Geography of Glaciers 3

GEOGRAPHY OF GLACIERS 3Roger BraithwaiteRoger Braithwaite

School of Environment and Development

1.069 Arthur Lewis Building

University of Manchester, UK

Tel: UK+161 275 3653

[email protected]@man.ac.uk

09/08/2012 Geography of glaciers June 2012 1

GEOGRAPHICAL VARIATIONS IN MASS BALANCE

• Observed mass balance variations

• Modelled mass balance variations• Modelled mass balance variations


CONTINENTAL/MARITIME

• Not just temperature temperature range but also precipitation

• Breaks down at low at low latitudes or in South America


MASS BALANCE ON SOME GLACIERS MORE VARIABLE THAN OTHER

(Braithwaite, 2006)

Order of magnitude difference in


Order of magnitude difference in variability!

BIAS IN MASS BALANCE MEASUREMENTS

(Braithwaite, 2009)


Measured glaciers are biased towards wetter conditions

WGMS HEALTH WARNING


Tables like this (from WGMS website) are nonsense because they don’t take account of geographical differences in MB variability

GEOGRAPHICAL VARIATIONS IN MASS BALANCE

• Observed mass balance variations

• Modelled mass balance variations• Modelled mass balance variations


“TUNING” AND “TWEAKING” DEGREE-DAY MODEL

• Glacier-level precipitation poorly precipitation poorly constrained

• Tune model by adjusting glacier precipitation to make model mass balance fit balance fit measured mass balance

Geography of glaciers June 2012 809/08/2012

“TUNING” DEGREE-DAY MODEL(Braithwaite and Zhang, 1999)

Vary precipitation to fit model

P= 4m 3

Model is now fitted

a)

-1

4P= 4m

P= 3m

P= 2m

P= 1m

Ma

ss b

ala

nc

e (m

a)

-1

-1

-2

0

3

1

2

Wa

ter

eq

uiv

ale

nt

(m w

ate

r a

-2

0

-1

1

2

3

13001200 1600 17001400 15001100-3

Altitude (m a.s.l.)13001200 1600 17001400 15001100

Altitude (m a.s.l.)13001200 1600 17001400 15001100


“TWEAKING” DEGREE-DAY MODELChange temperature by 1 K

a)

-1 2

3

Vary temperature ±5 K

a)

-1 4

6

Ma

ss b

ala

nce

(m

wa

ter

a

-3

1

0

-1

-2

Altitude (m a.s.l.)13001200 1500 16001400 17001100

Ma

ss b

ala

nce

(m

wa

ter

a

-6

2

0

-2

-4

Temperature change ( C)�

-2-4 2 40 6-6Altitude (m a.s.l.) Temperature change ( C)�

Geography of glaciers June 2012 10

Balance-temperature is nonlinearAccumulation falls with temperature increase due to reduced

snowfall

09/08/2012

TEMPERATURE-SENSITIVITY OF MASS BALANCE

Degree-day and energy balance models agree reasonably well

Whole-glacier sensitivity is similar to sensitivity at glacier mean elevation


MB SENSITIVITY AND CLIMATIC SETTING

Large difference between maritime and continental settings

Large changes on maritime glaciers and small changes on continental

glaciersglaciers


GEOGRAPHY OF GLACIERS1º lat/long cells with more than c. 20 km2 glacier cover,

according to GGHYDROBraithwaite (2002)


ELA IN DIFFERENT REGIONS(Braithwaite and Raper, 2007)

• Small difference between true ELA and between true ELA and median glacier elevation

• Large variation within each region

• Large differences between different between different region

• Axel Heiberg Island generally drier than Svalbard


ACCUMULATION AT ELA(Braithwaite and Raper, 2007)

• Use degree-day factor for snow

Warm-wet

factor for snow

• Large differences between different regions

• Arctic (Axel Heiberg Island and Svalbard) Island and Svalbard) is “dry” and New Zealand is “wet”


Cold-dry

MASS BALANCE SENSITIVITY AT ELA

(Braithwaite and Raper, 2007)

• Use degree-day factor for icefactor for ice

• Large differences between different regions

• Arctic (Axel Heiberg Island and Svalbard) Island and Svalbard) glaciers not very sensitive to temperature change


MASS BALANCE SENSITIVITY

• Temperature sensitivity scales sensitivity scales onto MB amplitude AMP

• Also scales on continental/maritime variables like RANG and PRECand PREC

• Also scales onto other sensitivity variables


GEOGRAPHY OF GLACIERS

Expect low MB sensitivity here

1 degree grid

MY LAST PROJECT?Develop a “geography of glaciers” so

Expect high MB

sensitivity here

Geography of glaciers June 2012 18

1 degree grid squares with glacier areas >20 km2

Develop a “geography of glaciers” so we can understand/predict glacier

behaviour in every part of the World

09/08/2012

ENERGY BALANCE AT ELA

• Assumed melting depends upon temperature, e.g. using degree-day modele.g. using degree-day model

• Topographic controls on temperature and precipitation are dominant in glacier geography

• However, melt energy does come from different sources and sinks and we probably different sources and sinks and we probably need energy balance to explain fine details in glacier geography


ENERGY BALANCE AND CLIMATE(From lecture 15)

High humidity and cloudiness favour • low shortwave↓ radiation

Wet?

• low shortwave↓ radiation• high longwave↓ radiation• high latent heat flux↓ (condensation)High temperature favours• high longwave↓ radiation

Warm?

• high longwave↓ radiation • high sensible heat flux↓• high latent heat flux↓ (condensation)


ENERGY BALANCE AND CLIMATE(From lecture 15)

Low humidity and cloudiness favour • high shortwave radiation↓

Dry?

• high shortwave radiation↓• low longwave radiation↓• low latent heat flux↓ (sublimation)Low temperature favours• low longwave radiation↓

Cold?

• low longwave radiation↓ • low sensible heat flux↓• low latent heat flux↓ (sublimation)


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Geography of Glaciers 3