The method employs comprehensive three-dimensional cloud simulations to replace the traditional approximations of cloud processes currently used in global climate models. The coupling of these simulations to global models is done with the help of software technology originally developed for astrophysical research. The team’s method and initial findings are published in the Journal of Advances in Modeling Earth Systems.
Superparameterization as alternative approach
It is notoriously difficult for climate modelers to accurately depict cloud processes. Cloud dynamics act on scales from millimeters to several kilometers, but present-day global climate models typically operate on scales from 100 km and coarser. To overcome this scale gap in the models, researchers have traditionally used approximate formulae, known as parameterizations, to represent cloud physics. Parameterizations are simplified numerical representations of physical processes based on measurements and physical understanding.
In their paper, the team, comprising researchers from the Royal Netherlands Meteorological Institute (KNMI), Delft University of Technology (TU Delft), the Centrum Wiskunde & Informatica (CWI) and the Netherlands eScience Center, presents a computationally attractive alternative. They have replaced the parameterizations of clouds in a global-scale model developed by the European Centre for Medium-Range Weather Forecasts (ECMWF) with a turbulence-resolving model (in technical terms a large-eddy simulation) developed by the KNMI and Dutch universities.
A 3D rendering of simulated cloud data in an artistic setting, illustrating the power of high-resolution calculations of 3D cloud structure at chosen geographic locations in a global weather-modeling context. Credits: Johanna Grönqvist
This approach of replacing parameterizations by explicit simulations, called superparameterization, has the advantage that all the relevant processes for the formation and development of clouds are actually resolved and represented in the model. This will increase the reliability of the simulations by, for example, improved predictions of cloud cover, cloud amount and cloud top heights.
“It is incredible that we can now represent clouds so realistically in global weather and climate models”, says lead author Dr Fredrik Jansson from CWI. “It is still a big computation to resolve clouds globally, but by applying our method to the regions where traditional approximations fail, I think we can gain a huge understanding of the role of clouds in the present and future climate.”
The authors plan to apply their method to quantify the response of the global model to the changes in cloud representation, which will allow improvements in weather prediction with respect to rainfall and ultimately quantify the global role of clouds in the future warming earth climate.
A side-by side view of simulated cloud fields over the Netherlands and a satellite image. The blue squares show the detailed cloud-resolving simulations which are coupled to the global weather model shown in purple. The satellite image is from Terra MODIS, NASA/Goddard Space Flight Center Earth Science Data and Information System.
A proud partner
“It is very exciting and satisfying to be able to take technologies that were developed to understand celestial bodies and interstellar clouds and apply these to better understand the weather on earth”, says Dr Inti Pelupessy, co-author and research engineer at the eScience Center.
Fellow author and research engineer Dr Gijs van den Oord agrees: “In many ways clouds on Earth are more complicated than interstellar clouds, and it was a big challenge to let the global model talk correctly to our cloud modelling code. I am extremely proud and satisfied that we as the Netherlands eScience Center were able to contribute our expertise to this project.”
Fredrik Jansson, Gijs van den Oord, Inti Pelupessy, Johanna H. Grönqvist, A. Pier Siebesma, Daan Crommelin, ‘Regional Superparameterization in a Global Circulation Model Using Large Eddy Simulations’ in Journal of Advances in Modeling Earth Systems (12 July, 2019). DOI: 10.1029/2018MS001600.
Read the full article.
Read more about the project.