dsergeev-cyclone-workshop-2017.pdf

Transcript

1. Modification of polar low development by orography and sea ice Denis Sergeev, Ian Renfrew, Thomas Spengler 18th Cyclone Workshop | Sainte-Adèle, QC, Canada | 06 October 2017
2. What are polar lows? • Polar lows are small (100-500 km in diameter), short-lived maritime cyclones with near-surface winds exceeding 15 m s-1 • Typical conditions give an estimate of 1018J of the total kinetic energy ≈ 100 thunderstorms. • A hazard to ships, oil rigs and coastal communities in the Arctic, North Atlantic and North Pacific Introduction Data and methods Importance of orography Importance of sea ice Summary Results Credit: Met Norway Credit: U. Tokyo
3. Polar lows & cold air outbreaks Introduction Data and methods Importance of orography Importance of sea ice Summary Results The annual mean density of polar lows over the northeast Atlantic Ocean. [Condron and Renfrew, 2013] Norwegian Sea Study area G reenland Fram Strait Iceland Barents Sea
4. Motivation • Does Svalbard serve as a trigger mechanism for polar lows? Introduction Data and methods Importance of orography Importance of sea ice Summary Results STARS dataset: http://polarlow.met.no/stars-dat/ ?
5. Motivation • Does Svalbard serve as a trigger mechanism for polar lows? • Is sea ice configuration crucial for convergence zone and its evolution into a cyclone? Introduction Data and methods Importance of orography Importance of sea ice Summary Results STARS dataset: http://polarlow.met.no/stars-dat/ ? ?
6. Motivation • Does Svalbard serve as a trigger mechanism for polar lows? • Is sea ice configuration crucial for convergence zone and its evolution into a cyclone? • Will location and impact of polar lows change with different sea conditions in the warming climate? Introduction Data and methods Importance of orography Importance of sea ice Summary Results Polar low approaching Lofoten Islands (Credit: NRK.no) STARS dataset: http://polarlow.met.no/stars-dat/ ? ?
7. Data and methods
8. Numerical simulations with Met Office Unified model Horizontal grid spacing 2.2 km Nested domain size 1540 km x 1320 km Nested domain centre 76°N, 10°E Vertical grid 70 levels (16 below 1 km) Run duration 48 h Time step 60 s Initial and boundary conditions MetUM global run Introduction Data and methods Importance of orography Importance of sea ice Summary Results
9. Can MetUM reproduce polar low dynamics? Introduction Data and methods Importance of orography Importance of sea ice Summary Results YES* *Cloud structure is not fully reproduced
10. The polar low cases: STARS database • We use STARS database to find PL cases • A dozen cases have been simulated • Chosen by their southward propagation and proximity to Svalbard Introduction Data and methods Importance of orography Importance of sea ice Summary Results Case A 5-6 April 2007 Case B 30-31 January 2008 AVHRR imagery
11. Results
12. Upper level conditions (500 hPa) Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE A • Geopotential height (m) • Temperature (K)
13. Upper level conditions (500 hPa) Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B CASE A CASE B • Geopotential height (m) • Temperature (K)
14. Polar lows’ evolution Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B L Shown: Sea ice edge SLP contours (every 2 hPa) L L L L
15. Polar lows’ evolution Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B L Shown: Sea ice edge SLP contours (every 2 hPa) L L L L PL PL PL PL PL PL
16. Polar lows’ evolution Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B ζ Shown: Sea ice edge SLP contours (every 2 hPa) Cyclonic vorticity (10-4 s-1) at 950 hPa L L L L PL PL PL PL PL
17. Polar lows’ evolution Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B PV 500hPa anomaly ζ Shown: Sea ice edge Cyclonic vorticity (10-4 s-1) at 950 hPa SLP contours (every 2 hPa) PV at 500 hPa (every 2 PVU)
18. Polar lows’ evolution Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B CASE A CASE B TOA OLR & precipitation rate ζ PV 500hPa anomaly
19. Polar lows’ evolution: close-ups along the track Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B
20. Polar lows’ evolution: close-ups along the track Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B Shown: Cyclonic vorticity (10-4 s-1) at 950 hPa SLP contours (every 2 hPa) time
21. Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B Shown: Potential temperature (K) Wind speed (m s-1) PV (every 2 PVU) LLJ
22. Sensitivity experiments
23. Run Svalbard Sea ice CTRL NOSVA Replaced by sea ice Control SVA200 Scaled by 200% Control Sensitivity runs configuration: OROGRAPHY Introduction Data and methods Importance of orography Importance of sea ice Summary Results CTRL Sfc Temp Height Non-linearity of flow regime • Upstream Fr < 1 prior to PL genesis • => Flow splitting occurs, possibly contributing to the formation of mesoscale vortices downstream
24. Sensitivity to Svalbard’s orography Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B ζ Shown: Cyclonic vorticity (10-4 s-1) at 950 hPa SLP contours (every 2 hPa) Sea ice edge
25. Sensitivity to Svalbard’s orography Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B ζ Shown: Cyclonic vorticity (10-4 s-1) at 950 hPa SLP contours (every 2 hPa) Sea ice edge Polar low tracks
26. Sensitivity to Svalbard’s orography Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE B CASE B ζ • Svalbard serves as an auxilary source of vorticity filaments which merge with their counterparts on the periphery of the synoptic cyclone NOSVA • Shift of PL tracks • No reinforcement from Svalbard SVA200 • Barrier effect of Svalbard • More vorticity from tip jets, PL further north
27. Run Svalbard Sea ice CTRL ICE76N Control Zonally straight edge at 76°N ICE82N Control Zonally straight edge at 82°N Sensitivity runs configuration: SEA ICE Introduction Data and methods Importance of orography Importance of sea ice Summary Results CTRL Sfc Temp
28. Sensitivity to sea ice cover Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B Shown: Cyclonic vorticity (10-4 s-1) at 950 hPa SLP contours (every 2 hPa) Sea ice edge
29. Sensitivity to sea ice cover Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B Shown: Cyclonic vorticity (10-4 s-1) at 950 hPa SLP contours (every 2 hPa) Sea ice edge Polar low tracks SHF LHF
30. Sensitivity to sea ice cover Introduction Data and methods Importance of orography Importance of sea ice Summary Results Shown: Cyclonic vorticity (10-4 s-1) at 950 hPa SLP contours (every 2 hPa) Sea ice edge Polar low tracks CASE A CASE B SHF LHF
31. Run Svalbard Sea ice CTRL Introduction Data and methods Importance of orography Importance of sea ice Summary Results Along-track cyclonic vorticity CASE B CASE A
32. Run Svalbard Sea ice CTRL NOSVA Replaced by sea ice Control Introduction Data and methods Importance of orography Importance of sea ice Summary Results Along-track cyclonic vorticity CASE B CASE A
33. Run Svalbard Sea ice CTRL NOSVA Replaced by sea ice Control SVA200 Scaled by 200% Control Introduction Data and methods Importance of orography Importance of sea ice Summary Results Along-track cyclonic vorticity CASE B CASE A
34. Run Svalbard Sea ice CTRL NOSVA Replaced by sea ice Control SVA200 Scaled by 200% Control ICE76N Control Edge at 76°N Introduction Data and methods Importance of orography Importance of sea ice Summary Results Along-track cyclonic vorticity CASE B CASE A
35. Run Svalbard Sea ice CTRL NOSVA Replaced by sea ice Control SVA200 Scaled by 200% Control ICE76N Control Edge at 76°N ICE82N Control Edge at 82°N Introduction Data and methods Importance of orography Importance of sea ice Summary Results Along-track cyclonic vorticity • Most perturbations simulated here lead to the weakening of the polar lows • Removal of Svalbard does not lead to unambigous effect in polar low dynamics • More convectively-driven polar low (case B) is more influenced by the change in sea ice cover • No significant change in baroclinicity over the Norwegian Sea CASE B CASE A
36. Preliminary conclusions • Many polar lows develop due to merging of vorticity filaments • Svalbard plays a role of additional, but not a primary source of low-level cyclonic vorticity • It plays a role in directing polar lows to the south instead of SW, but upper-level PV anomaly can counteract this effect • For our PLs convection is important => change in sea ice and SST leads to greater changes in PL intensity • Future plans: climatological study & composite analysis using automatic tracking Introduction Data and methods Importance of orography Importance of sea ice Summary Results 1 2
37. Preliminary conclusions • Many polar lows develop due to merging of vorticity filaments • Svalbard plays a role of additional, but not a primary source of low-level cyclonic vorticity • It plays a role in directing polar lows to the south instead of SW, but upper-level PV anomaly can counteract this effect • For our PLs convection is important => change in sea ice and SST leads to greater changes in PL intensity • Future plans: climatological study & composite analysis using automatic tracking Introduction Data and methods Importance of orography Importance of sea ice Summary Results 1 2 Thank you!
38. Extra
40. Run Svalbard Sea ice NOSVA Replaced by sea ice Control SVA200 Scaled by 200% Control ICE76N Control Edge at 76°N ICE82N Control Edge at 82°N Sensitivity runs configuration (CASE A example) Introduction Data and methods Importance of orography Importance of sea ice Summary Results
41. Sensitivity to orography Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B Shown: Equivalent potential temperature SLP contours (every 2 hPa) Sea ice edge
42. Sensitivity to sea ice cover Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A CASE B Shown: Equivalent potential temperature SLP contours (every 2 hPa) Sea ice edge
43. Introduction Data and methods Importance of orography Importance of sea ice Summary Results Much stronger cyclone (-10 hPa in the centre) Additional experiments: with uniform SST change CASE B SST CTRL +5 K
44. Additional experiments: with uniform SST change Introduction Data and methods Importance of orography Importance of sea ice Summary Results SST CTRL -5 K The mesoscale vortex still develops, but is even weaker than in ICE76N CASE B
45. Backward trajectory analysis Introduction Data and methods Importance of orography Importance of sea ice Summary Results CASE A • Most air parcels travel close to the surface, gaining heat and moisture as they move across over sea • Descending air from ~700 hPa also contributes to the formation of the core - To examine how the PL forms and develops a warm core • Model: LAGRANTO 2.0 [Sprenger and Wenrli, 2015] • Starting positions: between 950 and 850 hPa within 150 km around the PL centre