4/7/2010
Profiling of Winter Storms Project
(PLOWS)
NCAR C-130
Data Quality Report
This summary has been written to outline basic
instrumentation problems affecting the quality of the data set
and is not intended to point out every bit of questionable data.
It is hoped that this information will facilitate use of the data
as the research concentrates on specific flights and times.
The following report covers only the RAF supplied
instrumentation and is organized into two sections. The first
section lists recurring problems, general limitations, and
systematic biases in the standard RAF measurements. The second
section lists isolated problems occurring on a flight-by-flight
basis. A discussion of the performance of the RAF chemistry
sensors will be provided separately, as will the respective data
sets.
Section I: General Discussion
1. RAF staff have reviewed the data set for
instrumentation problems. When an instrument has been found
to be malfunctioning, specific time intervals are noted. In
those instances the bad data intervals have been filled in
the netCDF data files with the missing data code of -32767.
In some cases a system will be out for an entire flight.
2. Position Data. Both a Garmin Global Positioning
System (_GMN) and a Novatel Global Positioning System (GGPS)
were used as more accurate position references during the
program. The systems generally performed well. With no
real differences between the two sets of data, it was
decided to only output the Novatel variables in order to
avoid confusion in the data analysis. The algorithm
referred to in (3) below also blends the GPS and IRS
position to yield a best position (LATC, LONC) that
generally removes the GPS spikes.
3. 3D- Wind Data. The wind data for this project were
derived from measurements taken with the radome wind gust
package. As is normally the case with all wind gust
systems, the ambient wind calculations can be adversely
affected by either sharp changes in the aircraft's flight
attitude or excessive drift in the onboard inertial
reference system (IRS). Turns, or more importantly,
climbing turns are particularly disruptive to this type of
measurement technique. Wind data reported for these
conditions should be used with caution.
Special sets of in-flight calibration maneuvers were
conducted on PLOWS flights TF01, RF08 and RF13 to aid in the
performance analysis of the wind gust measurements. The
calibration data identified a systematic bias in the pitch
and sideslip parameters. These offsets have been removed
from the final data set. The time intervals for each set of
maneuvers have been documented in both the flight-by-flight
data quality review and on the individual Research Flight
Forms prepared for each flight. Drift in the IRS
accelerometers are removed using an algorithm that employs a
complementary high-pass/low-pass filter that removes the
long term drift with the accurate GPS reference and
preserves the shorter term fluctuations measured by the IRS.
Both the GPS corrected and basic uncorrected values are
included in the final data set for the purpose of data
quality review. RAF strongly recommends that the GPS
corrected inertial winds be used for all research efforts
(WSC,WDC,UXC,VYC,WIC,UIC,VIC).
4. SPECIAL NOTE: RAF flies redundant sensors to assure
data quality. Performance characteristics differ from sensor
to sensor with certain units being more susceptible to
various thermal and dynamic effects than others. Good
comparisons were typically obtained between the two static
pressures (PSFDC,PSFC), the two standard temperatures (ATRL,
ATRR, ATWH), three dynamic pressures (QCRC, QCFC, QCFRC),
and the two dew pointers (DPT,DPB). Exceptions are noted in
the flight-by-flight summary. The two remote surface
temperature sensors (RSTB, RSTB1) generally functioned well
but a failure of a sensor head heater in the RSTB system led
to excessive drift in these data during the second half of
the program. The backup static pressure system showed
smaller turbulent fluctuations in the signal (PSFRD) and
therefore was selected as the reference pressure (PSXC) used
in all of the derived parameters.
5. Ambient Temperature Data. Temperature measurements
were made using the standard heated (ATWH) and unheated
(ATRR) Rosemount temperature sensors and an OPHIR-III
radiometric temperature sensor. Some initial problems with
ATWH were tied to a bad data system interface card and were
corrected after flight rf03. Performance of all three
“insitu” sensors remained stable throughout the rest of the
project and showed excellent agreement. Due to its fast
response, ATRR was selected as the reference value (ATX)
used in calculating the derived parameters for most flights.
ATWH was used as the reference temperature on a few selected
flights where significant icing conditions adversely
affected the response of the ATRR system.
The OPHIR-III sensor was flown because it is not sensitive to
interference from sensor wetting or icing. Measurements are
derived from near field radiometric emissions in an infrared
frequency band. The integrated sample volume of the unit is
designed to extend roughly 10 meters out from the aircraft. In
actual practice there appears to have been some degradation of
the filters serving to limit this viewing depth. Since the unit
points out roughly horizontally, the increased viewing depth is
not a problem during normal straight and level flight. During
significant right hand turns where the ROLL angle exceeds +15
degrees, however, the OPHIR temperature will be influenced by the
presence of the surface in the field of view. Typical differences
between ATX and OAT during these turns are around +0.1 oC. While
the unit performed quite well and its output was generally well
correlated to the in-situ temperature sensors, it is susceptible
to in-flight calibration drift and intermittent signal drop outs.
The OPHIR-III sensor has a certain amount of drift, primarily
associated with significant and rapid altitude changes. Due to
the limited altitude changes during most of the PLOWS flights,
this drift is fairly minimal in this data set and the link to the
reference temperature (ATX) was not employed. Due the potential
for some residual impact for short intervals after significant
climbs and descents, however, use of the OPHIR data should be
strictly limited to the direct cloud penetrations where the
standard sensors have a problem with sensor wetting.
6. Humidity Data. Humidity measurements were made using two
collocated thermoelectric dew point sensors and one experimental
TDL hygrometer. A comparison of the dew point sensors (DPBC,
DPTC) yielded good correlation in instrument signatures during
the largest portions of the flights when both instruments were
functioning normally. Under conditions where the units had been
cold soaked at high altitude or experienced a rapid transition
into a moist environment, both units showed a tendency to
overshoot. Extended legs in cloud and/or icing conditions showed
some degradation in the data.
Oscillations in the signals were
common under these conditions and restricted sample flows led to
unrealistically high dew point temperatures during some of these
passes. In general, DPBC seemed less susceptible to these
problems and was selected as the reference output (DPXC) for most
flights.
The experimental TDL hygrometer has a separate data
collection system and requires fairly extensive post processing
to produce the final data products. It is not clear what the
quality of the data will be at this time.
Note: Even at their best, the response of the thermoelectric dew
point sensors is roughly 2 seconds. Response times are dependent
upon ambient dew point depression and can exceed 10-15 seconds
under very dry conditions.
7. Surface Temperature Data. Heimann radiometric sensors
were used to remotely measure surface temperature (RSTB & RSTB1
the surface, RSTT cloud base.
A failure of a sensor head heater
on the RSTB system led to excessive drift and oscillations in the
signal during the early stages of the deployment. The sensor was
replaced at the mid-project break. It is recommended that RSTB1
be used as the reference system for this measurement. RSTT also
functioned well. Note that when no clouds are present above the
aircraft the RSTT signal will be pegged at its maximum “cold”
limit of roughly -60 oC.
8. Altitude Data. The altitude of the aircraft was measured
in several ways. A pressure based altitude (PALT,PALTF) is
derived from the static pressure using the hydrostatic equation
and normally using the U.S. Standard Atmosphere, which assumes a
constant surface pressure of 1013mb and a mean surface
temperature of 288 K.
The GPS positioning systems also provide altitude readouts
(GGALT & GGALT_GMN). These outputs normally provide a fairly
accurate MSL altitude based on a ellipsoid model of the Earth
(WGS-84).
A radar altimeter was onboard the aircraft for the project.
The unit functioned extremely well. The standard output
(HGM232)is in ft AGL. Instrument normally turned on after
takeoff and off prior to landing - resulting in short data gaps
at both ends of most data files.
9. Liquid Water Content Data. One hot wire liquid water
sensor (King Probe: PLWCC1) and one optical (PVM-100: XGLWC,
PLWCG) liquid water sensor were mounted on the C-130 for the
program. King probe does not function on the ground. Short data
gaps in PLWCC1 at the start and end of most data files is normal.
Liquid water content is also derived from the concentration and
size distributions measured by some of the optical particle
probes. Most flight data show good agreement between all of the
systems (see exceptions in the CDP probe discussion). Excessive
zero drift in XGLWC led to the derivation of a new variable
(PLWCG) that baselines the zero value against on of the PMS cloud
probes. Note that the PVM-100 also outputs droplet surface area
(XGSFC) and an effective droplet radius (XGRFF). These
measurements are more problematic in mixed phase clouds and RAF
does not recommend using the PVM-100 particle sizing data from
PLOWS to assess particle size.
10. CN Concentration Data (0.01 to 3 um). The calculation
of CN sized aerosol particle concentrations (CONCN) is dependent
upon total particle counts (CNTS) and the measurement of sample
flow (FCN,FCNC). The internal sample flow (FCN) has been
corrected (FCNC) to ambient conditions, only, and not to STP for
the calculation of particle concentration. The special inlet for
this measurement is not susceptible to the normal droplet
splashing effects typically noted in all clouds. RAF believes
that the in-cloud measurements taken with this system are
accurate and represent a good representation on interstitial CN
concentrations.
Note: The location of the inlet on the aircraft and length of the
tubing connecting the inlet to the counter will induce a lag in
the system response to changes in particle concentration.
11. Aerosol & Cloud Droplet Sizing Data. Three PMS 1D
particle probes (SPP100-std, SPP100-mod,CDP) were used on the
project. Some specific details on each of the probes are
summarized below:
SPP100-LWI(FSSP-mod) - The probe is a modified system
(SN - #109) physically designed to reduce droplet
shattering ahead of the optics. The unit was somewhat
noisy in the smaller size bins requiring the 1st 3 bins
to be edited out of the overall concentration data.
While the data are considered to be acceptable it
appears that the probe had difficulty sampling ice
particles in the upper end of the sampling range. The
probe worked better in mixed phase clouds. Under
the conditions targeted by the PLOWS flights, the
particle concentration data and calculated liquid water
content will under estimate the true values of these
variables.
SPP100-LWO(FSSP-std) - The probe is the standard model
FSSP with the flow straightening shroud. This unit
seemed to function well under all conditions,
particularly in the ice clouds encountered at higher
altitudes. In liquid precipitation, some droplets
shattering likely shifts the size spectra toward higher
concentrations of small particles. It is recommended
that the cloud size particle data from this system be
used as a better representation of the conditions than
the other two 1-D probes.
CDP - This probe basically matches the same droplet
size distribution as covered by the SPP100 probes but
has difficulty in sampling ice particles. Under
the conditions targeted by the PLOWS flights, the CDP
particle concentration data and calculated liquid water
content will under estimate the true values of these
variables.
12. Precipitation Sizing Data. Three OAP probes were flown
during the project. Unit one was a standard 2D-C probe with 25
um resolution. The second unit was a standard 2D-P probe with
200 um resolution. Both systems functioned well though out the
entire project. A newly modified 2D-C with 10 um resolution was
added to the payload just prior to departure. The system
functioned well but suffered from a problem with the new optical
diode array. Most images will have “feathered” boundaries on the
back side of the particles. This will be a problem for analysis
software packages attempting to automatically produce size
distributions from this data set.
13. TECO Ozone Data. This is a dual channel system that
cycles between channels every 10 seconds – thus leading to the
stair step nature of the data. Both the raw signal (TEO3) and a
pressure corrected value (TEO3C) appear in the final data set.
The unit was normally turned off prior to landing to avoid
contamination. Short data gaps at the end of each flight are
normal.
14. SPECIAL NOTE: Virtually all measurements made on the
aircraft require some sort of airspeed correction or the systems
simply do not become active while the aircraft remains on the
ground. None of the data collected while the aircraft is on the
ground should be considered as valid.
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Section II:
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Flight-by-Flight Summary
RF01 Bad ADS interface channel on heated temperature probe.
ATWH data bad for entire flight.
Intermittent power drop outs to the OPHIR-III sensor.
data bad for entire flight.
TTWH &
OAT
PVM-100 probe not responding normally. XGLWC & PLWCG data bad
For entire flight.
Excessive drift in one of the down looking remote surface
temperature sensors. RSTB data bad for entire flight.
RF02 Bad ADS interface channel on heated temperature probe.
ATWH data bad for entire flight.
No power to RAF CN Counter system.
flight.
TTWH &
CONCN data bad for entire
Excessive drift in one of the down looking remote surface
temperature sensors. RSTB data bad for entire flight.
Loss of communication between both SPP-100 probes and the ADS
system. No SPP-100 data from 1658 to 1729 CUT.
Uncharacteristic response from top dew point sensor.
data bad from 1658 to 1845 and 1939 to 2022 CUT.
DPTC
Uncharacteristic response from the bottom dew point sensor.
DPBC, DPXC and all humidity related variables bad from 1658 to
1745 CUT.
Radome icing affecting 3-D wind data from 1711 to 1750 and
1956 to 2009 CUT.
RF03 Bad ADS interface channel on heated temperature probe.
ATWH data bad for entire flight.
Power drop out to OPHIR-III temperature sensor.
From 213126 to 213245 CUT.
TTWH &
OAT data bad
Loss of communication between Inertial Reference Unit (IRU)
and ADS data system. No aircraft attitude of 3-D wind data
from 1948 to 2114 CUT.
Icing rate indicator heater not functioning.
RICE response for entire flight.
Uncharacteristic
RF04 Power drop out to OPHIR-III temperature sensor.
from 012606 to 012649 CUT.
OAT data bad
TECO ozone analyzer failed to initiate correctly during preflight. TEO3 & TEO3C data bad for entire flight.
Radome icing affecting 3-D wind data from 030813 to 031445,
040316 to 040807 and 055430 to 061043 CUT.
Uncharacteristic response from bottom dew point sensor likely
due to water ingestion or sensor head icing. DPBC data bad
from 0022 to 0132 CUT.
Uncharacteristic response from the top dew point sensor.
DPTC, DPXC and all humidity related variables bad from 0022 to
0132 & 061447 to 062400 CUT.
RF05 Excessive oscillation in DPBC signal. DPTC selected as
reference input (DPXC) to all derived humidity variables.
Intermittent spikes in OPHIR-III temperature data between
1538 and 1648 CUT.
RF06 Power drop out to OPHIR-III temperature sensor.
from 2436 to 2438 CUT.
OAT data bad
Excessive drift in one of the down looking remote surface
temperature sensors. RSTB data bad for entire flight.
Radome icing affecting 3-D wind data from 2302 to 2312 CUT.
RF07 Uncharacteristic response from bottom dew point sensor likely
due to water ingestion or sensor head icing. DPBC data bad
from 0346 to 0429 CUT.
Uncharacteristic response from the top dew point sensor.
DPTC, DPXC and all humidity related variables bad from 0336 to
0408 CUT.
Radome icing affecting 3-D wind data from 0336 to 0429 CUT.
RF08 Calibration flight for RAF 3-D winds and Wyoming Cloud
Radar (HCR). Lenschow maneuvers and 20 degree circles.
Power drop out to OPHIR-III temperature sensor.
from 164034 to 164040 CUT.
OAT data bad
RF09 Radome icing affecting 3-D wind data from 1801 to 1813 CUT.
Uncharacteristic response form King Probe. PLWCC1 data bad
From 192600 to 192612 and 173417 to 173500 CUT.
RF10 Excessive drift in one of the down looking remote surface
temperature sensors. RSTB data bad from 1628 to 2219 CUT.
RF11 Loss of power to heated temperature probe.
Bad from 2150 to 2210 CUT.
TTWH & ATWH data
Excessive drift in one of the down looking remote surface
temperature sensors. RSTB data bad from 1824 to 2210 CUT.
RF12 Uncharacteristic response from top dew point sensor likely
due to water ingestion or sensor head icing. DPTC data bad
from 025300 to 031238 CUT.
Uncharacteristic response from the bottom dew point sensor.
DPBC, DPXC and all humidity related variables bad from 025300
to 030754 CUT.
Excessive drift in one of the down looking remote surface
temperature sensors. RSTB data bad from 0659 to 1149 CUT.
RF13 Loss of GPS signal to both receivers. No GPS position, ground
speed or altitude data from 2014 to 2022 CUT
Intermittent spikes in OPHIR-III temperature data between
2106 and 2132 CUT.
PVM-100 probe not responding normally. XGLWC & PLWCG data bad
for entire flight.
TECO ozone analyzer failed to initiate correctly during preflight. TEO3 & TEO3C data bad for entire flight.
No power to RAF CN Counter system.
flight.
CONCN data bad for entire
Excessive oscillation in DPBC signal. DPTC selected as
reference input (DPXC) to all derived humidity variables.
RF14 Loss of GPS signal to both receivers. No GPS position, ground
speed or altitude data from 230556 to 231848 CUT.
Pump for RAF CN Counter not turned on.
entire flight.
CONCN data bad for
Excessive oscillation in DPBC signal. DPTC selected as
reference input (DPXC) to all derived humidity variables.
Data recording terminated prior to landing.
RF15 Excessive drift in one of the down looking remote surface
temperature sensors. RSTB data bad from 1821 to 2218 CUT.
RF16 Intermittent spikes in OPHIR-III temperature data between
1728 and 1837 CUT.
Excessive drift in one of the down looking remote surface
temperature sensors. RSTB data bad from 2013 to 2532 CUT.
RF17 Intermittent spikes in OPHIR-III temperature data between
1811 and 1824 CUT.
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