Picture: Rain in tropics. Credit: Bibhukalyan Acharya (Pexels)

Climate models have long struggled to represent tropical precipitation, with modelled outputs often not matching observations. This discrepancy matters to our understanding of regional changes to rainfall due to global warming and also because tropical rainfall events impact atmospheric circulation, which can lead to indirect climate impacts in other parts of the world. For this reason, it is important to know if there have been any consistent improvements in modelling tropical rainfall.

To investigate this, CLEX researchers and international colleagues examined the models used by the Intergovernmental Panel for Climate Change (IPCC) over three model phases linked to IPCC reports – CMIP3, CMIP5, and the most recent, CMIP6. They found that while there were improvements in the modelling aspects of the pattern of mean precipitation, as well as the precipitation signature of modes of internal variability, the models still fell short in other areas. The researchers noted that in many metrics, such as in the diurnal or seasonal cycle, there was no clear sign of a continuous improvement in the latest CMIP6 models. And for some quantities, such as the fraction of precipitation that comes from low-level cloud regimes (warm rain), or in measures of the summer monsoon, the CMIP6 model mean departs more from the observations than do earlier CMIP phases.

The researchers say this shows that coarse climate models may have some structural issues that will take many decades to correct before they can accurately capture modelled outputs that correspond with observations. With this in mind, they propose that storm resolving models may offer a solution. They acknowledge that this approach has been dismissed previously as something for the distant future but recent studies have suggested that if existing models could be run on today’s largest supercomputer, global simulations on grids as fine as 1.5 km could already, today, deliver one simulated year per day.

While these models may not solve all of the current issues, they appear to have a more solid physical basis for simulating processes relevant to tropical precipitation, such as the coupling to mesoscale atmospheric dynamics. 

Paper: Fiedler, S., and Coauthors, 2020: Simulated Tropical Precipitation Assessed across Three Major Phases of the Coupled Model Intercomparison Project (CMIP). Mon. Wea. Rev., 148, 3653–3680, https://doi.org/10.1175/MWR-D-19-0404.1.