Transition Reynolds number R_T versus shear rate q in the cyclonic regime. Top panel: (1) squares represent DNS by LL05, see their Fig. 4; (2) triangles represent DNS performed in this work employing a cubic box; (3) circles represent DNS performed in this work employing a tall box with box ratio l_z = 2; (4) the solid curve shows R_nl(q) corresponding to constant maximum growth factor of the cross-rolls given by equation (69) and obtained using the procedure (i) introduced in Sect. 5; (5) the dashed curve shows the same as described in (4), but with g_{z max}/ϵ obtained using the procedure (ii) introduced in Sect. 5; bottom panel: error bars for tall box DNS represented by circles on top panel are plotted in the range q = −10 ÷ −3 according to the values of R_damp and R_turb given in Table 1. The corresponding solid and dashed lines represent the same as on top panel but for g_{z max}/ϵ evaluated for R_T(q = −4)=15 000 (lower lines) and for R_T(q = −4)=17 500 (upper lines).