RELEASE NOTES FOR CRU TS v3.25: 15 September 2017 The CRU TS dataset was developed and has been subsequently updated, improved and maintained with support from a number of funders, principally the UK's Natural Environment Research Council (NERC) and the US Department of Energy. Long-term support is currently provided by the UK National Centre for Atmospheric Science (NCAS), a NERC collaborative centre. The 3.25 release of the CRU TS dataset covers the period 1901-2016, and supercedes the 3.24.01 release and all earlier releases. The reference to use is: Harris, I., Jones, P.D., Osborn, T.J. and Lister, D.H. (2014), Updated high-resolution grids of monthly climatic observations – the CRU TS3.10 Dataset. Int. J. Climatol., 34: 623–642. doi: 10.1002/joc.3711 Differences Between CRU TS v3.25 and previous releases, Known Issues, the relationship with the co-released v4.01, and future plans. 1. Differences Between Releases 1.1 Between 3.24.01 and 3.25 In addition to updating the dataset with 2016 data, several specific areas have been addressed: • A bug in the way some CLIMAT 'WET' updates were processed was detected and resolved; this has resulted in small changes for many areas from 2000 onwards. • In an effort to fill gaps in the observed record, particularly in the first half of the C20th, observations from GHCN-M were selectively added where they offered new coverage. This has resulted in some new cover for TMP, TMN and TMX, and thus DTR, VAP, FRS (where applicable) and PET, in various parts of the world. These include small islands, the Amazon Basin, and areas of central Africa. Of course, there are still gaps to be filled, especially for PRE. 1.2 Between 3.25 and 4.01 As before, the differences between 3.xx and 4.xx releases represent the different interpolation philosophies inherent in their production. The 'original' 3.xx approach used IDL routines and (in theory) allowed observations to have influence far beyond the defined correlation decay distance. The updated approach in 4.xx releases does not allow this, with an associated loss of cover. This demonstrated the gaps that the efforts outlined above seek to address. 2. Known Issues 2.1 Interpolation near the International Date Line Affects: Eastern Siberia, all variables. In April 2014, a user notified BADC that they had discovered a discontinuity in data fields near to the International Date Line (ie, Eastern Siberia). This manifests as a vertical (ie, along a line of longitude) discontinuity approximately 2.5° (five gridcells) to the West of the Date Line (so, at about 177.5°E). Investigation of this phenomenon concluded that it was due to the gridding approach used: this is explained in the documentation, and consists of triangulation, followed by interpolation to the chosen grid points. The triangulation is not 'spherical', and so triangles do not form across the Date Line, this is causing the anomalous data. Despite exhaustive testing of available routines within the package that is being used, none have been found to deliver fields that are free from unwanted artifacts. This has been a significant driver in the move to ADW gridding (v4.xx). 2.2. Anomalous Canadian data. Affects: Canada (esp. Eastern Canada), temperature variables. In May 2014, a user contacted CRU with statistical work showing a cold bias in CRU TS mean temperature data (TMP) over Eastern Canada, when compared to other datasets and models. This bias apears to begin in 2005 and runs until the present. Resources have not been available to investigate thoroughly. Affects: Canada, wet days variable. The country-level spatially-averaged wet day series shows unlikely behaviour, with the day count steadily rising until 1990, at which point there is a precipitous fall (of approximately two days) to levels previously seen in the 1930s. 2.3. General differences from previous release. Affects: All variables, all releases At first glance, an update process should not change any existing published data unless it includes retrospective data for the previous time period, or introduces new stations. However, these are several ways in which new updates to existing stations may affect published data from any time: a) New data for a station will change the standard deviation of that station. This is used to assess each observation of that station and to eliminate excessive values. So in 'borderline' cases, new data may block a previously-allowed datum, or alternatively may allow through a previously-blocked value. b) New metadata for any station can result in a relocation. This may simply be a correction, or improved accuracy: but any move will alter the triangulation process, and this can propogate over a wide area as the triangles are not constructed independently of each other. So a station move of perhaps 0.05 degrees could change an interpolated value many degrees away (and all of the others in between). This change will also apply to the whole station series, changing all triangles at every time step. This has been another significant driver in the imminent move to ADW gridding. 3. The Accompanying 4.01 Release and the future of v3.xx releases As for 4.00, the 4.01 release utilises Angular-Distance Weighting (ADW) gridding, giving more accurate results with much improved control and logging. It covers the same spatial, temporal and variate spaces as version 3.25, and uses the same observations. It is intended that this will be the last 'parallel' release; the next scheduled update (1901-2017, in Summer 2018) will be v4.02, barring unanticipated eventualities or requests. As always, please contact BADC in the first instance if you have any questions, observations or suggestions. Ian Harris NCAS-Climate Climatic Research Unit School of Environmental Sciences University of East Anglia Norwich NR4 7TJ