Background
The authors write that "about half of the human population lives in urban areas (Martine et al., 2007) in contrast with only about 10 percent a century ago (Grimm et al., 2008)," and they say that "changes in extreme precipitation as the climate warms may pose challenges for stormwater management in urban areas, because most stormwater infrastructure was designed under the assumption of stationary climate that is 'dead' as argued by Milly et al. (2008)."

What was done
To better assess both the likelihood and significance of this potential problem, Mishra et al. compared precipitation output from all regional climate models (RCMs) that participated in the North American Regional Climate Change Assessment Program (NARCCAP) - which is described by Mearns et al. (2009) - with observations made at 100 urban U.S. weather stations that all had data for the period 1950-2009. This work involved two distinct RCM simulations: one forced by output from the National Center for Environmental Prediction/Department of Energy (NCEP/DOE) reanalysis (Kanamitsu et al., 2002) for the period 1979-2000, and one forced by selected global circulation models (GCMs) that provided RCM boundary conditions for the period 1968-2000.

What was learned
"For most urban areas in the western and southeastern U.S.," in the words of the three scientists, "the seasonality of 3-hour precipitation extremes was not successfully reproduced by the RCMs with either reanalysis or GCM boundary conditions," since "the RCMs tended to predict 3-hour precipitation maxima in winter, whereas the observations indicated summer." They also report that the RCMs "largely underestimated 3-hour precipitation maxima means and 100-year return period magnitudes at most locations across the United States for both reanalysis and GCM boundary conditions." And for both 3- and 24-hour annual precipitation maxima, they say that "RCMs with reanalysis boundary conditions underestimated interannual variability, while they "overestimated interannual variability with GCM boundary conditions."

What it means
With respect to the ultimate utility of the RCM projections, Mishra et al. say that performance deemed acceptable for stormwater infrastructure design was only adequate at about 25% of the urban areas, while they state that, regardless of boundary conditions, "RCM-simulated 3-hour precipitation maxima at a 100-year return period could be considered acceptable for stormwater infrastructure design at less than 12% of the 100 urban areas [italics and bold added for emphasis, which surely seems needed in this specific instance]."

And that about says it all: there is still a long, long way to go before RCMs are likely to be telling us anything of value. Right now, in fact, they could well be considered to be dangerously misleading.

References
Grimm, N.B., Faeth, S.H., Golubiewski, N.E., Redman, C.L., Wu, J., Bai, X. and Briggs, J.M. 2008. Global change and the ecology of cities. Science 319: 756-760.

Kanamitsu, M., Ebisuzaki, W., Woollen, J., Yang, S.K., Hnilo, J., Fiorino, M. and Potter, G. 2002. NCEP-DOE AMIP-II reanalysis (R-2). Bulletin of the American Meteorological Society 83: 1631-1643.

Martine, G. et al. 2007. State of World Population 2007: Unleashing the Potential of Urban Growth. United Nations Population Fund, New York, New York, USA.

Mearns, L.O., Gutowski, W., Jones, R., Leung, R., McGinnis, S., Nunes, A. and Qian, Y. 2009. A regional climate change assessment program for North America. EOS: Transactions, American Geophysical Union 90: 311.

Milly, P.C.D., Betancourt, J., Falkenmark, M., Hirsch, R.M., Kundzewicz, Z.W., Lettenmaier, D.P. and Stouffer, R.J. 2008. Stationarity is dead: Whither water management? Science 319: 573-574.