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Ice clouds in the Arctic are expected to have different radiative properties compared to mid latitude cirrus, because of the different humidity and temperature profile and also the prevalent aerosol loading in northern latitudes which govern their formation. During the late winter and early spring 2022 the HALO-(AC)3 campaign was conducted out of Kiruna (Sweden) to probe artic clouds with an airborne remote sensing payload. For this purpose, the German research aircraft HALO was equipped with a water vapor Differential Absorption Lidar (DIAL), a cloud radar, micro-wave radiometers, radiation measurements in the visible, near infrared and thermal region and a drop-sonde dispenser. A total of 25 flights where performed mainly over the sea between Svalbard and Greenland and up to nearly 90°N.The primary observable to study ice cloud formation is the relative humidity, which is not directly measurable by lidar, but can only be computed with the aid of additional temperature information. By comparison with a large number of dropsondes launched during flight, we will show that the temperature field from ECMWF IFS analyses and short-term forecasts provides sufficient accuracy to retrieve the relative humidity for ice cloud studies. Using this method we will analyse different scenarios of arctic cirrus formation: under stable artic conditions, during a warm air intrusion and while a cold air outbreak. An interesting special case is the modification of cirrus properties by the presence of an aerosol layer which is most probably composed of long range transported Sharan dust.

Mineral dust is one of the major contributors to the global aerosol load with the Sahara being its largest source. Dust particles can be transported over many days and thousands of kilometers. The main transport route spans from Africa over the Atlantic Ocean towards the Caribbean. Most of the time dust-transport takes place in the so-called Saharan Air Layer (SAL). During its transport the SAL affects the Earth’s atmosphere by scattering and absorption of solar and terrestrial radiation, and by changing cloud evolution and cloud properties. The main season for the transatlantic dust transport is during the boreal summer months. However, dust can be transported towards the Caribbean also during wintertime, although this happens with less frequency.Airborne lidar measurements with the combined water vapor differential absorption and high spectral resolution lidar system WALES provide simultaneous measurements of the water vapor mixing ratio and of aerosol properties. We use the measurements during the NARVAL-II experiment in August 2016 and during the EUREC4A experiment in January/February 2020 to characterize the long-range transported SAL in summer- and in wintertime, and to investigate its radiative effect and its impact on the subjacent shallow marine trade wind convection. We found, that a small amount of water vapor embedded in the SAL has a strong impact on the radiative heating effect of this layer and consequently also on the atmosphere’s stability. During summertime, when the SAL is well separated from the marine boundary layer, the radiative effect of the SAL dominates. The evolution of shallow marine clouds below the SAL is suppressed. In wintertime, the SAL is transported at lower altitudes and the dust layer is frequently mixed into the marine boundary layer. During this time of the year the effect of the SAL on the evolution and lifetime of marine trade wind convection is much more complex, as the dust particles within the SAL might additionally act as cloud or ice nuclei.In our presentation we will give an overview of the performed measurements and the radiative transfer calculations. We will present the radiative effects of the separated summertime SAL, and show first results of the impact of the wintertime SAL on the atmosphere’s stability and cloud properties.

In the last decades scientist have noticed that the average global temperature of the Earth has been increasing. Moreover, the arctic is warming significantly faster than the global average, a phenomenon labeled Arctic Amplification. Two atmospheric components contributing to the warming of the atmosphere in the arctic are water vapor and cirrus clouds. Both have an effect on the radiation budget of the atmosphere and more specifically the longwave radiation. A Warm Air Intrusion (WAI) event is defined as the meridional transport of warm, water-vapor-rich airmasses into the arctic. During such events large amounts of water vapor can be transported into the arctic, which also leads to high supersaturations aiding the formation and longevity of cirrus clouds. There is a strong hypothesis that WAI events in the high arctic are becoming more frequent, so it is important to study the effects these events have on the macrophysical and optical properties of cirrus clouds in the arctic.The HALO-(AC)3 flight campaign was conducted in March/April 2022 with the central goal of studying WAI events in the arctic regions of the Northern Hemisphere. For this campaign the German research aircraft HALO was equipped with remote sensing instrumentation, including the airborne LIDAR system WALES which we use in this study. WALES is a combined water vapor differential absorption and high spectral resolution lidar. It provides water vapor measurements in a 2D field along the flight track. We combine these measurements with ECMWF temperature data and calculate the Relative Humidity with respect to ice (RHi) inside and in the vicinity of cirrus clouds. For each flight we studied the synoptic situation and created two groups: One containing flights were cirrus that formed in arctic airmasses were measured and another were cirrus were measured during WAI events, henceforth arctic cirrus and WAI cirrus respectively. Our main goal is to compare the humidity characteristics inside and in the vicinity of arctic cirrus clouds and WAI cirrus clouds.For the arctic cirrus we find that 49 % of the in-cloud data points are supersaturated with RHi mostly below the lower threshold for heterogeneous nucleation (low HET). The cloud-free air around these clouds has a supersaturation percentage of 8.5 %. The WAI cirrus are measured in a wider temperature range and also have a significantly higher supersaturation percentage inside as well as in the cloud-free air, 61.7 % and 9.3 % respectively. The majority is again in the low HET regime. Additionally, WAI cirrus are on average geometrically thicker than arctic cirrus. Finally, regarding the vertical distribution of RHi within these clouds we find that WAI cirrus have their highest supersaturations near the cloud top and become gradually subsaturated towards cloud-bottom. On the other hand, arctic cirrus have their highest supersaturations near cloud-middle, with lower supersaturations at cloud-top and subsaturated cloud-bottom.

The CoMet 2.0 Arctic airborne greenhouse gas measurement campaign took place over Canada in Summer of 2022. For the campaign, the German research aircraft HALO has been equipped with various instruments for remote-sensing and in-situ measurements of CO2 and CH4 and flown over target areas with potential sources of greenhouse gases, either natural (wetlands, thawing permafrost, etc.) or anthropogenic (oil and gas drilling sites, oil-sand mining, open-pit coal mines, landfills as well as biomass-burning in forest fires). With the city of Edmonton, Alberta as campaign base, a variety of sources of methane released due to human activity and adding substantially to the Canadian anthropogenic CH4 budget were conveniently within reach for our measurements.This presentation focuses on a selection of anthropogenic sources of CH4 in Canada as well as the Valdemingómez and Pinto landfill sites near Madrid, which were targeted during a test flight. We show first results of the evaluation of active remote-sensing measurements that were conducted with DLR's CHARM-F lidar system. By using the Integrated-Path Differential-Absorption (IPDA)-lidar technique, CHARM-F enables measurements of total column concentrations of methane and carbon dioxide along flight tracks. After further adding wind information from auxiliary measurements or models, emission fluxes from localized sources can be estimated. We will highlight the top emitters in terms of estimated emission rate of CH4 (in the 10kt/year range). Those are likely the most promising candidates for mitigation attempts.

The distribution of H2O and O3 in the midlatitude UTLS is of key relevance for the Earth’s weather and climate. Tropospheric and stratospheric dynamical processes, acting on different time-scales, interact with chemistry and determine the composition of the UTLS and the extratropical transition layer (ExTL) therein. In this study, we investigate how strongly the fine-scale trace gas distribution in the UTLS/ExTL is related to interacting, tropospheric weather systems on synoptic time scales, which shape transport and mixing.We present range-resolved, collocated lidar H2O and O3 measurements from a research flight during the Wave-driven ISentropic Exchange (WISE) campaign, which was conducted across a jet stream located over the eastern North Atlantic on 1 October 2017. The observations are combined with 10-day backward trajectories along which meteorological parameters and turbulence diagnostics are traced. The derived transport and mixing characteristics are projected to the vertical cross sections of the lidar measurements and to the H2O–O3 phase space (Tracer-Tracer space) to explore linkages with the evolution of synoptic-scale weather systems and their interaction.We find that the formation of H2O and O3 filaments in the troposphere and stratosphere, the high variability of tropospheric H2O and the formation of the ExTL mixing layer can, to a large extent, be explained by transport and mixing associated with interacting tropical, midlatitude and arctic weather systems in the region of the jet stream on synoptic time scales. The mixed ExTL air exhibits a strong influence of turbulent mixing in the jet stream during the two days before the flight. The diagnosed non-local and transient character of mixing points to the complexity of the formation and interpretation of mixing lines in T–T space. The presented work is accepted for publication in ACP:Schäfler, A., Sprenger, M., Wernli, H., Fix, A., and Wirth, M.: Case study on the influence of synoptic-scale processes on the paired H2O-O3 distribution in the UTLS across a North Atlantic jet stream, Atmos. Chem. Phys. Discuss. [preprint], https://doi.org/10.5194/acp-2022-692, accepted, 2022.

During a research flight of the Wave-driven ISentropic Exchange (WISE) campaign, which was conducted over the eastern North Atlantic on 1 October 2017, the composition of the upper troposphere and lower stratosphere (UTLS) across the North Atlantic jet stream was observed by airborne, range-resolved differential absorption lidar (DIAL) profiles. We investigate how the high variability in the paired H2O and O3 distribution along the two-dimensional lidar cross section is affected by synoptic-scale weather systems, as revealed by the Lagrangian history of the observed air masses. To this aim, the lidar observations are combined with 10 d backward trajectories along which meteorological parameters and derived turbulence diagnostics are traced. The transport and mixing characteristics are then projected to the vertical cross sections of the lidar measurements and to the H2O-O3 phase space to explore linkages with the evolution of synoptic-scale weather systems and their interaction. Tropical, midlatitude, and arctic weather systems in the region of the jet stream and the related transport and mixing explain the complex H2O and O3 distribution to a large extent: O3-rich stratospheric air from the high Arctic interacts with midlatitude air from the North Pacific in a northward-deflected jet stream associated with an anticyclone over the US and forms a filament extending into the tropopause fold beneath the jet stream. In the troposphere, lifting related to convection in the intertropical convergence zone (ITCZ) and two tropical cyclones that continuously injected H2O into dry descending air from the tropical Atlantic and Pacific form filamentary H2O structures. One tropical cyclone that transitioned into a midlatitude cyclone lifted moist boundary layer air, explaining the highest tropospheric H2O values. During the two days before the observations, the air with mixed tropospheric and stratospheric characteristics experienced frequent turbulence along the North Atlantic jet stream, indicating a strong influence of turbulence on the formation of the extratropical transition layer (ExTL). This investigation highlights the complexity of stirring and mixing processes and their close connection to interacting tropospheric weather systems from the tropics to the polar regions, which strongly influenced the observed fine-scale H2O and O3 distributions. The identified non-local character of mixing should be kept in mind when interpreting mixing lines in tracer-tracer phase space diagrams., Atmospheric Chemistry and Physics, 23 (2), ISSN:1680-7375, ISSN:1680-7367

As part of the EUREC4A (Elucidating the role of cloud–circulation coupling in climate) field campaign, the German research aircraft HALO (High Altitude and Long Range Research Aircraft), configured as a cloud observatory, conducted 15 research flights in the trade-wind region east of Barbados in January and February 2020. Narrative text, aircraft state data, and metadata describing HALO's operation during the campaign are provided. Each HALO research flight is segmented by timestamp intervals into standard elements to aid the consistent analysis of the flight data. Photographs from HALO's cabin and animated satellite images synchronized with flight tracks are provided to visually document flight conditions. As a comprehensive product from the remote sensing observations, a multi-sensor cloud mask product is derived and quantifies the incidence of clouds observed during the flights. In addition, to lower the threshold for new users of HALO's data, a collection of use cases is compiled into an online book, How to EUREC4A, included as an asset with this paper. This online book provides easy access to most of EUREC4A's HALO data through an intake catalogue. Code and data are freely available at the locations specified in Table 6.

Contrail cirrus constitute the largest radiative forcing (RF) component of the aviation effect on climate. However, the difference of microphysical properties and radiative effects between contrails, contrail cirrus and natural cirrus clouds are still not completely resolved. Motivated by these uncertainties, we investigate the cirrus perturbed by aviation in the North Atlantic Region on 26 March 2014 during the Mid Latitude Cirrus (ML-CIRRUS) experiment. In the synoptic context of a ridge cirrus cloud, an extended thin ice cloud with many persistent contrails can be observed for many hours with the geostationary Meteosat Second Generation (MSG)/Spinning Enhanced Visible and InfraRed Imager (SEVIRI) from the morning hours until dissipation close to 14 UTC. Airborne lidar observations aboard the German High Altitude and LOng Range Research Aircraft (HALO) suggest that this cloud is mainly of anthropogenic origin. We develop a new method to distinguish between contrails, contrail cirrus and natural cirrus based on in situ measurements of ice number and NO gas concentrations. It turns out that effective radii (Reff) of contrail cirrus and contrails are in the range of 3 to 53 µm and about 18 % smaller than that of natural cirrus, hence a difference in Reff is still present. Ice particle sizes in contrail cirrus are on average 114 % larger than in contrails. The optical thickness of natural cirrus, contrail cirrus and contrails derived from satellite data has similar distributions with average values of 0.21, 0.24 and 0.15 for these three cloud types, respectively. As for radiative effects, a new method to estimate top-of-atmosphere instantaneous RF in the solar and thermal range is developed based on radiative transfer model simulations exploiting in situ and lidar measurements, satellite observations and ERA5 reanalysis data for both cirrus and cirrus-free regions. Broadband irradiances estimated from our simulations compare well with satellite observations from MSG and the Geostationary Earth Radiation Budget (GERB), indicating that our method provides a good representation of the real atmosphere and can thus be used to determine RF of ice clouds probed during this flight. Contrails net RF is smaller by a factor of 4 compared to contrail cirrus. On average, the net RF of contrails and contrail cirrus is more strongly warming than that of natural cirrus. For a larger spatial area around the flight path, the RF is well related to that along the flight track. We find warming contrail cirrus and cirrus in the early morning and cooling contrail cirrus and cirrus during the day. The results will be valuable for research to constrain uncertainties in the assessment of climate impacts of natural cirrus and contrail cirrus and for the formulation and evaluation of contrail mitigation options.

We develop a new method to describe the total cloud cover including optically thin clouds in trade wind cumulus cloud fields. Climate models as well as Large Eddy Simulations commonly underestimate the cloud cover, while estimates from observations largely disagree on the cloud cover in the trades. Currently, trade wind clouds contribute significantly to the uncertainty in climate sensitivity estimates derived from model perturbation studies. To simulate clouds well and especially how they change in a future climate we have to know how cloudy it is. In this study we develop a method to quantify the cloud cover from a clear-sky perspective. Using well-known radiative transfer relations we retrieve the clear-sky contribution in high-resolution satellite observations of trade cumulus cloud fields during EUREC4A. Knowing the clear-sky part, we can investigate the remaining cloud-related contributions consisting of areas detected by common cloud masking algorithms and those undetected areas related to optically thin clouds. We find that the cloud-mask cloud cover underestimates the total cloud cover by a factor of 2. Lidar measurements on board the HALO aircraft support our findings by showing a high abundance of optically thin clouds during EUREC4A. Mixing the undetected optically thin clouds into the clear-sky signal can cause an underestimation of the cloud radiative effect of up to −32 %. We further discuss possible artificial correlations in aersol-cloud cover interaction studies that might arise from undetected optically thin clouds. Our analysis suggests that the known underestimation of trade wind cloud cover and simultaneous overestiamtion of cloud brightness in models is even higher than assumed so far.

Cirrus clouds have a large impact on the Earth’s climate system. Overall this impact is positive, but depending on their macrophysical and optical properties, the effect of single clouds can be quite different. Thus, cirrus clouds still introduce large uncertainties in climate change predictions. To gain better knowledge of the impact of cirrus clouds, it is of importance to study their macrophysical and optical properties and their dependence on formation processes, environmental conditions, and their evolution with time.To improve our knowledge, the ML-Cirrus mission took place in March/April 2014. Research flights with the German research aircraft HALO; equipped with remote sensing and in-situ measurements, were performed over Central Europe and over the Northeast Atlantic Ocean. In this study we use measurements taken from the airborne LIDAR system WALES, which is a combined water vapor differential absorption and high spectral resolution lidar. Our main focus is on the humidity distribution within cirrus clouds and in the cloud-free air in their vicinity. For that we use Relative Humidity with respect to ice (RHi), calculated form the WALES water vapor measurements in a 2D field along the flight track together with ECMWF temperature data interpolated to the same grid. We identify cirrus clouds using the following criteria: a) backscatter ratio >= 3 b) linear depolarization ratio >= 20% and c) temperature < 235 K. We further split the cirrus clouds into two main categories according to their formation process: a) in-situ formed clouds and b) liquid-origin clouds. Overall, we find that, 34.1% of in-cloud data points are supersaturated with respect to ice. Supersaturation is also detected in 6.8% of the cloud-free data points. Regarding their vertical structure, most clouds have higher supersaturations close to cloud-top and become subsaturated near the cloud bottom. When the probability densities of RHi are calculated with respect to temperature, the in-cloud data points seem to have two peaks. One around 225K and close to saturation, RHi=100%. And a second one at colder temperatures around 215K and subsaturated, RHi = 90%. This means, that most cirrus clouds are measured either in a warmer saturated environment or a colder subsaturated environment. These two regions seem to represent the two cirrus cloud categories mentioned above. In-situ formed clouds are mostly cold and unsaturated, with RHi values below the lower threshold for heterogeneous nucleation. Liquid-origin clouds are usually warmer and supersaturated, with RHi values commonly up to the high threshold for heterogeneous nucleation. Finally, regarding the temporal evolution of cirrus clouds, we find that the vertical structure of RHi within the clouds is indicative of their life stage. RHi skewness tends to go from positive to negative values as the cloud ages. RHi modes are subsaturated in young clouds, supersaturated in mature clouds and return to subsaturated in dissipating clouds.

Current NWP analyses and reanalyses are known to possess a moist bias in the lower stratosphere of the mid-latitudes [1]. An accurate representation of water vapor in the extratropical upper troposphere and lower stratosphere (UTLS), however, is crucial to correctly predict weather but also when climate models are verified against reanalysis products. This presentation uses a unique airborne multi-campaign water vapor profile data set to better characterize the vertical structure of this bias and to investigate its connection to mixing processes.Highly-resolved water vapor profiles have been recorded with the differential absorption lidar (DIAL) WALES onboard the research aircraft HALO on various field campaigns since 2013. The high-resolution humidity profiles along the flight path provide high data availability across the entire UTLS in cloud-free situations. We analyzed mid-latitude data from more than 40 flights over the Northern Atlantic and Europe that cover a broad spectrum of synoptic situations and different seasons.This comprehensive data set is used for a comparison with the European Centre for Medium-Range Weather Forecast’s (ECMWF) ERA5 reanalysis. First, we show an example specific humidity distribution along a cross-section in the surrounding of an extratropical cyclone. The comparison to ERA5 indicates the largest positive and negative deviations in the UT, but on average no systematic differences. In contrast, we find a coherent layer of strongly overestimated humidity above the thermal tropopause (TP) persisting along the whole flight path. Second, the vertical structure of deviations is verified for all flights. On average, deviations in the UT are relatively weak (+15%) and the minimum bias (+10%) is found at the thermal tropopause. Above the TP, within a layer of 1-1.5 km the bias rapidly increases up to a maximum of +52% while it decreases again to 15-20 % by 4 km. Although the shape of the vertical structure is similar for each flight, variations of the moist bias are observed for different seasons. For instance, the overestimation in summer is more than twice as high as for autumn observations.A possible explanation for this systematic moist bias is overestimation of mixing of water vapor into the LS. During one field campaign, WALES additionally observed ozone profiles which allow a classification of the observations into tropospheric, stratospheric and mixed air using H2O-O3 correlations in tracer-tracer space [2]. We demonstrate that the bias is particularly increased in air that was mixed in its history which indicates that mixing processes are not sufficiently well represented by ERA5. References[1]Bland, J., Gray, S., Methven, J. and Forbes, R.: Characterising the extratropical near-tropopause analysis humidity biases and their radiative effects on temperature forecasts, Q.J.R. Met. Soc., 147(741), 3878-3898, https://doi.org/10.1002/qj.4150, 2021.[2]Schäfler, A., Fix, A., and Wirth, M.: Mixing at the extratropical tropopause as characterized by collocated airborne H2O and O3 lidar observations, Atmos. Chem. Phys., 21, 5217–5234, https://doi.org/10.5194/acp-21-5217-2021, 2021.

Wintertime Saharan dust plumes in the vicinity of Barbados are investigated by means of airborne lidar measurements. The measurements were conducted in the framework of the EUREC4A field experiment (Elucidating the Role of Cloud-Circulation Coupling in Climate) upstream the Caribbean island in January/February 2020. The combination of the water vapor differential absorption and high spectral resolution lidar techniques together with dropsonde measurements aboard the German HALO (High Altitude and Long-Range) research aircraft enable a detailed vertical and horizontal characterization of the measured dust plumes. In contrast to summertime dust transport, mineral dust aerosols were transported at lower altitudes and were always located below 3.5 km. Calculated backward trajectories affirm that the dust-laden layers have been transported in nearly constant low-level altitude over the North Atlantic Ocean. Only mixtures of dust-particles with other aerosol species, i.e. biomass burning aerosol from fires in West Africa and marine aerosol, were detected by the lidar. No pure mineral dust regimes were observed. Additionally, all the dust-laden airmasses that were observed during EUREC4A came along with enhanced water vapor concentrations compared to the free atmosphere above. Such enhancements have already been observed during summertime and were found to have a great impact on radiative transfer and atmospheric stability.

Using dropsondes and advanced remote sensing instrumentation to configure the German High-Altitude LOng range research aircraft (HALO) is configured for use as an airborne cloud observatory. A configuration of HALO (the High-Altitude LOng-range research aircraft) as a remote sensing cloud observatory is described and its use is illustrated with results from the first and second NARVAL (Next-generation Aircraft Remote-sensing for VALidation) field studies. Measurements from NARVAL2 are used to highlight the ability of HALO, when configured in this fashion, to characterize not only the distribution of water condensate in the atmosphere, but also its impact on radiant energy transfer and the co-varying large-scale meteorological conditions – including the large-scale velocity field, and its vertical component. The NARVAL campaigns with HALO demonstrate the potential of airborne cloud observatories to address long-standing riddles in studies of the coupling between clouds and circulation, and are helping to motivate a new generation of field studies.

As part of the EUREC4A field campaign, the German research aircraft HALO, configured as a cloud observatory, conducted 15 research flights in the trade wind region east of Barbados in January and February 2020. Narrative text, aircraft state data, and meta data describing HALO's operation during the campaign are provided. Each HALO research flight is segmented by time-stamp intervals into standard elements to aid the consistent analysis of the flight data. Photographs from HALO's cabin and animated satellite images synchronized with flight tracks are provided to visually document flight conditions. As a comprehensive product from the remote sensing observations, a multi-sensor cloud mask product is derived and quantifies the incidence of clouds observed during the flights. In addition, to lower the threshold for new users of HALO's data, a collection of use cases is compiled into an online book "How to EUREC4A", included as an asset with this paper. This online book provides easy access to most of EUREC4A's HALO data through an intake catalogue.

The interaction of aerosol, clouds, and water vapor is still a major source of uncertainty in projections of Earth’s future climate. Especially in the trades, the response of shallow marine trade wind convection to external forcings is poorly understood. These low-level clouds have an important cooling effect on surface temperatures, while their amount and height are directly influenced by the radiative cooling by aerosols and water vapor aloft. Furthermore, there is evidence that aerosols can modify the microphysical properties (e.g., by glaciation) and the precipitation formation inside these clouds while water vapor above the trade inversion influences the atmospheric stability in which they form. Due to the small horizontal scale of these clouds, the vertical separation of atmospheric layers, and the temporal evolution of precipitation, the observation of this interplay by geostationary satellites is scarce.To alleviate this observational data gap over the tropical North-Atlantic region, airborne lidar and cloud radar measurements were performed in the vicinity of Barbados and complemented with dedicated weather radar measurements during the EUREC4A campaign in February 2020. Aerosol properties and the vertical water vapor profile were characterized with simultaneous high spectral resolution and differential absorption measurements using the WALES lidar onboard the German research aircraft HALO. On the same platform, the vertical cloud extent and the presence of precipitation were sampled with the high-power Ka-band cloud radar HAMP MIRA. To capture the temporal evolution of precipitation patterns, these measurements were complemented with measurements of the C-band polarimetric weather radar POLDIRAD which was installed on the windward side of Barbados. During EUREC4A, measurements flights were conducted in high and low aerosol loads to sample its influence on the marine trade wind convection.This presentation will briefly introduce the instrumentation, data processing, and availability and give an overview of gained insights and ongoing studies. By means of case studies, we will give first impressions of the complementary nature of the collocated, highly resolved airborne measurements and the POLDIRAD measurements which provide the horizontal context and temporal evolution of the precipitation formation. By combining the cross-sectional snapshots with the temporal evolution of the precipitation pattern we will provide a detailed insight into the interplay between the aerosol and water vapor layer and the precipitation formation in the shallow marine trade wind convection.

Aeolian Saharan mineral dust particles can be transported over long distances. Great amounts of Saharan mineral dust particles are transported westwards over the Atlantic Ocean towards the Caribbean islands especially during the boreal summer months. During the transport they can either have a direct environmental effect by absorbing, emitting and scattering radiation or an indirect effect by changing cloud micro-physical properties and by modifying cloud lifetime or formation.Our recent studies indicate that elevated transported Saharan dust layers, so-called Saharan Air Layers (SALs), come along with enhanced concentrations of water vapor compared to the surrounding atmosphere. Radiative transfer simulations reveal that not the dust particles inside the SALs but the enhanced concentrations of water vapor play the dominant role for atmospheric heating in dust-laden subtropical regions. In this way water vapor has the potential to impact both atmospheric stability and turbulent properties not only inside the SALs but also at lower atmospheric levels. To study the effects of water vapor on atmospheric turbulence and stability in SAL-regions, we performed wavelet analyses as well as calculations of power spectra on the basis of airborne lidar backscatter and water vapor measurements by the DLR lidar system WALES during the NARVAL-II research campaign. For an in-depth investigation of SAL-properties, several research flights during NARVAL-II were designed to lead over dust-laden regions upstream the Caribbean island of Barbados. Our analysis shows that water vapor heating does not only have an effect on the stability and turbulence of SALs by maintaining their confining inversions and promoting vertical mixing in their interior, but also hinders the development of shallow marine convection below.In our presentation we will give an overview of the performed measurements and radiative transfer simulations as well as of the conducted stability and turbulence analyses by means of calculated power spectra and wavelet analyses.

A large fraction of global anthropogenic greenhouse gas emissions originates from localized point sources. International climate treaties foresee their independent monitoring. Given the high number of point sources and their global spatial distribution, local monitoring is challenging, whereas a global satellite-based observing system is advantageous. In this perspective, a promising measurement approach is active remote sensing by airborne lidar, such as provided by the integrated-path differential-absorption lidar CHARM-F. Installed onboard the German research aircraft HALO, CHARM-F serves as a demonstrator for future satellite missions, e.g. MERLIN. CHARM-F simultaneously measures weighted vertical column mixing ratios of CO2 and CH4 below the aircraft. In spring 2018, during the CoMet field campaign, measurements were taken at the largest European point sources of anthropogenic CO2 and CH4 emissions, i.e. coal-fired power plants and ventilation shafts of coal mines. The measurement flights aimed to transect isolated exhaust plumes, in order to derive the corresponding emission rates from the resulting enhancement in concentration, along the plume crossing. For the first time, multiple measurements of power plant emissions were made using airborne lidar. On average, we find that our measurements are consistent with reported numbers, but observe high discrepancies between successive plume crossings of up to 50 %. As an explanation for these high discrepancies, we assess the influence of inhomogeneity in the exhaust plume, caused by atmospheric turbulence. This assessment is based on the Weather Research and Forecasting Model (WRF). We find a pronounced diurnal cycle of plume inhomogeneity associated with local turbulence, predominately driven by midday solar irradiance. Our results reveal that periods of high turbulence, specifically during midday and afternoon, should be avoided whenever possible. Since lidar is intrinsically independent of sun light, measurements can be performed under conditions of weak turbulence, such as at night or in the early morning.

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 https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement.

Installed onboard the German research aircraft HALO, the integrated-path differential-absorption (IPDA) lidar CHARM-F measures weighted vertical columns of both greenhouse gases (GHG) below the aircraft and along its flight track, aiming at high accuracy and precision. Results will be shown from the deployment during the CoMet field campaign that was carried out in spring 2018, with its main focus on one of the major European hot spots in methane emissions: the Upper Silesian Coal Basin (USCB) in Poland. First analyses reveal a measurement precision of below 0.5% for 20-km averages and also low bias, which was assessed by comparison with in-situ instruments. The measurements flights were designed to capture individual CH4 and CO2 plumes from e.g. coal mine venting and coal-fired power plants, respectively, but also to measure large and regional scale GHG gradients and to provide comparisons with the Total Carbon Column Observing Network (TCCON). Many other different instruments, both airborne and ground-based, complemented the lidar measurements to provide a comprehensive dataset for model analyses. CHARM-F also acts as the airborne demonstrator for MERLIN, the “Methane Remote Lidar Mission”, conducted by the German and French space agencies, DLR and CNES, with launch foreseen in ~ 2024. In this context, the airborne lidar data are likewise important for mission support such as for e.g. algorithm development and improvement and, moreover, the CoMet mission was also an important step for MERLIN validation preparation.

The 29th International Laser Radar Conference (ILRC 29); International audience; Particle attenuated backscatter and depolarization ratio at 532 nm from the WALES (Water Vapor Lidar Experiment in Space) instrument are used in combination with the radar reflectivity from the 35 GHz MIRA35 cloud radar in order to perform an aerosol-cloud target classification on the lidar/radar collocated observations carried out with the High Altitude and LOng-range research aircraft (HALO), in high temporal and spatial resolution.The methodology is applied to the measurements conducted during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) in fall 2016. Here, we present one case study to show the feasibility and information context of the aerosol-cloud discrimination, which can serve as a good complement to the Cloud target categorization applied already in the measurements. Our results demonstrate that the developed mask is capable to identify complex stratifications with different aerosol and cloud types and even aerosol layers of low signal.

The paper investigates different approaches to the construction of a term structure of interest rates—reference rates that are the base for a Fund Transfer Pricing mechanism (FTP). While many positions in the literature focus on FTP mechanism as a part of asset liability management (ALM) process without any closer look at term structure construction, we identify features that let measure the behavior of the yield curve and detect the consequences of the model’s choice. The results show that the arbitrarily chosen model of the reference yield could have significant consequences for risk management process of a financial institution. The study provides a twofold contribution to the literature describing FTP mechanism. First it introduces more complex approach to the reference rate modeling inside FTP mechanism and shows the differences between considered models. Moreover it focuses on the construction of the reference curve itself and shows two different approaches covering a parsimonious model as well as Smith-Wilson one.

Airborne lidar observations of long-range transported Saharan air layers in the western North Atlantic trades indicate increased amounts of water vapor within the dust layers compared to the surrounding dry free atmosphere. This study investigates the impact of such enhanced water vapor concentrations on radiative heating. Therefore, spatially high resolved airborne high spectral resolution and differential absorption lidar measurements are used for the parametrization of aerosol optical properties and water vapor concentrations in radiative transfer calculations. Heating rates that are calculated under consideration of the measured water vapor distribution strongly differ from heating rates that are derived under assumption of an atmospheric reference water vapor profile which is steadily decreasing with altitude. Results highlight that water vapor represents a major radiative driver for dust layer vertical mixing and the maintenance of bounding inversions at the top and bottom of the dust layer.

A new, combined, lidar system has been developed that is able to simultaneously measure profiles of ozone and water vapor onboard aircraft. The concurrent measurement of these complementary trace species in the upper troposphere and lower stratosphere allows inferring exchange processes in the tropopause region. Whereas an advanced

Spectral radiance measurements collected in nadir and sideward viewing directions by two airborne passive solar remote sensing instruments, the Spectral Modular Airborne Radiation measurement sysTem (SMART) and the Differential Optical Absorption Spectrometer (mini-DOAS), are used to compare the remote sensing results of cirrus optical thickness τ. The comparison is based on a sensitivity study using radiative transfer simulations (RTS) and on data obtained during three airborne field campaigns: the North Atlantic Rainfall VALidation (NARVAL) mission, the Mid-Latitude Cirrus Experiment (ML-CIRRUS) and the Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems (ACRIDICON) campaign. Radiative transfer simulations are used to quantify the sensitivity of measured upward radiance I with respect to τ, ice crystal effective radius reff, viewing angle of the sensor θV, spectral surface albedo α, and ice crystal shape. From the calculations it is concluded that sideward viewing measurements are generally better suited than radiance data from the nadir direction to retrieve τ of optically thin cirrus, especially at wavelengths larger than λ = 900 nm. Using sideward instead of nadir-directed spectral radiance measurements significantly improves the sensitivity and accuracy in retrieving τ, in particular for optically thin cirrus of τ ≤ 2. The comparison of retrievals of τ based on nadir and sideward viewing radiance measurements from SMART, mini-DOAS and independent estimates of τ from an additional active remote sensing instrument, the Water Vapor Lidar Experiment in Space (WALES), shows general agreement within the range of measurement uncertainties. For the selected example a mean τ of 0.54 ± 0.2 is derived from SMART, and 0.49 ± 0.2 by mini-DOAS nadir channels, while WALES obtained a mean value of τ = 0.32 ± 0.02 at 532 nm wavelength, respectively. The mean of τ derived from the sideward viewing mini-DOAS channels is 0.26 ± 0.2. For the few simultaneous measurements, the mini-DOAS sideward channel measurements systematically underestimate (−17.6 %) the nadir observations from SMART and mini-DOAS. The agreement between mini-DOAS sideward viewing channels and WALES is better, showing the advantage of using sideward viewing measurements for cloud remote sensing for τ ≤ 1. Therefore, we suggest sideward viewing measurements for retrievals of τ of thin cirrus because of the significantly enhanced capability of sideward viewing compared to nadir measurements.

The Midlatitude Cirrus experiment (ML-CIRRUS) deployed the High Altitude and Long Range Research Aircraft (HALO) to obtain new insights into nucleation, life cycle, and climate impact of natural cirrus and aircraft-induced contrail cirrus. Direct observations of cirrus properties and their variability are still incomplete, currently limiting our understanding of the clouds’ impact on climate. Also, dynamical effects on clouds and feedbacks are not adequately represented in today’s weather prediction models. Here, we present the rationale, objectives, and selected scientific highlights of ML-CIRRUS using the G-550 aircraft of the German atmospheric science community. The first combined in situ–remote sensing cloud mission with HALO united state-of-the-art cloud probes, a lidar and novel ice residual, aerosol, trace gas, and radiation instrumentation. The aircraft observations were accompanied by remote sensing from satellite and ground and by numerical simulations. In spring 2014, HALO performed 16 flights above Europe with a focus on anthropogenic contrail cirrus and midlatitude cirrus induced by frontal systems including warm conveyor belts and other dynamical regimes (jet streams, mountain waves, and convection). Highlights from ML-CIRRUS include 1) new observations of microphysical and radiative cirrus properties and their variability in meteorological regimes typical for midlatitudes, 2) insights into occurrence of in situ–formed and lifted liquid-origin cirrus, 3) validation of cloud forecasts and satellite products, 4) assessment of contrail predictability, and 5) direct observations of contrail cirrus and their distinction from natural cirrus. Hence, ML-CIRRUS provides a comprehensive dataset on cirrus in the densely populated European midlatitudes with the scope to enhance our understanding of cirrus clouds and their role for climate and weather.

In situ measurements of cloud droplet number concentration N are limited by the sampled cloud volume. Satellite retrievals of N suffer from inherent uncertainties, spatial averaging, and retrieval problems arising from the commonly assumed strictly adiabatic vertical profiles of cloud properties. To improve retrievals of N it is suggested in this paper to use a synergetic combination of passive and active airborne remote sensing measurement, to reduce the uncertainty of N retrievals, and to bridge the gap between in situ cloud sampling and global averaging. For this purpose, spectral solar radiation measurements above shallow trade wind cumulus were combined with passive microwave and active radar and lidar observations carried out during the second Next Generation Remote Sensing for Validation Studies (NARVAL-II) campaign with the High Altitude and Long Range Research Aircraft (HALO) in August 2016. The common technique to retrieve N is refined by including combined measurements and retrievals of cloud optical thickness τ, liquid water path (LWP), cloud droplet effective radius reff, and cloud base and top altitude. Three approaches are tested and applied to synthetic measurements and two cloud scenarios observed during NARVAL-II. Using the new combined retrieval technique, errors in N due to the adiabatic assumption have been reduced significantly.

Liquid water path (LWP) is an important quantity to characterize clouds. Passive microwave satellite sensors provide the most direct estimate on a global scale but suffer from high uncertainties due to large footprints and the superposition of cloud and precipitation signals. Here, we use high spatial resolution airborne microwave radiometer (MWR) measurements together with cloud radar and lidar observations to better understand the LWP of warm clouds over the tropical North Atlantic. The nadir measurements were taken by the German High Altitude and LOng range research aircraft (HALO) in December 2013 (dry season) and August 2016 (wet season) during two Next-generation Advanced Remote sensing for VALidation (NARVAL) campaigns. Microwave retrievals of integrated water vapor (IWV), LWP, and rainwater path (RWP) are developed using artificial neural network techniques. A retrieval database is created using unique cloud-resolving simulations with 1.25 km grid spacing. The IWV and LWP retrievals share the same eight MWR frequency channels in the range from 22 to 31 GHz and at 90 GHz as their sole input. The RWP retrieval combines active and passive microwave observations and is able to detect drizzle and light precipitation. The comparison of retrieved IWV with coincident dropsondes and water vapor lidar measurements shows root-mean-square deviations below 1.4 kg m−2 over the range from 20 to 60 kg m−2. This comparison raises the confidence in LWP retrievals which can only be assessed theoretically. The theoretical analysis shows that the LWP error is constant with 20 g m−2 for LWP below 100 g m−2. While the absolute LWP error increases with increasing LWP, the relative one decreases from 20 % at 100 g m−2 to 10 % at 500 g m−2. The identification of clear-sky scenes by ancillary measurements, here backscatter lidar, is crucial for thin clouds (LWP < 12 g m−2) as the microwave retrieved LWP uncertainty is higher than 100 %. The analysis of both campaigns reveals that clouds were more frequent (47 % vs. 30 % of the time) in the dry than in the wet season. Their average LWP (63 vs. 40 g m−2) and RWP (6.7 vs. 2.7 g m−2) were higher as well. Microwave scattering of ice, however, was observed less frequently in the dry season (0.5 % vs. 1.6 % of the time). We hypothesize that a higher degree of cloud organization on larger scales in the wet season reduces the overall cloud cover and observed LWP. As to be expected, the observed IWV clearly shows that the dry season is on average less humid than the wet season (28 vs. 41 kg m−2). The results reveal that the observed frequency distributions of IWV are substantially affected by the choice of the flight pattern. This should be kept in mind when using the airborne observations to carefully mediate between long-term ground-based and spaceborne measurements to draw statistically sound conclusions.

Saharan dust is known to have an important impact on the atmospheric radiation budget, both directly and indirectly by changing cloud properties. However, up to now it is still an open question if elevated and long-range transported Saharan dust layers have an effect on subjacent marine trade wind cloud occurrence. Shallow trade wind clouds have a significant impact on the Earth's radiation budget and still introduce large uncertainties in climate sensitivity estimates, because of their poor representation in climate models. The Next-generation Aircraft Remote-Sensing for Validation studies (NARVAL) aimed at providing a better understanding of shallow marine trade wind clouds and their interplay with long-range transported elevated Saharan dust layers. Two airborne campaigns were conducted – the first one in December 2013 and the second one in August 2016; the latter one during the peak season of transatlantic Saharan dust transport. Airborne lidar measurements in the vicinity of Barbados performed during the second field campaign are used to investigate possible differences between shallow marine cloud macro-physical properties in dust-free regions and regions comprising elevated Saharan dust layers. The cloud top height distribution derived in dust-laden regions differs from the one derived in dust-free regions and indicates that clouds are shallower and convective development is suppressed. Furthermore, regions comprising elevated Saharan dust layers show a larger fraction of small clouds and larger cloud free regions, compared to dust-free regions. The cloud fraction in dusty regions is only 14 % compared to a fraction of 31 % in dust-free regions. Moreover, a decreasing trend of cloud fractions and cloud top heights with increasing dust layer vertical extent as well as aerosol optical depth is found.

Low planetary wave activity led to a stable vortex with exceptionally cold temperatures in the 2015–2016 Arctic winter. Extended areas with temperatures below the ice frost point temperature Tice persisted over weeks in the Arctic stratosphere as derived from the 36-year temperature climatology of the ERA-Interim reanalysis data set of the European Centre for Medium-Range Weather Forecasts (ECMWF). These extreme conditions promoted the formation of widespread polar stratospheric ice clouds (ice PSCs). The space-borne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument on board the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite continuously measured ice PSCs for about a month with maximum extensions of up to 2×106 km2 in the stratosphere. On 22 January 2016, the WALES (Water Vapor Lidar Experiment in Space – airborne demonstrator) lidar on board the High Altitude and Long Range Research Aircraft HALO detected an ice PSC with a horizontal length of more than 1400 km. The ice PSC extended between 18 and 24 km altitude and was surrounded by nitric acid trihydrate (NAT) particles, supercooled ternary solution (STS) droplets and particle mixtures. The ice PSC occurrence histogram in the backscatter ratio to particle depolarization ratio optical space exhibits two ice modes with high or low particle depolarization ratios. Domain-filling 8-day back-trajectories starting in the high particle depolarization (high-depol) ice mode are continuously below the NAT equilibrium temperature TNAT and decrease below Tice∼10 h prior to the observation. Their matches with CALIPSO PSC curtain plots demonstrate the presence of NAT PSCs prior to high-depol ice, suggesting that the ice had nucleated on NAT. Vice versa, STS or no PSCs were detected by CALIPSO prior to the ice mode with low particle depolarization ratio. In addition to ice nucleation in STS potentially having meteoric inclusions, we find evidence for ice nucleation on NAT in the Arctic winter 2015–2016. The observation of widespread Arctic ice PSCs with high or low particle depolarization ratios advances our understanding of ice nucleation in polar latitudes. It further provides a new observational database for the parameterization of ice nucleation schemes in atmospheric models.

The stringent requirements for energy reference measurement represent a challenging task for integrated path differential absorption lidars to measure greenhouse gas columns from satellite or aircraft. The coherence of the lidar transmitter gives rise to speckle effects that have to be considered for accurate monitoring of the energy ratio of outgoing on- and off-line pulses. Detailed investigations have been performed on various measurement concepts potentially suited for deployment within future satellite missions.

International audience; MEthane Remote LIdar missioN (MERLIN) is a German‐French space mission, scheduled for launch in 2024 and built around an innovative light detecting and ranging instrument that will retrieve methane atmospheric weighted columns. MERLIN products will be assimilated into chemistry transport models to infer methane emissions and sinks. Here the expected performance of MERLIN to reduce uncertainties on methane emissions is estimated. A first complete error budget of the mission is proposed based on an analysis of the plausible causes of random and systematic errors. Systematic errors are spatially and temporally distributed on geophysical variables and then aggregated into an ensemble of 32 scenarios. Observing System Simulation Experiments are conducted, originally carrying both random and systematic errors. Although relatively small (±2.9 ppb), systematic errors are found to have a larger influence on MERLIN performances than random errors. The expected global mean uncertainty reduction on methane emissions compared to the prior knowledge is found to be 32%, limited by the impact of systematic errors. The uncertainty reduction over land reaches 60% when the largest desert regions are removed. At the latitudinal scale, the largest uncertainty reductions are achieved for temperate regions (84%) and then tropics (56%) and high latitudes (53%). Similar Observing System Simulation Experiments based on error scenarios for Greenhouse Gases Observing SATellite reveal that MERLIN should perform better than Greenhouse Gases Observing SATellite for most continental regions. The integration of error scenarios for MERLIN in another inversion system suggests similar results, albeit more optimistic in terms of uncertainty reduction.