Image by Hans Braxmeier from Pixabay

 

By adapting scaling formulas that are traditionally used to diagnose the overall behavior of extreme rainfall events in a warming climate, atmospheric scientists at Lawrence Berkeley National Laboratory and UC Irvine have demonstrated that the potential exists for these formulas to capture the dynamics of individual rainstorms globally.

Heavy rainstorms that can cause floods, cyclones, and other damage are expected to last longer and bring higher winds as temperatures rise. These convective storms are predicted to become especially intense in the tropics, due to its added moisture and greater atmospheric instability. Yet current global climate models fail to accurately replicate extreme rainfall patterns, according to previous research led by Bill Collins, director of Berkeley Lab’s Climate and Ecosystem Sciences Division.

Using scaling formulas, UC Berkeley graduate student Benjamin Fildier, Collins, and colleagues test the ability of a standard computer model of global climate to simulate heavy rainstorms (the Community Atmosphere Model version 5, or CAM). They did so by comparing this model to SPCAM, a superparameterized version of CAM that incorporates a cloud-resolving model (This complex, SPCAM, approach had previously been found to improve rainfall statistics by better simulating the processes involved in shallow and deep-convective cloud formations.) A paper describing their research appears in the December issue of the Journal of Advances in Modeling Earth Systems.

The scientists then compared CAM and SPCAM performance using scaling formulas that are normally applied to estimating the changing dynamics of extreme rainstorms with climate change. These formulas are designed to approximate the storm’s rainfall intensity from the knowledge of dynamic and thermodynamic variables such as vertical updraft velocity and temperature.

These formulas had never been used before to simply test and compare climate models’ ability to simulate severe rainstorm events. To Fildier and colleagues’ surprise, these simple formulas were able to mimic the complex behavior of CAM and SPCAM at simulating the amount of rain produced by severe storms over broad scales of time and space.

The success of the scaling formulas at estimating severe rainstorms’ intensity, frequency and location might also apply outside of the tropics. As a result, the Berkeley Lab scientists’ approach could broadly permit an easier characterization of extreme rainfall events in future climates, based on the evolution of other physical quantities (such as temperature and updraft velocities). Moreover, the findings show that this approach works for extreme storms ranging in size between 200 km and 2,000 km.