Abstract
Passive microwave measurements at L-band from ESA’s Soil Moisture and Ocean Salinity (SMOS) mission can be used to retrieve sea ice thickness of up to 0.5–1.0 m. Since 2015, NASA’s Soil Moisture Active Passive (SMAP) mission provides brightness temperatures (TB) at the same frequency. Here, we explore the possibility of combining SMOS and SMAP TBs for sea ice thickness retrieval. First, we compare daily TBs over polar ocean and sea ice regions. For this purpose, the multi-angular SMOS measurements have to be fitted to the SMAP incidence angle of 40 ∘ . Using a synthetical dataset for testing, we evaluate the performance of different fitting methods. We find that a two-step regression fitting method performs best, yielding a high accuracy even for a small number of measurements of only 15. Generally, SMOS and SMAP TBs agree very well with correlations exceeding 0.99 over sea ice but show an intensity bias of about 2.7 K over both ocean and sea ice regions. This bias can be adjusted using a linear fit resulting in a very good agreement of the retrieved sea ice thicknesses. The main advantages of a combined product are the increased number of daily overpasses leading to an improved data coverage also towards lower latitudes, as well as a continuation of retrieved timeseries if one of the sensors stops delivering data.
Highlights
In light of a rapidly changing Arctic sea ice cover, measuring sea ice thickness remains an important task [1]
We demonstrate the impact of different fitting methods on the day-to-day TB variability over a stable target, such as an ice shelf (Section 3.1)
We start by comparing Soil Moisture and Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) sea ice thicknesses (Section 3.3.1) to address the following questions: How do the TB differences and biases found in Section 3.2 translate to SIT differences? Which differences remain after TB bias adjustment? In addition, we evaluate the differences caused by changing the retrieval algorithm from near-nadir to an incidence angle of 40◦ (Section 3.3.2)
Summary
In light of a rapidly changing Arctic sea ice cover, measuring sea ice thickness remains an important task [1]. After the second L-band sensor Aquarius [7] stopped delivering data in June 2015, there is currently a third L-band sensor in orbit: NASA’s Soil Moisture Active Passive (SMAP) mission delivers data since April 2015 and carries a passive microwave radiometer measuring at the same frequency as SMOS and with a comparable footprint size. This is an excellent opportunity to compare brightness temperature (TB) measurements from SMOS and SMAP and to analyze the benefits of a combined product for sea ice thickness retrieval. As a first step for a brightness temperature comparison, SMOS TBs have to be fitted to the SMAP incidence angle
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