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Federal Department of Home Affairs FDHA
Federal Office of Meteorology and Climatology MeteoSwiss
The new algorithm for
gridpoint - station
correspondence
Pirmin Kaufmann, MeteoSwiss
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%)
COSMO General Meeting 2007, Athens, Greece
[email protected]
land?
water?
2
Search radius
• Search radius over land
= 1.415 gp (~2)
• Search radius over
water = 2 gp
• Rationale: many stations
on small peninsulas are
almost 2 gp from the
nearest model land point
COSMO General Meeting 2007, Athens, Greece
[email protected]
??
3
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)
COSMO General Meeting 2007, Athens, Greece
[email protected]
4
Optimize horizontal and vertical
distance
• Calculate horizontal distance dhor and vertical distance dz
in meter
• Look for station with smallest „optimization-distance“
according to:
• Dist = dhor + 500*dz
• Example: to avoid a vertical distance of 20 m, we
allow a horizontal distance up to 10 km
z
Lines of equal dist
x, y
COSMO General Meeting 2007, Athens, Greece
[email protected]
5
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.
COSMO General Meeting 2007, Athens, Greece
[email protected]
6
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 = 1.415 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 dhor and vertical distance dz in
meter
• Look for station with smallest total distance according to
• Dist = dhor + 500*dz
COSMO General Meeting 2007, Athens, Greece
[email protected]
7
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