Anna Trosits

As part of BOWTIE (German: Beobachtung von Ozean und Wolken – Das Trans ITCZ Experiment), the German research vessel FS Meteor navigated the moist tropics of the Atlantic Ocean for 40 d in summer 2024, with an east-west trajectory. The journey started in the port of Mindelo, Cape Verde, on 16 August, and finished in the port of Bridgetown, Barbados, on 24 September. The objective was to measure properties of the atmosphere, upper ocean, and air-sea interface within the Intertropical Convergence Zone (ITCZ), under a variety of wind, convection, and sea surface temperature regimes. Using a set of 29 instruments/platforms, BOWTIE sampled the near-surface conditions of the atmosphere and ocean with high temporal resolution. This included continuous measurements of: the 2D wind field within the lowest 2 km of the atmosphere, near-surface ocean currents, cloud and precipitation properties. Profiles of the ocean state and atmospheric thermodynamics and kinematics were obtained both continuously and at discrete intervals. Furthermore, biochemical properties of the upper ocean were sampled during dedicated stations. Complementing BOWTIE observations, FS Meteor hosted further dedicated field campaigns for 3D cloud and precipitation properties, as well as intensive measurements of the atmospheric boundary layer. This manuscript provides an overview of the extensive instrumentation and data collected during BOWTIE. In addition, it addresses two key aspects based on these observations. First, it examines the range and uncertainties of selected quantities measured by multiple instruments, including column-integrated water vapor, rain detection, surface ocean currents, and sea surface temperature. Second, it illustrates the diversity of sampled weather regimes through two representative cases: calm doldrum conditions and a gusty, precipitating convective state. The links and DOIs of the data sets presented in this study are listed in the code and data availability section.

Clouds over the tropical Atlantic remain under-represented in ground-based observational studies, despite their key role in the regional radiative and hydrological budgets. This observational gap contributes to substantial uncertainties in the representation of these clouds in climate models. The recent BOWTIE (German: Beobachtung von Ozean und Wolken – Das Trans ITCZ Experiment) campaign aimed to address this deficiency through a six-week, ship-borne measurement campaign across the tropical Atlantic, spanning from Cape Verde to Barbados. This study investigates the influence of Saharan Air Layers (SALs) on cloud properties using vertically pointing remote sensing observations collected during the research cruise. Cloud types are classified and combined with Cloudnet-derived microphysical products to assess both macrophysical and microphysical cloud characteristics. The analysis reveals a shift in the cloud characteristics toward less convective regimes during SAL presence, accompanied by changes in cloud base height (CBH) and cloud top height (CTH). Microphysically, the suppression of convection associated with SAL leads to a greater vertical separation of cloud layers, meaning that droplet and ice effective radii (DER and IER) differ across layers. However, hydrometeor populations across all observed layers are comparable to those observed in marine cloud regimes reported in previous studies. Overall, the thermodynamic structure of the SAL emerges as the dominant driver of the observed changes in cloud properties. These findings highlight the importance of accounting for SAL influences in understanding cloud development over the tropical Atlantic and improving their representation in atmospheric models.

Clouds over the tropical Atlantic remain under-represented in ground-based observational studies, despite their key role in the regional radiative and hydrological budgets. This observational gap contributes to substantial uncertainties in the representation of these clouds in climate models. The recent BOWTIE (German: Beobachtung von Ozean und Wolken – Das Trans ITCZ Experiment) campaign aimed to address this deficiency through a six-week, ship-borne measurement campaign across the tropical Atlantic, spanning from Cape Verde to Barbados. This study investigates the influence of Saharan Air Layers (SALs) on cloud properties using vertically pointing remote sensing observations collected during the research cruise. Cloud types are classified and combined with Cloudnet-derived microphysical products to assess both macrophysical and microphysical cloud characteristics. The analysis reveals a shift in the cloud characteristics toward less convective regimes during SAL presence, accompanied by changes in cloud base height (CBH) and cloud top height (CTH). Microphysically, the suppression of convection associated with SAL leads to a greater vertical separation of cloud layers, meaning that droplet and ice effective radii (DER and IER) differ across layers. However, hydrometeor populations across all observed layers are comparable to those observed in marine cloud regimes reported in previous studies. Overall, the thermodynamic structure of the SAL emerges as the dominant driver of the observed changes in cloud properties. These findings highlight the importance of accounting for SAL influences in understanding cloud development over the tropical Atlantic and improving their representation in atmospheric models.

Clouds over the tropical Atlantic remain under-represented in ground-based observational studies, despite their key role in the regional radiative and hydrological budgets. This observational gap contributes to substantial uncertainties in the representation of these clouds in climate models. The recent BOWTIE (German: Beobachtung von Ozean und Wolken – Das Trans ITCZ Experiment) campaign aimed to address this deficiency through a six-week, ship-borne measurement campaign across the tropical Atlantic, spanning from Cape Verde to Barbados. This study investigates the influence of Saharan Air Layers (SALs) on cloud properties using vertically pointing remote sensing observations collected during the research cruise. Cloud types are classified and combined with Cloudnet-derived microphysical products to assess both macrophysical and microphysical cloud characteristics. The analysis reveals a shift in the cloud characteristics toward less convective regimes during SAL presence, accompanied by changes in cloud base height (CBH) and cloud top height (CTH). Microphysically, the suppression of convection associated with SAL leads to a greater vertical separation of cloud layers, meaning that droplet and ice effective radii (DER and IER) differ across layers. However, hydrometeor populations across all observed layers are comparable to those observed in marine cloud regimes reported in previous studies. Overall, the thermodynamic structure of the SAL emerges as the dominant driver of the observed changes in cloud properties. These findings highlight the importance of accounting for SAL influences in understanding cloud development over the tropical Atlantic and improving their representation in atmospheric models.

As part of BOWTIE (German: Beobachtung von Ozean und Wolken - Das Trans ITCZ Experiment), the German research vessel FS Meteor navigated the moist tropics of the Atlantic Ocean for 40 days in summer 2024, with an east-west trajectory. The journey started in the port of Mindelo, Cape Verde, on August 16, and finished in the port of Bridgetown, Barbados, on September 24. The objective was to measure properties of the atmosphere, upper-ocean, and air-sea interface within the Intertropical Convergence Zone (ITCZ), under a variety of wind, convection, and sea surface temperature regimes. Using a set of 29 instruments/platforms, BOWTIE sampled the near-surface conditions of the atmosphere and ocean with high temporal resolution. This included continuous measurements of: the 2-D wind field within the lowest 2 km of the atmosphere, near-surface ocean currents, cloud and precipitation properties. Profiles of the ocean state and atmospheric thermodynamics and kinematics were obtained both continuously and at discrete intervals. Furthermore, dedicated stations sampled biochemical properties of the upper-ocean. Complementing BOWTIE observations, FS Meteor hosted further dedicated field campaigns for 3D cloud and precipitation properties, as well as intensive measurements of the atmospheric boundary layer. This manuscript provides an overview of the extensive instrumentation and data collected during BOWTIE. In addition, it addresses two key aspects based on these observations. First, it examines the range and uncertainties of selected quantities measured by multiple instruments, including column-integrated water vapor, rain detection, surface ocean currents, and sea surface temperature. Second, it illustrates the diversity of sampled weather regimes through two representative cases: calm doldrum conditions and a gusty, precipitating convective state.

The radiative effect of clouds is determined by their development, lifetime and microphysical characteristics. The processes influencing cloud properties are diverse and require further investigation to gain a more profound understanding. However, process studies cannot be conducted by modelling alone, as observations are rarely available, particularly over the oceans. This leads to uncertainties in global weather modelling and climate forecasts. In order to disentangle the effects of various influencing factors on clouds in the intertropical convergence zone (ITCZ), recent observations from the BOWTIE campaign are being analysed with regard to the effects of Saharan dust. During the BOWTIE (Beobachtung von Ozean und Wolken – das Trans-ITCZ Experiment) campaign, which was part of the ORCESTRA (Organized Convection and EarthCARE Studies over the Tropical Atlantic) campaign, the research vessel (RV) Meteor crossed the tropical Atlantic from Cape Verde to Barbados in August and September 2024. The proximity to the African continent and the Sahara led to some trajectories of the Saharan air layers (SAL) traversing the atmosphere above the RV Meteor. Three episodes of SAL lasting between two and three days were observed at altitudes spanning from one to five km. The dry and dust-loaden character of the SAL is determined by their origin in the Saharan desert. Analysis of the Raman lidar observations provides information about the time and altitude of the SAL. In synergy with other instruments, such as radiosondes and a motion-stabilised 94GHz cloud radar, the effect on clouds can be investigated. The stabilisation of the cloud radar, which was monitored by STARPAS (STAbilized Radar Platform Alignment Sensor), ensures reliable vertically pointing cloud observations. The findings indicate that SAL reduces cloud vertical development and suppresses weak convection. This behaviour is due to the thermodynamic structure of the SAL with low relative humidity, nearly dry adiabatic temperature gradients and inversions at the top and bottom. Particle-specific properties of acting as cloud condensation nuclei or ice nucleating particles are of secondary order.

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.