How far can global models predict extreme hydrological events

Hervé Le Treut

21/04/23

Plan of the talk:

- French interest on the subject

- Remarks on AR4 models

- One test case: South-East South-America

21/04/23

Two French zoomed climate models

LMDZ-Mediterranean(IPSL, Paris)

Arpege-Mediterranean(Météo-France, Toulouse)

Global

Regional

buffer zone

buffer zone

Schematic of the quardruple coupling in IPSL

Global O-A coupled model: LMDZ-global / ORCA2Regional O-A coupled model: LMDZ-regional / MED8

Two atmospheric models are coupled through buffer zonesTwo oceanic models are also coupled through buffer zones

Applications to:

EuropeHYmex (Hydrology in the Mediterranean area)AMMA (West Africa)ChinaSouth America

Wüst, 1961

A downscaling study for France (Li et al):

•Three versions: Global / Europe / France

•Two-way nesting between Global/Europe

•One-way nesting from Europe to France

Annual-mean precipitation (mm) in three LMDZ models: Globe (top), Europe (middle) and France (bottom)

Intense precipitations (mm/day), return level at 50 years

Marseille Obs 300km 100km 20km

1961/1990 145 43 42 62

2021/2050 38 56 93

Paris Obs 300km 100km 20km

1961/1990 84 31 40 37

2021/2050 26 39 48

Strasbourg Obs 300km 100km 20km

1961/1990 65 40 49 41

2021/2050 32 61 44

Observations and three versions LMDZ

Future evolution of extremesFuture evolution of extremes

SON DJF MAM JJA

Precipitation (mm/day)averages

30-year return levels

2070/2099 minus 1970/1999

Goubanova et al. 2007, Planetary and Global Change

0 25 50 75 100 125 150 175 200 225

1E-5

1E-4

1E-3

0.01

0.1 OBS LMDZ-Global LMDZ/CTRL LMDZ/CTRL2

Fra

ctio

n

Bin(mm/day)

0 25 50 75 100 125 150 175 200 225

1E-5

1E-4

1E-3

0.01

OBS LMDZ-Global LMDZ/CTRL LMDZ/CTRL2

Nor

ma.

Pre

cip

Bin(mm/day)

(a) (b)

(a) Normalized frequency and (b) amount of precipitation as a function of daily intensity for observation and the reference simulations of LMDZ-Global、 LMDZ/CTRL and LMDZ/CTRL2.

Added values of LMDZ-regional: extremes

Chen et al. 2010, Cli Dyn

Hadley mass transport (DJF, winter cell) and its dependence on CO2 doubling

AR4 models

Hadley cell latitudinal extension (JJA, Southern limit) and its dependence on CO2 doubling

AR4 models

•Extension vers les pôles des branches subsidentes,

S JJA

(en °

lati

tude)

Gastineau et al, 2008

Main features of tropical response to CO2 increase in current GCMs

• Hadley cell intensity diminished (consistent with changes in vertical stability of the atmosphere, with radiative forcing)

• Precipitation rates are increased (increase in water vapour, in convective precipitation)

• The position of the main tropical system undergoes only moderate changes

SENTIVITY EXPERIMENTS in reponse to prescribed SSTS show that these agreements between models reflect a response to globally average SST changess, or zonally averaged SST changes. The impact of zonal asymmetries is much more complex and involves competition between Walker and Hadley cells.

A useful paradigm to understand tropical circulations: the two-column model.

The dynamic link between two cells determines specific scales (Bellon, Ghil, Le Treut, GRL, 2006)

R-2

R-1

Coupling two adjacent cells sharing a common subsidence zone (through a modification the low level heat transport) clearly shows the possibility of non-linear behaviour

(Le Treut and Bellon, 2005)

Differences of simulated seasonal mean precipitation between 2079-2999 and 1979-1999 periods from IPCC-AR4 models (see list in table 1). Color scale interval is 0.2 mm day-1. The black contour indicates the 0 level.

(Vera, Junquas, Li and Le Treut)

EOF1 of DJF (2001-2098) rainfall from IPCC-AR4 model. Color scale interval is 0.2 non-dimensional units. The black contour indicates the 0 level.

ZG PSE 2001−2049− ZG NSE 2001−20498 models

ZG PSE 2050−2098− ZG NSE 2050−20988 models

a.

b.

Figure 14: Composite differences of mean DJF geopotential height (zg) at 500hPa between the positive and the negative EOF1-SE for (2001-2049) (a) and (2050-2098) (b) from 8 of the 9 selected models .Color scale and contour interval is 10 m. The black contour indicates the 0 level.

Conclusions:

- Inceasing Model resolution is a necessity

- But the scale interactions are complex, and non-linearity can arise from the large-scale synoptic circulations

- Using a GCM for regional studies, and more so for the deterination of extreme events, requires a combination of approaches, and careful process studies

21/04/23