Cloudbursts

 Understanding Global Warming

Extreme cloudbursts are rare!

The Danish Meteorological Institute (DMI) has measured the rainfall in Denmark from 1874 to today - hyperlink. In the decade 1874-83, 659 mm/year fell as a national average. In 2007-16, this average had increased by 120 mm to 779 mm/year. Warmer sea and land have given cause to an increased evaporation, which mercifully returns to us as precipitation.

 

Figure 1. Rainfall is steadily increasing with no accelerating trend. Black line is 11 year moving average.

A cloudburst is an extreme amount of precipitation in a short period of time. DMI defines a cloudburst as 15 mm of rainfall within a maximum of 30 minutes. Cloudbursts are regional and are only measured when they hit a measuring station. 

The IDA Wastewater Committee has now a total of 171 rain gauge stations in a system started in 1979. The closest and one of the oldest in continuous operation for 40 years is at Silkeborg Vandværk. Principal engineer Troels Hosbjerg has kindly provided us with data for all 11,092 rainfall events over 0.2 mm since the station's inception. Troels Hosbjerg has also extracted all cloudbursts. There are too few cloudbursts to show a trend. Therefore, figure 2 also includes all 168 heavy rain showers down to 5 mm per 30 minutes.

The latest cloudburst in Silkeborg took place in 2016. We must go back 15 years to find others. Nor does heavy rainfall down to 5 mm per 30 minutes show any increasing intensity.

In an IDA analysis described in "SVK Document No. 28. Regional variation of extreme rainfall in Denmark 1979-2005" of 61 data series, significant trends were found in both positive and negative direction. For durations up to 3 hours, most rainfall series have positive trends, while for durations of 24 to 46 hours, most rainfall series have negative trends - hyperlink.

The header of the dataset from Silkeborg Waterworks.

The table above shows the header of the dataset from Silkeborg Waterworks. Each rain shower is represented by a row. The entire dataset can be downloaded as a text file - hyperlink. The figure below shows a subset with the strongest rain showers.

Figure 2. Out of 11,000 data sets, rain showers are extracted with intensity higher than 5 mm per 30 minutes. Cloudburst have more than 15 mm in 30 min. Courtesy Silkeborg Vandværk.

False or true

According to Mainstream, more extreme weather is already upon us. On the contrary. We've got more rain in Denmark, but the intensity has not been increasing. There are not many cloudbursts at all, and they have not become more frequent in the last 40 years.

Understanding Global Warming, Oversigt - LINK


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