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Since the start of the European Space Agency's Aeolus mission in 2018, various studies were dedicated to the evaluation of its wind data quality and particularly to the determination of the systematic and random errors in the Rayleigh-clear and Mie-cloudy wind results provided in the Aeolus Level-2B (L2B) product. The quality control (QC) schemes applied in the analyses mostly rely on the estimated error (EE), reported in the L2B data, using different and often subjectively chosen thresholds for rejecting data outliers, thus hampering the comparability of different validation studies. This work gives insight into the calculation of the EE for the two receiver channels and reveals its limitations as a measure of the actual wind error due to its spatial and temporal variability. It is demonstrated that a precise error assessment of the Aeolus winds necessitates a careful statistical analysis, including a rigorous screening for gross errors to be compliant with the error definitions formulated in the Aeolus mission requirements. To this end, the modified Z score and normal quantile plots are shown to be useful statistical tools for effectively eliminating gross errors and for evaluating the normality of the wind error distribution in dependence on the applied QC scheme, respectively. The influence of different QC approaches and thresholds on key statistical parameters is discussed in the context of the Joint Aeolus Tropical Atlantic Campaign (JATAC), which was conducted in Cabo Verde in September 2021. Aeolus winds are compared against model background data from the European Centre for Medium-Range Weather Forecasts (ECMWF) before the assimilation of Aeolus winds and against wind data measured with the 2 µm heterodyne detection Doppler wind lidar (DWL) aboard the Falcon aircraft. The two studies make evident that the error distribution of the Mie-cloudy winds is strongly skewed with a preponderance of positively biased wind results distorting the statistics if not filtered out properly. Effective outlier removal is accomplished by applying a two-step QC based on the EE and the modified Z score, thereby ensuring an error distribution with a high degree of normality while retaining a large portion of wind results from the original dataset. After the utilization of the described QC approach, the systematic errors in the L2B Rayleigh-clear and Mie-cloudy winds are determined to be below 0.3 m s−1 with respect to both the ECMWF model background and the 2 µm DWL. Differences in the random errors relative to the two reference datasets (Mie vs. model is 5.3 m s−1, Mie vs. DWL is 4.1 m s−1, Rayleigh vs. model is 7.8 m s−1, and Rayleigh vs. DWL is 8.2 m s−1) are elaborated in the text.

Numerical weather prediction models (NWP) are known to possess a distinct moist bias in the mid-latitude lower stratosphere which is expected to affect the ability to accurately predict weather and climate. This paper investigates the vertical structure of this bias in the European Centre for Medium-Range Weather Forecast’s (ECMWF) latest global reanalysis ERA5 using a unique multi-campaign data set of highly-resolved water vapor profiles observed with a differential absorption lidar (DIAL) onboard the High Altitude and LOng Range Research Aircraft (HALO). In total, 41 flights in the midlatitudes provide more than 31000 humidity profiles varying by four orders of magnitude. The data set covers different synoptic situations and seasons and thus is suitable to characterize the strong vertical gradients in the upper troposphere and lower stratosphere (UTLS). The comparison to ERA5 indicates high positive and negative deviations in the UT which on average lead to a slightly positive bias (+20 %). In the LS, the bias rapidly increases up to a maximum of +55 % at 1.3 km altitude above the thermal tropopause (tTP), and decreases again to 15–20 % at 4 km altitude. This vertical structure is reproduced in all flights. The depth of the layer of increased bias is smaller at high tropopause altitudes and larger when the tropopause is located low. Our results also suggest a seasonality of the bias, with the maximum in summer exceeding fall by up to a factor of 3. During one field campaign, co-located ozone and water vapor profile observations enable a classification of the observations into tropospheric, stratospheric and mixed air using H2O-O3 correlations. It is shown that the bias is higher in the mixed air while being small in tropospheric and stratospheric air which highlights that excessive transport of moisture into the LS plays a decisive role for the formation of the bias. Future climatological studies should consider the analysed lower-stratospheric moist bias in ERA5. Our results show that a better representation of mixing processes in NWP models could lead to a reduced LS moist bias that, in turn, may have a positive impact on weather and climate forecasts. The moist bias should be borne in mind for climatological studies using reanalysis data.

The composition of the extratropical transition layer (ExTL), which is the transition zone between the stratosphere and the troposphere in the midlatitudes, largely depends on dynamical processes fostering the exchange of air masses. The Wave-driven ISentropic Exchange (WISE) field campaign in 2017 aimed for a better characterization of the ExTL in relation to the dynamic situation. This study investigates the potential of the first-ever collocated airborne lidar observations of ozone (O3) and water vapor (H2O) across the tropopause to depict the complex trace gas distributions and mixing in the ExTL. A case study of a perpendicular jet stream crossing with a coinciding strongly sloping tropopause is presented that was observed during a research flight over the North Atlantic on 1 October 2017. The collocated and range-resolved lidar data that are applied to established tracer–tracer (T–T) space diagnostics prove to be suitable to identify the ExTL and to reveal distinct mixing regimes that enabled a subdivision of mixed and tropospheric air. A back projection of this information to geometrical space shows remarkably coherent structures of these air mass classes along the cross section. This represents the first almost complete observation-based two-dimensional (2D) illustration of the shape and composition of the ExTL and a confirmation of established conceptual models. The trace gas distributions that represent typical H2O and O3 values for the season reveal tropospheric transport pathways from the tropics and extratropics that have influenced the ExTL. Although the combined view of T–T and geometrical space does not inform about the process, location and time of the mixing event, it gives insight into the formation and interpretation of mixing lines. A mixing factor diagnostic and a consideration of data subsets show that recent quasi-instantaneous isentropic mixing processes impacted the ExTL above and below the jet stream which is a confirmation of the well-established concept of turbulence-induced mixing in strong wind shear regions. At the level of maximum winds reduced mixing is reflected in jumps in T–T space that occurred over small horizontal distances along the cross section. For a better understanding of the dynamical and chemical discontinuities at the tropopause, the lidar data are illustrated in isentropic coordinates. The strongest gradients of H2O and O3 are found to be better represented by a potential vorticity-gradient-based tropopause compared to traditional dynamical tropopause definitions using constant potential vorticity values. The presented 2D lidar data are considered to be of relevance for the investigation of further meteorological situations leading to mixing across the tropopause and for future validation of chemistry and numerical weather prediction models.

International audience; ESA’s Aeolus satellite observations are expected to have the biggest impact for the improvement of numerical weather prediction in the Tropics. An especially important case relating to the evolution, dynamics, and predictability of tropical weather systems is the outflow of Saharan dust, its interaction with cloud microphysics and impact on the development of tropical storms over the Atlantic Ocean. The Atlantic Ocean off the coast of West Africa and the eastern Caribbean uniquely allows the study of the Saharan Aerosol layer, African Easterly Waves and Jet, Tropical Easterly Jet, as well as the deep convection in the Intertropical Convergence Zone and their relation to the formation of convective systems, and the long-range transport of dust and its impact on air quality. The Joint Aeolus Tropical Atlantic Campaign (JATAC) deployed on Cabo Verde and the US Virgin Islands is addressing the validation and preparation of the ESA missions Aeolus, EarthCARE and WIVERN, as well as supporting the related science objectives raised above. The JATAC campaign started in July 2021 with the deployment of ground-based instruments at the Ocean Science Center Mindelo (OSCM, Cabo Verde), including the EVE lidar, the PollyXT lidar, a W-band Doppler cloud radar and a sunphotometer. By mid-August, the CPEX-AW campaign started their operations from the US Virgin Islands with NASA’s DC-8 flying laboratory in the Western Tropical Atlantic and Caribbean with the Doppler Aerosol Wind Lidar (DAWN), Airborne Precipitation and Cloud Radar (APR-3), the Water Vapor DIAL and HSRL (HALO), a microwave sounder (HAMSR) and dropsondes. In September, a European aircraft fleet was deployed to Sal (Cabo Verde) with the DLR Falcon-20 carrying the Aeolus Airborne Demonstrator (A2D) and the 2-µm Doppler wind lidar, and the Safire Falcon-20 carrying the high-spectral-resolution Doppler lidar (LNG), the RASTA Doppler cloud radar, in-situ cloud and aerosol instruments among others. The Aerovizija Advantic WT-10 light aircraft with filter-photometers and nephelometers for in-situ aerosol characterisation was operating in close coordination with the ground-based observations from Mindelo. More than 35 flights of the four aircraft were performed. 17 Aeolus orbits were underflown, four of which completed by simultaneous observations of three aircraft, with a perfect collocation of Aeolus and the ground-based observation for two cases. Several flights by the NASA DC-8 and the Safire Falcon-20 have been dedicated to cloud microphysics and dust events. The EVE lidar has been operating on a regular basis, while the PollyXT and several other ground-based instruments were continuously operating during the campaign period. For further characterisation of the atmosphere, radiosondes were launched up to twice daily from Sal airport. Additionally, there were radiosonde launches from western Puerto Rico and northern St Croix, US Virgin Islands. The JATAC was supported by dedicated numerical weather and dust simulations supporting the forecasting efforts needed for successful planning of the flights and addressing open science questions. While the airborne activities were completed end September, the ground-based observations are continuing into 2022. The paper will present an overview and initial results of JATAC. In memory of our colleague and friend Gail.

Warm conveyor belts (WCBs) are important airstreams in extratropical cyclones, often leading to the formation of intense precipitation and the amplification of upper-level ridges. This study presents a case study that involves aircraft, lidar and radar observations in a WCB ascending from western Europe towards the Baltic Sea during the Hydrological Cycle in the Mediterranean Experiment (HyMeX) and T-NAWDEX-Falcon in October 2012, a preparatory campaign for the THORPEX North Atlantic Waveguide and Downstream Impact Experiment (T-NAWDEX). Trajectories were used to link different observations along the WCB, that is, to establish so-called Lagrangian matches between observations. To this aim, an ensemble of wind fields from the global analyses produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) Ensemble of Data Assimilations (EDA) system were used, which allowed for a probabilistic quantification of the WCB occurrence and the Lagrangian matches. Despite severe air traffic limitations for performing research flights over Europe, the German Aerospace Center (DLR) Falcon successfully sampled WCB air masses during different phases of the WCB ascent. The WCB trajectories revealed measurements in two distinct WCB branches: one branch ascended from the eastern North Atlantic over southwestern France, while the other had its inflow in the western Mediterranean. Both branches passed across the Alps, and for both branches Lagrangian matches coincidentally occurred between lidar water vapour measurements in the inflow of the WCB south of the Alps, radar measurements during the ascent at the Alps and in situ aircraft measurements by Falcon in the WCB outflow north of the Alps. An airborne release experiment with an inert tracer could confirm the long pathway of the WCB from the inflow in the Mediterranean boundary layer to the outflow in the upper troposphere near the Baltic Sea several hours later. The comparison of observations and ensemble analyses reveals a moist bias in the analyses in parts of the WCB inflow but a good agreement of cloud water species in the WCB during ascent. In between these two observations, a precipitation radar measured strongly precipitating WCB air located directly above the melting layer while ascending at the southern slopes of the Alps. The trajectories illustrate the complexity of a continental and orographically influenced WCB, which leads to (i) WCB moisture sources from both the Atlantic and Mediterranean, (ii) different pathways of WCB ascent affected by orography, and (iii) locally steep WCB ascent with high radar reflectivity values that might result in enhanced precipitation where the WCB flows over the Alps. The linkage of observational data by ensemble-based WCB trajectory calculations, the confirmation of the WCB transport by an inert tracer and the model evaluation using the multi-platform observations are the central elements of this study and reveal important aspects of orographically modified WCBs.

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 H2O differential absorption lidar at 935 nm has successfully been developed and extensively tested at DLR in the past, we describe here an amendment of this lidar by the addition of an ultraviolet (UV) channel to measure ozone. The transmitter of the ozone differential absorption lidar (DIAL) is based on a near-IR optical parametric oscillator that is frequency-converted into the UV spectral range by intracavity sum frequency mixing. Hereby, a continuous UV tuning range of ∼297–317 nm has been achieved. The average output power in this range is higher than 1 W corresponding to more than 10 mJ per pulse at a repetition rate of 100 Hz. The ozone DIAL system has been carefully characterized both on the ground and in flight. The first simultaneously measured two-dimensional cross-sections of ozone and water vapor in the upper troposphere and lower stratosphere have been recorded during the Wave-driven Isentropic Exchange (WISE) field campaign in 2017 demonstrating the high potential of this system for studying exchange processes in this region of the atmosphere.

Since the successful launch of ESA's Earth Explorer mission Aeolus in August 2018, atmospheric wind profiles from the ground to the lower stratosphere are being acquired on a global scale deploying the first-ever satellite-borne wind lidar system ALADIN (Atmospheric LAser Doppler INstrument). ALADIN provides one component of the wind vector along the instrument's line-of-sight (LOS) with a vertical resolution of 0.25 km to 2 km depending on altitude, while the precision in wind speed is between 2 m/s to 4 m/s. The near-real-time wind observations contribute to improving the accuracy of numerical weather prediction and advance the understanding of tropical dynamics and processes relevant to climate variability. Already several years before the satellite launch, an airborne prototype of the Aeolus payload, the ALADIN Airborne Demonstrator (A2D), was developed at DLR (German Aerospace Center). Due to its representative design and operating principle, the A2D has since delivered valuable information on the wind measurement strategies of the satellite instrument as well as on the optimization of the wind retrieval and related quality-control algorithms. Broad vertical and horizontal coverage across the troposphere is achieved thanks to the complementary design of the A2D receiver, which, like ALADIN, comprises a Rayleigh and Mie channel for analysing both molecular and particulate backscatter signals. In addition to the A2D, DLR's research aircraft carries a well-established coherent Doppler wind lidar (2-µm DWL) which allows determining the wind vector with accuracy of better than 0.1 m/s and precision of better than 1 m/s. Hence, both instruments represent key instruments for the calibration and validation activities during the Aeolus mission.

After its launch in autumn 2018, the spaceborne wind lidar ALADIN (Atmospheric LAser Doppler INstrument) on-board ESA's Earth Explorer satellite Aeolus will allow for global observation of atmospheric wind profiles. Being the first ever satellite-borne Doppler wind lidar instrument, ALADIN will significantly contribute to the improvement in numerical weather prediction by providing one component of the wind vector along the instrument's line-of-sight (LOS) from ground throughout the troposphere up to the lower stratosphere. The vertical resolution is 0.25 km to 2 km depending on altitude, while the precision in wind speed is envisaged to be between 1 m/s to 3 m/s.

In preparation of ESA's upcoming Earth Explorer mission Aeolus which strives for the global observation of wind profiles from the ground to the lower stratosphere deploying the first-ever satellite-borne wind lidar system ALADIN, the ALADIN airborne demonstrator (A2D) has been developed at DLR (German Aerospace Center). Due to its representative design and operating principle, the A2D provides valuable information on the wind measurement strategies of the satellite instrument as well as on the optimization of the wind retrieval and related quality-control algorithms. Hence, it represents an essential testbed for the planned calibration and validation activities after the launch of Aeolus which is scheduled for end of August 2018.