The Annual Cycle of Precipitation Over the Indian Subcontinent: Daily, Monthly and Seasonal Statistics
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NCAR/TN-401+STR NCAR TECHNICAL NOTE m January 1995 The Annual Cycle of Precipitation over the Indian Subcontinent: Daily, Monthly and Seasonal Statistics Dennis J. Shea Climate Analysis Section, NCAR N. A. Sontakke Indian Institute of Tropical Meteorology, Pune, India CLIMATE AND GLOBAL DYNAMICS DIVISION - NATIONAL CENTER FOR ATMOSPHERIC RESEARCH BOULDER, COLORADO TABLE OF CONTENTS Preface .... Acknowledgments . v 1. Introduction ... .......................... 15V Physiography .. ·.......................... 35 . .· . ·. .. .. ..... 3 Seasons .... Surface Winds and Pressure . *.*.. 5 Temperature ..... ....... ... .. 7 Precipitation . .......... .. .. 2. Data . ............... 15 3. Methodology ............... .. .. 17 Unconditional and Conditional Probabilities . .............. 18 Objective Analysis Procedure ....... .................. 19 4. Daily Annual Cycle ........... ..... .. .21 Detailed description for one station .... ................. .21 Annual Cycle at Selected Indian Stations .. .................. 26 Annual Cycle at Selected Stations from Bangladesh, Pakistan, Sri Lanka and Tibet . 33 5. Seasonal Precipitation Statistics . .................... 35 6. Summary ..................... ............ 37 References .... 39 Appendix A: "Day-of-Year" to "Calander Day" table ..... 43 Appendix B: Listing of Stations .......... 45 Appendix C: Daily Annual Cycles at Selected Indian Stations .... 55 Appendix D: Daily Annual Cycles at Selected Stations from 125 Bangladesh, Pakistan, Sri Lanka and Tibet Appendix E: Seasonal Statistics over the Subcontinent ...... 156 iii I Preface This technical note documents the annual cycle of precipitation over the Indian subcontinent. Particular attention is focused upon India, the largest country within the region. Gaussian (i.e., means and standard deviations) and robust (i.e., medians and sextiles) statistics are used to describe the climatology and variability (both temporal and spatial) of precipitation on a daily, monthly and seasonal basis. These statistics should be useful to meteorologists, hydrologists, atmospheric modelers, the agricultural community and policy makers. Acknowledgements N. A. Sontakke (Indian Institute of Tropical Meteorology) was supported under the World Me- teorological Organization Fellowship Program and was visiting Dr. Roland Madden for the pe- riod September through December 1993. The data were obtained from the India Meteorological Department and the Climate Analysis Center, Washington D.C. The data were archived and sup- plied by NCAR's Data Support Section. Drs. P. Das (Texas A&M), S. Gadgil (Indian Institute of Science), G.B. Pant (IITM) and S.V. Singh (IITM) graciously reviewed the manuscript. NCAR's Graphics Group produced several of the figures. Finally, Ronna Terrell-Baily typed much of the manuscript and provided considerable technical support. v 1. INTRODUCTION The climate of the Indian subcontinent" is dominated by the south Asiatic monsoons (Fig. 1) which are important components of the global circulation of the atmosphere [see, for exam- ple, Ramage (1971), Rao (1976, 1981), Das (1986), Chang and Krishnamurti (1987), Fein and Stephens (1987) and references therein]. The northeast (NE) monsoon affects the subcontinent during winter, however, the influence of the NE monsoon is weak and limited in spatial extent. The southwest (SW) monsoon affects the subcontinent during summer, June to September. It brings humid air masses and the Inter-Tropical Convergence Zone reaches its maximum pole- ward displacement over the subcontinent. Rainfall associated with the SW monsoon accounts for 70 to 90% of the annual total over much of the subcontinent. However, this seasonal rainfall exhibits large temporal and spatial variability. As such, it has significant social and economic impacts. Agriculture, industry, hydroelectric power generation, irrigation and reservoir man- agement are directly affected. During pre-monsoon and winter months, rainfall mainly occurs over the southern portions of peninsular India and northern/northwestern India. The principal rainy season over southeast- ern India and part of Sri Lanka is due to the NE monsoon advancing across the Bay of Bengal from October to December. Rainfall over India has been the subject of scientific investigations for well over a century (e.g., Blanford 1884, 1886). The India Meteorological Department has published reports and atlases (1906, 1943, 1962, 1971a, 1971b, 1981) on climatology and rainfall characteristics on different time scales over India and its vicinity. Rao (1981) described the general characteristics of the climate of the Indian subcontinent in World Survey of Climatology. More recently, Sontakke (1993a) and Sontakke et al. (1993b) have constructed SW and NE monsoon rainfall series from 1844 and 1871 onwards, respectively. Most previous climatological studies of precipitation over the subcontinent have focused upon monthly or seasonal precipitation. In this note, the monthly and seasonal aspects of the annual cycle are briefly discussed. However, the emphasis is upon documenting the daily climatological annual cycle of precipitation using selected stations from various parts of the subcontinent. Particular attention is focused on India, the largest country within the region. Daily rainfall data from approximately 1600 spatially well distributed Indian stations generally spanning 60 1The Indian subcontinent is defined as India, Pakistan, Bangladesh, Bhutan, Nepal and Sri Lanka. 1 July January Figure 1. Schematic of the mean surface pressure and wind patterns of the south Asiatic mon- soon in July and January. [Rumney (1968); reprinted with permission.] or more years are used. The daily rainfall climatologies from several other countries within the subcontinent are derived using data from about 60 stations. A maximum of 14 years of daily data is available from these non-Indian stations. 2 The results include: * Robust and gaussian statistics on daily, monthly and seasonal bases. * Plots from selected stations over the subcontinent which display (a) the unconditional prob- ability of precipitation on any day of the year and tabulated weekly information, (b) estimates of median and sextile precipitation amounts for each day of the year using wet days only (see Section 3) and tabulated monthly robust and gaussian statistics, and (c) histograms indicating the distribution of daily precipitation amounts. Before proceeding to the details of the methodology and the results, a brief overview of the physiography and surface climatology of the subcontinent will be provided. The purpose is to provide the reader with a framework for interpreting the statistics. Physiography One reason the SW monsoon has such a marked affect on the subcontinent is due to the phys- iography of the subcontinent (see Figs. 2 and 3). Peninsular India is bordered on the west, east and south by water (the Arabian Sea, the Bay of Bengal and the Indian Ocean, respectively). The northern limit is defined by high mountains including the Himalayas. These features isolate the subcontinent from the rest of Asia and exert a profound influence on the climate of the region. The Western Ghats are parallel to the west coast of India, extending from 10°N to 22°N. Gen- erally, this mountain range is within 100 km of the coast. Several peaks exceed 2000 m in the southern portions of this range (e.g. Anai Mudi [2695 m] and Nilgiri Hills [2636 m]). The Eastern Ghats are further inland (200 km) from the Bay of Bengal than the Western Ghats are from the Arabian Sea and are not as high. The highest peaks, located near 20°N, are a little over 1000 m while in the south, where the two mountain ranges merge, peaks as high as 1628 m occur in Tamil Nadu. Several other, smaller, mountain ranges (Aravali, Vindhya and Satpura) are present in west-central India. Between the Western and Eastern Ghats and south of about 22°N lies the Deccan Plateau. The Chota Nagpur Plateau is between 80°E and Bangladesh and is centered at the Tropic of Cancer. North of the Chota Nagpur Plateau and south of Nepal and the Himalayas is the Ganges Plain. The Great Indian Desert (sometimes called the Thar Desert) is in the northwestern part of India and borders Pakistan. 3 AREA AVERAGE TOPOGRAPHY OF INDIAN SUBCONTINENT 30 20 10 70 80 90 Figure 2. The physiography of the Indian subcontinent. (Area averaged elevations are in meters. Data source: RAND l°topography.) SCHEMATIC REPRESENTATION OF INDIAN SUBCONTINENT r,.---j ,, --- H1 I, I I I L ,-^· ts@ !iKunlunshanmai r ..I: ," .. [' >- be CHINA c:_ ~ ' /-, Plateau 30 X C - . / . .. Indian 62 "/ Y)- A i Desert , ' A-- Naga- l P a in '\..1 '". ' rY SnlS S -Hill ' ] -- /, Nagpur ( , ateau - -jI 20 Bay of Bengal 10 I I[1, --IiI I I I t I t 70 80 90 Figure 3. Schematic of the Indian subcontinent. 4 Seasons The climate over India is broadly divided into four seasons. The seasons and other names which are often used when describing them are: January-February (JF; winter), March-April-May (MAM; spring; "hot-weather" season), June through September (JJAS; summer; SW monsoon) and October through December (OND; autumn; northeast monsoon; post-monsoon season). Because the subcontinent is large, this seasonal segmentation is not "perfect". However, these unconventional seasonal designations best segment the large-scale annual variations of the ma- jor components of the subcontinent's climate: surface wind/surface pressure, temperature and precipitation. Surface Wind and Pressure Patterns Over the subcontinent, a large-scale reversal of the wind