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NAWDEX-Influence: The North Atlantic Waveguide and Downstream Impacts Experiment - Influence on weather in western Europe

Start date: 01-09-2016 - End date: 30-11-2016

Status: Confirmed

Open to sharing: Yes

Confidential: No

Transnational Access: Yes

Open to training: No

Grounded / Maintenance: No


Aircraft name: FA20 - DLR

Airport: The team led by DLR intend to base the DLR Falcon in Iceland during the NAWDEX campaign with the possibility of temporary operation from Shannon, Ireland. On the days when we use the aircraft, we intend to operate closer in to the UK observation network, most likely to the north of Ireland and west of Scotland. This is feasible from Iceland or Shannon in one flight, or perhaps double flights to maximise time on the science objectives.

Project description

Project theme: TA-008: Applications of atmospheric in-situ measurements - dynamics of weather systems

Science context: The worst forecasts for Europe share a distinct Rossby wave precursor with a prominent ridge bringing moist air across the eastern USA (Rodwell, 2013, BAMS). They hypothesised that the representation of diabatic processes over the USA is at fault. However, there are 3 other plausible reasons for the large forecast error: i) The negative PV anomaly in the outflow from ascending warm conveyor belts resembles a thin lens. Forecast models cannot represent the vertical structure of the lens, with consequences for Rossby wave propagation. ii) Air adjacent to the tropopause is modified by diabatic processes which are not well represented in models. In particular, longwave cooling is sensitive to the profile of water vapour and cirrus cloud. iii) The PV gradient at the tropopause is smoothed by numerical dissipation in NWP models and this affects jet strength and Rossby wave propagation. A unique opportunity exists to stage an international experiment spanning the North Atlantic in autumn 2016, working with the HIAPER and HALO aircraft flying near North America and Iceland. In this project, the DLR-Falcon would characterise the structure of the negative PV lens next to the tropopause as it approaches Europe. In concert, the aircraft will conduct a quasi-Lagrangian experiment to quantify the time-integrated diabatic transformation of air masses. An enhanced ground-based network across the UK will observe high impact weather events and their interaction with waveguide disturbances. For example, tropopause mesovortices that are associated with the roll-up of streamers (called "dry eyes") are often implicated in the development of severe weather.

Measurements to be made by aircraft: Scientific objectives Overarching scientific aim of NAWDEX: to increase physical understanding and quantify the effects of diabatic processes on disturbances to the jet stream, their influence on downstream propagation across the North Atlantic, and consequences for high impact weather in Europe. The specific objectives below are classified in two ways. A) denotes objectives addressed by the international NAWDEX campaign where the UK team have taken a central role in devising the experimental plan and have published relevant papers. F) denotes objectives relating particularly to the DLR Falcon flights proposed here as a EUFAR project. A.1: Conduct an observation experiment, spanning the North Atlantic with international partners to determine the integrated effects of diabatic processes following air masses that influence the jet stream and its properties as a waveguide across the Atlantic. The specific role of the DLR Falcon in the project proposed here will be to: F.1: examine the structure of the PV gradient at the jet stream and the negative PV lenses that are advected from the outflow of WCBs under the tropopause where the jet stream forms a ridge. F.2: observe features that connect high impact weather in western Europe (especially as it crosses the UK) and detailed structure on the jet stream, such mesovortices associated with “dry eyes” and multiple tropopause folds. A.2: Advance the theory describing the dynamical coupling between diabatic processes, disturbances growing and propagating along the jet stream, and downstream weather. A.3: Quantify the predictability of high impact weather (precipitation and strong surface winds) and associated risks in Europe, conditional on knowledge of the jet stream disturbance upstream. Utilising the new observations to distinguish the 3 mechanisms proposed in Section 1 The primary scientific hypothesis for NAWDEX is that the poorest forecasts for European weather are related to systematic errors in the representation of diabatic processes and the North Atlantic waveguide. At the upstream end of the North Atlantic, the air in the warm sector of each cyclone flows polewards and ascends in a warm conveyor belt (WCB) from the boundary layer near SE USA into the ridge at tropopause level. Diabatic processes, such as turbulent fluxes, cloud microphysics and convection in the WCB influence the net heating, the level of the outflow layer and the direction taken by outflow air masses (Martinez-Alvarado, QJRMetS, 2014). Greater net heating amplifies the negative potential vorticity (PV) anomaly in the shallow outflow layer, which is associated with stronger anticyclonic circulation. Therefore misrepresentation of these diabatic processes could affect downstream development of Rossby waves. The systems in the west Atlantic will not be investigated directly by the EUFAR Falcon flights, although it will be examined using the data from NAWDEX partners combined with NWP model simulations and sensitivity experiments. The EUFAR flights will focus on the 3 mechanisms described in the Summary relating to the structure of the jet stream and the negative PV lenses as they approach Europe and their influence on high impact weather in western Europe. Mechanism (i) will be addressed by the EUFAR Falcon flights. Measurements of the vertical structure in wind, static stability and water vapour of air masses near the tropopause and the jet stream will be compared with the structures simulated by NWP models. The ramifications of differences in PV structure for the “induced flow” and influence on Rossby wave propagation will be assessed using a PV inversion tool and quantitative comparison with water vapour channel satellite imagery (as developed both at the Met Office and Meteo France). A similar approach will be taken in the examination of mesoscale features at tropopause level, including the “dry eyes” identified in satellite imagery. Mechanism (ii) will be addressed in two ways. The net diabatic transformation within air masses adjacent to the jet stream will be quantified using connected observations between the DLR Falcon and upstream flights of the HIAPER and/or HALO aircraft. This has been shown to be possible in the atmospheric chemistry context by Methven et al (2006, JGR). Secondly, the DLR Falcon will be able to observe cloud top using the downward pointing LIDAR. This data can be compared with simulated data derived from NWP models. Mechanism (iii) relates to the smoothing effects of numerical dissipation on the PV gradient in NWP models and ramifications for Rossby wave evolution. The Falcon flights (and those of upstream aircraft) will provide detailed observations of the PV gradient structure (wind shear and static stability contrast) across the poleward flank of the jet stream and the related sharpness of the jet maximum. Research outcomes O.1: A unique observational dataset, sampling triggering, propagation and the downstream impact of disturbances along the North Atlantic waveguide associated with the jet stream. O.2: Detailed observation of structure in winds, temperature and water vapour near the tropopause in jet stream ridges with particular focus on the thin lenses associated with negative PV anomalies in air that has recently ascended from the lower troposphere to tropopause level. O.3: Quantification of the physical processes that have most effect on the upstream waveguide disturbances and their subsequent propagation across the Atlantic. O.4: Quantification of the enhanced predictability of European weather phenomena that are associated with mesoscale structure at the tropopause (such as dry eyes). The delivery of objectives A.2 and A.3 will be dependent on research staff time that will form a major component of a proposal intended for the NERC standard grant round in July 2015. If successful, the EUFAR funding of DLR Falcon flight hours will be treated as co-funding which will lend added weight to the case for support to NERC.

Season: The dates are set for the international NAWDEX experiment and the ground-based IOP, as described above. The maximum aircraft window is 12/9/16-23/10/16. Agreement to share aircraft time The transit of the DLR Falcon aircraft from southern Germany to Iceland (or Ireland, depending on the jet stream) and its return will be covered by the DLR project during NAWDEX. This will enable us to maximise the proportion of EUFAR-funded flight hours spent on task. We will work with the scientific coordinator for NAWDEX in DLR (Andreas Schaefler).

Weather constraints: Most of the Falcon flight time will be spent at high altitude, either crossing the jet in the upper troposphere or above the tropopause using the downward pointing wind LIDAR. The target location for objective F.1 will be in the large-scale ridges at tropopause level. The LIDAR return is attenuated by cloud and therefore cloud free conditions (at least in the upper troposphere are desirable). However, where there is extensive cirrus just below the tropopause it may be better to fly below the cirrus (but above lower cloud decks). The target may be air masses that have already been sampled upstream by the HIAPER or HALO aircraft and this is likely to introduce a strong time constraint based on the forecast air mass trajectories and locations when they are in range of the Falcon. Otherwise the requirement to fly within a large-scale ridge is a weak time constraint on take-off. Clear air turbulence is likely on flight levels crossing the jet, especially where it has strong curvature. The target for objective F.2 are the dry eye features that can be identified first in forecasts and then refined using near real-time satellite images. The lack of cloud within a "dry eye" makes them particularly suitable for observation from above using the wind LIDAR.

Time constraints: The time window for the international NAWDEX campaign was determined to be the September/October 2016 period at a meeting in March 2014 convened by the THORPEX PDP Working Group (the PI is a member). Since then the HIAPER aircraft and HALO aircraft have been confirmed for 12/9/16-23/10/16. DLR intend to use the DLR Falcon aircraft during the window, as discussed in Section 1. The enhanced ground-based observation network will operate for a longer period starting in early September 2016 until the end of October 2016.

Flights (number and patterns): Ideally we require 18 flight hours for 5 reasonably long flights – some of the time will be spent getting into the task area, depending on the location of the jet stream and the location of the Falcon when it switches use from DLR to this team. Ten hours would be our minimum (equating to 3 flights) since any less and we would be unlikely to be able to utilise effectively the link with the ground-based network, given the variability in the jet stream and the requirement to link with upstream international aircraft activity. The proposed research is made feasible with the EUFAR flight hours suggested because there is a strong link across the whole NAWDEX experiment and established collaboration across the group. Flight patterns: P.1: Negative PV lens Fly to location in mid-troposphere under PV lens. Profile up through lens into the stratosphere, flying in the along-flow direction. Conduct a LIDAR cross-section in the across-flow direction. Profile down through the lens and back up along a parallel track to obtain two further profiles in the along-flow direction. Conduct a second LIDAR cross-section. Profile back down through the lens and return to base. P.2: Dry eye Similar to P.1 but the feature is much smaller scale a moving fast. The initial way-point will be very important to ensure that the first upward profile passes through the feature. Then conduct two shorter LIDAR cross-sections perpendicular to each other by flying over the dry eye. Profile down through the centre of the dry eye. If time conduct in situ legs at a range of levels ahead of the dry eye feature (where ascent occurs). P.3: Quasi-Lagrangian interception The location of the target air mass is forecast using an ensemble of forecast forward trajectories from an upstream flight that has just completed. The greatest uncertainty in achieving a matching Lagrangian sample is the position of the air mass in the vertical. Chances are maximised by changing altitude until observed theta-e matches the value covered by the linking legs of the upstream aircraft. Vertical profiles through the air mass are best conducted in the along-flow direction to reduce variability and enhance information gained from the time elapsed between the matching flights. Using the experimental methodology established by Methven et al (2006, JGR).

Instruments: None

Other constraints: None.

Scientific contact

Name: METHVEN John

PI email: