The files in the subdirectories off of this directory contain Precip
Accumulation ("rain" or "precip") scan files for S-Pol in TiMREX. The
individual sweepfiles are in DORADE format, and have been created
after applying all known corrections to the S-Pol TiMREX data.
Corrections have included applying a bias to Zdr and corrections to
some of the raw S-Pol covariances to remove a numeric scale-wrap
issue.
The input files for precip generation were required to be in DORADE
format, and were derived from the original S-Pol data files after
doing the following:
-- applying an appropriate bias correction to Zdr
-- correcting for a numeric scale-wrap problem in the original data
-- re-scaling KDP so the it appeared in the DORADE files with a
scale factor of 0.01 and an offset of 0.00 (required to use the
legacy precip accumulation code)
The final precip accum files vary from original field precip accum
files in a number of ways:
-- a bias-corrected Zdr was used in all calculations
-- accumulations were computed for a .75-degree beam spacing, as
opposed to the 1-degree spacing used in the field
-- the time interval between basic scans was changed from the
original field value of 10 minutes to an interval of 7.5 minutes,
now more closely matching the actual interval between low-level
SUR scans.
-- higher surveillance tilts were used than in the field (original
computations used a .5-degree elevation for the SUR, whereas the
current computations used a mix of tilts at either 1.1 or 1.5 deg
elevation); the .5-degree scan was eliminated as being too prone
to clutter effects.
-- daily accumulations ("storm" accumulations) were computed from
08 UTC and for a period of 24 hours
Three subdirectories are available for each day of data:
basic the estimated accumulation or rain rate over a single
base period (7.5 minutes)
hourly the estimated accumulation for an hour
storm a running total of all the basic accumulations from
08 UTC on the indicated day, through the next 24 hours.
Five different precip accumulations are computed:
DAC estimated accumulation based upon reflectivity, and implicitly
using assumed droplet size distribution information (similar to
NEXRAD rain rate, but with modified coefficients)
ZAC estimated accumulation based upon reflectivity and differential
reflectivity, allowing dynamic modification of the assumed droplet
size distribution
KAC estimated accumulation based upon specific differential phase
HAC a hybrid estimate; estimated accumulation based upon selection
of "best" estimate, with reflectivity used as a selection
criteria (uses a DAC, ZAC, or KAC estimate, depending upon
the value of refectivity)
NAC modified hybrid estimate, using the NSSL criteria; very similar
to HAC
Note that for different conditions, arguments can be made that one
estimate should be more robust than another. For example, the KAC
estimate should be somewhat immune to blockage and clutter effects.
The preference for one particular estimate over another is a matter of
informed decision, best left to the scientist/analyst.
Precip accumulations were derived by Bob Rilling with software support
by Dennis Flanigan, both of NCAR EOL.
8-Jan-2009