Warm conveyor belts Heini Wernli – ETH Zurich, Switzerland With contributions from: Maxi...

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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

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

Airstreams in extratropical cyclones

e.g., Browning 1990

Coherent ascent from theboundary layer to the uppertroposphere

Maximum cloud andprecipitation producing airflow

Wernli and Davies 1997Wernli 1997 (QJ)

950 hPa

320 hPawithin 2 days:

- ascent > 600 hPa- polew. transport > 3500 km- latent heating > 20 K

flow structure in extratropical cyclones with strongest latent heat release & precipitation

Warm conveyor belts: maximum ascending airstream

colors indicate pressure

0.5 pvu

0.5 pvu- PV anomaly

characteristic evolution of potential vorticity (PV) along WCBs:

in low troposphere:increase ~0.5 ~1.5 pvudue to dH/dz > 0

in upper troposphere:decrease ~1.5 ~0.5 pvudue to dH/dz < 0

Warm conveyor belts: characteristic PV evolution

1.5 pvu+ PV anomaly

Warm conveyor belts

Joos and Wernli 2012 (QJ)

Warm conveyor belts

Warm conveyor beltsImportant cross-isentropic transport of low-PV air

Case 2: WCB after ET of Hanna

PV on 320 K

Grams et al. 2011 (QJ)

Case 3: WCBs and Xynthia

PV@320K 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

+6 h +12 h +18 h0 h

WCB small-scale Rossby wave generation

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)

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

2003 01 26/12 -15.9 hPa02 02/00 -13.4 hPa12 07/12 -13.8 hPa12 27/12 -15.6 hPa

2004 12 23/12 +15.7 hPa winter forecasts2005 01 13/12 +20.1 hPa

01 14/00 +17.6 hPa02 08/12 +18.1 hPa02 09/00 +18.2 hPa10 21/12 -14.0 hPa ET of “Wilma”

Interesting: only one similarly “bad” fc in 2006-2010!

CE forecast busts: example 6

ana +3.5

fc +3.5

ana +5

T850 and SLP

CE forecast busts: example 6

ana +3.5

fc +3.5

ana +5

PV on 320 K

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” trajectoriesana fc

2003 01 26/12 64 3102 02/00 19 512 07/12 164 134

2004 12 23/12 291 1522005 01 13/12 115 2

02 08/12 45 010 21/12 31 14

Forecast busts have too weak WCBs over North Atlantic !

generation of a positive PV anomaly(downstream trough)

WCB amplified upper-level ridge downstream troughWCB triggers / enhances downstream Rossby wave activity

Hypothesis: errors in WCBs amplify downstream

Forecast busts: how well represented by EPS?

Look at bust no.1:

20050113_12 + 5 days SLP average over C. Europe

analysis 1003 hPa

deterministic fc 1024 hPa

EPS 1012 – 1037 hPa !!

WCB climatology

ERAinterim Dataset (T255L60), 1989-2009Forward trajectory calculationTrace 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

North Atlantic NA

Climatology of WCBs (DJF 1989-2009)

WCB starting points (t=0)

from Erica Madonna

Time evolution from t = -96 h to +96 h

pre-ascent post-ascentascent

Start at 935 hPa and rise to 310 hPa, non uniform

Pressure evolution along N Atlantic WCBs

from Erica Madonna

Initial moisture of 9.4 g/kg,final 0.1 g/kg

time [h]

pre-ascent post-ascentascent

Specific humidity evolution along N Atl WCBs

from Erica Madonna

WCB meeting 2012 – Erica

LWC and IWC evolution along N Atlantic WCBs

DJF JJA

Climatology of WCB starting regions in North Pacific 2001-2010

Where are evaporative moisture sources of WCBs? do Lagrangain moisture uptake analysis (Sodemann et al. 2008, JGR)along 10-day backward extensions of WCBs

DJF JJA

Climatology of WCB moisture uptakes

Moisture uptake …… quasi in-situ, purely oceanic, … involves long-range transportno transport from tropics from tropics and land evapotransp. from Stephan Pfahl

Warm conveyor beltsHydrometeors

Warm conveyor beltsMicrophysical processes

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

Heavy precipitation events & cyclones

HPE: >99 percentile at every grid point (ERAinterim, 1989-2010)

Pfahl and Wernli 2012 (J. Clim., in press)

Warm conveyor belts & HPE in autumn 2011

from Maxi Böttcher

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)

T-NAWDEX: International experiment in 2015?

from Pat Harr

Thank you for your attention !

Warm conveyor belts Heini Wernli – ETH Zurich, Switzerland With contributions from: Maxi...

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