1. Warm conveyor belts Heini Wernli – ETH Zurich, Switzerland
With contributions from:
Maxi Böttcher, Christian Grams, Hanna Joos, Erica Madonna, Stephan Pfahl, Nicolas Piaget
PDP WG meeting, Reading, 19 June 2012

2. Outline of the talk The concept of WCBs
WCBs and amplification of upper-level ridges
WCBs and forecast busts
Climatology of WCBs
Moisture sources of WCBs
Microphysical processes in WCBs
WCBs and HPEs

3. Airstreams in extratropical cyclones
WCB:
Coherent ascent from the
boundary layer to the upper
troposphere
Maximum cloud and
precipitation producing airflow
e.g., Browning 1990

4. Warm conveyor belts: maximum ascending airstream
320 hPa
within 2 days:
ascent > 600 hPa
polew. transport > 3500 km
latent heating > 20 K
flow structure in extratropical cyclones with strongest latent heat release & precipitation L
950 hPa
colors indicate pressure
Wernli and Davies 1997
Wernli 1997 (QJ)

5. Warm conveyor belts: characteristic PV evolution
0.5 pvu
- PV anomaly
characteristic evolution of potential vorticity (PV) along WCBs:
in low troposphere:
increase ~0.5  ~1.5 pvu
due to dH/dz > 0
in upper troposphere:
decrease ~1.5  ~0.5 pvu
due to dH/dz < 0 L
1.5 pvu
+ PV anomaly
0.5 pvu

6. Warm conveyor belts
Joos and Wernli 2012 (QJ)

7. Warm conveyor belts

8. Warm conveyor belts

9. Warm conveyor belts

10. Warm conveyor belts

11. Warm conveyor belts

12. Warm conveyor belts Important cross-isentropic transport of low-PV air
Joos and Wernli 2012 (QJ)

13. Case 2: WCB after ET of Hanna
PV on 320 K
Grams et al (QJ)

14. Case 3: WCBs and Xynthia
and SLP at 00 UTC 26 Feb with WCB intersection points
Intense WCBs associated with US snowstorm and early phase of Xynthia
from Nicolas Piaget

15. WCB small-scale Rossby wave generation
0 h
+6 h
+12 h
+18 h

16. Identification of top-10 Central European forecast busts in ECMWF 5-day forecasts during 2003-2005
based upon simple error measure:
SLP-error = difference in domain-averaged SLP (fc - ana)
domain: Central Europe (0-30E, 45-60N)

17. Identification of top-10 Central European forecast busts in ECMWF 5-day forecasts during 2003-2005
select forecasts with 5 largest positive and 5 largest negative SLP errors over Central Europe
YEAR MONTH DAY/HH
/ hPa
02 02/ hPa
12 07/ hPa
12 27/ hPa
/ hPa winter forecasts
/ hPa
01 14/ hPa
02 08/ hPa
02 09/ hPa
10 21/ hPa ET of “Wilma”
Interesting: only one similarly “bad” fc in !

18. CE forecast busts: example 6
T850 and SLP
ana +3.5
ana +5
fc +3.5
fc +5

19. CE forecast busts: example 6
PV on 320 K
ana +3.5
ana +5 T1 R1 T1 R1
fc +3.5
fc +5 T1 T1 R1 R1

20. Forecast busts: common dynamical pattern?
In all cases: forecasts have too weak UT ridges (not broad enough, PV values not low enough)
Backward trajectory analysis of these UT ridges, look for “WCB-like ascent” into ridges (criterion  > 15K)
# of “WCB-like” trajectories
ana fc /
02 02/
12 07/ / /
02 08/
10 21/
Forecast busts have too weak WCBs over North Atlantic !

21. - - Hypothesis: errors in WCBs amplify downstream+
generation of a positive PV anomaly
(downstream trough)
WCB  amplified upper-level ridge  downstream trough
WCB triggers / enhances downstream Rossby wave activity

22. Forecast busts: how well represented by EPS?
Look at bust no.1:
_ days SLP average over C. Europe
analysis hPa
deterministic fc hPa
EPS – 1037 hPa !!

23. WCB climatology ERAinterim Dataset (T255L60), 1989-2009
Forward trajectory calculation
Trace TH, THE, Q, LWC, IWC, PV
WCBs selection criteria
1) start in the atmospheric boundary layer ( p > 790 hPa) and ascent larger than 600 hPa within 2 days (48 hrs)
2) ascent in the vicinity of extratropical cyclones
To identify WCBs we start forward trajectories calculation in the ABL (p>790) and we select those trajectories that ascend more than 600hPa within 48 hours. Afterward we consider the physical relationship between WCBs and extratropical cyclone to filter out trajectories related to tropical convection.

24. Climatology of WCBs (DJF 1989-2009)
WCB starting points (t=0)
North Atlantic NA
The situation for DJF is displayed here. In North Hemisphe we observe more maxima than for JJA, a shifting of the maximum from chine over the North Pacific and a strong intensification of the maximum over the North Atlantic.
In the Southern hemisphere there is less variability than in the northen and we obsere still a peak in the lee of the Andes.
from Erica Madonna

25. Climatology of WCBs (DJF 1989-2009)
Time evolution from t = -96 h to +96 h

26. Climatology of WCBs (DJF 1989-2009)
Time evolution from t = -96 h to +96 h

27. Climatology of WCBs (DJF 1989-2009)
Time evolution from t = -96 h to +96 h

28. Climatology of WCBs (DJF 1989-2009)
Time evolution from t = -96 h to +96 h

29. Climatology of WCBs (DJF 1989-2009)
Time evolution from t = -96 h to +96 h

30. Climatology of WCBs (DJF 1989-2009)
Time evolution from t = -96 h to +96 h

31. Start at 935 hPa and rise to 310 hPa, non uniform
Pressure evolution along N Atlantic WCBs
ascent
pre-ascent
post-ascent
Start at 935 hPa and rise to 310 hPa, non uniform
from Erica Madonna

32. Initial moisture of 9.4 g/kg,
Specific humidity evolution along N Atl WCBs
Initial moisture of 9.4 g/kg,
final 0.1 g/kg
time [h]
For the next we focus on the North Atlantic region in the wintertime. We look at all the trajectories that starts in this regions and performed a statistic of the temporal evolution of the pressure and specific humidity and like before 0 refer to the moment where the ascent begin. In the representation is to see the mean and the median (black and red) and the 50 and 90 percentile as green areas.
In Winter WCBs start above 935 hPa and rise up to 310 hPa in two day but not linearly. In average the WCBs contain around 10 g of water vapor pro Kg air and they lose almost all the vapor in the first 36 hours, first trough condensation and deposition. Then most of the water will leave the air parcel and reach the ground in form of precipiation.
ascent
pre-ascent
post-ascent
from Erica Madonna

33. LWC and IWC evolution along N Atlantic WCBs
This graphics represent all the WCBs that start in the NA during Decemer, Januaray and February in the last 21 year. In the representation I plot tge mean and the median (black and red) and the 50 and 90 percentile as green areas. Display is the temporal evolution (like before 0 refer to the moment where the ascent begin) of the pressure, specific humidity, lWC, IWC.
In Winter WCBs start above 900 hPa and rise up to 300 hPa in two day but not linearly. In average the WCBs contain around 10 g of water vapor pro Kg air and they lose almost all the vapor in the first 36 hours. As the air parcel rise vapor condense and as we can see from the following pictures, we observe first an increase in the LWC and then, as the parcel reach higher altitude, an increase in LWC.
Most of the water will leave the air parcel and reach the ground in form of precipiation as it can be seen in the amount of water after 48h: the parcels contain almost no water.
However we still have some ice, which indicate the capacity of WCBs to form cirrus clouds.
WCB meeting – Erica

34. Climatology of WCB starting regions in North Pacific 2001-2010
DJF
JJA
Where are evaporative moisture sources of WCBs?
 do Lagrangain moisture uptake analysis (Sodemann et al. 2008, JGR)
along 10-day backward extensions of WCBs

35. Climatology of WCB moisture uptakes
DJF
JJA
Moisture uptake …
… quasi in-situ, purely oceanic, … involves long-range transport
no transport from tropics from tropics and land evapotransp.
from Stephan Pfahl

36. Warm conveyor belts
Hydrometeors
Joos and Wernli 2012 (QJ)

37. Warm conveyor belts
Microphysical processes
Joos and Wernli 2012 (QJ)

38. Warm conveyor belts & HPE
Climatology: what percentage of HPE occurs simultaneously with the presence of a cyclone?
HPE and WCBs in pre-HYMEX autumn 2011

39. Pfahl and Wernli 2012 (J. Clim., in press)
Heavy precipitation events & cyclones
HPE: >99 percentile at every grid point (ERAinterim, )
Pfahl and Wernli 2012 (J. Clim., in press)

40. Warm conveyor belts & HPE in autumn 2011
from Maxi Böttcher

41. Warm conveyor belts & HPE in autumn 2011
from Maxi Böttcher

42. Warm conveyor belts & HPE in autumn 2011
from Maxi Böttcher

43. Warm conveyor belts & HPE in autumn 2011
from Maxi Böttcher

44. Summary WCBs are key airflows in extratropical cyclones
strong ascent and cross-isentropic transport
preferred regions of occurrence
some WCBs associated with HPE
characteristic PV evolution
impact on downstream flow evolution
critical process for medium-range forecasting
associated with different microphysical processes
 Several aspects that could be investigated within aircraft field experiments (DIAMET, T-NAWDEX-Falcon)

45. T-NAWDEX: International experiment in 2015?
from Pat Harr

46. Thank you for your attention !