Lagrangian formation pathways of moist anomalies in the trade-wind region during the dry season: two case studies from EUREC4A

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

Water isotopic characterisation of the cloud–circulation coupling in the North Atlantic trades – Part 1: A process-oriented evaluation of COSMOiso simulations with EUREC4A observations

Naturally available, stable, and heavy water molecules such as HDO and HO have a lower saturation vapour pressure than the most abundant light water molecule HO; therefore, these heavy water molecules preferentially condense and rain out during cloud formation. Stable water isotope observations thus have the potential to provide information on cloud processes in the trade-wind region, in particular when combined with high-resolution model simulations. In order to evaluate this potential, nested COSMOiso (isotope-enabled Consortium for Small Scale Modelling; Steppeler et al., 2003; Pfahl et al., 2012) simulations with explicit convection and horizontal grid spacings of 10, 5, and 1 km were carried out in this study over the tropical Atlantic for the time period of the EUREC4A (Elucidating the role of clouds-circulation coupling in climate; Stevens et al., 2021) field experiment. The comparison to airborne in situ and remote sensing observations shows that the three simulations are able to distinguish between different mesoscale cloud organisation patterns as well as between periods with comparatively high and low rain rates. Cloud fraction and liquid water content show a better agreement with aircraft observations with higher spatial resolution, because they show strong spatial variations on the scale of a few kilometres. A low-level cold-dry bias, including too depleted vapour in the subcloud and cloud layer and too enriched vapour in the free troposphere, is found in all three simulations. Furthermore, the simulated secondary isotope variable d-excess in vapour is overestimated compared to observations. Special attention is given to the cloud base level, which is the formation altitude of shallow cumulus clouds. The temporal variability of the simulated isotope variables at cloud base agrees reasonably well with observations, with correlations of the flight-to-flight data as high as 0.7 for δ2H and d-excess. A close examination of isotopic characteristics under precipitating clouds, non-precipitating clouds, clear-sky and dry-warm patches at the altitude of cloud base shows that these different environments are represented faithfully in the model with similar frequencies of occurrence, isotope signals, and specific-humidity anomalies as found in the observations. Furthermore, it is shown that the δ2H of cloud base vapour at the hourly timescale is mainly controlled by mesoscale transport and not by local microphysical processes, while the d-excess is mainly controlled by large-scale drivers. Overall, this evaluation of COSMOiso, including the isotopic characterisation of different cloud base environments, suggests that the simulations can be used for investigating the role of atmospheric circulations on different scales for controlling the formation of shallow cumulus clouds in the trade-wind region, as will be done in part 2 of this study

Isotopic measurements in water vapor, precipitation, and seawater during EUREC4^4A

n early 2020, an international team set out to investigate trade-wind cumulus clouds and their coupling to the large-scale circulation through the field campaign EUREC$^4$A: ElUcidating the RolE of Clouds-Circulation Coupling in ClimAte. Focused on the western tropical Atlantic near Barbados, EUREC$^4$A deployed a number of innovative observational strategies, including a large network of water isotopic measurements collectively known as EUREC$^4$A-iso, to study the tropical shallow convective environment. The goal of the isotopic measurements was to elucidate processes that regulate the hydroclimate state – for example, by identifying moisture sources, quantifying mixing between atmospheric layers, characterizing the microphysics that influence the formation and persistence of clouds and precipitation, and providing an extra constraint in the evaluation of numerical simulations. During the field experiment, researchers deployed seven water vapor isotopic analyzers on two aircraft, on three ships, and at the Barbados Cloud Observatory (BCO). Precipitation was collected for isotopic analysis at the BCO and from aboard four ships. In addition, three ships collected seawater for isotopic analysis. All told, the in situ data span the period 5 January–22 February 2020 and cover the approximate area 6 to 16° N and 50 to 60° W, with water vapor isotope ratios measured from a few meters above sea level to the mid-free troposphere and seawater samples spanning the ocean surface to several kilometers depth. This paper describes the full EUREC$^4$A isotopic in situ data collection – providing extensive information about sampling strategies and data uncertainties – and also guides readers to complementary remotely sensed water vapor isotope ratios. All field data have been made publicly available even if they are affected by known biases, as is the case for high-altitude aircraft measurements, one of the two BCO ground-based water vapor time series, and select rain and seawater samples from the ships. Publication of these data reflects a desire to promote dialogue around improving water isotope measurement strategies for the future. The remaining, high-quality data create unprecedented opportunities to close water isotopic budgets and evaluate water fluxes and their influence on cloudiness in the trade-wind environment. The full list of dataset DOIs and notes on data quality flags are provided in Table 3 of Sect. 5 (“Data availability”)

Large-scale circulation drivers and stable water isotope characteristics of shallow clouds over the tropical North Atlantic

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

Early instrumental meteorological observations in Switzerland: 1708–1873

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)

Lagrangian formation pathways of moist anomalies in the trade-wind region during the dry season: two case studies from EUREC4A

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.ISSN:2698-4016ISSN:2698-400

An open-source radar-based hail damage model for buildings and cars

Severe hailstorms result in substantial damage to buildings and vehicles, necessitating the quantification of associated risks. Here, we present a novel open-source hail damage model for buildings and cars based on single-polarization radar data and 250 000 geolocated hail damage reports in Switzerland from 2002 to 2021. To this end, we conduct a detailed evaluation of different radar-based hail intensity measures at 1 km resolution and find that the maximum expected severe hail size (MESHS) outperforms the other measures, despite a considerable false-alarm ratio. Asset-specific hail damage impact functions for buildings and cars are calibrated based on MESHS and incorporated into the open-source risk modelling platform CLIMADA. The model successfully estimates the correct order of magnitude for the number of damaged building in 91 %, their total cost in 77 %, the number of damaged vehicles in 74 %, and their total cost in 60 % of over 100 considered large hail events. We found considerable uncertainties in hail damage estimates, which are largely attributable to limitations of radar-based hail detection. Therefore, we explore the usage of crowdsourced hail reports and find substantially improved spatial representation of severe hail for individual events. By highlighting the potential and limitations of radar-based hail size estimates, particularly MESHS, and the utilization of an open-source risk modelling platform, this study represents a significant step towards addressing the gap in risk quantification associated with severe hail events in Switzerland.ISSN:1561-8633ISSN:1684-998