https://crudata.uea.ac.uk/cru/data/drought/

This is the readme file for the self-calibrating Palmer Drought Severity Index
(scPDSI) data for the global land surface (excluding Antarctica), spanning the
period 1901-2024 with monthly resolution. The spatial resolution is 0.5 x 0.5 degrees.
The information below relates to global scPDSI files.

The format of the data is a self-describing netCDF file, with information about
grid coordinates and time coordinates provided within the file.
The grid size is 720 longitudes by 360 latitudes and the coordinates of the
CENTRE of the grid cells end in 0.25 or 0.75 (i.e. the EDGES of the grid cells
lie on either 0.00 or 0.50 degrees).
The grid cells run from 89.75N, 179.75W (centre) to 89.75S, 179.75E (centre).

We have no longer provide a plain text format file as well. Please request this only
if you cannot progress without it.

The data run from January 1901 to December 2024, driven by the
CRU-TS v4.09 monthly climate dataset.
The final version of the CRU-TS data will be available from here:
https://crudata.uea.ac.uk/cru/data/hrg/


CAVEATS and NOTES:

1. Values during 1901 are unreliable because scPDSI was initialised in 1901
and no CRU-TS climate data were available for 1900.

2. In CRU-TS, grid cell climate values are relaxed towards the 1961-1990 mean
climatology when there are no or few weather station observations within the
Correlation Decay Distance (CDD).  The CDD is 1200 km for monthly temperature
but only 450 km for monthly precipitation.  This means that the scPDSI of grid
cells may be closer to zero (i.e. normal conditions), underestimating the
occurrence of anomalously dry or wet spells, in regions and times with poor
data coverage.

3. The underestimation of anomalously dry or wet spells (see note 2) will
affect the early decades of the twentieth century the most, but it can also
occur during recent years because not all observations are easily available in
near-real time for updating CRU-TS.  A rough indication of the scale of this
limitation is given by the number of precipitation observations incorporated into
the preliminary version of CRU-TS v3.25 (the version used for the 1901-2016 scPDSI dataset).
For the 1990s and 2000s, approximately 5,000 observations were available per month.
For 2011-2015, the mean number of precipitation obserations per month in CRU-TS v3.25
was 2200, 2166, 2064, 1725 and 1676, respectively.

4. The scPDSI data values in 2024 are based on CRU-TS v4.09 created in February 2025, and
future versions will likely incorporate additional observations for 2024 that were not available
to us so early in 2025.


When using these data, please cite:

van der Schrier G, Barichivich J, Briffa KR and Jones PD (2013) A scPDSI-based
global data set of dry and wet spells for 1901-2009. J. Geophys. Res. Atmos.
118, 4025-4048 (10.1002/jgrd.50355). 

and

Barichivich J, Osborn TJ, Harris I, van der Schrier G and Jones PD (2025)
Monitoring global drought using the self-calibrating Palmer Drought Severity Index
[in "State of the Climate in 2024"]. Bull. Amer. Meteor. Soc., 106, S77-S78
https://doi.org/10.1175/BAMS-D-25-0102.1


The publication which introduced the self-calibrating PDSI, and gives a large
amount of detailed information about its calculation, is:
Wells, N., Goddard, S. and Hayes, M. J. (2004)
A Self-Calibrating Palmer Drought Severity Index, J. Climate 17, 2335-2351


Support for the 2024 update was provided by Horizon Europe research and innovation
programme (ERC-starting grant CATES, grant agreement No. 101043214) and UK NERC
through the GloSAT project (NE/S015582/1) and for the 2024
update of CRU-TS v4.09 by the NERC National Centre for Atmospheric Science (NCAS).