<p>The Arctic region attracts considerable scientific interest in these years. Some of the Earth's most pronounced signs of the recent climate change are found here. The summer sea-ice cover is shrinking at an alarming rate. At the same time the region warms faster than the rest of the globe.</p><p>The sea-ice reduction implies an increase of solar-radiation absorption at the surface leading to warming which is expected to be larger at higher than at lower latitudes. It is therefore often assumed that the sea-ice reduction is a major cause of the observed Arctic temperature amplification. However, results presented in this thesis suggest that the snow and ice-albedo feedbacks are a contributing but not dominating mechanism behind the Arctic amplification. A coupled climate-model experiment with a doubling of the atmospheric CO2 concentration reveals a considerable Arctic surface-air-temperature amplification in a world without surface-albedo feedback. The amplification is only 8 % larger when this feedback is included. Instead the greenhouse effect associated with an increase of humidity and cloud cover over the Arctic seems to play a major role for the amplification.</p><p>Reanalysis data, which are partly based on observations, show Arctic temperature amplification well above the surface in the troposphere. In the summer season, the amplification has its maximum at ~ 2 km height. These trends cannot be explained by the snow- and ice-albedo feedbacks which are expected to induce the largest amplification near the surface. Instead, a considerable part of the trends aloft can be linked to an increase of the atmospheric energy transport into the Arctic.</p><p>A major topic of this thesis is the linkage between the mid-latitude circulation and the Arctic warming. It is suggested that the atmospheric meridional energy transport is an efficient indicator of this linkage.</p>

The Arctic region attracts considerable scientific interest in these years. Some of the Earth's most pronounced signs of the recent climate change are found here. The summer sea-ice cover is shrinking at an alarming rate. At the same time the region warms faster than the rest of the globe. The sea-ice reduction implies an increase of solar-radiation absorption at the surface leading to warming which is expected to be larger at higher than at lower latitudes. It is therefore often assumed that the sea-ice reduction is a major cause of the observed Arctic temperature amplification. However, results presented in this thesis suggest that the snow and ice-albedo feedbacks are a contributing but not dominating mechanism behind the Arctic amplification. A coupled climate-model experiment with a doubling of the atmospheric CO2 concentration reveals a considerable Arctic surface-air-temperature amplification in a world without surface-albedo feedback. The amplification is only 8 % larger when this feedback is included. Instead the greenhouse effect associated with an increase of humidity and cloud cover over the Arctic seems to play a major role for the amplification. Reanalysis data, which are partly based on observations, show Arctic temperature amplification well above the surface in the troposphere. In the summer season, the amplification has its maximum at ~ 2 km height. These trends cannot be explained by the snow- and ice-albedo feedbacks which are expected to induce the largest amplification near the surface. Instead, a considerable part of the trends aloft can be linked to an increase of the atmospheric energy transport into the Arctic. A major topic of this thesis is the linkage between the mid-latitude circulation and the Arctic warming. It is suggested that the atmospheric meridional energy transport is an efficient indicator of this linkage.

Dimma är ett komplext småskaligt väderfenomen som gäckat meteorologerna ända sedan väderprognosernas start och varit en bidragande orsak till att många mist livet inte minst inom flyget. Än idag är prognostisering av dimma mycket svårt i synnerhet strålningsdimma. Studier (COST, 2007) har visat att för att en modell ska kunna fånga formation av dimma krävs det i vissa fall att den skall kunna lösa upp inversionsskikt på 10m.Därtill uppgår osäkerheten i många fall för numeriska modeller vid fastställande av sikten i dimma till 50 %. Då kan en statistisk kartläggning specifikt för varje dimtyp utgöra ett bra komplement till modellerna för att få en uppfattning om exempelvis sikten i dimman. I den här studien användes observationer gjorda med en halvtimmes mellanrum från Arlanda under perioden 1993-07-01 – 2009-11-03.I denna kartläggning identifierades de fyra dimtyperna: strålningsdimma, nederbördsdimma, advektionsdimma samt dimma orsakad av en sänkning av molnbasen till marknivå (molndimma). Här framkom att strålningsdimman var den vanligaste dimtypen (43,6%) och mest frekvent under sommaren och hösten, till skillnad från nederbördsdimma och advektionsdimma som var vanligast under vinterhalvåret. För strålningsdimman fann man också de sämsta siktvärdena följt av molndimma. Det framkom också att frekvensen av de fall där strålningsdimman täcker hela flygplatsen avsevärt minskat sedan början på 2000-talet, detta samtidigt som utbyggnaden av flygplatsen stod klar.Vid studierna av isdimma och vattendimma visade det sig att den lägsta sikten för vattendimmor uppkom då temperaturen och den specifika fuktigheten var som högst. Medan för isdimma fann man de lägsta siktvärdena vid temperaturer runt -20 °C / Since the beginning of weather forecasting fog has caused many meteorologists severe problems, and in aviation it has been one of the main reasons for several crashes. Even today with the latest numerical models, predicting fog is a difficult task. New studies have shown that to predict fog the model must be able to resolve inversion layers down to 10 m. Even then the precision in determination of the visibility inside the fog is down to 50 % (Gultepe, 2007). In those cases a statistical mapping of fog at a certain place could be a good complement to the numerical models in determining time of formation and visibility. In this study I have used METAR observations made at Arlanda airport from 1993-07-01 to 2009-11-03, which are made every half an hour. In the mapping Radiation fog, Advection fog, Precipitation fog and fog caused by lowering of clouds (cbl. fog) was sorted out. I also looked into the behavior of ice fog vs. water fog.The results showed that radiation fog was the most common fog type (43,6 %) and had the lowest rate of visibility followed by cbl. fog. Radiation fog also had its highest frequency on summer and autumn, while advection fog and precipitation fog was concentrated into wintertime. Additionally the study showed that the frequency of the times when radiation fog covers the whole airport started to decrease at the beginning of the 21st century. Coinciding with when the expansion of the airport was finished including a new runway. In the comparison between water fog and ice fog it was clear that the distribution of visibility was similar. The lowest visibilities in water fog was found at the highest temperatures and highest specific liquid water content while in ice fog the lowest visibilities where found for temperatures in the neighborhood of - 20 °C. For temperatures below - 20 °C there were no observations of fog.

<p>A new large eddy simulation (LES) code is developed and used to investigate non-local features of turbulent planetary boundary layers (PBLs). The LES code is based on filtered Navier-Stokes equations, which describe motions of incompressible, Boussinesq fluid at high Reynolds numbers. The code computes directly large-scale, non-universal turbulence in the PBL whereas small-scale, universal turbulence is parameterized by a dynamic mixed subgrid closure. The LES code is thoroughly tested against high quality laboratory and field data. </p><p>This study addresses non-local properties of turbulence which emphasis on the stable stratification. Its basic results are as follows. The flow stability in PBLs is generally caused by two mechanisms: the negative buoyancy force (in the stable density stratification) and the Coriolis force (in the rotating system). The latter stabilizes the flow if the earth’s vorticity and the turbulent vorticity are anti-parallel. The Coriolis force stability suppresses large-scale turbulence and makes large eddies asymmetric. The density stratification suppresses vertical scales of turbulence. Joint actions of the Coriolis and the buoyancy forces result in a more complex behavior of turbulence. Particularly, the layers of vigorous turbulence may appear in the course of development of low-level jets in baroclinic atmosphere. </p><p>Non-local effects determine integral measures of PBLs, first of all the PBL depth. This study clearly demonstrates its pronounced dependences on the Coriolis parameter, the Kazanski-Monin internal stability parameter, and newly introduced imposed-stability and baroclinicity parameters. An LES database is created and used to validate an advanced PBL-depth formulation. LES support the idea that PBLs interact with the stably stratified free flow through the radiation of gravity waves, excited by large turbulent eddies at the interface. </p>

A new large eddy simulation (LES) code is developed and used to investigate non-local features of turbulent planetary boundary layers (PBLs). The LES code is based on filtered Navier-Stokes equations, which describe motions of incompressible, Boussinesq fluid at high Reynolds numbers. The code computes directly large-scale, non-universal turbulence in the PBL whereas small-scale, universal turbulence is parameterized by a dynamic mixed subgrid closure. The LES code is thoroughly tested against high quality laboratory and field data. This study addresses non-local properties of turbulence which emphasis on the stable stratification. Its basic results are as follows. The flow stability in PBLs is generally caused by two mechanisms: the negative buoyancy force (in the stable density stratification) and the Coriolis force (in the rotating system). The latter stabilizes the flow if the earth’s vorticity and the turbulent vorticity are anti-parallel. The Coriolis force stability suppresses large-scale turbulence and makes large eddies asymmetric. The density stratification suppresses vertical scales of turbulence. Joint actions of the Coriolis and the buoyancy forces result in a more complex behavior of turbulence. Particularly, the layers of vigorous turbulence may appear in the course of development of low-level jets in baroclinic atmosphere. Non-local effects determine integral measures of PBLs, first of all the PBL depth. This study clearly demonstrates its pronounced dependences on the Coriolis parameter, the Kazanski-Monin internal stability parameter, and newly introduced imposed-stability and baroclinicity parameters. An LES database is created and used to validate an advanced PBL-depth formulation. LES support the idea that PBLs interact with the stably stratified free flow through the radiation of gravity waves, excited by large turbulent eddies at the interface.

<p>The thermodynamic conditions for the behaviour of silicon in the lower part of the blast furnace have been the focus of the thesis. More specifically, the influences of temperature, carbon activity, total gas pressure and Fe reoxidation on silicon have been studied.</p><p>Calculations show that an increased temperature gives higher equilibrium ratio between silicon in hot metal and slag. Furthermore, laboratory reduction studies shows that the carbon activity in the cohesive zone increase with an increased reduction time. Increased carbon activity will increase the equilibrium silicon content in liquid metal.</p><p>Equilibrium calculations based on tapped hot metal and slag shows that the equilibrium silicon content of the liquid metal phase is higher than measured at tapping. Around the raceway area the equilibrium silicon content is very high. The high equilibrium silicon content makes it important to differ between the conditions under operation and the conditions of samples taken out of the blast furnace before studied. The equilibrium silicon content is strongly correlated to the CO gas partial pressure. Often this partial pressure is changed during sampling and cooling of samples. At tapping the equilibrium partial pressure of CO has been calculated to higher values than the total gas pressure inside the blast furnace.</p><p>Metal droplets found in tapped slags are probably formed by reduction of FeO. In the periphery part of the lower part of the blast furnace, it is believed that mainly FeO oxidises silicon in hot metal. It is not expected that the metal droplets in the slag is formed if FeO oxidises dissolved silicon. Instead, the iron droplets may form at reactions with gas, coke carbon or coal powder carbon. Around some droplets increased magnesium content has been found. This may be due to reactions with gaseous magnesium that, according to thermodynamic conditions, is easy to form. It has been reported that much FeO may be formed in the raceway area. The metal droplets may indicate how much FeO that reacts with other components than liquid iron. The iron found in metal droplets in the slag corresponds to between 0.02 and 0.2 wt-% FeO in the slag.</p>

<p>A faithful depiction of the tropical atmosphere requires three-dimensional sets of observations. Despite the increasing amount of observations presently available, these will hardly ever encompass the entire atmosphere and, in addition, observations have errors. Additional (background) information will always be required to complete the picture. Valuable added information comes from the physical laws governing the flow, usually mediated via a numerical weather prediction (NWP) model. These models are, however, never going to be error-free, why a reliable estimate of their errors poses a real challenge since the whole truth will never be within our grasp. </p><p>The present thesis addresses the question of improving the analysis procedures for NWP in the tropics. Improvements are sought by addressing the following issues:</p><p>- the efficiency of the internal model adjustment, </p><p>- the potential of the reliable background-error information, as compared to observations,</p><p>- the impact of a new, space-borne line-of-sight wind measurements, and</p><p>- the usefulness of multivariate relationships for data assimilation in the tropics.</p><p>Most NWP assimilation schemes are effectively univariate near the equator. In this thesis, a multivariate formulation of the variational data assimilation in the tropics has been developed. The proposed background-error model supports the mass-wind coupling based on convectively-coupled equatorial waves. The resulting assimilation model produces balanced analysis increments and hereby increases the efficiency of all types of observations.</p><p>Idealized adjustment and multivariate analysis experiments highlight the importance of direct wind measurements in the tropics. In particular, the presented results confirm the superiority of wind observations compared to mass data, in spite of the exact multivariate relationships available from the background information. The internal model adjustment is also more efficient for wind observations than for mass data. </p><p>In accordance with these findings, new satellite wind observations are expected to contribute towards the improvement of NWP and climate modeling in the tropics. Although incomplete, the new wind-field information has the potential to reduce uncertainties in the tropical dynamical fields, if used together with the existing satellite mass-field measurements.</p><p>The results obtained by applying the new background-error representation to the tropical short-range forecast errors of a state-of-art NWP model suggest that achieving useful tropical multivariate relationships may be feasible within an operational NWP environment.</p>

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.

<p>The present thesis deals with the ocean circulation from two viewpoints: <i>Pro primo</i>, the dependence of the global thermohaline ocean circulation (THC) on the parameterization of the small-scale vertical mixing processes in the interior of the ocean, and, <i>pro secundo</i>, the dynamics of the circulation in the Nordic Seas. The THC is found be crucially dependent on the parameterization of the small-scale vertical mixing, two types of which have been compared: The commonly used constant diffusivity and a, physically more plausible, stability-dependent parameterization. For constant diffusivity the circulation weakens when the equator-to-pole surface density difference is decreased, consonant with commonly held prejudices. However, for stability-dependent diffusivity the circulation is enhanced. This conclusion has been reached using two investigative techniques, viz. a scale analysis as well as a numerical zonally-averaged and equatorially symmetric THC model. However, if asymmetric flows are considered, the dynamics become more complex to interpret. It has, nevertheless, been concluded that when the degree of asymmetry of the surface-density distribution is taken to be fixed, the response of the circulation to changes of the surface-density distribution corresponds to that from the symmetric investigation.</p><p>The studies of the Nordic Seas are mainly based on satellite-altimetric data providing Sea-Level Anomalies (SLAs). These are utilized to estimate the seasonal cycle as well as the inter-annual variability of the depth-integrated flows. The seasonal cycle is examined using the winter-to-summer difference of the barotropic flow, with focus on the entire region as well as on two sections extending from a common point in the central Norwegian Sea to Svinøy on the Norwegian coast and to the Faroe Islands, respectively. The total barotropic transport is estimated to be around 10 Sv larger during winter than in summer, of which 8 Sv are associated with the barotropic re-circulation gyre in the interior of the Norwegian Sea, the remainder being linked to the Atlantic inflow across the Iceland-Scotland Ridge. The inter-annual variability of the circulation in the Nordic Seas is investigated on the basis of a theoretical analysis permitting independent calculation of the barotropic flow along closed isobaths using SLA data as well as wind data. The barotropic flow based on SLA data is found to co-vary with the flow estimated using wind data.</p>

<p>Two types of mesoscale wind-speed jet and their effects on boundary-layer structure were studied. The first is a coastal jet off the northern California coast, and the second is a katabatic jet over Vatnajökull, Iceland. Coastal regions are highly populated, and studies of coastal meteorology are of general interest for environmental protection, fishing industry, and for air and sea transportation. Not so many people live in direct contact with glaciers but properties of katabatic flows are important for understanding glacier response to climatic changes. Hence, the two jets can potentially influence a vast number of people.</p><p>Flow response to terrain forcing, transient behavior in time and space, and adherence to simplified theoretical models were examined. The turbulence structure in these stably stratified boundary layers was also investigated. Numerical modeling is the main tool in this thesis; observations are used primarily to ensure a realistic model behavior.</p><p>Simple shallow-water theory provides a useful framework for analyzing high-velocity flows along mountainous coastlines, but for an unexpected reason. Waves are trapped in the inversion by the curvature of the wind-speed profile, rather than by an infinite stability in the inversion separating two neutral layers, as assumed in the theory. In the absence of blocking terrain, observations of steady-state supercritical flows are not likely, due to the diurnal variation of flow criticality.</p><p>In many simplified models, non-local processes are neglected. In the flows studied here, we showed that this is not always a valid approximation. Discrepancies between simulated katabatic flow and that predicted by an analytical model are hypothesized to be due to non-local effects, such as surface inhomogeneity and slope geometry, neglected in the theory. On a different scale, a reason for variations in the shape of local similarity scaling functions between studies is suggested to be differences in non-local contributions to the velocity variance budgets.</p>