The dynamics of cut-off lows and their vertical extension to the surface

Abstract

English: Cut-off low (COL) pressure systems are critical weather systems which can result in resulting various severe weather events on South Africa. COLs are often associated with surface cyclones which can enhance weather-related impacts on affected communities. This is shown by a case study of Cape Storm, a COL extension associated with explosive cyclogenesis that had dire effects on the Western Cape of South Africa. The stratospheric effects that determine a COLs extension to the surface and eventual impact on South Africa are studied from a potential vorticity (PV) perspective in terms of a case study, a climatology and an idealised framework. Deep COLs (which extend to the surface) are most frequent in the autumn months, are longer lasting, are more mobile and found most frequently in the higher latitudes. Stratospheric, high-PV intrusions that are associated with COLs are studied in detail with respect to a COLs extension. PV intrusions which extend closer to the surface more frequently result in surface cyclogenesis. This is independent on the size (or intensity) of the intrusion as PV intrusions which extend from higher tropopause heights tend to result in shallow COL development. COLs, have in the past, been viewed as upper-level systems that systematically extend to the surface. Composite analysis however shows that deep COLs associated with deep PV intrusions result in the simultaneous amplification of troughs throughout the troposphere, with the surface cyclone developing within a day of the COL.

Sepedi: Mananeotshepedišo a go hlola phetogo ya boso (COL) ke mananeotshepedišo a bohlokwa a kgatelelo ya boso ao a ka hlolago ditiragalo tše šoro tša boso ka Afrika Borwa. Di-COL gantši di tswalanywa le madimo a godimo ao a ka godišago ditlamorago tše di amanago le boso metseng yeo e amegilego. Se se bontšhwa ka nyakišišo ya mohlala ya Cape Storm, katološo ya COL yeo e amanago le cyclogenesis ya go thuthupa yeo e bilego le ditlamorago tše šoro ka Kapa Bodikela bja Afrika Borwa. Ditlamorago tša stratospheric tšeo di laolago katološo ya COLs go ya godimo le khuetšo ya yona mafelelong go Afrika Borwa di ithutwa go tšwa go pono ya kgonagalo ya vorticity (PV) go ya ka thuto ya mohlala, thuto ya tlelaemete le tlhako ye e kgonthišitšwego. Di-COL tše di tseneletšego (tšeo di katologelago godimo) di direga gantši kudu dikgweding tša seruthwane, di swarelela nako ye telele, di sepelasepela kudu gomme di hwetšwa gantši kudu go di-latitude tša godimo. Ditsenogare tša stratospheric, tša PV ya godimo tšeo di amanago le di-COL go ithutwa ka tšona ka botlalo mabapi le katološo ya di-COL. Ditsenogare tša PV tšeo di atologelago kgauswi le bokagodimo gantši di feletša ka cyclogenesis ya bokagodimo. Se se ikemetše ka bogolo (goba bogale) bja ditsenogare ka ge PV tšeo di atologelago go tšwa bophagamong bja godimo bja "tropopause" di na le go feletša ka tlhabollo ya COL ye e sa tsepamago. Di-COL, nakong e fetilego, di be di lebelelwa e le ditshepedišo tša maemo a godimo tšeo ka mo go rulagantšwego di atologelago godimo. Le ge go le bjalo,Tshekatsheko ye e tswakaganego e bontšha gore di-COL tše di tseneletšego tšeo di amanago le go tsenelela ga PV mo go tseneletšego di feletša ka go godišwa ga diforo ka nako e tee go ralala le troposphere, ka ledimo la godimo le bopega letšatši le tee gare ga COL.

Afrikaans: Afsnylaagdrukstelsels (COL) is kritieke weerstelsels wat verskeie strawwe weersomstandighede in Suid-Afrika tot gevolg kan hê. COL's word dikwels geassosieer met oppervlaksiklone wat weerverwante impakte op geaffekteerde gemeenskappe kan help bestuur. Dit word getoon deur 'n gevallestudie van Cape Storm, 'n COL-uitbreiding wat verband hou met plofbare siklogenese wat 'n ernstige uitwerking op die Wes-Kaap van Suid-Afrika gehad het. Die stratosferiese effekte wat 'n COL-uitbreiding na die oppervlak en uiteindelike impak op Suid-Afrika bepaal, word bestudeer vanuit 'n potensiële vortisiteit (PV) perspektief in terme van 'n gevallestudie, 'n klimatologie en 'n geïdealiseerde raamwerk. Diep COL's (wat na die oppervlak strek) kom die meestal voor in die herfsmaande, hou langer, is meer beweeglik en kom veral voor in die hoër breedtegrade. Stratosferiese, hoë-PV-indringings wat met COL's geassosieer word, word in detail bestudeer met betrekking tot 'n COL-uitbreiding. PV-indringings wat nader aan die oppervlak strek, lei meer gereeld tot oppervlaksiklogenese. Dit is onafhanklik van die grootte (of intensiteit) van die indringing aangesien PV-indringings wat vanaf hoër tropopousehoogtes strek, geneig is om vlak COL-ontwikkeling tot gevolg te hê. COL's is in die verlede beskou as hoërvlakstelsels wat stelselmatig na die oppervlak strek. Saamgestelde ontleding toon egter dat diep COL's geassosieer met diep PV-indringings lei tot die gelyktydige versterking van trôe regdeur die troposfeer, met die oppervlaksikloon wat binne 'n dag na die COL ontwikkel.

IsiZulu: Izinhlelo zengcindezi enamandla (ama-COL) yizinhlelo zesimo sezulu ezibalulekile ezingaholela ekutheni kubekhona izehlo ezehlukahlukene zesimo sezulu esingesihle eNingizimu Afrika. Ama-COL imvamisa zihlotshaniswa nezishingishane ezingaphezu komhlaba ezingandisa imithelela ehlobene nesimo sezulu emiphakathini ethintekile. Lokhu kukhonjiswa ucwaningo lwesehlo sesiphepho saseKapa, lapho ukunwebeka kwe-COL okuhlobene nesishingishane esibhamukayo kwaba nemithelela esabekayo eNtshonalanga Kapa yaseNingizimu Afrika. Imithelela kwistrathosfiye enquma ukunwebeka kwe-COL ngaphezu komhlaba futhi ekugcineni umthelela eNingizimu Afrika uyafundwa ngokombono wesilinganiso sokuzungeza komoya (i-PV) maqondana nocwaningo lwesehlo, izifundo zesimo sezulu kanye nohlaka olucatshangelwayo. Ama-COL angenele ezweni (ezinwebekela ngaphezu komhlaba) enzeka kaningi ngezinyanga zasekwindla, futhi athatha isikhathi eside, ahamba kakhudlwana futhi enzeka kaningi ezindaweni eziphakeme. Imingenela yomoya ekustrathosfiye futhi ephakeme ehlobene nama-COL ifundwa ngokuphelele maqondana nokunwebeka kwezinhlelo zengcindezi enamandla. Imingenelo ye-PV ekhulela eduze komhlaba kaninginingi kaningi iholela ekwakhekeni kwezishingishane ngaphezu komhlaba. Lokhu akuyi ngobukhulu (noma ngamandla) emingenelo njengoba imingenelo yama-PV ikhula isuka kuthrophophozi ivamise ukuholela ekukhuleni kwama-COL okungatheni. Ama-COL, esikhathini esedlule, ebeke abukwe njengezinhlelo zezinga eliphezulu elikhulela ngokuhlelekile ngaphezu komhlaba. Uhlaziyo oluyinhlanganisela nokho lukhombisa ukuthi ama-COL ahlobene nemingenela ejulile yama-PV aholela ekukhuleni kanyekanye kwezindawo ezinengcidezi ephansi kuthrophosfiye yonkana, nesishingishane esingaphezu komhlaba esikhula ngosuku olulodwa lwe-COL.