Finnish Meteorological Institute
Observations Branch
R. King

Analaysis of NORDRAD paired-radar comparison images produced by NRDTOOLS software.

1.Some general observations

2.Procedures for analysis of paired difference images.

A satisfactory analysis of paired difference images requires a computer program approach, since Even a manual/visual approach requires the reading of the values at some 200 points from a minimum of 5 images for each radar pair and rainfall situation. Such an analysis and the preparation of its visualisation can take 1 - 2 hours, i.e. the evaluation and display of the Finnish chain results (STO - KOR - VAN - ANJ - IKA - KUO - UTA - LUO (+ UTA - LUL) requires about 2 working days.

The automatised approach can take at least one of the following paths:

While waiting for the writing and testing of the program embodying the first (or second) approach, it may be useful to pursue the manual method to the extent allowed by resources. The usefulness of the manual method lies in its possibility of activating new insights into the part played by the factors enumerated above: the analyst is in direct visual contact with the material. For this reason the manual method, in the state of development which its has reached now (November, 2000), is briefly described.

3.Manual analysis procedure

The procedure requires for each radar pair to be analysed a set of transparent templates, which are marked as follows:

Such a transparency, whose points are plotted from x,y values generated by a program ( is needed for all elevation pairs near to that used, i.e. for a 0.6/0.6 nominal pair, the following may be required: 0.5/0.6, 0.5/0.7, 0.5/0.8, 0.4/0.8. It is assumed to a first approximation that the results obtained from, say, 0.5/0.7 are very similar to those from 0.6/0.8, though this still has to be demonstrated.

Fig. 3. The abscissa represents the difference in nominal data collection elevation angles, i.e. 0 represents here 0.6/0.6 deg. The y-axis is the average value obtained from each perimeter set: for the upper curve the value is the standard deviation of the perimeter set averages. In this case the calibration bias (difference between radar 1 and radar 2) appears to be approximately -3.2 dB, and the antenna elevation difference about 0.25 degrees, with radar1 below radar2.

The analysis proceeds as follows: