RELEASE NOTES FOR CRU TS v4.00: 31 January 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 4.00 release of the CRU TS dataset covers the period 1901-2015. It is the first release using a new process, in particular the gridding approach has now changed (see below). This release covers the same temporal and spatial extents as v3.24.01, and uses the same source databases. Users may therefore satisfy themselves as to the differences between these two versions. It is the intention that CRU TS moves to this new process exclusively in the future, but the two processes will run in parallel until at least the next full release (1901-2016) in late Summer 2017. There is currently no published reference for this version; work is in progress to accomplish this. In the meantime the description below will need to suffice. 1. The 4.00 Process, and how it differs from the previous process (3.xx). The main process change in 4.00 is the move to Angular Distance Weighting (ADW) for gridding the monthly anomalies. Compared to the previous approach, which used IDL routines TRIANGULATE and TRIGRID to effect triangulated linear interpolation, ADW allows us total control over how station observations are selected for gridding, and complete traceability for every datum in the output files. For secondary variables, this means that observed and synthesised data values are used in the same way in the gridding process. ADW was previously used for the production of CRU TS 2.0: New, M., M. Hulme, and P. Jones, 2000: Representing Twentieth-Century Space–Time Climate Variability. Part II: Development of 1901–96 Monthly Grids of Terrestrial Surface Climate. J. Climate, 13, 2217–2238, http://dx.doi.org/10.1175/1520-0442(2000)013<2217:RTCSTC>2.0.CO;2 The configuration for ADW in v4.00 is largely based on that described in the above paper. Some aspects of that configuration are: • Between 1 and 8 stations contributing to a gridcell at any time step • The exponent in the distance weighting calculation is 4 • Observations take precedence over synthetic data (where both present) • Synthetic data stations that lie within 45° of a 'real' data station are not used • Gridcells with no stations in range are set to 0 anomaly (ie the climatology) All the supporting code for CRU TS is now in Fortran, improving maintenance, speed and portability when compared to the previous mixture of IDL and Fortran. There have been changes to the synthetic variable generators. The most significant of these is that variables are now synthesised for discrete stations, whereas before they were produced as a lower-resolution (2.5°) grid. This enables them to be used by the ADW gridding process. Additionally, synthetic WET is now produced as an absolute number of wet days, (it was previously converted to an anomaly). It then passes through the same processes as regular observations and is thus anomalised using its own 1961-90 normals. This approach may be extended to other synthetic variables in the future. 2. Differences in output files For now, the approach of issuing NetCDF and ASCII files in parallel will remain, as will the publications of decadal files as well as full-length versions. For 4.00, station counts have changed. In 3.xx it was effectively impossible to know which stations had contributed to each datum, so two approximations were published: the number of stations reporting within the CDD ('.stn'), and the number of reporting stations in each gridcell ('.st0'). In 4.00 a count of observations, (including synthetic observations where used), is produced by the gridder. These counts are published as ASCII '.stn' files, and are also embedded into the NetCDF files as a second variable ('stn'). It is anticipated that this will encourage users to use this additional information to better understand the dataset they are working with. 3. Differences between 3.24.01 and 4.00 The change in approach has affected coverage. The interpolation process in 3.xx runs could not prevent the triangulation exceeding the defined Correlation Decay Distance (CDD) for the variable being gridded (it merely ensured that 'dummy' stations were inserted in unobserved regions). In 4.00, gridcells outside the CDD of any observations (actual or synthetic) are set to the climatology, so discs of influence with sharp edges can appear in plots of sparsely-observed regions. This lacks aesthetic qualities but is more justifiable; it also highlights areas where more observations are needed. The same is true for numerous small islands, which will have lost cover either partly or completely. In terms of comparisons, the extent to which 4.00 agrees with 3.24.01 varies with station density, unsurprisingly. Comparison plots of country averages have been made and are included in the release: against 3.24.01 and against 2.10. The longstanding issue of discontinuous interpolation along the dateline in Eastern Siberia is resolved with the new process. Please contact me with any observations or questions. This is, in many ways, a new dataset: I expect it to evolve. Ian Harris NCAS-Climate Climatic Research Unit School of Environmental Sciences University of East Anglia Norwich NR4 7TJ i.harris@uea.ac.uk