Low Clouds Contribute to Weather Prediction Model Bias

Long-term measurement records improve the representation of clouds in climate and weather forecast models.

Image courtesy of ARM Climate Research
Millimeter wavelength cloud radar, located at the Southern Great Plains site, probes the extent and composition of clouds at millimeter wavelengths.

The Science

This study investigated and addressed biases and compensating errors in the surface radiation (radiation hitting the surface of the Earth), which are directly linked to cloud amount and properties, using data from the Southern Great Plains Atmospheric Radiation Measurement (ARM) Climate Research Facility.

The Impact

These findings will provide guidance for a targeted improvement of cloud parameterizations in climate and weather forecast models.


The long-term measurement records from the Atmospheric Radiation Measurement site on the Southern Great Plains (SGP) show evidence of a surface irradiance bias in the global Numerical Weather Prediction model from the European Centre for Medium-range Weather Forecasts (ECMWF). Surface temperature, cloud cover and associated radiative processes need to be accurately represented. This has been a long-standing problem in the model and previous studies have suggested that low clouds may contribute. In order to guide improvements to the model's cloud and radiation parameterizations, the origin of the bias was explored for different cloud regimes in order to highlight the particular cloud processes that are contributing to the error. Comparisons between observed and modeled cloud fraction profiles over six years at the SGP site identified overcast low cloud conditions during the spring and fall seasons are identified as a major contributor to the model bias.


Dr. Maike Ahlgrimm
European Centre for Medium-Range Weather Forecasts
Great Britain


Basic Research: DOE Office of Science, Office of Biological and Environmental Research


Ahlgrimm, Maike, Richard Forbes, 2012: The Impact of Low Clouds on Surface Shortwave Radiation in the ECMWF Model. Mon. Wea. Rev., 140, 3783–3794.  DOI: http://dx.doi.org/10.1175/MWR-D-11-00316.1

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