Comments on “Mesoscale cellular convection: is it convection?”

Fiedler (1985) demonstrates the inadequacy of three convective instability theories in explaining the large aspect ratio of mesoscale cellular convec-tion (MCC). As an alternative explanation, he suggests mesoscale entrainment instability (MEI), an instability resulting from the entrainment fluxes at the top of the mixed layer. Fiedler uses the acronym MCC to designate “the cellular cloud pattern in marine strato-cumulus cloud decks”, i.e. closed convection cells. In fact, “Mesoscale cellular convection ” was first used by Hubert (1966) as a collective name for open as well as closed convection cells (see also Agee (1984) for some nice examples). Nowhere does Fiedler allude to the large differences between open and closed cells. Cloud cover in open cells may be only 10%. while Fiedler assumes 100 % cloud cover. Since a coupling between entrainment and conden-sation of water vapour is definitely necessary for the occurrence of ME1 (Fiedler. 1984. p. 98). it is very doubtful whether ME1 can explain the existence, let alone the structure and large aspect ratio, of open cells. The conditional nature of latent heat release seems to be a much better candidate (Van Delden, 1985). To me it even seems doubtful that ME1 can explain the large aspect ratio of closed cells with a cloud cover close to 100%. This can be seen from eq. (3.22) (Fiedler, 1985) which gives the small-scale cut-off. According to this equation ME1 occurs only whe