Alpine Precipitation Analyses
from High-Resolution Rain-Gauge Observations

Christoph Frei, Institute for Atmospheric and Climate Science ETH (IACETH), Zuerich, Switzerland

  • Project Overview

  • Analysis Version 4.0

  • Interpretation Hints

  • Rain-gauge Database

  • Sample Charts

  • Future Improvements

  • Spatial Analysis

  • Digital Access

  • Publications

  • SOP Precipitation Analysis


  • Dr. Christoph Frei
    Institute for Atmospheric and Climate Science ETH
    Winterthurerstr. 190
    CH-8057 Zuerich
    Tel  +41 (0) 1 635 52 32
    Fax +41 (0) 1 362 51 97
     Email address: IACETH-Homepage, then click on "Staff" and type "frei".

    Project Overview

    This project aims at deriving spatial analyses and climatological evaluations of precipitation in the region of the European Alps. The project makes use of an unique dataset of rain gauge observations from the high-resolution networks of all Alpine countries. These networks constitute one of the densest meteorological observing systems over complex topography world-wide. The analyses are suitable for research activities on regional climate dynamics, mesoscale meteorology and the evaluation of weather- and climate models.

    The project is allocated at the Swiss Federal Institute of Technology Zuerich (ETH, Institute for Atmospheric and Climate Science ETH). The project is closely linked to the Mesoscale Alpine Programme MAP an international research initiative on mesoscale meteorology in the Alpine region.


    Rain-gauge Database


    The climatology is based on an extensive dataset of rain-gauge observations from the operational high-resolution networks of all Alpine countries. The data has been provided by a number of national and regional meteorological and hydrological services. At present a total of about 6700 time series is included with daily observations for at least part of the period 1966-1995. In the average there are about 5000 observations available per day, and this corresponds to a station spacing of 10-15 km. The same region encompasses only about 250 SYNOP stations reporting on the Global Telecomunication System (GTS).

    The distribution of stations is fairly balanced over the northern and western portions of the analysis domain, while the coverage is more variable for the southern region. The diagram depicts the station coverage representative for the 20 years 1971-1990. For the more recent years (1991-1995), the station coverage is similar except for northern Italy where the station density is lower than in previous years.

    Some of the data supplying institutions have extensively checked the quality of their observations. In other cases only unproofed data was available. For the actual analyses (version 4.0) all daily observations have been passed through a common quality controle procedure, checking for the spatial consistency of the daily reports. Suspect data was then ingnored during the spatial analysis.

    Previous versions of the climatology were based on a subset of the rain-gauge database described. A detailed description of the database for version 4.0 is given in Frei and Schär (1998). Check Versions for recent changes in the database.


    Spatial Analysis

    Mesoscale Alpine precipitation fields are constructed by spatial analysis of quality proofed rain-gauge observations onto a regular grid in geographical latitude and longitude coordinates. The analysis is performed on a daily basis and with a grid resolution of about 25 km.

    The applied analysis technique is a modified version of the SYMAP algorithm of Shepard (1968, 1984) and Willmott et al. (1985). It is designed to calculate local area means from a set of station values in the neighbourhood of the analysis grid-points, taking into account the distance and the directional isolation of the station data. The search area is locally adapted to the density of available point observations, resulting in a variable effective spatial resolution. Through this procedure optimal use can be made of high station coverage while still providing a coarser resolution analysis for areas with sparser coverage.

    The accuracy of the mesoscale precipitation analysis is influenced by the systematic biases of the rain-gauge measurements and by the limited spatial sampling for the area-mean estimation. Future improvements of the analyses will focus on an extension of the station sample over data-sparse areas and considerations of the non-representative gauge distributions with respect to topography.

    A detailed description of the applied analysis scheme, the modifications to the standard procedure and a discussion of the error sources is given in Frei and Schär (1998).

  • Shepard D.S., 1968: A two-dimensional interpolation function for irregularly-spaced data. Proceedings of 1968 ACM National Conference, 517-524.

  • Shepard D.S., 1984: Computer mapping: The SYMAP interpolation algorythm. In: Spatial Statistics and Models, (Eds. G.L. Gaile and C.J. Willmott), 133-145.

  • Willmott C.J., Rowe C.M., Philpot W.D., 1985: Small-scale climate maps: a sensitivity analysis of some common assumptions associated with grid-point interpolation and contouring. The American Cartographer, 12, 5-16.


    Analysis Version 4.0

    The actual version 4.0 of the Alpine climatology was released in October 1997 and covers the period 1971-1990. A detailed description of the dataset and analysis method for version 4.0 is given in Frei and Schär (1998) and also includes a climatological analysis over this 20 year reference period. In March 1998 analyses for the recent years (1992-1995) were made accessible. While dense for most regions, this update is based on coarser data over Northern Italy (SYNOP coverage only) however.

    Versions of the Alpine precipitation climatology:
    Version Nr. Date of Release Changing Characteristics
    V4.0 October 1, 1997
  • Newly includes data from 68 records for Slovenia (south-eastern Alps, full period);
  • Additional 48 records for the northern Appennino during 1987-1990;
  • Some refinements in the extent and magnitude of the moist anomaly over the Carnic and Julian Alps, minor changes elsewhere.
  • V3.0 May 1, 1997
  • Newly includes more than 3000 records from France (full climatological period);
  • Significant refinements of the distribution over France and the Southern Alps.
  • V2.0 Jan 1, 1997
  • Newly incorporates high-resolution rain-gauge networks for large parts of northern Italy (mostly for 1971-1986) and for Croatia (full period);
  • Reference period changed to 1971-1990;
  • Improved distributions over the Po valley, the Appenines and the eastern Adriatic coast.
  • V1.5 Jan 1, 1996
  • Analyses on regular grid in conventional coordinates of geographical latitude/longitude;
  • Database similar like for V1.0.
  • V1.0 July 1, 1995
  • Original public version;
  • Analyses on rotated spherical grid (0.25 degrees resolution);
  • Based on high-resolution networks for Austria, Germany and Switzerland, SYNOP coverage elsewhere;
  • Reference period 1973-1992.

  • Sample Charts

    A number of climatological charts from the actual analyses (V4.0) is available directly in picture format. Select the desired field, month and preferred format to get the chart. These sample fields can also be accessed in digital format (Digital Access).


    Field:

    Month: Format:


    Digital Access to Gridded Analyses

    This site offers access to several products from the latest version (see Versions) of the Alpine Precipitation Climatology in the form of ASCII-files:

    Available Fields

    Conditions for Access

    File Format:

    Download ...


    Interpretation Hints

    The nature of rain-gauge data and the variable density both in time and space require some care in the interpretation of the results of the Alpine precipitation climatology. Here we list some of the most important items, a detailed discussion can be found in Frei and Schär (1998).


    Future Activities

    Future activities of the project will focus on improvements of the Alpine precipitation analyses. Further extensions of the current database are sought to supplement areas and time-periods with currently still week observational basis. In this respect, high priority is devoted to the compilation of high-resolution rain gauge data from Northern Italy of the years (1987-1995).

    An update of the analyses for more recent years is planned but will be undertaken for bundels of years after major data compilation steps.

    Activities are currently underway to produce a climatological analysis at high spatial resolution (a few kilometers) using a more sophisticated analysis technique that takes account of the statistical relationship of precipitation with topography. (See Schwarb 2000).

    Actual efforts are also undertaken to extend the present mesoscale analyses over the entire 20th century. For this purpose an objective statistical analysis technique is developed which allows to optimally combine data from a dense network and limited time period with sparser data for the entire century. These reconstructions are planned for monthly precipitation on the 25 km grid of the present analyses. (See Schmidli et al. 2001 and Schmidli 2000).

    The Alpine rain-gauge dataset is currently exploited with respect to the daily precipitation statistics aiming at a climatological analysis of heavy precipitation. The set of mesoscale analyses is also used for the study of the relationship between regional precipitation and the large-scale dynamical fields on a day-by-day time scale. The analyses also serve as a reference for systematic evaluations of mesoscale numerical weather-prediction and climate models in the Alpine region.


    Relevant Publications


    Acknowledgements

    We are indebted to the following institutes for providing access to daily precipitation data: Deutscher Wetterdienst, Offenbach a.M.; Hydrographisches Zentralbüro des Bundesamtes für Land- und Forstwirtschaft, Wien; Zentralanstalt für Meteorologie und Geophysik, Wien; Météo France, Toulouse; MeteoSwiss, Zuerich; Servizio Idrografico e Mareografico Nazionale, Roma; Ufficio Centrale di Ecologia Agraria, Roma; the Italian MAP working group on climatology; Croatian Meteorological and Hydrological Service, Zagreb; Hydrometeorological Institute of Slovenia, Ljubljana.


    Last Change: February 2004

    Send Questions and Comments to Ch. Frei