16.3 Assimilation of Satellite Data




In the previous section, the last valid time of the particle position file was 12 UTC on the 15th of April. We can compare the particle positions from the calculations with observations by various satellites. Many of these have sun synchronous orbits, which means that the pass over Iceland-England would occur around local solar noon or about 1200 UTC. Examples include NASA TOMS, OMI, and MODIS.

  1. The image by the NASA / MODIS Rapid Response team for the afternoon overpass by Satellite Aqua at 13:30 UTC on April 15th shows the ash plume extending from Iceland to Scotland.

  2. Actually the overlap between the model calculation and observation is quite good, except that the calculation appears slightly further to the north. The small difference may be just due to variations in release height. There is a procedure we can use to adjust the particle positions before continuing the calculation. Press the Concentration / Utilities / Particle Adjustment menu tab to open the shift menu.

  3. There are two adjustment options, defining a window and translating particles, or rotating them about a point. We will choose the rotation option. Select the rotation radio-button. The rotation point defaults to the source location and enter a rotation angle of 10 degrees (positive = clockwise) to a distance of 1000 km. All particles within this distance will be rotated. The menu already defaults to writing the adjusted particles to PARINIT which would then default to the input file for the next simulation cycle. Process the points and the script will automatically open the particle display where there is an obvious discontinuity at 1000 km. This could be eliminated by selecting the blend option.

On your own try a few of the other adjustment options and then restart the calculation. The fit with observations for this case is somewhat fortuitous as no particular effort was made to get detailed source amount or release height information.