Abstract
The sensitivity of weather and climate system to sea ice thickness (SIT) in the Arctic is recognised from various studies. Decrease of SIT will affect atmospheric circulation, temperature, precipitation and wind speed in the Arctic and remotely. Ice thermodynamics and dynamic properties depend strongly on ice and snow thickness. The heat transfer through ice critically depends on ice thickness. Long term accurate SIT records with corresponding uncertainties are required for improved seasonal weather forecast and estimate of the sea ice mass balance. Satellite radar and Laser Altimeter (LA) provide long term records of sea ice freeboard. Assuming isostatic equilibrium, SIT is retrieved from the freeboard, extracted from radar altimeter (RA) or LA, where the snow depth, density, ice and water density are input variables in the equation for hydrostatic equilibrium to derive SIT from LA or RA. Different input variables (snow depth, density, ice and water density) with unknown accuracy have been applied from various authors to retrieve SIT and Sea Ice Draft (SID) from RA or LA, leading to not comparative results. Sea ice density dependence on ice type, thermodynamic properties and freeboard is confirmed with different studies. Sensitivity analyses confirm the great impact of sea ice density, snow depth and density on accuracy of the retrieved SIT and the importance of inserting variable ice density (VID) in the equation for hydrostatic equilibrium for more accurate SIT retrieval, weather and climate forecast. The impact of sea ice density and snow depth and density on retrieved SIT from the freeboard derived from LA and RA have been analyzed in this study using the equation for hydrostatic equilibrium, statistical and sensitivity analyses. An algorithm is developed to convert the freeboard, derived from LA in SIT, inserting VID in the equation for hydrostatic equilibrium. The algorithm is validated with field, laboratory studies and collocated SIT retrieved from RA on board Envisat. The accuracy of the developed algorithm is analyzed, using statistical and uncertainty analyses. It is found that the uncertainty of the retrieved SIT from LA is decreased 7.6 times (from rhi = 59 cm for fixed ice density) if variable ice density is inserted in the equation for hydrostatic equilibrium. The SIT, which has been retrieved from the freeboard derived from LA is validated with collocated SIT derived from RA2 on Envisat, using variable ice density. The bias of the mean SIT derived from LA and RA has been reduced from -1.1 m to about one millimeter when VID is applied to retrieve SIT from LA and RA. The results and algorithms, discussed in this paper are essential contribution to SIT and SID retrieval, satellite remote sensing, cryosphere, meteorology and improved weather and climate forecast.
Highlights
Sea ice plays an important role in the global climate system via feedbacks and ice-atmosphere heat exchange [1] [2]
The sea ice thickness (SIT) which is derived from the freeboard, retrieved from Laser Altimeter (LA) (Airborne Topographic Mapper (ATM) on board OIB) is calculated by inserting in the equation of hydrostatic equilibrium fixed sea ice, water and snow densities, as well as snow thickness derived from the snow radar on board OIB [5], which is different from the snow depth, which was used to retrieve SIT from ICESat [7]
The paper is organized as follows: 1) The hydrostatic equations to convert the freeboard derived from radar altimeter (RA) and LA into SIT and the impact of contributing factors are discussed in Section 2; 2) Sensitivity, uncertainty analysis and retrieval of VID to derive SIT from LA are provided in Section 3; 3) Validation of the derived SIT from LA, using variable ice density and collocated SIT from RA is given in Section 4; 4) the final conclusions are provided in the last Section 5
Summary
Sea ice plays an important role in the global climate system via feedbacks and ice-atmosphere heat exchange [1] [2]. The SIT which is derived from the freeboard, retrieved from LA (Airborne Topographic Mapper (ATM) on board OIB) is calculated by inserting in the equation of hydrostatic equilibrium fixed sea ice (ρi = 914.3 kg/m3), water (ρw = 1023.9 kg/m3) and snow (ρs = 264 kg/m3 during the expedition in 2009 and ρs = 320 kg/m3 during the expedition in 2010) densities, as well as snow thickness derived from the snow radar on board OIB [5], which is different from the snow depth, which was used to retrieve SIT from ICESat [7]. The uncertainty analyses and validation studies confirm the increased accuracy of the derived SIT from RA if variable ice density (VID) is inserted in the equation of hydrostatic equilibrium to convert the freeboard into SIT [12]. The purpose of this study is to develop and validate algorithm with corresponding error analysis to retrieve sea ice density and convert the freeboard (derived from LA) in SIT for climate applications, providing minimum bias with collocated SIT from independent satellite observations from RA. The paper is organized as follows: 1) The hydrostatic equations to convert the freeboard derived from RA and LA into SIT and the impact of contributing factors are discussed in Section 2; 2) Sensitivity, uncertainty analysis and retrieval of VID to derive SIT from LA are provided in Section 3; 3) Validation of the derived SIT from LA, using variable ice density and collocated SIT from RA is given in Section 4; 4) the final conclusions are provided in the last Section 5
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