85th AMS Annual Meeting

Wednesday, 12 January 2005
On the sensitivity of cloud related radiative processes on the initiation and rate of melt over snow covered landfast sea ice
Xin Jin, Centre for Earth Observation Science, Winnipeg, MB, Canada; and D. G. Barber
Thermodynamic evolution of the snow/sea ice system is a key element in the overall mass balance of sea ice in the northern hemisphere and it plays a central role in processes operating across the ocean-sea ice-atmosphere (OSA) interface. In the Arctic regions, cloud is a key factor. Typically, clouds warm the surface in winter and cool it in summer by reflecting the incident solar radiation and reducing the outward surface longwave flux. With partial cloud cover the relationship between cloud fraction and surface solar forcing (SCF), is usually treated by simple linear interpolation or related statistical parameterization. We also know that the high spatial and temporal variability in cloud forcing complicates our ability to model OSA thermodynamic processes at the regional and hemispheric scales.

In this paper, we propose a new method to retrieve the relationship between the cloud fraction (CF) and the integrated incident surface solar flux (0.3-3um) by considering the multi-reflection effect between cloud ceiling and the snow/ice surface. Using in situ observations from the Canadian High Arctic we examine the sensitivity of each parameter in this new relationship using a range of observed melt onset conditions. Numerical sensitivity tests and observed data show that the derived parabolic curve can fit the observed data with a higher correlation coefficient (0.86) than other contemporary methods. Snowmelt processes are then examined using the CF-SCF relationship. Results show that, when the daily averaged net surface flux approaches 40 Wm-2, the snow cover above a first year landfast sea ice begins to melt. However, the initiation moments of snowmelt are significantly advanced and delayed correspondingly, and the melting periods are shortened and elongated correspondingly, with the assumption of different cloud fractions. Analysis of the efficiency of the SCF shows that for selected cloudy days SCF is highly variable diurnally due to the interplay of solar zenith angle with surface insolation. We conclude the paper with an examination of how the SC-SCF relationship can be formulated using optical and thermal infrared Earth Observation data from MODIS and related satellite and how these data may be assimilated into a thermodynamic snow/sea ice model.

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