Sunday, May 16, 2010

Rainfall on the Meghalaya plateau in Northeastern India

Monthly and daily variations in rainfall over Cherapunjee and Mawsynram on the Meghalaya plateau of northeastern India are analysed. Cherapunjee and Mawsynram are well known as two of the places with the heaviest rainfall in the world. The daily rainfall variation is attributed to the influence of synoptic scale disturbances, with a periodicity of 10–20 days, and the orographic interaction. The annual and monthly highest rainfalls over Cherapunjee during the 31 years from 1973 to 2003 were much larger than mean values.

The Meghalaya plateau is located on the northeastern part of the Indian subcontinent and consists of relatively high hills. The highest hill peak of the region is about 1,965 m above mean sea level (Fig. 1). Meghalaya means ‘‘adobe of clouds’’. Cherapunjee is well known as the place with the world record for the highest rainfall of 26,461 mm, during August 1860 to July 1861. It is located on the southern slope of the Meghalaya plateau and inside a deep canyon with steep cliffs. Four seasons are experienced in Cherapunjee, viz., winter (December–February),pre-monsoon (March–May), monsoon (June–September) and post-monsoon (October–November). It was opined that Mawsynram might be the rainiest station in India. Mawsynram is the neighbouring village of Cherapunjee, and a rain gauge was installed in 1941 and hence the mean annual rainfall over Mawsynram and Cherrapunjee was compared simultaneously for 10 years and the annual mean rainfall over Mawsynram was larger than that over Cherrapunjee. However, they indicated the difficulty of this comparison, because of the limited data of 10 years. The rainfall records for Cherrapunjee and Mawsynram are available for 100 and 60 years, respectively. After comparison of annual mean rainfall over both stations, Cherrapunjee still has the maximum rainfall over 24 hours, 1,036.3 mm in 1867, in India. On the basis of hourly rainfall analyses for 4 years, indicated that rainfall over Cherrapunjee occurred mostly in the early morning (0100–0700 IST). It was speculated the existence of the frontal zone with dry easterly or northerly to the north and maritime wet southerly to the south to explain the morning rainfall and the effect of topography along with the front near Cherrapunjee caused the extraordinary rainfall. Recently, observations using the tipping-bucket rain gauge revealed that severe hourly rainfall occurred during the night.

Analyzing the variations in annual, monthly and daily rainfall over Cherrapunjee was done, Cherrapunjee is located on the southern slope of the Meghalaya plateau, most of the rain flows directly down into Bangladesh; the amount of rain falling in the catchment area upstream of rivers is important for the forecast of severe floods in Bangladesh. By comparing the rainfall over Cherrapunjee and the water level in adjoining rivers in Bangladesh, The water level in Bangladeshi rivers is significantly related to rainfall over Cherrapunjee.

By utilizing the monthly rainfall data from January 1973 to December 2003 and daily rainfall data from June 2003 to July 2004, supplied by IMD. Figure 2 shows the annual rainfall variation at Cherrapunjee for 31 years from 1973 to 2003. The maximum rainfall was 24,555.3 mm in 1974 and the minimum was 6,950.3 mm in 1978. The average annual rainfall during this period (referred to as l) was 11,987 mm with a standard deviation (referred to as r) of 3,324 mm. Most of the rainfall occurs during the summer monsoon season, and the amount of rain in winter is much less. Rainfall begins to increase in March, with 300 mm, and the rainy season continues until October. The peak of monthly rainfall comes around July, at more than 3,000 mm. The variation of monthly rainfall during 1974 (the year of highest annual rainfall) is shown as a dashed line. The monthly highest rainfalls during the 31 years under consideration were recorded in the 4 months of April, July, September and October in that year.

Source:Fumie Murata, Taiichi Hayashi, Jun Matsumoto & Haruhisa Asada for Nat Hazards (2007)
Photo:-My Collections

1 comment:

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