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Understanding intensification of short-duration rainfall extremes

Scientific meeting

Starts:

February
052020

09:00

Ends:

February
062020

17:00

Location

Kavli Royal Society Centre, Chicheley Hall, Newport Pagnell, Buckinghamshire, MK16 9JJ

Overview

Satellite meeting organised by Professor Hayley Fowler.

Cyclist on Chillingham Road in Heaton, Newcastle upon Tyne. Photo from the 28 June 2012 "Toon Monsoon" event in Newcastle upon Tyne, UK, where there was almost 50mm rainfall in 90 mins. Credit: ncjMedia Ltd, Lewis Arnold.

The satellite meeting continued the discussion meeting Intensification of short-duration rainfall extremes and implications for flash flood risks but with a clearer focus on process understanding and integrating/synthesising research results from different international groups working in this area with identification of key research gaps and research studies that can address these.

Speaker biographies and abstracts are available below. Recorded audio of the presentations is also available below. 

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Event organisers

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Schedule of talks

05 February

09:00-12:30

Session 1

5 talks Show detail Hide detail

Chairs

Dr Andreas Prein, National Center for Atmospheric Research, USA

09:05-09:30 Aims and objectives of the meeting

Professor Hayley Fowler, Newcastle University, UK

Show speakers

09:30-09:50 Discussion

09:50-10:10 Dependence of short-duration rainfall to temperature across Australia

Dr Conrad Wasko, University of Melbourne, Australia

Abstract

Predicting future short-duration rainfall extremes is difficult. Therefore, many studies condition rainfall extremes on exogenous variables to gain insights on how extremes may change in the future. This presentation reviews literature using Australian data to study the historical day-to-day dependence of rainfall extremes on various climatic variables. Daily rainfall extremes show little homogeneity in their dependence with temperature across Australia. Contrary to expectation, many sites exhibit decreasing rainfall extremes with increasing temperatures. However, a dependence close to the theoretical Clausius-Clapeyron relationship is obtained when dew point temperature, a measure of absolute humidity, is matched to daily rainfall extremes. As rainfall intensity has a positive dependence with integrated water vapour across Australia, it is reasoned that when moisture is available, excess heat results in more evaporation as opposed to an increased air temperature, causing negative relationships between rainfall and temperature. When sub-daily rainfall extremes are examined, the dependence between rainfall and dew point temperature is consistently greater than Clausius-Clapeyron suggesting dynamic intensification of rainfall extremes with higher temperatures. Confounding factors such as embedded storms and mixing of storm durations are discussed with the suggestion that the most intense portion of a storm is the most likely to intensify with higher temperatures.

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10:10-10:30 Strong relationship between observed sub-daily precipitation extremes to dewpoint temperature

Dr Haider Ali, Newcastle University, UK

Abstract

The intensity and frequency of extreme precipitation events have increased globally and are likely to rise further under the warming climate. The Clausius-Clapeyron (CC) relationship (scaling) provides a physical basis to understand the relationship of precipitation extremes with temperature. Recent studies have used global sub-daily precipitation data from satellite, reanalysis and climate model outputs (due to the limited availability of long term observed sub-daily data at global scales) and have reported a higher sensitivity of sub-daily precipitation extremes to surface air temperature than for daily extremes. Moreover, at higher temperatures, moisture availability becomes the dominant driver of extreme precipitation, therefore, dewpoint temperature can be a better scaling variable to overcome humidity limitations as compared to air temperature. Here, the group used hourly precipitation data from the Global Sub-daily Rainfall (GSDR) dataset and daily dewpoint temperature data (DPT) from the Met Office Hadley Centre observations dataset (HadISD) at 6695 locations across the United States of America, Australia, Europe, Japan, India and Malaysia. The team found that more than 60% of locations (scaling estimated for individual location) show scaling greater than 7%/K (CC rate). Moreover, more than 55% of locations across Europe, Japan, Australia and Malaysia show scaling greater than 1.5CC. Furthermore, when locations across selected regions are pooled within similar climatic zones (based on Koppen Geiger classification), scaling curves show around 7%/K scaling. The scaling curves for locations at greater altitude (>400m MSL) are flat compared to locations at relatively lower altitude. The difference in scaling rates at-station and for pooled regions highlight the importance of understanding the thermodynamic and dynamic processes governing precipitation extremes at different spatial scales and indicate that local processes are driving the super-CC sensitivities in most regions.

Show speakers

10:30-11:00 Coffee

11:00-12:30 Discussion

12:30-13:30 Lunch

13:30-17:00

Session 2

7 talks Show detail Hide detail

Chairs

Dr Michael Wehner, Lawrence Berkeley National Laboratory, USA

13:30-13:50 Non‐stationarity and flooding

Dr Gabriele Villarini, University of Iowa, USA

Abstract

Flooding is one of the costliest natural hazards and it is often associated with a high toll in terms of fatalities. Over the past several decades, the frequency and magnitude of these events have been changing, complicating the capability to prepare to and respond against this hazard. Most of the literature has focused on the detection of changes in flooding, with much less emphasis on their attribution. Improving our understanding of the physical drivers that are responsible for the observed changes in this natural hazard can enhance our capability of predicting and projecting these changes, with large implications for water resources management and the design of hydraulic structures. This presentation will discuss some of the issues associated with the nonstationarity in the flood records, and provide an overview of some of the methodologies to deal with these changes. 

 

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13:50-14:10 A synthesis of hourly and daily precipitation extremes in different climatic regions

Dr Renaud Barbero, National Research Institute of Science and Technology for Environment and Agriculture, France

Abstract

Climatological features of observed annual maximum hourly precipitation have not been documented systematically compared to those on daily timescales due to observational limitations. Drawing from a quality-controlled database of hourly records sampling different climatic regions including the United States, Australia, the British Isles, Japan, India and peninsular Malaysia available over multiple decades, we examined climatological features of annual maximum precipitation (AMP) across timescales ranging from 1-hr to 24-hr. Climatological features include seasonal and diurnal distribution of AMP, the storm duration triggering AMP, as well as the relation with the convective available potential energy. This study provides insights on climatological features of hourly precipitation extremes and how they contrast with the daily extremes examined in most studies.

Show speakers

14:10-15:00 Discussion

15:00-15:30 Tea

15:30-15:50 The role of short-duration high-intensity precipitation in geohazards and information needs in a changing climate

Dr Nina S Oakley, Western Regional Climate Center, Desert Research Institute, USA

Abstract

There is a well-established connection between short-duration, high-intensity rainfall and geohazards such as shallow landslides and post-wildfire debris flows. The applied meteorology and geomorphology research communities in the western United States are currently working together to improve early warning and forecasting of potential impacts of these geohazards. One of the questions these scientists often hear from floodplain managers and community planners is, “How will the frequency and impacts of geohazards change in a warming climate?” To address this question, sub-daily precipitation projections at spatial scales relevant to the geohazards in question are necessary. For application in the far western United States, there must be consideration of short-duration precipitation changes within cool season mid-latitude cyclones, as they produce the highest precipitation intensities in these regions. This poster demonstrates the common meteorological drivers of short-duration, high-intensity precipitation producing cool-season shallow landslides and post-wildfire debris flow events in the western United States. Additionally, it describes the temporal and spatial resolutions needed in precipitation projections such that they can be applied to these geohazards. Current research goals on climate change, precipitation intensification, and geohazards are also discussed to stimulate a conversation about data needs from the applied research communities to promote collaboration and progress.

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15:50-16:10 What can the UK rain gauge network tell us about changes to intense rainfall?

Dr Stephen Blenkinsop, Newcastle University, UK

Abstract

It is widely expected that anthropogenic warming will lead to an increase in the intensity of extreme rainfall. The Clausius-Clapeyron (CC) relationship indicates an increase of ~7% per degree of warming, but in some regions the scaling rate for hourly extremes has been observed to be higher, leading to concerns over increased risk of flash flooding in the future. The group has collected and quality-controlled data from ~2000 rain gauges across the UK, providing high resolution rainfall data to assess the climatology of extremes at timescales from 5 minutes through to 5 days, leading to improved understanding of when and where intense rainfall is more likely. They have also quantified changes in hourly extremes and find evidence of recent intensification in summer. However, although intense UK summer rainfall was found to scale in accordance with the CC relationship suggesting a possible thermodynamic cause, the group cannot rule out the influence of large-scale modes of variability as potential mechanisms, especially as they note the dependence of intense rainfall on weather patterns. These results highlight the difficulties of both quantifying, and understanding the processes behind, observed changes in intense rainfall, and confirm the need to combine knowledge derived from observational and modelling studies.

Show speakers

16:10-17:00 Discussion

17:00-18:00

Poster session

06 February

09:00-12:30

Session 3

5 talks Show detail Hide detail

Chairs

Professor Hayley Fowler, Newcastle University, UK

09:00-09:30 Review of previous day

Professor Hayley Fowler, Newcastle University, UK

Show speakers

09:30-10:00 Discussion

10:00-10:30 Accounting for large-scale circulation effects on extreme precipitation

Professor Richard Allan, University of Reading, UK

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10:20-10:40 UKCP: Understanding uncertainty in future changes in precipitation extremes at convection-permitting scale

Dr Giorgia Fosser, Met Office Hadley Centre, UK

Abstract

Convection-permitting models (CPMs) provide a better representation of sub-daily precipitation statistics and convective processes, both on climate and NWP time scales, mainly thanks to the possibility to switch off the parameterisation of convection. The improved realism of these models gives us greater confidence in their ability to project future changes in short-duration precipitation extremes. The first 12-member ensemble of convection-permitting climate simulations over the UK was completed within the latest updates to the UK Climate Projections (UKCP). The 20-year long CPM simulations for present-day and end of century periods are nested in an ensemble of regional climate model (RCM) simulations over Europe driven by a global climate model ensemble. In the driving ensembles, uncertain parameters in the model physics are varied within plausible bounds to sample uncertainty. Although no perturbations are applied directly to the CPMs, this project allow to provide a first-ever estimate of uncertainty at convection-permitting scale and thus provide UK risk assessment studies with more reliable climate change projections at local and hourly scales. The presentation will show results looking at the uncertainty in future changes in hourly precipitation extremes across the CPM ensemble, and how this differs from the driving RCM ensemble.

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10:40-11:10 Coffee

11:10-12:30 Discussion

12:30-13:30 Lunch

13:30-16:45

Session 4

4 talks Show detail Hide detail

Chairs

Dr Conrad Wasko, University of Melbourne, Australia

13:30-13:50 Implications of small‐scale processes on convection

Dr Peter Berg, Swedish Meteorological and Hydrological Institute, Sweden

Abstract

Several studies have shown that convective extremes become more intense at increasing temperatures. They even show scaling beyond the rate indicated by the increase in atmospheric moisture given by the Clausius-Clapeyron (CC) relation. However, the process which drives this intensification is not yet well established, but is likely a combination of both thermodynamical and dynamical components. In earlier work, idealized temperature perturbed large eddy simulator (LES) experiments suggest a strong component through interaction between convective events when studied in the form of raincell tracks. Convective tracks occurring in isolation, i.e. those that develop and decay as a single track, cannot reach super-CC scaling. However, those tracks that form by the merging of two or more previous track do show super-CC behavior. To further explore this effect in less idealized settings, precipitation tracking is performed in both observations, in the form of radar measurements, and in convective permitting simulations at 3 km grid resolution over southern Sweden for historical and future climate projections. Raincell tracking is performed to gain insights into the formation and decay of tracks, as well as to determine general statistical character of the tracks, such as duration, size, intensity, etc.

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13:50-14:10 Climate change’s influence on June 2009 extreme precipitation event over Southeast Austria

Aditya Narayan Mishra, University of Graz, Austria

Abstract

During 22–24 June 2009, Austria witnessed a rampant rainfall spell that spread across populated areas of the country. High intensity rainfall caused 3000+ landslides in Feldbach, and property damages worth €10,000,000 in Styria itself. In a warmer climate change scenario, such an extreme precipitation event may become more intense due to higher water holding capacity of air with increased temperatures, but this reasoning may not be so straightforward considering the complex physics of precipitation. CCLM regional model is used recreate the event over the same location as the original event. Further we use CMIP5 global climate models (at the RCP8.5) scenario. In particular, these will be applied in the ‘surrogate climate change’ method. Here, the climate change signals are calculated by computing the difference between the thermodynamic fields of the CMIP5 simulations for the future and the past. A similar approach is to be applied for the ‘past’ case simulations. The idea behind this experimental setup is to establish a ‘storyline’ for the event as it would have occurred in the past, present and the future.

 

Show speakers

14:10-15:00 Discussion and planning future research

15:00-15:30 Tea

15:30-16:15 Concluding remarks

Related events

Understanding intensification of short-duration rainfall extremes

Satellite meeting organised by Professor Hayley Fowler

Kavli Royal Society Centre, Chicheley Hall Newport Pagnell Buckinghamshire MK16 9JJ
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