Abstract:
The daily weather at a particular place is largely influenced by the synoptic circulation and
thermodynamic profile of the atmosphere. Heavy rainfall occurs from a particular subset of synoptic and
thermodynamic states. Baseline climatologies provide objective information on heavy rainfall-producing
circulation patterns and thermodynamic variables. This is how climatologically large or extreme values
associated with heavy rainfall are identified. The aim of this research is to provide a heavy rainfall
sounding climatology in austral summer over Gauteng, South Africa, using self-organising maps (SOMs).
The results show that the SOM captures the intra-seasonal variability of heavy rainfall soundings by
clearly distinguishing between the atmospheric conditions on early summer (October to December) and
late summer (January to March) heavy rainfall days. Conditions associated with heavy early summer
rainfall are large vertical wind shear and conditional instability, while the atmosphere is drier and cooler
than when heavy rainfall occurs in late summer. Late summer heavy rainfall conditions are higher
convective instability and small vertical wind shear values. The SOM climatology shows that some heavy
rainfall days occur in both early and late summer when large-scale synoptic weather systems cause strong
near-surface moisture flux and large values of wind shear. On these days, both the conditional and
convective instability of the atmosphere are low and heavy rainfall results from the strong synoptic
forcing. In contrast, heavy rainfall also occurs on days when synoptic circulation is not very favourable
and the air is relatively dry, but the atmosphere is unstable with warm surface conditions and heavy rainfall develops from local favourable conditions. The SOM climatology provides guidelines to critical
values of sounding-derived parameters for all these scenarios.
