The character of fog at Cape Town International Airport (CTIA), South Africa, is investigated, using 13 yr of historical hourly data during the period 1997–2010. Hourly surface observations are used to identify fog types that most frequently affect CTIA, by using an objective fog-type classification method that classifies fog events according to their primary formation mechanisms. Fog-type characteristics, such as the minimum visibility, duration, and time of onset and dissipation, are determined. Self-organizing maps (SOMs) are used to determine the dominant synoptic circulation types associated with fog at CTIA. Results show that radiative processes are the most common cause of fog, with an enhanced likelihood of radiation fog in winter. Cloud-base-lowering fog and advection fog events are more likely at the start of the fog season. As the fog season (March–August) progresses, synoptic circulations associated with fog change from a dominant low pressure pattern along the west coast in March and April to a dominant interior high pressure pattern toward July and August. The techniques presented may be used to provide aviation forecasters with a detailed description of the types of fog that frequently occur, their characteristics, and the synoptic circulation associated therewith.
A climatology of synoptic drylines on the subtropical southern African interior plateau (SAP) is developed using ERA5 reanalysis specific humidity and surface temperature gradients and an objective detection algorithm. Drylines are found to occur regularly during spring and summer (September – March), almost daily during December, but rarely in winter. A westward shift in peak dryline frequency takes place through the summer. Drylines peak first over the eastern parts of the SAP during November with a mean of 10 drylines, but over the central (mean of 12) and western SAP (mean of 20) in December. During mid-summer, drylines over the eastern SAP are negatively correlated with drylines in the west. Between 1980 and 2020, a significant correlation exists between ENSO and dryline days over the eastern (r=0.44, p-value=0.004) and central (r=0.41, p-value=0.008) SAP with fewer drylines (up to 10) occurring during years with increased surface moisture and more drylines (up to 45), occurring during years with decreased surface moisture. Drylines forming over the eastern parts of the SAP were more likely to move westwards, than drylines over the central and western parts. Onset times across the SAP show that drylines have a tendency to form during either the late morning to early afternoon (11:00 and 14:00 LST) or during the early evening hours (17:00 and 20:00 LST), suggesting that the surface heat trough (Kalahari Heat Low) and westward moisture transport mechanisms such as the Limpopo low-level jet and ridging highs, are responsible for the formation of most drylines across the SAP.
<p>Surface moisture boundaries or drylines are considered important role players on severe thunderstorm days by South African weather forecasters. On 11 December 2017 one of several thunderstorms which occurred along a dryline, developed into a supercell thunderstorm that resulted in a destructive tornado over the Vaal Marina south of Johannesburg. While the importance of drylines for convection initiation in the region is relatively well understood, the climatological characteristics of drylines and dryline related convection were unknown until recently. Making use of an objective dryline identification algorithm and ERA5 reanalysis data, a climatology of synoptic drylines for the subtropical southern African interior is developed for 1979-2021. The Southern African Plateau (SAP) domain falls within 16-32&#176;S; 18-29&#176;E. Subsequently cloud-to-ground lightning stroke data from the South African Weather Service&#8217;s lightning detection network are used to identify days where drylines result in convection between 2010-2021 for a smaller domain within 24-30&#186;S; 18-24.5&#186;E. This region is characterized by its high rainfall variability and has some of the highest dryline frequencies in southern Africa. Drylines are found to occur regularly during spring and summer (September - February), almost daily during December, but rarely in winter. Dryline frequencies show large interannual variability with as few as 47 drylines in the summer of 2015/16 of which 38 were convective dryline days, and as many as 73 drylines in the summer of 2016/17 of which 61 were convective. A westward shift in peak dryline frequency takes place through the summer. Drylines peak first over the eastern parts of the SAP during November, but over the central and western SAP during December. Synoptic composite maps are used to gain insight into the different formation environments of drylines that result in convection and those that do not.</p>
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