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Shallow clouds in the trade-wind region over the North Atlantic contribute substantially to the global radiative budget. In the vicinity of the Caribbean island of Barbados, they appear in different mesoscale organization patterns with distinct net cloud radiative effects (CREs). Cloud formation processes in this region are typically controlled by the prevailing large-scale subsidence. However, occasionally weather systems from remote origin cause significant disturbances. This study investigates the complex cloud–circulation interactions during the field campaign EUREC4A (Elucidate the Couplings Between Clouds, Convection and Circulation) from 16 January to 20 February 2020, using a combination of Eulerian and Lagrangian diagnostics. Based on observations and ERA5 reanalyses, we identify the relevant processes and characterize the formation pathways of two moist anomalies above the Barbados Cloud Observatory (BCO), one in the lower troposphere (∼ 1000–650 hPa) and one in the middle troposphere (∼ 650–300 hPa). These moist anomalies are associated with strongly negative CRE values and with contrasting long-range transport processes from the extratropics and the tropics, respectively. The first case study about the low-level moist anomaly is characterized by an unusually thick cloud layer, high precipitation totals, and a strongly negative CRE. The formation of the low-level moist anomaly is connected to an extratropical dry intrusion (EDI) that interacts with a trailing cold front. A quasi-climatological (2010-2020) analysis reveals that EDIs lead to different conditions at the BCO depending on how they interact with the associated trailing cold front. Based on this climatology, we discuss the relevance of the strong large-scale forcing by EDIs for the low-cloud patterns near the BCO and the related CRE. The second case study about the mid-tropospheric moist anomaly is associated with an extended and persistent mixed-phase shelf cloud and the lowest daily CRE value observed during the campaign. The formation of the mid-level moist anomaly is linked to “tropical mid-level detrainment” (TMD), which refers to detrainment from tropical deep convection near the melting layer. The quasi-climatological analysis shows that TMDs consistently lead to mid-tropospheric moist anomalies over the BCO and that the detrainment height controls the magnitude of the anomaly. However, no systematic relationship was found between the amplitude of this mid-tropospheric moist anomaly and the CRE at the BCO. This is most likely due to the modulation of the CRE by above and below lying clouds and the fact that we used daily mean CREs, thereby ignoring the impact of the timing of the synoptic anomaly with respect to the daily cycle. Overall, this study reveals the important impact of the long-range moisture transport, driven by dynamical processes either in the extratropics or the tropics, on the variability of the vertical structure of moisture and clouds, and on the resulting CRE in the North Atlantic winter trades., Weather and Climate Dynamics, 3 (1), ISSN:2698-4016, ISSN:2698-4008

Shallow trade-wind cumulus clouds have been a research focus in the recent decade due to their uncertain response to climate change. While in some models the occurrence of these clouds is strongly reduced under future climate conditions, it remains nearly invariant in others. These contradictory results reflect our limited understanding of the mechanisms involved in the formation and spatial organisation of these clouds. Processes on different scales are at play: turbulent fluxes, convective mixing, large-scale subsidence, and the clouds themselves, which affect the thermodynamical conditions in their surroundings through diabatic processes (i.e., cloud radiative effects, warming through condensation, evaporative cooling). To shed light on the complex cloud-circulation coupling, the field campaign Elucidate the Couplings Between Clouds, Convection and Circulation (in short EUREC4A) was carried out in 2020 in the vicinity of the Caribbean island Barbados. The intensive observation period took place from 20 January to 20 February. The overarching goal of the campaign was to provide observational constraints for these low-level cloud processes and their interaction with atmospheric circulations. Already before EUREC4A, it was known that these clouds frequently organise themselves in different mesoscale patterns referred to as Sugar, Gravel, Flower, and Fish. These mesoscale cloud organisation patterns were readily adopted by the EUREC4A community to classify the cloudiness during the campaign. The campaign included a component called EUREC4Aiso, which operated a stable water isotopologue measurement network consisting of two aircrafts, four ships, and the Barbados Cloud Observatory (BCO), which was the major island-based observational site of EUREC4A. Stable water isotopologues (hereafter water isotopes or isotopes) are non-radioactive water molecules with different molecular structures and weights due to a heavy hydrogen (2H) or oxygen (18O) atom. Here, the isotopes 1H216O, 1H2H16O, and 1H218O were considered and the isotope parameter d2H and d-excess were used to assess isotope effects. The above-mentioned isotopes have different saturation vapour pressures and diffusion velocities, which means that their relative abundance in a given water sample changes during phase transition. The isotopic composition, thus, reflects the accumulated record of all moist atmospheric processes (e.g., evaporation, condensation and mixing) that the sample has experienced since evaporation from the ocean (which is the reference used for the normalisation of atmospheric isotope ratios). Thus, isotopes provide a link to the Lagrangian history of atmospheric water and have the ability to link processes on different scales. The measurements conducted by the author include measurements of water vapour isotopes in the lower troposphere onboard the French aircraft ATR-42 during 19 research flights that took place between 25 January and 13 February 2020, the continuous measurements of water vapour isotopes at the BCO from 13 January to 17 February 2020, and the collection of 42 rain samples at the BCO from 16 January to 18 February 2020. High consistency was found between the isotope measurements from the different EUREC4Aiso platforms except for an unexpected offset between the two instruments deployed for the vapour measurements at the BCO. Overall, the comparison of the isotope measurements from the different platforms showed that our measurements are of high quality and suitable for model evaluations and process-oriented analyses. The EUREC4A observations were complemented with ERA5 reanalyses and numerical simulations performed with the isotope-enabled, regional model COSMOiso in a setting with fully resolved convection. Applying a spectral nudging, nested simulations with spatial resolutions of 10 km, 5 km, 1km (referred to as COSMOiso,10km, COSMOiso,5km, COSMOiso,1km), were performed. The simulated cloud organisation patterns approximate the ones found in satellite images. Fish, Flower, and finer-structured (i.e., Gravel and Sugar) cloud patterns are clearly distinguishable in the simulations. However, cold pools, which are closely connected to the different patterns, are only evident in the COSMOiso,1km data. Precipitation is simulated in smaller regions compared to satellite observations. Cloud fraction as well as liquid water contents at cloud base are larger than in aircraft observations. These variables are strongly dependent on the spatial model resolution and generally, they approximate the observations better the higher the resolution. Contrastingly, temperature, humidity, and isotopes in vapour are nearly insensitive to the spatial model resolution. While their temporal variability (except for temperature) matches well with observations, deviations were found with respect to absolute values. The simulations are characterized by a low-level cold-dry bias associated with too strongly depleted vapour and an overly high d-excess. Despite these shortcomings, the high correlation between simulated and measured time series at the synoptic timescale showed that the simulations represent the large-scale forcing in an adequate way. Furthermore, a feature-based analysis of the isotope variability at cloud base showed that the key processes of the mesoscale moisture cycling are well captured by the model. The role of the large-scale circulation for the trade-wind region and its cloudiness was addressed using trajectories that reach 10 d (using ERA5 winds) or 6 d (with winds from COSMOiso,10km) backward in time. They showed that the air parcels arriving in the lower troposphere near Barbados typically take one of three pathways. Either they cross the North Atlantic at low latitudes with little vertical displacement (typical trade-wind flow) or they descend from high latitudes and altitudes within an extratropical dry intrusion in the rear of a Rossby wave breaking. If the Rossby wave breaking occurs over the eastern North Atlantic/Europe, the air parcels cross the North Atlantic (similarly to the typical trade-wind flow) after completing their descent. If the Rossby waver breaking is situated over the central/western North Atlantic, the air parcels arrive much more directly after their descent (extratropical dry intrusion flow). The different pathways are reflected in the subsidence rate of the trajectories, which is weakest for air parcels following a tropical trade-wind flow and strongest for air parcels travelling within an extratropical dry intrusion flow regime. A link could be established between the subsidence rate and the isotopic composition in the trade-wind region: the stronger the subsidence, the lower the d2H and the higher the d-excess of the vapour and precipitation. In other words, flow regime-specific processes lead to contrasting isotope signals and, therefore, isotopes can be used to identify changes in the large-scale circulation. The extratropical dry intrusion flow regime was shown to play a crucial role for the occurrence of the Fish cloud pattern. The dry intrusion air parcels either arrive together with the trailing cold front leading to moist and cloudy conditions or they arrive shortly afterwards in the cold sector leading to dry and clear-sky conditions. Independently of this, on a daily basis, the arrival of dry intrusion air parcels in Barbados generally leads to a radiative cooling through the shortwave effect of the Fish cloud and the longwave effect of the dryness in the free troposphere (caused by the strengthened large-scale subsidence within the dry intrusion). Another process that is induced by the large-scale circulation and alters the radiative balance, although not directly connected to shallow clouds, is the formation of an altostratus cloud layer in the mid troposphere. Such an event occurred towards the end of EUREC4A. The trajectory analysis showed that this mid-level cloud layer was formed due to detrainment of moist air parcels from tropical convection at the 0°C-isotherm and subsequent northward transport towards Barbados. The search for the origin of the isotopic contrasts between cloudy and clear-sky cloud base environments led to novel insights into the overturning circulation associated with clouds. A link was found between the isotopic characteristics of clear-sky environments and the altitude at which detrainment from shallow clouds takes place. Moreover, the comparison of the COSMOiso,1km data to a simple model simulating fractionation due to condensation in a moist adiabatically cooled air parcel during ascent revealed that vapour in COSMOiso,1km is enriched through evaporation in the lower part of the cloud layer and depleted through mixing with surrounding vapour in the upper part of the cloud layer. Finally, strong up- and downdrafts in clouds (assumed to represent shallow convective mixing) were shown to moisten and enrich the lower troposphere more than weak shallow convective mixing. The work carried out in the framework of this thesis clearly reveals the prominent role of the large-scale circulation for the trade-wind region and its cloudiness. Furthermore, it shows that isotopes reflect changes in the large-scale circulation and provide constraints for processes embedded in the overturning circulation associated with clouds. By adding a complementary perspective to conventional meteorological variables, isotopes substantially contribute to the understanding of atmospheric processes. Combining the isotope signal and the trajectory of an air parcel, one can identify couplings across large distances, which would be difficult to diagnose otherwise.

The science guiding the EUREC4A campaign and its measurements is presented. EUREC4A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic - eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC4A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air-sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC4A explored - from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation - are presented along with an overview of EUREC4A's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at 10.25326/165 , and a film documenting the campaign is provided as a video supplement. ISSN:1866-3516 ISSN:1866-3508

The decadal variability of weather and its extremes are still poorly understood. This is partly due to the scarcity of records, which, for many parts of the world, only allow for studies of 20th century weather. However, the 18th and early 19th centuries saw some pronounced climatic variations, with equally pronounced impacts on the environment and society. Considerable amounts of weather data are available even for that time but have not yet been digitised. Given recent progress in the quantitative reconstruction of subdaily weather, such data could form the basis of weather reconstructions. In Switzerland, measurements before 1864 (the start of the national network) have never been systematically compiled except for three prominent series (Geneva, Basel, Great St. Bernard Pass). Here we provide an overview of early instrumental meteorological measurements in Switzerland resulting from an archive survey. Our inventory encompasses 334 entries from 206 locations, providing an estimated 3640 station years and reaching back to the early 18th century. Most of the data sheets have been photographed and a considerable fraction is undergoing digitisation. This paper accompanies the online publication of the imaged data series and metadata. We provide a detailed inventory of the series, discuss their historical context, and provide the photographed data sheets. We demonstrate their usefulness on behalf of two historical cases and show how they complement the existing series in Europe. If similar searches in other countries yield similarly rich results, an extension of daily weather reconstructions for Europe back to the 1760s is possible.

We describe a dataset of recently digitised meteorological observations from 40 locations in today's Switzerland, covering the 18th and 19th centuries. Three fundamental variables – temperature, pressure, and precipitation – are provided in a standard format after they have been converted into modern units and quality-controlled. The raw data produced by the digitisation, often including additional variables and annotations, are also provided. Digitisation was performed by manually typing the data from photographs of the original sources, which were in most cases handwritten weather diaries. These observations will be important for studying past climate variability in Central Europe and in the Alps, although the general scarcity of metadata (e.g. detailed information on the instruments and their exposure) implies that some caution is required when using them. The data described in this paper can be found at https://doi.org/10.1594/PANGAEA.909141 (Brugnara, 2020).

We describe a dataset of recently digitised meteorological observations from 40 locations in today's Switzerland, covering the 18th and 19th centuries. Three fundamental variables – temperature, pressure, and precipitation – are provided in a standard format after they have been converted into modern units and quality-controlled. The raw data produced by the digitisation, often including additional variables and annotations, are also provided. Digitisation was performed by manually typing the data from photographs of the original sources, which were in most cases handwritten weather diaries. These observations will be important for studying past climate variability in Central Europe and in the Alps, although the general scarcity of metadata (e.g. detailed information on the instruments and their exposure) implies that some caution is required when using them. The data described in this paper can be found at https://doi.org/10.1594/PANGAEA.909141 (Brugnara, 2020)., Earth System Science Data, 12 (2), ISSN:1866-3516, ISSN:1866-3508

The interaction between low-level tropical clouds and the large-scale circulation is a key feedback element in our climate system, but our understanding of it is still fragmentary. In this paper, the role of upper-level extratropical dynamics for the development of contrasting shallow cumulus cloud patterns in the western North Atlantic trade wind region is investigated. Stable water isotopes are used as tracers for the origin of air parcels arriving in the sub-cloud layer above Barbados, measured continuously in water vapour at the Barbados Cloud Observatory during a 24 d measurement campaign (isoTrades, 25 January to 17 February 2018). These data are combined with a detailed air parcel back-trajectory analysis using hourly ERA5 reanalyses of the European Centre for Medium-Range Weather Forecasts. A climatological investigation of the 10 d air parcel history for January and February in the recent decade shows that 55 % of the air parcels arriving in the sub-cloud layer have spent at least 1 d in the extratropics (north of 35∘ N) before arriving in the eastern Caribbean at about 13∘ N. In 2018, this share of air parcels with extratropical origin was anomalously large, with 88 %. In two detailed case studies during the campaign, two flow regimes with distinct isotope signatures transporting extratropical air into the Caribbean are investigated. In both regimes, the air parcels descend from the lower part of the midlatitude jet stream towards the Equator, at the eastern edge of subtropical anticyclones, in the context of Rossby wave breaking events. The zonal location of the wave breaking and the surface anticyclone determine the dominant transport regime. The first regime represents the “typical” trade wind situation, with easterly winds bringing moist air from the eastern North Atlantic into the Caribbean, in a deep layer from the surface up to ∼600 hPa. The moisture source of the sub-cloud layer water vapour is located on average 2000 km upstream of Barbados. In this regime, Rossby wave breaking and the descent of air from the extratropics occur in the eastern North Atlantic, at about 33∘ W. The second regime is associated with air parcels descending slantwise by on average 300 hPa (6 d)−1 directly from the north-east, i.e. at about 50∘ W. These originally dry airstreams experience a more rapid moistening than typical trade wind air parcels when interacting with the subtropical oceanic boundary layer, with moisture sources being located on average 1350 km upstream to the north-east of Barbados. The descent of dry air in the second regime can be steered towards the Caribbean by the interplay of a persistent upper-level cut-off low over the central North Atlantic (about 45∘ W) and the associated surface cyclone underneath. The zonal location of Rossby wave breaking and, consequently, the pathway of extratropical air towards the Caribbean are shown to be relevant for the sub-cloud layer humidity and shallow-cumulus-cloud-cover properties of the North Atlantic winter trades. Overall, this study highlights the importance of extratropical dynamical processes for the tropical water cycle and reveals that these processes lead to a substantial modulation of stable water isotope signals in the near-surface humidity., Weather and Climate Dynamics, 2 (1), ISSN:2698-4016, ISSN:2698-4008

In 1970, the Institute of Geography of the University of Berne initiated the phenological observation network BernClim. Seasonality information from plants, fog and snow originally served for applications in urban and regional planning, agricultural and touristic suitability and are now a valuable data set for climate change impacts studies. Covering the growing season volunteer observers record the dates of key development stages of hazel (Coryllus avellana), dandelion (Taraxacum officinale), apple tree (Pyrus malus) and beech (Fagus sylvatica). All observations consist of detailed site information including location, altitude, exposition and inclination that make BernClim unique in detail-richness on decadal time-scales. Quality control (QC) by experts and statistical analyses of the data has been performed to flag impossible dates, dates outside the biologically plausible range, repeated dates in the same year, stretches of consecutive identical dates, and statistically inconsistent dates (outliers in time or in space). Here, we report BernClim data of 7414 plant phenological observations from 1970 to 2018 from 1304 sites at 110 stations, the QC procedure and selected applications (Rutishauser et al. 2019, doi: https://doi.org/10.1594/PANGAEA.900103). The QC points to a very good internal consistency (only 0.2% were flagged as internally inconsistent) and likely a high quality of the data. BernClim data indicate a trend towards an extended growing season. They also well track the regime shift in the late 1980s.