Elizabeth J. Kendon

Future changes to the intensity and frequency of short‐duration extreme rainfall

Evidence that extreme rainfall intensity is increasing at the global scale has strengthened considerably in recent years. Research now indicates that the greatest increases are likely to occur in short-duration storms lasting less than a day, potentially leading to an increase in the magnitude and frequency of flash floods. This review examines the evidence for subdaily extreme rainfall intensification due to anthropogenic climate change and describes our current physical understanding of the association between subdaily extreme rainfall intensity and atmospheric temperature. We also examine the nature, quality, and quantity of information needed to allow society to adapt successfully to predicted future changes, and discuss the roles of observational and modeling studies in helping us to better understand the physical processes that can influence subdaily extreme rainfall characteristics. We conclude by describing the types of research required to produce a more thorough understanding of the relationships between local-scale thermodynamic effects, large-scale atmospheric circulation, and subdaily extreme rainfall intensity.

Realism of Rainfall in a Very High-Resolution Regional Climate Model

AbstractThe realistic representation of rainfall on the local scale in climate models remains a key challenge. Realism encompasses the full spatial and temporal structure of rainfall, and is a key indicator of model skill in representing the underlying processes. In particular, if rainfall is more realistic in a climate model, there is greater confidence in its projections of future change.In this study, the realism of rainfall in a very high-resolution (1.5 km) regional climate model (RCM) is compared to a coarser-resolution 12-km RCM. This is the first time a convection-permitting model has been run for an extended period (1989–2008) over a region of the United Kingdom, allowing the characteristics of rainfall to be evaluated in a climatological sense. In particular, the duration and spatial extent of hourly rainfall across the southern United Kingdom is examined, with a key focus on heavy rainfall.Rainfall in the 1.5-km RCM is found to be much more realistic than in the 12-km RCM. In the 12-km RCM, hea...

Robustness of Future Changes in Local Precipitation Extremes

Abstract Reliable projections of future changes in local precipitation extremes are essential for informing policy decisions regarding mitigation and adaptation to climate change. In this paper, the extent to which the natural variability of the climate affects one’s ability to project the anthropogenically forced component of change in daily precipitation extremes across Europe is examined. A three-member ensemble of the Hadley Centre Regional Climate Model (HadRM3H) is used and a statistical framework is applied to estimate the uncertainty due to the full spectrum of climate variability. In particular, the results and understanding presented here suggest that annual to multidecadal natural variability may contribute significant uncertainty. For this ensemble projection, extreme precipitation changes at the grid-box level are found to be discernible above climate noise over much of northern and central Europe in winter, and parts of northern and southern Europe in summer. The ability to quantify the chan...

The value of high-resolution Met Office regional climate models in the simulation of multi-hourly precipitation extremes

AbstractExtreme value theory is used as a diagnostic for two high-resolution (12-km parameterized convection and 1.5-km explicit convection) Met Office regional climate model (RCM) simulations. On subdaily time scales, the 12-km simulation has weaker June–August (JJA) short-return-period return levels than the 1.5-km RCM, yet the 12-km RCM has overly large high return levels. Comparisons with observations indicate that the 1.5-km RCM is more successful than the 12-km RCM in representing (multi)hourly JJA very extreme events. As accumulation periods increase toward daily time scales, the erroneous 12-km precipitation extremes become more comparable with the observations and the 1.5-km RCM. The 12-km RCM fails to capture the observed low sensitivity of the growth rate to accumulation period changes, which is successfully captured by the 1.5-km RCM. Both simulations have comparable December–February (DJF) extremes, but the DJF extremes are generally weaker than in JJA at daily or shorter time scales. Case st...

Using and Designing GCM–RCM Ensemble Regional Climate Projections

Abstract Multimodel ensembles, whereby different global climate models (GCMs) and regional climate models (RCMs) are combined, have been widely used to explore uncertainties in regional climate projections. In this study, the extent to which information can be enhanced from sparsely filled GCM–RCM ensemble matrices and the way in which simulations should be prioritized to sample uncertainties most effectively are examined. A simple scaling technique, whereby the local climate response in an RCM is predicted from the large-scale change in the GCM, is found to often show skill in estimating local changes for missing GCM–RCM combinations. In particular, scaling shows skill for precipitation indices (including mean, variance, and extremes) across Europe in winter and mean and extreme temperature in summer and winter, except for hot extremes over central/northern Europe in summer. However, internal variability significantly impacts the ability to determine scaling skill for precipitation indices, with a three-...

Challenges in Quantifying Changes in the Global Water Cycle

AbstractUnderstanding observed changes to the global water cycle is key to predicting future climate changes and their impacts. While many datasets document crucial variables such as precipitation, ocean salinity, runoff, and humidity, most are uncertain for determining long-term changes. In situ networks provide long time series over land, but are sparse in many regions, particularly the tropics. Satellite and reanalysis datasets provide global coverage, but their long-term stability is lacking. However, comparisons of changes among related variables can give insights into the robustness of observed changes. For example, ocean salinity, interpreted with an understanding of ocean processes, can help cross-validate precipitation. Observational evidence for human influences on the water cycle is emerging, but uncertainties resulting from internal variability and observational errors are too large to determine whether the observed and simulated changes are consistent. Improvements to the in situ and satellit...

Temperature influences on intense UK hourly precipitation and dependency on large-scale circulation

Short periods of intense rainfall may be associated with significant impacts on society, particularly urban flooding. Climate model projections have suggested an intensification of precipitation under scenarios of climate change. This is in accordance with the hypothesis that precipitation intensities will increase with temperature according to the thermodynamic Clausius–Clapyeron (CC) relation (a rate of ~6–7% °C−1)—a warmer atmosphere being capable of holding more moisture. Consequently, CC scaling between temperature and extreme precipitation has been demonstrated in numerous studies and in different locations, with higher than CC scaling (so-called super CC scaling) observed for sub-daily extremes. Here we use a new dataset of UK hourly precipitation to identify seasonal scaling relationships between mean daily temperature and 99th percentile hourly precipitation intensities. Pooling the data for the whole UK indicates only slightly higher than CC scaling in spring and summer at higher temperatures, notably less than the 2xCC scaling observed in other regions. Both the highest hourly intensities and the highest scaling in the UK occur in summer and so for this season the dependency of the scaling relationship on large scale circulation conditions is examined using a set of air flow indices. A shear vorticity index (indicative of large-scale flow cyclonicity) is noted to have the greatest influence on the relationship, approaching 2xCC at higher temperatures when shear vorticity is negative (anticyclonic rotation). An examination of the occurrence of intense events indicates that these can occur under cyclonic and anticyclonic conditions but that in the south-east of England the latter conditions disproportionately favour their occurrence. These results suggest that changes in circulation regimes could modify the expected changes in precipitation intensities prescribed by CC scaling and arising as a consequence of future warming.

Projected increases in summer and winter UK sub-daily precipitation extremes from high-resolution regional climate models

Summer (June–July–August; JJA) UK precipitation extremes projections from two UK Met Office high-resolution (12 km and 1.5 km) regional climate models (RCMs) are shown to be resolution dependent. The 1.5 km RCM projects a uniform () increase in 1 h JJA precipitation intensities across a range of return periods. The 12 km RCM, in contrast, projects decreases in short return period (5 years) events but strong increases in long return period (≥20 years) events. We have low physical and statistical confidence in the 12 km RCM projections for longer return periods. Both models show evidence for longer dry periods between events. In winter (December–January–February; DJF), the models show larger return level increases (≥40%). Both DJF projections are consistent with results from previous work based on coarser resolution models.

Do convection-permitting regional climate models improve projections of future precipitation change?

AbstractRegional climate projections are used in a wide range of impact studies, from assessing future flood risk to climate change impacts on food and energy production. These model projections are typically at 12–50-km resolution, providing valuable regional detail but with inherent limitations, in part because of the need to parameterize convection. The first climate change experiments at convection-permitting resolution (kilometer-scale grid spacing) are now available for the United Kingdom; the Alps; Germany; Sydney, Australia; and the western United States. These models give a more realistic representation of convection and are better able to simulate hourly precipitation characteristics that are poorly represented in coarser-resolution climate models. Here we examine these new experiments to determine whether future midlatitude precipitation projections are robust from coarse to higher resolutions, with implications also for the tropics. We find that the explicit representation of the convective st...

The characteristics of summer sub-hourly rainfall over the southern UK in a high-resolution convective permitting model

Flash flooding is often caused by sub-hourly rainfall extremes. Here, we examine southern UK sub-hourly 10 min rainfall from Met Office state-of-the-art convective-permitting model simulations for the present and future climate. Observational studies have shown that the duration of rainfall can decrease with temperature in summer in some regions. The duration decrease coincides with an intensification of sub-hourly rainfall extremes. This suggests that rainfall duration and sub-hourly rainfall intensity may change in future under climate change with important implications for future changes in flash flooding risk. The simulations show clear intensification of sub-hourly rainfall, but we fail to detect any decrease in rainfall duration. In fact, model results suggest the opposite with a slight (probably insignificant) lengthening of both extreme and non-extreme rainfall events in the future. The lengthening is driven by rainfall intensification without clear changes in the shape of the event profile. Other metrics are also examined, including the relationship between intense 10 min rainfall and temperature, and return levels changes; all are consistent with results found for hourly rainfall. No evaluation of model performance at the sub-hourly timescale is possible, highlighting the need for high-quality sub-hourly observations. Such sub-hourly observations will advance our understanding of the future risks of flash flooding.