My work is on sub-task 1.3 which will analyse atmospheric precursors for flooding from intense rainfall which will investigate the atmospheric conditions at the large-scale which lead to intense flooding events. We will also use results and techniques from the NERC Changing Water Cycle projects HyDEF and CONVEX projects.
Flooding From Intense Rainfall (FFIR) Programme
Extreme rainfall events continue to be one of the largest natural hazards in the UK. In winter, heavy precipitation and floods have been linked with intense moisture transport events associated with atmospheric rivers (ARs), yet no large-scale atmospheric precursors have been linked to summer flooding in the UK. This study investigates the link between ARs and extreme rainfall from two perspectives: (1) Given an extreme rainfall event, is there an associated AR? (2) Given an AR, is there an associated extreme rainfall event? We identify extreme rainfall events using the UK Met Office daily rain gauge data set and link these to ARs using two different horizontal resolution atmospheric data sets (ERA-Interim and Twentieth Century Reanalysis). The results show that less than 35% of winter ARs and less than 15% of summer ARs are associated with an extreme rainfall event. Consistent with previous studies, at least 50% of extreme winter rainfall events are associated with an AR. However, less than 20% of the identified summer extreme rainfall events are associated with an AR. The dependence of the water vapor transport intensity threshold used to define an AR on the years included in the study, and on the length of the season, is also examined. Including a longer period (1900-2012) compared to previous studies (1979-2005) reduces the water vapor transport intensity threshold used to define an AR.
Atmospheric moisture characteristics associated with the heaviest 1% of daily rainfall events affecting regions of the British Isles are analysed over the period 1997–2008. A blended satellite/rain-gauge data set (GPCP-1DD) and regionally averaged daily rain-gauge observations (HadUKP) are combined with the ERA Interim reanalysis. These are compared with simulations from the HadGEM2-A climate model which applied observed sea surface temperature and realistic radiative forcings. Median extreme daily rainfall across the identified events and locations is larger for GPCP (32 mm day−1) than HadUKP and the simulations (∼25 mm day−1). The heaviest observed and simulated daily rainfall events are associated with increased specific humidity and horizontal transport of moisture (median 850 hPa specific humidity of ∼6 g kg−1 and vapour transport of ∼150 g kg− 1m s− 1 for both observed and simulated events). Extreme daily rainfall events are less common during spring and summer across much of the British Isles, but in the south east region, they contribute up to 60% of the total number of distinct extreme daily rainfall events during these months. Compared to winter events, the summer events over south east Britain are associated with a greater magnitude and more southerly location of moisture maxima and less spatially extensive regions of enhanced moisture transport. This contrasting dependence of extreme daily rainfall on moisture characteristics implies a range of driving mechanisms that depend upon location and season. Higher spatial and temporal resolution data are required to explore these processes further, which is vital in assessing future projected changes in rainfall and associated flooding.
Champion, A.J., Allan, R.P., Lavers, D.L., 2015: Atmospheric Rivers do not explain UK Summer Extreme Rainfall, J. Geophys. Res. Atmos., 120 (14), 6731-6741, doi:10.1002/2014JD022863. http://dx.doi.org/10.1002/2014JD022863
Allan, R.P., Lavers, D.L., Champion, A.J., 2015: Diagnosing links between atmospheric moisture and extreme daily precipitation over the UK, Int. J. Clim., doi:10.1002/joc.4547. http://dx.doi.org/10.1002/joc.4547