshold (which varies from region to region depending upon the rainfall climatology of the region) are being widely used for the regions having large spatial variations (Revadekar and Preethi, 2010). In this context, we selected 90th percentile and 10th percentile threshold to represent heavy and low rainfall events respectively and the rainfall corresponding these thresholds are computed for each octets at each grid point over the subcontinent. These thresholds are based on the rainfall corresponding to the 90% (10%) of observations while sorted in ascending order and is called 90th (10th) percentile. The number of events when rainfall is higher (lower) than 90th percentile (10th percentile) threshold and also the amount of rainfall received from such events have been computed for all years for all the octets. Further, the spatial distribution of climatology of the extremes and their diurnal variation are explored. A discussion on the spatial variation
in the hours of maximum and minimum precipitation is also been made on this study. 3. Climatology of seasonal rainfall Climatology of rainfall, frequency of rain events and intensity of rainfall are computed at each grid point based on the 3-h data of 122 days of the monsoon season of 12 years (1999–2010) and is presented in Fig. 2. The regions along the west coast and northeastern parts of India receive large amount of rainfall during summer monsoon season as revealed by the earlier study (Rao, 1976). Orography plays an important role in the spatial distribution of rainfall over the country. Due to the orientation of western ghats from north to south, southwesterly monsoon winds shed most of the moisture over west coast of the country located along the windward side of mountains. The mountain ranges over northeastern parts are responsible for the heavy rainfall activity over these regions. The extreme northern part of the country is shut off from the southwest monsoon current by orographic barriers of Himalayas, however the regions gets rainfall from western disturbances,
Fig. 1. The topographic map of India.
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(a)
(b)
(c)
Fig. 2. Seasonal climatology (1999–2010) of (a) rainfall (mm/h), (b) frequency of rain events (numbers/season) and (c) mean rainfall intensity (mm/rainy hours) as derived from TRMM 3 h rain rate data for summer monsoon season.
which originates from the Mediterranean Sea. Northwest India and southeastern parts of the Indian peninsula receives lowest amount of rain (Fig. 2a). It is observed from the Figure that the heavy rainfall regions of west coast, northeast India and the foothills of Himalaya posses higher frequency of rainfall events (Fig. 2b). These regions receive seasonal rainfall of 400 mm from 150 rainy hours. However, the intensity of rainfall (Fig. 2c) is high over western part of the country, over the Gujarat region (more that 4 mm/h) and also over the regions of heavy rainfall, where the intensity exceeds 3.5 mm/h. The amount of rainfall received during the season has large diurnal variability. The spatial distribution of seasonal rainfall and the corresponding frequency of rain events during the different octets of a day can be observed from Figs. 3 and 4 respectively. The figures show that the west coastal regions of India receives heavy rainfall throughout the day. High amount of rainfall is also received over the foothills of Himalayas and north Bay of Bengal. Frequency of rain events is also high in the regions of high rainfall, however, the pattern of rain intensity is different (Fig. 5) from the pattern of seasonal rainfall and number of events (Figs. 3 and 4). Over central India, the rainfall amount varies from 0.2 to 0.6 mm/h and has a large diurnal variation with maximum rainfall during evening hours around 1730 IST and minimum during morning hours around 0830 IST. Over the region, the frequency of rainfall events vary from 10–15 in the morning hours of 0830 IST to 20–30 in the evening hours of 1730 IST. Large diurnal variation (similar to central India) is also seen in the foot hills of Himalayan regions with maximum around early morning hours of 0230 IST (with more than 32 rain events) and minimum around 1130 IST (with around 16 events). The diurnal variation over the north Bay of Bengal is slightly different from that of the foot hills. The minimum rainfall occurs around 2030 h and maximum rainfall around 0830 h. The rain events also show similar spatial pattern as that of the rainfall. During the minimum rainfall hours, the frequency of rain events are less than 32 per season and during the maximum rainfall hours the rain events are more than 44 per season. Rainfall intensity is calculated from the total rainfall and total number of rain events. Average seasonal rain intensity over 12 year period for the octets are given in Fig. 5. On the spatial domain, the hourly intensity is high over the Gujarat region in all the time. It indicates that high intensity regions are receiving rainfall from very few events, viz these regions receive considerable amount of rainfall within a short period of time. The peak in intensity of rainfall is at 1730 IST in the west coastal regions and minimum during
predawn and dawn hours. Gujarat region recorded the maximum intensity (more than 5.5 mm/rainy hours) at almost all the hours of a day. An intensity of 3 mm/rainy hours is observed along the west coast of India throughout the day. Large diurnal variation in the intensity of rainfall is seen over the foothills of Himalayas with minimum around 1130 and 2330 IST, and maximum in the early morning hours from 0230 to 0530 IST. Diurnal variation is less over central India, where early morning hours receive intense rainfall activity compared to the evening hours. The rainfall intensity over Central India is between 3.5 and 4.5 mm/rainy hours, therefore, we can conclude that the high amount of rainfall within a short interval of time are very few over the region. Contribution of hourly rainfall to the seasonal rainfall is studied to understand the time in which the maximum contribution takes place in a spatial domain. The percentage contribution of each mode of the octet has been computed and is provided in Fig. 6. The figure reveals that over most part of the country, more than 20% of rainfall is obtained during the evening hour of 1730 IST, with more contribution along the Indo-Gangetic plain, except over the foothills of Himalayas. A minimum contribution (<10%) of rainfall over the country is observed during 0830 IST. Whereas in the foothills of Himalayas, maximum contribution comes from the rainfall during morning hours and minimum from the evening hours. Over the Indo-Gangetic planes, the percentage contribution shows higher from 1430 to 2030 IST, however, over the peninsular region, high contribution is observed from 1730 to 2330 IST. From 0230 to 1130 IST the rainfall contribution in continental areas of Indian region is comparatively much less and it is below 9%. Foothills of Himalayas receives it is 70% of seasonal rainfall from the hours of 0230 and 1130 IST and diurnal peak is observed at 0230 IST. In the western Bay of Bengal the high contribution of rainfall is seen from 0230 to 1130 IST and low contribution from 1430 to 2330 IST. 4. Seasonal rainfall extremes Goswami et al. (2006) reported that heavy rain events are increasing and low rain events are decreasing over Central India during the recent decades. Based on finer scale analysis of the ISMR, Ghosh et al. (2009) also reported, that some places are having increasing trends in heavy rain events and some other regions with decreasing trends in low rain events. Frequent occurrence of low (deficient) rainfall has large negative impact on the crop compared to heavy rainfall (Revadekar and Preethi, 2010). In this context, it is important to see the spatial variation in rainfall extremes
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Fig. 3. Hourly climatology (1999–2010) of summer monsoon rainfall (mm/h) corresponding to the octets, 0230 IST through 2330 IST at 3-h interval.
Fig. 4. Hourly climatology (1999–2010) of frequency of rain events (numbers/season) during summer monsoon season corresponding to the octets, 0230 IST through 2330 IST at 3-h interval.
during different hours of a day. As the amount of rainfall received over the country has large spatial variability (Ghosh et al. 2009), percentile threshold have been used to define an extreme rainfall activity. 90th percentile of rainfall have been used to define heavy rainfall threshold while a 10th percentile of rainfall is used to show the lower end of rainfall. Similar to the spatial variation of seasonal rainfall obtained at different hours, both the extremes in rainfall has large spatial and temporal variations. The spatial variation of rainfall corresponding to the heavy (lower) rainfall thresholds,
the frequency of events with rainfall greater (lesser) than the threshold value are analyzed to obtain the diurnal variation in the heavy and low intensity of rainfall. 4.1. Rainfall corresponds to 90th percentile The amount of rainfall corresponding to the 90th percentile has adverse effect on agriculture because showers may damage the crop system. Here, we present the hourly rainfall that corresponds
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Fig. 5. Hourly climatology (1999–2010) of rainfall intensity (mm/rainy hours) during summer monsoon season corresponding to the octets, 0230 IST through 2330 IST at 3-h interval.
Fig. 6. Percentage contribution of rainfall (%) at different octet modes, 0230 IST through 2330 IST at 3-h interval, to the summer monsoon seasonal rainfall.
to 90th percentile to understand its variability in space and time (Fig. 7). Over the space, high amount of rainfall are seen over the western coastal belt, foothills of Himalayas and northeastern regions. Out of these three prominent areas, maximum rainfall is found over the Gujarat region. In addition to the spatial variation, rainfall exhibits dominant diurnal variation. Over west coastal region, it has maximum value, above 11.5 mm, during 0830 IST. The foothills of Himalayas and northeastern parts of the country
has its largest value (above 8.5 mm) during early morning hours (0530 IST). The region of Gujarat posses largest value of the 90th percentile compared to other parts of the country during all the hours, with maximum above 15 mm during 0830 IST and minimum around 8 mm in the evening hours. Over central Indian region, the 90th percentile of rainfall varies from 4–8 mm, with maximum during 0230 and 0530 IST hours. Southeastern part shows high amount of rainfall corresponding to the 90th percentile
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Fig. 7. Rainfall (mm/h) corresponding to 90th percentile at different octets, 0230 IST through 2330 IST at 3-h interval, during summer monsoon season.
during evening hours with a maximum of 8 mm at 1730 IST, and a minimum of 2 mm at 1130 IST. The number of events exceeding the rainfall amount corresponding to the 90th percentile in the southwest monsoon season follows the spatial distribution of seasonal rainfall. Average number of rain events exceeding the 90th percentile at each location is given in Fig. 8. High number of events are found over the north Bay of Bengal and it exceeds more than 4 events per season. Over this region, dominant peak is observed at 1130 IST and minimum at 2030 IST. However, the diurnal variation over the west coastal belts are not so prominent and it has been noted that the number of events exceeding the 90th percentile is between 2.5 and 3.5 events per season. Even though the frequency of occurrence of rain events above the 90th percentile is low, the relative contribution from these events are high (Fig. 9). In the west coastal areas, the rainfall is more than 28 mm/h in all the octets. Similarly, over the north Bay of Bengal, the amount of rainfall from high rain events are more than 40 mm/h from 2330 to 1130 IST (from the average number of events 4.5 per season). In other periods the amount of rainfall is above 32 mm/h from the 3.5 events. Over Central India, the contribution from the high intensity rainfall is more during 1430 to 2030 with a peak at 1730 IST and the minimum is observed at 0830 IST and there the average rainfall amount is less than 8 mm/h from about 1 event per season. In general, more than 50% of rainfall is contributing from the high intensity rain events or from the rain spell above the 90th percentile. 4.2. Rainfall corresponds to 10th percentile It is observed that the diurnal variation of high intensity events (events above 90th percentile) has prominent diurnal pattern. In the similar way, the study of the low intensity rainfall is also imperative to understand its spatial diurnal pattern. Spatial variation of rainfall corresponding 10th percentile at different octets is presented in Fig. 10. During southwest monsoon period, the low rainfall has been spreaded in large part of the Indian subcontinent, especially over Indo-Gangetic plains and west coastal areas. The rainfall shows low amount over the southwestern regions. The low intensity rainfall shows a coherent diurnal pattern over the
Indo-Gangetic plain and west coastal regions. The peak in rainfall is seen at 1730 and minimum is found at 2330 IST. The rain amount over both the regions exceeded 0.55 mm/h during the peak time. The Bay of Bengal show high amount of rainfall at early morning hours around 0530 IST and minimum at 2330 IST. During the peak time, the rainfall amount over most of the areas in the Bay of Bengal is about 0.5 mm/h, however, in certain pockets, it exceeded more than 0.55 mm/h. In general the value of rainfall amount below 10th percentile ranges from 0.1 to 0.7 mm/h over the country. The lowest value for 10th percentile of rainfall, indicates that the mid-night hours (2330 IST) are comparatively dry hours during the Indian summer monsoon season. The frequency of rainfall events having rainfall less than the 10th percentile and the amount of rainfall received due to these events has shown a similar spatial variation as that of the rainfall exceeding 90th percentile (Figure not shown). Obviously, the contribution of the low rainfall towards the seasonal mean is less.
4.3. Heavy and low rainfall hours In addition to above studies, we describe the climatological peak of maximum rainfall and minimum rainfall and it is given in Fig. 11. This information is important to understand the hours at which heavy and low rainfall activity occurs over the country. This is also a manifestation of diurnal pattern showing the minimum and maximum rainfall at their octet modes. Large spatial variations in the time of occurrence of heavy rainfall activity has been observed over the country. Over most part of the country the heavy rainfall occurs in the afternoon hours, the regions of west coast, Indogangetic plain, the state of Orissa, reports a rainfall maxima around 1730 IST, however, the rain shadow regions of northern Tamil Nadu, Andhra Pradesh, Maharashtra, northern parts of Gujarat, show rainfall maxima during night hours, around 2030 to 2330 IST. The foot hills of Himalayas, the northeastern states like Assam and some regions in the east coast of Andhra Pradesh, experiences heavy rainfall activity during early morning hours of 0230 to 0830 IST. Just south of the foot hills of Himalaya, the heavy rainfall peaked at 1430 IST. In general, maximum rainfall
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Fig. 8. Frequency of rain events (number/season) corresponding to 90th percentile at different octets, 0230 IST through 2330 IST at 3-h interval, during summer monsoon season.
Fig. 9. Rainfall (mm/season) received from the rain events exceeding the value corresponding to 90th percentile at different octets, 0230 IST through 2330 IST at 3-h interval, during summer monsoon season.
hours show systematic southward propagation from early morning to late night. In the case of low rainfall, we also noticed spatial distribution of trough in rainfall during different octet modes (Fig. 11b). The lowest rainfall occurs over the Indian subcontinent in most of the regions are between 0830 and 1130 IST, except in certain areas like foot hills of Himalayas and right over the coastal belts. The foot
hills of Himalayas, the northern and northeastern parts of the country experiences minimum rainfall during evening hours from 1730 to 2330 IST. The state of Jammu and Kashmir recorded it minimum rainfall at 1430 IST. Some regions in the northern part of west and east coast reports minimum rainfall during night hours, 2330 IST. The windward and leeward sides of the mountain ranges of western ghats does not show any difference in the hours of
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Fig. 10. Rainfall (mm/h) corresponding to 10th percentile at different octets, 0230 IST through 2330 IST at 3-h interval, during summer monsoon season.
Fig. 11. Peak octets (IST) of the (a) maximum and (b) minimum rainfall.
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minimum rainfall activity, suggesting that the orography does not play an important role in the spatial distribution of minimum rainfall. However there is considerable difference in the hour of occurrence of maximum rainfall between the windward and leeward sides of the western ghats, with west coastal regions reporting maximum rainfall activity during evening hours (1730 IST) and the rain-shadow regions during night hours (2030 to 2330 IST and even the early morning hours of 0230 IST). Most parts of the Arabian Sea and the Bay of Bengal show minimum rainfall during night hours at 2330 IST.
5. Summary and discussion Spatial structure of diurnal variation of mean rainfall and extremes in rainfall during the Indian summer monsoon season have been discussed in the present study. The study used 3-h high resolution (0.25° 0.25° latitude-longitude grid) Tropical Rainfall Measuring Mission (TRMM) 3B42 V6 rain rate for the recent 12 year period from 1999 to 2010 for the southwest monsoon period. We explained the diurnal pattern of number of events in seasonal rainfall and also the number of events above 90th percentile and below the 10th percentile, representing the heavy and low rainfall activities respectively. The spatial variation in the hours of maximum and minimum rainfall activity over the country has also been explored. In general, the heavy rainfall regions are observed over the west coast, northeast, north Bay of Bengal regions and the foothills of Himalayas. Similarly the rain events are also registered more over the high rainfall areas. The maximum rain events are noticed over the north Bay of Bengal and it is more than 250 events/season. On an average, the regions of high rainfall receive a seasonal rainfall of 400 mm from about 150 rainy events. However, the intensity of rainfall is high over western part of the country, over Gujarat region (more that 4 mm/rainy hours) and also over the regions of heavy rainfall, where the intensity exceeds 3.5 mm/rainy hours. The rainfall shows its peak during evening hours over the west coastal stations, wee hours over the foot hills of Himalayas and early morning hours over the north Bay of Bengal regions. Variability of rain events are almost very similar to that of rainfall in both the spatial and diurnal domains. However, the rain intensity is different from the rainfall and rain events. Here, we observed two peaks, one in the early morning hours and other one at evening/ night hours. Relative contribution of the hourly rainfall to the seasonal rainfall in its octet modes is more during the hours between 1430 and 2030 IST over the continental regions except over the foot hills of Himalayas, where the contribution is high during the wee hours. Moreover, the extremes of rainfall and their spatial and temporal distribution is also studied by considering percentile threshold of 90th percentile. We observed that the events and contribution of rainfall above and corresponding to 90th percentile has also registered prominent diurnal variation. In addition, we studied the low intensity rainfall with a threshold of 10th percentile in the same way. However, the diurnal variation is more dominant in the high intensity rainfall than the low intensity rainfall. This study provides detailed understanding of the diurnal and spatial variation of Indian summer monsoon rainfall and can also provide an insight into possible impact on various sectors including agriculture.
Acknowledgment The authors are thankful to the Director, IITM, Pune for providing the necessary facilities to carry out this research work.
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