Downscaling of global circulation model predictions to daily rainfall over the upper Olifants River catchment
| dc.contributor.advisor | Walker, Sue | |
| dc.contributor.advisor | Engelbrecht, Francois A. | |
| dc.contributor.author | Steyn, Abraham Stephanus | |
| dc.date.accessioned | 2018-01-11T10:30:44Z | |
| dc.date.available | 2018-01-11T10:30:44Z | |
| dc.date.issued | 2008 | |
| dc.description.abstract | English: Climate change could have far reaching consequences for all spheres of life. Continued greenhouse gas (GHG) emissions at or above current rates will cause further warming and induce further changes in the global climate system. This is particularly true for southern Africa where an ever-increasing population is already causing an increase in the demand for fresh water and much of the agricultural food production depends on rain. Global Circulation Models (GCMs) are the main source of climate projections under varying GHG emission scenarios. The spatial resolution of GCMs is too coarse to resolve sub-grid processes such as convection and precipitation. However, agrohydrological application models often require information at a network of point locations, implying the need to downscale the GCM output. Downscaling approaches have subsequently emerged as a means of employing large-scale atmospheric predictor variables (such as the 500 hPa meridional velocity) to develop station-scale meteorological series. Variables such as daily rainfall, which are not always accurately represented by the GCMs, can be derived using statistical approaches to build relationships between the required forecast parameter and variables that are simulated more accurately. Previous investigators have used the statistical downscaling model (SDSM) to downscale climate projections of daily rainfall over North America and Europe. A similar methodology was adopted to downscale daily rainfall projections under the A2 and B2 emission scenarios at five selected quaternary catchments (QCs) within the Upper Olifants River catchment. The downscaling was performed for the summer months of December, January and February (DJF). The set of generic predictors which were identified across all five QCs included surface airflow strength, vorticity, divergence and specific humidity, 850 hPa wind direction and relative humidity as well as 500 hPa relative humidity and meridional wind velocity. Generally, all the predictors exhibited a reasonably low explanatory power. The considerable variation in the resultant correlations between the large-scale predictors and the observed daily precipitation at the selected QCs may very well have stemmed from the convective nature of the rainfall patterns, being irregularly distributed in space and time. Generally, the downscaling model results were not very encouraging as the model failed to produce satisfactory results for four of the five QCs. For one of the QCs, namely Groblersdal, the projected changes for the future climate were assessed by calculating several delta-statistics. Only a few of the indices revealed a clear change, while most indices exhibited inconsistent changes for DJF across three future periods centred on the 2020s, 2050s and 2080s. Similar changes in the characteristics of the daily rainfall series are projected for the early and mid 21st century under the A2 and B2 scenarios. Differences in the expected GHG forcing under the B2 scenario does not seem to affect any of the rainfall indices differently from the A2 scenario until the late 21st century. It should however be noted that the projected changes are often smaller than the model errors which implies that the downscaling model is simply not sensitive enough for the projected changes to be taken at face value. Therefore the results should only be used with caution. The fact that the downscaling procedure provides similar results for the A2 and B2 scenarios suggests that it is at least to some extent robust and stable. | en_ZA |
| dc.description.abstract | Afrikaans: Klimaatverandering kan verreikende gevolge inhou vir alle vlakke van die samelewing. Volgehoue kweekhuisgas (KHG) vrylatings teen vlakke wat die huidige tempo ewenaar of oorskry, sal verdere verwarming teweeg bring en verdere veranderinge in die globale klimaatstelsel veroorsaak. Dit is veral waar vir suider-Afrika waar ʼn steeds groeiende bevolking reeds ʼn toename in die vraag na vars water veroorsaak en ʼn groot gedeelte van die landboukundige voedselproduksie van reënval afhanklik is. Globale Sirkulasiemodelle (GSMs) is die hoofbron van klimaatvooruitskouings onder veranderende KHG vrystellingscenario’s. Die ruimtelike resolusie van GSMs is te grof om prosesse soos konveksie en reënval wat kleiner as die roosterveld is te hanteer. Landbou-hidrologiese toepassingsmodelle vereis dikwels inligting by ʼn netwerk punte wat dan die behoefte om die GSM uitvoer af te skaal beklemtoon. Afskalingsbenaderings het gevolglik ontluik as ʼn middel om groot-skaalse atmosferiese voorspellersvelde (soos die 500 hPa meridionale windspoed) in te span om stasievlak weerkundige reekse te ontwikkel. Veranderlikes soos die daaglikse reënval, wat nie altyd akkuraat deur GSMs voorgestel word nie, kan afgelei word deur middel van statistiese metodes wat verwantskappe vaslê tussen die vereiste parameter en veranderlikes wat meer akkuraat gesimuleer word. Vorige navorsers het die statistiese afskalingsmodel (SDSM) ingespan om klimaatprojeksies van daaglikse reënval oor Noord-Amerika en Europa af te skaal. ʼn Soortgelyke metodologie is aangeneem om daaglikse reënvalprojeksies onder die A2 en B2 vrystellingscenario’s by vyf gekose sub-opvanggebiede binne die Bo-Olifantsrivier af te skaal. Die afskaling is uitgevoer vir die somermaande Desember, Januarie en Februarie (DJF). Die stel generiese voorspellers, wat oor al vyf sub-opvanggebiede geïdentifiseer is, sluit oppervlak windsterkte, vortisiteit, divergensie en spesifieke humiditeit, 850 hPa windrigting en relatiewe humiditeit asook 500 hPa relatiewe humiditeit en meridionale windspoed in. Oor die algemeen het al die voorspellers relatief lae verklarende vermoëns getoon. Die aansienlike variasie in die gevolglike korrelasies tussen die groot-skaalse voorspellers en die waargenome daaglikse reënval by die gekose sub-opvanggebiede mag teweeg gebring word deur die konvektiewe aard van die reënvalpatrone wat onreëlmatig in tyd en ruimte versprei is. In die algemeen was die afskalingsmodel se resultate nie baie bemoedigend nie aangesien dit gefaal het om aanvaarbare resultate vir vier uit die vyf sub-opvanggebiede te verskaf. Vir een van die sub-opvanggebiede, naamlik Groblersdal, is die vooruitgeprojekteerde veranderinge vir die toekomstige klimaat geevalueer aan die hand van ʼn aantal delta-statistieke. Slegs ʼn paar van die indekse het ʼn duidelike verandering getoon, terwyl meeste indekse vir DJF onkonsistente veranderings oor drie toekomstige periodes, wat op die 2020s, 2050s en 2080s fokus, getoon het. Soortgelyke veranderinge in die eienskappe van die daaglikse reënvalreeks word onder die A2 en B2 scenario’s voorspel vir die vroeë- en mid-21ste eeu. Verskille in die verwagte KHG forserings tussen die A2 en B2 scenario’s blyk nie ʼn invloed op enige van die reënvalindekse te hê tot die laat 21ste eeu nie. Daar moet gelet word dat die geprojekteerde veranderinge dikwels kleiner is as die modelfoute wat dan impliseer dat die afskalingsmodel eenvoudig nie sensitief genoeg is om die geprojekteerde veranderinge blindelings te aanvaar nie. Die resultate moet gevolglik versigtig gebruik word. Die feit dat die afskalingsprosedure soortgelyke resultate vir die A2 en B2 scenario’s lewer toon dat dit ten minste rigied en stabiel is. | af |
| dc.description.sponsorship | Water Research Commission (WRC) | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/11660/7576 | |
| dc.language.iso | en | en_ZA |
| dc.publisher | University of the Free State | en_ZA |
| dc.rights.holder | University of the Free State | en_ZA |
| dc.subject | Precipitation forecasting | en_ZA |
| dc.subject | Rain and rainfall -- Mathematical models | en_ZA |
| dc.subject | Climatic changes -- South Africa -- Mathematical models | en_ZA |
| dc.subject | Rainfall probabilities -- South Africa | en_ZA |
| dc.subject | Dissertation (M.Sc.Agric. (Soil, Crop and Climate Sciences))--University of the Free State, 2008 | en_ZA |
| dc.title | Downscaling of global circulation model predictions to daily rainfall over the upper Olifants River catchment | en_ZA |
| dc.type | Dissertation | en_ZA |