Background
The authors begin by noting that precipitation is one of the most poorly parameterized physical processes in general circulation models (GCMs). In fact, they say that it is difficult for them to simulate precipitation features as fundamental as diurnal variation, frequency and intensity.

What was done
To see what progress may have recently been made along these lines, Song et al. evaluated the performances of seven single-column models (SCMs) by comparing model-simulated surface precipitation with observations made during the Atmospheric Radiation Measurement (ARM) Program at the Southern Great Plains (SGP) site from January 1999 to December 2001.

What was learned
The eight researchers report that (1) "in the warm season, most SCMs produce more rain events in the daytime than in the nighttime, while the observations have more rain events in the nighttime," (2) "the mean intensities of rain events in most SCMs are much stronger (weaker) in the daytime (nighttime) than the observations," (3) "in the daytime, most SCMs have a higher frequency of moderate-to-strong precipitation events than the observations for both the warm and cold seasons," (4) "for the precipitation events [for which] all the SCMs simulate the total precipitation well, different SCMs achieve the good performance by different combinations of compensating errors between the number of precipitation events and the mean precipitation intensity," (5) "most SCMs produce a spurious precipitation peak around the regime of weak vertical motions," (6) "model underestimation events occur in the strong ascending regimes with negative low-level horizontal heat and moisture advection," while (7) "model overestimation events occur in the weak (in the daytime) or moderate (in the nighttime) ascending regimes with positive low-level horizontal heat and moisture advection."

What it means
The seven deadly sins of the current batch of SCMs look like they will likely be keeping most SCMs out of climate-model heaven for quite some time to come.