Tao, Ran
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Item type:Publication, Melt pond fractions on Arctic summer sea ice retrieved from Sentinel-3 satellite data with a constrained physical forward modelThe presence of melt ponds on Arctic summer sea ice significantly alters its albedo and thereby the surface energy budget and mass balance. Large-scale observations of melt pond coverage and sea ice albedo are crucial to investigate the role of sea ice for Arctic amplification and its representation in global climate models. We present the new Melt Pond Detection 2 (MPD2) algorithm, which retrieves melt pond, sea ice, and open-ocean fractions as well as surface albedo from Sentinel-3 visible and near-infrared reflectances. In contrast to most other algorithms, our method uses neither fixed values for the spectral albedo of the surface constituents nor an artificial neural network. Instead, it aims for a fully physical representation of the reflective properties of the surface constituents based on their optical characteristics. The state vector X, containing the optical properties of melt ponds and sea ice along with the area fractions of melt ponds and open ocean, is optimized in an iterative procedure to match the measured reflectances and describe the surface state. A major problem in unmixing a compound pixel is that a mixture of half open water and half bright ice cannot be distinguished from a homogeneous pixel of darker ice. In order to overcome this, we suggest constraining the retrieval with a priori information. Initial values and constraint of the surface fractions are derived with an empirical retrieval which uses the same spectral reflectances as implemented in the physical retrieval. The snow grain size and optical thickness change with time, and thus the ice surface albedo changes throughout the season. Therefore, field observations of spectral albedo are used to develop a parameterization of the sea ice optical properties as a function of the temperature history of the sea ice. With these a priori data, the iterative optimization is initialized and constrained, resulting in a retrieval uncertainty of below 8 % for melt pond and 9 % for open-ocean fractions compared to the reference dataset. As reference data for evaluation, a 10 m resolution product of melt pond and open-ocean fraction from Sentinel-2 optical imagery is used.journal article309 175 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Spatial heterogeneity and seasonal evolution of surface properties and radiative fluxes of Arctic sea ice(2024-05-31); ; ; ; In recent years, the Arctic sea ice has experienced a significant decline, characterised by the smaller extent, longer melt season, and a shift from thick multi-year ice to thinner first-year ice. As a result, more solar radiative energy is deposited into the Arctic sea ice and the ocean underneath, further enhancing sea ice melt and ocean heat. When the Arctic is transitioning from melt onset to freeze onset, the sea ice surface spatial variability becomes stronger, altering the spatial distribution of radiative energy deposition. Understanding the seasonal evolution and spatial variability of solar radiative fluxes is a key step to broadening our knowledge of the changing Arctic sea ice. In this thesis, I investigate the year-round changes in solar radiative fluxes within the Arctic sea ice, both temporally and spatially. I examine the changes in optical properties during the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition (MOSAiC) in 2020. This thesis utilises a wide range of sensors and platforms, ranging from long-term continuous point measurement, to weekly under-ice mapping of light field, and to ice-floe size parameterization. This thesis highlights the spatial variability of the solar radiative fluxes of Arctic sea ice: under the same atmospheric condition and located on the same ice floe, different locations show highly variable evolution. The largest variability is in the middle of the melt season, due to the changing melt pond coverage and status. The sea ice types and surface conditions are crucial for the sea ice energy budget, thus further controlling the melting process. This thesis provides a comprehensive 3-dimensional view of the sea ice radiative fluxes and improves the parameterization of sea ice optical properties. Also, by investigating the effects of spatial surface variability, which is a function of time and area, this thesis guides future observations of the new Arctic sea ice regime. This study bridges in-situ observation to floe-size parameterisation, advances our understanding of the upscaling of solar radiative energy fluxes both onto and through the Arctic sea ice, and deepens our understanding of the impact of sea ice heterogeneity on the large-scale energy budget of the melting Arctic sea ice.doctoral thesis270 783