Extreme Precipitation

Some time ago, I saw a poster that showed observed extreme precipitation increasing in time along the east coast and Gulf coast of the US, suggesting increasing extremes due to land falling hurricanes (Ashouri et al, 2012). There is also some supporting analysis of increasing precipitation trends and extremes in the recent National Climate Assessment report (Figures 2.15 and 2.16). To narrow the results to potential hurricane sources, Figure 1 here evaluates the trend of maximum daily precipitation, each season from 1979-2012, where hurricane season is defined as June through November.  Significant trends are seen along the northeast US track as well as some trends along the Gulf coast track in the south east US.

Figure 1 Trend of maximum daily precipitation in each hurricane season from 1979-2012. Trends significantly different from zero at 90% confidence are outlined in white contours.

The MERRA reanalysis is able to reproduce, generally this pattern of increasing extreme precipitation (Figure 2). MERRA's increasing trends in the southeast have a wider area, and in the northeast, the strongest trends  do not extend through the New England states, as observed. Still the reproduction of the trends of such a specialized diagnostic in a relatively coarse reanalysis is noteworthy.


Figure 2 As in Figure 1, except for the precipitation produced in the MERRA reanalysis.

As a further test of these trends, we area average the observed hurricane season maximum precipitation for the North Atlantic states in MERRA and the CPC observations. The interannual variability of the extreme precipitation is well reproduced, though, MERRA's mean value tends to be less than observed. Figure 3 shows increases in time for the northeast, and not just some end point variation caused be recent very large storms (e.g. Irene). though, low anomalies can occur in the recent few years, as well.

Remnants of Tropical Storm Karen produced heavy precipitation over a substantial portion of the Northeast, so that the 2013 season in the northeast will likely also be a positive anomaly (here is some result of that storm). The southeast may not have comparable extreme precipitation in 2013, at least related to tropical storms and hurricanes. We will come back to this as the 2013 hurricane season closes and MERRA is extended through it.


Figure 3 Time series of area averaged extreme precipitation anomalies from CPC gauge observations and MERRA reanalysis. The mean value removed for comparing anomalies is presented in the legend.