Background
In introducing their important new paper, Stephens et al. write that in prior studies of the subject, "land surface observations of the daily-accumulated rainfall intensities of rates >1 mm/day were compiled from the Global Historical Climatology Network by Sun et al. (2006) and compared to analogous model accumulated precipitation," and they report that "as in other studies (e.g., Dai and Trenberth, 2004), the Sun et al. comparison revealed a general overestimate in the frequency of modeled precipitation and an associated underestimate of intensity," while noting that "Wilcox and Donner (2007) reached a similar conclusion."

What was done
To further examine the issue -- and to extend the scope of its relevance -- the nine researchers focused on the much larger portion of the planet that is occupied by its oceans, where they used "new and definitive measures of precipitation frequency provided by CloudSat [e.g., Haynes et al., 2009]" to assess the realism of global model precipitation via an analysis that employed five different computational techniques representing "state-of-the-art weather prediction models, state-of-the-art climate models, and the emerging high-resolution global cloud 'resolving' models."

What was learned
Stephens et al. determined that "the character of liquid precipitation (defined as a combination of accumulation, frequency, and intensity) over the global oceans is significantly different from the character of liquid precipitation produced by global weather and climate models," noting that "the differences between observed and modeled precipitation are larger than can be explained by observational retrieval errors or by the inherent sampling differences between observations and models." More specifically, they say that for the global ocean as a whole, "the mean model intensity lies between 1.3 and 1.9 times less than the averaged observations," while occurrences "are approximately twice the frequency of observations." They also say the models "produce too much precipitation over the tropical oceans" and "too little mid-latitude precipitation." And they indicate that the large model errors "are not merely a consequence of inadequate upscaling of observations but indicative of a systemic problem of models more generally."

What it means
In the final sentence of their paper, the US, UK and Australian researchers say their results imply that state-of-the-art weather and climate models have "little skill in precipitation calculated at individual grid points," and that "applications involving downscaling of grid point precipitation to yet even finer-scale resolution has little foundation and relevance to the real earth system," which is not too encouraging a result, considering it is the "real earth system" in which we live and for which we have great concern.

References
Dai, A. and Trenberth, K.E. 2004. The diurnal cycle and its depiction in the Community Climate System Model. Journal of Climate 17: 930-951.

Haynes, J.M., L'Ecuyer, T.S., Stephens, G.L., Miller, S.D., Mitrescu, C., Wood, N.B. and Tanelli, S. 2009. Rainfall retrieval over the ocean with spaceborne W-band radar. Journal of Geophysical Research 114: 10.1029/2008JD009973.

Sun, Y., Solomon, S., Dai, A. and Portmann, R.W. 2006. How often does it rain? Journal of Climate 19: 916-934.

Wilcox, E.M. and Donner, L.J. 2007. The frequency of extreme rain events in satellite rain-rate estimates and an atmospheric General Circulation Model. Journal of Climate 20: 53-69.