First detailed observations of the Congo Air Boundary at the southern African tropical edge.

The Congo Air Boundary (CAB) is a synoptic-scale dryline and convergence zone that demarcates the southern edge of the tropical rainbelt during austral spring. The feature plays a key role in regulating spring rainfall across subtropical southern Africa, with early (late) CAB breakdown associated with early (late) onset of the summer rains. The CAB is also central to projections of future regional rainfall change, with successive CMIP model generations indicating early summer drying as a result of increased CAB persistence. However, owing to observational scarcity across subtropical southern Africa, the structure and circulation associated with the CAB is not well understood beyond a set of hypotheses stemming from model and reanalysis datasets.

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 100 km. On the dry side of the CAB, the afternoon boundary layer exhibits characteristics of a heat low, with near dry-adiabatic lapse rates (9.6 ⁰C km^-1) capped by a mid-level inversion at 550 hPa. On the moist side of the CAB, the boundary layer is more stably stratified (9.1 ⁰C km^-1) and mid-level inversions are 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 layer of moist (15 g kg^-1) north-westerlies.

Our field 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, establishes a baseline for model evaluation, and highlights parallels with subtropical drylines in other regions.