Abstract
Abstract. The European Space Agency's Climate Change Initiative for Soil Moisture (ESA CCI SM) merging algorithm generates consistent quality-controlled long-term (1978–2018) climate data records for soil moisture, which serves thousands of scientists and data users worldwide. It harmonises and merges soil moisture retrievals from multiple satellites into (i) an active-microwave-based-only product, (ii) a passive-microwave-based-only product and (iii) a combined active–passive product, which are sampled to daily global images on a 0.25∘ regular grid. Since its first release in 2012 the algorithm has undergone substantial improvements which have so far not been thoroughly reported in the scientific literature. This paper fills this gap by reviewing and discussing the science behind the three major ESA CCI SM merging algorithms, versions 2 (https://doi.org/10.5285/3729b3fbbb434930bf65d82f9b00111c; Wagner et al., 2018), 3 (https://doi.org/10.5285/b810601740bd4848b0d7965e6d83d26c; Dorigo et al., 2018) and 4 (https://doi.org/10.5285/dce27a397eaf47e797050c220972ca0e; Dorigo et al., 2019), and provides an outlook on the expected improvements planned for the next algorithm, version 5.
Highlights
The European Space Agency’s Climate Change Initiative for Soil Moisture, hereafter referred to as ESA CCI SM, is dedicated to the development of consistent satellite-based long-term climate data records (CDRs) for soil moisture, aiming to serve climate science as well as numerous other communities (Dorigo et al, 2017)
The ESA CCI SM merging algorithm produces three individual products: (i) the activemicrowave-based only (ACTIVE) product, which is generated by merging soil moisture retrievals from activemicrowave-based sensors only, (ii) the passive-microwavebased only (PASSIVE) product, which is generated by merging soil moisture retrievals from passive-microwave-based sensors only, and (iii) the COMBINED product, which is generated by merging soil moisture retrievals from both active-microwave-based and passivemicrowave-based sensors
Since this product is provided in the ASCAT data space, i.e. as the degree of saturation, a porosity map derived from the Harmonized World Soil Database (HWSD; Nachtergaele and Batjes, 2012) is provided alongside the data in order to allow the soil moisture estimates to be converted to volumetric units if required
Summary
The European Space Agency’s Climate Change Initiative for Soil Moisture (http://www.esa-soilmoisture-cci.org/, last access: 17 May 2019), hereafter referred to as ESA CCI SM, is dedicated to the development of consistent satellite-based long-term climate data records (CDRs) for soil moisture, aiming to serve climate science as well as numerous other communities (Dorigo et al, 2017). The ESA CCI SM serves more than 6000 registered users, providing the basis for a host of scientific publications and data set applications (Dorigo et al, 2017). Central to the ESA CCI SM is a merging algorithm, which, in essence, merges soil moisture retrievals from various satellites that have finite lifetimes and significantly different instrument characteristics (frequency, spatial resolution, temporal coverage, polarisation, revisit time, etc.) into three consistent multi-decadal data sets. This process faces innumerable scientific challenges and is subject to continuous research and development. An outlook on the expected developments that are planned for the iteration, version 5, which is foreseen to be released in 2019, is provided
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