Richardson number,vertical wind shear and storm occurrences at Kano,Nigeria

Radiosonde data for thirteen summer months have been used to relate Richardson under Ri and vertical wind shear to storm occurrences at Kano. It is shown that thunderstorms occur most frequently in association with low-level shears, ΔU_L, below the Africal Easterly Jet (surface to 700 mb) within −20⩽ΔU_L⩽−5 ms⁻¹ and for the 700-400 layer, ΔU_m, in the range 0<ΔU_m<10 m s⁻¹. The Richardson number with which storm occurrences are most common is bi-modal in both lower and middle troposphere: −2⩽Ri⩽0 and Ri⩽ −10 in the boundary layer (surface to 900 mb); 1⩽Ri⩽4 and Ri⩾15 in the inflow region (origin) of the downdraft air between 800 and 600 mb.Storms rarely occur (one in every seven cases) if boundary layer Ri>0 and virtually no storm should be expected if the boundary layer Ri>0 and for DDR (the regional origin of down draft air) Ri satisfying 4<Ri<15 simultaneously. The lower cut-off (Ri ⩾ 1) for the inflow air is close to the value (Ri⩾2) obtained by Moncrieff and Miller (1976) for propagating tropical storms.