Diurnal variations of rainfall over Niger in the West African Sahel: a comparison between wet and drought years

A comparative study has been conducted of the diurnal variations of 3‐hourly rainfall for Niamey, located in Niger, in the West African Sahel among four extreme precipitation periods: the wet periods of the 1950s and late 1980s to early 1990s versus the drought periods of the early 1970s and 1980s. In general, the 3‐hourly rainfall amounts and frequencies for Niamey exhibited a primary maximum during the hours 03:00–06:00 h local standard time (LST). The most striking differences between the wet and dry periods were found in the rainfall amounts and frequencies from midnight to morning, especially during 03:00–06:00 h LST. For the two drought periods, the rainfall amounts, frequencies, and intensities during 03:00–06:00 h LST decreased to 42, 55, and 75% of the respective rainfall parameters for the wet periods. The drought conditions during the early 1970s and 1980s were characterized by a reduced frequency of continuous heavy rainfall (total amounts exceeding 30 mm) that occurred mainly during 00:00–09:00 h LST. The phase of peak rainfall in the diurnal cycle varied from region‐to‐region over Niger. This greatly depended on the timing of the passage of mesoscale convective systems (MCSs), such as squall lines preferentially generated near the Aïr Mountains and Jos Plateau regions during the late afternoon, which then propagated westward. For the drought year 1987, reduced rainfall occurred widely over Niger during the diurnal phase of peak rainfall. This was likely due to the southward movement of the tracks of the MCSs associated with a similar displacement of the West African rainbelt. Copyright © 1999 Royal Meteorological Society