Understanding intensification of short-duration rainfall extremes

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.

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.

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. 

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.

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.

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.