When it Rains, it Pours: Extreme Precipitation and its Observation

Over the last decades, more and more extreme and episodic precipitation events have taken place. Has extreme precipitation increased alongside global warming? The answer is yes – based on the intensification of the hydrological cycle by a warming climate, wet places get wetter. A recent study suggested that climate change is also leading to an increase in extreme precipitation even in arid regions [1]. In any case, a significant change in precipitation patterns will result in direct damage, economic losses and high response costs.

Global warming is a driver for heavier precipitation events that are much more localised.

Increases in extreme precipitation events are disproportionate to overall trends in precipitation. In other words, more extreme precipitation does not always mean an increase in total or mean amounts of precipitation. This is because global annual mean precipitation is constrained by the energy budget of the troposphere, and conversely, extreme precipitation is restricted by moisture content in the atmosphere [2]. Under a warming climate, the warmer atmosphere holds more water vapour, termed the Clausius-Clapyetron rate, so the chances of heavier precipitation are higher than before.

At the same time, global warming may lead to an increase in localized precipitation events. For example, convective precipitation responds sensitively to atmospheric temperature increases; an unstable state of the atmosphere resulting from a temperature difference between the surface and higher altitudes is a main driver of deep cumulonimbus cloud systems. In many cases, convective precipitation covers a small area for a short period as convective clouds have a limited horizontal extent.

Observations are the most fundamental means for understanding extreme precipitation

Owing to the high variability both on temporal and spatial scales, there are difficulties in obtaining ‘good’ observational data on extreme precipitation. This is one of reasons why there is no robust agreement between model projections on how climate change will affect intensities and frequencies of precipitation, particularly when it comes to extreme precipitation on a local scale. Indeed, the Intergovernmental Panel on Climate Change (IPCC) has assigned only ‘medium confidence’ to occurrence of increases in heavy precipitation and has indicated that there are strong regional and sub-regional variations in trends in the number of heavy precipitation events [3].

However, observations are among the most fundamental methods to comprehend precipitation as well as all other climate variables. Note that the World Meteorological Organization’s Global Framework for Climate Service has defined observation and monitoring of Essential Climate Variables as key elements in the framework.

Why observations? Well, the process of forecasting weather or projecting climate involves: observation, theoretical understating, modelling and dissemination/communication [4]. First, weather phenomena and its response to climate change needs to be understood and based on a physical basis – explained by observations. Secondly, results from numerical modelling, built on a series of theories, have to be consistent with observations to ensure reliability. Finally, observations (historical weather data) are necessary to identify communities vulnerable to extreme events and mitigate their potential damage through  effective communication.

As a result, ‘observation’ will be a good starting point on understanding climate change and its impacts. In that context, my articles in the future will highlight observations, mainly for precipitation.