what can the mass balance of lewis glacier ......determined speci˜c mass balance (prinz et al.,...

Institute of Meteorology and Geophysics (IMGI), University of Innsbruck, AustriaCenter for Climate and Cryosphere, University of Innsbruck, Austria

Lindsey Nicholson, Rainer Prinz, Georg [email protected]

WHAT CAN THE MASS BALANCE OF LEWIS GLACIER TELL US ABOUT EAST AFRICAN AND TROPICAL CLIMATE DYNAMICS?

Cogley, J. G. (2005) Mass and energy balances of glaciers and ice sheets, in Encyclopedia of Hydrological Sciences, vol. 4, edited by M. G. Anderson, pp. 2555 – 2573, John Wiley, Hoboken, New Jersey. Hastenrath, S. (2005) Glaciological studies on Mount Kenya 1971 – 2005. University of Wisconsin, Madison. Kaser, G., Cogley, J. G., Dyurgerov, M. B., Meier, M. F., & Ohmura, A. (2006). Mass balance of glaciers and ice caps: Consensus estimates for 1961–2004. Geophysical Research Letters, 33(19), 1-5. Mölg, T., Cullen, N. J., Hardy, D. R., Kaser, G., & Klok, E. J. (2008) Mass balance of a slope glacier on Kilimanjaro and its sensitivity to climate. International Journal of Climatology, 28(7), 881–892. Prinz, R., Fischer, A., Nicholson, L. I., & Kaser, G. (2011) Seventy-six years of mean mass balance rates derived from recent and re-evaluated ice volume measurements on tropical Lewis Glacier, Mount Kenya. Geophysical Research Letters, 38(20), 2-7.

Oct-09 Jan-10 Apr-10 Jul-10 Oct-10 Jan-11 Apr-11 Jul-11 Oct-11 Jan-125

101520


101520



101520


101520


101520

2468

1q [g

/kg]

200600

10001400

1

SW[W

/m2 ]

200

300

1

LW[W

/m2 ]

26

10

1V [m

/s]

568570572574

1

P [h

Pa]

-5

0

5

1

T [�

C] °

time

of d

ay [h

our i

n lo

cal t

ime]

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0

surf

ace

heig

ht c

hang

e [m

]

measured surface height from 29th Sep. 2009


0.05

0.1daily snow accumulation

modelled surface height for 2011

80 82 84 86 88 90 92 94 96-2.5

-1.5

-0.5

0.5

1.5

2.5

mass balance year [March-March]

mass balance [1000kg m-2]std. mass balancestd. rainy season precip.

1940 1950 1960 1970 1980 1990 2000-2800

-2600

-2400

-2200

-2000

-1800

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

spec

i�c

mea

n su

rfac

e m

ass

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nce

[kg

m-2

a-1

]

-50 0 50 100

Q swQ lw

Q sQ l

Q cQ m

complete 2011

-50 0 50 100

Q swQ lw

Q sQ l

Q cQ m

jan - feb 2011

-50 0 50 100

Q swQ lw

Q sQ l

Q cQ m

mean energy �ux density [W m-2]

oct - nov 2011

-5 0 5

accumulationdeposition

sublimationmelt

internal meltrefreezing

complete 2011

-5 0 5


sublimationmelt


jan - feb 2011

-5 0 5


sublimationmelt


mean daily mass exchange [kg m-2]

oct - nov 2011

+1K -1K +10% precip -10% precip150

100

50

0

50

100

150

200

% o

f unp

ertu

rbed

mas

s ba

lanc

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2011 (-849) JF (-134) ON (+139)

more positiveless positive

less negativemore negative

INTRODUCTION

Low latitude glaciers o�er potential for providing evidence of conditions in the tropical mid-troposphere, and how they have changed over recent decades.

HISTORICAL RECORDS OF LEWIS GLACIER IN THE CONTEXT OF GLOBAL MASS BALANCE

(1) Location of Lewis Glacier (00°09’S; 37°30’E) with respect to key tropical convergence zones (image modi�ed from wikigraphics). On the right is the glacier as it was after a snowfall in 2010.

(2) Lewis Glacier margins mapped at ca.10 year intervals since 1934 and the resultant geodetically determined speci�c mass balance (Prinz et al., 2011), indicate mass balance was broadly in-line with global estimates up until 1974, when a period of much more rapid mass loss ensued until 2004.

Lewis glacier in 2010: total ice volume =1.90 ± 0.30 x 106 m3, ice area = 0.105 ± 0.001 x 106 m2, mean (max) thickness =18 ± 3 m (45 ± 3 m).

(3) Interannual variability of directly measured mass balance measurements 1978-1996 (Hastenrath, 2005), is strongly related to total rainy season (MAM and OND) precipitation at 4800m, but precipitation at thhis elevation is not well corre-lated with regional precipitation records.

(Cogley, 2005) Pentadal global arith-metic mean (± 2 std error)(Kaser et al., 2006) Pentadal mean of 3 global mass balance records (± total uncertainty)(Prinz et al., 2011) Quasi decadal Lewis glacier geodetic mass balance (± total error)

RELATIONSHIP OF HISTORICAL MASS BALANCE WITH PRECIPITATION RECORDS

CHARACTERISTIC METEOROLOGICAL CONDITIONS

(4) Day-hour isopleth plots of meteorological records from the glacier surface (4830m) show that afternoon cloud cover is a persistent feature throughout the year with maximum speci�c humidity at 15:00. The regional wet (MAM & OND) and dry (JF & JJAS) seasons are not strongly evident in the available records.

The box plots show median values, with boxes extending to the interquartile range and whiskers to 1.5 times the interquartile range. The color scale in the isop-leth diagram is adjusted to the full range of the data shown in the boxplots.

(6) Modelled energy (left) and mass (right) �uxes for all of 2011 and a sub-sample with dry conditions (JF in green) and wet condi-tions (ON in blue) show that:

Shortwave radiation is the dominant energy source

Sublimation and melt consume almost the same amount of energy on an annual scale, but Ql dominates in the dry season, while Qm dominates in the wet season.

Melt rate is higher during the wet season than the dry season

(5) Results from a point surface energy and mass balance model (Mölg et al., 2008) shows generally good correspondance to the measured surface change. Deviations between the measured and modelled surface changes are related to occasionally poor per-formance of the albedo module within the model.

(7) Sensitivity to 4 single parameter perturbations are shown expressed as mass balance totals as a % of that modelled using unperturbed meteorological inputs. The unperturbed mass balance total in kg m-2 is given in parenthesis in the legend. Note that all y axis values are positive and the bars below the x axis simply indicate changes in mass balance for periods of mass loss.

Mass balance over 2011 appears more sensitive to changes in temperature than precipitation. However, the meteorological records and the energy �ux suggest that Lewis glacier annual mass balance is likely to be strongly a�ected by the distri-bution and duration of wet and dry conditions experienced at the glacier through-out the year.

SURFACE ENERGY BALANCE MODELLING

SURFACE ENERGY BALANCE SENSITIVITY

ITCZ July

ITCZ January

July

January

what can the mass balance of lewis glacier ......determined speci˜c mass balance (prinz et al.,...

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