Project #2001-161 EPIC 2001
Eastern Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System
Principal Investigators: David Raymond, et al.
NSF/NCAR EC-130Q Hercules (N130AR)
Data Quality Report
Prepared by: Allen Schanot (NCAR/RAF)
12 December 2001
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 and RAF chemistry sensors will be provided separately, as will their respective data sets.
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.
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 and RF17 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, UIC, VIC, WIC, UXC, VYC).
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 along with the OPHIR-III near-field radiometric temperature sensor (OAT, XOAT). The sensing element from ATRL became damaged during flight RF04 and was subsequently replaced. Both unheated sensors (ATRR, ATRL) were damaged during icing incidents encountered on flight RF07, resulting in fixed offset adjustments to the calibration factors for both systems. During the following flights, all of the standard temperature sensors performed reasonably well, encountering the usual problems with sensor wetting during cloud passes. Post project calibration of the resistance changes in the elements in both sensor heads quantified the adjustments needed for the production processing of the final data set. A comparison of the data sets when all systems were functional yielded good correlation in instrument signatures and only small differences in absolute value (±0.2 °C) throughout the program. ATWH 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 infared 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.
In order to assist in the analysis of these data, RAF has output two different OPHIR temperatures in the final data set. The standard OAT data were obtained from a direct calculation of ambient temperature using a calibration fit to clear-air data taken from the reference Rosemount temperature sensor during level flight. RAF has great confidence in this calibration fit, because it works well for all 19 research flights. No flight-by-flight adjustments were necessary. A second value (XOAT) was produced through the use of our 'loose couple' data-processing technique that has been modified slightly to recognize and adjust for the level shifts caused by the malfunction noted above.
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.
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. 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.
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 use different housing types. The stub unit (MRLA, RHOLA, DPLA, RHLA) 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. To avoid any confusion over possible bad data from this system, all of the related variables have been removed from the final data set. The cross-flow unit (MRLA1, RHOLA1, DPLA1, RHLA1) functioned well under all conditions and should be perfectly adequate as the reference high rate sensor for basic analysis. For flux calculations, MRLA1 should be used.
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 measure 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 should only be used to identify trend problems with the primary humidity sensors (ie: overshooting or oscillation in dew pointers). We expect a subsequent release of a 'corrected' set of TDL data (MRLHS) at a later date.
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 these data RAF has added a special calculation of sea surface temperature (XTSURF) to the final data set 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 normally using the U.S. Standard tmosphere, which assumes a constant surface pressure of 1013 mbar and a mean surface temperature of 288 °K. Due to the tropical nature of the research area for EPIC2001, better results were obtained by using a mean surface temperature of 305 °K.
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 GPS 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 data when the GPS signal is interrupted.
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 frequent damage caused by the large precipitation encountered during the routine cloud penetrations. In order to conserve enough sensing elements to complete the field deployment, only one system was flown on each flight (PLWCC1) after flight RF07. 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 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. During final data processing, the RAF loose-couple technique was used to remove some of the remaining baseline drift. 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. Both of the direct-measurement systems are less effective in measuring the water in the larger droplet sizes (greater than 100 µm) and are basically unresponsive to ice particles, whereas the SPP-100 probe samples all particles but can't differentiate between ice and water.
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: For the calculation of particle concentration the internal sample flow (FCN) has been corrected (FCNC) to ambient conditions only and not to STP.
Note: the length of the tubing connecting the inlet to the counter will induce a 2 second lag in the system 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 inlet system has been in use for several projects, specifically designed to avoid this problem and provide good interstitial, in-cloud aerosol measurements. However, based on the results from this project, it now appears that the new inlet can still be affected by this problem in warm clouds with large droplets.
Six PMS particle probes (SPP-100, SPP-200, SPP-300, 260X, 2D-C, 2D-P) were used on the project. Some specific details on each of the probes are summarized below:
The SPP-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 SPP-100 has no way of distinguishing between ice particles and water droplets. Therefore, the liquid water content calculated from this probe (PLWCF_IBR) should be used with caution in this application.
The SPP-200 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_WDL, 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.
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 to 20 µm). Like all 1-D optical probes, however, the SPP-300 has no way to distinguish between aerosols, ice or water. Counts in the lowest 3 bin sizes were contaminated by excessive electronic noise. Data from those channels have been removed from the calculation of total particle concentration.
Note: The bin sizes vary significantly in the particle-sizing routines for this probe.
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 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 to 640 µm when it is mounted on the C-130. The unit functioned well throughout the program.
2D-C and 2D-P
Both of the two-dimensional imaging probes 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.
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.
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).