Abstract
ABSTRACTAtmospheric 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.
Highlights
Flooding associated with heavy rainfall can lead to extensive damage to society (Penning-Rowsell, 2014), the severity of which is anticipated to increase as the climate warms in response to rising atmospheric concentrations of greenhouse gases (Pall et al, 2011; Arnell and Gosling, 2014)
We considered 0.75 × 0.75 degree latitude/longitude resolution 6-h data, extracting specific humidity (q), zonal wind (u) and meridional wind (v) components at 850 hPa and column-integrated water vapour (IWV, units of kg m−2 which approximates to mm assuming a water density of 1000 kg m−3) and westerly and southerly vertically integrated horizontal water vapour transport (IVT) computed by Berrisford et al (2011)
The heaviest 1% of daily rainfall in multiple grid points covering the British Isles are analysed in a blended satellite/rain-gauge dataset (GPCP-1 Degree Daily (1DD)), regionally averaged daily rain-gauge observations (HadUKP) and a climate model simulation applying prescribed observed sea surface temperature and realistic radiative forcing (HadGEM2-A) over the period 1997–2008
Summary
Flooding associated with heavy rainfall can lead to extensive damage to society (Penning-Rowsell, 2014), the severity of which is anticipated to increase as the climate warms in response to rising atmospheric concentrations of greenhouse gases (Pall et al, 2011; Arnell and Gosling, 2014). Atmospheric rivers (ARs), narrow bands of strong moisture transport within the warm sector of extra-tropical cyclones (Browning and Pardoe, 1973; Zhu and Newell, 1998; Dacre et al, 2014; Gimeno et al, 2014), have been linked with heavy rainfall and high river flows in the winter half year (Ralph et al, 2006; Dettinger et al, 2011; Lavers et al, 2011; Neiman et al, 2013) The strength of these synoptic features is anticipated to grow as temperatures rise due to concurrent increases in atmospheric moisture (Dettinger et al, 2011; Lavers et al, 2013; Warner et al, 2015). While it is important to analyse and understand the full intensity/duration characteristics of rainfall (Kendon et al, 2014; Blenkinsop et al, 2015), here we concentrate on characterizing moisture and its transport by the winds associated with heavy daily rainfall totals across the British Isles region
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