Noise Weather

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A night with inversion

To highlight the power of the idea of the sound weather, Figure 3 shows the series of noise roses for a night time inversion. The noise roses in the second column of Figure 3 follow the setting for the noise rose in Figure 2. The noise roses in the right most column are noise roses with a (10 m/1°)-resolution up to a distance of 5 km.

The columns in between the noise roses indicate the profile forecasts for each hour. The x-axis each is the height above ground scaled in meters from 0 m to 200 m. The left diagram shows the wind profile. The left y-axis refers to the wind speed (blue line) in meters per second and is always scaled from 0 m/s to 8 m/s. The compass wind direction (red line) is assigned to the right y-axis scaled in degrees from 110° to 280°. The right diagram indicates temperature (blue line, left y-axis scaled in degrees centigrade, 0 °C to 6 °C) and the relative humidity (red line, right y-axis, scaled in percent from 50% to 100%).

Figure 3 – Time line of noise roses for a night with temperature inversion

At the beginning of the time line the inversion is already there but due to the still blowing easterly winds increasing with height the sound levels only react at the northeast and southwest direction. These are the ‘neutral’ directions if it would be a pure wind driven regime. Due to the decreasing wind along with the proceeding night the sound weather indicates the transition into a clear inversion regime during the early morning hours. At 8 am the inversion disappears, beginning at the ground and ending up with a normal downwind propagation to the east due to the freshening westerly winds.

This is what the noise roses up to 2 km indicate. The noise roses up to 5 km are telling a slightly different story complementing the picture for larger distances. The noise rose for the first hour in the time line points out that the downwind regime applies not to a sector but to a region that is more or less an elliptical ring.

People who are living in the south of the source may be in a ‘shadow’ if they are close enough; at greater distances they experience high levels during that hour. In addition, the downwind regime to the northwest stops at roundabout 2 km distance from the source entering a weak shadow zone. The upcoming inversion is a more or less local phenomenon in this case; there is no inversion above 100 m and only a weak change in wind speed. Further interpretation is left to reader.

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