RELEASE NOTES FOR CRU TS v3.22: 4 July 2014

The 3.22 release of the CRU TS dataset covers the period 1901-2013. It
supercedes 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 v3.21 and v3.22, and Known Issues

In addition to updating the dataset with 2013 data, the v3.22 release
corrects an error in the v3.21 dataset. This is summarised in the
document, 'CRU_Advisory_v3.2x_NE_Africa.txt', and affects PRE and WET
variables only. There are several known issues with the current dataset
which cannot be resolved in the timeframe of this release; they will be
addressed in the future.

1. Corrected Error: Mozambique Stations

New data including some stations in Mozambique was incorporated into the
databases that were used to construct the v3.21 release of CRU TS; the
Mozambique stations had the signs of their latitudes reversed. This
resulted in those stations appearing in North-Eastern African countries.
Algeria was particularly affected, as it has areas with low station
counts. Corrected files were made available on request; this is the
first full release to incorporate the corrections.


2. Known Issue: 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 indicates that the gridding process will have
to move to another platform, which is not possible in the timeframe of
this release.


3. Known Issue: 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.


4. Known Issues: 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. However, these are several ways in which new updates 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. For example, in the
     current 3.22 release, a particular cell in Nicaragua is showing
     differences from the 3.21 release, for Wet Days ('WET'):

     Year   Month   Lat Cell   Lon Cell   Difference
     1968      11        205        188         4.85
     1972      11        205        188        -2.67
     1978       8        205        188        -3.70
     1980       8        205        188        -2.28
     2011       8        205        188        -3.97

     There are no useful station observations near to this location,
     nor are there any changes to station data on these dates within
     20° of this location. The changes are to the triangulation network,
     as several stations have moved location slightly.


As always, please contact BADC in the first instance if you have any
questions, obervations or suggestions.

Ian Harris
Climatic Research Unit
School of Environmental Sciences
University of East Anglia
Norwich
NR4 7TJ