RG5-LF: An improved Arctic-wide winter lead-fraction dataset over 2002–2015 for climate-oriented studies and model development
The project is devoted to improvement of lead fraction retrievals in the Arctic from satellites. These will then be used to study processes such as heat exchange between the ocean and the atmosphere and brine rejection during the ice formation.
Objectives
The main goal of the project is to improve the lead fraction dataset and consequently assess the lead fraction trend over the Arctic during 2002–2015 in order to quantify the associated heat and water vapor flux, brine rejection, and the wind work, and assess their role for ocean mixing/circulation. The improved retrievals will also be used for further development and evaluation of the new state-of-the-art sea ice model neXtSIM developed at NERSC. This is an important step before it is possible to implement this new and much improved sea ice model into NorESM and couple to MICOM.
Project Summary
Participants: N. Ivanova, P. Rampal, S. Bouillon, E. Ólason (NERSC), I. Fer, L.H Smedsrud (GFI), and M. Ilicak (Uni-Research)
Leads in the Arctic sea ice pack serve as an indicator of the ice break-up and following divergence. In winter leads control the heat and water vapor exchange between ocean and atmosphere. In addition, new ice formation in leads causes brine rejection into the upper ocean, which significantly affects vertical mixing and stratification. Open water in the leads is exposed to wind forcing and can lead to substantial upwelling of deeper warm waters. Therefore the lead fraction and its trend over the last decades are important climate parameters. A lead fraction dataset obtained from satellite passive microwave data (AMSR-E) is available, but clearly overestimates the values by a factor of 5-8 when compared to sea ice divergence from RGPS-processed SAR images. It is thus not suitable for climate investigations, model evaluation or data assimilation at its present quality.
Project plan.
Mar-Dec 2015: i) Quantification of the errors in the existing AMSR-E lead-fraction (LF) dataset by SAR data (a simple threshold technique will be applied to the radar backscatter field). ii) Improvement of the AMSR-E method by applying image processing techniques and adjusting the algorithm parameters with help of optical and SAR data. iii) Production of the new LF dataset for 2002–2015. iv) Improvement of a standard sea ice concentration (SIC) dataset using iii).
Jan-Dec 2016: i) NorESM 0.25 degree ocean+sea ice coupled simulations with CORE-II atmospheric forcing and comparison of the simulated SIC to the satellite datasets. ii) Calibration/validation of neXtSIM against the new LF and SIC datasets. iii) Process studies: impact on the heat fluxes and salt rejection (neXtSIM), variability and correlations between the atmospheric forcing and upper ocean stratification, vertical mixing and circulation (NorESM).
Interaction with other projects: The project will strengthen collaboration between the Bjerknes partners (here NERSC, Uni-Research and GFI), the ongoing internal SKD project BASIC, and improve extrapolation results of field observations from the ongoing Lance drift project. The work at NERSC will build on and interact with already existing projects on remote sensing (H2020/SPICES and ESA SICCI) and sea ice modeling (NFR SIMech).
