Scientific Challenges for NOAA Atlas 15 Projections of Heavy Rainfall Design Values

An update to NOAA’s heavy precipitation frequency atlas (NOAA Atlas 14) is currently underway. This update, to be called NOAA Atlas 15 (NA15), will consist of two volumes. Volume 1 will incorporate non-stationarity into estimates of current values. Volume 2 will provide rainfall frequency design values that incorporate potential future climate change. The qualitative basis for incorporating potential future changes is clear. First, there is a strong monotonic statistical relationship between extreme rainfall amounts and atmospheric water vapor content in historical observations. Second, in a warmer world, the saturation water vapor pressure will increase following the Clausius-Clapeyron relationship. However, the quantitative effects are much more complex. NA15 will cover a range of durations from 5 minutes to 60 days and average recurrence intervals (ARIs) out to 1000 years. This talk will address some of the scientific challenges in estimating future changes to design values for the full suite of duration and ARI combinations. In particular, sub-daily durations and very long ARIs pose challenges because of limited availability of climate model simulation data at high temporal frequency and for large model ensembles.