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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Land Cover Change and Climate on the North American Great Plains

Gerstein, Shira January 2014 (has links)
Changing land cover from prairie grasslands to intensive, primarily cereal agriculture, over the North American Great Plains since the mid-19th century, has had a hydrological and climatological impact on that ecosystem (Pielke, Sr., et al., 2011). Agriculture has introduced timed harvest seasons, irrigation, and C3 photosynthesizing crops with poorer water efficiency than the grasses it replaced. All of these changes have been linked to exacerbated drought conditions and warmer temperatures; however, few studies have quantified this relationship at the continental scale. In order to evaluate the change imposed by this shift in land use and land cover, the observation based 20th Century Reanalysis Project (20CR) was used to quantify the climatological differences in temperature and humidity between areas of natural prairie and agriculture over the 20th century. An additional analysis used the Observation Minus Reanalysis (OMR) technique to isolate the surface climate signal found in the 20CR. We find indications that changing land cover had an impact on climate. However, using observation based data returned no evidence of a statistically significant change due to the small land use and land cover change (LULCC) signal within the larger climate noise. Therefore, an idealised modelling experiment was undertaken using the Geophysical Fluid Dynamics Laboratory (GFDL) AM2-LM2 atmosphere-land model to remove these other influences. This experiment compared the results of two model simulations: one where the entirety of the prairie was preserved as grassland (GRASS), and another where the entire prairies had been converted into an agricultural area (AGRIC). Relative to GRASS, the AGRIC simulation has reduced surface albedo and root zone depth, and increased roughness length over the prairies, which collectively cause a significant summer drying. This occurs when the shallower rooting zone limited potential evapotranspiration (PET) forcing the additional energy created by turbulent mixing and a lower surface albedo to warm the air, surpassing PET and reaching drier conditions faster. While not conclusive, the results presented in this thesis represent a step towards filling the gaps in understanding land-atmosphere interactions and connecting LULCC to climate.
42

Wind Turbine Production losses in Cold Climate : case study of ten wind farms in Sweden

Malmsten, Jon January 2011 (has links)
As wind power expands rapidly worldwide, it is becoming more common to build wind farms in alpine locations where the wind resources often are good and conflicting interests are few. This is evident in Sweden where a substantial portion of the large wind parks planned are to be built in cold climate locations. The fact that icing of turbine blades and sensors can severely impact the production raises the question how large the losses are. In this thesis 10 wind parks comprising 45 turbines, well dispersed throughout Sweden are investigated. Daily production figures are compared to wind data from the MERRA reanalysis data-set in order to see if it is possible to determine the level of losses during the winter period caused by cold climate. A method is suggested where a relationship between daily production and daily average wind speed is established using representative summer days. This relationship is then used to calculate an expected production for the winter period. Losses are concluded as the difference between expected and actual production. The method did not produce a consistent and reliable result for the sites investigated. However, the method captures the overall trend with higher losses in the north of Sweden compared to the sites in the south where little or no icing is likely. At the sites where icing is expected, losses in the range of 10 to 20% of the annual production were calculated.
43

Análise da variabilidade espaço-temporal da água precipitável na atmosfera utilizando dados do ERA– INTERIM/ECMWF.

CABRAL, Danilo Ericksen Costa. 27 August 2018 (has links)
Submitted by Lucienne Costa (lucienneferreira@ufcg.edu.br) on 2018-08-27T15:03:16Z No. of bitstreams: 1 DANILO ERICKSEN COSTA CABRAL – DISSERTAÇÃO (PPGMET) 2018.pdf: 3103151 bytes, checksum: 21c39f5e775ca2cffae4904318258171 (MD5) / Made available in DSpace on 2018-08-27T15:03:16Z (GMT). No. of bitstreams: 1 DANILO ERICKSEN COSTA CABRAL – DISSERTAÇÃO (PPGMET) 2018.pdf: 3103151 bytes, checksum: 21c39f5e775ca2cffae4904318258171 (MD5) Previous issue date: 2017-12-22 / Analisou-se neste estudo a tendência anual e a média global da Água Precipitável (AP) no período de 1986 a 2016. Foram utilizados dados extraídos das reanálises do ERAInterim/ European Center for Medium-Range Weather Forecasts (ECMWF). A partir destes dados avaliou-se a distribuição espacial da tendência de Água Precipitável. Para a análise da tendência climática aplicou-se testes estatísticos de Mann-Kendall e t de Student observandose o comportamento das séries globais. Para a variável AP, foi encontrado o valor médio de 24,4mm para todo o período, correspondendo a um volume estimado para toda a atmosfera. Os resultados apontam tendências significativas de 0,0246; 0,0121 e 0,0203 mm/ano para o HN; HS e todo o Globo, respectivamente. Foi observada uma tendência positiva para algumas áreas do globo bem como outras negativas, resultando em uma distribuição não homogênea, denotando-se uma provável tendência de aumento do vapor d’água no globo. / In this study was analyzed the annual trend and the global mean of Precipitation Water (WP) from 1986 to 2016. Data extracted from the ERA-Interim / European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis were used. From these data was evaluated the Precipitation Water tendency spatial distribution. For the analysis of the climatic trend, Mann-Kendall and t-Student statistical tests were applied, observing the behavior of the global series. For the WP variable was found the average value of 24.4 mm for the whole period, corresponding to estimated volume for the whole atmosphere. The results indicate significant trends of 0.0246, 0.0121 and 0.0203 mm /year for HN; HS and the whole Globe, respectively. A positive trend was observed for some areas of globe as well as negative values, resulting the not homogeneous distribution, denoting a probable tendency to increase water vapor in the globe.
44

Coastal marine heatwaves: Understanding extreme forces

Schlegel, Robert William January 2017 (has links)
Philosophiae Doctor - PhD (Biodiversity and Conservation Biology) / Seawater temperature from regional to global scale is central to many measures of biodi- versity and continues to aid our understanding of the evolution and ecology of biolog- ical assemblages. Therefore, a clear understanding of the relationship between marine biodiversity and thermal structures is critical for effective conservation planning. In the an- thropocene, an epoch characterised by anthropogenic forcing on the climate system, future patterns in biodiversity and ecological functioning may be estimated from projected climate scenarios however; absent from many of these scenarios is the inclusion of extreme thermal events, known as marine heatwaves (MHWs). There is also a conspicuous absence in knowl- edge of the drivers for all but the most notorious of these events. Before the drivers of MHWs along the coast of South Africa could be determined, it was first necessary to validate the 129 in situ coastal seawater temperature time series that could be used to this end. In doing so it was found that time series created with older (longer), lower precision (0.5 Degrees Celsius) instruments were more useful than newer (shorter) time series produced with high precision (0.001 Degrees Celsius) instruments. With the in situ data validated, a history of the occurrence of MHWs along the coastline (nearshore) was created and compared against MHWs detected by remotely sensed data (offshore). This comparison showed that the forcing of offshore temperatures onto the nearshore was much lower than anticipated, with the rates of co-occurrence for events between the datasets along the coast ranging from 0.2 to 0.5. To accommodate this lack of consistency between datasets, a much larger mesoscale area was then taken around southern Africa when attempting to determine potential mesoscale drivers of MHWs along the coast. Using a self organising-map (SOM), it was possible to organise the synoptic scale oceanographic and atmospheric states during coastal MHWs into discernible groupings. It was found that the most common synoptic oceanographic pattern during coastal MHWs was Agulhas Leakage, and the most common atmospheric pattern was anomalously warmoverland air temperatures.With these patterns known it is now necessary to calculate how often they occur when no MHW has been detected. This work may then allow for the development of predictive capabilities that could help mitigate the damage caused by MHWs.
45

Um método de reanálise adaptativa para otimização estrutural usando um algoritmo genético não-geracional

Loureiro, Michelli Marlane Silva 26 February 2016 (has links)
Submitted by Renata Lopes (renatasil82@gmail.com) on 2016-05-05T14:31:22Z No. of bitstreams: 1 michellimarlanesilvaloureiro.pdf: 1130544 bytes, checksum: 086b7a6420ee519ed479ec2df829bc04 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2016-06-07T15:38:53Z (GMT) No. of bitstreams: 1 michellimarlanesilvaloureiro.pdf: 1130544 bytes, checksum: 086b7a6420ee519ed479ec2df829bc04 (MD5) / Made available in DSpace on 2016-06-07T15:38:53Z (GMT). No. of bitstreams: 1 michellimarlanesilvaloureiro.pdf: 1130544 bytes, checksum: 086b7a6420ee519ed479ec2df829bc04 (MD5) Previous issue date: 2016-02-26 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Algoritmos Genéticos (AG’s) são ferramentas de grande poder computacional quando utilizadas em problemas de otimização em que a solução não pode ser obtida de forma trivial. Apesar de ser uma excelente ferramenta, o AG tem um alto custo computacional devido ao elevado número de avaliações necessárias para a obtenção de uma solução factível. Além disso, em otimização estrutural, a avaliação de uma solução candidata pode resultar na resolução de um sistema de equações. A solução deste sistema na maioria das vezes demanda alto custo computacional. Dessa forma, a fim de reduzir o custo computacional, uma técnica de reanálise por aproximação combinada é utilizada. Esta técnica reduz a dimensão do sistema para um número de vetores base previamente definido. Na formulação padrão, a escolha do número de vetores base não é feita de forma automática. Então, o método de reanálise será usado em um esquema adaptativo, proposto aqui, em que, o número de vetores base serão escolhidos e alterados ao longo do processo de otimização. Com o objetivo de obter o menor número possível de análises completas, a matriz de rigidez inicial também será alterada ao longo do processo de otimização. Para ilustrar a eficácia da formulação proposta, exemplos clássicos de otimização estrutural são analisados. Os exemplos numéricos mostram que o esquema adaptativo proposto fornece resultados com boa precisão e, em algumas situações, melhores, ou seja, menor peso com menor custo computacional, comparados com outras estratégias da literatura. / Genetic Algorithms (GA’s) are tools of great computational power when employed in optimization problems for which the solution cannot be obtained by a trivial way. Despite being an effective tool, GA has a high computational cost due to the high number of structural analysis needed to obtain a feasible solution. Furthermore, in structural optimization, the evaluation of a candidate solution may lead to the solution of a system of equations. The solution of such a system in some cases demands a great computational effort. In this way, in order to reduce the high computational cost, a reanalysis technique using the combined approximations method is employed. This technique reduces the dimension of the system to a number of basis vectors previously defined. In the standard formulation, the choice of the number of basis vectors is not carried out in an automatic way. Hence, the reanalysis technique will be employed in a proposed adaptive scheme for which the number of basis vectors will be chosen and modified along with the optimization process. With the objective of obtaining the smallest number of complete analyses, the initial stiffness matrix will also be modified along with the optimization process. In order to illustrate the effectiveness of the proposed formulation, classical examples of structural optimization are analyzed. The numerical examples reveal that the proposed adaptive scheme provides accurate and, in some cases, better results, i.e. lowest weight with lowest computational cost, when compared to other schemes in the literature.
46

Temporal Consistency of the UERRA Regional Reanalysis: Investigating the Forecast Skill / Tidsmässig konsistens i UERRA-återanalysen: Undersökning av prognoskvaliteten

von Kraemer, Adam January 2018 (has links)
Weather forecasting has improved greatly since the middle of the 20th century, thanks to better forecasting models, an evolved weather observing system, and improved ways of assimilating the observation data. However, these large systematical improvements make it difficult to use the weather data for climatological studies. Furthermore, observations are scarce, and they cannot be made everywhere. One way to solve this problem is to produce reanalyses, where a fixed version of a numerical weather prediction (NWP) model is used to produce gridded analysis and forecast data with detailed descriptions of the weather by assimilating observation data for a determined time period. One of the newest regional reanalyses is UERRA (Uncertainties in Ensembles of Regional Re-Analyses), which spans over the time period 1961-2015 and covers the whole Europe. By using a fixed NWP model, the only two factors that might influence the temporal quality of a regional reanalysis dataset are the varying number and quality of weather observations, and the quality of the global driving model which gives information about boundaries and large-scale features. In this report, data from one of the UERRA products has been used with the aim to investigate the temporal consistency of the 30-hour forecast skill regarding three parameters; temperature at 2 meters height (t2m), wind speed at 100 meters height (ws100) and 500 hPa geopotential (Φ500). The work has been focused on only land points over Europe during winters and summers, as this enables to investigate the model behaviour at the lowest and highest temperatures. The 30-hour forecast skill was estimated throughout the time period from how well it performed compared to the 6-hour forecast. Temporal inconsistencies were found throughout the reanalysis, with the largest temporal differences being present for Φ500, followed by ws100. UERRA shifts its global driving model in 1979 from ERA-40 (ECMWF Re-Analysis 40) to ERA-Interim (ECMWF Interim Re-Analysis), which ends up as a significant improvement of forecast skill for all investigated parameters. Furthermore, ws100 also shows a significant skill improvement in wintertime from 1979 onwards, while Φ500 shows a systematical improvement for both seasons. In general, the forecast skill is lower in wintertime than in summertime, which might be a result from higher natural variability of the weather in winters. A quick study of forecast data from ERA-Interim shows that the same improving trend in Φ500 can be seen also in that dataset, while the two model drifts differ completely. It was concluded that the addressed issues with temporal inconsistency should be communicated to end users utilizing the UERRA datasets, as knowledge about this can be greatly beneficial when studying climatological trends and patterns and when using the model to reforecast weather events. / Väderprognostisering har utvecklats betydligt sedan mitten på 1900-talet, tack vare bättre prognosmodeller, fler väderobservationer och förbättrade sätt att samla in och nyttja observationerna. Den snabba utvecklingen gör det dock svårt att på ett tillförlitligt sätt kunna jämföra väderdata från olika tidsperioder med varandra, då det är svårt att säkerställa kvaliteten på observationer från flera decennier tillbaka. Ett sätt att lösa det här problemet är att framställa så kallade återanalyser, vilka använder en enskild väderprognosmodell för att uppskatta vädret historiskt i varje punkt i ett förutbestämt rutnät, som sträcker sig över en enskild kontinent eller hela Jorden. En av de nyaste återanalyserna är UERRA, vilket är en regional återanalys över Europa som sträcker sig över tidsperioden 1961–2015. Då en och samma modell används för att beräkna vädret över hela perioden så påverkas inte kvaliteten på datat av den historiska utvecklingen av prognosmodeller. De enda två faktorerna som kan påverka datakvaliteten är den varierande tillgängligheten till väderobservationer, samt kvaliteten på den globala modellen vilken ger information om vädret utanför Europa. För att undersöka om det finns tidsmässiga skillnader i hur konsistent eller inkonsistent kvaliteten på UERRA-återanalysen är, har väderdatat från denna analyserats med avseende på temperatur, vindstyrka och lufttryckshöjd. Arbetet har fokuserats på enbart landpunkter över Europa för sommar och vinter, då detta möjliggör att kunna se hur bra modellen presterar vid de allra lägsta och högsta temperaturerna. Datat har utvärderats genom att undersöka hur tillförlitlig en prognos för 30 timmar framåt är jämfört med en prognos för 6 timmar framåt. Resultaten visar att kvaliteten på återanalysdatat i UERRA inte är konsistent genom hela tidsperioden, där de största skillnaderna hittades för lufttryckshöjden följt av vindstyrkan. För alla tre parametrar hittades betydande kvalitetsskillnader från vilken typ av global modell som används för att ge väderinformation utanför Europa, då UERRA byter global modell under år 1979. För lufttryckshöjden sågs även att datakvaliteten ökar konsekvent även efter 1979 och framåt, vilket därmed är ett resultat från den ökande mängden väderobservationer. Generellt sågs en högre prognoskvalitet sommartid än vintertid, vilket tros vara ett resultat från att vädret varierar mycket mer vintertid vilket därmed bör göra det mer svårprognostiserat. Dessa skillnader i datakvaliteten bör tydliggöras för alla användare av UERRA-återanalysen, då det är viktigt att ha kännedom om detta före eventuella slutsatser dras från återanalysdatat om hur vädret har varit historiskt sett.
47

Atmospheric Change in Antarctica since the 1957-1958 International Geophysical Year

Nicolas, Julien Pierre 09 September 2014 (has links)
No description available.
48

Probabilistic and Prominence-driven Incremental Argument Interpretation in Swedish

Hörberg, Thomas January 2016 (has links)
This dissertation investigates how grammatical functions in transitive sentences (i.e., `subject' and `direct object') are distributed in written Swedish discourse with respect to morphosyntactic as well as semantic and referential (i.e., prominence-based) information. It also investigates how assignment of grammatical functions during on-line comprehension of transitive sentences in Swedish is influenced by interactions between morphosyntactic and prominence-based information. In the dissertation, grammatical functions are assumed to express role-semantic (e.g., Actor and Undergoer) and discourse-pragmatic (e.g., Topic and Focus) functions of NP arguments. Grammatical functions correlate with prominence-based information that is associated with these functions (e.g., animacy and definiteness). Because of these correlations, both prominence-based and morphosyntactic information are assumed to serve as argument interpretation cues during on-line comprehension. These cues are utilized in a probabilistic fashion. The weightings, interplay and availability of them are reflected in their distribution in language use, as shown in corpus data. The dissertation investigates these assumptions by using various methods in a triangulating fashion. The first contribution of the dissertation is an ERP (event-related brain potentials) experiment that investigates the ERP response to grammatical function reanalysis, i.e., a revision of a tentative grammatical function assignment, during on-line comprehension of transitive sentences. Grammatical function reanalysis engenders a response that correlates with the (re-)assignment of thematic roles to the NP arguments. This suggests that the comprehension of grammatical functions involves assigning role-semantic functions to the NPs. The second contribution is a corpus study that investigates the distribution of prominence-based, verb-semantic and morphosyntactic features in transitive sentences in written discourse. The study finds that overt morphosyntactic information about grammatical functions is used more frequently when the grammatical functions cannot be determined on the basis of word order or animacy. This suggests that writers are inclined to accommodate the understanding of their recipients by more often providing formal markers of grammatical functions in potentially ambiguous sentences. The study also finds that prominence features and their interactions with verb-semantic features are systematically distributed across grammatical functions and therefore can predict these functions with a high degree of confidence. The third contribution consists of three computational models of incremental grammatical function assignment. These models are based upon the distribution of argument interpretation cues in written discourse. They predict processing difficulties during grammatical function assignment in terms of on-line change in the expectation of different grammatical function assignments over the presentation of sentence constituents. The most prominent model predictions are qualitatively consistent with reading times in a self-paced reading experiment of Swedish transitive sentences. These findings indicate that grammatical function assignment draws upon statistical regularities in the distribution of morphosyntactic and prominence-based information in language use. Processing difficulties in the comprehension of Swedish transitive sentences can therefore be predicted on the basis of corpus distributions.
49

Meteorological Investigation of Preconditions for Extreme-Scale Wind Turbines in Scandinavia

Hallgren, Christoffer January 2013 (has links)
During the last three decades, the hub height of wind turbines has increased from 24 to 162 meters and with an increasing demand for break-through innovations in green energy production it seems likely that this trend will continue. The meteorological preconditions for extreme-scale wind turbines are investigated for Scandinavia using 33 years of reanalysis data from MERRA (Modern-Era Retrospective Analysis for Research and Applications). Second degree polynomials are fitted to the wind and temperature profiles and evaluated at 100, 200 and 300 m above ground level (AGL). The spatial and temporal variation of average wind speed and median wind power density is studied. Simple metrics such as the wind shear and risk of icing, measured as occasions with temperature below freezing, are used to give an idea of the loads on the wind turbines. Winter is the windiest season, and generally the wind speed is highest over sea and in the Scandinavian mountain range. Going from 100 to 300 m AGL the average wind speed increases with 1 m/s over sea and 2 m/s over land. During night the wind speed increases over land but decreases over sea compared to daytime values. On average the wind shear is about 3.6 times larger in the 50-100 m layer than in the 100-300 m layer. The calculated wind field at 100 m AGL has been compared with results from the MIUU-model, developed at the Department of Meteorology, Uppsala University. The general features are captured but there are important discrepancies between the coast and the mountains in the northern part of Sweden. MERRA data has been validated in different ways, for example by comparing with measured wind speed and temperature profiles. The temperature profiles are in good agreement while the wind profiles differ significantly. It is also shown that MERRA data is not internally consistent in the mountain range, causing a large uncertainty. In future studies, the risk of icing could be explored further. Also, the distribution of sound from extreme-scale wind turbines could be investigated. / Under de senaste tre decennierna har navhöjden för vindkraftverk ökat från 24 till 162 meter och med en ökande efterfrågan på banbrytande innovationer inom produktion av grön energi är det troligt att denna trend kommer att fortsätta. De meteorologiska förutsättningarna för extremskaliga vindkraftverk i Skandinavien undersöks baserat på 33 års återanalysdata från MERRA (Modern-Era Retrospective Analysis for Research and Applications). Andragradspolynom anpassas till vind- och temperaturprofilerna och evalueras på höjderna 100, 200 och 300 meter över markytan. Variationen i rummet och med tiden av medelvindhastigheten och medianen av vindenergiintensiteten studeras. Enkla mått som vindskjuvningen och risken för nedisning, mätt som antalet tillfällen då temperaturen understiger fryspunkten, används för att ge en uppfattning om risken för belastningarna på vindkraftverken. Vintern är den årstid då det blåser mest och i allmänhet är vindstyrkan högst över hav och i fjälltrakterna. Förflyttar man sig från 100 till 300 m över markytan ökar medelvindhastigheten med 1 m/s över hav och med 2 m/s över land. Under natten ökar vinden över land men minskar över hav i jämförelse med värdena under dagen. I medeltal är vindskjuvningen 3.6 gånger större i 50-100 m skiktet jämfört med 100-300 m skiktet. Det beräknade vindfältet på 100 m över markytan har jämförts med resultat från MIUU-modellen, utvecklad vid institutionen för meteorologi, Uppsala universitet. De allmänna dragen är samma men det finns viktiga avvikelser mellan kusten och fjälltrakterna i norra Sverige. MERRA-data har validerats på olika sätt, till exempel genom att jämföra med uppmätta vind- och temperaturprofiler. Temperaturprofilerna visar god överensstämmelse men det är signifikanta skillnader mellan vindprofilerna. Det visas också att MERRA-data inte är konsistent i fjälltrakten, vilket medför en stor osäkerhet. I framtida studier kan risken för nedisning studeras utförligare liksom ljudutbredningen från extremskaliga vindkraftverk.
50

Regional Precipitation Study in Central America, Using the WRF Model

Maldonado, Tito January 2012 (has links)
Using the regional climate model WRF, and the NCEP-NCAR Reanalysis Project data asboundary and initial conditions, regional precipitation was estimated by means of thedynamical downscaling technique for two selected periods, January 2000 and September2007. These months show very particular climatic characteristics of the precipitationregimen in Central America, like dry (wet) conditions in the Pacific (Caribbean) coast of theCentral American isthmus, in January, and wet (dry) conditions, respectively in each coast,during September. Four-nested-domains, each grids of resolution of 90 km (d01), 30 km(d02), 10 km (d03), and 3.3 km (d04), were configured over this region. The runs werereinitialized each 5 days with 6 hours of spin-up time for adjustment of the model. A total of8 experiments (4 per month) were tested in order to study: a) two important CumulusParameterization Schemes (CPS), Kain-Fritsch (KF) and Grell-Devenyi (GD); and b) thephysical interaction between nested domains (one- and two-way nesting), during eachsimulated month.January 2000 results showed that the modeled precipitation is in agreement withobservations, and also captured the mean climate features of rainfall concerning magnitude,and spatial distribution, like the particular precipitation contrast between the Pacific and theCaribbean coast.Outputs from September 2007 revealed significant differences when a visual comparison ismade to the spatial distribution of each coarse domain (d01, d02, and d03) with theirrespective domain in each experiment. However, the inner grids (d04) in all theexperiments, showed a similar spatial distribution and magnitude estimation, mainly inthose runs using one-way nesting configuration. Furthermore, the results for this mothdiffer substantially with observations, and the latter could be related with associateddeficiencies in the boundary condition that do not reproduce well the transition periodsfrom warm to cold El Niño episodes.Moreover, in all the experiments, the KF scheme calculated more precipitation than the GDscheme and it is associated to the ability of the GD scheme to reproduce spotty but intenserainfall, and apparently, this scheme is reluctant to activate, frequently yielding little or norain. However, when rainfall does develop, it is very intense.Also, the time series do not replicate specific precipitation events, thus, the 5-daysintegration period used in this study, is not enough to reproduce short-period precipitationevents.Finally, physical interaction issues between the nested domains are reflected indiscontinuities in the precipitation field, which have been associated to mass fieldadjustment in the CPS. / Nederbörden i Central Amerika har uppskattats med dynamisk nedskalning för två utvaldaperioder, januari 2000 och september 2007. Global återanalysdata från NCEP-NCARsåteranalysprojekt har använts som randdata och initialdata till den regionalaklimatmodellen WRF. De studerade månaderna uppvisar stora variationer inederbördsmönster, t ex lite (mycket) nederbörd under januari och mycket (lite) nederbördunder september för kustområdena längs Stilla havet (Karibiska havet). Fyra nästladedomäner över Central Amerika har använts med en upplösning på 90 km (d01), 30 km (d02),10 km (d03) och 3,3 km (d04). Simuleringarna initialiserades var 5:e dag och de första 6timmarna efter varje initialisering används för modellens anpassning till initialtillståndet.Totalt 8 experiment genomfördes (4 för varje månad) för att studera: (a) två olika sätt attparameterisera konvektion i Cumulusmoln (CPS), Kain-Fritsch (KF) och Grell-Devenyi (GD)och (b) den fysikaliska interaktionen mellan de nästlade domänerna (en- respektive tvåvägsnästlade scheman).För januari 2000 var det god överensstämmelse mellan modellerad och observeradnederbörd. Modellen beskriver väl såväl mängden nederbörd som den rumsligafördelningen, t ex den stora kontrasten mellan kustområdena längs Stilla havet och Karibiskahavet.För september 2007 uppvisar den modellerade nederbörden stora skillnader i de olikaexperimenten för de yttre domänerna (d01, d02, d03). För den inre domänen (d04) ärresultaten från de olika experimenten betydligt mer lika, särskilt för experimenten medenvägs nästlade scheman. Vidare skiljer sig den modellerade nederbörden väsentligt frånobserverad nederbörd under september 2007. Detta kan förklaras med felaktiga randdatapå grund av problemet i återanalys data att reproducera perioder med övergång från varmtill kall El Niño. I alla experiment gav KF mer nederbörd än GD, det kan förklaras med att GDbättre reproducerar kortvarig, intensiv nederbörd. Det finns en viss tröghet innannederbörden i GD aktiveras, vilket innebär större frekvens av lite eller ingen nederbörd. Närnederbörden väl utvecklas blir den dock intensiv. WRF-modellen klarar inte av att återgespecifika nederbördshändelser för de genomförda experimenten, vilket betyder att 5-dagarär för lång simuleringstid för att kunna reproducera specifika händelser. Slutligen,interaktion mellan de nästlade domänerna skapar diskontinuiteter i nederbördsmöns.

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