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Many atmospheric processes can be characterized by the rotation and divergence of the surface wind field. An accurate portrayal of these phenomena is very important for numerical weather prediction. This project focused on the global and regional model bias between the rotation and divergence of near-surface ocean wind observations, measured by satellite scatterometers, and numerical model fields of the European Center for Medium-range Weather Forecasting operational model (ECMWF-Ops) for the year 2022. The observations of two different scatterometer types are used over a total of five satellites: ASCAT (MetOp-B and -C) and HSCAT (HY-2B, -2C and -2D). On global and yearly time scales, modelled wind convergence is weaker than conver- gence found in the observations. Also, the bias in the wind rotation is minor, less than 10 % of the mean values. However, on some occasions, regional biases are significant. Turbulent island wakes are one of the features that show a significant bias in wind rotation, with an overestimation in strength for the modelled wake near Madagascar and a slightly different orientation for the modelled wakes of Hawaii, which are most likely caused by smoothing of the island topography in ECMWF-Ops. The gap wind in South Mexico, the Tehuano Wind, is well represented in ECMWF- Ops, both in the wind divergence and rotation, resulting in no significant biases. Yet, the gap winds near Hawaii show a positive bias in the divergence fields as well as the wind vector field, likely a result of the much smaller scale of the gaps with respect to the Tehuano Wind. The most significant bias in the wind rotation is located over the Gulf Stream in the Western North Atlantic. Here, ASCAT wind observations show a dual curl-band over the Gulf Stream, with negative rotations on the west-side and positive rotations on the east-side. These rotations, caused by the surface winds responding to large sea surface temperature gradients, are not seen in the model fields, res

Windstorm Poly, which developed overnight from 4th to 5th, 2023, was a major summer windstorm in the Netherlands, characterized by rapid intensification and severe wind gusts. The Royal Netherlands Meteorological Institute (KNMI) issued severe weather warnings, including code red, as the storm’s windfield intensified. The storm resulted in wind gusts of up to 146 km/h in IJmuiden and significant damage from Zandvoort to Alkmaar. This study investigates the development and unique characteristics of Storm Poly, focusing on the influence of record-high sea surface temperatures and atmospheric dynamics. We use the Regional Atmospheric Climate Model (RACMO), Harmonie and ERA5 reanalysis data to analyze Poly’s formation and evolution. Our findings suggest that the interaction between an upper-level trough and convective showers was the primary driver of the storm’s rapid intensification. The RACMO simulations demonstrate that while Sea Surface Temperatures (SSTs) variations impacted the storm’s characteristics, including wind speed, precipitation, and storm scale, they are not the dominant factor in Poly’s rapid cyclogenesis. The study also explores the hypothesis of a sting-jet phenomenon contributing to Poly’s severe wind gusts. However, analyses from ERA5 and the new cycle of HARMONIE (CY46) datasets showed no evidence of typical sting-jet dynamics. Instead, the strongest winds are attributed to surface processes and warm-core dynamics, indicative of a hybrid cyclone undergoing subtropical transition. This research highlights the complexity of storm dynamics and underscores the need for improved high-resolution modeling to understand and predict similar events. Future investigations should focus on past severe summer storms in the Netherlands to determine if Storm Poly is unique in its hybrid characteristics and consider the implications of climate change on the future frequency and intensity of such storms.

Change of land use may be an important tool in mitigating future climate extremes. Studies in the past have shown that deforestation in the tropics has led to a decrease in precipitation, but there is less evidence that such an effect exists in the Netherlands, on a smaller scale. In order to explore the possibilities of this, simulations with the regional climate model HCLIM from 2018-2022 are performed in which the input files for land use, ECOCLIMAP-SG, are manipulated. Within a square around the Dutch province Limburg, all vegetation types are set to either temperate deciduous broadleaf, temperate evergreen needleleaf, or kept the same. A resulting difference is more average precipitation in Limburg in winter in the forest experiments, which is explained by effects associated with the higher roughness length for trees in comparison to low vegetation. In addition, a case study of August 2019 shows higher precipitation values for two precipitation events in the forest simulations, compared to the control simulation. This difference may be explained by a higher sensible heat flux in the forest simulations in the days before these precipitation events. Furthermore, it is found that also the yearly average latent heat flux is smaller over Limburg in the forest experiments. Three additional simulations indicate that the chosen values for the minimal stomatal resistance in HCLIM may be partly, but cannot be solely responsible for the difference in latent heat flux.

Changes in ocean density influence ocean sea level and global ocean overturning circulation. Therefore, understanding ocean density changes is crucial to understand ocean circulation and its role in the climate system, by influencing heat and carbon uptake, storage and distribution. In this thesis, we use observational data of temperature and salinity to calculate global density changes and investigate its effects on ocean circulation and sea level change. We focus on the contributions related to mesoscale and small-scale mixing processes, where the former are separated in a horizontal component near the surface and an isoneutral component in the interior. We further distinguish between processes that only redistribute density and processes that have a net effect on global mean density in the ocean (Cabbeling and Thermobaricity). To relate density changes to the ocean circulation, we use the water mass transformation framework, which quantifies mass transport due to density changes. Analysing the different mixing processes, we can show that Cabbeling and Thermobaricity are important for the production of intermediate water along the Antarctic Circumpolar Current, the Gulf Stream and the Kuroshio, and for the production of Nord Atlantic Deep Water in the Atlantic and Antarctic Bottom Water in the Southern Ocean; where the effect of Thermobaricity is one order of magnitude smaller than the effect of Cabbeling. Further, we identify that horizontal mixing processes, redistributing density in the boundary layer, are strongly related to ocean surface currents. Redistribution by vertical mixing depends predominantly on vertical buoyancy gradients and bathymetry. When combining all terms, we find a resulting global mean sea level change of -2.4 mm/yr, which is of the same order of magnitude as the +3.7 mm/yr observed global mean sea level rise. This emphasises the importance of accurately representing ocean mixing, to understand ocean density change and its impact on circul

Tropical cyclones are both deadly and costly disasters, though their impact on Europe is infrequently studied due to relatively few occurrences. However, recent studies suggest the number and impact of tropical-origin storms affecting Europe is likely to increase with future warming. We present a case study of one recent Europe-impacting storm, 2017's Hurricane Ophelia, examining its representation in analysis datasets (ERA5, GFS, and the ECMWF Operational) as well as in simulations of these datasets downscaled with the regional model RACMO. The ECMWF-based models do not accurately simulate Ophelia, especially in the tropical phase where Ophelia's central pressure is overestimated by more than 30 hPa. Applying a uniform temperature forcing ranging between -2 to +4 $^\circ{}$C to the GFS-driven RACMO simulations allows us to model alternative climates. In warmer climates the storm grows larger and stronger than in the present-day scenario, moving faster and further from land. Despite this, the wind speeds experienced on the Irish Coast are higher than when it impacts the coast directly in the present-day scenarios. Additionally, its extratropical transition proceeds differently: in the warmest scenarios Ophelia does not complete extratropical transition, but continues to resemble a warm-core tropical cyclone.

Estimates of global mean near-surface air temperature (global SAT) for the Cenozoic era rely largely on paleo-proxy data of deep-sea temperature (DST), with the assumption that changes in global SAT covary with changes in the global mean deep-sea temperature (global DST) and global mean sea-surface temperature (global SST). We tested the validity of this assumption by analyzing the relationship between global SST, SAT, and DST using 25 different model simulations from the Deep-Time Model Intercomparison Project simulating the early Eocene Climatic Optimum (EECO) with varying CO2 levels. Similar to the modern situation, we find limited spatial variability in DST, indicating that local DST estimates can be regarded as a first order representative of global DST. In line with previously assumed relationships, linear regression analysis indicates that both global DST and SAT respond stronger to changes in atmospheric CO2 than global SST by a similar factor. Consequently, this model-based analysis validates the assumption that changes in global DST can be used to estimate changes in global SAT during the early Cenozoic. Paleo-proxy estimates of global DST, SST, and SAT during EECO show the best fit with model simulations with a 1,680 ppm atmospheric CO2 level. This matches paleo-proxies of EECO atmospheric CO2, indicating a good fit between models and proxy-data.

Changes in ocean density influence ocean sea level and global ocean overturning circulation. Therefore, understanding ocean density changes is crucial to understand ocean circulation and its role in the climate system, by influencing heat and carbon uptake, storage and distribution. In this thesis, we use observational data of temperature and salinity to calculate global density changes and investigate its effects on ocean circulation and sea level change. We focus on the contributions related to mesoscale and small-scale mixing processes, where the former are separated in a horizontal component near the surface and an isoneutral component in the interior. We further distinguish between processes that only redistribute density and processes that have a net effect on global mean density in the ocean (Cabbeling and Thermobaricity). To relate density changes to the ocean circulation, we use the water mass transformation framework, which quantifies mass transport due to density changes. Analysing the different mixing processes, we can show that Cabbeling and Thermobaricity are important for the production of intermediate water along the Antarctic Circumpolar Current, the Gulf Stream and the Kuroshio, and for the production of Nord Atlantic Deep Water in the Atlantic and Antarctic Bottom Water in the Southern Ocean; where the effect of Thermobaricity is one order of magnitude smaller than the effect of Cabbeling. Further, we identify that horizontal mixing processes, redistributing density in the boundary layer, are strongly related to ocean surface currents. Redistribution by vertical mixing depends predominantly on vertical buoyancy gradients and bathymetry. When combining all terms, we find a resulting global mean sea level change of -2.4 mm/yr, which is of the same order of magnitude as the +3.7 mm/yr observed global mean sea level rise. This emphasises the importance of accurately representing ocean mixing, to understand ocean density change and its impact on circul

Understanding surface wind divergence is crucial for multiple physical mechanisms such as convection and pressure adjustments. During the entire year 2022 five scatterometer instruments on MetOp-B, MetOp-C, HY-2B, HY-2C and HY-2D satellites were operational at the same time. Taking advantage of this opportunity, this thesis conducts an analysis of the differences in surface wind divergence between observations from the available satellites and the operational IFS model of ECMWF. In addition, comparison between observations of different scatterometers are performed. The analyses focus on the North Atlantic and the Tropical Atlantic regions, where two important phenomena for the surface wind divergence can be found: the Gulf Stream and the ITCZ. Overall, ASCAT observations show more variability and a higher proportion of extreme wind divergence values in comparison with HSCAT. This is particularly noticeable in the ITCZ and Gulf Stream regions, where convergence extremes are more prevalent. These study’s findings indicate that the operational ECMWF model exhibits a greater bias in wind divergence when compared to the ASCAT satellites (MetOp-B and MetOp-C) than with HSCAT satellites (HY-2B, HY-2C and HY-2D). Particularly, on the Tropical Atlantic, significant bias is found on the ITCZ area with ASCAT and HSCAT instruments for the winter and summer months of 2022. Furthermore, the results reveal more consistency between MetOp-B and MetOp-C observations over the Tropical Atlantic than over the North Atlantic. However, the model biases perceived by ASCAT and HSCAT satellites is greater over the Tropical Atlantic than over the North Atlantic. In consequence, it demonstrates that the operational ECMWF model presents a greater bias on the Tropical Atlantic.

Change of land use may be an important tool in mitigating future climate extremes. Studies in the past have shown that deforestation in the tropics has led to a decrease in precipitation, but there is less evidence that such an effect exists in the Netherlands, on a smaller scale. In order to explore the possibilities of this, simulations with the regional climate model HCLIM from 2018-2022 are performed in which the input files for land use, ECOCLIMAP-SG, are manipulated. Within a square around the Dutch province Limburg, all vegetation types are set to either temperate deciduous broadleaf, temperate evergreen needleleaf, or kept the same. A resulting difference is more average precipitation in Limburg in winter in the forest experiments, which is explained by effects associated with the higher roughness length for trees in comparison to low vegetation. In addition, a case study of August 2019 shows higher precipitation values for two precipitation events in the forest simulations, compared to the control simulation. This difference may be explained by a higher sensible heat flux in the forest simulations in the days before these precipitation events. Furthermore, it is found that also the yearly average latent heat flux is smaller over Limburg in the forest experiments. Three additional simulations indicate that the chosen values for the minimal stomatal resistance in HCLIM may be partly, but cannot be solely responsible for the difference in latent heat flux.

The onset of the rainy season is crucial for rain-fed farming in tropical countries. The timing of the onset can drastically influence the performance of the growing season, which subsequently impacts the food security of rural communities. Despite the high societal importance, the availability of operational forecast of the onset is limited, especially across the African continent. While there is little ambiguity about the overall behaviour of the West African Monsoon (WAM), there are several man-made definitions of its onset. Those definitions can address the onset both regionally and locally, the latter ones being the most commonly used. In this study we tackle the operational prediction of the WAM onset, focusing on Ghana and neighbouring countries. The regional approach is the main focus of both the climatological research and the development of the forecasting algorithm. However, the local perspective has been analysed too, representing the common practise for onset’s forecasting. Firstly, the correlation between the onset and atmospheric variables is investi- gated in the period 1981-2021, using satellite-based rainfall records (CHIRPS dataset) and ERA5 re-analysed wind fields at different pressure levels. Afterwards, the operational prediction of the onset is produced by post-processing operational weather forecasts, issued by the European Center for Medium-range Weather Forecasts (ECMWF). It is found that ECMWF rainfall forecasts (both medium-range and sub-seasonal) are the necessary ingredients of any prediction of the onset. Other atmospheric variables, such as wind fields at 925, 850, and 200 hPa, revealed to bring little to no advantage for the operational forecast of the onset, despite showing a clear correlation with the WAM in the climate. The best forecast performances are obtained with a threshold- based algorithm, which detects the onset imposing conditions on the amount and the temporal distribution of rainfall. Both regional and local onset’s fo

Soil moisture plays a crucial role in land-atmosphere interactions through its ability to divide the incoming radiation into latent heating and sensible heating. Accurate modelling of soil moisture could significantly improve numerical weather prediction, but observational data is difficult to obtain. In this research, the Tiled ECMWF Scheme for Surface Exchanges over Land (TESSEL) is coupled to Whiffle’s large eddy simulation model called GRASP and the behaviour of this system is explored. It is concluded that TESSEL is highly sensitive to initial conditions of soil moisture and soil parameters. In 6 months of model runs, rain-soil moisture coupling improves the model results through increased latent heating. Differentiation between soil types in Cabauw and Gilze-Rijen also increases the model skills, concluded from decreased biases and increased correlations with observations of surface fluxes, temperature, humidity and wind speed. This effect is strongest in summer and also influences wind speed and cloud formation. In a radiation fog case study, initialization with atmospheric observations produced a fog layer close to observations in depth, with the dissipation delayed by an hour. For this model set-up, it is recommended to permanently add differentiation between soil types, so that the initial soil moisture matches the soil parameters. Some improvements made to TESSEL should be added such as the conductivity formulation, the bare soil evaporation and the addition of surface run-off. In future research the behaviour of the system during heavy rainfall could be studied, as well as the influence of open boundary conditions and the use of other data sources for initialization.

The mid-Pliocene is the most recent geological period with a greenhouse gas concentration of approximately 400 ppmv, similar to the present day. Proxy reconstructions indicate enhanced warming in the high North Atlantic in the mid-Pliocene, which has been suggested to be a response to a stronger Atlantic Meridional Overturning Circulation (AMOC). Ensemble results from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) show a stronger AMOC and North Atlantic sea surface temperatures (SSTs) that match reconstructions better than PlioMIP1. A major difference between PlioMIP1 and PlioMIP2 is the closure of the Bering Strait and Canadian Archipelago in PlioMIP2. Previous studies have shown that closure of these Arctic gateways leads to an intensified AMOC due to altered freshwater fluxes in the Arctic. For this study, we compared results from our Community Earth System Model (CESM1) simulations, using two pre-industrial runs with a CO2 concentration of 280 and 560 ppmv and three mid-Pliocene runs with PlioMIP2 boundary conditions and a CO2 concentration of 280, 400 and 560 ppmv. Our analysis shows that the simulated intensified AMOC in the mid-Pliocene is a result of the palaeogeographic boundary conditions, in particular the closure of the Arctic gateways. In the mid-Pliocene simulations, the AMOC intensifies by 2.6 - 4.2 Sv as a result of a higher salinity in the Labrador Sea. The stronger AMOC is accompanied by enhanced Atlantic northward ocean heat transport, which is the cause for amplified SST warming in the high North Atlantic in the mid-Pliocene. Further analysis on the variability of the AMOC suggests that the simulated mid-Pliocene AMOC shows fundamentally different behavior from the pre-industrial AMOC, where it appears that the mid-Pliocene AMOC is driven more strongly by salinity and has a weakened temperature feedback. This is supported a high sensitivity of the mid-Pliocene AMOC to the surface freshwater flux in the Labrador Sea.

The topic of extreme weather in the mid-latitudes remains a subject of discussion. The response of Rossby waves (and the jetstream that tracks them) to a warming climate is very uncertain, while this large scale flow is known to be the driver for synoptic weather. A comprehensive theory on Rossby wave growth and breaking is hard to determine due to its nonlinear nature. This study advocates for the use of vertical velocity relative to isentropic surfaces to describe the conversion of available potential energy into kinetic energy. It is found that large areas of baroclinic energy conversion (BEC) can be found in the mid-latitudes in the middle as well as the upper troposphere. Intense regions of (BEC) can be found along the meridional moving jetstream in the vicinity of relative vorticity minima and maxima, meaning this could play an important role in amplifying Rossby waves. In an Eulerian frame it is found that regions of BEC coincide with jetstreaks. Results from the Lagrangian framework suggest that BEC might be the energy source of the formation or strengthening of jetstreaks.

While the early Eocene has been considered in many modelling studies, detailed simulations of the middle and late Eocene climate are currently scarce. To understand Antarctic glaciation at the Eocene-Oligocene Transition (~ 34 Ma) as well as middle Eocene warmth, it is vital to have an adequate reconstruction of the middle-to-late Eocene climate. Here, we present a set of high resolution coupled climate simulations using the Community Earth System Model (CESM) version 1. Two middle-to-late Eocene cases are considered with new detailed 38 Ma geographical boundary conditions with a different radiative forcing. With 4 × pre-industrial concentrations of CO2 (i.e. 1120 ppm) and CH4 (~ 2700 ppb), the equilibrium sea surface temperatures correspond well to available late middle Eocene (42–38 Ma) proxies. Being generally cooler, the simulated climate with 2 × pre-industrial values is a good analog for that of the late Eocene (38–34 Ma). Deep water formation occurs in the South Pacific Ocean, while the North Atlantic is strongly stratified and virtually stagnant. A shallow and weak circumpolar current is present in the Southern Ocean with only minor effects on southward oceanic heat transport within wind-driven gyres. Terrestrial temperature proxies, although limited in coverage, also indicate that the results presented here are realistic. The reconstructed 38 Ma climate has a reduced equator-to-pole temperature gradient and a more symmetric meridional heat distribution compared to the pre-industrial reference. Climate sensitivity is similar (~ 0.7 °C/Wm2) to that of the present-day climate (~ 0.8 °C/Wm2; 3 °C per CO2 doubling), with significant polar amplification despite very limited sea ice and snow cover. High latitudes are mainly kept warm by albedo and cloud feedbacks in combination with global changes in geography and the absence of polar ice sheets. The integrated effect of geography, vegetation and ice accounts for a 6–7 °C offset between pre-industrial and 38 Ma Eo