Abstract
Convective initiation is a challenge for convection‐permitting models due to its sensitivity to sub‐km processes. We evaluate the representation of convective storms and their initiation over South Africa during four summer months in Met Office Unified Model simulations at a 1.5‐km horizontal grid length. Storm size distributions from the model compare well with radar observations, but rainfall in the model is predominantly produced by large storms (50 km in diameter or larger) in the evening, whereas radar observations show that most rainfall occurs throughout the afternoon, from storms 10–50 km in diameter. In all months, the modelled maximum number of storm initiations occurs at least 2 hr prior to the radar‐observed maximum. However, the diurnal cycle of rainfall between the model and observations compares well, suggesting that the numerous storm initiations in the simulations do not produce much rainfall. Modelled storms are generally less intense than those in radar observations, especially in early summer. In February, when tropical influences dominate, the simulated storms are of similar intensity to observed storms. Simulated storms tend to reach their peak intensity in the first 15 min after initiation, then gradually become less intense as they grow. In radar observations, storms reach their peak intensity 15 min into their life cycle, stay intense as they grow larger, then gradually weaken after they have reached their maximum diameter. Two November case studies of severe convection are analysed in detail. A higher resolution grid length initiates convection slightly earlier (300 m as opposed to 1.5 km) with the same scientific settings. Two 1.5 km simulations that apply more subgrid mixing have delayed convective initiation. Analysis of soundings indicates little difference in the convective indices, suggesting that differences in convection may be attributed to the choice of subgrid mixing parameters.
Highlights
Skilful prediction of the timing and location of convective storms reduces our risk from hazards such as lightning, hail and wind gusts, and forms one of the principal goals of numerical weather prediction
None of the convection-permitting models (CPMs) run by South African Weather Service (SAWS) operationally use data assimilation, so our evaluation will focus on the physical characteristics of convection and convective inhibition (CIN) and the distribution of timings and locations of convection initiation, rather than the predictive skill of forecasting individual events
The SAWS 1.5 km simulations used in this article are one-way nested in a GA6.1 simulation initiated from the 1800 UTC global analyses for a domain covering all of South Africa
Summary
Convective initiation is a challenge for convection-permitting models due to its sensitivity to sub-km processes. We evaluate the representation of convective storms and their initiation over South Africa during four summer months in Met Office Unified Model simulations at a 1.5-km horizontal grid length. The diurnal cycle of rainfall between the model and observations compares well, suggesting that the numerous storm initiations in the simulations do not produce much rainfall. Simulated storms tend to reach their peak intensity in the first 15 min after initiation, gradually become less intense as they grow. Storms reach their peak intensity 15 min into their life cycle, stay intense as they grow larger, gradually weaken after they have reached their maximum diameter. Two 1.5 km simulations that apply more subgrid mixing have delayed convective initiation. Convection-permitting models, convective initiation, model evaluation, radar observations, storm life cycles
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