Soil Moisture CCI: Production of the Most Complete and Consistent Global Soil Moisture Data Record
Haas, Eva Maria1; Wagner, W.2; Dorigo, W.2; Chung, D.2; de Jeu, R.3; Parinussa, R.3

The important role of soil moisture for the environment and climate system is well known and it was recognised as an Essential Climate Variable (ECV) in 2010. ESA's Programme on Global Monitoring of Essential Climate Variables (ECV), better known as the Climate Change Initiative (CCI), initiated in 2010 for a period of 6 years. The CCI Programme wants to contribute to the data bases collecting ECVs required by GCOS (Global Climate Observing System) and other international parties. The CCI Soil Moisture project started in January 2012 with the objective to produce the most complete and most consistent global soil moisture data record based on active and passive microwave sensors.

The project focuses on C-band scatterometers (ERS-1/2 scatterometer, MetOp Advanced Scatterometer) and multi-frequency radiometers (SMMR, SSM/I, TMI, AMSR-E) as these sensors are characterised by their high suitability for soil moisture retrieval and a long technological heritage. Other microwave sensors suitable for soil moisture retrieval, including the Soil Moisture and Ocean Salinity (SMOS) mission, Synthetic Aperture Radars (SARs) and radar altimeters are not considered in the first phase of the CCI programme due to their recentness and/or their unfavourable spatio-temporal coverage. Nevertheless, the ECV production system will be set up in such a way to allow the integration of all these sensors in the next phase(s) of the CCI programme.

In order to make best use of existing European and international programmes, and to ensure that new sensors such as SMOS can be integrated quite easily, the design of the ECV production system is as modular as possible. The main characteristic of the approach is that the merging of the different sensor-specific data sets takes place at the level of the retrieved surface soil moisture data (Level 2). The conversion of the Level 1 backscatter- and brightness temperature measurements to surface soil moisture is hence not considered to take place within the ECV production system itself, but before the ECV production system. In this way it becomes possible to make best use of already established Level 2 services of ESA, EUMETSAT, NASA, JAXA, etc. for the different satellites and sensors. The CCI Soil Moisture project expects that this approach will encourage international cooperation in multiple ways, e.g. in terms of the ECV production process itself or in terms of defining GCOS standards.

In June 2012 the project made the first public release of the ECV soil moisture data set generated within the WACMOS and CCI Soil Moisture projects. The global soil moisture data set has been generated using active and passive microwave spaceborne instruments and covers the 32 year period from 1978 to 2010. Since its publication more than 470 users requested the 30 years of ECV soil moisture data with about three new requests daily. The ECV soil moisture data set was globally recognised with user requests originating from over 30 different countries largely stemming from research organisations, but also private companies and public bodies. The users have wide application areas and cover all nine Global Earth Observation System of Systems (GEOSS) societal benefit areas with focus on climate and water applications.