Den stora torkan som rådde i Sahel under 1970 och 1980-talen är unik i modern tid. Nederbördsmängderna i regionen har ökat sedan dess, men perioder av torka är fortfarande återkommande. Variationer i klimat likt dessa gör att Sahel är ett av de mest utsatta områdena i världen för klimatrelaterade förändringar. Området är starkt förknippat med begreppet ökenspridning, och det har spekulerats i att klimatvariationerna som drabbar regionen är ett resultat av mänsklig verkan inom och utanför Sahel, såsom överbetning och utsläpp av växthusgaser. Invånarna i Sahel står inför en framtid som är högst osäker; trots klimatmodellers framgångar med att beskriva klimatvariationerna som präglade 1900-talet tenderar modellernas simuleringar för framtida klimat att skilja sig åt. Detta projekt är en kombinerad litteraturstudie och laborativ studie. Simuleringar framtagna med SMHI Rossby Centers regionala klimatmodellen RCA4, driven av den globala modellen EC-EARTH är inkluderade. Simuleringarna har gjorts för framtida klimat (2071- 2100) och en historisk period (1971-2000), för klimatvariablerna temperatur och nederbörd. Litteraturstudien redogör för en del av den stora mängd forskning som bedrivits som ämnar beskriva orsakerna till torkan i Sahel, samt vad forskningen säger om framtida klimat i regionen. Mer precist undersöks hur ökenspridning och klimatförändringar samverkar, samt vilka konsekvenser dessa har på Sahel-områdets klimat. Jämförelser mellan de två tidsperioderna görs sedan, detta för tre olika RCP-scenarior för framtida klimat: RCP 2.6, 4.5, och 8.5. Med utgångspunkt från dessa jämförelser undersöks hur det framtida klimatet kommer att bli i Sahel. Ökning i medeltemperatur sker för samtliga RCP-scenarier. Ökningen är 1.2 till 5 C, beroende på vilket scenario som undersöks. Inga tydliga trender för nederbörd kan urskiljas. För RCP 2.6 blir medelvärdet för årsnederbörd i hela Sahel 0-100 mm mindre perioden 2071- 2100, jämfört med perioden 1971-2000. För RCP-4.5 ökar årsmedelnederbörden med 0-100 mm i västra Sahel, och minskar med 0-100 mm i öster. För RCP-8.5 får centrala Sahel en ökning på 0-100 mm, och i de västra och östra delarna minskar nederbörden med 0-100 mm. Osäkerhet i resultaten bedöms utifrån jämförelse med medelvärde från en klimatmodellensemble beräknade av SMHI. Resultaten för temperatur anses rimliga och är därför trovärdiga. Osäkerhetsnivån för den simulerade årsmedelnederbörden i framtiden bedöms vara hög. Det är troligt att ökade antropogena utsläpp av växthusgaser och förändringar i globala havsytetemperaturer har stor betydelse för det framtida klimatet i Sahel. Ökenspridning har påverkat klimatet historiskt sett, och bedöms ha en viss inverkan på det framtida klimatet i regionen.

Due to climate change and arctic amplification, the avalanche activity is expected to increase at higher latitudes (Hall et al., 1994). The arctic settlement of Longyearbyen, Svalbard, situated inside the valley Longyeardalen, is yearly threatened by avalanche activity (Eckerstorfer & Christiansen, 2011a). The surrounding slopes are known to produce avalanche, and during the last century they have proven to be able to cause substantial damages and even fatalities (Hallerstig, 2010). Previous studies investigated magnitude and forcing of the avalanches, including a meteorological perspective (Eckerstorfer & Christiansen, 2011b). This allowed for the usage of forecasts from the weather model AROME-Arctic in order to have an avalanche bulletin.The forecast for the area of Longyearbyen suffers from the location and the insufficient resolution of its source data. The data are obtained from an AWS located at the local airport, at the mouth of a relatively wide NW/SE oriented valley. Conversely, Longyeardalen is oriented NE/SW and is narrower. Because of the topography, channeling of winds is expected to produce difference weather conditions at the two sites, generating two distinct local weather conditions (Whiteman, 2000). If these different weather conditions are not taken into account, the weather model may provide forecasts that are not reliable for the area of Longyeardalen, hence resulting in biased avalanche bulletins.In this work I compare the data from the airport, from Longyeardalen and from the plateau above in order to assess if relevant differences exist in some important meteorological parameters (temperature, wind speed and direction, precipitation) between these sites. The weather station at the airport is an official AWS while the data from the other two sites were obtained using portable weather stations deployed during a field campaign between March 1st and April 11th 2018. During this time, snow data from the slopes surrounding Longyeardalen were also obtained. These data have been used to look for correlations among the wind conditions in the valley and the depth of the snow, as it is known that snow transport is a major factor determining snow accumulation in the area (Jaedicke and Sandvik, 2002; Hestnes, 2000).Temperature and precipitation have been found to be consistent among the two investigated valleys, while wind parameters differed significantly. Wind speed in Longyeardalen is on average overestimated by 3 m/s if only the data from the airport are used while the direction data are uncorrelated. This is due to the different circulations that occur at the two sites. Adventdalen is mostly influenced by southeasterly winds that are forcedly channeled or induced by the synoptic circulation, while in the smaller Longyeardalen southerly winds prevail due a thermal circulation induced by the presence of two glaciers on the top part of the valley. Snow depth is altered by the wind transport but it was not possible to find any correlation due to the low resolution of the snow depth data.

The simplest form of a snow particle is a hexagonal prism which can grow into a stellar crystal by growing branches from the six corners of the prism. The snow particle is affected by the temperature and supersaturation in the air, giving its unique form. Manual classification of snow particles based on shape is tedious work. Convolutional Neural Network (CNN) can therefor be of great assistance and are common in automatic image processing. From a data set consisting of 3165 images sorted into 15 shape classes, a sub set of 2193 images and 7 classes was used. The selected classes had the highest number of snow particle images and were used to train, validate and test on. Four data sets were constructed and eight models were used to classify the snow particles into seven classes. To reduce the amount of training data needed pretrained versions of neural networks AlexNet and ResNet50 were used with a technique called transfer learning. The 2193 images make up the first data set, Data set 1. To handle unbalanced classes in the first data set Synthetic Minority Oversampling Technique (SMOTE) was used to increase the number of snow particles in classes with few examples, creating Data set 2. A third data set was constructed to mimic a real world application. The data for training and validation was increased with SMOTE, while the test data only consisted of real snow particles. The performance of both ResNet50 and AlexNet on the data met the requirements for a practical application. However, ResNet50 had a higher overall accuracy, 72%, compared to AlexNet 69% on the evaluated data set. A t-test was conducted with a significance of p < 1·10−8. To enhance the shape of the snow particles a Euclidean Distance Transform (EDT) was used, creating Data set 4. However, this did not increase the accuracy of the trained model. To increase the accuracy of the models more training data of snow particles is needed, especially for classes with few examples. A larger data set would also allow more classes to be included in the classification.

Wind is used as an energy source all over the world. To be able to use this effectively, there is a need for as good forecasts and forecast models as possible. One of these models is Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) that is used to calculate short time forecasts. This model is used here to calculate wind speeds at two different areas in Västra Götaland, Bengtsfors and Vänersborg. There are also wind measurements with SODAR stations for these areas. The first part of this work is to investigate the difference between two model resolutions, 36 and 12 km, for the model results compared with the measurements. The comparison is done by calculating some different statistical values. The results of these parameters show that the difference between the two resolutions is fairly small and that the lower resolution gives a slightly better result. The second and major part of this work is to use two different regression models to adjust the result of the forecast models to the result of the measurements. These regression models will then be possible to use even when there are no measurements to compare with. The idea of these regression models is to find a way to describe the difference between the result of the forecast model and the SODAR measurements. This difference is then subtracted from the result of the forecast model so that you get an adjustment and more accurate result. The first regression model calculates the difference according to time of the day, the other model calculates the difference according to the wind speed. Furthermore, the measurements used are taken from 75 meters height above the ground. These are then compared to some different results from the forecast model, for example different model heights and different resolutions, and also the model results adjusted with the regression models. The comparison is done by calculating the same statistic values as before, both with and without an adjustment with the regression models, and also to look at histograms that show the distribution of the difference. It is shown that with the regression adjustment, there is a clear improvement of the statistical values compared to the original results of the forecasts. For example the value of the absolute mean difference is reduced with approximately 0.4-0.7 m/s with an adjustment of the regression model. The histograms clearly show that a more even distribution occurs after the adjustment with the regression models. From having a major part of the differences at 1-2 m/s to now having the major part at around 0 m/s and furthermore there is also generally a lower difference between the measurements and the results from the forecast model.

As a part of the European research program CAL, sprite observations were carried out from the OMP observatory in the French Pyrenees during the summer 2003. Images of the sprites were taken by two remotely controlled CCD cameras. The 23 July was considered particularly interesting because we then had access to data concerning both cloud-to-ground and intracloud lightning activity. This day was therefore chosen as the object of the present study. A large thunderstorm with two convective cores, one to the north and the other to the south, developed over the South of France during the late afternoon, and about two hours after sunset, the first sprite was detected. During a little more than three hours, 13 sprites were observed, 7 over the northern system and 6 over the southern system. The images enabled us to determine the azimuth angle of each sprite from the OMP observatory. 12 of the 13 sprites could be associated to positive cloud-to-ground flashes, and by putting together the sprite directions and the locations of the associated flashes on the radar images, we managed to get a rough idea of the position of the sprites in the storm system, and also to estimate their vertical and horizontal extent. Satellite images were included at this point of the study, and it appeared clear that sprites tend to occur over the stratiform region of the storm system in the area with the coldest (highest) cloud tops. The associated positive flashes were also within or close to this portion of the storm. The sprite occurrences were studied in relation to the cloud-to-ground and to the intracloud activity. We found that sprites seem to occur in a late stage of each storm system, when the rate of negative cloud-to-ground flashes has considerably decreased, and when the ratio of positive cloud-to-ground flashes is much higher then during the most active phase of the storm. Globally, the intracloud activity is also low during the sprite-producing periods, but sudden "bursts" of intracloud lightning could frequently be observed at the moment of the sprite. The peak current of the positive flashes was found to be rather weakly correlated to their sprite-generating capacity. The available Schumann resonance measurements seem to indicate that the charge moment is a much more adequate parameter in this respect. The areal coverage of the radar echo was calculated. The result supports the idea that sprite events tend to appear almost exclusively over large thunderstorm systems.

The purpose of this thesis is to study the visibility conditions during snowfall events in order to study the possibility of improvement of the visibility forecast currently in use. Measurement data for visibility and precipitation rate was gathered from Storöns automatic weather station at Storön in the Kalix archipelago situated in most northern part of the Bay of Bottnia. The elevation of weather station at Storön is fairly low. Examination of data gathered from the automatic weather station at Storön shows a large degree of scatter in the visibility for a given precipitation rate. An estimation of visibility function from the gathered data exhibits some discrepancy from the visibility function in the operational model. However this discrepancy is not statistically significant. On comparison with previous studies of the relationship between visibility and precipitation rate, the estimated visibility function was found to be somewhere in the middle of the previous estimates. In order to try to explain some of the deviation in the measured visibility from the value estimated by the visibility function, three factors were studied. These three were cloud temperature, the average temperature in the layers between ground and approximately 850 hPa and the time/month during the year when the snowfall occurred. The reason behind the choice of these factors were that literature shows that the underlying physics is the same in both the case of visibility reduction due to snowfall and density of freshly fallen snow, and in the latter studies has shown that these factors to some extent explain the variability in snow density. Based on the available data the results never the less show that these factors does not influence the variability of the measured visibility. / Syftet med denna uppsats är att studera siktförhållanden under perioder med snöfall för att undersöka om det finns möjlighet till förbättringar av dagens siktprognoser. Mätdata över sikt och nederbördsintensiteter kom från Storöns automatstation som ligger på en relativt låg ö i Kalix skärgård, i den nordligaste delen av Bottenviken. Undersökningen av mätdata från Storöns automatstation visar på en stor spriding av siktvärden vid givna nederbördsintensiteter. En skattning av siktfunktionen utifrån mätdata gav en viss skillnad jämfört med dagens prognosmodell. Dock var skillnaden inte statistiskt signifikant. Vid en jämförelse med tidigare studier av sikten vid snöfall befanns denna skattning ligga någonstans i mitten av skattade siktfunktioner. I ett försök att förklara en del av avvikelserna från den skattade siktfunktionen studerades huruvida tre faktorer, som visat sig ha betydelse för prognostisering av snödensiteter även påverkade siktvärden. Dessa tre var molntemperatur, medeltemperatur i de lägsta skikten samt när under året snöfallet ägde rum, uppdelat månadsvis. Skälet till att dessa tre faktorer studerades var likheten mellan den bakomliggande fysiken hos siktreduktionen respektive snödensiteten. Både snöns densitet och siktreduktion till följd av snöpartiklar i luften beror på egenskaper hos de enskilda snöflingorna. Resultaten baserade på tillgängliga mätdata gav dock inget stöd för att dessa faktorer även påverkar sikten.

I kvällstidningen Aftonbladet har det under flera år publicerats artiklar med långtidsprognoser för sommaren. För prognoserna står en tysk man vid namn Wolfgang Röder, som tidigare arbetat på meteorologiska institutionen vid Freie Universität i Berlin. Röders metod att ställa långtidsprognoser är hemlig, men han har avslöjat för Aftonbladet att han bl.a. studerar vädret under våren och jämför med statistik från tidigare år. Han gör dessutom en rad påståenden om aprilvädrets påverkan på sommarvädret, framförallt juli månad. Syftet med undersökningen var att ta reda på hur pass bra Röders lång- tidsprognoser är med det verkliga utfallet och utifall hans påståenden om april- vädrets påverkan på sommarvädret stämmer. I denna undersökning granskades Röders prognoser för Östra Svealand, med Stockholm-Bromma som represente- rande mätstation. För att bedömma Röders prognoser antogs månadsmedeltemperaturerna föl- ja en normalfördelning under en 30-årsperiod. I och med normalfördelningsan- tagandet kunde en standardavvikelse ifrån temperaturmedelvärdet för en viss månad beräknas. Med hjälp av sannolikhetstabeller för standardiserad normal- fördelning beräknades sedan avvikelseintervall från månadsmedelvärdet, vilka delades in i fem klasser: “Mycket kallt”, “kallt”, “normalt”, “varmt” och “myc- ket varmt”. Denna indelning skulle göra det enkelt att avgöra om Röder haft rätt eller fel. Vidare undersöktes också korrelation mellan aprils månadsme- deltemperatur och julis månadsmedeltemperatur samt julinederbörd under en 49-årsperiod. Det gjordes även detaljstudier av somrar som följt av varma re- spektive kalla aprilmånader. Detta för att eventuellt kunna urskilja ett mönster beroende på vädret i april. Resultatet blev inte särskilt tillfredställande. Med gällande klassindelning fick Röder endast rätt vid 10 tillfällen av 33, vilket ger en tillförlitlighet på cirka 30%. Hans påståenden om aprilvädrets påverkan på sommarvädret kunde inte heller styrkas. Korrelationskoefficienterna mellan aprils månadsmedeltem- peratur och julis månadsmedeltemperatur samt julinederbörd hamnade på 0.11 respektive 0.015, vilket är mycket lågt. Ur detaljstudien av ett antal somrar som följt av kall respektive varm april, kunde heller inga tydliga mönster urskiljas. Sammansfattningsvis visade undersökningen att det praktiskt taget är omöjligt för Röder att ställa en långtidsprognos för sommaren. Det går lika bra att singla slant!

Referat De tropiska cyklonerna har både skrämt och fascinerat människor genom tiderna och det pågår en hel del forskning kring dem. I detta arbete studeras om El niño‐Southern ocsillation påerkar antalet, intensiteten eller livslägden påde tropiska cyklonerna i Stilla havet. Till detta anväds temperaturdata frå bojar som den amerikanska väertjästen National Oceanic and Atmospheric Administration placerat ut i Stilla havet och insamlad information kring de tropiska cyklonerna. Studien baseras pådata frå en 15ås period under åen 1995 till 2009. Studien visar bland annat att antalet tropiska cykloner påerkas relativt mycket av de olika faserna i El niñ‐ Southern oscillation. Ötra Stilla havet påerkades mest dä det under vad som visade sig vara ett starkt El niñå var uppemot 30 tropiska cykloner medan det under ett La niñå endast var strax öer 10. Äen livslägden påde tropiska cyklonerna visade sig påerkas nåot av de olika faserna i El niñ‐Southern oscillation men dåtill stösta del i vätra Stilla havet. Intensiteten däemot visade sig inte direkt påerkas.

This thesis presents new insights into the chemical composition of semi-volatile compounds in aerosol samples collected in the central Arctic Ocean close to the North Pole in September 2018. The central Arctic Ocean is an inaccessible location due to the lack of land areas along with heavy pack ice conditions. Therefore, large knowledge gaps remain to understand the Arctic climate system, and in particular the role of aerosol particles in its pristine atmosphere. The chemical composition of the aerosol samples was analysed on a molecular level using High Resolution Time-of-Flight Chemical Ionization Mass Spectrometry coupled to a Filter Inlet for Gases and AEROsols (FIGAERO-HRToF-CIMS). The analysis revealed a significant signal from compounds that are likely from marine sources. One important precursor for marine aerosols is dimethyl sulfide (DMS), a gas released by phytoplankton and ice algae in the Arctic Ocean. DMS oxidises in the atmosphere to produce oxidation products that can contribute to aerosol growth. Analysis of air mass origin with backward trajectories showed that the highest ambient DMS concentrations originated from marine areas around the pack ice. However, no correlation could be shown within the pack ice between ambient DMS and its oxidation product methanesulfonic acid (MSA) in the particle phase. As FIGAERO-HRToF-CIMS is commonly used in areas with higher particle concentrations and has never been used in the central Arctic before, this thesis further demonstrates its suitability for measurements of aerosol chemical composition in this remote region. / Denna avhandling ämnar att presentera nya kunskaper om den kemiska sammansättningen av halvflyktiga föreningar i aerosolprover som samlades in i Norra ishavet nära Nordpolen i september 2018. Den innersta delen av Norra ishavet är svårtillgängligt på grund av bristen på fasta landområden ihop med kompakt packis. Därför kvarstår stora kunskapsluckor i förståelsen av klimatsystemet i Arktis, och i synnerhet aerosolpartiklars roll i dess orörda atmosfär. Den kemiska sammansättningen av aerosolproverna analyserades på molekylnivå med högupplöst kemisk joniseringsmasspektrometri kopplad till ett filterinsläpp för gaser och aerosoler (FIGAERO-HRToF-CIMS). Analysen visade på en tydlig signal från föreningar som sannolikt har marina källor. En viktig kemisk föregångare till marina aerosoler är dimetylsulfid (DMS), en gas som frigörs av fytoplankton och isalger i Ishavet. DMS oxiderar i atmosfären till oxidationsprodukter som kan bidra till en storleksökning av aerosoler. Genom analys av provluftens ursprung med trajektorieanalys visades att de högsta DMS-koncentrationerna kom från havsområdena runt packisen. Ingen korrelation kunde emellertid visas inom packisområdet mellan DMS i gasfas och dess oxidationsprodukt metansulfonsyra (MSA) i partikelfas. Eftersom FIGAERO-HRToF-CIMS ofta används i områden med högre partikelkoncentrationer och aldrig har använts i de inre delarna av Arktis tidigare, visar denna avhandling även att tekniken är lämplig för att mäta den kemiska sammansättningen av aerosoler i detta avlägsna polarområde.

The atmospheric spatial variability over Isfjorden, Svalbard, was investigated through statistical data analysis. The data used in the thesis was measured by mobile weather stations installed on three ships crossing the fjord, and two stationary weather stations close to the shoreline, during the autumn of 2021 and spring to autumn 2022. The spatial distributions of temperature, specific humidity, and wind speed were filtered into categories of season, ambient wind direction, and ambient wind speed, before they were studied. Gradients in temperature and humidity are re-occurring patterns along the Isfjorden axis during various seasons and wind directions. The temperature gradient was found to be partly generated by local drainage winds and further to be connected to the difference between air and sea surface temperature. The humidity gradient was consistent across all seasons and predominantly controlled by advection and differences in available water from surrounding land surfaces. A peak in wind speed around the mouth of Sassenfjorden indicated forced channeling effects, as the pattern dominated when the ambient wind aligned with the axis of the fjord. The results indicate local processes that can be expected from previous research.