Irina Gorodetskaya, Michael S. Town, Hubert Gallée

Laboratoire de Glaciologie et Géophysique de l’Environnement, Grenoble,France

EGU, Vienna 23 Apr. 2009

Mechanisms behind synoptic-scale variability

in South Pole meteorologyfrom observations and a regional model

acknowledgements:

Gerhard Krinner for support and discussions

Von P. Walden for providing computer time and space

Stephen G. Warren for antarctic cloud discussions

Ells Dutton and Tom Mefford of NOAA-GMD, and BSRN for radiation and meteorology data and advice.

Gorodetskaya, Town, Gallée, LGGE : EGU 2009

•importance of synoptic activity over Antarctica

•data sets and model description

•the climate of the South Pole

•model evaluation: wavelets

•cluster analysis

•conclusions

Gorodetskaya, Town, Gallée, LGGE : EGU 2009

Outline

1911

1960 1970 1980 1990 20001950

1957 1975 2003

surface meteorology/observations

radiosondes

radiation

accumulation

clouds

snow temperatures

NOAA CMDL/GMD

South Pole climate data set: A review

See poster M. Town and V. Walden, Session AS2.4, XY105

Atmospheric model: mesoscale hydrostatic primitive equation model (Gallée 1994, 1995)

Terrain following vertical coordinates (normalized pressure) Turbulence: 1 1/2 closure (Duynkerke 1988) Bulk cloud microphysics (Kessler 1962 and Lin et al 1983 + improvements of Meyers et al. 1992 and Levkov et al. 1992) Solar and infrared radiative transfer scheme (Morcrette 2002, Ebert and Curry 1992) Snow fall included into infrared radiation scheme

Snow model: conservation of heat and water (solid and liquid), description of snow properties (density, dendricity, sphericity and size of the grains), melting/freezingBlowing snow model (Gallée et al, 2001)

FS

FS

FL T4 HLatHSen

Sn

ow

HMelt HFreez

HCond

Tsfc

Pe

rco

lati

on

Liquid water

Blowingsnow

coupling to sea ice, land ice, vegetation...

Horizontal resolution 80 km 33 vertical levels (lowest ~9m, one level each 10 m below 50 m; top = 10hPa) Initial and boundary conditions: ECMWF ERA-40

Modèle Atmosphérique Régional (MAR)

Gorodetskaya, Town, Gallée, LGGE : EGU 2009

The climate of the South Pole

altitude = 2835 m

accumulation rate = 8 cm yr-1

mean temperature = -50oC

Gorodetskaya, Town, Gallée, LGGE : EGU 2009

Sfc air temperature

MAR. ERA40. South Pole. 1994

a

..-65oC

..-45oC

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Importance of synoptic activity over Antarctic interior

Time series of the five snow accumulation eventsclose to the South Pole (860S, 460W)from acoustic depth gauge

Braaten 2000

Nor

mal

ized

w.e

.A

ccum

ulat

ion

(10-3

m)

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Importance of synoptic activity over Antarctic interior

The 700hPa height and 500hPa wind field at 1200 UTC on Nov 5, 1997

Noone, Turner, Mulvaney 1999

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Directional distribution of hourlynear surface winds during warm and cold events

Isobaric temperature advectionwhen 300 hPa wind is from SW or NW (warm events)and from SE (cold events)

Neff, JGR (104) 1999

Warming eventsCooling events

Down-slope (“East”)Along-slope (“North”)

Warming Cooling

SE

SW

NW

Thermal advection (0C/day)Direction Class Intervals

Nu

mb

er i

n i

nte

rval

Hei

gh

t, m

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Convolve wavelets of increasing sizewith time series to obtain scaling coefficients.

T(a,b) = w(a) x(t) dt

Wavelets applied to time series:

t-ba

power spectrum

time

b

a

T(a,b)

Wavelets give information in temporal and frequency domains.

Gorodetskaya, Town, Gallée, LGGE : EGU 2009

Model validation : wavelet analysis

Power spectrum (units2/time) Goro

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Variables measuredat South Pole:

Sfc temperature Water vapor pressure (from frost point) Sfc wind speed downwelling LW flux downwelling SW flux

6 hour time step, 1994

5 variables...

Cluster analysis applied to time series: G

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• Sfc air temperature amplitude is good in MAR (both synoptic and seasonal)

• Wind speed underestimated during some warm events

• Increased humidity and LW fluxes during warm events in obs and MAR

Variables simulated by MAR:

Sfc temperature Sfc pressure tropospheric water vapor downwelling LW flux downwelling SW flux U,V near surface U,V at 300 hPa tropospheric cloud liquid tropospheric cloud ice stratospheric cloud ice

6 hour time step, 1994

Cluster analysis applied to time series:

12 variables...

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MAR : 6 meteorological regimes

Tair,

0C

Rad Trop

Hum

Trop

Clds

Strat

Clds

Sfc

wind

300hPa

wind

Cold -60 - - -

Warm-I -40 LW -

Warm-II -45..

-50

LW -

Warm-III -40..

-60

LW -

Summer -20..

-40

SW

LW - -

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

E

NW NE

NE

NE

E

NE

E

SW

SE

NW N

SES

E

SW...

Gorodetskaya, Town, Gallée, LGGE : EGU 2009

7%

24%

54%

11%

4%

Accum,

%

warm events

Sno

w a

ccum

ulat

ion,

mm

.w.e

Inte

grat

ed s

now

,m

m.w

.e

Snow accumulationG

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Con

clu

sio

ns

III. warm events correlated with high stratospheric ice content together with slight increase in tropospheric moisture content

7% snow accumulation

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Modèle Atmosphérique Régional (MAR): shows good skill in synoptic-scale simulations

Cold events are more or less similar: - low tropospheric humidity, clear sky, low downwelling LW flux - NE-E near surface wind (“inversion” wind) - weak SE wind at 300 hPa

11% snow accumulation

Warm events happen for a variety of reasons:

I. warm air advection from W-SW (West Antarctica) with increase in tropospheric humidity and tropospheric cloud liquid 54% snow accumulation

II. warm air advection from N-NW (Weddell Sea) - slight increase in tropospheric moisture content- no tropospheric clouds but stratospheric clouds form

24% snow accumulation

Plans

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Extend cluster analysis to the entire period (1994-2000)

Upper air charts for each type of warm event