Mesoscale Atmospheric Systems

Heavy Precipitation Eventsin the Mediterranean

30 May 2017Stephan Pfahl

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HPE in the Mediterranean region

13.11.2012 Tuscany

29.9.2012 Malaga

8.1.2013 Tel Aviv

16.6.2010 SE France

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Seasonal distribution of top 50 events at every grid point

Raveh-Rubin and Wernli 2015 (QJ)3

Overview

Upper-level forcingKey role of tropopause-level Rossby wave breaking

Low-level featuresSynoptic setting: Med cyclonesOrographic flow modificationTriggering of convection

Interaction between moist processes and dynamicsMoisture sourcesStable water isotopes

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Upper-level forcing

Case studies reveal characteristic pattern of upper-level PV associated with western Med HPEs

Brig (23.09.1993) Piedmont (05.11.1994) Ticino (13.09.1995)

Massacand et al. 1998 (GRL)

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Upper-level forcing

Characteristic pattern confirmed by composite / cluster analysis

Gray: averaged upper-level PV Contours: averaged Z@500hPa

Martius et al. 2006 (IJC) Nuissier et al. 2011 (QJ)

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Upper-level forcing

This pattern („PV streamer“) is result of Rossby wave breaking at the end of the North Atlantic storm track

PV streamers typically propagate slowly

Effect on HPE is potentially 4-fold:

- reduced static stability / increased CAPE beneath PV streamer- forcing for ascent (Q-vector convergence)- strong low-tropospheric moisture flux- increased evaporation due to increased surface winds

Q: What is relative importance of these effects?

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Upper-level forcing

Many other cases with pronounced UL PV feature

Algier (10.11.2001) Mallorca (04.10.2007) Israel (04.12.2001)

Argence et al. 2008 Cohuet et al. 2011 Krichak et al. 2007 (QJ) (AR) (NHESS)

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Upper-level forcing

Model studies à strong sensitivity of QPF to UL PV structure

Fehlmann et al. 2000 (WaF)Homar et al. 2003 (QJ)Romero et al. 2005 (NHESS)etc.

e.g., HPE inCevennes (09.09.2002)

Significant predictabilitylimit due to sensitivity tostructure of UL precursor!

Q: until which fc step are these uncertainties at UL relevant?9

Low-level processes: Surface cyclones

Cyclone frequency % of HPE colocated with cyclones

Pfahl and Wernli 2012 (J. Clim.)

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Low-level processes: Orographic flows

e.g., flow channeling, splitting, blocking, ...

precipitation (04.10.1999) near-surface winds

Ivancan-Picek et al. 2003 (Met. Z.)

bora

jugo

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Low-level processes: Triggering of convection

Gard (08.09.2002)

-first cells develop during night over sea, ahead of front-organized convection (MCS) quasi-stationary during 24 h-merging of cold front with MCS

Nuissier et al. 2008 (QJ)

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Low-level processes: Triggering of convection

Lee et al. 2016 (QJ)13

Satellite-derived brightness temperature and near-surface winds

Low-level processes: Triggering of convection

Q: What triggers convection over the sea?

Synoptic lifting? Low-level wind convergence? Cold air outflowfrom earlier deep convection?

Cohuet et al. 2011 (AR)

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Low-level processes: Stability vs. transport

What is more important: conditional instability (CAPE) or a strong horizontal moisture flux?

Nuissier et al. 2008 (QJ)

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Importance of moisture supply

Composites for HPE and non-HPE with similar UL flow

Nuissier et al. 2011 (QJ)

v at 925 hPa

qv at 925 hPa

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Interaction of moist processes & dynamics

Evaporative moisture sources for Northern Med HPEs

Winschall et al. 2014 (JGR)17

Interaction of moist processes & dynamics

Evaporative moisture sources for top 50 Northern Med HPEs in SON

Winschall et al. 2014 (JGR)

Histogram of evaporation in western Med on dayspreceding HPEs (red) compared to clim (black)

Q: is very intense evaporation required to get HPE?

WINSCHALL ET AL.: MEDITERRANEAN PRECIPITATION EXTREMES X - 37

(c) SON

0ºE40ºW

20ºN

40ºN

60ºN

Figure 7. Composites of the diagnosed moisture sources combined from boundary layer and

free tropospheric uptakes (shading, in mmd�1) for the 50 HPEs in (a) MAM, (b) JJA, (c) SON,

and (d) DJF.

D R A F T February 18, 2014, 9:52am D R A F T

WINSCHALL ET AL.: MEDITERRANEAN PRECIPITATION EXTREMES X - 37

0.020.040.060.080.010.120.140.160.180.200.240.280.320.360.400.4mm d-1

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0ºE40ºW

20ºN

40ºN

60ºN

Figure 7. Composites of the diagnosed moisture sources combined from boundary layer and

free tropospheric uptakes (shading, in mmd�1) for the 50 HPEs in (a) MAM, (b) JJA, (c) SON,

and (d) DJF.

D R A F T February 18, 2014, 9:52am D R A F T

X - 36 WINSCHALL ET AL.: MEDITERRANEAN PRECIPITATION EXTREMES

(a) MAM

(c) SON (d) DJF

(b) JJA

−250 −200 −150 −100 −50 0

05

1015

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Surface latent heat flux in W m−2

Freq

uenc

y in

%

−250 −200 −150 −100 −50 0

05

1015

20

Surface latent heat flux in W m−2

Freq

uenc

y in

%

−250 −200 −150 −100 −50 0

05

1015

20

Surface latent heat flux in W m−2

Freq

uenc

y in

%

−250 −200 −150 −100 −50 0

05

1015

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Surface latent heat flux in W m−2

Freq

uenc

y in

%Figure 6. Histograms of the domain-mean surface latent heat flux in Wm�2 at grid points in

the western Mediterranean (red region in Figure 1), averaged over the day before and the day of

the 50 HPEs (red shading) and before all other days between 1989 and 2009 (black line).

D R A F T February 18, 2014, 9:52am D R A F T

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Interaction of moist processes & dynamics

COSMOiso simulation of Brig 1993 HPE

precipitation d-excess

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Interaction of moist processes & dynamics

COSMOiso simulation of Brig 1993 HPE

precipitation d-excess

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Interaction of moist processes & dynamics

COSMOiso simulation of Brig 1993 HPE

precipitation d-excess

Interaction of moist processes & dynamics

COSMOiso simulation of Brig 1993 HPE

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Open questions & research challenges

General: Do we understand the stationarity of HPE flows?

Upper-level forcingPV streamer à stability, ascent, moisture advection, surface evap?What are key structural details of PV streamers for HPE?Essential upstream processes for formation of PV streamers?

Low-level featuresTriggering of convection over ocean?Resolution required to resolve orographic effects?Role of SSTs & need for coupled O-A modeling?

Interactions of moist processes & dynamicsLinkage between intense evaporation and HPE?How do microphysical processes impact on storm dynamics?Will stable water isotopes provide novel insight?

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Oral Exam

Either in 2nd or 4th week of August (7/8 or 21/22 Aug)

30 Minutes questions from Heini & Stephan

Language: English or German (your choice)

Key aspect: Explain, discuss, interpret figures and formulae

Don’t learn things by heart

Slides from lectures as starting point for discussion “What do you see on this figure? What is this equation telling us? What can we learn from this study? What are general characteristics of XY? …”

Preparation: Speak, discuss, think (What did I learn about XY? What is key in this chapter? …)

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