Project #2003-109 AIRS Phase 2

Alliance Icing Research Study

J. Hallett, et al.

NSF/NCAR C-130Q Hercules (N130AR)

Data Quality Report

Prepared by: Jørgen Jensen (NCAR/RAF)

8 Oct 2004

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 standard RAF-supplied instrumentation and is organized into two sections. Section I lists recurring problems, general limitations, and systematic biases in the standard RAF measurements. Section II lists isolated problems occurring on a flight-by-flight basis. A number of other instruments were flown in a purely test mode. This discussion does not cover any of those sensors.

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.      The target conditions for AIRS-II often lead to build-up of icing on the aircraft, and this can adversely affect a number of sensors. When analyzing the AIRS-II data set, the users should carefully consider if measurements are affected by icing. There are a number of obvious occasions of sensors affected by aircraft icing, and these are noted in Section II. At other times the effects of aircraft icing may be more subtle, and users should evaluate this possibility.

3.      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 flights TF02 to aid in the performance analysis of the wind gust measurements. The calibration data identified a systematic bias in the pitch and sideslip measurements. 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 flight-by-flight data quality review 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 short-term fluctuations measured by the IRS.

Both the GPS-corrected and basic, uncorrected values are included in the final data set. RAF strongly recommends that the GPS-corrected inertial winds be used for all research efforts (WSC, WDC, UXC, VYC, WIC, UIC, VIC).

4.      A Garmin Global Positioning System (GPS) was used as a more accurate position reference during the program. The system performed extremely well, and the GPS position values should be used for all research efforts (GGLAT, GGLON). There may be occasional spikes in these measurements due to satellite geometry and aircraft maneuvering. The algorithm referred to in (2) above also blends the GPS and IRS position to yield a best position (LATC, LONC) that generally removes the GPS spikes.

5.      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 three standard temperatures (ATRL, ATRR, ATWH), three dynamic pressures (QCRC, QCFC, QCRF), three liquid water sensors (PLWCC, PLWCC1, NLWCC), and the two dew pointers (DPT, DPB). Exceptions are noted in the flight-by-flight summary. The digital static pressure (PSFDC) was selected as the reference pressure (PSXC) used in all of the derived measurements. Due to occasional icing of the radome gust probe and in order to maintain continuity from flight-to-flight, the fuselage pitot dynamic pressure (QCFC, TASF) was used as the reference sensor in calculations for all research flights. The two remote-surface temperature sensors (RSTB, RSTB1) generally functioned well, but there were times when the two measurements differ significantly.

6.      Temperature measurements were made using the standard heated (ATWH) and unheated (ATRR, ATRL) Rosemount temperature sensors along with the OPHIR-III near-field radiometric temperature sensor (OAT). All of the standard temperature sensors performed reasonably well, encountering the usual problems with sensor wetting during cloud passes. ATWH was selected as the reference value (ATX) used in calculating the derived measurements, but the sensor has slightly slower time response that the un-heated sensors. Accordingly ATWH was used as the reference sensor. If flux analysis is to be done, then the recommendation is to use ATRR.

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 cloudy air and more in clear air. 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 experienced some intermittent segments where the system seemed to lose its lock. Segments where direct sunlight affects the sensor have been marked as bad data; this is easily done for obvious segments which may create errors of several degrees, but more subtle segments where the error is closer to a degree have not been removed. Users should carefully evaluate if their segments are affected by sunlight. This is not normally a problem in cloudy air. Because this is not an independent, stand-alone measurement, use of the OPHIR data should be strictly limited to the direct cloud penetrations where the standard sensors have a problem with sensor wetting.

7.      Humidity measurements were made using two, collocated, thermoelectric dew point sensors and an experimental TDL laser hygrometer. Generally speaking, the humidity sensors performed well. As is typically the case, the two dew point sensors (DPBC, DPTC) were set up differently to provide the best coverage under the widest range of ambient conditions. DPTC was set up for fast response, but its dynamic range was more limited. DPBC had a slower response but had the capability of measuring greater dew point depressions. A comparison of the data sets yielded good correlation in instrument signatures during the largest portions of the flights when both instruments were functioning normally. However, some problems with water ingestion occurred which resulted in some sensor drift. Each flight was evaluated on a case-by-case basis to see which dew point sensor performed the best on that particular flight. Except where noted in the flight-by-flight review, DPBC was used as the reference humidity sensor (DPXC) in calculating all of the derived measurements.

8.      A set of standard upward- and downward-facing radiometers was used to measure shortwave, ultraviolet, and infrared irradiance. It should be noted that all units are hard mounted and that none of the raw data have been corrected for changes in the aircraft's flight attitude.

9.      Thermal temperature chamber experiments have indicated that the Heimann sensors used to remotely measure the surface temperature (RSTB, RSTB1, RSTT) are sensitive to some thermal drift. In an attempt to deal with these problems the units were equipped with a temperature-control heater system. Generally speaking, the heater system stabilized the signals fairly well and either RSTB or RSTB1 can be used for the down-looking radiometric temperature. RSTT provided a measurement of cloud base temperature for any cloud layers higher than the C-130 flight level. Note that clear skies are characterized by a flat line response with the unit pegged at its extreme lower limit (approximately -60° C).

10.  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 1013 mbar and a mean surface temperature of 288° K.

A radar altimeter (HGM232) was onboard the aircraft for the project. This unit worked well.

The GPS also provides an altitude measurement (GGALT). The military has removed the electronic dithering of this signal that used to prevent its use by research aircraft. This output now provides a fairly accurate altitude (MSL) based on an ellipsoid model of the Earth (WGS-84). The signal recorded on the C-130 can still be interrupted during steep turns.

To aid the Users in choosing a common altitude to use in their analyzes, RAF now calculates a "reference" altitude (GGALTC). The output is based directly upon GGALT and uses basic smoothing techniques to fill gaps with inertial altitude data when the GPS signal is interrupted.

11.  Two hot-wire liquid water sensors were used on the C-130 at the start of the program. The PMS King Probes (PLWCC, PLWCC1) worked well during the program, but the sensing elements were susceptible to random damage caused by the large precipitation encountered during the routine cloud penetrations. A comparison of the two units yielded a good correlation in instrument signatures and only small differences in absolute value. Special note should be made of the fact that both these instruments are calibrated for a specific range of aircraft speeds. Small changes in the baseline are apparent with airspeed changes, as are small zero offsets. Each cloud penetration will require a baseline adjustment with the relative change providing the sampled liquid water content. Due to the nature of this sampling technique, it should be clear that water contained in ice particles will not be observed. This fact should be taken into account when comparing data from these sensors with the calculated liquid water content obtained from the optical particle probes.

A Nevzorov total water probe was included in the research instrumentation package on an experimental basis. The unit has separate channels designed to differentiate between liquid water (NLWCC) and combined total ice/water content (NTWCC). The unit responded well to the presence of water, and the differences between the two channels appeared to correspond to the presence of solid particles, as evidenced by other sensors. However, both channels exhibited short periods of large baseline offset that were strongly dependent upon changes in the aircraft's true airspeed and/or altitude. The offset had to set manually during flight; when changing altitude the offset would have to be re-set, and this was not always possible. Accordingly there may be periods when the Nevzorov data has an incorrect offset, and this would also imply that cloudy values be smaller than they should have been. There is no way to recover the true offset from these periods. Calibration for this unit comes from the manufacturer and cannot be confirmed by any process available to the RAF. Note that a comparison between these data, the King Probe measurements and the equivalent liquid water content calculated from the SPP-100 probe can vary significantly, depending on the cloud type and droplet spectra. Caution should be used in mixed phase clouds where the SPP-100 probe can't differentiate between ice and water. The Nevzorov probe was not turned on until after takeoff, and it was turned off prior to landing. This probe was also occasionally zeroed in flight. The probe also suffered from 'stickiness' in the bearing and vane system; this system was designed to always having the probe directed into wind, regardless of the aircrafts attack angle. The probe could get stuck in certain positions, and this could result in offset changes and, presumably to a lesser extent, gain changes.

12.  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 length of the tubing connecting the inlet to the counter will induce a 2-second lag in the system's response to changes in particle concentration.

In the past, droplet shattering during cloud penetrations would sharply increase the number of counts and falsely increase indicated CN concentrations by several orders of magnitude. A new rear-facing inlet system has been in use for several projects, specifically designed to avoid this problem and provide good interstitial, in-cloud aerosol measurements.

13.  Five standard particle probes (SPP100, modified SPP100, 2D-C, 2D-P, 2D-HVPS) were used on the project. Some specific details on each of the probes are summarized below:

o        SPP-100S
The standard SSP100 (CONCF_LPO) cloud droplet probe functioned normally. Regular servicing and recalibration of the sensor optics provided good documentation for data processing. Being an optical scattering probe, the SSP100 has no way of distinguishing between ice particles and water droplets. Therefore, the liquid water content calculated from this probe (PLWCF_LPO) should be used with caution in this application. The probe tends to undercount droplets in the lowest sizing bins. Care should be used in using data from channels 1 and 2 in the particle size distribution. This unit is equipped with a shroud around the optics that is intended to strengthen the overall structure of the probe and to straighten the flow through the sample volume. In the presence of large droplets or frozen particles, measured particle size distributions can be skewed toward high concentrations of smaller particles due to shattering of the large particles on this shroud. The SPP-100 was only flown on the last flight, RF16.

o        SPP-100M
The modified German SPP100 (CONCF_LPC) functions in pretty much the same way as the standard unit and exhibits most of the same sampling problems. The shroud has been removed from this unit, however, avoiding the problem with large-particle shattering. This probe was flown on all flights. The probe was calibrated using water drops prior to flights. Glass bead calibrations were done late in the flight period. The values for PLWCF_LPO (the standard EOL probe) and PLWCF_LPC (the modified German probe) were in close agreement during the last flight in water clouds. SPP-100M was flown on all AIRS-II flights.

o        SPP-200
The SPP200 aerosol particle probe functioned extremely well during the project. The probe being flown has been modified in order to directly measure the sample flow through the instrument. These data, recorded as PFLWC_LWI, have been used in the calculation of particle concentrations to provide a more accurate measurement of aerosol concentrations. Due to the sampling technique employed by this probe, it is not suitable for use in clouds where droplet break-up may lead to errors. The expansion of the number of sizing bins has exceeded the true capability of this probe. Due to this fact, and the tendency for excessive noise in the lowest size bins led to the removal of data from bins 1 to 5 from the calculation of total particle concentration. This probe was not flown on the last flight (RF15) of the project in order to compare the two SPP100 probes.

o        260X
While the range of this probe is specified to be 10 to 640 µm in 10 µm increments, it has some problems sampling the smaller sizes when mounted on an aircraft. NCAR data processing uses the Baumgardner correction algorithms (Baumgardner, D., Korolev, A., 1997: Airspeed Corrections for Optical Array Probe Sample Volumes. J. Atmos. Oceanic Tech.,14, 1224-1229) to correct the 260X data for these problems. Effectively this changes the range of the unit to 50 to 640 µm when it is mounted on the C-130. The unit functioned well throughout the program.

o        2D-C
The two-dimensional imaging probe performed extremely well throughout the program. New data processing techniques convert the image data into 1-D equivalent particle size distribution and total particle concentration data that have been included in the general data set. The actual image data have been archived separately. Access to the images will require the use of specialized software routines that may necessitate direct assistance from the RAF staff.

14.  The SPEC Cloud Particle Imager (CPI) was operated on all flights. This microphysical digital camera has a 1024x1024 pixel array, with an effective pixel size of 2.3 µm. The camera was operated with intelligent software to set the particle detection threshold (i.e., variable- as opposed to fixed-detection threshold). The CPI developed two problems during IDEASA-3, but mostly gave good images. A camera line driver went intermittently faulty, and this led to the generation of "squiggle" particles on some images. At other times the camera would intermittently trigger and take images when no particles were present in the detection system.

RAF has analysis software for the display of CPI images (cpidisplay), and this software is freely available from RAF (contact Janet Scannell). SPEC also sells commercial software for the analysis of CPI images. RAF does not, at present, provide derived data from the CPI data, such as particle concentrations and spectra; this is left up to the individual user.

15.  The TECO Model 49 ozone analyzer is a slow-response instrument which provides 10-second, averaged values. The corrected output (TEO3C) has been adjusted for variations in sample pressure and temperature. Generally, the instrument performed well. Intermittent spikes do occur but are fairly isolated and obvious to any User.

16.  Data recording typically begins well in advance of the actual aircraft takeoff time. 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.


Section II: Flight-by-Flight Summary

Note: All times listed below are Coordinated Universal Time (UTC).












Last update: Wed Oct 13 17:35:11 MDT 2004