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2006-08-17T00:00:00 |
This spatial data set was prepared by spatially interpolating point precipitation
frequency estimates. The precipitation frequency estimates were calculated using regional
L-moment statistics based on an annual maximum data series. The spatial interpolation
of the point precipitation frequency estimates was accomplished with PRISM coupled
with a derivation process called the cascade, residual add-back (CRAB) procedure.
So-called "index flood," otherwise known as the mean annual maximum, grids were created
using PRISM, Parameter-elevation Regressions on Independent Slopes Model, at Oregon
State University. PRISM is an analytical tool that uses point data, a digital elevation
model, and other spatial data sets to generate gridded estimates of climatic parameters.
PRISM is uniquely designed and constantly updated to map climate in the most difficult
situations, including high mountains, rain shadows, temperature inversions, coastal
regions, and other complex climatic regimes. Meanwhile, CRAB is used to actually derive
the precipitation frequency grids. CRAB is a derivation process that utilizes the
strong, linear relationship between a particular duration and frequency (e.g. 50-year
24-hour) and the next higher frequency (e.g. 100-year 24-hour). In fact, within an
L-moment region this relationship is constant and known as the Regional Growth Factor
(RGF). With the CRAB procedure however, a global (all-region) relationship is developed
based on actual observing-site data, then the linear relationship is applied to the
preceding grid (i.e. 50-year 24-hour) to establish a first guess 100-year 24-hour
grid. Knowing regional differences occur, residuals (predicted minus observed) are
calculated for each observing-site and then normalized (divided by) by the preceding
estimate (50-year 24-hour). These (point) normalized residuals are then spatially
interpolated to a grid. The resultant grid is then un-normalized by multiplying it
by the preceding grid to obtain a grid of actual residuals, in inches. The last step
is to simply add the residual grid to the first guess grid to arrive at the final
100-year 24-hour grid. The process, as the term cascade implies, utilizes a previously
derived grid to derive the next grid. So the same process is followed for deriving
the 200-year 24-hour grid, but instead of the 50-year 24-hour grid being used as the
predictor, the new 100-year 24-hour grid is used. For more details about the processes
used to create this grid, please see the project quarterly reports posted here: <http://www.nws.noaa.gov/ohd/hdsc/current-projects/project.html>
or the NOAA Atlas 14 Documention.
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