A natural hazard is a naturally occurring extreme event that has a negative effect on people and society or the environment. Natural hazards may have severe implications for human life and can potentially generate economic losses and damage ecosystems. A better understanding of their major causes, probability of occurrence, and consequences enables society to be better prepared to save human lives as well as to invest in adaptation options. Natural hazards related to climate change are identified as one of the Grand Challenges in the Baltic Sea region. Here, we summarize existing knowledge about extreme events in the Baltic Sea region with a focus on the past 200 years as well as on future climate scenarios. The events considered here are the major hydro-meteorological events in the region and include wind storms, extreme waves, high and low sea levels, ice ridging, heavy precipitation, sea-effect snowfall, river floods, heat waves, ice seasons, and drought. We also address some ecological extremes and the implications of extreme events for society (phytoplankton blooms, forest fires, coastal flooding, offshore infrastructure, and shipping). Significant knowledge gaps are identified, including the response of large-scale atmospheric circulation to climate change and also concerning specific events, for example, the occurrence of marine heat waves and small-scale variability in precipitation. Suggestions for future research include the further development of high-resolution Earth system models and the potential use of methodologies for data analysis (statistical methods and machine learning). With respect to the expected impacts of climate change, changes are expected for sea level, extreme precipitation, heat waves and phytoplankton blooms (increase), and cold spells and severe ice winters (decrease). For some extremes (drying, river flooding, and extreme waves), the change depends on the area and time period studied.

The decay of turbulence kinetic energy (TKE) and its budget in the afternoon period from midday until zero-buoyancy flux at the surface is studied in a two-part paper by means of measurements from the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign for 10 intensive observation period days. Here, in Part 1, near-surface measurements from a small tower are used to estimate a TKE budget. The overall boundary layer characteristics and mesoscale situation at the site are also described based upon taller tower measurements, radiosoundings and remote sensing instrumentation. Analysis of the TKE budget during the afternoon transition reveals a variety of different surface layer dynamics in terms of TKE and TKE decay. This is largely attributed to variations in the 8 m wind speed, which is responsible for different amounts of near-surface shear production on different afternoons and variations within some of the afternoon periods. The partitioning of near-surface production into local dissipation and transport in neutral and unstably stratified conditions was investigated. Although variations exist both between and within afternoons, as a rule of thumb, our results suggest that about 50 % of the near-surface production of TKE is compensated for by local dissipation near the surface, leaving about 50 % available for transport. This result indicates that it is important to also consider TKE transport as a factor influencing the near-surface TKE decay rate, which in many earlier studies has mainly been linked with the production terms of TKE by buoyancy and wind shear. We also conclude that the TKE tendency is smaller than the other budget terms, indicating a quasi-stationary evolution of TKE in the afternoon transition. Even though the TKE tendency was observed to be small, a strong correlation to mean buoyancy production of −0.69 was found for the afternoon period. For comparison with previous results, the TKE budget terms are normalized with friction velocity and measurement height and discussed in the framework of Monin–Obukhov similarity theory. Empirically fitted expressions are presented. Alternatively, we also suggest a non-local parametrization of dissipation using a TKE–length scale model which takes into account the boundary layer depth in addition to distance above the ground. The non-local formulation is shown to give a better description of dissipation compared to a local parametrization.

The marine boundary layer is known to be influenced by fast long ocean swell waves travelling away from their generation area, where they were initiated by momentum transferred to the ocean wave field during storms. The atmospheric boundary layer during wind‐following swell and various stability states has been investigated using large‐eddy simulation (LES) data. The dominant energy‐containing motions in the near‐neutral atmospheric boundary layer over flat terrain are known to be dominated by near‐ground shear‐induced regions of high‐ and low‐speed flow. Wind fields and momentum fluxes from LES for swell‐dominated situations have been used to interpret field measurements suggesting that these motions are disrupted by effects related to the underlying wave field in the presence of swell waves. Statistical analysis and visualization are used to further describe the effects of stratification during swell for convective boundary‐layer winds and fluxes. A mechanism for transport of momentum to the upper levels of the boundary layer is suggested from interpretation of LES data. Coherent detached eddies from the directly wave‐induced motions near the surface are found to maintain an upward momentum transfer. This mechanism is found to strengthen during stronger swell conditions and also during slightly convective conditions. In this way, it is argued that processes related to both the wave field and surface convection can have a significant influence on the global structure of neutral and convective boundary layers during swell. This has implication for the turbulence length‐scales during wind‐following swell. Copyright © 2012 Royal Meteorological Society

Air‐sea exchange of gaseous elemental mercury (Hg⁰) is not well constrained, even though it is a major component of the global Hg cycle. Lack of Hg⁰ flux measurements to validate parameterizations of the Hg⁰ transfer velocity contributes to this uncertainty. We measured the Hg⁰ flux on the Baltic Sea coast using micrometeorological methods (gradient‐based and relaxed eddy accumulation [REA]) and also simulated the flux with a gas exchange model. The coastal waters were typically supersaturated with Hg⁰ (mean ± 1σ = 13.5 ± 3.5 ng m⁻³; ca. 10% of total Hg) compared to the atmosphere (1.3 ± 0.2 ng m⁻³). The Hg⁰ flux calculated using the gas exchange model ranged from 0.1–1.3 ng m⁻² h⁻¹ (10th and 90th percentile) over the course of the campaign (May 10–June 20, 2017) and showed a distinct diel fluctuation. The mean coastal Hg⁰ fluxes determined with the two gradient‐based approaches and REA were 0.3, 0.5, and 0.6 ng m⁻² h⁻¹, respectively. In contrast, the mean open sea Hg⁰ flux measured with REA was larger (6.3 ng m⁻² h⁻¹). The open sea Hg⁰ flux indicated a stronger wind speed dependence for the Hg⁰ transfer velocity compared to commonly used parameterizations. Although based on a limited data set, we suggest that the wind speed dependence of the Hg⁰ transfer velocity is more consistent with gases that have less water solubility than CO₂ (e.g., O₂). These pioneering flux measurements using micrometeorological techniques show that more such measurements would improve our understanding of air‐sea Hg exchange.

A night-time turbulence regime classification, the so-called “HOckey-Stick Transition ” (HOST) theory, proposed by Sun et al. (2012) from the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) is explored using data from the Boundary-Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign which took place during summer 2011 in the central French Pyrenean foothills. Results show that the HOST turbulence relationships for the BLLAST field campaign data are strongly dependent on both the meteorological and orographic features. The HOST pattern only appears for nights when a stably stratified boundary layer can be developed, corresponding to fair-weather and clear-sky nights, when the flow is generated by the nearby orography, from the south and the south-east. Those flows strongly influenced by the orography may generate intermittent or enhanced turbulence. When considering the whole nocturnal dataset for these flow directions, several enhanced turbulence points are found to be associated with sudden wind speed and directional shear transitions. In contrast, flows from other directions do not reproduce the HOST relationships and the turbulence relationship is almost linear, independent of vertical temperature gradients, corresponding to flows driven by synoptic scales. In addition we identify examples of gravity waves and top-down turbulent events that lead to transitions between the turbulence regimes.

This study evaluates the ability of three operational models, with resolution varying from 2.5 to 16 km, to predict the boundary-layer turbulent processes and mesoscale variability observed during the Boundary Layer Late-Afternoon and Sunset Turbulence (BLLAST) field campaign. We analyse the representation of the vertical profiles of temperature and humidity and the time evolution of near-surface atmospheric variables and the radiative and turbulent fluxes over a total of 12 intensive observing periods (IOPs), each lasting 24 h. Special attention is paid to the evolution of the turbulent kinetic energy (TKE), which was sampled by a combination of independent instruments. For the first time, this variable, a central one in the turbulence scheme used in AROME and ARPEGE, is evaluated with observations.In general, the 24 h forecasts succeed in reproducing the variability from one day to another in terms of cloud cover, temperature and boundary-layer depth. However, they exhibit some systematic biases, in particular a cold bias within the daytime boundary layer for all models. An overestimation of the sensible heat flux is noted for two points in ARPEGE and is found to be partly related to an inaccurate simplification of surface characteristics. AROME shows a moist bias within the daytime boundary layer, which is consistent with overestimated latent heat fluxes. ECMWF presents a dry bias at 2 m above the surface and also overestimates the sensible heat flux. The high-resolution model AROME resolves the vertical structures better, in particular the strong daytime inversion and the thin evening stable boundary layer. This model is also able to capture some specific observed features, such as the orographically driven subsidence and a well-defined maximum that arises during the evening of the water vapour mixing ratio in the upper part of the residual layer due to fine-scale advection. The model reproduces the order of magnitude of spatial variability observed at mesoscale (a few tens of kilometres). AROME provides a good simulation of the diurnal variability of the turbulent kinetic energy, while ARPEGE shows the right order of magnitude.