The diurnal cycle of rainfall in South Africa in the austral summer

Abstract

ABSTRACT:The diurnal cycle of precipitation over South Africa during summer is analysed for the first time using hourly precipitation data from 103 stations for the period 1998-2007. Using harmonic analysis, we found the presence of a distinct diurnal cycle over most of South Africa regarding both the frequency and amount of precipitation events. The standardized amplitudes, indicative of the strength of the diurnal cycle across a region, are strongest over the interior and along the east coast of South Africa with up to 70% explained variance associated with the diurnal cycle. The time of maximum precipitation is late afternoon to early evening in the interior, and midnight to early morning along the Agulhas Current as well as inland in the northeast of the county. The proximity of the warm Agulhas Current plays a role in the diurnal cycle of rainfall at coastal stations. There is an early morning maximum in precipitation in the South West of the country with small amplitude in the diurnal cycle there. On average, the peak in precipitation amount leads the peak in frequencies of precipitation by 30 min to 1 h. Together with a high resolution climatological summer rain rate presented here and a detailed table for all stations, this study is a benchmark upon which model output or satellite estimate of the diurnal cycle can be compared within the region.

Atmospheric Signature of the Agulhas Current

“…The positive anomalies of vertical motion ( Figure S6) along the inland coast in summer (DJF) are collocated with the increased in rainfall when CTL is compared to SMTH. The diurnal cycle is an important driver for terrestrial rainfall in summer there (Pohl et al, 2014;Rouault et al, 2013), and low-level convergence associated with the orography could also be a factor in enhancing summer rainfall. A complete understanding of how the Agulhas Current drives terrestrial precipitation in this region will therefore involve in depth analysis of both the diurnal cycle and seasonality of various parameters.…”

“…Radiosondes and heat flux measurements above the Agulhas Current show that the vigorous exchange of moisture and energy above the current penetrates into the troposphere to at least 1,500 m (Lee- Thorp et al, 1999;Rouault et al, 2000). In particular, rain rate and the diurnal cycle of rainfall along the eastern coast of South Africa are related to the proximity of the Agulhas Current (Jury et al, 1993;Rouault et al, 2013). Moreover, moisture produced by the Agulhas Current can be advected inland (Jury et al, 1997;Lee-Thorp et al, 1999;Rouault et al, 2000).…”

A climatology of potential severe convective environments across South Africa

“…Only one of the four daily time steps available in the reanalysis will be used; namely, 12h00 UTC (14h00 LST) since most local convective activity in South Africa occurs in the afternoon (Tyson and Preston-Whyte 2000). The timing of regional rainfall patterns is linked to the diurnal cycle in convective activity, where summer rainfall in South Africa generally falls during the afternoon in the interior of the country and during the evening along the east coast region (Rouault et al 2013). Long-lived convective systems in South Africa, one of the largest heavy rainfall producers, tend to initiate during the early afternoon (Blamey and Reason 2012).…”

ENSO–Kalahari Desert linkages on southern Africa summer surface air temperature variability

“…This means that maximum precipitation tends to occur in the late afternoon/early evening after the optimum development of convective clouds. Several studies have emphasized the presence of strong diurnal cycles related to localized convective processes in low latitude tropical areas (Machado and Rossow, ; Rouault et al , ). In summer, the regional SAT attains maximum values (SAT max ) through the increase in shortwave radiation and minimum (SAT min ) levels late at night through heat loss from long wave radiation.…”