NAWDEX - North Atlantic Waveguide and Downstream Impact Experiment

Upper level PV

Potential vorticity, with its property of being conserved in adiabatic motions, is a key quantity to investigate the diabatic influence on the dynamic structure at upper levels. Despite its outstanding dynamical importance, PV is difficult to measure in the atmosphere as this requires the simultaneous observation of horizontal wind and vertical potential temperature gradients, which is a large challenge for available observing systems. Chagnon et al. (2013) investigated diabatic PV production and showed that both negative PV in warm conveyor belt outflows related to latent heating and positive PV on the stratospheric side of the tropopause related to radiative processes, i.e. LW cooling impact the tropopause PV structure. These processes can change the PV gradient, which in turn is highly relevant for the downstream propagation of Rossby waves. Since significant uncertainties exist in the representation of diabatic processes affecting these PV gradients in numerical models, it is highly desirable to confront model-derived PV structures with those estimated from direct wind and temperature observations.

Several earlier studies have estimated PV from aircraft data (e. g., Shapiro, 1978; Hartmann et al., 1989; Vaughan et al., 1996). They used in-situ wind observations to calculate along-track wind gradients. To gather vertical temperature information, the in-situ wind observations had to be combined with lapse-rate data from dropsondes or a microwave temperature profiler. However, these observations were limited to PV estimates along the flight track. Information about PV at different altitudes could only be obtained by flying the aircraft at different vertical levels. This limits the possibility of temporally collocated measurements and only very limited vertical resolution can be achieved with flight legs at different altitudes.

During NAWDEX range-resolved temperature and wind profiles will be observed by a dropsondes and a wind lidar. Unlike the earlier studies, this new effort will use range resolved remote sensing to estimate PV in a 2-dimensional cross-section. The range resolved temperature profiles allow determining lapse rates at different altitudes and the scanning wind-lidar that measures the wind vector beneath the aircraft allows to estimate horizontal wind gradients in flight direction.

Upper level PV
Research Questions	How much of the total PV is reproduced by approximated PV determined from observations? How much information is contained in the wind field versus thermodynamic gradients? Can diabatically generated PV features be tracked to evaluate model (non) conservation?
Planned Observations	HALO & Falcon (wind lidar, dropsondes & radiometer temperatures)
Obsevation strategy	repeated across-jet flight legs perpendicular to the wind direction enter a region of divergent WCB outflow along-jet control leg to investigate the assumption of small variation flight in region where dropsondes are allowed, drop at high frequency
Schematic flight plan:

Chagnon J, Gray S, Methven J. 2013. Diabatic processes modifying potential vorticity in a North Atlantic cyclone. Q. J. R. Meteorol. Soc., 139: 1270-1282.

Hartmann, D. L., et al., 1989: Potential vorticity and mixing in the south polar vortex during spring. J. Geophys. Res. Atmos., 94: 11 625-640.

Shapiro MA. 1978. Further evidence of the mesoscale and turbulent structure of upper level jet stream-frontal zone systems. Mon. Wea. Rev., 106: 1100-1111.

Vaughan, G., S. J. Pepler, and D. S. McKenna, 1996: Aircraft measurements of potential vorticity and Richardson number in baroclinic zones. Quart. J. Roy. Meteor. Soc., 122: 721-735.

NAWDEX goals:

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