Spatial and Temporal Variability of Minimum Brightness Temperature at the 6.925 GHz Band of AMSR2 for the Arctic and Antarctic Oceans

Young-Joo Kwon,Jeong-Won Park,Seung Hee Kim,Sungwook Hong,Jong-Min Kim,Hyun-Cheol Kim

doi:10.3390/rs13112122

Young-Joo Kwon, Jeong-Won Park + Show 4 more

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https://doi.org/10.3390/rs13112122

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Abstract

The minimum brightness temperature (mBT) of seawater in the polar region is an important parameter in algorithms for determining sea ice concentration or snow depth. To estimate the mBT of seawater at 6.925 GHz for the Arctic and Antarctic Oceans and to find their physical characteristics, we collected brightness temperature and sea ice concentration data from the Advanced Microwave Scanning Radiometer 2 (AMSR2) for eight years from 2012 to 2020. The estimated mBT shows constant annual values, but we found a significant difference in the seasonal variability between the Arctic and Antarctic Oceans. We calculated the mBT with the radiative transfer model parameterized by sea surface temperature (SST), sea surface wind speed (SSW), and integrated water vapor (IWV) and compared them with our observations. The estimated mBT represents the modeled mBT emitted from seawater under conditions of 2–5 m/s SSW and SST below 0 °C, except in the Arctic summer. The exceptional summer mBT in the Arctic Ocean was related to unusually high SST. We found evidence of Arctic amplification in the seasonal variability of Arctic mBT.

Highlights

The brightness temperature (BT) of seawater measured by passive microwave (PMW)satellite data is approximately proportional to sea surface emissivity and sea surface temperature
From the BT data measured by Advanced Microwave Scanning Radiometer 2 (AMSR2) for eight years, we found a similar minimum brightness temperature (mBT) in in the Arctic and Antarctic Oceans: 157.852 K (Arctic) and 157.654 K (Antarctic) for Vthe Arctic and Antarctic Oceans: 157.852 K (Arctic) and 157.654 K (Antarctic) for V-polarpolarization and+ 74.949 K (Arctic) and 74.784 K (Antarctic) for H-polarization
We determined the mBT for V- and H-polarization with the average of the lower 0.1% of data based on the points projected along the trend line

Summary

Introduction

The brightness temperature (BT) of seawater measured by passive microwave (PMW)satellite data is approximately proportional to sea surface emissivity and sea surface temperature. The brightness temperature (BT) of seawater measured by passive microwave (PMW). The emissivity of seawater depends on the salinity of the sea surface (SSS), sea surface temperature (SST), and sea surface wind speed (SSW) for the given observation frequency band and incidence angle of the sensor. The polar region is where the phase transition occurs between seawater and sea ice. Because solar radiation incidents in the Arctic and Antarctic oceans are low, the SST can drop to the freezing point. When seawater is frozen and sea ice is formed, emissivity in the microwave region increases from a value of 0.2–0.5 for the seawater to 0.9 for sea ice. even if the sea ice has the same temperature as the ocean, the BT of sea ice measures higher than that of the ocean

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