A study of the structure and dynamics of the Saharan air layer over the northern equatorial Atlantic during BOMEX

The present-day conception of the trade-wind regime has emerged from the research of many investigators over the last few decades. A generally accepted description of the vertical structure of the lower atmosphere in the trade-wind zone is provided by Malkus (1958). A moist convective layer, the so-called "trade—wind moist layer, typically, extends from the sea surface to a height of about 6000 ft where it is topped by much drier air. The characteristics of this layer develop as a consequence of both dry and moist convective processes. Dry convection predominates in approximately the lowest 2500 ft (called the homogeneous mixing layer), while moist adiabatic ascent and dry adiabatic descent occur above cloud base. The upper limit of this convection (i.e., the top of the moist layer) is marked by an inversion; the inversion serves as a zone of transition between the lower layer, which is in convective contact with the sea, and the air above it, which is essentially isolated from the surface. This so-called "trade-wind inversion" is typically several hundred feet in depth; across this region, the potential temperature and humidity undergo considerable changes.