RELEASE NOTES FOR CRU TS v4.04: 22 April 2020 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 4.04 release of the CRU TS dataset covers the period 1901-2019. This is the fourth release of the new interpolation algorithm, which is unchanged since the previous release (v4.03). To understand this dataset, it is important to understand its construction and limitations. It is therefore recommended that all users read the relevant paper: Harris, I., Osborn, T.J., Jones, P. et al. Version 4 of the CRU TS monthly high-resolution gridded multivariate climate dataset. Sci Data 7, 109 (2020). https://doi.org/10.1038/s41597-020-0453-3 1. What's new in 4.04 1.1 Process changes The process whereby VAP is synthesised, using TMP and DTR, has been altered so that it delivers VAP absolutes, rather than anomalies as before. This means that synthetic VAP stations are introduced to the main process flow, and are anomalised in the same fashion as actual VAP observations. One of the main inputs to CRU TS is the MCDW update. This was previously a monthly update, however, it is now a monolithic file in .csv format. This resulted in a redesign and recode of the entire assimilation process, as all months from all sources now need to be reviewed in tandem. It has also had far-reaching effects on the output files, as some observations have been revised, and some stations added or deleted. If you need more information, please contact me directly. 1.2 Bug fixes The work on VAP uncovered a previously-undetected bug that, in some circumstances, resulted in TMP observations persisting when the station concerned suffered a break in observations. This resulted in extra synthetic VAP values at times and places where none should have been. The effects may be seen in the VAP comparison plots, though these are, of course, also subject to the MCDW changes. In general, quite a lot of uncharacteristic behaviour has been removed, though in some sparsely- observed places (some islands, for instance), coverage has been lost for early periods. If you need more information, please contact me directly. A peculiar bug in the anomaly process was uncovered during the rewriting and testing of the assimilation process. This meant that any set of station observations that ended in 1983 would be omitted. This has not significantly affected outputs. 2. 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. However, decadal files may not be archived when superceded, in order to make best use of space. As always, please contact BADC in the first instance if you have any questions, observations or suggestions. If, however, you wish to contact CRU directly about these datasets, please contact me at i.harris@uea.ac.uk, as mail to other members of the Unit will be passed through to me anyway. Ian Harris NCAS-Climate Climatic Research Unit School of Environmental Sciences University of East Anglia Norwich NR4 7TJ i.harris@uea.ac.uk