ALBERT

Description: The tropical circulation exhibits a prominent two-layer structure in the vertical: a moist-unstable lower layer (the "trade layer") with an ITCZ-ward directed mass transport, and a stable upper layer. The aerolo�gical measurements of the German research vessel "Meteor" in Nov.-Dec. 1965 over the Eastern Atlantic (19° W) show that this mean structure is valid even in actual meridional sections, if disturbances are absent. It follows that vertical averages over each of the two layers are appropriate to characterize the tropical atmosphere. In this paper only the trade layer is studied. Its upper boundary is defined by (a) the vertical minimum of static energy; (b) the vertical extreme value of various physical quantities such as temperature and humidity gradients; ( c) constant surfaces of a quantity c which proved useful for the data evaluations. All these definitions are more or less equivalent. The boundary surface is permeable for vertical property transports which are parameterized in terms of the hot tower- and subsidence-process (RIEHL & MALKUS). It is not possible, however, to calculate these fluxes, since meridional profiles of the horizontal mass and energy transport divergence cannot be inferred from the "Meteor" data. The main results are: (1) the static energy surface (a) is always parallel to, but systematically several 100 meters above, the surfaces (b ). (2) The sea surface pressure is practically independent of the trade layer thickness. (3) The components of the horizontal transport of momentum and energy are practically uncorrelated in the vertical; this demonstrates the Hadley-like character of the trade layer. (5) Water vapour plays with 〉 50% contribution the dominant role in the horizontal energy transport. ( 6) In the diabatic forcing function the flux of latent heat across the sea surface contributes the biggest part.These results show that vertically integrated two-layer models represent the gross features of the tropical circulation. Such formulations are thus a good approach towards simple and straightforward models for numerical experiments.