To increase public awareness of extreme precipitation
events, it is important that the severity of
precipitation events be translated into meaningful
terms that can be understood easily by the public and
decision makers. ‘‘Return period’’ or ‘‘average recurrence
interval’’ (ARI) has been used in the hydrological
community to depict the rarity of flood events for
decades. An event with ARI of 20 yr means the probability
of occurrence in any given year is 1 in 20 or 0.05.
This should not be interpreted as only one such event
occurring in 20 yr; a 20-yr ARI event in one year does
not preclude the same kind of event the next year. The
National Weather ServiceHydrometeorologicalDesign
Studies Center (HDSC) has updated its series of rainfall
frequency atlases in terms of ARI (e.g., 100-yr 24-h
rainfall depth) in recent years for regions of United
States (Bonnin et al. 2011), except for the states in the
Northeast and Northwest and Texas. This updated atlas,
known as NOAA Atlas 14, not only provided updated
information but also established clearer terminology.
Near-real-time and prediction maps in ARI are being
produced for the contiguous United States (CONUS)
from radar and gauge precipitation estimates together
with historical ARI statistics from Atlas 14 (http://
metstat.com; Parzybok et al. 2011). However, since
the NOAA atlas is station based, uses point precipitation
frequency estimates, and only covers the
United States and selected Pacific Islands, near-realtime
global precipitation ARI maps are still lacking. A
product such as the TRMM near-real-time 3B42RT
data is a good candidate to fill the gap in this regard.
Obviously, the utility of 3B42RT for climate purposes is
still limited because of the short data record. However,
theARI maps combined with available local land-based
data can provide much-desired coverage for warning of
unusual heavy rain events in many rural areas and
developing nations that lack dense surface gauge and
radar observations. Additionally, TRMM rainfall ARI
maps over the ocean will provide useful scientific
information regarding the response of rainfall to climatic
fluctuations such as El Niño–Southern Oscillation.
As the global rainfall satellite record grows in
length from the merging of TRMM and GPM data in
the near future, the usefulness of such rainfall ARI
maps will become increasingly important for climate
change studies.