T-NAWDEX Pilot - Flight Planning Document

Transcription

1 T-NAWDEX Pilot - Flight Planning Document John Methven and Tom Frame October 30, Experiment Aims The main goal of this experiment is to trial flights designed to quantify the net effects of diabatic processes within extratropical cyclones. The results will be used as proof of concept for proposal bids to fund a major international experiment in autumn 2012 called T-NAWDEX (THORPEX North Atlantic Waveguide and Downstream impacts EXperiment). THORPEX is a project of the World Weather Research Programme which is coordinated by the World Meteorological Organization. T-NAWDEX is proposed as the next major campaign for THORPEX. Its focus is the diabatic modification of air masses brought into contact with the tropopause, the resulting changes to waves on the jetstream at tropopause level and consequences for downstream propagation of Rossby waves and their impacts on forecasts for Europe. 2 Contacts The PIs of the T-NAWDEX Pilot project are: Ian Renfrew; University of East Anglia John Methven, Suzanne Gray, Jeffrey Chagnon and Tom Frame; University of Reading Geraint Vaughan; University of Manchester Doug Parker, Stephen Mobbs, Peter Knippertz and John Marsham; University of Leeds Phil Brown; Met Office See the campaign website: swrmethn/tnawdex/ 3 Situations of scientific interest I. Warm conveyor belt (WCB) transect This is our primary focus and would be an absolute must if the situation arises during the campaign window. The aim is to intercept a warm conveyor belt where it is ascending and perform at least two high level dropsonde transects across it, separated as far as possible in time and distance along the WCB. We would also like to fly lower legs measuring water in all phases and temperature within the active latent heating region. The scientific interest is the inference of volume averaged non-conservative processes following the warm conveyor belt air mass and its relation to the same processes represented in numerical forecast models. Ozone and CO will be useful for labelling air masses. 1

2 II. Influence of boundary layer fluxes on low level cyclone development Addresses a second type of European forecast bust, when a cyclone develops rapidly at low levels below a jet exit with particular focus on the gradients of moisture and temperature at low levels, especially over the ocean, and the boundary layer (BL) fluxes there. Recent research by Boutle et al has shown that horizontal fluxes of moisture within the BL are crucial to system development. III. Bent-back warm fronts and cloud heads Essentially a back-up plan if WCBs do not pass within range on a suitable day for a flight. Bent-back warm fronts and cloud heads are also regions where diabatic processes are especially strong within cyclones. They are regions of high SCAPE (slantwise convective available potential energy) which can result in slantise convection, banding in fronts and rolls. All these phenomena are not well represented in NWP models and yet result in strong diabatic modification of air masses. 4 Proposed Sorties Situations I and III require rather similar flight plans, but within different air masses of a cyclone. Both involve long dropsonde transects which would best be performed over the sea. Situation II requires low level flying over the ocean. Therefore all flights are most likely to be to the west of the UK in NOTAM areas A or B in Fig. 1, in Irish airspace or in the East Atlantic maritime sector. I. Warm conveyor belt (WCB) transect Two types of sortie are described here: a single flight and double header. Our preference would be for a double header with a refuel stop closer to the operational area in the west (Exeter, Newquay, Prestwick or Shannon). A stop of several hours would allow the airmass to move several hundred kilometres so that the first flight can sample the WCB as far downstream as possible and the second as far upstream as possible. It is probably not feasible to do the flights on consecutive days because the system will move too far in this time. Also less time would be wasted on transit. However, operational constraints during the period mean that this would only work if the first flight took off in the morning so that the second flight returns in the evening before closure of Cranfield. If the system is only in a suitable operating area in the afternoon it may be better to do a single flight. I.1 Single flight sortie brief 1. Transit to first waypoint which will be the starting point of a section across the front sampling the WCB and crossing the jetstream. Aim of transit is to be as fuel efficient as possible while attaining highest feasible altitude for the start of the section. In Fig. 2 this point is to the SE of the ascending airmass at low levels. 2. The aircraft then flies towards the NW crossing above the WCB which slants upwards (Fig. 3), crossing through the jetstream and eventually crossing the tropopause (marked by high PV in Fig. 3). If the system were closer to Cranfield it maybe better to cross the 2

3 Figure 1: NOTAM area map used to indicate area of operation to UK air traffic control. Ignore the hand written coordinates which are incorrect. front in the opposite sense (from stratosphere to troposphere). The aim is to drop at least 5 sondes along this downstream section. 3. Turn onto upstream heading. Make this leg last as long as flight will allow to give greatest spacing between upstream and downstream sections. Make good use of the leg by descending to a level within WCB (as identified from sondes) and then crossing front at a shallow angle while heading mainly upstream. This will give good resolution across the tropopause, jetstream and WCB. 4. Profile ascent to maximum feasible altitude, positioning to one side of upper front. 5. Turn in cross-front direction for upstream section. Again could be from stratosphere to troposphere or vice versa, depending on greatest time saving for return to base. 6. If sufficient time, turn about and descend to level of interest within WCB (identified from sondes) and perform level run across with in situ measurements. 7. If possible cross again on another leg below this, perhaps with dip down into the BL to complete the vertical profile with in situ measurements. 3

4 8. Return to base at cruise altitude. Figure 2: Single flight illustration using a WCB example from 12UT 20/10/08. Left: Change in pressure following trajectories arriving at 12UT on a fine grid covering the 800hPa surface. Yellow/green/blue indicates air that has ascended over the last 1.5 days. The green strip running across Cornwall is the bottom edge of the WCB. Right: Potential vorticity (PV) on 300hPa surface. Red indicates stratospheric air. Figure 3: Curtains along the flight tracks sketched above. Left: change in pressure following trajectories. The aircraft first crosses the WCB centre at 600km along the section. Note two distinct centres: the active WCB over the UK has maximum ascent at FL180, while the air in the jetstream FL300 ascended along East Coast USA and then travelled rapidly across the Atlantic over the last day. Right: PV along the curtain. Note how the downstream section leg crosses into the stratosphere (red) before turning upstream. Double header sortie brief 4

5 1. Take-off in morning considering maximum duty hours for pilots and latest landing at Cranfield (18:45). 2. Transit to first waypoint which will be the starting point of a downstream section across the front sampling the WCB as for single flight. This is likely to be a long transit and should reach maximum altitude towards the first waypoint on nearside of front. 3. Head in cross-frontal direction for downstream dropsonde leg as item 2 of single flight. 4. Turn upstream as item 3 of single flight. 5. Consider flight time remaining and total constraint on day. If time perform another crossfrontal section as in items 4+5 of single flight. 6. Head for refuel base, preferably at lower altitude with active cloud processes. 7. Time on ground is important. Minimum refuel time is about 90 minutes. However, longer on the ground gives a greater interval between sections across the WCB. Wait for a long as possible given constraint on return to Cranfield after second 4 hour flight. 8. Head for closest area within or below WCB, preferably over the sea. 9. Turn upstream and descend to minimum safe altitude and perform leg sampling BL fluxes. 10. Ascend to level above BL to one side of the front. 11. Turn across front and fly through WCB for in situ measurements. 12. Repeat legs a higher, stepping up in altitude. 13. Attain maximum altitude for final upstream section directly across front (but in the towards home sense). Drop sondes as closely as possible. 14. Return to Cranfield. II. Influence of boundary layer fluxes on low level cyclone development 1. Transit towards region of strong BL θ e gradients where developing system is forecast. 2. Profile descent into marine BL at point before forecast gradient on low pressure side. 3. Long level run across gradient well into high pressure area. 4. Reciprocal turn, profile descent to minimum safe altitude and run across gradient. 5. Profile ascent to highest altitude, zig-zagging across front several times. 6. Drop curtain of sondes across front if time allows. 7. Return at altitude. III. Bent-back warm fronts and cloud heads Similar to situation (I), except that target airmass will be air drawn into occlusion around bentback warm front or wrapping into a cloud head, rather than WCB ahead of a long cold front. This is a lower priority than the WCB, but depends on the type of cyclones that arise during the campaign window. 5

6 Figure 4: The precipitation in the WCB forecast to occur on 30/10/09. Figure 5: RDF trajectory analysis showing the same WCB and a double flight plan. Left: Change in pressure along trajectories arriving at 500hPa - blue indicating the WCB centre. Right: PV at upper levels showing the location of the tropopause to the west of Ireland. Note that the system moves slowly eastwards and the air is also flowing rapidly towards the NNE along the WCB through the system. Over the duration of two flights, the air will travel 500km. This plays in our favour since the downstream section will be far away by the time the upstream section is sampled giving a greater separation. This is better for seeking a signal in net diabatic influence. 6