A Protocol to Determine Interannual Variation of North-East Monsoon (Nlaha rainfalls): A Case Study in Anuradhapura, Sri Lanka

Abstract

A protocol was developed to analysis the interannual variation of North-East monso on (Maha rainflalls] and the protocol was validated with daily rainfall data of Anuradhapura for 1980-2010 period. Cumulative distribution of rainy days was plotted against the date of the year and a Gaussian model was fitted. The maximum positive and maximum negative curvatures of the Gaussian model were selected as the onset and the retreat ofthe North-East monsoon rainy season and the linear trends ofonset, retreat and length of the growing season were checked for the significant deviation from zero. Accordingly, the mean onset and retreat of the period for 1981-200 \was ZZ"d of October and 29th of fanuary, respectively. The average length of the growing season was 97 days. Onset dates for 2002-2010 period variecl from 13rh of October (2008] to 15th of November [2009J with the mean of 26thoctober. The earliest and the most delayed retreat dates were 31't December [2006) and Bd, March [2010), respectively. The length of the season varied from 58 days [2006J to 128 days [2010J with a mean of 90 days. The regression analysis revealed that there is a positive trend, i.e. delayed onset and retreat, and increased growing length in future though the interannual variation is not significant during zooz-2010.