The warm conveyor belt (WCB) is one of the three coherent airstreams described by, e.g., and in the so-called conveyor belt model of extratropical cyclones. The WCB is the airstream that ascends the most, rising from the boundary layer in the cyclone's warm sector to the upper troposphere downstream of the cyclone within about 1 d e.g.. Climatologically, WCBs are most frequent in the winter season and they preferentially occur in the main oceanic storm track regions in both hemispheres . WCBs transport heat, moisture and atmospheric pollutants typically poleward e.g.. They are, due to the intense formation of clouds in the ascending airstream, among the strongest precipitating weather systems at the mid-latitudes . Early studies about WCBs focused on clouds and precipitation structure . They mainly relied on cyclone-relative isentropic flow analysis, satellite images and radar observations to identify the WCB, which appear as hammer-shaped cloud bands that are characteristic for extratropical cyclones e.g.. The recent study by investigated the substantial radiative forcing exerted by the extended cloud shields associated with WCBs. Observational studies e.g. revealed the differing microphysical properties of WCB cirrus compared to other types of cirrus and highlighted their potential importance for radiation and climate.

This brief summary of general characteristics of WCBs emphasises their importance for mid-latitude weather including heavy precipitation e.g.. For numerical weather prediction (NWP), WCBs still constitute one of the most challenging weather elements due to their troposphere-deep motion accompanied by cloud thermodynamical processes from typically +20 to -50 ∘C. The presented study investigates a special case of an orographically influenced WCB over central Europe in autumn 2012 using a sequence of measurements taken during the Hydrological Cycle in the Mediterranean Experiment (HyMeX) and T-NAWDEX-Falcon, a preparatory field campaign for the THORPEX North Atlantic Waveguide and Downstream Impact Experiment (T-NAWDEX). This sequence of measurements that are physically related to each other by air parcel trajectories is referred to as Lagrangian matches . To the best of our knowledge, this is the first successful study that describes Lagrangian matches between ground-based and aircraft observations in a WCB, confirmed by a tracer release experiment with a large horizontal and vertical separation between release and intercept. The unique airborne tracer release experiment was performed as part of T-NAWDEX-Falcon to provide direct experimental evidence for the long-range transport by the WCB. The measurements mainly portray the evolution of moisture along the WCB ascent, which is, on the one hand, one of the most important characteristics of the WCB and, on the other hand, still seen as an obstacle for accurate weather forecasts e.g.. The following paragraphs will further discuss (i) why moisture in the WCB inflow is critical for the WCB ascent, (ii) why the evolution of moisture and related diabatic processes along the WCB ascent is important, and (iii) why observations of moisture in WCBs are a valuable supplement to model studies.

Moisture and moist processes, which we address by our measurements, are essential characteristics of the WCB. The cloud formation in the WCB can modify the dynamics of the governing weather system, and the associated latent heat release is essential to achieve the troposphere-deep moist isentropic ascent of WCBs . In addition, latent heat release feeds back on the flow dynamics by modifying potential vorticity (PV) e.g.. To first order, latent heat release due to condensation of water vapour in the ascending WCB leads to positive and negative PV changes below and above the level of maximum condensation, respectively . For instance, the diabatically produced low-level positive PV anomaly during WCB ascent can have an impact on the intensity of the surface cyclone , while the negative PV anomaly in the WCB outflow can interact with the upper-tropospheric waveguide, leading to wave amplification, Rossby wave breaking and the formation of PV streamers e.g..

The simple concept of a diabatically produced PV dipole during the WCB ascent introduced by has been refined in the last decade in at least two important ways. First, it became clear that the microphysical processes within WCBs are more complex, and, second, convection was found to potentially occur embedded within the large-scale cloud band of the WCB. Considering microphysics, were the first to study the detailed cloud structure of a WCB in a regional NWP model. The water species in a WCB evolve along the ascent from low-level water vapour via condensation to liquid clouds and via freezing to mixed-phase clouds and then cirrus near the tropopause. Phase transitions also occur beneath the strongly ascending WCB air parcels due to sedimenting and potentially sublimating or melting hydrometeors e.g.. Governed by such a variety of microphysical processes, this leads to a complex pattern of latent heating in the WCB and regions with latent cooling beneath. The modelling study by showed that the vertical cloud structure of a WCB is affected by seemingly small changes in the implementation of the microphysics, with direct implications for the detailed shape and amplitude of the upper-tropospheric ridge influenced by the WCB outflow. More recently, systematically compared the vertical structure of WCB-related clouds in the latest European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset, ERA5 , with profiles derived from satellite-based lidar and radar observations and concluded that the model reproduces the overall cloud structure quite well but underestimates ice and snow water content in the mixed-phase layer in WCBs above the melting layer. Such weaknesses in models might arise from various assumptions made in microphysical parameterisations that account for the subgrid-scale nature of the cloud processes and often lead to uncertainties in NWP . The phenomenon of WCB-embedded convection was already pointed out in very early observational WCB studies e.g. and then brought up again by . Recently, and confirmed the presence of convection embedded in WCBs with radar observations. and showed that WCB-embedded convection can result in locally enhanced precipitation and that diabatic heating by convection can influence the upper-level jet by the formation of PV dipole bands.

This brief summary reveals the importance of WCBs for understanding precipitation in and the dynamics of extratropical cyclones, but it also indicates the complexity of their ascent behaviour and the involved microphysical processes. It is conceivable that the detailed properties of a WCB strongly depend on the humidity in its inflow. For numerical weather prediction, the correct representation of humidity, particularly in strongly cloudy situations as in WCBs, is still one of the main challenges . However, most of the climatological investigations e.g. and case studies of WCBs e.g. have been based on reanalyses or model simulations only. Comparatively few studies used observations to evaluate the humidity in WCBs. investigated airborne lidar measurements of humidity in the low-level inflow of a WCB over Spain and found an overestimation of humidity in ECMWF analyses. A similar result was found by for the inflow of a marine WCB over the eastern North Pacific. Their sensitivity experiments with the ECMWF forecast model revealed that with the corrected low-level moisture in the initial conditions the WCB outflow would have occurred at a lower potential temperature level and produced a less developed upper-level ridge. The humidity in the WCB inflow, however, is not only determined by boundary layer ventilation . It can also be affected by a recycling of moisture within the WCB, which occurs, e.g., when raindrops from an elevated layer of the WCB fall into a sub-saturated lower layer of the WCB inflow where they evaporate .

Most observational studies of WCBs so far, e.g. based on surface radar measurements or more recently on aircraft and satellite data , provide limited information about the variability of the WCB characteristics along the ascent, which typically covers spatial and temporal dimensions of >1000 km and 1 d, respectively. To better address the Lagrangian nature of these airstreams, a few pioneering field campaign studies attempted to follow the pathway of a WCB with an aircraft and to measure the same WCB air parcels multiple times e.g.. In principle, such Lagrangian matches enable investigating the material evolution of thermodynamic variables along a WCB. A major challenge of such experiments is the planning of the Lagrangian matches realised by aircraft. In contrast to the boundary layer experiment ASTEX in 1992, where additional balloons were used to follow the mainly two-dimensional movement of trade wind clouds e.g., the prediction of the ascending WCB relies on air parcel trajectories using three-dimensional wind fields from forecasts, which are inherently uncertain. To cope with this uncertainty, the planning of Lagrangian matches is best done with data from ensemble forecasts . An interesting observational approach to identifying Lagrangian matches is the use of a physical tracer that is measured at consecutive times to experimentally corroborate the pathway of air parcels. Former experiments have proven that naturally occurring boundary layer trace gases and pollutants can effectively be transported by the WCB to upper levels of the free troposphere . Using this property, an experiment in 2004 described in aimed at realising Lagrangian matches between airborne measurements in the free troposphere to study intercontinental transport of pollutants. One case of the campaign involved a WCB, for which the natural occurrence of a physical tracer was used to mark air parcels. Another extensive experiment over Europe in 1994 simulated an emergency situation by the release of perfluorocarbon (PFC) tracers in the planetary boundary layer . Their second tracer release, lasting 12 h, involved a WCB-like situation of a cold front passage, and it was reported that an aircraft sampled the tracer 700–800 km away from the source . The results of these upper air samples, however, do not appear in the literature and most likely failed. The approach in our study is, for the first time, to specifically focus on studying the transport along a WCB by the release and re-sampling of a synthetic inert tracer . For completeness, we briefly note that Lagrangian matches have also been applied in research on stratospheric chemistry, for instance, by to infer ozone loss rates in the Arctic stratosphere from ozonesonde measurements and by to study the Lagrangian evolution of polar stratospheric clouds from consecutive airborne lidar observations.

Since the majority of the WCBs occur over ocean e.g., not many studies exist about continental WCBs and the potential effects when they rise over mountains. In their climatological study, found precipitation maxima in regions with orographically enhanced WCB ascent. For the European area, a hotspot for WCB ascent and increased precipitation is found along the Mediterranean side of the Alps. There, over 10 % of total precipitation is related to WCBs, and they contribute up to 60 % of extreme precipitation events . In a case study, investigated the flood in the Piedmont region south of the Alps in November 1994, which was associated with a WCB. Using numerical experiments with flattened orography, they could show that the WCB-related precipitation was distinctly enhanced by orography. But so far, no study has investigated how the pathway of WCB trajectories is affected by the interaction with orography.

In October 2012, a team from the German Aerospace Center (DLR) in Oberpfaffenhofen and ETH Zurich organised a small aircraft research campaign, devoted to obtaining in situ measurements of moisture and thermodynamic parameters in different phases of WCBs over Europe . This campaign was termed T-NAWDEX-Falcon and occurred in parallel to the comprehensive HyMeX Special Observation Period 1 (SOP1) . The WCB presented in this study occurred during T-NAWDEX-Falcon's Intensive Observation Period 2 (IOP2), and it ascended from the western Mediterranean across the Alps towards the Baltic Sea. The WCB was successfully sampled by two Falcon flights. The analysis of the airborne observations benefits from additional surface lidar observations made in the framework of HyMeX and from operational radar observations by MeteoSwiss.

In addition, a tracer experiment with the physical release of a passive tracer by a small aircraft in the inflow of the WCB was conducted as part of the T-NAWDEX-Falcon IOP2. The main objective was to “label” WCB air in the inflow and then later “catch” the same air during its ascent by the Falcon aircraft. However, such an experiment can only be successful (i) if a tracer gas is released that otherwise does not exist in the atmosphere or has a very low atmospheric background concentration like PFCs, such that any observation of the tracer can be uniquely associated with the release experiment; (ii) if a suitable and highly sensitive airborne sampling and analysis technique is available, which allows measuring potentially low concentrations after long-range transport and dilution; (iii) if the forecast is exact enough to reproduce a fairly realistic picture of the actual flow situation that serves as the basis for the flight planning; and (iv) if the flight planning method is appropriate such that the tracer is indeed released in the WCB inflow and that the subsequent Falcon flights targeting the ascending WCB air have a chance of sampling the tracer. In this study we report to what degree we were successful with this challenging experiment.

As shown in detail below, the campaign successfully collected valuable observations in the WCB and benefited from overall good ECMWF forecasts and from the sophisticated multi-stage flight planning procedure . This set of observations will enable us to address the following specific questions related to this orographic WCB case study:

How can ensemble analyses be used to provide probabilistic information about the location of WCBs and about Lagrangian matches between observations?

This study uses observations from in situ aircraft measurements, from ground-based lidar and radar, and from a tracer release experiment. The observational datasets are briefly introduced in the following subsections. Then a special diagnostic method, based on ECMWF ensemble analysis data, is introduced that serves to identify WCBs and Lagrangian matches between measurements.

The 2 d IOP2 of the T-NAWDEX-Falcon campaign was planned to take measurements along a WCB ascending from the western Mediterranean over the Alps towards the Baltic Sea. The WCB was induced by a low-pressure system that originated in the central North Atlantic before 13 October 2012. WCB air masses start from the warm sector already during this earlier phase of the cyclone while it moves eastward, steered at the southern flank of a complex upper-level PV cutoff (Fig. a, b; the cyclone is marked by a red “L” in a). Downstream of the cyclone a zonal wind direction prevails at low levels over France at 18:00 UTC on 13 October (Fig. a). North of the Pyrenees and further east over the Mediterranean, the low-level flow is orographically influenced and arrives with increased wind speed and reduced humidity at the northwestern Mediterranean, as reflected by low equivalent potential temperature values. This local wind, called “Tramontane”, is the subject of a case study by as part of the HyMeX campaign. They use observations of the HyMeX lidar that started operating at 21:44 UTC on 13 October (see westernmost red marker for the lidar in Fig. ). Within the westerly flow, WCB trajectories originating over the Atlantic are embedded and start to ascend prior to passing the lidar (Fig. b). At the same time, another WCB branch north of the one from the Atlantic starts to rise over France. These two northern WCB branches with inflow from the Atlantic are directly steered by the surface cyclone that enters France from the Bay of Biscay at 06:00 UTC on 14 October (not shown). The cloud band related to this ascending part of the WCB is visible east and north of the cyclone centre a few hours later in the satellite image (Fig. d and black dots at 12:00 UTC on 14 October in Fig. b).

The cold front of the cyclone is located over the northwestern part of the Iberian Peninsula and western France at 12:00 UTC on 14 October, when the wind direction over the northwestern Mediterranean turned to a moist southwesterly flow onshore (Fig. c). In this flow, ahead of the surface cold front, another WCB branch is discernible at low levels (Fig. b) while this region is covered by heavy convective clouds in the satellite image (Fig. b, d; see also , about the associated heavy convective precipitation event). The inflow of this Mediterranean WCB branch passes the HyMeX lidar just southeast (Fig. b). On a smaller scale, after 15:00 UTC on 14 October a secondary lee cyclone occurs east of the Maritime Alps in the Italian Piedmont region (red “L2” for the mature lee cyclone in Fig. b).

The preceding upper-level PV cutoff forms a streamer-like structure during the development of the main cyclone L and subsequently extends southward over the western Mediterranean (Fig. b). The low-level lee cyclone L2 intensifies due to the interaction with the upper-level PV streamer. The Mediterranean WCB branch shown in Fig. b moves northeastward ahead of the PV streamer. Between 00:00 and 06:00 UTC on 15 October, the WCB branch passes cyclone L2 (Fig. b) and impinges upon the Alps, where it is forced to ascend. There the WCB air stream crosses the Monte Lema radar in the early phase of the ascent (red marker in Fig. b). Within the next 6 h, the Mediterranean WCB air parcels reach the middle troposphere and proceed with a moderate ascent rate over Germany towards the Baltic Sea, where it approaches the Atlantic WCB branch. Along with the WCB, the cold front of the main cyclone L proceeds into central Europe. Together they form the stratiform and scattered cloud band that is visible in the satellite image at 06:00 UTC on 15 October (Fig. a). The research flight IOP2b was designed to climb stepwise along the WCB ascent towards northeast Germany and then to cross the outflow in the area of northern Germany, Poland and the Baltic Sea approximately in an east–west direction (red/violet flight route in Fig. b). In the northernmost part, the flight route traverses the centre of the surface cyclone. As with flight IOP2b, flight IOP2c (blue flight route in Fig. b) also occurs in the region of the WCB and the cold front, which is crossed several times.

South of the Alps the upper-level PV streamer cuts off, and the former lee cyclone L2 evolves into a wide cyclone below the PV cutoff (not shown). The initial cyclone L, however, weakens over the Baltic Sea and re-intensifies later below a newly formed short-wave trough in the hours after the research flights. In the upper levels, the WCB outflow is directed northeastward by the short-wave trough (not shown).

This detailed WCB case study revolves around a unique set of airborne in situ observations taken during the T-NAWDEX-Falcon field experiment, which happened in parallel to the HyMeX SOP1 in autumn 2012. The key object of this study is an orographically influenced WCB ascending with two inflow branches across the Alps towards the Baltic Sea. The WCB shows a rather complex flow behaviour in the vicinity of the Alpine mountains. There is low-level inflow of air into the WCB from both the North Atlantic and the Mediterranean, where the former mainly ascends ahead and the latter rises above the Alps, with some of the lifting being clearly due to mechanically forced orographic ascent at the southern Alpine slopes. This is so far the most detailed WCB case study near complex topography, illustrating that dynamic and orographic lifting can interact in the formation and evolution of a WCB. Assessing the generality of this finding for other WCBs near mountains is left for further research. Two inflow branches with moisture sources in the North Atlantic and the Mediterranean were also found in studies of heavy precipitation events on the south side of the Alps , which emphasises the potential of WCBs in contributing to severe weather in this region . North of the Alps, where the two branches of the WCB further ascend towards the Baltic Sea, two research flights intersected both branches, one at mid-level during ascent and the other later in the outflow.

In order to clearly attribute certain periods of the in situ aircraft observations to the WCB, we used a sophisticated trajectory approach. Kinematic air parcel trajectories are subject to uncertainties in the wind field and to numerical errors in the trajectory computation. To address these uncertainties and to provide more confidence about the identification of the WCB, we determined the WCB probabilistically by calculating trajectories in each of the 11 members of the ECMWF's EDA – an operational dataset that quantifies uncertainties in the representation of the state of the atmosphere.

Observational evidence for the long-range transport in the WCB was obtained from an airborne tracer release experiment. The experiment involved the release of an inert tracer gas in the WCB inflow region in the western Mediterranean by a small aircraft and the sampling of the tracer by the Falcon aircraft 30 h later and 1000 km further north over Germany. The setup of this experiment was unique as previous releases of the same passive tracer were conducted in smaller-scale convective flow systems and included a much smaller spatial and temporal separation of the release and sampling. Before we summarise the results, we need to consider that such an experiment requires (i) a tracer gas with very low natural atmospheric concentrations, (ii) a suitable and highly sensitive airborne sampling and analysis technique, (iii) accurate forecasts for reliable flight planning, and (iv) a sophisticated flight planning method. We can report that increased values of the collected tracer gas coincided with sections of the Falcon flights where trajectory calculations suggested increased probabilities for the tracer air mass. However, the locations of the sampled tracer were characterised by rather low probabilities (i.e. they were not confirmed by many EDA members), and the tracer sampling just occurred at the edge of the WCB. What we cannot evaluate by using trajectories is dispersion or dilution of the tracer gas that possibly occurred by convective mixing in the WCB inflow and at the sampling location near the Baltic Sea for IOP2b and also by turbulence due to wind shear along the cold front. These processes may have occurred in reality, potentially leading to a more widespread tracer plume as shown by our trajectories. Since we regard our technical setup for the experiment and the flight planning as sophisticated , we attribute the relatively low tracer probabilities at the edge of the WCB – instead of the targeted higher tracer probability in the core of the WCB – to a non-perfect forecast as mentioned in (iii) above, to obviously substantial uncertainties in the EDA due to strong wind shear in the region of the WCB and finally to the limited area where the tracer gas was released into the WCB. The fact, however, that the observed transport of the tracer qualitatively agrees well with the WCB pathway as indicated by EDA trajectories provides highly valuable evidence that WCBs, identified on the basis of trajectory calculations in many studies in the last 25 years, are meaningful Lagrangian flow features in the atmosphere.

WCB probabilities were also used to link the Falcon in situ measurements in space and time with earlier observations in the WCB, which we refer to as Lagrangian matches. This approach revealed that part of the WCB inflow was already observed at low levels by a HyMeX lidar near Montpellier and later at the southern slope of the Alps as the WCB passed a MeteoSwiss radar. These Lagrangian matches between in situ airborne and ground-based observations occurred within the WCB air as confirmed by the tracer. The Lagrangian matches were not planned beforehand but were beneficially included later during the analysis of this WCB. EDA members are also used for evaluating humidity and cloud properties with the measurements and to characterise the evolution of the moisture in the model. The key findings from this evaluation can be summarised as follows:

Lidar humidity measurements sampled both branches of the WCB inflow from the Atlantic and Mediterranean during a time period of almost 1 d. Irrespective of the changing wind direction due to the approaching cyclone and the local wind circulation , the observations reveal an almost continuously too-moist boundary layer in the ECMWF analyses. This highlights the difficulty of correctly representing moisture in this complicated flow situation of the WCB inflow, which was influenced by multi-scale processes. Our results agree with earlier observations by and , who also found a too-humid boundary layer in WCB inflows in analysis data.

Apart from these appealing results, it is important to emphasise the limitations of this campaign-driven study. The limitations of the study are related to the fact that, even with sophisticated flight planning, perfect flight routes in terms of maximising WCB encounters and Lagrangian matches have not been possible for several reasons. As an effect, the number of Lagrangian matches between flights and the duration of in situ observations within the WCB are limited. We think that this is the price to pay for such a challenging observational study that essentially relies on flight planning, which in turn was based on non-perfect forecasts. However, we think that this multi-faceted study reveals a range of interesting and, for some of them, unique features of WCBs. This study presented the first case of a WCB with measurements following the Lagrangian ascent and describing the moisture and cloud evolution along the flow. The limited number of observations, however, only allowed to analyse a limited set of processes contributing to the total moisture budget of this WCB. We expect that repeating these kinds of Lagrangian measurements in a WCB with a more complete instrumental package will be rewarding. In addition, the theme of orographic WCBs definitively deserves further attention as an orographically induced change in updraught velocities may modify the dynamics and the water partitioning within the WCB.