Contribution of turbulent heat fluxes to surface ablation on the Greenland ice sheet.
- 1Utrecht University, Institute for Marine and Atmospheric Research, Utrecht, Netherlands (m.vantiggelen@uu.nl)
- 2Utrecht University, Department of Physical Geography, Utrecht, Netherlands
Over ice sheets and glaciers, the turbulent heat fluxes are, next to the radiative fluxes, the second largest source of energy driving the ablation. In general, most (climate) models use a bulk turbulence parametrization for the estimation of these energy fluxes. Recent work suggest that the turbulent heat fluxes might be greatly underestimated by such models. Unfortunately, only a few direct and long-term observations of turbulent fluxes are available over ice sheets to evaluate their inclusion in models.
In this study, we developed a vertical propeller eddy-covariance method to continuously monitor the sensible heat fluxes over the Greenland ice sheet (GrIS). We quantify its contribution to surface ablation using three years of data from the K-transect, located in the western ablation area of the GrIS. The direct flux measurements are also compared to those from several bulk turbulence models, and to a high-resolution regional climate model (RACMO2), in order to quantify modelling uncertainty.
The differences between observations and models highlight the need for upgrading the bulk turbulence parameterizations and especially the model parameters, such as the surface roughness lengths. We also find that during short but extreme warm events, the turbulent heat fluxes become the largest source for surface ablation. Typical for such intense events on the K-transect are fast changes in wind direction, which cause changes in the surface roughness parameters due to the anisotropic feature of the ice hummocks. These parameters are critical for modelling the turbulent fluxes in bulk parameterizations, but are often variable and unknown. We conclude with drone topography measurements to better constrain the surface roughness locally, and discuss methods to improve the modelling of turbulent surface fluxes on the whole GrIS.
How to cite: Van Tiggelen, M., Smeets, P., Reijmer, C., Noël, B., Steiner, J., Nieuwstraten, E., Immerzeel, W., and van den Broeke, M.: Contribution of turbulent heat fluxes to surface ablation on the Greenland ice sheet., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15227, https://doi.org/10.5194/egusphere-egu2020-15227, 2020.
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interesting, thanks. I dont really understand how you derive the time series of roughness length in your last slide as this is n. Is this a sort of residual as it barely can be a measured quantity?
Hello Jakob, we estimate the aerodynamic roughness length using the measured friction velocity (momentum flux measured by eddy-covariance), measured wind speeds (Young vane), sensor height (sonic ranger), Monin-Obukhov length, and then by extrapolating the semi-logarithmic wind profile to the height where the wind speed drops to zero, which yields z0. Kind regards, Maurice
Hi Maurice
Do you use a stability correction for this? Given that the flow is katabatic, I am not very convinced that classical MO approximation works.
Ivana
Hello Ivana,
Yes we use the integrated stability functions from Holstag and De Bruin (1988) and we reject all the measurements when stability z/L > 0.2. (This filter rejects not more than ~10% of the data at this location)
Concerning the validity of M-O: Near the margins of the Greenland ice sheet, the katabatic wind maximum is typically higher than 50m. We thus assume that the instruments (4m +- 1m above the surface) are located within the inertial sublayer, such that the fluxes are not affected by the wind maximum. The wind profile is not regularly measured anymore but there are many documented experiments at this location, e.g., Van den Broeke (1994), Heinemann (1999).
I hope this asnwers your question,
Maurice
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Van den Broeke MR, Duynkerke PG, Oerlemans J (1994) The observed katabatic flow at the edge of the greenland ice sheet during GIMEX-91. Global Planetary Change 9:3–15
Heinemann G (1999) The KABEG’97 experiment: an aircraft-based study of katabatic wind dynamics over the greenland ice sheet. Boundary–Layer Meteorol 93:75–116