Project #2001-131 ACE-Asia
Asian Pacific Regional Aerosol Characterization Experiment
Barry Huebert, et al.
NSF/NCAR EC-130Q Hercules (N130AR)
Data Quality Report
by Allen Schanot & Dick Friesen
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, LTI inlet system, and RAF chemistry sensors will be provided separately, as will their respective data sets.
RAF staff has 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 instances a system will be out for an entire flight.
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 TF03 and RF15 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. Late in the program excessive drift in the position output of the Inertial Reference System (IRS) resulted in some errors in the basic wind data. Such cases are noted in the flight-by-flight section of this summary. Drifts in the IRS accelerometers are removed using an algorithm that employs a complementary high-pass/low-pass filter that removes the longer-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).
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). There may be occasional spikes in these 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.
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), two dynamic pressures (QCRC, QCFC), three liquid water sensors (PLWCC, PLWCC1, XGLWC), 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.
Temperature measurements were made using the standard heated (ATWH) and unheated (ATRR,ATRL) Rosemount temperature sensors. All of the standard temperature sensors performed reasonably well, encountering the usual problems with sensor wetting during cloud passes. A comparison of the data sets yielded good correlation in instrument signatures and only small differences in absolute value (&plusm;0.2°C) throughout the program. A comparison of pre- and post-program calibrations indicated that all units maintained stable and independent calibrations. ATRR was selected as the reference value (ATX) used in calculating the derived measurements.
Humidity measurements were made using two collocated thermoelectric dew point sensors, two Lyman-alpha fast-response hygrometers 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 was functioning the best on that particular flight. The selection of a reference humidity sensor (DPXC) for use in calculating all of the derived measurements was varied accordingly.
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 two systems remained well correlated for clear air sampling but showed significant differences during cloud penetrations. The two Lyman-alpha hygrometers used different housing types. The stub unit (MRLA, RHOLA, DPLA, RHLA) tends to be slightly faster but is more susceptible to in-cloud wetting and thermal drift. This unit failed during the project and had to be replaced. The replacement system also failed late in the project. The cross-flow unit (MRLA1, RHOLA1, DPLA1, RHLA1) is therefore recommended as the reference sensor for basic analysis. For flux calculations, MRLA1 should be used.
A TDL (tunable diode laser) 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 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 in a form that was suitable for inclusion. We expect a subsequent release of a "corrected" set of TDL data (MRLH) at a later date. The TDL data have been removed from the current release to avoid possible confusion.
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 raw data have been corrected for changes in the aircraft's flight attitude. RAF has recently added a new set of irradiance measurements which have been corrected for aircraft attitude and relative sun angle. A description of this correction process is provided in RAF Bulletin No. 9 and RAF Bulletin No. 25.
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 exhibited better accuracy in the regular, single-point, precision tests run prior to each flight. RSTB1 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. Using a compilation of vertical soundings from all of the research flights, RAF was able to empirically generate a mean-moisture vertical profile that could be used to characterize the conditions encountered during the research flights. To aid in the analysis of the data RAF has added a special calculation of sea surface temperature (XTSURF) which makes a rough attempt to account for this dependency.
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 the U.S. Standard Atmosphere, which assumes a constant surface pressure of 1013mbar and a mean surface temperature of 288 °K.
A radar altimeter (HGM232) was on board 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 GPS (Global Positioning System) 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.
To aid the Users in choosing a common altitude to use in their analyses, RAF now calculates a "reference" altitude (ALTX). The output is based directly upon GALT and uses basic smoothing techniques to fill gaps with Inertial altitude (ALT) data when the GPS signal is interrupted.
Two hot-wire liquid-water sensors were used on the EC-130Q during the program. The PMS King Probes (PLWCC, PLWCC1) worked well during the program. A comparison of the two units yielded a good correlation in instrument signatures and only small differences in absolute value throughout the program. 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 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.
A Gerber Model PVM-100 liquid-water probe was included in the research instrumentation package. The unit responded well to the presence of liquid water but exhibited a large baseline offset that was strongly dependent upon changes in the aircraft true airspeed. Calibration for this unit comes from the manufacturer. It was returned for a full post-program evaluation and calibration. During final data processing, the RAF loose-couple technique was used to remove some of the remaining baseline drift.
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.
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 inlet system was used on this project, specifically designed to avoid this problem and provide good interstitial, in-cloud aerosol measurements.
Five PMS particle probes (FSSP-100, SPP-100, SSP-300, PCASP, 260X) were used on the project. Some specific details on each of the probes are summarized below:
The FSSP-100 and SPP-100 probes use the same optics but different electronics to measure both particles and small cloud droplets. During the project the FSSP-100 was primarily operated in the cloud-droplet mode (3 to 50 µm). However, for the first 3 flights it was operated on the same range as the SPP-100 (0.5 to 8.0 µm). Cloud penetrations were limited, but the system performed well. Comparisons of particle sizing showed good agreement with the other particle probes in the ranges where the measurements overlapped. The droplet concentration data appear in the data set as CONCF_RPC. Other related measurements can be identified by the _RPC suffix. Like all 1-D optical probes, however, the FSSP-100 has no way to distinguish between aerosols, ice or water.
The SPP-100 and FSSP-100 probes use the same optics but different electronics to measure both particles and small cloud droplets. During the project the SPP-100 was operated in the aerosol mode (0.5 to 8 µm). Comparisons of particle sizing showed good agreement with the other particle probes in the ranges where the measurements overlapped. The particle concentration data appear in the data set as CONCF_LPI. Other related measurements can be identified by the _LPI suffix. Like all 1-D optical probes, however, the SPP-100 has no way to distinguish between aerosols, ice or water.
The SPP-300 aerosol probe covers a range of particle sizes that bridges the gap between the true aerosols and the smaller droplets (0.3 - 20 µm). Like all 1-D optical probes, however, the SPP-300 has no way to distinguish between aerosols, ice or water. In addition, the optical particle sizing of the SPP-300 can be affected by heavy concentrations of smaller hygroscopic particles. An intermittent malfunction in the probe caused spiking in the data during certain flights. Most of these intervals have been filled with the "missing data" flag, but some will still be present in the data set. The bad data can be clearly identified as periods with excessive values of particle concentration (CONC3). The remainder of the data from this probe look good, with comparisons of particle sizing showing good agreement with the other particle probes in the ranges where the measurements overlapped.
Note: The bin sizes vary significantly in the particle-sizing routines for this probe.
Throughout the project the probe consistently produced particle concentrations 5 to 6 times expected values. While the instrument was clearly responding to changes in aerosol concentrations (as determined by comparisons with CONCN and CONC3) and the resulting size distributions seem reasonable, RAF could find no reason to account for the discrepancy in the total concentrations. Therefore all of the data from this probe are considered questionable and have been removed from this data release. If further analysis results in a resolution to this problem, the data will be provided in a subsequent release.
While the range of this probe is specified to be 10 - 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 alogrithms (Baumgardner, D., Korolev, A., 1997: Airpseed 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 - 640 µm when it is mounted on the EC-130Q. The unit functioned well throughout the program, but most cloud penetrations were limited to small cumulus clouds or scattered alto-stratus. This has resulted in a very small sampling of droplets large enough to trigger the new minimum threshold on this precipitation probe.
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 they are fairly isolated and obvious to any User.
On 19 December 2001, it was called to our attention by Steve Howell, University of Hawaii, that the ACE-Asia flights RF01 through RF08 need to have a factor of 2 applied to the measurement XUCN - ""Clarke - Ultrafine CN Concentrations." This is now a known problem with the data set, but at this time, no action will be taken to change our production data files. Investigators using these data should simply multiply the concentration by two for flights before RF09. For more information you may contact Steve via email or phone: (808)956-5185.
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.
Note: All times listed below are Coordinated Universal Time (UTC).