Analysis of Storm Events (and Interstorm Dry Periods)

Abstract

In the East Sahel, it is important to know whether monsoon rains occur over extended periods of contiguous days or are episodic in nature. Not only are the statistics of storm duration important for proper water management, but also statistics on the intervals between storms—intraseasonal and interannual dry periods. Storm statistics reported here for three study sites in the East Sahel accord well with results reported by others for elsewhere in the Sahel. There are typically 33 storm events annually at Gedaref (GED), 20 events at Kassala (KAS), and 11 events at Khartoum (KHA); their respective mean storm totals are 18 mm (GED), 13 mm (KAS) and 12 mm (KHA). Statistical metrics on storm total and duration are derived from the percentage of the annual number of storms and from the percentage of the annual total rainfall that each storm contributes. Much like what is reported for the West Sahel, there are significantly more smaller events than larger ones at these three East Sahel stations; however, it is the fewer, larger storms that deliver the greater percentage of the annual total rainfall. Storm totals greater than 50 mm are an important class of extreme events, with approximately 5 % of the storm events at these stations falling into this category. Such extreme events tend to occur between Day 150 and Day 300 of the calendar year. Although there appears to be little tendency for storms to cluster at particular times of the season, their temporal center of mass tends to fall in the July to August sector. Interstorm dry periods during the monsoon or growing season commonly exceed 20 days. Interannual dry periods between the monsoons at GED exceed 200 days approximately 25 % of the time (one out of 4 years), 230 days at KAS, and 250 days at KHA.