The atmospheric boundary layer over melting glaciers

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The atmospheric boundary layer over melting
glaciers

• By J. Oerlemans
• Presentation by Jasper Donker

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Outline

• Introduction
• Measurments on katabatic flow
• Modelling katabatic flow

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Introduction

• Melting is sensitive to air temperature
  variations
• Downward sensible heat flux increases with
  increasing temperatures
• The surface stays at melting point for long
  periods of time
• Causing a katabatic flow down the glaciers

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Experiment on the kathabatic winds above glaciers

• Done at 3 different locations
• Pasterze (Austria)
• West of greenland
• Vatnajökull(ice land)
• Weatherstations at the surface measure wind
  direction and speed
• Vertical structure is determined with the help of
  zeppelins caring sensors

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Greenland measurments on katabatic wind
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The Prandl model

• The prandl model was used to make a model for the
  vertical structure of the katabatic glacial wind
• This model couldn't describe the decrease in eddy
  diffusivity when approaching the surface
• The steep velocity gradient close to the surface
  could not be produced
• Katebatic jet is more pronounced
• With increasing strength the height of the
  katebatic jet is increased this is not reproduced

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Problems with applying flux theory

• The top of the boundary layer is hard to identify
• Its hard to determine the dept of the surface
  layer because of unevenly melt.

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Determine surface fluxes

• In times of continues melt fluxes can be
  calculated by using radiation balance.
• Profiling method doesnt work
• Microtopograpic method has much better results

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Modified model for the surface layer a constant
flux layer
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Flow dependent eddy diffusivity

• Values for the eddy diffusivity were taken
  constant in previous model
• Due to dependence of turbulent mixing on the
  strength of the katabatic flow this is not
  realistic
• This modification gives better estimates.

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Conclusions

• Katabatic flow is present over melting glaciers
• Its difficult to apply existing stable
  boundary-layer theory to glacier wind.
• Micrographic method is best method to determine
  roughness of the surface
• Modified model should be tested.

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The End