Throughout the late 20th and early 21st century, the global temperature has been on the rise, a process that has been accelerated in the Arctic. The Arctic surface temperatures have risen at a factor of 3 greater rate than the global average, leading to the term Arctic Amplification of climate change. In this study, the enhanced warming of the Arctic, and the enhancement at the Arctic surface in comparison to the warming of the atmosphere aloft, is investigated through a reconstruction of the past surface energy balance by a model driven by downwelling irradiance reconstructed using radiosonde profiles and the radiative transfer code SBDART. The downwelling irradiance is shown to be increasing over the time-period of 1994-2009, and the sources of this increase are diagnosed. The time-evolution of the surface flux terms are discussed, and the sensitivity of the surface temperature to changes in atmospheric temperature is investigated.

We propose the use of a small unmanned aerial vehicle (UAV) equipped with temperature, pressure, and relative humidity sensors to estimate sensible and latent heat fluxes over an active agricultural area in east-central Mississippi. The Bowen ratio method is applied to vertical soundings from the surface to 120 meters at 10-meter intervals. A number of flights were conducted at Mississippi State University during the late stages of the growing season with the purpose of obtaining heat flux estimates over different land surface/cover types. Results show that the UAV platform is able to provide reasonable heat and moisture flux estimates, and that the fluxes show substantial variability among different land cover types over a small spatial scale. Future work must be done to quantify the diurnal and seasonal changes in heat flux estimates over various crop types and investigate flight plans and sensor mounting options to maximize sensor precision.

Ocean-atmosphere fluxes on the West Florida Shelf (WFS) coastal ocean region are investigated using observations and derived surface fluxes from an array of buoys deployed between 1998 and 2003. The observed annual cycle shows that water column temperatures increase and are stratified when heat flux is positive, and they decrease and are well mixed when it is negative. Water temperature is minimum (maximum) when heat flux switches sign from negative (positive) to positive (negative) in early spring (autumn). Tropical and extra-tropical events help define the seasonal characteristics of the water temperature. Despite considerable daily and synoptic variability in relative humidity, observations on the WFS show that the monthly mean values are nearly constant at about 75%. Winter relative humidity varies from less than 50% to over 100% (supersaturation values of up to 3% are recorded and coincide with fog on shore) as extra-tropical fronts move over the WFS. Sensor distribution shows small spatial variations in relative humidity in the coastal ocean environment that depends on high frequency variability in meteorological conditions and low-frequency variability in oceanic conditions. Comparisons with observations show that standard climatologies are unable to reproduce spatial variability on the WFS, especially in relative humidity and surface heat flux components that are dependent on sea surface temperature. Model experiments show that careful attention must be paid in calculating and applying surface heat fluxes. Observations and models are employed to assess the relative importance of surface fluxes and convergence of heat flux by the ocean circulation in controlling ocean temperature. In spring and autumn, seasonal change in water temperature is mainly controlled by surface heat flux with smaller contributions by ocean convergence, but synoptic scale variability is controlled by both surface heat flux and ocean circulation. Surface fluxes are of primary importance in determining water temperature during the passage of tropical storms or extra-tropical fronts. The coastal ocean temperature balance is fully three-dimensional. Models must be supported by adequate surface heat flux boundary conditions. These require sufficient numbers of in situ measurement points for constraining atmospheric models. The number of observations will depend on the spatial scales of SST variability.

This thesis examines a method for estimating the daytime fluxes of heat, water vapour and carbon dioxide at regional scales by using simple models to combine spatially resolved surface properties with bulk meteorological quantities measured at a central location. The central themes of this thesis are that the spatial and temporal variability of regional scale fluxes are contained in the surface properties and meteorology respectively and that the surface properties can be interpolated across a heterogeneous landscape using remotely sensed data. The regional scale fluxes estimated using this technique are compared to the values from three other methods and this allows some conclusions to be made regarding the relative strengths and weaknesses of each method. The surface property approach yields robust estimates of the fluxes that will be useful in researching exchange processes at regional scales, providing input parameters for, and validation of, the biosphere components of General Circulation Models and testing inventory estimates of CO2 budgets. The surface properties are derived using data from 33 aircraft flights and eight ground-based sites along a 96 km transect established during the 1995 Observations At Several Interacting Scales experiment held near Wagga Wagga, New South Wales, Australia. Surface properties examined are the evaporative fraction (ratio of evapotranspiration to available energy), the Bowen ratio (ratio of sensible heat flux to evapotranspiration), the maximum stomatal conductance (maximum stomatal opening under optimal conditions) and the water-use efficiency (ratio of CO2 flux to evapotranspiration). Maximum stomatal conductance is calculated using a simple model of the stomatal response to light and water vapour deficit assuming soil evaporation occurs at the equilibrium rate. The diurnal trend and day-to-day variability in the surface properties is found to be significantly less than the spatial variability. All of the surface properties examined show some sensitivity to the synoptic conditions. The relationships between the surface properties and the Normalised Difference Vegetation Index (NDVI) are examined using a 130 km by 50 km sub-scene from a Landsat 5 Thematic Mapper (TM) image obtained five days before the start of the experiment period. The ground-based and aircraft observations are used to calculate the source-area influencing each measurement and this is combined with the Landsat 5 TM data to produce an average, source-area weighted NDVI for each ground-based site and each aircraft location. The source-area model is important because it provides the link between the observations and the remotely sensed data by identifying the surface patch that influences the measurements. Linear relationships are found between the source-area weighted NDVI and the surface properties. The observed relationships are used to interpolate the surface properties over the region covered by the satellite image and spatial variations in water loss and CO2 uptake by the surface vegetation are identified that are not resolved by the ground-based network. Analysis of the ground-based data showed that the spatial variability of the bulk meteorological quantities used in the surface property approach was much less than the diurnal trend in these data. With the small temporal variation in the surface properties noted before, this confirms the utility of assigning the spatial and temporal variability of the fluxes to the surface properties and the meteorology respectively. The combination of surface properties derived from the aircraft data and meteorology measured at a single location at the centre of the transect shows good skill in predicting the observed fluxes. Furthermore, the discrepancies between the predictions and the observations are explained by the different source-areas of the aircraft and ground-based data and much of the bias is removed when the surface properties are scaled from the NDVI of the aircraft source-area to the NDVI of the ground-based sites. Regional scale fluxes of heat and water vapour calculated using the surface property approach agree with averages of the ground-based data and this indicates that the ground-based network was representative of the OASIS region. Estimates of regional scale CO2 fluxes are not available from the ground-based network due to the lack of measurements at the driest ground-based site but the surface property approach yields plausible values. The results demonstrate the utility of extrapolating surface properties across heterogeneous landscapes using remotely sensed data.

Water is essential to life on Earth. Since water exists in all three phases (solid, liquid, and gas) on Earth, it exists in various reservoirs throughout the planet that compose the hydrologic cycle, and its movement through these reservoirs requires energy. Thus, water is a key component of the energy balance of the Earth. Despite its importance, its representation in modeling and dataset generation is problematic. Here, the depiction of three phenomena, ocean surface turbulent fluxes, humidity inversions, and groundwater, are assessed, and suggestions for improvements of their representations are made. First, ocean surface turbulent fluxes, including those of moisture (latent heat flux), heat (sensible heat flux), and momentum (wind stress), from reanalysis, satellite-derived, and combined products which are commonly used to produce climatologies and to evaluate global climate models are compared to in situ observations from ship cruises to ascertain which products are the least problematic. The National Aeronautics and Space Administration’s reanalysis, the Modern Era Retrospective Analysis for Research and Applications, is the least problematic for all three fluxes, while a couple of others are the least problematic for only one of the three fluxes. Also, the product biases are disaggregated into uncertainties from the grid cell mean quantities, or bulk variables, used plus the residual uncertainties which includes the algorithm uncertainties due to the parameterization used to relate the small-scale turbulent processes to the large-scale bulk variables. The latter contribute the most to the majority of product latent heat fluxes, while both uncertainties can contribute the most to product sensible heat fluxes and wind stress. Thus, both algorithms and bulk variables need to be improved in ocean surface flux datasets. Second, humidity inversion climatologies in five reanalyses are evaluated. Humidity inversions, similar to its thermal counterpart, are layers in which specific humidity increases with height rather than the usual decrease with height. These are especially persistent in the polar regions in autumn and winter. However, Arctic inversions are the strongest in summer corresponding to the time of year that low cloud cover is the highest. Comparing the reanalysis inversions to radiosonde observations reveals some problems with the realization of humidity inversions in reanalyses including the misrepresentation of the diurnal cycle and of the overproduction of inversions in areas outside the polar regions. Finally, the simulation of groundwater in the Community Land Model (CLM) as used in the Community Earth System Model is made more realistic by including variable soil thickness. Because the bottom of the model soil column is placed at effectively bedrock, the unconfined aquifer model currently used in CLM is removed and a zero bottom water flux is put in place. The removal of the unconfined aquifer allows the simulation of groundwater to not be treated separately from soil moisture. The model is most affected where the number of soil layers is reduced from the original constant 10 layers and largely unaffected where the number of soil layers is increased except for baseflow where the mean annual range in rainfall is large.

In this study data from the regional climate model RCA1 is compared to measured data to investigate how good the model is during ice conditions and mainly how well the turbulent surface fluxes are described by the model. Comparisons of the sensible heat flux and the momentum flux as well as mean parameters are included. The measured data used are from the Bothnian Bay measured during the BASIS field experiment in February to March 1998. RCA1 (Rossby Centre regional Atmospheric model) is a regional climate model for Northern Europe based on HIRLAM and forced by ERA-40 data. Two different grid points of the RCA1 44 km grid have been chosen with geographical coordinates as close as possible to the two measuring sites. The first site is a small peninsula south of the town Umeå at the east coast of Sweden and the second one is the ship R/V Aranda anchored in the sea ice outside the Finnish west coast. The model presents generally too large negative (downward) sensible heat fluxes and too large momentum fluxes over ice. The largest difference between modelled and measured sensible heat fluxes are seen after warm front passages due to melting conditions. There are some uncertainties in the comparison of modelled fluxes and measured fluxes at Umeå due to a complex and varying ice cover around this measuring site. The vertical structure in the atmosphere has also been studied and modelled temperature, wind and humidity profiles were compared to radiosondes at the Umeå site. Two periods with on-ice flow and off-ice flow was analysed and one of the main differences was that for the on-ice situation an internal boundary layer was built up over the ice. Modelled profiles are generally smoother then measured profiles and inversions and other small-scale phenomena like low-level jets are almost never described correctly by the model but there are tendencies of the phenomena to occur. / Sammanfattning av ”En undersökning i hur bra den regionala klimatmodellen RCA1 beskriver olika parametrar över is, i huvudsak de turbulenta flödena”. I denna undersökning har den regionala klimatmodellen RCA1 jämförts med mätdata för att se hur bra modellen är över is och framförallt hur bra den beskriver de turbulenta flödena över is. RCA1 är en regional klimatmodell för norra Europa baserad på HIRLAM och som drivs av ERA-40 data. Modelldata har jämförts med mätdata från två platser i Bottenviken, en liten halvö utanför Umeå på den svenska östkusten och forskningsfartyget Aranda som var fastankrad i havsisen utanför Finlands västkust. Mätningarna gjordes under BASIS fältexperiment i februari till mars 1998. Modellen ger generellt för stora negativa (nedåtriktade) sensibla värmeflöden över is och likaså för stora impulsflöden över is. Den största skillnaden mellan modellvärden och mätningar förekom efter det att varmfronter passerat och detta beror delvis på smältförhållanden. I jämförelsen av de turbulenta flödena beräknade av RCA och de uppmäta turbulenta flödena från Umeå finns det vissa osäkerheter pga. den komplicerade issituationen runt denna mätplats med mycket varierande isförhållanden. Den vertikala strukturen i atmosfären har också undersökts och temperatur, vind och fuktighetsprofiler har jämförts. Två perioder varav en där det blåste från havet mot isen (on-ice) och en där det blåste från isen mot havet (off-ice) valdes ut och en av skillnaderna var att det bildades ett internt gränsskikt över isen i on-ice perioden. Modellen har en tendens att släta ut profilerna och småskaliga fenomen som inversionsskikt, inversionshöjder och low-level jets är nästan aldrig korrekt beskrivna i modellen men det finns tendenser till inversioner och vindmaximum.

Les milieux complexes sont une source d'incertitude importante notamment lorsqu'il s'agit de développer des modèles climatique ou météorologique. Le développement de la couche limite atmosphérique à l'intérieur d'une vallée encaissée, incluant des vents de vallée et de pente, n'est, par exemple, pas résolu, ce qui a un impact considérable sur la prévision de la convection, du transport de polluants, etc... La simulation des grandes échelles de la turbulence (SGE) est un outil qui a montré sa capacité à reproduire finement les structures turbulentes dans ce type de contextes au travers des approches idéalisées. L'extension de la SGE aux milieux naturels est réalisée dans cette thèse qui s'articule en trois parties. La première partie présente les équations et les notions nécessaires à la compréhension des problèmes de turbulence dans la couche limite atmosphérique. On s'attache à décrire le cadre des lois issues de la théorie des similitudes et le cadre de la turbulence optique. La seconde partie présente des résultats issus de deux simulations idéales dans lesquelles nous comparons les résultats issus de la simulation avec d'une part les lois issues de la théorie des similitudes et d'autre part les données radar disponible lors de la campagne IHOP. L'utilisation d'un maillage raffinée près du sol permet d'améliorer les profils verticaux des champs turbulents en améliorant la prédiction des gradients à l'interface sol/atmosphère. De plus, le profil de couche limite est bien reproduit par les profils du paramètre de structure des fluctuations de l'indice de réfraction de l'air simulé si on compare avec les résultats issus du radar. Globalement, la dynamique des champs turbulents résolus par le modèle reproduise une dynamique et des ordres de grandeurs corrects conformes à nos attentes. Une fois l'évaluation du modèle effectuée, une simulation a été mise en place autour de la campagne d'observation VOTALP située dans le sud de la Suisse. Cette simulation comprend 5 domaines emboités allant de 16 kms de résolution horizontales pour le plus grand domaine à 100m pour le plus petit domaine. L'important dispositif déployé durant la campagne VOTALP située dans une vallée dans le sud de la Suisse a permis de confronter les résultats issus de la modélisation à haute résolution avec ces données d'observation. Les résultats obtenus ont, entre autre, montré que la simulation à haute résolution est un outil adapté pour l'étude des phénomènes de basses couches et notamment la turbulence optique en milieu complexe. / Complex environments are an important source of uncertainty especially when it comes to developing climate models and weather. The development of the atmospheric boundary layer within a valley, including valley and slope winds is, for example, unresolved, which has a significant impact on the prediction of convection, of transport of pollutants, etc ... The large eddy simulation of turbulence (LES) is a tool that has demonstrated its ability to reproduce turbulent structures in such idealized contexts. The extension of the LES to natural environments is performed in this thesis divided into three parts. The first part presents the equations and the concepts necessary to understanding the problems of turbulence in the atmospheric boundary layer. It attempts to describe the laws for the similarity theories and the context of optical turbulence. The second part presents the results of two ideals simulations and we compare the results of the simulation with one hand the laws of the similarity theories and in other hand with radar data available in the IHOP campaign. The use of a refined mesh near the floor improves vertical profiles of turbulent fields improving prediction gradients at the interface soil / atmosphere. In addition, the boundary layer profile seems to be well reproduced by Cn2 profiles when compared with the results from the radar. Overall, the dynamics of turbulent fields solved are in good agreement with our expectations. Once the model evaluation performed, a simulation was set up around the measurement campaign VOTALP located in the south of Switzerland. This simulation includes five nested domains ranging from 16 km horizontal resolution for the largest to 100m for the smallest area. The important device deployed during the campaign VOTALP located in a valley in southern Switzerland has to confront the results of modeling the high-resolution observational data. The results obtained, among others, showed that the high-resolution simulation is a suitable tool for the study of the phenomena of lower layers including optical turbulence in complex environments.

<p>The boreal forests represent 8 % of all forested areas on the earth and have a significant role in the control of greenhouse gases and an impact on global climate change. The main objective of this thesis is to increase the understanding of how evaporation and heat-flux processes in the boreal forest zone are affecting the regional and global climate.</p><p>A meteorological mesoscale model with an advanced land-surface parameterization has been utilized to study aggregation of fluxes of water vapour and heat. The model has been compared against four other methods for flux estimation in a southern boreal landscape. The results show that the mesoscale model is successfully reproducing 24-hour averages of fractionally weighted mast measurements of sensible and latent heat flux.</p><p>The model was also evaluated against in-situ observations of surface fluxes and other meteorological variables. The results reveal that a correct initialization of soil moisture is crucial to simulate a realistic partitioning of the sensible and latent heat fluxes. Significant differences in surface fluxes and friction velocities between two apparently similar forest sites indicate the need for careful assessment of areal representativity when comparing mesoscale model results with in-situ observations.</p><p>A parameterization for the absorption of solar radiation of high-latitude sparse forests was implemented and tested in the model that significantly improved the simulation of high wintertime midday sensible heat fluxes. A scheme for heat storage in vegetation was also implemented which improved the results, but the scheme needs further evaluation for high latitude forests.</p><p>Two commonly used strategies for the description of land-surface heterogeneity, the effective parameter approach and the mosaic approach, were tested in the mesoscale model against airborne observations of sensible and latent heat fluxes. The results show that the mosaic approach produces better results especially when small lakes are present in model grid-squares.</p> / <p>Norra halvklotets barrskogsbälte representerar 8 % av all skogsbeklädd mark på jorden och har stor betydelse för kontrollen av växthusgaser och påverkan på globala klimatförändringar. Syftet med denna avhandling är att öka förståelsen av hur avdunstning och värmeflöden i den boreala skogszonen påverkar klimatet regionalt och globalt.</p><p>En meteorologisk mesoskalemodell med en avancerad landyteparameterisering har använts för att studera aggregering av avdunstning och värmeflöden. Modellen jämfördes med fyra andra metoder för uppskattning av värmeflöden i den boreala skogszonens södra delar. Resultaten visade att mesoskalemodellen reproducerar 24-timmarsmedelvärden av sensibelt och latent värmeflöde från areellt viktade mastmätningar med bra resultat.</p><p>Modellen utvärderades även mot markbaserade mätningar av sensibelt och latent värme och andra meteorologiska variabler. Resultaten visar att en korrekt initialisering av markvatteninnehållet är avgörande för att simulera en realistisk uppdelning av de sensibla och latenta värmeflödena. Markanta skillnader i markyteflöden och friktionshastigheter mellan två liknande skogsmätstationer påvisar nödvändigheten av en noggrann bedömning av den areella representativiteten när man jämför resultat från mesoskalemodellen med markbaserade mätningar.</p><p>En parameterisering för absorption av solstrålning i glesa skogsbestånd på höga breddgrader infördes och testades i modellen vilket markant förbättrade simuleringen av de höga sensibla värmeflöden som observerats vid middagstid på vintern. Ett uttryck för att beskriva värmelagring i vegetationen infördes också vilket förbättrade resultaten, men uttrycket behöver vidare utvärdering för skogsbestånd på höga breddgrader.</p><p>Två ofta använda strategier för att beskriva markytans heterogenitet, effektiva parametermetoden och mosaikmetoden, testades i mesoskalemodellen mot flygburna observationer av sensibla och latenta värmeflöden. Resultaten visar att mosaikmetoden ger bättre resultat särskilt när mindre sjöar förekommer i modellrutorna.</p>

The boreal forests represent 8 % of all forested areas on the earth and have a significant role in the control of greenhouse gases and an impact on global climate change. The main objective of this thesis is to increase the understanding of how evaporation and heat-flux processes in the boreal forest zone are affecting the regional and global climate. A meteorological mesoscale model with an advanced land-surface parameterization has been utilized to study aggregation of fluxes of water vapour and heat. The model has been compared against four other methods for flux estimation in a southern boreal landscape. The results show that the mesoscale model is successfully reproducing 24-hour averages of fractionally weighted mast measurements of sensible and latent heat flux. The model was also evaluated against in-situ observations of surface fluxes and other meteorological variables. The results reveal that a correct initialization of soil moisture is crucial to simulate a realistic partitioning of the sensible and latent heat fluxes. Significant differences in surface fluxes and friction velocities between two apparently similar forest sites indicate the need for careful assessment of areal representativity when comparing mesoscale model results with in-situ observations. A parameterization for the absorption of solar radiation of high-latitude sparse forests was implemented and tested in the model that significantly improved the simulation of high wintertime midday sensible heat fluxes. A scheme for heat storage in vegetation was also implemented which improved the results, but the scheme needs further evaluation for high latitude forests. Two commonly used strategies for the description of land-surface heterogeneity, the effective parameter approach and the mosaic approach, were tested in the mesoscale model against airborne observations of sensible and latent heat fluxes. The results show that the mosaic approach produces better results especially when small lakes are present in model grid-squares. / Norra halvklotets barrskogsbälte representerar 8 % av all skogsbeklädd mark på jorden och har stor betydelse för kontrollen av växthusgaser och påverkan på globala klimatförändringar. Syftet med denna avhandling är att öka förståelsen av hur avdunstning och värmeflöden i den boreala skogszonen påverkar klimatet regionalt och globalt. En meteorologisk mesoskalemodell med en avancerad landyteparameterisering har använts för att studera aggregering av avdunstning och värmeflöden. Modellen jämfördes med fyra andra metoder för uppskattning av värmeflöden i den boreala skogszonens södra delar. Resultaten visade att mesoskalemodellen reproducerar 24-timmarsmedelvärden av sensibelt och latent värmeflöde från areellt viktade mastmätningar med bra resultat. Modellen utvärderades även mot markbaserade mätningar av sensibelt och latent värme och andra meteorologiska variabler. Resultaten visar att en korrekt initialisering av markvatteninnehållet är avgörande för att simulera en realistisk uppdelning av de sensibla och latenta värmeflödena. Markanta skillnader i markyteflöden och friktionshastigheter mellan två liknande skogsmätstationer påvisar nödvändigheten av en noggrann bedömning av den areella representativiteten när man jämför resultat från mesoskalemodellen med markbaserade mätningar. En parameterisering för absorption av solstrålning i glesa skogsbestånd på höga breddgrader infördes och testades i modellen vilket markant förbättrade simuleringen av de höga sensibla värmeflöden som observerats vid middagstid på vintern. Ett uttryck för att beskriva värmelagring i vegetationen infördes också vilket förbättrade resultaten, men uttrycket behöver vidare utvärdering för skogsbestånd på höga breddgrader. Två ofta använda strategier för att beskriva markytans heterogenitet, effektiva parametermetoden och mosaikmetoden, testades i mesoskalemodellen mot flygburna observationer av sensibla och latenta värmeflöden. Resultaten visar att mosaikmetoden ger bättre resultat särskilt när mindre sjöar förekommer i modellrutorna.

Majority of the rain producing monsoon systems in India form or intensify over the Bay of Bengal and move onto the land. We expect the air-sea interaction to be a crucial factor in the frequent genesis and intensification of monsoon systems over the Bay. Knowledge of air-sea fluxes is essential in determining the air-sea interactions. However, the Bay remains a poorly monitored ocean basin and the state of the near surface conditions during the monsoon months remains to be studied in detail. For example, we do not know yet which among the various flux formulae used in the General circulation models are appropriate over the Bay since there are no direct measurements of surface fluxes here during the peak monsoon months. The present thesis aims towards filing that gap. In this thesis fluxes were computed using the Bulk method, Inertial dissipation method and direct covariance method. The flux comparisons were reasonable during certain flow conditions which are clearly identified. When these conditions are not met the differences among the fluxes from these methods can be larger than the inherent uncertainties' in the methods. Stratification, flow distortion and averaging time are the key variables that give rise to the differences in the fluxes. It is found that there are significant differences in the surface flux estimates computed from different atmospheric General Circulation Model bulk parameterization schemes. In this thesis, the flow gradients are estimated by taking advantage of the natural pitch and roll motion of the ship. A attempt is made to gain insight into the flow distortion and its influence on the fluxes. In our analysis it is found that the displacement of the streamlines is an important component in quantifying flow distortion.