Steering height of a semi-infinite Eady edge wave ($z_c=|f_0|/(NK)$)

The critical level of the regular edge wave $c=\Lambda /m$, $U=\Lambda z$, is $z_c=1/m$ for $\Lambda \ne 0$. At that height $U=c$, so mean advection and the phase term oppose in the wave frame. A literal condition $U=-c$ instead gives the unphysical height $-1/m$ below the lower wall.