The BOWTIE ship-based field experiment explored the influence of convective storm processes and their oceanic interactions on the Atlantic Inter-Tropical Convergence Zone (ITCZ) in August and September 2024. This research is driven by evidence suggesting that storm-scale dynamics is pivotal for shaping the broader structure of the ITCZ and its connection to global circulation patterns and energy transport. Utilizing the German research vessel METEOR, the expedition involved extensive sampling within the ITCZ from its Northern to Southern edge - while transiting the tropical Atlantic from East to West - with detailed atmospheric and oceanic vertical profiling. The research focuses on obtaining vertically resolved cross-sections of the ITCZ and its surrounding conditions, with an emphasis on the atmosphere-ocean boundary layers. Key measurements include precipitation, cloud and humidity profiles, wind, sea-surface temperature, as well as physical and biogeochemical upper ocean characteristics with state-of-the-art instrumentation. BOWTIE is embedded within the international initiative ORCESTRA that combines eight different sub-campaigns. During BOWTIE the research vessel METEOR also served as a platform for two additional subcampaigns of ORCESTRA, which contributed UAVs (STRINQS) and a dual-polarization C-band radar (CSU’s SEA-POL, through PICCOLO) to the ship’s instrumentation. We will present an overview of BOWTIE and first results from the comprehensive measurments.

How to cite: Klocke, D., Wing, A., Dengler, M., Segura, H., Georges, G., Nuijens, L., Bell, M., Kalesse-Los, H., Ruppert, J., and Schmidt, H.: BOWTIE: A ship-based field campaign to explore the inner life of the Atlantic ITCZ, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16165, https://doi.org/10.5194/egusphere-egu25-16165, 2025.

Near-surface winds are a key component for the coupling of the atmosphere- ocean system. Convergence and divergence patterns can be inferred from measurements of the surface wind vector, enabling the characterization of meso- and synoptic-scale atmospheric dynamics. Observations over remote areas, such as the Atlantic ocean, are mostly limited to satellites and buoys. Geostationary satellites derive wind data primarily from cloud tracking and thus do not measure surface winds. In contrast, polar-orbiting satellites can provide surface wind data predominantly using active remote sensing instruments, such as scatterometers, wind-lidars or synthetic aperture radars, providing better spatial but certainly lower temporal resolution. Finally, buoy measurements are point-observations and cannot be employed for large-scale wind field analyses. This work aims to explore an alternative approach for quantifying surface wind fields over the ocean by analysing high-spatial resolution imagery from airborne observations.

Measurements of specularly reflected solar radiation (sunglint) by the hyperspectral and polarized imager specMACS aboard the German research aircraft HALO are employed for the development of a surface wind retrieval. Size and shape of the sunglint are predetermined by wind speed and direction: With ocean surface roughness directly corresponding to near-surface wind speed, the specMACS retrieval makes use of the relationship between ocean wave slope distribution and angular variation of sunglint radiance. SpecMACS measurements of spectral radiances are evaluated against simulated spectral radiances for different solar zenith and azimuth angles, as well as surface wind speeds and directions. The radiative transfer simulations are done with the Monte Carlo (MYSTIC) solver of the libRadtran radiative transfer package.

The overarching goal of this work is the development of an operational surface wind retrieval after analysing selected cases as an initial step. The retrieval requires a view of the ocean surface from the aircraft. We aim to explore to what extent the wind retrieval can be employed for (a) partially cloud-covered scenes or (b) scenes with an optically thin cirrus layer above or below the aircraft. A first application will be the analysis of data acquired during the recent ’Persistent EarthCare underflight studies of the ITCZ and organized convection’ (PERCUSION) sub-campaign in the tropical Atlantic. Research concerned with the horizontal wind field structure of atmospheric phenomena, e.g. the doldrums in the deep tropics, will benefit from along flight-track surface wind observations. Continuous surface wind data also further supplement dropsonde point-measurements.

How to cite: Stallmach, A., Weber, A., and Mayer, B.: Development of a surface wind retrieval by analysing sunglint geometry from specMACS radiance measurements, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20399, https://doi.org/10.5194/egusphere-egu25-20399, 2025.

Convection, which influences cloud and precipitation properties as well as, the radiative effects of clouds, needs to be better understood for a comprehensive picture of interconnected climate processes all over the globe and requires a better representation for accurate climate projections. Meanwhile, the modelling of these phenomena is already a challenge itself, the quality and quantity of high-resolution observational data of convective clouds are limited, especially over the (tropical) oceans. The recent measurement campaign BOW-TIE (“Beobachtung von Ozean und Wolken – das Trans ITCZ Experiment”) on board the research vessel Meteor in August and September 2024 focussed on atmospheric and oceanic measurements inside the Atlantic ITCZ (intertropical convergence zone), between Mindelo (Cabo Verde) and Bridgetown (Barbados). Our working group from the Leipzig Institute for Meteorology concentrated on the investigation of the microphysical properties of clouds and precipitation by deploying a continuously measuring remote sensing suite mainly consisting of a motion-stabilized, vertically-pointing 94 GHz cloud radar and a microwave radiometer (MWR; to derive liquid water path (LWP) and integrated water vapor (IWV)) on the research vessel. In combination with data obtained with the ceilometer deployed by the MPI for Meteorology Hamburg, the synergetic Cloudnet processing chain can be employed. Cloudnet products include a hydrometeor target classification as well as cloud microphysical properties like the effective radius of cloud droplets and ice crystals, the hydrometeors phase, ice water content (IWC), and liquid water content (LWC). All observed and derived cloud and precipitation characteristics are contrasted between the Eastern and Western Atlantic. A first insight reveals the dominance of shallow convective clouds in the Eastern Atlantic, while the Western Atlantic also evinces convective clouds with a height up to six kilometers and cirrus clouds more frequently. Related to the frequency of occurrence of the different cumulus cloud types, the distribution of the microphysical properties like the effective radius of hydrometeors, IWC, and LWC allows for a more detailed glimpse into differences of the processes in the Eastern and Western ITCZ. With that, the convective processes above the tropical Atlantic Ocean are studied concerning microphysics and their distribution and contribute to an improved understanding of cloud-affecting processes.

How to cite: Trosits, A., Foth, A., and Kalesse-Los, H.: First insights into the diverse remote sensing observations of convection during BOW-TIE, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6335, https://doi.org/10.5194/egusphere-egu25-6335, 2025.

As convective aggregation has been found to be supported by radiative heating in idealized simulations, this study seeks to answer whether such a property of convection exists in the observed convective organization. Data was collected during the ORCESTRA field campaign over the tropical Atlantic in August and September 2024. Cloud properties, precipitation, atmospheric radiative fluxes, and the derived degree of convective organization are measured by the Sea-Pol radar and other instruments on a shipborne platform (RV Meteor) supported by sub-campaigns including PICCOLO and BOW-TIE. We analyze how atmospheric radiative effects support the spatial organization of tropical deep convection, and also inversely, how the convective organization affects the strength of convective-radiative feedback. In particular, the strength of convective-radiative feedback is assessed by the temporal covariance between atmospheric radiative heatings and either moist static energy tendency or precipitation rate. We will show the observed dependency of convective-radiative feedback on the occurrence of various convective organization phenomena, including mesoscale convective organization and the passage of tropical waves. The effect of radiative heating and its induced circulation on the state of convective organization during the field campaign will also be derived from mechanism-denial numerical simulations.

How to cite: Hsiao, W.-T., Wing, A., Kennison, S., Bell, M., and Ruppert, J.: Interaction between cloud-radiative effects and convective systems measured during the ORCESTRA field campaign in Aug-Sep 2024, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7132, https://doi.org/10.5194/egusphere-egu25-7132, 2025.

STRINQS stands for Soundings and TuRbulent eddy measurements in the ITCZ with a Network of QuadcopterS and is one of the sub-campaigns under the umbrella of ORCESTRA, an international collaboration of measurement campaigns with the larger goal of understanding mesoscale organization of convection in the tropical Atlantic. STRINQS made measurements of the ITCZ boundary layer by employing four meteorological quadcopters, designed and developed by Menapia. Co-ordinated flights were conducted with the German research vessel Meteor as base, a part of the BOWTIE subcampaign in ORCESTRA. The quadcopters, designed to sustain performance in heavy rain and strong wind, provide high-resolution atmospheric soundings of temperature, humidity, pressure, and winds while allowing for high flight ceilings. There are two sets of meteorological sensors in each quadcopter, with a sampling frequency of 10 Hz. Additionally, a sonic anemometer configured on a 1 m arm above the quadcopter body helps provide wind measurements without disturbances due to the wake of the propellers during non-descending trajectories. The team had to initially overcome multiple logistical and technical challenges, unfortunately including mishaps. However eventually, between the period of 30th August and 9th September, the team successfully conducted around 45 vertical profiles reaching altitudes of up to 1500 m (the permitted flight ceiling for STRINQS) in addition to flying horizontal hexagonal patterns that traversed distances between 2 and 4 km horizontally. Some flights recorded intriguing case-studies such as profiling the boundary-layer thrice in a span of 40 minutes as an organized squall-line rain event passed over the ship, thus providing contrasting conditions before, during and after the storm. The meteorological sensors' data show promising results in the drone's capability to sample the boundary layer, but some corrections still need to be made to the retrieval of wind measurements, particularly vertical wind, which is known to be a challenging measurement from UAV (uncrewed aerial vehicle) platforms. Post data-processing and preliminary analyses, the data will be made publicly available in state of the art data formats. Although STRINQS only partially achieved its scientific goal of statistical sampling, the learnings on the measurement capabilities of such methods have been significant. With this demonstration of using a ship as a launchpad for coordinated flights of heavyduty quadcopters even in heavy rain events, STRINQS signals the possibilities of such strategies in future campaigns to provide a rich spatial characterization of the boundary layer.

How to cite: George, G., Mackenzie, R., O'Driscoll, O., Klocke, D., Nuijens, L., Siebesma, P., and Menapia, T.: Boundary-layer measurements of the ITCZ with meteorological quadcopters off a trans-Atlantic ship expedition, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17730, https://doi.org/10.5194/egusphere-egu25-17730, 2025.

Two thirds of variability in cloud cover in the trade-wind regions is associated with cloudiness near the top of the cloud layer, which mostly occurs in the form of stratiform layers. Stratiform inversion cloud is also the cloud component that changes most across different patterns of cloud organization and potentially also under climate change. Unfortunately, our understanding of the factors controlling the occurrence and lifetime of stratiform layers and how they link to the structure of the trade inversion is limited. Because the trade inversion can be as thin as 10-100m, even high-resolution large-eddy simulations have serious issues in accurately representing the sharpness of the inversion and its associated cloudiness. The EUREC⁴A field campaign released >850 dropsondes from the HALO aircraft upstream Barbados in January-February 2020 in 200 km diameter circles, constituting a sounding dataset which lends itself particularly well to investigate the structure and variability of the trade inversion. Here we use the dropsonde data in conjunction with lidar-retrieved cloud-top height distributions and high resolution large-eddy simulations (the cloud botany ensemble) to investigate controls on inversion strength and height, document their variability at different scales, and assess their connection with cloudiness.    

How to cite: Vogel, R. and Janssens, M.: Mesoscale controls on trade-wind inversion structure and cloudiness during EUREC4A, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7235, https://doi.org/10.5194/egusphere-egu25-7235, 2025.

Clouds and convection play a key role in structuring atmospheric circulation and in determining the climate sensitivity. However, it is still not understood how clouds and convection will respond to warming of the atmosphere. This is due to an insufficient representation of clouds and moist convection in climate models. A better understanding of the coupling between water vapor, convection, cloud formation and circulation is needed. Shallow marine convection shows the largest frequency of occurrence amongst clouds. But besides being uniform clouds of similar structure, they can occur in different larger scale patterns of organization. The trade wind region is characterized by a complex structure of water vapor, aerosols and clouds. Depending on the season and larger scale circulation, it was found, that lofted layers of water vapor and aerosols can have a quite significant impact on the atmospheric stability, and with that on cloud structure and evolution.

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. The WALES instrument was deployed in a series of airborne experiments aiming to better understand the coupling of clouds and convection over the sub-tropical and tropical Atlantic Ocean. The first campaign of this series, the NARVAL experiment, was conducted in wintertime out of Barbados. It was followed by the NARVAL-II experiment in August 2016, the EUREC4A experiment in January/February 2020 and the PERCUSION campaign in August and September 2024. The latter especially focused on the transition of shallow to deep convection and the ITCZ. Another add on of this campaign was the contrasting measurements over the east and west Atlantic Ocean. We used these measurements to investigate how the complex structure of water vapor and aerosol impact the stability of the atmosphere and with that the evolution and structure of clouds. We found that the impact is different, if the water vapor and aerosol are within distinct layers. During summertime, when they are well separated from the marine boundary layer, the radiative effect of the layers dominates. The evolution of shallow marine clouds below the SAL is suppressed. In wintertime, the e.g. dust 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 layer 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 that were performed based on our findings. Those calculations together with the observations better help to understand the impact of lofted layers on cloud evolution and structure.

How to cite: Gross, S., Gutleben, M., Wirth, M., and Ewald, F.: Impact of elevated water vapor and aerosol layers on the stability of the sub-tropical atmosphere and the structure and evolution of shallow marine clouds, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18408, https://doi.org/10.5194/egusphere-egu25-18408, 2025.

The temporal development of cloud properties along the trajectory of a tracked clouds can show a behavior that is clearly unrealistic. For instance, a low-level cloud that is partly obscured by faint high-level clouds above can show development of cloud-top temperature with jumps of 40 K or more. To assess the general development of clouds tracked over central Europe, we have applied the ISCCP cloud classification to those clouds. It is found that the majority of the tracked clouds shows transitions between time steps that are either between identical cloud types (e.g. cumulus to cumulus) or represent reasonable development (e.g. stratus to stratocumulus or vice versa). The additional consideration of objective weather types enables an assessment of the large-scale conditions under which different cloud types are most abundant.

How to cite: Tesche, M., Müller, F., and Seelig, T.: Track ‘n’ Type: Do tracked clouds show a realistic behavior with respect to cloud type?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16875, https://doi.org/10.5194/egusphere-egu25-16875, 2025.

We compare satellite data from the EUREC⁴A campaign (observed by the Advanced Baseline Image onboard the GOES-16 satellite) and model output from ICON-LEM tailored for the EUREC⁴A campaign [1]. All datasets are located east of Barbados in the Caribbean Sea. We build on previous cloud tracking analyses for the GOES satellite dataset [2].

We use a cloud tracking algorithm [2] to find lifetime and cloud size distributions. The trajectories can be classified into the four mesoscale cloud patterns “sugar” to “fish” based on C³ONTEXT data which makes it possible to investigate how well these patterns are represented in the model data. We compare the distributions of cloud sizes, lifetimes and the average development of cloud size over the cloud’s lifetimes.

Cloud modelling is a very important tool for climate research. However, it is not an easy task to validate model data and assess a model’s performance. The cloud tracking enables us to provide a unique quality assessment of the model data. Lifetime information is interesting because it encompasses multiple dynamic scales from micro to planetary regimes, while cloud size and cloud cover are important factors for the radiative properties of the clouds in a region and characterise the clouds’ general behavior.

[1] Schulz, Hauke & Stevens, Bjorn (2023) “Evaluating Large-Domain, Hecto-Meter, Large-Eddy Simulations of Trade-Wind Clouds Using EUREC4A Data” in Journal of Advances in Modeling Earth Systems, doi: 10.1029/2023MS003648

[2] Seelig et al. (2023) “Do optically denser trade-wind cumuli live longer?”, in Geophysical Research Letters, doi: 10.1029/2023GL103339

How to cite: Müller, F., Seelig, T., Schulz, H., Villanueva, D., and Tesche, M.: Tracking Clouds: Assessing the representation of mesoscale cloud patterns in the EUREC4A model data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17353, https://doi.org/10.5194/egusphere-egu25-17353, 2025.

Tropical anvil clouds have significant impacts on the atmosphere due to their cloud radiative effect (CRE), and their response to warming remains one of the largest uncertainties in future climate projections. Recent research has highlighted both the importance of changes in anvil cloud structure and changes in convective mass flux in a warmer climate to CRE feedbacks. However, understanding of these processes is limited due to a lack of observations linking convective processes to anvil cloud properties across their entire lifetimes. We apply the tobac-flow algorithm to a year of Meteosat SEVIRI observations over Africa and the tropical Atlantic to detect and track convective cores and their subsequent anvil clouds to investigate the impact of convective dynamics on anvil clouds. By combining this cloud tracking dataset with retrieved cloud properties and broadband fluxes, changes in the intensity and organisation of convection can be linked to changes in anvil CRE. Overall, both more intense and more organised convection tends to result in anvils with positive CRE, as these storms produce higher, colder anvil clouds and, over land, anvils that exist for longer at night. However, when controlling for the anvil temperature and time of day, more intense convection tends to result in positive CRE, while more organised convection results in a negative CRE. We attribute these differences to changes in anvil structure, as we observe that more organised convection tends to produce thicker anvils, while more intense convection results in thinner anvils. The contrasting effects of different convective processes on anvil CRE highlight the importance of understanding the mechanisms through which convective dynamics affect anvil structure, and indicate that different changes in convective processes may lead to regional differences in anvil cloud feedbacks.

How to cite: Jones, W. and Stier, P.: Contrasting effects of convective intensity and organisation on anvil cloud radiative effect observed using cloud tracking, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19918, https://doi.org/10.5194/egusphere-egu25-19918, 2025.