The estimation of rainfall in the Sahelian squall line by the area-threshold method

Radar observations from Niamey (Niger), in the soudano-sahelian region of West Africa, are used to show the applicability of the area-threshold method to the estimation of the mean areal rainfall in squall lines. A squall line, the main rain-bearing system in the sahelian area, is made up of two strongly different entities: a narrow convective line producing heavy rainfall, followed by a wide area of light stratiform rain. The extent of the system is such that it usually cannot be entirely observed in the frame of a single radar snapshot. So the ratio of convective to stratiform rain in the radar snapshot changes while the squall line moves over the radar-observed area. It is demonstrated that, in this case, the area-threshold method can be separately applied to the convective line and to the stratiform area.