Syftet med den här studien är att studera vindprofiler och klassificera Low Level Jets (LLJ) och även analysera den termiska vinden över Östergarnsholm. Östergarnsholm är en liten ö som ligger 4 kilometer öster om Gotland. Viseringar och sonderingar från åren 1995 till 2001 samt år 2003 har använts. Det kriteriet som har använts för att klassificera LLJs är att det ska finnas ett vindhastighetsmaximum under 500 meters höjd. De undersökta orsakerna till uppkomsten av en LLJ är sjöbris, tröghetssvängning och termisk vind. För att kunna ta ut LLJs orsakade av sjöbris har vindprofiler över hela dygn använts. Vindens vridning vid marken, under dagen, jämfört med den överlagrade vindens riktning vid 2000 meters höjd har undersökts. LLJs som uppkommit på grund av tröghetssvängning har analyserats. Metoden för tröghetssvängning bygger på att analysera den geostrofiska vindens hastighet och vindriktning för att sedan kunna räkna ut hur vinden har blåst och var en eventuell LLJ skulle kunna bildas. Här har vindriktningar mellan 20º – 220º använts eftersom det medför att vinden då blåser från havet. Resterande vindriktningar medför att vinden har blåst över Gotland vilket resulterar i att en eventuell tröghetssvängning skulle störas och en LLJ skulle försvinna innan den når Östergarnsholm. Vindhastigeter och vindriktningar har jämförts med teoretiskt uträknade värden från tryckmätningar. Från alla 245 viseringar fanns 103 vindprofiler med LLJs. Utav dessa var 27 stycken, under 12 dygn, orsakade av sjöbris. Hur många LLJs som bildats av tröghetssvängning är oklart. Detta eftersom olika resultat erhålles beroende på om vindhastigheter och vindriktningar tas från viseringarna eller är beräknade från tryckmätningar, samt om beräkningarna av transporttiden sker med raka eller krökta trajektorier. Totalt hittades 9 LLJs som orsakats av tröghetssvängning. Metoden som använts för att analysera tröghetssvängning ger förmodligen bättre resultat över land än över hav. Detta eftersom det är svårt att mäta de exakta vindförhållandena längs luftens transport över hav. Den geostrofiska vindens ändring med höjden över ön, det vill säga den termiska vinden, har analyserats genom att undersöka alla vindprofiler, även de utan LLJs. Dessa har jämförts med den geostrofiska vinden beräknad från tryckmätningar, som representerar vinden vid marken, för att se om det uppkommer termiska vindar över Östergarnsholm. En LLJ som orsakats av termisk vind uppkommer när den geostrofiska vinden avtar med höjden, det vill säga vid negativ termisk vind. Antalet fall där den geostrofiska vinden avtar med höjden och där vinden är konstant med höjden var ungefär lika många. Däremot fanns något fler fall där den geostrofiska vinden ökade med höjden, dessa uppgick även till högre hastigheter än när vinden avtog med höjden. Det finns inget samband för vindvridningen med höjden då det förekommer LLJs på grund av termisk vind. Däremot finns ett tydligt samband mellan den negativa termiska vindens u- och v-komponent och den geostrofiska vindens u- respektive v-komponent. I båda fallen så tenderar vinden att gå mot noll med höjden. Totalt hittades 41 LLJs som var orsakade av termisk vind. Vid ungefär 50% av alla vindprofiler, både när den geostrofiska vinden avtog och ökade med höjden, var den termiska vindens nord-sydliga komposant positiv och den ost-västliga komposanten negativ. Detta ger att den varmare luften finns i nordost. / The aim of this study is to classify Low Level Jets (LLJ) and analyze the thermal wind over Östergarnsholm. Östergarnsholm is a small island that is situated 4 kilometres east of Gotland in the Baltic Sea. Pibal trackings and soundings from 1995 to 2001 and 2003 have been used in the study. The criteria that have been used to classify the LLJs is that there must be a maximum of the wind speed below 500 meters. Wind profiles from a specific day have been used to determine if the LLJs is caused by sea breeze. The shift of wind direction at ground level, during the day, compared to the geostrophic wind at 2000 meters has been analyzed. LLJs caused by internal oscillation have been analyzed. In the used method the geostrophic wind speed and wind direction have been analyzed to determine how the wind has blown and where a LLJ possibly could be formed. Only wind directions between 20º and 220º have been used to avoid that the wind should have passed Gotland. If the winds have passed Gotland the internal oscillation has been disturbed and no LLJ can appear. Measured wind speeds and directions have been compared to theoretical values from pressure measurements. 103 wind profiles with LLJs were found from all pibal trackings. In 27 of these pibal trackings, from 12 days, were LLJs caused by sea breeze. It is difficult to say how many LLJs that are caused by an internal oscillation. This due to the different results obtained when using wind speeds and wind directions from pibal trackings or pressure measurements. Also using straight line trajectories or trajectories following the isobars gives different results. Totally 9 LLJs were found, caused by an internal oscillation. Probably this way of analyze the internal oscillations is a better method for measurements over land than over sea. This due to the difficulties in measuring the exact wind speed and wind direction over the whole traveling distance over sea. The thermal wind over the island has been analyzed by studying all pibal trackings, even those without LLJs. The pibal trackings have been compared to the geostrophic wind calculated from pressure measurements to determine if thermal winds occur over Östergarnsholm. A LLJ caused by thermal wind occur when the geostrophic wind decreese with height, i.e. negative thermal wind. There was no difference in the amount of negative thermal winds and cases with no thermal wind. There were a few more positive thermal winds and they had higher wind speeds than the negative ones. There is no connection in the shift of wind direction when there is a LLJ caused by thermal wind. But there is a significant connection between the u- and v-component of the negative thermal winds and the u- and v-component of the geostrophic wind. In both cases the geostrophic wind tends to decreese towards zero with height. 41 of all LLJs were caused by thermal winds. About 50% of both negative and positive thermal winds have a positive v-component and a negative u-component. This means that the warmer region is in the north east.

Vid luftens passage från en typ av yta till en annan byggs ett internt gränsskikt upp där luftens egenskaper förändras successivt. Detta är grunden för bl.a. bildandet av advekterade low-level jets orsakade av en tröghetssvängning, vilken har sitt ursprung i att under vissa förhållanden erhålls obalans mellan de krafter som påverkar vinden. Målet med arbetet var att kartlägga inflytandet av low-level jets på vindprofilen på Näsudden, Gotland, ovan 100 m höjd. För detta syfte användes mätningar under perioden maj 2000 till juli 2001 gjorda i tre mätmaster i området. En mätmast var placerad på land 1.5 km från Näsuddens kust-linje, en andra mast precis vid kustlinjen och en tredje mast ute i Östersjön 4.6 km från kusten. Eftersom masten ute i havet var placerad mitt i Bockstigens vindpark, omgiven av fem vindturbiner, påverkades vindmätningarna av turbinvakar i vissa riktningssektorer. Genom jämförelser mellan vindmätningar i masten och på turbinerna i för masten ostörda sektorer, togs en korrektion fram så att vind-mätningarna gjorda på turbinerna kunde användas i de sektorer mastmätningarna stördes av turbinvakar. För att kunna studera påverkan av low-level jets på vindprofilen användes data från Bockstigens vindpark tillsammans med en vindmodell, byggd på rörelseekvationerna, Monin-Obukhovs similaritetsteori och K-teori. Modellen, som ej kan generera low-level jets, användes för att beräkna vindprofilerna över havet och hur dessa vind-profiler modifierades vid passagen av kusten. Resultatet från modellen jämfördes sedan med observationer från den 145 m höga masten på Näsudden, 1.5 km från kusten. Det visade sig att de observerade vindhastigheterna på 145 m höjd i medeltal var ca 0.8 m/s högre än vad som beräknats med modellen vid vindriktningar från havet in över Näsudden, vilket kan antas bero på inflytandet från low-level jets. Under månaderna mars till maj är vattnet i Ösersjön klimatologiskt sett mycket kallare än luften, ett förhållande som är fördelaktigt för bildandet av low-level jets. Det visade sig att under dessa månader var vindhastigheten mätt på 145 m höjd inom vind-riktningssektorn 220°-250° hela 1.7 m/s högre än vad som beräknades med vind-modellen, vilket understryker betydelsen av low-level jets under denna del av året. Under resten av året befanns skillnaden mellan observerad och modellberäknad vindhastighet på 145 m höjd vara ca 0.8 m/s för samma vindriktningar. / When air is flowing from one type of surface to another, an internal boundary layer is built up where the properties of the air gradually change. This is the basic condition for the development of for example advected low-level jets caused by inertial oscillations, whose origin is that under certain conditions an imbalance will occur between the forces driving the wind. The purpose with the present work was to map the influence from low-level jets on the wind profile at Näsudden, Gotland, above 100 m height. For this purpose, measurements taken on three towers in the area during the period May 2000 to July 2001 were used. One tower was located inland 1.5 km from the coastline of Näsudden, a second tower was located just at the coastline, and a third one was located offshore 4.6 km from the coast. Since the offshore tower was located in the middle of the Bockstigen wind farm, surrounded by five wind turbines, the wind measurements were affected by turbine wakes in certain wind direction sectors. By comparing the wind measurements taken on the tower with measurements taken on the turbines for directions where the tower was not in the wake, corrections were determined where by the wind measurements taken on the turbines could be used for sectors where the tower was located in wakes. To investigate the influence from low-level jets on the wind profile, data from Bockstigen were used together with a wind model based on the equations of motion, Monin-Obukhov’s similarity theory, and K-theory. The model, which is unable to generate low-level jets, was used to calculate the wind profiles over the sea and how these profiles were modified after passage of the coastline. The results from the model were compared to observations made on the 145 m high tower at Näsudden, 1.5 km from the coast. The comparison showed that the observed wind speed at 145 m height on the average was ca 0.8 m/s higher than the wind speed calculated with the model for wind directions from the sea at Näsudden, which could be assumed to be the effect of the presence of low-level jets. During the months March to May the water in the Baltic Sea is climatologically colder than the air temperature, which is favorable for the development of low-level jets. The results from these months showed that the measured wind speed at 145 m height in the wind direction sector 220°-250° was as much as 1.7 m/s higher than what was calculated with the model, which points at the importance of low-level jets during this part of the year. During the rest of the year the average difference between observed and modeled wind speed at the 145 m level was about 0.8 m/s for the same wind direction.

Low-level jets (LLJs) are local wind speed maximums in the atmospheric boundary layer. In the Baltic Sea, LLJs are frequently occurring in spring and summer. It is an important phenomena to consider for wind energy parks, and changes in turbulence during the jets can effect the efficiency of said parks. In this study, the effect that offshore LLJs have on turbulence intensity (TI) is analysed and the goal is tounderstand if TI significantly changes as the jets form, and if the changes aredifferent above and below the core. The theory of shear sheltering predicts that turbulence decreases below the core of a LLJ, and it has been experimentally tested previously with various results. However, turbulence characteristics above the core ofa LLJ has not been studied before. LiDAR measurements of wind speed and TI profiles, up to 300 m, from the island of Östergarnsholm in the Baltic Sea are used. The measurements are from the period 2016-2020 and are limited to a sector with unobstructed line-of-sight to the ocean. Complete LLJ-events, which includes non-LLJ profiles before and after the actual jets, are analysed. The LLJs are found to appear in low TI conditions related to stable stratification. Mean TI increases with 38 - 47% above the core as the jets appear, and then returns to approximately the initial values after the jets disappear. Below the core, mean TI instead decreases with 14 - 19% during the jets, which is compatible with the theory of shear sheltering. For future studies it is recommend to choose a location with larger unobstructed line-of-sight to the ocean, further optimise the LLJ-finding algorithm and also analyse other turbulent quantities. / Vindmaxima på låg höj (LLJ, för eng. Low-level jets) är lokala vindhastighetsmaximum i det atmosfäriska gränsskiktet. I Östersjön är LLJs vanliga, framförallt på våren och sommaren. Det är ett viktigt fenomen att beakta för vindkraftsparker, och turbulensförändringar i samband med LLJs kan påverka effektiviteten av vindkraftverk. I denna studie analyseras effekten som LLJs över havet har på turbulensintensiteten (TI) och målet är att förstå om TI förändras närströmmarna bildas, och om förändringarna är olika ovan och under kärnan. En teori förutspår att turbulens minskar under kärnan i en LLJ, s.k. skjuvningsblockering (eng. Shear sheltering), och den har testats tidigare med varierande resultat. Turbulens ovanför kärnan i en LLJ har dock inte studerats tidigare. LiDAR-mätningar av vindhastighets och TI-profiler, upp till 300 m, vid ön Östergarnsholm i Östersjön används. Mätningarna är från perioden 2016-2020 och är begränsade till en sektor med fri siktlinje mot havet. Kompletta LLJ-event, vilket inkluderar icke-LLJ-profiler före och efter själva strömmen, analyseras. Resultatet visar att LLJs förekommer vid låga TI-förhållanden relaterade till stabil skiktning. Medel TI ökar med 38 - 47% överkärnan när strömmarna dyker upp och återgår sedan till ungefär de ursprungligavärdena efter att strömmarna försvunnit. Under kärnan minskar medel TI istället med 14 - 19% i samband med strömmarna, vilket är förenligt med skjuvningsblockerings-teorin. För framtida studier är det rekommenderat att välja en plats med större fri siktlinje till havet, ytterligare optimera identifikationen av kompletta LLJ-event och även analysera andra variabler för att karakterisera turbulensen.

Wind turbine noise is a re-emerging issue in the wind industry. As the competition for sites with good wind potential on land is rising, offshore projects in coastal areas seem as a reasonable alternative to onshore. In this context offshore sound propagation is gaining more importance considering that sound will travel over longer distances on water, especially with regard to lower frequencies. Moreover different meteorological conditions that occur on sea may attenuate or enhance sound propagation on water. The prediction tools commonly used by developers are only partially taking these parameters into account. This will be investigated in this thesis.   Hence, different methods for predicting offshore wind turbine noise are going to be assessed. These methods can be divided in two approaches namely algebraic and Partial Differential Equation (PDE) based. The methods evaluated are the ISO 9613-2 standard for outdoor noise prediction, the Danish method and the Swedish method for wind turbines noise estimation over water.   For the PDE based approach, the Helmholtz Equation will be employed in order to examine different meteorological conditions and phenomena occurring over a flat reflecting surface. The experiments with the PDE include the simulation of meteorological conditions with different levels of refraction and changing ground impedance in order to take into account the effect of a shoreline. In addition a meteorological phenomenon called the low-level jet is investigated which is characterised by strong winds at relatively low altitude.   Noise prediction tools used by developers need to be able to consider these effects in order to allow for thorough planning of wind energy projects. Nonetheless, relatively more complex models such as the Helmholtz Equation require experienced users and significant computing time. Further research and development needs to be made in order to promote the wider use of noise prediction methods like the Helmholtz Equation in the wind industry.

In recent years there has been an increase in offshore wind energy, which poses the need for accurate wind speed estimates in the marine environment, especially in coastal areas where most wind turbines will be placed. This thesis is focused on the Baltic Sea, which is a small, semi-enclosed sea where land-sea interaction play an important role in explaining the wind patterns. Mesoscale model simulations can be used to study the marine environment, where observations are often scarce. In this thesis the Weather Research and Forecasting (WRF) model is used. In the first study simulations show that stable stratification over sea is very common in spring and summer and is associated with an increase in low-level jet occurrence and increased wind shear below 200 m, at heights where wind turbines are erected. The model performance in stable conditions is evaluated against aircraft measurements using several boundary layer parametrization schemes, and it is shown that the low-level jet height and strength is not accurately captured with any of the parametrizations. In the second study the advection of land features is investigated. From simulations, aircraft observations and satellite images it is shown that boundary layer rolls are created in the convective boundary layer over land, and advected several tens of kilometres out over sea surface, despite the stable stratification, where convective turbulence dissipates quickly. The occurrence of boundary layer rolls gives rise to horizontal wind speed variations of several meters per second over distances of kilometres, which can increase the uncertainty of short term wind speed forecasts in coastal areas with offshore flow. It is shown that mesoscale processes in and above the marine boundary layer are important in modifying the wind field in distances of at least 100 km from the coast and that models still need to be improved in order to capture these conditions. / Under de senaste decennierna har vindkraftsutbyggnaden ökat till havs, vilket innebär att det krävs tillförlitliga vindhastighetsuppskattningar över hav, särskilt i kustområden, där de flesta vindturbinerna kommer att placeras. Den här avhandlingen kommer att fokusera på Östersjön, vilket är ett relativt litet hav omgivet av landmassor, där land-hav-interaktion har en stor påverkan på vindmönstren över havet. Mesoskaliga modeller kan användas för att studera den lägre delen av den marina atmosfären. I den här avhandlingen används modellen "Weather Research and Forecasting" (WRF). I den första studien visar simuleringar att stabila förhållanden över havet är vanligt förekommande under sommar och vår, och sker i samband med en ökad förekomst av vindmaxima på låg höjd och ökad vindskjuvning under 200 m - alltså på höjder där vindkraftverk förekommer. Modellen, med flertalet gränskiktsparametriseringar, utvärderas för stabila fall mot flygplansmätningar, och resultaten visar att styrkan och höjden för vindmaxima är antingen över- eller underskattade oavsett parametrisering. I den andra studien undersöks advektiva fenomen. Simuleringar, flyplansobservationer och satellitbilder visar att avlånga rullvirvlar uppstår i det konvektiva gränsskiktet över land och advekteras flera tiotals kilometer över hav ut från kusten. Detta trots stabila förhållanden över havet, där den konvektiva turbulensen snabbt dör ut. Rullvirvlarna ger upphov till variationer i horisontell vindhastighet på flera meter per sekund över avstånd på några kilometer, vilket kan öka osäkerheten hos korttidsprognoser för vind när det är blåser från land. Sammanfattningsvis har det har visats att mesoskaliga processer i och ovanför det marina gränsskiktet har en stor inverkan på vindfältet åtminstone 100 km från kusten, och modeller behöver fortfarande förbättras för att kunna fånga dessa företeelser. / StandUp for Wind

Master of Arts / Department of Geography / John Harrington, Jr / Nighttime rainfall has long been thought of as an important component to the central Great Plains hydroclimate during the wettest three-month period known as the “late spring -early summer precipitation maximum.” Research has suggested that nocturnal rainfall in the region results from a phenomenon known as the nocturnal Great Plains Low-Level Jet (GPLLJ). The jet, which originates in the Gulf of Mexico, transports moisture into the Great plains during the nighttime hours and often provides fuel for nighttime convection. The climatological characteristics of nighttime rainfall, as well the configuration of the low-level winds and the mechanisms behind its formation during this three-month wet period, however; are not well understood. Using hourly rainfall data from Topeka, KS, the nighttime rainfall characteristics are examined Topeka, KS and other Kansas stations for a 63-year period from 1950-2012 for May-July. Additionally, using the NCEP/NCAR Reanalysis data, the structure and configuration of the southerly wind phenomenon was analyzed based on its horizontal and vertical characteristics for nighttime rainfall events in May, June and July. A subsequent analysis also analyzed the larger synoptic-scale environment in place for six half-month periods from May to July. The results indicate that nighttime rainfall is a major contributor to the overall moisture budget in the Great Plains, contributing close to 50% of the overall rainfall total for the three-month period. The percentage of nighttime rainfall increases from west-east across the state, as well as temporally from May to July. The southerly winds are at their strongest during May events, tends to reach its peak at 850 mb at 6z (0000LST) near south-central Oklahoma, and forms as the result of both synoptic and thermal mechanisms. The synoptic mechanisms in place that generate the a southerly wind component change by month, leading to incredible variation in terms of its characteristics during nighttime rainfall events.

O enfoque deste trabalho é contribuir para a compreensão dos processos que definem a propagação de linhas de instabilidade tropicais da Amazônia. Foi feita a análise de 9 anos de imagens de satélite definindo casos de linha de instabilidade e relacionando sua ocorrência com as do seu ambiente. Foi ressaltado o papel dos jatos de baixos níveis na propagação das linhas de instabilidade e verificadas possíveis diferenças entre os sistemas que se propagam e os que não se propagam. A partir da análise observacional foram feitas simulações numéricas com o modelo BRAMS para explorar a sensibilidade da propagação das linhas de instabilidade às características cinemáticas do ambiente. Como resultado principal, obteve-se que a maioria dos casos observados foi de linhas que não se propagaram, permanecendo quase estacionárias nas proximidades da costa norte do Brasil. Do total de casos de LI identificadas nas imagens de satélite, apenas 20% foram de linhas se propagaram mais de 400 km. O perfil do vento para seu ambiente de formação e desenvolvimento possui características que reúnem um jato em baixos níveis, um jato em altos níveis e pouco cisalhamento na intensidade e na direção. O jato em baixos níveis foi predominante de leste com intensidade média de 9 ms-1 e ocorreu em torno de 800 hPa. Ele foi mais intenso e mais profundo para as linhas que se propagaram mais de 400 km e menos intenso e menos profundo para as linhas costeiras. Os anos de 2005 e 2006 tiveram um aumento considerável de casos, em especial para as linhas que se propagaram. 2005 foi um ano atípico, pois foi verificado um aumento na temperatura da superfície do mar do Atlântico tropical, o que permitiu especular que aumentos como o que foi verificado, em certas áreas do Oceano Atlântico, pode levar ao aprofundamento do JBN através da intensificação das ondas de leste e assim favorecer a formação de mais LI na costa norte do Brasil. O JBN tem papel importante no processo de desenvolvimento das torres de cúmulos que compõe as linhas de instabilidade. Não só são responsáveis pela organização do sistema em linha, mas influenciam diretamente nos processos internos das nuvens, como ventilação e entranhamento, a formação de correntes descendentes e a formação de frentes de rajada em superfície que, em última instância, definem as características de propagação. / The aim of this work is to contribute to the understanding the processes that define the propagation of Amazon Tropical Squall Lines (ASL). 9 years of satellite images were analyzed defining ASL cases and their relationship with environment characteristics. The low level jet (LLJ) in ASL propagation has emerged as an important feature and possible differences between the systems that propagated and systems that did not propagate were analyzed. The observational analyzes suggested a series of numerical simulations with BRAMS model to explore the sensitivity of propagating ASL with respect to the kinematic characteristics of environment. Main results show that the larger number of ASL cases was of non propagating coastal lines. From the total, only 20% of ASL propagated more than 400 km (SL2). The wind profile of ASL formation and development environment had a LLJ, an upper level jet and to small values of speed and directional shear. The LLJ was from east with intensity of about 9 ms-1 and it was occurred around 800 hPa. It was more intense and deeper for SL2 than CSL. The 2005 and 2006 years had an considerable increase of ASL cases, specially for propagated lines. The 2005 year was atypical, because had an increase of sea surface temperature of Tropical Atlantic Ocean, allowing to speculate that this kind of increase, in certain areas of the Ocean, can lead to the deepening of LLJ through of easterly wave intensification, and thus favoring the formation of more ASL in the North coast of Brazil. The LLJ has a important role in the development processes of cumulus towers that compose the ASL and not only it were responsible for organization of the system, but it directly influence the internal processes of clouds, like inflow into the cloud.

A census of 24 coupled (CMIP) and 13 uncoupled (AMIP) models from the Intergovernmental Panel on Climate Change (IPCC) fourth assessment report (AR4) were compared with observations and reanalysis to show varied ability of the models to simulate Caribbean precipitation and mechanisms related to precipitation in the region. Not only were errors seen in the annual mean, with CMIP models underestimating both rainfall and sea surface temperature (SST) and AMIP models overestimating rainfall, the annual cycle was also incorrect. Large overestimates of precipitation at all SSTs (and particularly above 28 degrees C) and at vertical circulations less than -10 hPa/day (the deep convective regime) were inherent in the atmospheric models with models using spectral type convective parameterizations performing best. In coupled models, however, errors in the frequency of occurrence of SSTs (the distribution is cold biased) and deep convective vertical circulations (reduced frequency) lead to an underestimation of Caribbean mean precipitation. On daily timescales, the models were shown to produce too frequent light rainfall amounts (especially less than 1 mm/day) and dry extremes and too few heavy rainfall amounts and wet extremes. The simulation of the mid-summer drought (MSD) proved a challenge for the models, despite their ability to produce a Caribbean low-level jet (CLLJ) in the correct location. Errors in the CLLJ, such as too strong magnitude and weak semi-annual cycle, were worse in the CMIP models and were attributed to problems with the location and seasonal evolution of the North Atlantic subtropical high (NASH) in both CMIP and AMIP models. Despite these discrepancies between models and observations, the ability of the models to simulate the correlation between the CLLJ and precipitation varied based on season and region, with the connection with United States precipitation particularly problematic in the AMIP simulations. An observational study of intraseasonal precipitation in the Caribbean showed an explicit connection between the Madden-Julian oscillation (MJO) and Caribbean precipitation for the first time. Precipitation anomalies up to 50 percent above (below) the annual mean are observed in phases 1 and 2 (5 and 6) of the MJO and are related to changes in the CLLJ, that is also modulated by the MJO. Considerable progress has been made on identifying both problems and successes in the simulation of Caribbean climate in general circulation models, but many areas still require investigation.

O enfoque deste trabalho é contribuir para a compreensão dos processos que definem a propagação de linhas de instabilidade tropicais da Amazônia. Foi feita a análise de 9 anos de imagens de satélite definindo casos de linha de instabilidade e relacionando sua ocorrência com as do seu ambiente. Foi ressaltado o papel dos jatos de baixos níveis na propagação das linhas de instabilidade e verificadas possíveis diferenças entre os sistemas que se propagam e os que não se propagam. A partir da análise observacional foram feitas simulações numéricas com o modelo BRAMS para explorar a sensibilidade da propagação das linhas de instabilidade às características cinemáticas do ambiente. Como resultado principal, obteve-se que a maioria dos casos observados foi de linhas que não se propagaram, permanecendo quase estacionárias nas proximidades da costa norte do Brasil. Do total de casos de LI identificadas nas imagens de satélite, apenas 20% foram de linhas se propagaram mais de 400 km. O perfil do vento para seu ambiente de formação e desenvolvimento possui características que reúnem um jato em baixos níveis, um jato em altos níveis e pouco cisalhamento na intensidade e na direção. O jato em baixos níveis foi predominante de leste com intensidade média de 9 ms-1 e ocorreu em torno de 800 hPa. Ele foi mais intenso e mais profundo para as linhas que se propagaram mais de 400 km e menos intenso e menos profundo para as linhas costeiras. Os anos de 2005 e 2006 tiveram um aumento considerável de casos, em especial para as linhas que se propagaram. 2005 foi um ano atípico, pois foi verificado um aumento na temperatura da superfície do mar do Atlântico tropical, o que permitiu especular que aumentos como o que foi verificado, em certas áreas do Oceano Atlântico, pode levar ao aprofundamento do JBN através da intensificação das ondas de leste e assim favorecer a formação de mais LI na costa norte do Brasil. O JBN tem papel importante no processo de desenvolvimento das torres de cúmulos que compõe as linhas de instabilidade. Não só são responsáveis pela organização do sistema em linha, mas influenciam diretamente nos processos internos das nuvens, como ventilação e entranhamento, a formação de correntes descendentes e a formação de frentes de rajada em superfície que, em última instância, definem as características de propagação. / The aim of this work is to contribute to the understanding the processes that define the propagation of Amazon Tropical Squall Lines (ASL). 9 years of satellite images were analyzed defining ASL cases and their relationship with environment characteristics. The low level jet (LLJ) in ASL propagation has emerged as an important feature and possible differences between the systems that propagated and systems that did not propagate were analyzed. The observational analyzes suggested a series of numerical simulations with BRAMS model to explore the sensitivity of propagating ASL with respect to the kinematic characteristics of environment. Main results show that the larger number of ASL cases was of non propagating coastal lines. From the total, only 20% of ASL propagated more than 400 km (SL2). The wind profile of ASL formation and development environment had a LLJ, an upper level jet and to small values of speed and directional shear. The LLJ was from east with intensity of about 9 ms-1 and it was occurred around 800 hPa. It was more intense and deeper for SL2 than CSL. The 2005 and 2006 years had an considerable increase of ASL cases, specially for propagated lines. The 2005 year was atypical, because had an increase of sea surface temperature of Tropical Atlantic Ocean, allowing to speculate that this kind of increase, in certain areas of the Ocean, can lead to the deepening of LLJ through of easterly wave intensification, and thus favoring the formation of more ASL in the North coast of Brazil. The LLJ has a important role in the development processes of cumulus towers that compose the ASL and not only it were responsible for organization of the system, but it directly influence the internal processes of clouds, like inflow into the cloud.

Central America is a region regularly affected by natural disasters, with most of them having a hydro-meteorological origin. Therefore, the understanding of annual changes of precipitation upon the region is relevant for planning and mitigation of natural disasters. This thesis focuses on studying the precipitation variability at annual scales in Central America within the framework of the Swedish Centre for Natural Disaster Science. The aims of this thesis are: i) to establish the main climate variability sources during the boreal winter, spring and summer by using different statistical techniques, and ii) to study the connection of sea surface temperature anomalies of the neighbouring oceans with extreme precipitation events in the region. Composites analysis is used to establish the variability sources during winter. Canonical correlation analysis is employed to explore the connection between the SST anomalies and extreme rainfall events during May-June and August-October. In addition, a global circulation model is used to replicate the results found with canonical correlation analysis, but also to study the relationship between the Caribbean Sea surface temperature and the Caribbean low-level jet. The results show that during winter both El Niño Southern Oscillation and the Pacific Decadal Oscillation, are associated with changes of the sea level pressure near the North Atlantic Subtropical High and the Aleutian low. In addition, the El Niño Southern Oscillation signal is intensified (destroyed) when El Niño and the Pacific Decadal Oscillation have the same (opposite) sign. Sea surface temperature anomalies have been related to changes in both the amount and temporal distribution of rainfall. Precipitation anomalies during May-June are associated with sea surface temperature anomalies over the Tropical North Atlantic region. Whereas, precipitation anomalies during August-September-October are associated with the sea surface temperature anomalies contrast between the Pacific Ocean and the Tropical North Atlantic region. Model outputs show no association between sea surface temperature gradients and the Caribbean low-level jet intensification. Canonical correlation analysis shows potential for prediction of extreme precipitation events, however, forecast validation shows that socio-economic variables must be included for more comprehensive natural disaster assessments.