Abstract
I am grateful that A. van Delden has pointed out some inadequacies of mesoscale entrainment instability (MEI) as an explanation for mesoscale cellular convection (MCC). I agree that the MEI model for MCC suffers because of the requirement that lateral diffusion acts as the large wavenumber cut-off. In order for lateral diffusion to effectively damp out scales immediately smaller than those with the maximum growth rate, the diffusivity K is constrained by
where km is the horizontal wavenumber and σ3 the growth rate of the fastest growing mode. (The above criterion can also apply to linear convective models.) For MCC, km≈10-4m-1. Any mesoscale instability worthy of the name would need σm > 10-5 s-1. K must be at least 1000 m2 s-1 if MEI is to account for the cell-size of MCC.