unexpected behavior with floattoshort and _FillValue

I have an array of floating point values that I need to convert to
short values using floattoshort.
The array (fdata) doesn't have any missing values, but I define the
_FillValue to be 1.e30, for good measure.
The short array (sdata) that is the target of the floattoshort
function also has a _FillValue defined to be floattoshort(32767),
as I'm trying to avoid the default short value of -99. When I coerce
fdata to sdata using floattoshort, the values that
should be -99 are converted to the _FillValue of 32767.

Similarly, if I create the same floating point array (fdata2), but do
not define an _FillValue (use the default -999), the coercion to
sdata2 using floattoshort works as I expected -- floating point value
of -99.31335 becomes -99 -- not _FillValue.

Since I'm defining my short array to use 32767 as the _FillValue
(instead of the default -99), I expected this would avoid the problem
of floating point values
between -99 and -100 becoming _FillValue when they pass through
floattoshort. I was further surprised that using an _FillValue other
than the
default on the input floating point array made a difference to
floattoshort.

I don't understand why floattoshort gives different results dependent
on the value of _FillValue in the input floating point array. Does
this make
sense to anyone?

(I'm using ncl version 5.0.0 on OSX 10.4)

Here's a simple demonstation:
;;;;;;;;;;;;;;;;;;;;;;;;ncl code;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
fdata=new((/8/),"float",1.e30)
sdata=new((/8/),"short",floattoshort(32767))
fdata=
(/-101.1139,-100.7233,-100.3479,-99.99084,-99.99,-99.31335,-98.99597,-98
.6908/)
print(fdata(5))
sdata=floattoshort(fdata)
print(sdata(5))
print(ismissing(sdata(5)))

fdata2=new((/8/),"float") ;;don't define a missing value for float
array
sdata2=new((/8/),"short",floattoshort(32767))
fdata2=
(/-101.1139,-100.7233,-100.3479,-99.99084,-99.99,-99.31335,-98.99597,-98
.6908/)
print(fdata2(5))
sdata2=floattoshort(fdata2)
print(sdata2(5))
print(ismissing(sdata2(5)))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;

ncl printout::

Variable: fdata (subsection)
Type: float
Total Size: 4 bytes
             1 values
Number of Dimensions: 1
Dimensions and sizes: [1]
Coordinates:
Number Of Attributes: 1
   _FillValue : 1e+30
(0) -99.31335

Variable: sdata (subsection)
Type: short
Total Size: 2 bytes
             1 values
Number of Dimensions: 1
Dimensions and sizes: [1]
Coordinates:
Number Of Attributes: 1
   _FillValue : 32767
(0) 32767
(0) True

Variable: fdata2 (subsection)
Type: float
Total Size: 4 bytes
             1 values
Number of Dimensions: 1
Dimensions and sizes: [1]
Coordinates:
Number Of Attributes: 1
   _FillValue : -999
(0) -99.31335

Variable: sdata2 (subsection)
Type: short
Total Size: 2 bytes
             1 values
Number of Dimensions: 1
Dimensions and sizes: [1]
Coordinates:
Number Of Attributes: 1
   _FillValue : 32767
(0) -99
(0) False

---------------------

Jamie Scott
NOAA/ESRL/PSD
james.d.scott_at_noaa.gov

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Received on Thu Apr 03 2008 - 16:08:35 MDT