Changes to Sub-daily Rainfall Patterns in a Future Climate.

Westra, S., Evans, J.P., Mehrotra, R. and Sharma, A.
34th Hydrology and Water Resources Symposium, 19-22 November, Sydney, Australia.


This paper describes an algorithm for disaggregating daily rainfall into sub-daily rainfall ‘fragments’ (continuous fine-resolution rainfall sequences whose total depth sums to the daily rainfall amount) under a future, warmer climate. The basis of the algorithm is to re-sample sub-daily fragments from the historical record conditional on the total daily rainfall amount and a range of temperature-based atmospheric predictors representative of the future climate. The logic is that as the atmosphere warms, future rainfall patterns will be more reflective of historical rainfall patterns which occurred on warmer days at the same location, or at locations which have an atmospheric temperature profile more reflective of expected future conditions.
It was found that the daily to sub-daily scaling relationship varied significantly by season and by location, with rainfall patterns on warmer seasons or at warmer locations typically showing more intense rainfall occurring over shorter periods within a day compared with cooler seasons and locations. Importantly, by regressing against temperature-based atmospheric covariates, this effect was substantially reduced, suggesting that the approach may also be valid when extrapolating to a future climate. An adjusted method of fragments algorithm was then applied to nine stations around Australia, with the results showing that when holding total daily rainfall constant, the maximum intensity of short duration rainfall increased by a median of about 5% per degree change in temperature for the maximum 6 minute burst and 3.5% for the maximum one hour burst, whereas the fraction of the day with no rainfall increased by a median of 1.5%. This highlights that a large proportion of the change to the distribution of rainfall is likely to occur at sub-daily timescales, with significant implications for many hydrological systems.

Key Figure

fraction in 6 minute burst

Figure 2: Seasonal variation in the fraction of daily rainfall occurring in the maximum 6 minute rainfall burst.

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