Working Report No. 18 2007 – Noise mapping by use of Nord2000 – Reduction of number of meteo-classes from nine to four

Noise mapping by use of Nord2000 – Reduction of number of meteo-classes from nine to four

1 Background

In a joint Nordic work for calculation of noise from road traffic it was proposed that calculation of the yearly average of L_den and L_night should be based on 25 meteorological classes as described in [1] and [2]. In the work a format was defined for the probability of occurrence of each meteo-class together with the average air temperature and relative humidity in the meteo-class for the day, evening and night period, respectively. The meteo-class statistics vary with the direction of propagation, and it was decided that values should be available in the range 0-360° with a spacing of 10°.

In the work each participating country calculated the statistics according to this format. In the Danish statistics (as well as in the statistics of the other Nordic countries) a large number of the meteo-classes contained no occurrences, and a few classes contained so few occurrences that they could be moved to neighbouring classes. After this reduction the resulting statistics for Denmark included nine significant classes as described in [3].

For strategic noise mapping the use of nine meteo-classes has subsequently been found too calculation time consuming. As the calculation time is proportional to the number of meteo-classes, it has been found necessary to reduce the number of classes further.

In the European Harmonoise/IMAGINE workgroup accurate calculation of L_den and L_night is also based on the format with 25 classes but the number of noise classes has been reduced to four for strategic noise mapping in general and to one for city streets (ignoring meteorological effects).

On this basis, the aim is to reduce the number of meteo-classes for strategic noise mapping to the same magnitude as used in the European method. In the European method the four classes have been based on a parameter D/R where D is the propagation distance and R is the ray curvature. However, the parameter D/R is not an input parameter in Nord2000. Therefore, when using Nord2000, the format containing the 25 classes has to be kept, but the number of classes containing occurrences can be reduced by combining two or more classes into one. Another reason for using this approach is that new statistics can be derived from available statistics. The experience from the European work can support the process of reducing the number of meteo-classes.