Abstract
A frictional boundary layer is coupled to a two-level model of the free atmosphere to investigate the wave-CISK (conditional instability of the second kind) in the tropics. To relax several assumptions made in a previous analytical study (Wang, 1988), the full set of linear model equations is solved numerically. In the stable parameter regime for the inviscid wave-CISK, the frictional convergence in the boundary layer generates a slowly eastward-propagating, unstable mode which has finite growth rate even for the short wave. In contrast to Wang (1988), the unstable mode does not have a maximum of the growth rate at any finite wavenumber, and is not equatorially trapped but has a wavy meridional structure at the longwave limit.
