Environmental Research Letters LETTER • OPEN ACCESS The relationship of atmospheric air temperature and dew point temperature to extreme rainfall To cite this article: Andrew Bui et al 2019 Environ. Res. Lett. 14 074025 View the article online for updates and enhancements. This content was downloaded from IP address 128.250.0.113 on 05/08/2019 at 03:59 Environ. Res. Lett. 14 (2019) 074025 https://doi.org/10.1088/1748-9326/ab2a26 LETTER The relationship of atmospheric air temperature and dew point OPEN ACCESS temperature to extreme rainfall RECEIVED 21 March 2019 Andrew Bui1, Fiona Johnson1 and Conrad Wasko2 REVISED 1 School of Civil and Environmental Engineering, University of New South Wales, Sydney, Australia 9 May 2019 2 Department of Infrastructure Engineering, University of Melbourne, Parkville, Australia ACCEPTED FOR PUBLICATION 17 June 2019 E-mail: [email protected] PUBLISHED Keywords: precipitation, atmospheric temperature, atmospheric dew point, scaling, Clausius–Clapeyron, climate change 16 July 2019 Supplementary material for this article is available online Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 Abstract licence. To understand the expected changes of extreme rainfalls due to climate change, the sensitivity of Any further distribution of this work must maintain rainfall to surface temperature is often calculated. However, as surface temperatures may not be a attribution to the good indicator of atmospheric moisture, an alternative is to use atmospheric temperatures, but the use author(s) and the title of the work, journal citation of atmospheric temperatures lacks precedent. Using radiosonde atmospheric temperature data at a and DOI. range of geopotential heights from 34 weather stations across Australia and its territories, we examine whether atmospheric temperature can improve our understanding of rainfall-temperature sensitiv- ities. There is considerable variability in the calculated sensitivity when using atmospheric air temperature, while atmospheric dew point temperature showed robust positive sensitivities, similar to when surface dew point temperature measurements were used. We conclude atmospheric dew point temperature may be a promising candidate for future investigations of empirically calculated sensitivities of rainfall to temperature but does not appear superior to the use of surface dew point temperature measurements. 1. Introduction (Soden and Held 2006), and that extreme rainfall events precipitate all available moisture (Lenderink Climate change represents one of the most pressing and van Meijgaard 2010). However, the possible issues facing society due to its effect on meteorological violation of these assumptions, alongside artefacts and hydrological events. Rising temperatures can introduced due to the use of surface temperatures in directly impact rainfall patterns by increasing the the calculations results in deviations from the C–C atmospheric moisture holding capacity leading to relationship being observed. increased rainfall extremes (Trenberth et al 2003, Scaling of rainfall extremes much higher than C–C O’Gorman 2015). Hence, understanding the relation- (super C–C) has been found across a variety of cli- ship between temperature and extreme rainfall is a key mates and regions (Lenderink and van Meijgaard step towards understanding the effects of climate 2008, Liu et al 2009, Mishra et al 2012), particularly for change on rainfall. The association between rainfall short rainfall durations (Hardwick Jones et al 2010, and temperature, termed ‘scaling’, is linked to the Lenderink et al 2011, Panthou et al 2014, Busuioc et al Clausius–Clapeyron (C–C) relationship, which is the 2016, Wasko et al 2018, Wibig and Piotrowski 2018). theoretical exponential increase in the saturation Super C–C scaling has generally been attributed to the vapour pressure of approximately 7%/°C. If the convective nature of short duration storms (Berg and saturation vapour pressure increases at this rate it Haerter 2013, Berg et al 2013, Park and Min 2017). is plausible to suggest the maximum rainfall intensity During a storm the natural release of latent heat should increase at a similar rate (Trenberth 2011, could cause additional moisture convergence in the Westra et al 2014). This scaling relationship is based system, invigorating the storm and increasing the on two core assumptions; that relative humidity rainfall intensity beyond the theoretical C–C value will remain approximately constant in the future (Trenberth et al 2003, Westra et al 2014). Using © 2019 The Author(s). Published by IOP Publishing Ltd Environ. Res. Lett. 14 (2019) 074025 lightning (Molnar et al 2015) or cloud type (Berg and air temperature. It has been strongly advocated that Haerter 2013) as a proxy for identifying convective dew point temperature (e.g Lenderink and Attema events, super C–C scaling rates have been found for 2015, Barbero et al 2017) should be used as, by defini- convective storms. When convective storms are tion, the dew point is the temperature a parcel of air removed from the storm sample, the scaling reduces to needs to be cooled at constant pressure for saturation close or less than C–C. Hence changes in atmospheric (100% relative humidity) to occur. Hence, a 1 °C circulations which are associated with convection also increase in dew point temperature can be interpreted affect the scaling calculated (Blenkinsop et al 2015, as equivalent to a 7% increase in atmospheric moist- Chan et al 2016). ure content (Lenderink and van Meijgaard 2010).A Negative scaling has also been identified in many multitude of studies have investigated the sensitivity regions across the world (Utsumi et al 2011, Wasko of extreme rainfall with dew point temperature et al 2016) and is particularly associated with the (Lenderink and van Meijgaard 2010, Lenderink et al higher temperatures experienced in tropical regions 2011, Panthou et al 2014, Ali and Mishra 2017, Park (Hardwick Jones et al 2010, Maeda et al 2012). Nega- and Min 2017, Ali et al 2018, Wasko et al 2018). Most tive scaling contradicts observed historical increases in recently a global study (Ali et al 2018) showed that, for tropical rainfall extremes (Donat et al 2016, Guerreiro the majority of tropical areas, positive scaling closer to et al 2018). This has led to the explanation that, at the C–C relationship is obtained when surface dew higher temperatures, the relative humidity decreases, point temperature is used (instead of surface air temp- as there is no more moisture to be sourced (Hardwick erature). However, one of the main arguments, for not Jones et al 2010) and evaporation limitations using surface temperatures (air or dew point) is that (Priestley 1965, Roderick et al 2019) result in a reversal they are impacted by the rainfall from the storm event. from positive to negative scaling (Drobinski et al An alternative proposed is to use an upper tropo- 2016, 2018, Gao et al 2018). Indeed the most negative sphere temperature at a height sufficient enough to scaling is found in regions with the largest humidity avoid fluctuations due to the storm event (Mishra et al limitations (Wasko et al 2015). However, other expla- 2012) and prevent the dominance of solar surface nations are also possible. Warmer surface temperatures heating (Chan et al 2016). Using atmospheric temper- generally occur on less cloudy days with less rainfall, ature is physically more consistent with rainfall caus- resulting in negative scaling (Trenberth and Shea 2005), ing processes. Rainfall totals are constrained by the which would account for the observation that at higher amount of the moisture in the atmosphere and hence temperatures the proportion of the day experiencing exhibit a very strong correlation to the integrated rainfall is also smaller (Utsumi et al 2011).Ithasalso water vapour (Roderick et al 2019). The integrated been shown that storms of shorter duration generally water vapour is a function of the mean temperature in occur when temperatures are higher. As these storms the water vapour column (Hagemann et al 2003), and also have lower rainfall intensities due to their shorter rainfall resulting from convection is dependent on the duration there is a resulting negative bias in the scaling atmospheric temperature (Neelin et al 2009). Hence a relationship calculated (Wasko et al 2015). change in atmospheric air temperature (as opposed to Negative scaling has also been explained by con- surface temperature) should better capture a change in sidering the timing of measured temperatures in rela- the saturation vapour pressure that is linked to tion to the storm arrival. Temperatures can decrease extreme rainfall. during a rainfall event for many reasons, including The use of reanalysis atmospheric air temperature evaporative cooling, or movement of cold air asso- at 850 hPA above India largely resulted in positive ciated with the rainfall event (Bao et al 2017).Asa scaling, in line with expectations, including instances result the temperature coincident with the rainfall may of super C–C scaling slightly above 7%/°C (Ali and not truly reflect the temperature that occurred when Mishra 2017). To that extent climate modelled the rainfall was actually generated (Ali and Mishra 850 hPA air temperatures have also resulted in positive 2017, Bao et al 2017). Using the temperature three scaling across the United Kingdom where surface tem- days prior to rainfall events has returned positive scal- peratures did not (Chan et al 2016). Atmospheric air ing in some locations where previously negatively scal- temperature has also been used for developing non- ing was calculated (Ali and Mishra 2017). However, as stationary IDF curves (Ali and Mishra 2017, Gol- it has been refuted that local cooling is the reason for roudbary et al 2019). negative scaling (Barbero et al 2017), there continues The notion that surface temperature may (a) not to be an assertion that moisture limitations are the be physically well linked to atmospheric moisture, and most likely physical explanation for negative scaling (b) subject to many (statistical) artefacts, gives cre- (Lenderink et al 2018).