Federal Department of Home Affairs FDHA Federal Office of Meteorology and Climatology MeteoSwiss The new algorithm for gridpoint - station correspondence Pirmin Kaufmann, MeteoSwiss

2 COSMO General Meeting 2007, Athens, Greece Find grid cell to which station belongs Find grid cell within which station (red star) is located (nearest neighbour: round grid coordinates of station) Determine if cell has land or water surface (water: SOILTYP = 9 or FR_LAND < 50%) land? water?

3 COSMO General Meeting 2007, Athens, Greece Search radius Search radius over land = gp (~  2) Search radius over water = 2 gp Rationale: many stations on small peninsulas are almost 2 gp from the nearest model land point ? ?? ?

4 COSMO General Meeting 2007, Athens, Greece Search within radius If land is found, only consider these cells If only water grid points are within radius, continue with all grid points within radius Rationale Some parameters are quite different over sea and over land (especially temperature) Stations are usually placed on land Some stations are far from any land (e.g. oil rigs in the north sea)

5 COSMO General Meeting 2007, Athens, Greece Optimize horizontal and vertical distance Calculate horizontal distance d hor and vertical distance dz in meter Look for station with smallest „optimization-distance“ according to : Dist = d hor + 500*dz Example: to avoid a vertical distance of 20 m, we allow a horizontal distance up to 10 km x, y z Lines of equal dist

6 COSMO General Meeting 2007, Athens, Greece Advantages Better verification results because height differences of less than 100 m still matter (and sometimes quite a lot) Simple optimization algorithm Avoid using more distant grid point just because of slightly smaller height difference (above 100 m) Avoid using grid point with more height difference (below 100 m) when grid point with smaller height difference is almost as near.

7 COSMO General Meeting 2007, Athens, Greece Summary Find grid cell within which station is located (=round grid coordinates of station) Determine if SOILTYP = 9 (or FR_LAND < 50%) Search radius over water = 2 gp Search radius over land = gp (~  2) Search within this radius If solid ground is found, only consider these cells If only water is found, continue with all cells within radius Calculate horizontal distance d hor and vertical distance dz in meter Look for station with smallest total distance according to Dist = d hor + 500*dz