IASI L2Pcore sea surface temperature. By Anne O Carroll, Thomas August, Pierre Le Borgne and Anne Marsouin

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1 IASI L2Pcore sea surface temperature By Anne O Carroll, Thomas August, Pierre Le Borgne and Anne Marsouin Abstract Anne O Carroll EUMETSAT Eumetsat Allee Darmstadt Germany Tel: E mail: Anne.Ocarroll@eumetsat.int The IASI L2 Product Processing Facility (PPF) at EUMETSAT retrieves surface skin temperature over land and sea and has been running operationally since April The data are available via EUMETCast together with vertical temperature and humidity profiles. Since March 2011, the IASI SSTs have been available from EUMETSAT in the format of the Group for High Resolution Sea Surface Temperature (GHRSST) project as a demonstrational product called IASI L2Pcore SST. In the Continuous Development and Operations Phase 2 (CDOP 2) of the OSI SAF, the IASI L2Pcore SST will be ingested and the remainder of the auxiliary meteorological fields will be filled. The product will be released in the full GHRSST L2P format. An associated in situ matchup database will be compiled for validation purposes. Introduction Recent validations have shown IASI SSTs to have a global mean cool bias of 0.32K and standard deviation of 0.32K, with the highest quality results having a cool bias of 0.09K [1]. Three way collocations, using the method described in [2], have shown IASI SSTs to have a global mean uncertainty of ±0.28K [1]. The IASI L2Pcore SSTs are produced following the Group for High Resolution Sea Surface Temperature (GHRSST) Data Specification 2.0 [3] format, and contain skin SSTs, Sensor Specific Error Statistics, quality levels, flags and collocated model surface winds. They are provided in swath format with the IASI observations having an IFOV of 12km at nadir. The IASI L2Pcore product is available via ftp access from the EUMETSAT Data Centre and FTP server, with registration possible through the EUMETSAT EO Portal. To analyse validation statistics IASI SSTs are collocated with the OSI SAF Metop A/AVHRR matchup dataset (mdb1 full version) [4]. Global comparisons with drifting buoy SSTs Time series of IASI minus drifting buoy differences (figure 1) show how the standard deviations range between 0.3 and 0.4K, and with a bias mostly under 0.4K for most recent results. The global mean uncertainties, derived using the method in [2], are mostly under 0.3K for IASI SSTs. The global map of IASI minus buoy differences, in figure 2, shows mainly green points where the IASI SSTs are slightly cooler than buoy SSTs. There are larger excursions in aerosol regions, or where there may be possible cloud detection issues such as in the Southern Ocean.

2 Figure 1 Monthly time series of IASI minus buoy differences, showing mean difference (left), standard deviation (centre), and standard deviation of error (right) over the period March 2010 to June Figure 2 Locations of buoy and IASI collocations from July 2011 to June Influence of cloud Global collocations of IASI, AVHRR and drifting buoys SST observations were analysed, for night time only observations where the AVHRR quality level is 3 or above. The criteria were varied to select cases where the IASI IFOV is most likely to be cloud free by looking at the number of cloudy AVHRR pixels in a 21x21 box. In addition, collocations using only IASI quality level 5 (highest) were analysed. Since the IASI skin SSTs were converted to a sub skin SST by adding a 0.17K offset [5] to all observations, rather than filtering for only high wind speed cases only, an additional criterion was applied to select where the wind speed at the observation is greater than 6m/s. Statistics are shown in table 1. For the most stringent criteria, the IASI observations have a cool bias with respect to drifting buoys of around 0.2K. Table 1 Statistics of IASI, AVHRR and buoy differences from July 2011 to June Criteria (night, AVHRR QL>=3) IASI buoy mean diff K IASI buoy st.dev. K AVHRR IASI mean diff K AVHRR IASI st.dev. K Number matchups Standard Cloud free Cloud free, IASI QL

3 Cloud free, wind>6m/s Cloud free, wind>6m/s IASI QL Using information within the OSI SAF MDB, collocations of IASI minus buoy, AVHRR minus IASI, and buoy minus AVHRR were compared to box rate (number clear observations/number water observations in 21x21 AVHRR box) and the standard deviation of AVHRR SST within the 21x21 pixel box, see figure 3. It is possible to observe how the biases vary according to likely cloud cover in the IASI IFOV, and the homogeneity of SST over the IFOV. For clear views and homogeneous scenes, the IASI cool bias is more constant at around 0.2K. Where cloud increases, the cool bias also increases, and when the standard deviation of SSTs in the box increase the cool bias fluctuates and has increased uncertainties. Figure 3 IASI minus buoy SST differences versus box rate (left), AVHRR standard deviation in 21x21 box (centre), AVHRR standard deviation for when box rate is equal to 1.0 (cloud free), July 2011 to June Regional statistics Figure 4 shows histograms of IASI minus buoy differences for different regions. Regionally, standard deviations of IASI minus buoy collocations are within around 0.35K, except for the NE Pacific and Arabian Sea where larger biases are shown too, and the latter may be influenced by aerosols. High standard deviations are also seen in the Saharan dust region.

4 Figure 4 Histograms of IASI minus buoy SST regional differences July 2011 to June IASI L2P from the EUMETSAT Ocean and Sea Ice Satellite Application Facility In 2012, the OSI SAF began a work package as part of the EUMETSAT SAF Continuous Development and Operations Phase 2 to produce fully GHRSST compliant IASI L2P SST products, using the EUMETSAT IASI L2Pcore products as input. There are two parts to the work package: firstly the operational chain to produce the full L2P, with the inclusion of missing auxiliary fields in the L2Pcore such as OSI SAF sea ice concentration, and either SEVIRI Saharan Dust Index or NAAPs aerosol data. Secondly, a validation processing chain is currently being developed by the OSI SAF team at CMS to produce an operational IASI/AVHRR/in situ MDB, and an additional research IASI/AVHRR/in situ MDB. The research MDB will contain AVHRR pixels in a 37x37 box to enable coverage of the entire IASI IFOV even at the edge of scan. The EUMETSAT OSI SAF IASI L2P GHRSST compliant products are planned to be available from the OSI SAF initially via FTP in Conclusions SSTs from the IASI L2Pcore have a slight cool bias of around 0.2K when compared to drifting buoys, and when effort has been made to filter those observations thought to be most cloudy. In the 2013 time frame work is on going at EUMETSAT to introduce a new cloud detection scheme based on artificial neural networks, and to include IASI band 3 during night time into the retrieval, with both updates expected to give some improvements in the SST bias and standard deviations. Work is currently on going at the OSI SAF to produce fully GHRSST compliant IASI L2P files, using the EUMETSAT IASI L2Pcore as input.

5 References 1] O Carroll, A.G., T. August, P. Le Borgne and A. Marsouin, The accuracy of SST retrievals from Metop A IASI and AVHRR using the EUMETSAT OSI SAF matchup dataset, RSE, ] O Carroll, A.G., R.W. Saunders and J.R. Eyre, Three Way Error Analysis between AATSR, AMSR E, and In Situ Sea Surface Temperature Observations, JAOT, Vol. 25, No. 7, p1197, ] The recommended GHRSST Data Specification Revision 2.0, 4] Le Borgne, P., G. Legendre, A. Marsouin, and S. Pere, Operational SST retrieval from METOP/AVHRR validation report, OSI SAF CDOP report, Version 2.0, July ] Donlon, C. J., P. J. Minnett, C. Gentemann, T. J. Nightingale, I. J. Barton, B. Ward, and M. J. Murray, Toward improved validation of satellite sea surface skin temperature measurements for climate research. J. Climate, 15, , 2002.