Daily temperature and precipitation maps with 1 km resolution derived from Norwegian weather observations

Daily mean temperature measurements as well as 24-hour accumulated daily precipitation observations are used to produce maps of temperature and precipitation with 1 km horizontal resolution for the whole of Norway. These maps are updated on a daily basis and published on the website "seNorge.no". For the process of spatially interpolating temperature and precipitation observations de-trended Kriging and Triangulation, respectively, are used.

Gridding of temperature and precipitation observations was previously done using ArcInfo routines. However, since ArcInfo is not an operational platform at met.no, it was decided that new routines should be developed in Fortran for the purpose of daily gridding of temperature and precipitation observations.

Measured 24-hour mean temperatures are first "de-trended" or "reduced" to sea level, to a common latitude and to a common longitude as described in the met.no report "Nordic Temperature Maps" (Tveito et al., 2000). Furthermore, the average and the lowest altitude in a circle of 20 km radius surrounding each station are used in order to determine whether a particular station is situated in a valley or on a mountain top. All five dependencies are treated as linear expressions. Regression coefficients were calculated for each month separately from long-term monthly mean temperature data for Norway, Sweden, Denmark and Finland using step-wise linear regression (Tveito et al., 2000). The same regression coefficients are used for the daily data, only depending upon the month when the data was measured.

For the spatial interpolation of "reduced" temperatures, Kriging is used in conjunction with the semi-variogram parameters for the de-trended monthly mean temperatures of the respective month (Tveito et al., 2000). The method is also called residual kriging or de-trended kriging. The Geostatistical Software Library (GSLIB) was used for this purpose.

For the spatial interpolation of precipitation, the method of triangulation is used. Gridded precipitation values are corrected for the altitude of the respective grid cell, using a vertical precipitation gradient of 10% per 100 m height difference and a gradient of 5% per 100 m height difference above an altitude of 1000 m above sea level (see met.no report "NORDGRID -a preliminary investigation on the potential for creation of a joint Nordic gridded climate dataset"; Jansson et al., 2007). The triangulation from GEOMPACK was used for this purpose. Precipitation measurements are operationally corrected for (mostly aerodynamic) "gauge losses" using an exposure class between 1 and 5 for each station. Correction coefficients for accumulated daily precipitation range from 1.02 for sheltered stations and liquid precipitation to 1.80 for very exposed stations and solid precipitation (snow). The method is described in detail in the met.no report "Manual for Operational Correction of Nordic Precipitation Data" (Förland et al., 1996).

The new routines are examined using cross validation in conjunction with daily data for a period of several years. The strengths and weaknesses of the above described methodology are shown and improvements to the methodology are suggested.