The NMIMET Method of Analysis

Data Retrieval

The required statistics of visual wind and wave observations were extracted from the Marine Databank for each of the areas covered, and put on a magnetic tape suitable for input to the NMIMET process. For each of the 104 sea areas in the world-wide database the tapes contained a set of 45 arrays corresponding to the data categories ('Annual' + four seasons) * ('All Directions' + 8 directional sectors).

A sample of one such array is shown in Figure 2 and a detailed account of the retrieval procedure is given in Appendix A2. The array consists of a joint frequency distribution or scatter diagram of wave height against wind speed (as reported in terms of Beaufort scale), and a separate set of normalized wind frequencies.

Figure 2 - Sample of Data Array as Retrieved from the Marine Databank.

The statistics of wave height and wind speed were derived from what may be termed '2-group' wave/wind data. These are the frequencies of occasions when estimates of the heights H1 and H2 of both sea and swell wave groups have been reported and are expressed in terms of the so-called 'resultant' height, Hr. When the sea and swell are in the same 45 sector (which has been found to be so in the majority of cases) then:

Otherwise Hr is equated to the greater of the two heights, H1 and H2. The separate set of normalized wind frequencies was derived from what may be termed 'All Wind' data, covering all occasions when estimates of wind speed have been reported in the years 1854 to 1983. In this case the frequencies are much higher than those in the '2-group' data; they have, however, been subjected to a normalizing process designed to reduce bias due to annual and monthly variations of the reporting rate. The resulting normalized frequencies are numerically smaller than the actual frequencies to an extent which increases with the total number of observations for the area concerned and may often be smaller than the corresponding frequencies of the '2-group' data which are not normalized. At the beginning of the data for each area information on the actual frequencies of the 'All Wind' data is stored in an array listing the number of observations in each month of each of the years 1854 to 1983.