Rainfall uncertainty in the Mediterranean: Definition of the rainy season - A methodological approach

Two different approaches exist for defining a rainy season; the Meteorological year (from January 1st till December 31st) is usually used in regions with rainfall all year round, and the Hydrological year is used in regions with a long dry period. The Starting Analysis Date (SAD) refers to the beginning of a new rainy season and it characterizes a certain region, whereas, the Rainy Season Beginning Date (RSBD) and the Rainy Season Ending Date (RSED) determine the Rainy Season Length (RSL). The RSBD can be defined in two ways: on the day in which a certain threshold amount of rainfall is accumulated, or on the day in which a certain percentage of rainfall is accumulated. The RSL is influence by both the SAD and the RSBD/RSED. An unsuitable SAD will alter the analyses results and may lead to misleading conclusions. The goals of the present study are: to suggest a new approach for the determination of the SAD and to apply it to the Mediterranean region. For this end, daily rainfall data for the period of 1931-2006 were analyzed. The RSL was defined as the period that elapsed from the day on which 10% of the annual rain was accumulated till the day on which 90% was accumulated. In order to find the shortest RSL, it was calculated 365 times for each Julian day as the SAD and for every available year. The median RSL for each Julian day was determined. A Cluster and a Factor analyses were performed on a correlation matrix that included 365 RSLs at 41 stations. Two distinguishable courses with uni-modal distribution appeared in all analyses and separated the area into two regions: a southern region with a minimum in the summer and a maximum in the winter, and a less pronounced annual course with a minimum in March and a maximum in June in the northern part of the area. Between these two regions, a transitional zone with a bi-modal course was identified. The main conclusions of the present study are that the Hydrological year should be adopted for rainfall analyses in all regions, regardless of their temporal distribution. It is suggested that the SAD should be set to July 1st, in the southern Mediterranean basin; February 1st in the transitional zone and to March 1st in the northern Mediterranean basin.