Occurrence and structure of the equatorial trough zone in Venezuela

Aircraft traverses were made from north to south across Venezuela during the rainy season of 1972 to determine whether a convectively active equatorial low pressure trough occurred and, if so, its structure, average properties and mechanics. The trough could indeed be located regularly near latitudes 5−7°N flying along the Orinoco River. Near cloud base (920mb) there was a wind shift from east to west and from north to south components. Farther south, winds from southeast were encountered. The low troposphere was warm with a maximum equivalent potential temperature at the axis. At 700mb neither the warm layer nor any wind shift could be found. The structure closely resembled that of the equatorial trough observed in the Garp Atlantic Tropical Experiment of 1974. The traverses largely indicated negative correlation between thermodynamic energy, mainly controlled by water vapour, and buoyancy, given by virtual temperature, on mesoscale distances of 30–50km. Occasional ‘hot tower’ penetrations were encountered. Surface air reached cloud base undiluted. Absolute vorticity was highest at cloud base just north of and at the trough, decreasing sharply to near zero to its south, just where heaviest convection was concentrated. Here also the largest vertical shear of the zonal wind component was located, again in agreement with GATE. The pressure‐height field was calculated from the wind data and indeed yielded a minimum value at the trough axis. To the north, the zonal wind increased with height though temperature decreased northwards. Calculations on the balance of forces with frictional retardation and on the origin of the low‐level warm layer give indications of the mechanics of the structure; the latter appears to be suggestive of a widely applicable equatorial trough model.