This summary has been written to outline basic instrumentation problems affecting 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 detailed discussion of the performance of the TDL Laser Hygrometer will be provided separately by Bruce Gandrud and appended to this document.

Section I: General Discussion

1. Some minor problems with the RAF airborne data system (ADS) occurred during the project. In some circumstances the entire onboard system crashed resulting in a complete loss of data for short intervals. In those instances the data gaps have been filled in the netCDF data files with the "missing_value" of -32767.

   The flights from JeffCo to Churchill stopped in Winnepeg to clear customs. Typically the data system was left running during the time on the ground, and the resulting data files include both flight legs in a single file. The exception to this practice was flight RF16A,B where repairs were made to the aircraft in Winnepeg, and the time interval was too great to keep the ADS system running.

2. 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, and 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 RF04 and RF26 to aid in the performance analysis of the wind gust measurements. The calibration data identified a systematic bias in pitch and sideslip. 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 Reports prepared for each flight.

   As an additional enhancement to this data set, a second-pass correction was applied to the wind data using position and ground speed updates provided by the Global Positioning System (GPS). Both the GPS-corrected and basic uncorrected values are included in the final data set. RAF strongly recommends that the GPS-corrected winds be used for all research efforts.

   Two sets of vertical winds were also calculated (WI, WIC). The two measurements are calculated using different aircraft vertical velocities. During flight level changes, the WIC values will be significantly better.

3. A Trimble 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 (GLAT, GLON).

4. 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), three standard temperatures (ATRL, ATFH, ATWH), two dynamic pressures (QCRC, QCFC), two liquid water sensors (PLWCC, PLWCC1), 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 differed significantly. A third, experimental TDL laser hygrometer system was also flown, and preliminary results showed good correlation with the standard dew pointers. Some issues related to pressure calibration errors at low altitudes are being resolved and should be corrected in a subsequent data release.

   Various derived measurements require input from one or more of the standard sensors. Reference sensors are selected for each project, depending on a review of their performance. Reference sensor designations can be found in the project's Aircraft Variable List.

5. One of the key inputs to most of the derived measurements is aircraft true airspeed (TASX) which is calculated using dynamic pressure. Normally the radome dynamic pressure (QCR, QCRC) would be used as the reference input, because the high-rate characteristics (10 Hz) from the radome gust probe system are better than the other options. Due to occasional problems with icing of the radome, however, the decision was made to use the fuselage dynamic pressure (QCF, QCFC) as the reference sensor in the standard low-rate data set. The two systems are well matched, so this change will not affect the overall accuracy of the data.

6. Temperature measurements were made using the standard heated (ATFH) and unheated (ATRL, ATRR) Rosemount temperature sensors. All of the standard temperature sensors performed reasonably well, encountering the usual problems with sensor wetting or icing during cloud passes. A comparison of the data sets yielded good correlation in instrument signatures and only small differences in absolute value (±0.2 °C) throughout the program. In-flight icing damaged both of the unheated sensors during the project requiring that both units be replaced. The unheated sensors tend to be faster, so either ATRL or ATRR were used as the reference temperature (ATX) whenever possible. For those flights where icing was encountered, the heated sensor (ATWH) was used as the reference.

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. DPBC was set up for fast response, but its dynamic range was more limited. DPTC was a little slower 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. Each flight was evaluated on a case-by-case basis to see which sensor was functioning better on that particular flight. The selection of a reference humidity sensor (DPXC) for use in calculating all of the derived measurements was selected accordingly.

8. 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. While the humidity values encountered during this deployment were in the normal operational range of the instrument, the bulk of the research flights were conducted at much lower altitudes. A fairly complex pressure calibration is required to correct the basic data. At the time of this data set's release, the pressure calibration curve had not been established for ambient pressures greater than 825 mbar. While we fully expect a subsequent release of a 'corrected' set of TDL data (MRLH) at some later date, data from the current release should not be used at ambient pressures greater than the specified value. For those intervals where the ambient pressure was within the valid range of the pressure calibration, the TDL output compared very well with the dew point sensors and exhibited a greatly-improved response time.

9. Thermal temperature chamber experiments have indicated that the Heimann sensors (RSTB, RSTB1), used to remotely measure the surface temperature, are sensitive to some thermal drift. In an attempt to deal with these problems, the units were equipped with a temperature-control heater system. In general, the heater system stabilized the signal fairly well. Some drift is still apparent in the data set, however, and the cyclical thermal signature of the heater system (TRSTB) can sometimes be seen in the system output when the surface temperature is fairly uniform. Total errors from the heater-induced drift are limited to ±0.3 °C. RSTB1 seemed to be the more stable of the two units and exhibited better accuracy in the regular, single-point, precision tests run during the project. RSTB1 is therefore recommended as the reference system for this measurement.

10. The altitude of the aircraft was measured in several ways. The primary measurement (PALT, PALTF) is derived from the static pressure using the hydrostatic equation and the U.S. Standard Atmosphere which assumes a constant surface pressure of 1013 mbar and a mean surface temperature of 288K. For this project, the surface temperature has been decreased to 268K to provide a more accurate representation of an Arctic atmosphere.

    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 calculated pressure altitude.

    The Inertial Reference System (IRS) outputs a measurement of altitude (ALT) by combining static pressure measurements with vertical accelerations. Because these data come directly from the IRS, RAF was unable to apply the same correction to the data for operation in an Arctic atmosphere. While this output has been included in the final data set, it should not be used in the scientific analysis.

    The Global Positioning System (GPS) also provides an altitude readout (GALT). Prior to 1 May 2000 the GALT signal had been "de-tuned" by the military and exhibited erratic oscillations of ±100 M. To avoid confusion, this measurement has been removed from the final data set.

11. Two hot-wire liquid-water sensors were used on the aircraft during the program. The PMS King Probes (PLWCC, PLWCC1) worked well, and a comparison of the two units yielded a good correlation in instrument signatures and showed only small differences in absolute value. Special note should be made that both of these instruments are calibrated for a specific range of aircraft speeds. Small changes in the baseline are apparent with speed 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.

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. If there are any questions about segments of the CONCN data, the flows should be examined to determine if the pump malfunctioned or if an obstruction in the inlet limited particle sampling.

13. Four PMS particle probes (FSSP-100, FSSP-300, PCASP, 260X) were used on the project. Some specific details on each of the probes are summarized below:

    • FSSP-100
      The FSSP-100 cloud droplet probe functioned extremely well. Weekly servicing and re-calibration of the sensor optics provided good documentation for data processing. Being an optical scattering probe, the FSSP has no way to distinguish between ice particles and water droplets. Therefore, the liquid water content calculated from this probe (PLWCF_LPI) should be used with caution in this application.

    • FSSP-300
      The FSSP-300 aerosol probe covers a range of particle sizes that bridges the gap between the true aerosols and the smaller droplets (0.3 to 20 µM). Like all 1-D optical probes, however, it has no way to distinguish between aerosols, ice or water.

    • PCASP
      The PCASP aerosol particle probe functioned extremely well during the project. This particular probe has been modified to directly measure the sample flow through the instrument. These data, recorded as PFLWC_RPO, have been used in the calculation of particle concentrations to provide a more accurate measurement of aerosol concentrations.

    • 260X
      While the 260X probe's range 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 alogrithms to correct the 260X data for these problems. (See Baumgardner and Korolev Reference below.) Effectively this changes the range of the unit to 50 to 640 µM when it is mounted on this aircraft. The unit functioned well throughout the program, but most cloud penetrations were limited. This has resulted in a very small sampling of droplets large enough to trigger the new minimum threshold on this precipitation probe.

      Like the FSSP probe, the 260X has no way to distinguish between ice and water. Therefore, the liquid water content calculated from this probe (PLWC6_LPO) should be used with caution in this application.

14. A set of upward- and downward-looking radiometers was deployed on the aircraft for the measurement of downwelling and upwelling ultraviolet (UV), short wave and infrared (IR) radiation. RAF recently implemented a processing algorithm that removes the effects of aircraft attitude from the upward-looking shortwave radiometer data. Users should note that the uncorrected upward-looking shortwave radiometer's variable name is SWT, while the corrected, upward-looking shortwave variable name is SWTC. A description of this correction process is provided in RAF Bulletin No. 25.

    The algorithm used to remove the effects of aircraft attitude from the upward-looking shortwave radiometer data requires estimates of the shortwave direct and diffuse radiation fractions. For this data set, a simple radiative transfer model was used to estimate these direct and diffuse fractions, and it was determined during this analysis that the same average fraction values (direct = 0.62, diffuse = 0.38) could be used for processing all of the research flights.

15. 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 are simply not active while the aircraft remains on the ground. None of the data collected while the aircraft is on the ground should be considered valid. This includes the intervals from the JeffCo to Churchill transits when the aircraft was on the ground in Winnepeg.

Reference

Baumgardner, D., Korolev, A., 1998: Airpseed Corrections for Optical Array Probe Sample Volumes. J. Atmos. Oceanic Tech., 14, 1224-1229.

Section II: Flight-by-Flight Summary

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

• TDL Hygrometer not on aircraft for this flight.
• Radar altimeter not turned on at start of flight. No HGM232 data from 15:28 to 15:56.
• Aircraft on the ground in Winnepeg. All data bad from 18:20 to 19:30.
• Data system crash. Data gap from 22:01:30 to 22:08:04.
• ATRR and DPB used as reference sensors.

• TDL Hygrometer not on aircraft for this flight.
• Element damage to ATRL temperature sensor.
• ATRR and DPB used as reference sensors.

• TDL Hygrometer not on aircraft for this flight.
• Radar altimeter not turned on at start of flight. No HGM232 data from 18:45 to 21:02.
• Dew pointers contaminated or sample flow obstructed. DPB/DPT data bad from 18:45 to 19:45.
• Excessive drift in RSTB surface temperature sensor during flight.
• ATRR and DPB used as reference sensors.

• Aircraft on the ground in Winnepeg. All data bad from 18:34 to 19:58.
• PITCH maneuvers conducted from 15:22 to 15:25. Maneuvers conducted from 15:18 to 15:38.
• Uncharacteristic response in DPB. Data questionable from 23:47 to 23:55.
• ATRR and DPB used as reference sensors.

• Failure of an onboard power converter prior to takeoff required a reduction in power usage. PI's decided to turn off RAF and Weber CN counters. No CONCN data for flight.
• PLWCC liquid water sensor showing unexplained level shifts. Data bad from 13:28 to 16:03.
• Excessive drift in RSTB surface temperature sensor during flight. Data questionable from 16:00 to 19:40.
• ATRR and DPT used as reference sensors.

• Failure of an onboard power converter prior to takeoff required a reduction in power usage. PI's decided to turn off RAF and Weber CN counters. No CONCN data for flight.
• PLWCC liquid water sensor showing unexplained level shifts. Data bad for entire flight.
• ATRR and DPT used as reference sensors.

• Failure of an onboard power converter prior to takeoff required a reduction in power usage. PI's decided to turn of RAF and Weber CN counters. No CONCN data for flight.
• PLWCC liquid water sensor showing unexplained level shifts. Data bad for entire flight.
• Uncharacteristic response in DPT. Data questionable from 16:54 to 19:54.
• Excessive oscillation in DPB from 21:36 to 21:54.
• DPB sensor balanced in flight. Data bad from 18:02 to 18:06.
• ATRR and DPB used as reference sensors.

• Aircraft on the ground in Winnepeg. All data bad from 17:52 to 19:00.
• Data system crashs. Data gaps from 15:01:10 to 15:07:57 and 18:52:58 to 19:02:21.
• PLWCC liquid water sensor showing unexplained level shifts. Data bad for entire flight.
• ATRR and DPB used as reference sensors.

• PLWCC liquid water sensor showing unexplained level shifts or excessive drift. Data bad for entire flight.
• Radar altimeter not turned on at start of flight. No HGM232 data from 13:25 to 13:32.
• Excessive drift in RSTB surface temperature sensor during flight. Data questionable from 16:00 to 19:40.
• Top shortwave radiometer (SWT) signal intermittent. SWT data bad from 15:19 to 15:50.
• ATRR and DPB used as reference sensors.

• PLWCC liquid water sensor showing unexplained level shifts or excessive drift. Data bad for entire flight.
• Light aircraft icing affected radome wind system. Vertical wind data (WIC, WI) questionable from 21:33 to 23:05.
• ATRR and DPB used as reference sensors.

• Aircraft on the ground in Winnepeg. All data bad from 18:25 to 19:45.
• Data system crashes. Data gaps from 16:23:25 to 16:40:57 and 18:30:46 to 18:45:32.
• PLWCC liquid water sensor showing unexplained level shifts. Data bad for entire flight.
• Light aircraft icing affected radome wind system. All wind data (WIC, WI) questionable from 20:12 to 21:18.
• ATRR and DPB used as reference sensors.

• PLWCC liquid water sensor showing unexplained level shifts. Data bad from 13:05 to 13:13.
• Excessive drift in RSTB surface temperature sensor during flight. Data questionable from 15:00 to 18:39.
• ATRR and DPB used as reference sensors.

• PLWCC liquid water sensor showing unexplained level shifts. Data bad for entire flight.
• Excessive drift in RSTB surface temperature sensor during flight. Data questionable from 14:18 to 18:00.
• ATRR and DPT used as reference sensors.

• CN counter pump not turned on. CONCN data bad from 14:30 to 14:36.
• Excessive drift in RSTB surface temperature sensor during flight. Data questionable from 18:23 to 19:50.
• Uncharacteristic response in DPT. Data questionable from 20:11 to 20:43.
• Uncharacteristic response in DPB. Data questionable from 20:30 to 20:43.
• Light aircraft icing affecting ATRL temperature sensor. ATRL data questionable from 20:26 to 20:43.
• ATRR and DPB used as reference sensors.

• DPB contaminated by deicing fluid prior to takeoff. DPB data bad for entire flight.
• DPT contaminated by deicing fluid prior to takeoff. DPT data bad from 17:03 to 20:48.
• Baseline offset in PLWCC data. PLWCC data bad from 17:03 to 20:12.
• ATRR and DPT used as reference sensors.

• Intermittent sample flow in CN counter. CONCN data bad from 15:35 to 16:34.
• Baseline offset in PLWCC data. PLWCC data bad from 15:35 to 16:33.
• ATRL and DPB used as reference sensors.

• Excessive drift in PLWCC and PLWCC1. Data questionable from 25:50 to 26:00.
• Excessive drift in RSTB surface temperature sensor during flight. Data questionable from 25:28 to 26:00.
• ATRL and DPB used as reference sensors.

• Data system crash. Data gap from 11:57:31 to 11:59:12.
• Baseline offset in PLWCC data. PLWCC data bad from 15:27 to 16:46.
• Excessive drift in RSTB surface temperature sensor during flight. Data questionable from 15:08 to 16:38 and 17:20 to 17:35.
• ATRL and DPB used as reference sensors.

• Excessive drift in RSTB surface temperature sensor during flight. Data questionable from 13:55 to 16:50.
• Element damage to ATRR temperature sensor.
• Intermittent signal in top shortwave radiometer. SWT data bad from 14:50 to 14:56.
• ATRL and DPB used as reference sensors.

• Radar altimeter not turned on at start of flight. No HGM232 data from 14:54 to 15:17.
• Data system crash. Data gap from 15:18:24 to 15:34:00.
• ATRL and DPB used as reference sensors.

• Radar altimeter not turned on at start of flight. No HGM232 data from 14:21 to 14:32.
• Excessive drift in RSTB surface temperature sensor during flight. Data questionable from 17:00 to 20:00.
• ATWH and DPT used as reference sensors.

• Failure of RSTB1 system. RSTB1 data bad for entire flight.
• ATWH and DPB used as reference sensors.

• FSSP-300 probe failure. No data for entire flight.
• PLWCC liquid water sensor showing unexplained level shifts. Data bad for entire flight.
• Aircraft on the ground in Winnepeg. All data bad from 18:49 to 20:07.
• Radar altimeter not turned on at start of flight. No HGM232 data from 20:07 to 20:15.
• Obstruction in ATTACK radome pressure system. ADIFR and vertical wind data bad from20:53 to 21:45.
• ATWH and DPB used as reference sensors.

• FSSP-300 probe failure. No data for entire flight.
• PLWCC liquid water sensor showing unexplained level shifts. Data bad from 11:43 to 12:12.
• Light aircraft icing affecting ATRL and ATRR temperature sensors during flight.
• Excessive drift in RSTB surface temperature sensor during flight. Data questionable from 14:42 to 16:30 and 17:06 to 18:00.
• ATWH and DPB used as reference sensors.

• FSSP-300 probe failure. No data for entire flight.
• Excessive drift in RSTB surface temperature sensor during flight. Data questionable from 14:21 to 15:15.
• Intermittent signal in top shortwave radiometer. SWT data bad from 15:33 to 15:50.
• ATWH and DPB used as reference sensors.

• FSSP-300 probe failure. No data for entire flight.
• PLWCC liquid water sensor showing unexplained level shifts. Data bad for entire flight.
• Excessive drift in RSTB surface temperature sensor during flight. Data questionable from 18:40 to 19:04.
• ATWH and DPB used as reference sensors.

• FSSP-300 probe failure. No data for entire flight.
• PCASP aerosol probe pump failure. CONCP data bad from 18:25 to 18:26.
• PLWCC liquid water sensor showing unexplained level shifts. Data bad for entire flight.
• Maneuvers conducted from 21:56 to 22:30.
• Light aircraft icing affecting radome wind measurements.
• QCR and vertical wind data bad from 16:59 to 18:05.
• ATWH and DPB used as reference sensors.

• Flight scrubbed shortly after takeoff due to maintenance problem. No data processed at PI's request.

• Aircraft on the ground in Winnepeg. All data bad from 19:34 to 20:38
• Excessive drift in RSTB surface temperature sensor during flight. Data questionable from 21:26 to 23:44.
• ATWH and DPB used as reference sensors.

• ATRR and DPT used as reference sensors.

• Radar altimeter not turned on at start of flight. No HGM232 data from 11:53 to 12:21.
• PITCH maneuver conducted from 16:55 to 16:58.
• ATRR and DPB used as reference sensors.

• Light aircraft icing affecting radome wind measurements.
• QCR and all 3-D wind data questionable from 18:45 to 20:03.
• ATWH and DPB used as reference sensors.

• Radar altimeter not turned on at start of flight. No HGM232 data from 15:55 to 16:10.
• ATRR and DPB used as reference sensors.