Abstract
Abstract. Hourly precipitation extremes in very long time series from the Hong Kong Observatory and the Netherlands are investigated. Using the 2 m dew point temperature from 4 h before the rainfall event as a measure of near surface absolute humidity, hourly precipitation extremes closely follow a 14% per degree dependency – a scaling twice as large as following from the Clausius-Clapeyron relation. However, for dew point temperatures above 23 °C no significant dependency on humidity was found. Strikingly, in spite of the large difference in climate, results are almost identical in Hong Kong and the Netherlands for the dew point temperature range where both observational sets have sufficient data. Trends in hourly precipitation extremes show substantial increases over the last century for both De Bilt (the Netherlands) and Hong Kong. For De Bilt, not only the long term trend, but also variations in hourly precipitation extremes on an inter-decadal timescale of 30 yr and longer, can be linked very well to the above scaling; there is a very close resemblance between variations in dew point temperature and precipitation intensity with an inferred dependency of hourly precipitation extremes of 10 to 14% per degree. For Hong Kong there is no connection between variations in humidity and those in precipitation intensity in the wet season, May to September. This is consistent with the found zero-dependency of precipitation intensity on humidity for dew points above 23 °C. Yet, outside the wet season humidity changes do appear to explain the positive trend in hourly precipitation extremes, again following a dependency close to twice the Clausius-Clapeyron relation.
Highlights
It is generally conceived that the intensity of precipitation extremes will increase as the climate warms (IPCC, 2007)
The primary reason for this expectation is the fact that the maximum moisture content of the atmosphere increases with approximately 7 % per degree temperature rise, which follows from the Clausius-Clapeyron relation
The difference in the corresponding dew point temperatures is only about 1–2 degrees and this would yield a dependency of the precipitation intensity of about 50 % per degree
Summary
It is generally conceived that the intensity of precipitation extremes will increase as the climate warms (IPCC, 2007). It has been argued that soil drying, and a resulting decrease in relative humidity at high temperatures, could cause a limit to the super C-C scaling of precipitation extremes (Berg et al, 2009) This may explain the lower dependency of hourly precipitation extremes, or even decrease with temperature, found in recent studies for Australia and the United States (Hardwick Jones et al, 2010; Shaw et al, 2011). For this reason, Lenderink and van Meijgaard (2010) introduced the dew point temperature as a direct measure of atmospheric absolute humidity. We will not investigate this further since sub-hourly data is not (yet) available for the data studied here, but we note that this could be an important factor which deserves further research
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
