Project #2004-109 GOTEX (Gulf Of Tehuantepec EXperiment)

K. Melville and Carl Friehe

NSF/NCAR C-130Q Hercules (N130AR)

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Data Quality Report

Prepared by: Allen Schanot (NCAR/RAF)

8 June 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 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 discussion of the performance of the SABL lidar, GPS dropsondes and RAF chemistry sensors will be provided separately, as will their 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. The flight conditions targeted by the research were extremely detrimental to the performance of many of the standard sensors. Salt deposits from ocean spray tended to accumulate on the sensors and fuselage over the course of each flight. In some instances sensor performance would degrade as the salt accumulations increased. All sensors were washed with fresh water after each flight and the airframe was washed twice during the duration of field operations.

  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 TF01, TF03, RF04 and RF11 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 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. RAF strongly recommends that the GPS corrected inertial winds be used for all research efforts (WSC,WDC,UXC,VYC,WIC,UIC,VIC).

    Note:   This data set was processed using the new pressure correction factors empirically derived from comparisons against the trailing-cone static pressure reference.

  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 or discontinuous shifts in these values due to satellite geometry and aircraft maneuvering. The algorithm referred to in (3) 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, QCFRC), 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. 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). The sensing element from ATRL tended to be slightly unstable showing small amounts of drift when compared to the other two sensors. Differences were less than 0.5 °C and often closer to the standard 0.2 °C considered to be a good comparison. An adjustment to the calibration of ATRL was made for flights after RF04 to account for a specific shift caused by minor damage to the sensing element. ATRR performance remained stable throughout the project so it was selected as the reference value (ATX) used in calculating the derived measurements.

    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 turns where the ROLL angle exceeds ±5 degrees, however, the OPHIR temperature will be influenced by the combination of the increased sampling depth and the vertical gradient in ambient temperature. Typically, differences in ATX and OAT during these turns are around ±0.75 °C. 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. These periods are characterized by a sudden level shift and, in some cases, the loss of response to the fine-scale structure in the temperature profile.

    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, one Lyman-alpha fast-response hygrometer and an experimental TDL laser hygrometer. Two Lyman-alpha sensors were installed at the beginning of the project, but after the failure of one unit on the initial research flight it was determined that the adverse conditions targeted by the research were detrimental to the survival of our spare (stub) units. The remaining flights were conducted with a single RAF sensor. Note that an additional HRT humidity measurement was being made by User-supplied instrumentation. 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. DPBC was used as the reference humidity sensor (DPXC) in calculating all of the derived measurements.

    Lyman-alpha hygrometers are susceptible to in-flight drift in the instrument's bias voltage. Due to this problem, RAF uses a special data-processing technique to remove the bias drift by referencing the long-term humidity values to one of the more stable thermoelectric dew point sensors. Measurements from the RAF and User systems remained well correlated for clear-air sampling. The RAF cross-flow unit (MRLA, RHOLA, DPLA, RHLA) functioned well under all conditions and should be perfectly adequate as the reference high-rate sensor for basic analysis and flux calculations. Note that there was some degradation in the RAF Lyman-alpha signal late in the project.

    A TDL (tunable diode laser) based hygrometer was flown on an experimental basis for this project. The system was originally designed for measuring extremely low absolute humidities at stratospheric altitudes. The path length was shortened for the unit when it was placed on the C-130 to allow it to function at the higher humidities common to the boundary layer and mid to lower troposphere. While the humidity values encountered during this deployment were within the expected operational range of the instrument, a complex pressure calibration and special data processing are required to correct the basic data. At the time of this data release, the TDL data were not fully corrected and have therefore been removed from this data set. We expect a subsequent release of a "corrected" set of TDL data (MRLH) at a later date.

  8. A set of standard upward- and downward-facing radiometers were used to measure shortwave, ultraviolet, and infrared irradiance. It should be noted that all units are hard mounted and that none of the data have been corrected for changes in the aircraft's flight attitude. It was necessary to replace the top-mounted UV sensor after flight RF02.

  9. Thermal temperature chamber experiments have indicated that the Heimann sensors used to remotely measure the surface temperature (RSTB, RSTB1) 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. Some drift is still apparent in the data set. RSTB1 seemed to be the more stable of the two units and is therefore recommended as the reference system for this measurement.

    In addition to their thermal sensitivity, the accuracy of the remote sensing measurement is also dependent upon the total amount of water in the sensing path. In such a moist, marine environment this sensitivity appears as an altitude dependence in the raw surface temperature. Due to the flight profiles, there was insufficient data to formulate a correction algorithm to adjust for this problem. Most of the flight tracks for this program were conducted at low altitudes, so this problem is minimized in this particular data set.

  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 oK. Due to the tropical nature of the research area for GOTEX, better results were obtained by using a mean surface temperature of 305 °K.

    The GPS also provides an altitude readout (GALT). 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 MSL latitude based on a ellipsoid model of the Earth (WGS-84). The signal recorded on the C-130 can still be interrupted during steep turns.

    A radar altimeter (HGM232) was onboard the aircraft for the project. This unit worked well and, due to the fact that most of the research was conducted over a water surface, showed an excellent correlation with the GPS altitude.

    To aid the Users in choosing a common altitude to use in their analyses, 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 mounted on the C-130 at the start of the program. One of the probes (PLWCC1) failed during the first flight. As there were no "in-cloud" objectives for the project, the unit was not replaced for the remainder of the experiment. The remaining PMS King Probe (PLWCC) worked well during the program, but the sensing elements were susceptible to drift resulting from salt accumulations.

  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). NOTE: the internal sample flow (FCN) has been corrected (FCNC) to ambient conditions, only, and not to STP for the calculation of particle concentration. NOTE: the length of the tubing connecting the inlet to the counter will induce a 1-2 second lag in the system response to changes in particle concentration.

  13. Four PMS particle probes (SPP100, SPP200, SPP300, 260X) were used on the project. Some specific details on each of the probes are summarized below:

  14. The RAF is currently testing a small, inexpensive inertial reference unit (CMIGITS) as a potential aircraft attitude backup system to the standard Honeywell Laseref IRU. While the unit was functional during the field project, the data are not yet suitable for use in any of the derived wind calculations. The raw data have been included in the final data set at the request of one of the Principle Investigators for use in their personal research into the performance of similar systems. No other use should be made of these variables (CLAT, CLON, CPITCH, CROLL, CTHDG, CVEW, CVNS, CVSPD, CALT).

  15. The trailing-cone static-pressure reference was only flown during research flight RF11. PCONE data are only valid for that flight.

  16. The RAF Giant Nuclei Impactor was only flown on flights RF02 and RF03. The associated variables (XGN1, XGN2) are only valid for those flights.

  17. 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).


RF01 RF02 RF03 RF04 RF05 RF06 RF08 RF09 RF11
Last update: Fri Jun 11 14:04:12 MDT 2004