Friday, August 1, 2014

New paper shows why climate models don't realistically simulate precipitation & convection

A paper published today in Climate Dynamics illustrates the reasons why climate models do not realistically simulate convection and convective precipitation, and thus why current model projections of regional [and global] precipitation cannot be relied upon. 

According to the authors,
Precipitation is one of the most crucial variables to assess in a changing climate. However, it is also one of the most difficult to simulate.
The primary reason why is that IPCC climate models have resolutions of at least 50 to 10,000 kilometers, and regional climate models have resolutions of 5-10 km at most, but several papers have shown that much higher resolutions of 1-2 km are required to realistically simulate convection, and thus convective precipitation (and clouds too). The computational time for global and regional climate models at 1-2 km resolution are beyond the capabilities of today's most powerful supercomputers. Thus, the models are programmed with "parameterizations," i.e. basically "fudge factors" for convection rather than properly and directly simulating convection at the required 1-2 km resolution.
A high temporal resolution of precipitation statistics is especially important in summer when precipitation over land is often convective. In regional climate models (RCMs) convective precipitation usually occurs too early in the day and the amplitude of its diurnal cycle is usually overestimated. The correct representation of convective precipitation is a very complicated task in modelling because it involves complex interactions between the surface, the boundary layer, and the free troposphere. Furthermore, vertical profiles of temperature and humidity define the convective available potential energy, thereby affecting the strength of the convective systems and the associated precipitation intensities. Since convection acts on a scale of only a few kilometres, it is necessary to use parameterisations [a "fudge"] for typical Regional Climate Model simulations with grid resolutions of 10–100 km.

According to Weisman et al. (1997), 4 km spatial resolution may be sufficient for non-hydrostatic models to explicitly represent convective systems. Previous studies (e.g. Hohenegger et al. 2008; Brockhaus et al. 2008; Bechtold et al. 2004) recognised the parameterisation of convection as a major source of uncertainties and errors in simulating the diurnal cycle of precipitation.
Increasing the spatial resolution towards convection permitting scales provides the possibility to switch off a major part of the convective parameterisations. In addition, a higher resolution model allows a better representation of orography and surface fields which are crucial for the initiation of convection in complex terrain (Hohenegger et al. 2008). Copious literature proves the added value of this spatial resolution in the representation of the precipitation field especially in cases of moist convection and/or in regions with strong orography (Langhans et al. 2013; Mass et al. 2002; Miura et al. 2007; Grell et al.2000; Richard et al. 2007; Lean et al. 2008; Schwartz et al. 2009; Weusthoff et al. 2010; Baldauf et al.2011; Hohenegger et al. 2008; Prein et al. 2013). Most of these studies are related to numerical weather prediction (NWP) or limited to few summers. The computational cost is high for running long-term high-resolution simulations, and there is furthermore a shortage of suitable observational datasets available for evaluation.

The authors find that using a regional climate model with 2.8 km resolution greatly improves simulation of convection and convective precipitation in comparison to a 7 km resolution model, and thus the "benefit of convection permitting climate model simulations in representing convective precipitation."

Other papers find it is necessary to use even higher 1-2 km resolution to realistically simulate convection. Clearly, low-resolution current IPCC regional and global climate models aren't close to realistically simulating convection or convective precipitation, and thus global and regional precipitation projections are not reliable. 

Benefit of convection permitting climate model simulations in the representation of convective precipitation

G. FosserS. Khodayar, P. Berg
Download PDF (1,519 KB)  View Article


A major source of uncertainty in regional climate model (RCM) simulations arises from the parameterisation of sub-grid scale convection. With increasing model resolution, approaching the so-called convection permitting scale, it is possible to switch off most of the convection parameterisations. A set of simulations using COSMO-CLM model has been carried out at different resolutions in order to investigate possible improvements and limitations resulting from increased horizontal resolution. For our analysis, 30 years were simulated in a triple nesting setup with 50, 7 and 2.8 km resolutions, with ERA40 reanalysis data at the lateral boundaries of the coarsest nest. The investigation area covers the state of Baden-W├╝rttemberg in southwestern Germany, which is a region known for abundant orographically induced convective precipitation. A very dense network of high temporal resolution rain gauges is used for evaluation of the model simulations. The purpose of this study is to examine the differences between the 7 and 2.8 km resolutions in the representation of precipitation at sub-daily timescales, and the atmospheric conditions leading to convection. Our results show that the highest resolution of Regional Climate Model simulations significantly improves the representation of both hourly intensity distribution and diurnal cycle of precipitation. In addition, at convection permitting scale [2.8 km] the atmospheric fields related to convective precipitation show a better agreement with each other. The results imply that higher spatial resolution [2.8 km] partially improves the representation of the precipitation field, which must be the way forward for regional climate modelling.

1 comment:

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