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A Comparison of T-Rex Avionics and Research In Situ Temperatures to Radiosondes
MJ Mahoney/JPL and Julie Haggerty/NCAR
Initial Page:
October 31, 2007
Last Revision: December 04, 2007
The NSF/NCAR GV (NGV) Microwave
Temperature Profiler (MTP) did not
participate in the
T-Rex field campaign. However, the MTP data analysis software was used
to assess the accuracy of the T-Rex in situ avionics (Tavi) and research (Tres) temperature
measurements.
This web page was created to summarize the results of our comparisons
with radiosondes launched near the NGV flight track. This included both
normal WMO radiosondes and radiosondes launched by Leeds University in
support of T-Rex. We find that
both Tres and Tavi have substantial warm biases
with respect to
raodiosondes launched near the NGV flight track. While Tres has the
largest warm bias, we also find that the Tavi warm bias is very
significantly pressure altitude dependent.
Introduction
Because radiosondes provide well-characterized temperature profiles of
the Earth's atmosphere and because research flights often pass near
radiosonde (RAOB) launch sites, JPL Microwave Temperature Profiler
(MTP) measurements at flight level are
generally calibrated against them; they are also used to evaluate the
MTP
performance away from flight level. Because there are known biases
during T-Rex between the NSF/NCAR GV (NGV) avionics outside air
temperature (Tavi) and the
completely independent research temperature
(Tres) measurement, we were
asked if the MTP data calibration/analysis
software could be used to evaluate the accuracy of Tavi and Tres. As
shown in Figure 1, the NGV
flew near several WMO/NWS radiosonde launch sites on its transit flight
from Jeffco airport and the study region near Independence, CA.
Figure 1. The NGV flight track
for the T-Rex flight on 20060314. Two flights took a more southerly
transit track.
These included radiosonde launch sites at Denver, CO (DNR), Grand
Junction, CO
(GJT), Salt Lake City, UT (SLC), Flagstaff, AZ (FZG), Albuquerque, NM
(ABQ), Desert Rock, NV (DRA), Reno, NV (REV), and Elko, NV (LKN) and
Edwards AFB, CA (EDW). In addition, the Leeds University launched
radiosondes from Independence, CA (INCA), which we obtained from the
project data archive. Both sets of radiosondes have an accuracy of 0.3
K.
Steps In the RAOB Comparisons
Sampling INCA Soundings
Because the INCA soundings had as many as 3000 levels, we used an
existing MTP program (RAOBconvert) to reduce the number of levels to
<153 while maintaining good fidelity to the original fully sampled
profile. The INCA
soundings have the advantage that the NGV spent much of its flight time
close to Independence, CA ( INCA in Figure
1), so it would be expected
that this would
provide the nearest (and therefore best) comparisons for calibration.
Determine Times of NGV Closest Approach to RAOB Launch Sites
Before comparisons between aircraft temperature measurements and RAOBs
can be made, we need to determine the times of closest approach of the
NGV to the RAOB launch sites. The MTP software includes a tool for
doing this, but before this could be done ASCII text files had to be
generated from the netCDF archive files. This was done for us by Dick
Friesen; these files are based on the raw aircraft data -- no
temperature corrections have been applied to the data. Data in these
files was provided every 10 seconds of flight time.
UTC
ATRL AT_A GGALT
GGLAT GGLON PALT
PITCH PSFC
ROLL TAS_A Time
18:10:04
5.22977 3.27061 1732.84 39.9154 -105.128 1676.13 -0.799787 827.372
-0.675202 0 6147
An example of the first line from the 20060314 flight is shown above.
Most of the parameters shown in the header line have obvious meanings.
ATRL=Tres is the research
temperature and AT_A=Tavi is
the avionics
temperature. Note that the true air speed (TAS_A) is 0 because the NGV
is on the ground. The MTP software would read these files and look for
times of closest approach to any of the RAOB sites mentioned above by
using a great circle calculation every 10 seconds. Every time a closest
approach was seen relevant information (date, time, distance, site
name, etc.) would be written to a file. When this was done for all the
flights, 648 closest approaches were logged. This is an order of
magnitude greater than what we normally find; this is a result of the
many orbits flown near Independence, CA.
Calibrate Tavi and Tres Against All The Closest Approaches
As a first pass we used MTP RAOB comparison software especially adopted
for the NGV to process all 648 closest approaches. Before discussing
these results, we must point out that doing these comparisons well
requires a great deal of care to ensure that there is not excessive
aircraft pitch or roll at the time of the comparisons (or any temporal
variation in them). For the T-Rex comparisons we averaged three samples
(30 seconds of data). During this time we required that the aircraft
altitude changed by <50 m to minimize the effects of a non-zero
lapse rate. The result of these requirements to reduce the number of
comparisons from 648 to 498.
We do not make comparisons with a single RAOB as both temporal and
spatial RAOB interpolation must be done. Historically the MTP software
handled both of these, but the spatial interpolations require a lot of
extra work. In recent years we have done only temporal interpolations,
but with care to assess the atmospheric variability. That is, we not
only examine the RAOBs launched immediately before and after the time
of closest approach of an aircraft to a RAOB launch site to see how
much the temperature has changed at flight level. We also examine those
launch two before and two after for variability.
Figure 2. A comparison of Tavi
with two interpolated Desert Rock, CA, radiosondes.
Figure 2 shows a comparison of
the NGV Tavi temperature
(center of horizontal white line at 9 km
altitude) with two Desert Rock, CA, (DRA) radiosondes launch on
20060414 at 1200UT (yellow trace) and 20060417 at 1200UT. The
comparison is done using the interpolated radiosonde (white trace)
corresponding to the time of closest approach (74.27 ks on April 15,
2006. The NGV was 112 km from DRA at the time of the comparison. Even
though the RAOBs used cover a span of three days, generally the
comparisons are done within hours of the radiosonde launch. This
particular comparison was kept because the comparison was made in the
troposphere which shows very little temporal variability. Had the NGV
been in the stratosphere, the comparison would have be rejected due to
the apparent gravity wave activity. With these and other precautions,
we found using all the 648 closest approaches that:
Tavi - Traob
= 1.34 +/- 0.12
Tres - Traob
= 2.18 +/- 0.12
Tavi - Tres
=-0.85 +/- 0.02
where Traob is the
interpolated RAOB temperature at flight level at the
time of the comparison. We see clearly that compared to radiosondes
Tres has a statistically
significant warm bias of 2.18 K, while Tavi
has a warm bias of 1.34 K. This would have been the end of the story,
but generally some editting is done during the comparisons if there are
objective reasons for doing so. One particular concern is that any one
RAOB site might have several entries in the closest appraoch list
because the aircraft was doing a lot of manuevering. This was certainly
the case near INCA.
In addition, when the first 498 RAOB comparisons were made, there were
occasions during level flight
when differences in excess of 10 K were observed. When the archived
netCDF
files were examined, it was clear that the large differences occurred
because gravity waves (GWs) were present. The wavelength of the GWs was
about 12 km, based on a period of one minute and an aircraft speed of
200 m/s. Including such data could degrade the quality of the
comparisons, so this had to be taken into account. While GWs were not
expected near the NWS launch sites, they were generally much farther
from the NGV, which can also compromise the quality of the comparsions.
The need for closer examination was driven home when comparisons were
done for individual flight days. When we evaluated several metrics such
as bias, rms and coefficient of variation (COV) with range and
temperature threshold, we found there was very significant day-to-day
variation. One culprit was obviously the stratospheric gravity wave
activity near INCA and possibly other RAOB launch sites.
Remove All Comparisons with Stratospheric Variability
The next step was to remove all soundings showing any temporal
variability, which tended to limit the INCA comparisons to the
troposphere only. This resulted in 151 comparisons (down from
498). The overall results for 151 comparisons was:
Tavi - Traob = 1.21 +/- 0.12
Tres - Traob = 2.37 +/- 0.12
Tavi - Tres =-1.17 +/- 0.02
which was not that significantly different from our original 498
comparisons. This did improve the comparison metrics from day to day,
but there were
still day-to-day bias differences begging for explanation.
We were concerned that this might be a result of overweighted use of
the INCA soundings, combined with the impact of NWS soundings generally
being at much greater range. To evaluate this we used only INCA
soundings. One day (20060415) showed some negative biases, which we did
not expect. We had included some stratospheric comparisons on this day
because the before and after soundings were very similar, but there was
some variability. So we removed all the stratospheric sounding and
received a big surprise. We had not understood how the Tavi-Tres bias
could be so well behaved with range and yet there could be strong
day-to-day
variability. Using only tropospheric comparisons on 20060415, we found
that the bias between Tavi and
Tres doubled from about 1 K to
2 K.
Clearly there was a pressure altitude dependence for Tavi-Tres. However
since the bias of Tres-Traob
hadn't changed, the only conclusion could
be that Tavi was pressure
altitude dependent.
The Bottom Line
Given this smoking gun, we went back to the second cut of 151
comparisons using all the RAOB sites, and studied the biases in two
altitude regimes: one using comparisons with pressure altitude from 7.5
to 9.5 km, and one from 11.5 to 13.5 km. The results are shown in
Figure 3, where the first range
is designated 8.5 km (the average layer height) and the other 12.5 km.
Figure 3. RAOB
comparison for two altitude layers with a mean difference of 4 km.
In Figure 3 the solid lines
are the 12.5 km comparisons between Tavi-Traob
(red) and Tres-Traob
(blue), and the dashed lines are the corresponding 8.5 km comparisons.
The Tres-Traob (blue)
comparisons at 12.5 and 8.5 km are not
statistically significant, because if the error bars at each altitude
were shown they would overlap. However, the Tavi - Traob comparisons are
statistically significant.
The bias goes from ~1 K at 8.5 km to more than 2 K at 12.5 km. Tavi has a problem. While not as
problematic as Tavi, Tres is 2.5 - 3.0 K too warm.