# Sea Surface Temperature in Global Analyses: Gains from the Copernicus Imaging Microwave Radiometer
## 1 Introduction  
Measurement of sea surface temperatures (SSTs) is crucial to the understanding and prediction of the atmosphere and ocean. The SST regulates the exchange of moisture and heat [1; 2] between the two and thus controls the thermal structure of the upper ocean and sets the boundary conditions needed for numerical weather prediction (NWP). Errors in SST can lead directly to a decrease in NWP skill [3]. SSTs retrieved from infrared (IR) sensor data generally have superior spatial resolution(\(\approx\)1 km) and retrieval accuracy (0.2 to 0.4 K) to passive microwave (PMW) sensors (\(\approx\)50 km and 0.35 to 0.5 K, respectively, for recent and current missions) [4; 5; 6; 7]. However, they are limited to cloud-free regions, whereas PMW sensors are able to make measurements through clouds. This greatly increases their coverage and makes them particularly important for regions of persistent cloud such as the high-latitudes in winter. The Copernicus Imaging Microwave Radiometer (CIMR) is a mission concept under consideration as a high-priority candidate mission for the expansion of the Copernicus space programme (the Sentinels). The concept includes the deployment of a \(\approx\)7 m mesh antenna capable of real aperture resolution \(<\) 15 km at 6.9 GHz (required for cool-water SST estimation). The CIMR mission is motivated by the European Union (EU) Arctic policy [8], whereby monitoring of the physical properties of the changing high-latitude environment is essential for sustainability, economic development and environmental protection. Its primary objectives are the retrieval of sea ice concentration (SIC) and SST. The requirement for SIC is for a spatial resolution \(\leq\) 5 km with a total standard uncertainty of \(\leq\)5%, and for SST, a spatial resolution \(\leq\) 15 km with a total standard uncertainty of \(\leq\)0.2 \(\pm\) 0.1 K is required [9]. The CIMR mission will deliver both high-latitude and global data with near-global coverage every day. CIMR SSTs will therefore improve the observability of ocean dynamics reflected in the SST compared to an infrared-only SST dataset, particularly in areas of persistent cloud: the Arctic, Southern Ocean, north Pacific and Atlantic oceans, marine stratiform regions and the inter-tropical convergence zone including the tropical warm pool. The targets for the uncertainty components are given in Table 1. Measurements from both the forward and rearward arcs of CIMR's conical scan will be available, enabling the averaging of retrievals to reduce noise. This combination of features implies that CIMR SSTs are expected to have simultaneously higher spatial resolution and lower noise uncertainty than present PMW SSTs [9]. The aim of this study is to examine the effect on SST analyses of adding CIMR to the IR-observing constellation, here represented by the Advanced Very High Resolution Radiometer (AVHRR) instrument on-board Met-Op A and by the Sea and Land Surface Temperature Radiometer (SLSTR) carried by Sentinel-3A. Synthesised observations were provided as inputs to the Operational Sea Surface Temperature and Sea Ice Analysis system (OSTIA) [10] in different combinations as set out below. The OSTIA system is in operational near-real time use by the Met Office. It operates by ingesting satellite and in situ SSTs and optimally combining them to produce estimates of daily, global, foundation SSTs (defined as SST free of diurnal variability [11; 12]). A climate configuration of the system generates SST analyses from satellite data only, which are representative of the daily average at a 20-cm depth [7] and this is used in this study. For each day, the system generates a background field based on the persistence of the previous day's analysis, with a weak relaxation to climatology. For the open ocean, the relaxation timescale is 30 days. The background is combined with the satellite SSTs with relative weights determined by their uncertainties.