International Journal of Climatology, 2015
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International Journal of Climatology, 2015

Rabatel. et al. . (. 2013). Exemple. : le Glacier Zongo (. Bolivie. , 16°S), zone . tropicale. . externe. Monitoring network on Zongo glacier, 16°S (Bolivia). GLACIOCLIM. Ablation stakes. Snow pits.

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International Journal of Climatology, 2015




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Presentation on theme: "International Journal of Climatology, 2015"— Presentation transcript:

Slide1

International Journal of Climatology, 2015

Slide2

Rabatel

et al

. (

2013)

Slide3

Exemple

: le Glacier Zongo (

Bolivie

, 16°S), zone

tropicale externe

Monitoring network on Zongo glacier, 16°S (Bolivia)

GLACIOCLIM

Ablation stakes

Snow pits

Proglacial

discharge

Rain gauges

Automatic

Weather

Station

Slide4

Veblen et al. (2007)

Wet summer season

Dry winter season

Slide5

Tropical glaciers: mass balance largely depends on

cloud and precipitation properties

Transition season SONRunoff peakClear-sky, summer solsticeLow glacier albedo

Wet season DJFClouds, SnowfallsHigh glacier albedo

sum of precipitation

5 stations on the Altiplano, daily averages over 1991-2008 Ramallo (2014)

Objectives: to relate locally observed radiative properties of clouds to the synoptic atmospheric circulation.

Dry season JJA

Little melt energy, Longwave radiation surface deficit

Slide6

C

loud

transmissivity for shortwave radiation (

T

n

 ≤ 

1)

:

S

=

T

n

S

clear

=

T

n

T

clear

S

extra

S

clear

is the shortwave clear-sky irradiance (W m

2

)

S

extra

is the theoretical shortwave irradiance at the top of the atmosphere (W m

−2

)

T

clear

is the bulk clear-sky

transmissivity (0.87

)

Daily values from 2005 to

2013

Slide7

The

sky longwave irradiance (

L

):

L

=

clear

F σ T

4

(2)

ε

clear

is the apparent clear sky

emissivity

 = 5.67 10

−8

 W m

−2

 K

−4

is the Stefan-

Boltzman

constant

F

 

 

1 is the cloud emission factor describing the increase in sky emissivity due to cloud emission.

ε

clear

= C (e / T)

1/m

(3)

e

is the vapor pressure (

hPa

) and

T

is the temperature (K) of the air near the ground

.

Daily values from 2005 to

2013

Slide8

The

cloud cover

index

CI

=

F

T

n

will be large for warm low clouds with high longwave emissivity and/or for thick clouds, which strongly attenuate shortwave

radiation.

The

cloud radiative forcing (

CF

)

is

the difference between all sky and clear sky down-welling fluxes:

CF

=

CF

SW

+

CF

LW

=

S

-

S

clear

+

L

-

L

clear

=

S

clear

(

T

n

-1) +

L

clear

(

F

-1)

(4)

where (

T

n

-1) is negative whereas (

F

-1) is positive

.

Daily values from 2005 to 2013

Analysis of regional atmospheric circulation

Daily 850, 500, and 200

hPa

wind and geopotential height anomalies from

ERA-interim

reanalysis

at 0.75× 0.75° horizontal resolution

Daily OLR data from NCAR/NOAA

at 2.5×2.5° horizontal: proxy for deep convection.

Composite analysis of atmospheric circulation was conducted considering the beginning of intense cloudy-sky events, defined as

the first day (D0) when the cloud cover index CI over Zongo was higher than the 90th percentile of all the years during the period 2005–2013

.

Slide9

850 hPa meridional wind anomalies in the 15–25°S and 65–57°W region. Days characterized by low-level incursions of southern winds to the east of the Andes were identified by positive anomalies of meridional wind in this region

Conceptual model of a cold air incursion over South America, generally applicable for wintertime and summertime episodes. Dark (light) thick arrows represent low-level wind advecting cold (warm) air. Thin contours represent surface isobars. Cold front atsurface is shown conventionally.Garreaud et al. (2000)

Slide10

Cloud long-wave emission:

F= L

/ σ

T

4

Slide11

Cloud long-wave emission

: F= L / σ T4

Cloud radiative forcing:CF = S - Sclear + L - Lclear = Sclear (Tn-1) + Lclear (F-1)where (Tn-1) is negative whereas (F-1) is positive.

Cloud

reduction in solar

radiation:

T

n

= S

/

T

clear

S

extra

Slide12

Cloud long-wave

emission:

F= L / σ T4

Cloud

reduction in solar

radiation:

T

n

= S

/

T

clear

S

extra

Slide13

Cloud radiative forcing:

CF = S - Sclear + L - Lclear = Sclear (Tn-1) + Lclear (F-1)where (Tn-1) is negative whereas (F-1) is positive.

Reduction

in solar radiation by clouds

exceeds

their increase in longwave radiation

largely because at low latitudes, solar irradiance is high all year

long.

Reduction

of the down-welling fluxes due to clouds

is

maximal in the wet season due to high extraterrestrial solar irradiance and low shortwave transmissivity, and despite high longwave emissivity of convective clouds

.

Slide14

CI = F – Tn largeDays with ‘thick’ cloud covers were identified by using the 90th percentile of CI over the eight years of study.

Dry season

Transition

Wet season

Slide15

Low Level Circulation (850

hPa

), June to August JJA

Cloud cover on Zongo glacier related

to low-level incursions of southern wind

Slide16

Cut off low events in

June

2011 producing

thick cloud

covers over the region of Zongo glacier

(500

hPa

)

Slide17

Composites anomalies of wind (vectors, m s-1) and geopotential height (contours at 40 m intervals, positive [negative] anomalies are in red [blue]) at 200 hPa from D–3 to D+3 from September to November (SON) in the period 2005-2012 when large cloud covers (CI > 0.75) over Zongo glacier were not related to low-level incursions of southern wind. Only wind anomalies higher than 0.5 standard deviation are plotted.

High level Circulation (200

hPa

), September

to

November SON, cloud cover on Zongo glacier NOT

related to low-level incursions of southern wind

Slide18

Low Level Circulation (850

hPa

), December to February DJF, cloud cover over Zongo Glacier NOT

related to low-level incursions of southern wind

Slide19

Some conclusions on regional atmospheric circulation…

In JJA and

SON, clouds mostly occurred during southern

wind incursions

at low levels

(80–87% of cases),

characterizing the

beginning of a cold surge episode, which

generally lasts

2–3 days in the Oriental Altiplano.

Other

episodes of

cloud cover

in the dry season (JJA) were linked

to low-pressure

conditions at 200

hPa

on the Chilean

coast (

including cut-off low episodes)

, whereas in SON,

they were

linked to early summer conditions

characterized

by an

active Bolivian High and easterly winds at 200

hPa

over

the Cordillera

Real

.

During

the wet season,

thick cloud

covers were still often associated with southern

wind incursions

(46% of cases), other cloud events being

associated with

the South American Monsoon

, intensification

of the

Bolivian High, and enhancement of the easterly

winds at

200

hPa

over the Altiplano

.

Slide20

Some perspectives…

To

quantify the effects of clouds on the glacier melt rate,

cloud forcing on the surface radiation balance

, including down- and up-welling fluxes, need to be investigated in relation with

glacier

albedo.

Cloud

properties need to be related to the

intensity, frequency, and phase of precipitation

to investigate the effects of the timing and duration of the wet season on tropical glaciers, and of the occurrence of cold surges, which generally cause large snowfalls that drastically reduce glacier melt

.

Similar work has been undertaken in

Ecuador

(climate more complicated

…)

THANK YOU!