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(Working Group on Climate and Hydrology, WMO RA VI)

(Last update: 15/05/2017)

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Presentation Data rescue Proxy data CDMS Quality control Homogenization Data repositories


The path to the provision of Climate Services to the public begins by the acquisition of data and their inclusion in homogeneous and quality controlled climatological series. This will allow a reliable analysis of climate and hydrology variability and trends, hence providing sound basis for expected changes in future at different scales (seasonal, decadal, centennial).

Data rescue

Current data acquisition is generally either made by automatic means or routinely entered into computers, ready to be used for different applications. But a good wealth of old data remains in original paper support, and Data Rescue (DARE) activities are needed to ensure their preservation and digitization in order to extend backwards our knowledge about climate. NMHS have DARE projects in different degrees of development (see the EUMETNET Data Rescue homepage), and there are several international initiatives promoting DARE activities worldwide (compiled at the I-DARE portal).

Data Rescue activities can be seen as composed of a few distinctive steps:

  1. Imaging of the historic data documents, either by photographing or scanning them in standard digital computer formats. Any analog micro-forms must also be converted to digital files. Afterwards, paper documents must be preserved in adequate archives, as digital images should be the primary source for the following steps. Conservation of digital images will include backup copies and a policy of conversion to open emerging formats to avoid obsolescence.
  2. Digitizing the images obtained in step 1 to allow computer processing of the data. Scanned typewritten documents can be treated with an OCR (Optical Character Recognition) program; otherwise human mechanization will be needed, either with ad hoc input programs or with simple spread sheets. After some basic Quality Controls on the data, they can be transferred to the preferred Climate Data Management System.
  3. Data rescue ends here, but before being used for climate analysis, rescued series must be further quality controlled and homogenized to remove the frequent alterations due to non-climatic factors. Errors originated during the digitization process must be corrected, but otherwise the raw series must be kept untouched, since different homogenization procedures can yield (slightly) different homogenized series. Homogenized series, however, will be preferred source for climate studies.

Additional information can be found at the WCDMP web page, including the publications Guidelines on Climate Data Rescue (WMO No. 1210) and Guidelines for Hydrological Data Rescue (WMO No. 1046). (See also other WCDMP reports).

Proxy data

In order to achieve long series of climatological or hydrological data, observational series can be extended backwards before the first instrumental measurements by means of proxy data (quantitative or even qualitative observations of strongly related variables). Examples are the width of the tree rings, polinic records or isotopic rates in drill cores, historical reports about extreme events, etc.

Documents on the use of proxy data in Hydrology:

Hydrological proxy data web sites:

Climate Data Management Systems

Efficient use of climate data, either rescued from documental sources or routinely acquired through modern observing systems, require their incorporation into a Climate Data Management System, usually implemented in advanced National Meteoro-Hydrological Services.

For less developed NMHS, WMO supports the use of several Climate Data Management Systems (CDMS), as those listed in Guidelines on Climate Data Management (WCDMP No. 60 / WMO-Td No. 1376) and, more recently, MCH (Meteorology, Climatology, Hydrology), which has the advantage of being based on free yet reliable open software, hence relieving NMH services of the burden of costly software licences.

Quality control

Quality control procedures must be applied to the data at several stages, from their acquisition through their transmission, storing and different processing operations. A broad variety of quality control rules can be applied, many of them depending of the variable and instrumentation, but they can be classified in five types:

The first three types of controls apply to individual series, while the others require at least two (of related variables at the same station in the forth type, and at different stations in the fifth).

These quality controls are normally part of Climate Data Management Systems, and some of them are generally applied in other processes, as during the homogenization of the series.

See here a proposal for the Guidelines on Quality Control Procedures for data from Automatic Weather Stations.


Changes in the observing conditions or in the environment of the meteorological stations introduce anomalous perturbations in the data series (inhomogeneities). Therefore, homogenization is of paramount importance to obtain reliable analysis of the variability of climatological series, and there exists an extensive literature about the different methodologies applied so far. This allows climatologists to choose their preferred methods for their research, often implementing those methods by themselves. But some of them have made their developments available to other users by preparing and documenting ready to use computer packages and distributing them in the Internet or by other means. This is very interesting for operational climatology units of many National Meteorological and Hydrological Services around the world, that sometimes can lack the needed expertise to make their own developments, or else can devote the required time to other priorities.

Tables with comparative characteristics of publicly available homogenization packages are maintained at the web site of the Task Team on Homogenization, and another page shows some preliminary results of some automatic benchmarking tests.

One key aspect to improve the assessment of inhomogeneities in the series is the ability to check whether the detected alterations are backed by events in the history of the observatories. Therefore, it is of paramount importance to keep records of any change in the conditions of observations (including changes in the land use of the surroundings) in proper meta-data archives, and to make them available to climate researchers. A couple of examples of such good practices can be seen in the meta-data pages of the Australian Bureau of Meteorology and of the USA National Oceanic and Atmospheric Administration.

Data repositories

National Meteorological and Hydrological Services are the primary source of updated climatological data. Moreover, some of them, together with universities and other research institutions, compile regional or world-wide climate data-sets, facilitating in this way climatological and hydrological studies at these scales.
(Additional contributed links can be sent to jguijarrop[AT]aemet.es).