A tentative climatology of the snow load on Arctic sea ice based on satellite
Abstract
Having a firm grasp of the sea ice extent carries over to the understanding of poleward energy transport, atmospheric heat exchange and high-latitude ocean dynamics at large. One reason to investigate the snow load is the insulation against exchange of heat. Another, regarding the intrinsic value of remote sensing, is that snow constitutes the greatest unknown in sea ice altimetry. The properties of snow can modify how deeply into the snow-ice system the altimeter signal penetrates. While Cryosat views to the ice surface, Icesat views to the snow surface. The freeboard cannot be measured and converted to ice thickness properly without compensation for the thickness and density of the snow cover. To identify the satellite channels with most information on the scenery, we made the standard assumption that the inversion of measured brightness temperature to physical parameters is sufficiently linear to converge for Gauss-Newtonian iteration. An optimal estimation scheme has been adopted and the information content in the averaging kernel matrix scrutinized for the parameters at stake. The a priori covariance and initial guess on parameters was computed by feeding the snow-ice model Memls with ERA40 atmospheric reanalysis over a range of locations, winters, and type of ice as having grown from either scratch (first-year) or not (multiyear). Each of the currently flown passive sounders under consideration, the Advanced Microwave Scanning Radiometer (AMSR), the Advanced Microwave Sounding Unit (AMSU), and the Microwave Humidity Sounder (MHS), is modelled with a measurement error taken as the sum of sensitivity and accuracy prior to launch. Covariance between the channels has been neglected. Simulation of the actual measurement discretizes the snow pack into ten numerical layers to resolve the steep temperature gradient and applies the model Rttov to represent the air column. Snow is taken to be fresh and dry, a valid assumption until melt sets in, and the density of multi-year ice is imposed a fixed decrease above the waterline. The correlation length in ice that governs scattering shifts from water content (brine) to air bubbles after the first year. The optimal set of satellite channels has been chosen, in part, by minimizing the number of platforms involved and the jumps in frequency between them. These channels provide the basis on which we intend to retrieve a snow climatology that spans the past few years. Construction requires iteration against the assumption that either type of ice alone was covering the surface pixel and then engagement with a lookup table to meet with the brightness temperature observed. Comparison of the seasonal and regional variability is made to reanalysis and in situ measurement.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.C23B..07S
- Keywords:
-
- 0736 Snow (1827;
- 1863);
- 0750 Sea ice (4540);
- 1621 Cryospheric change (0776);
- 1827 Glaciology (0736;
- 0776;
- 1863);
- 4540 Ice mechanics and air/sea/ice exchange processes (0700;
- 0750;
- 0752;
- 0754)