A potential rule of thumb for hourly rainfall?

Future global warming will be accompanied by more intense rainfall and flash floods due to increased evaporation, as a consequence of higher surface temperatures which also lead to a higher turn-around rate for the global hydrological cycle. In other words, we will see changing rainfall patterns. And if the global area of rainfall also shrinks, then a higher regional concentration of the rainfall is bound to lead to more intense downpours (the global rainfall indicator is discussed here). 

Even with successful mitigation and cut in CO2-emissions, there will be a need for climate change adaptation (e.g. keeping the global warming below 2°C according to the Paris accord). We must be better prepared for more water in inconvenient forms and critical infrastructure may need to be upgraded to withstand it.

Engineers typically make use of so-called intensity-duration-frequency (IDF) estimates to design buildings, bridges or roads that are dimensioned to weather these conditions. Farmers need to know how to manage their fields to avoid erosion and loss of crops. Hence, IDF curves are useful for climate adaptation.

The IDF curves traditionally need sub-daily rainfall measurements (e.g. rainfall recorded every hour) which are not so common everywhere. The standard commonplace rain gauge data, however, record the rainfall accumulated over 24-hr segments (Figure 1).

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Figure 1. Location of 24-hr rain gauge measurements from the global historical climate network (GHCN). 

The question is whether it is possible to make use of 24-hr rain gauge data to say something about IDF statistics, even if it doesn’t involve measurements accumulated over intervals of minutes to hours.

There have been some attempts to estimate IDF curves based on 24-hr data that recently were accompanied by a new “rule of thumb” that gives approximate results (Benestad et al., 2021):

a potential rule of thumb for hourly rainfall 1 - A potential rule of thumb for hourly rainfall?.

In this expression(illustrated in Figure 2), a potential rule of thumb for hourly rainfall 2 - A potential rule of thumb for hourly rainfall? is the return level for rainfall accumulated over time duration a potential rule of thumb for hourly rainfall 3 - A potential rule of thumb for hourly rainfall? (units in hours), a potential rule of thumb for hourly rainfall 4 - A potential rule of thumb for hourly rainfall? is a statistical correction factor (accounting for the fact that rainfall is not really exponentially distributed), a potential rule of thumb for hourly rainfall 5 - A potential rule of thumb for hourly rainfall? is the wet-day mean precipitation (here a threshold of 1 mm/day was used to distinguish ‘wet’ and ‘dry’ days), a potential rule of thumb for hourly rainfall 6 - A potential rule of thumb for hourly rainfall? describes the dependency between different temporal scales, a potential rule of thumb for hourly rainfall 7 - A potential rule of thumb for hourly rainfall? is the wet-day frequency, and a potential rule of thumb for hourly rainfall 8 - A potential rule of thumb for hourly rainfall? is return interval. 

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Figure 2 IDF- curve estimated for a location in Norway. 

The question is whether this formula can quantify sub-daily rainfall statistics outside Norway, so it should be tested for other parts of the world to see if it provides useful information on a more general basis.

It should be fairly straight-forward to calibrate a potential rule of thumb for hourly rainfall 6 - A potential rule of thumb for hourly rainfall? against already established IDF curves for sites with sub-daily rain measurement (see Benestad et al., (2021) for more details). This timescale dependency is expected to depend on relative occurrences of convective storms, cyclones, atmospheric rivers, weather fronts, and orographic rainfall (rainfall triggered by moist air blowing over hills and mountains forcing air ascent).

An advantage of this “rule of thumb” formula is its simplicity. It needs the two key rain fall parameters wet-day mean precipitation a potential rule of thumb for hourly rainfall 5 - A potential rule of thumb for hourly rainfall? and frequency a potential rule of thumb for hourly rainfall 7 - A potential rule of thumb for hourly rainfall?, and if the correction factor a potential rule of thumb for hourly rainfall 4 - A potential rule of thumb for hourly rainfall? and the timescale cross-dependency a potential rule of thumb for hourly rainfall 6 - A potential rule of thumb for hourly rainfall? have similar values across regions, then it may provide a quick first guess of the IDF curves for sites where hourly rainfall measurements are absent.

References


  1. R.E. Benestad, J. Lutz, A.V. Dyrrdal, J.E. Haugen, K.M. Parding, and A. Dobler, “Testing a simple formula for calculating approximate intensity-duration-frequency curves”, Environmental Research Letters, vol. 16, pp. 044009, 2021. http://dx.doi.org/10.1088/1748-9326/abd4ab

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