Observations of the atmosphere across the Congo Air Boundary

Abstract

The Congo Air Boundary (CAB) is a synoptic-scale dryline and convergence zone that is a defining feature of the southern African tropical-edge climate from August to November, playing a central role in both the timing of rainfall onset and projections of regional drying. Here we present the first detailed observations of the CAB using surface and upper-air data from the Decreasing Rainfall to Year 2100 - Role of the Congo Air Boundary (DRY-CAB) project. Based on observations between 1 October and 13 November 2022, the CAB is shown to feature a specific humidity gradient of up to 12.0 g kg −1 per 100km. On the dry side of the CAB, the afternoon boundary layer is near dry-adiabatic (9.6 °C km −1 ) and capped by a mid-level inversion, while on the moist side the boundary layer is more stably stratified (9.1 °C km −1 ) and mid-level inversions weaker. Low-level convergence at the CAB peaks at 05:00 local time and is accompanied by mid-level divergence, as a dry (7 g kg −1 ), 15.0 m s −1 easterly low-level jet is lifted above a shallow (~600m deep) layer of moist (15 g kg −1 ) north-westerlies. Observations evidence shallow overturning to the north of the CAB, comprising north-westerly low-level flow, ascent at the CAB, and a mid-level return flow, and suggest the CAB is not typically associated with deep convection. Results are synthesised in a conceptual model which illuminates the structure and maintenance of a feature not previously resolved in observations and establishes a baseline for model evaluation.