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

2 Method

The reduction of the number of meteo-classes is based on the quantity D/R where D is the propagation distance and R is the ray curvature. The procedure for calculating R is described in [4] ([5] contains slightly modified equations). [5] contains four propagation classes M1-M4 which will serve as a guideline for the reduction of meteo-classes.

Table 1: Propagation classes defined in [5].

In Nord2000 ([6], [7]) as well as in the Harmonoise/IMAGINE method ([4], [5]) the vertical effective sound speed profile is assumed to be a log-lin profile as shown in Eq. (1). The procedure for calculating the yearly average of the noise levels is based on log-lin profiles, and the meteo-classes are defined by A and B (as defined in [2]). In the IMAGINE method the definition of A and B has been interchanged.

       (1)

An overview of the meteorological statistics in [3] is given in Figure 1 where the percentage of occurrence in each meteo-class is the average of all propagation directions 0-360°. The figure shows that the nine significant meteo-classes are 3, 7, 8, 13, 18, 19, 20, 23, and 24. The five classes 3, 8, 13, 18, and 23 are the purely logarithmic sound speed profiles (B = 0) with increasing favourable propagation conditions ranging from strong upward propagation (class 3) over neutral propagation (class 13) to strong downward propagation (class 23). The other four classes have an additional linear component which increases the refraction effect relative to the purely logarithmic profile (A and B have the same sign which implies that the refraction effect is enhanced by B, e.g. class 7 is more unfavourable than 8, and 24 is more favourable than 23) . The nine classes are defined by A and B in Table 2.

Figure 1: Percentage of occurrences in 25 meteo-classes averaged for propagation directions 0-360°.

Table 2: The nine meteo-classes in the Danish statistics in [3] and the corresponding values of A and B.

For the nine classes in Table 2 the parameter D/R is calculated using the equations in [4]. For the logarithmic profiles the value of D/R will be independent of the distance while the numerical value of D/R will increase with the distance if the profile has a linear component. The result of the calculations is shown in Annex A. If the nine meteo-classes are reduced to four based on D/R solely, the outcome will be Proposal 1 in Table 3. On the basis of the results shown in Annex A, meteo-classes 8, 13, 18, and 24 has been selected to represent the four classes M1-M4 defined in Table 1.

Table 3: Two proposals for reducing the number of meteo-classes.

For propagation class “M4” combining 19, 20, 23, and 24, one of the classes with a moderate linear component seems to be the natural choice. However, it must be expected that the use of class 19 will underestimate the noise levels whereas class 24 may lead to an overestimation in the lower range of distances. Therefore, it was decided also to investigate a Proposal 2 where class 19 has been moved to the “M³” which is represented by class 18. In addition, a Proposal 3 has been defined where the four groups for combing the meteo-classes are the same as in Proposal 2, but where class 19 is used to represent “M³” instead of class 18.

For each of the three proposals new meteo-statistics have been determined using the 25-class format defined in [2]. Temperature and humidity in the new classes are obtained by calculating the weighted average (weighted by the percentages) of the values in the classes which are combined.

To test the effect of reducing the number of meteo-classes, calculation of the yearly average L_den has been carried out for a few cases based on the test cases described in [8]. Test case 1 and 2 among the test case group “Test Cases for the Yearly Average” in [8] have been selected for this purpose. Test case 1 and 2 cover propagation from a road placed north-south over flat terrain to a receiver east or west of the road, respectively, 100 m from the road. Calculations are carried out for these cases and for additional distances of 25, 50, 200, and 500 m. The calculations are repeated for a terrain corresponding to terrain group 7 (thin hard screen on flat ground) in test case group “Test Cases for a Straight Road” in [8]. The two groups are denoted “Flat” and “Screen”, respectively.

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Version 1.0 March 2007, © Danish Environmental Protection Agency