RICO Logo, 8,273 byte jpg

Project # 2005-135 RICO

Rain In Cumulus over the Ocean

Principal Investigator(s):  Bob Rauber, Harry Ochs, et al.

NSF/NCAR C-130Q Hercules (N130AR)

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Data Quality Report

Prepared by: Allen Schanot (NCAR/RAF)

27 April 2005

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. Section III has special notes on the high-rate data set.

A discussion of the performance of the SABL lidar, GPS dropsondes and RAF chemistry sensors will be provided separately, as will the 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.

  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, RF02, RF09, RF12 and RF18 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).

    Special Note:  The Inertial Reference Unit (IRS) was changed between flights RF14 and RF15. The systematic biases in the pitch and sideslip parameters were adjusted accordingly.

  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 differed significantly. The RSTB1 sensor failed during flight RF12. No replacement was available, so there are no RSTB1 data for flights RF13 through RF19.

  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. Typically, differences were less than 0.5 °C and often closer to the standard 0.2 °C considered to be a good comparison. However, midway through the project ATRL started exhibiting discontinuous jumps in signal output. Replacing the sensor did not fix the problem which could have been in the wiring. 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 right turns where the ROLL angle exceeds +15°, however, the OPHIR temperature will be influenced by the presence of the warm sea surface in the field of view. Typical differences between ATX and OAT during these turns are around +0.1 °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.

    The OPHIR-III sensor has a certain amount of drift, primarily associated with significant and rapid altitude changes. To further improve the data, a loose-coupled processing method is used to remove some of this drift The "corrected" OPHIR temperature appears in the data set as XOAT. It is this variable that should be used for analyses purposes. Because XOAT 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. 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. RAF cross-flow units function well under all conditions and should be perfectly adequate as the reference high rate sensor for basic analysis and flux calculations. About half way through the project, the primary sensor (VLA, RHOLA, MRLA, RHLA) began exhibiting short periods where the signal log amplification would shift. This resulted in some overshooting during cloud passes. A second unit (VLA1, RHOLA1, MRLA1, RHLA1) was installed prior to flight RF14. Both units were flown for the remaining flights. MRLA1 should be used as the reference high rate humidity sensor for flights RF14 through RF19.

    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.

  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. RSTB seemed to be the more stable of the two units and is therefore recommended as the reference system for this measurement. The RSTB1 sensor failed during flight RF12. No replacement was available, so there are no RSTB1 data for flights RF13 through RF19.

    In addition to their thermal sensitivity, the accuracy of this remote sensing measurement is also dependent upon the total amount of water vapor in the sensing path. In such a moist, marine environment this sensitivity appears as an altitude dependence in the raw surface temperature. RAF has added a special "corrected" surface temperature variable called "XTSURF" based on data collected over a uniform water surface at multiple altitudes during each flight. These data will only be useful when there are no clouds in the field of view of the sensors.

  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 calculated using the U.S. Standard Atmosphere 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 RICO, better results were obtained by using a mean surface temperature of 305 °K.

    The GPS also provides an altitude readout (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 MSL altitude based on a ellipsoid model of the Earth (WGS-84). The signal recorded on the C-130 can still be interrupted during steep turns and still experiences some long-term oscillation on the order of ±10 meters.

    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 for 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 IRS altitude data when the GPS signal is interrupted.

  11. Two hot-wire liquid water sensors and a PVM-100 laser-based liquid water sensor were mounted on the C-130 for the program. While all systems performed well, accumulation of salt on the hot-wire elements tended to increase the baseline drift in the raw output. RAF employed a loose-couple processing method to remove the excess drift. Both techniques, hot wire and PVM-100, have a tendency to underestimate the total liquid water in large drops or mixed phase clouds.

  12. The calculation of CN sized (0.01 to 3 µm) aerosol particle concentrations from the standard sensor (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. Due to the length of the tubing connecting the inlet to the counter, a 1- to 2-second lag in the system response to changes in particle concentration has been introduced into this measurement.

    A second, "ultra-fine" CN counter (XUFCT) was added to the research payload. The particle size range for this probe is 0.003 to 3 µm. The unit outputs a concentration already corrected to ambient conditions, not to STP. Due to the length of the tubing connecting the inlet to the counter, a 1- to 2-second lag in the system response to changes in particle concentration has been introduced into this measurement.

    NOTE:   Both CN sampling systems are susceptible to false particle counts resulting from cloud and precipitation droplet-shattering on the sampling inlets. Use of both data sets should be limited to clear-air sampling passes only.

  13. Five PMS particle probes (SPP-100, SPP-200, 260X, 2D-C and 2D-P) were used on the project. Some specific details on each of the probes are summarized below:

  14. 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).


FF02 (12/04/2004) RF01 (12/07/2004) RF02 (12/08/2004) RF03 (12/09/2004) RF04 (12/10/2004) RF05 (12/13/2004) RF06 (12/16/2004) RF07 (12/17/2004) RF08 (12/19/2004) RF09 (12/20/2004) RF10 (01/05/2005) RF11 (01/07/2005) RF12 (01/11/2005) RF13 (01/12/2005) RF14 (01/14/2005) RF15 (01/16/2005) RF16 (01/18/2005) RF17 (01/19/2005) RF18 (01/23/2005) RF19 (01/24/2005) FF05 (01/26/2005)

Section III:   Special Notes on the High-Rate Data Set

The high rate (HRT) data files produced for the RICO project are subsets of the total list of available variables provided in the standard (1 sps) data archive. Only key variables were included in the HRT archive in order to minimize the overall size of the data files and make them more manageable for the average participant. A separate HRT variable list has been included in the on-line project documentation. Both the standard and HRT data files can be accessed via the RAF web site.

  1. Different sensors have different response times. In order to make the resultant data more useable, C-130 HRT data files output most of the variables at a standard rate of 25 sps. For sensors samples at higher rates, the data are averaged down to 25 sps. Slower sensors, sampled at 5 sps or even 1 sps, are handled on a case-by-case basis. Key low-rate variables, such as GPS altitude and ground speed, are interpolated up to the standard 25 sps output rate. Other low-rate variables are simply output at the standard 1 sps rate.

  2. PMS 1-D particle probe data are sampled at 10 sps and output at the same rate in the HRT data files. Concentration data from the PMS 2-D probes are output at 1 sps in the HRT files because they are recorded asynchronously and have no set rate.

  3. RAF special software programs designed to reduce excess baseline drift in certain variables are not available in HRT data set. Take special note of the fact that the King Probe liquid water data (PLWCC, PLWCC1) fall into this category. Excess zero drift in these two variables has been removed from the data in the standard archive. This is not the case with the HRT data set. Comparisons of PLWCC and PLWCC1 values between the two data sets (1 sps & 25 sps) will show some differences in absolute value due to this difference in zero offset. On some flights this difference can be as large as 0.05 gram/m3.

  4. RAF has special software programs designed to calculate new, special function variables for selected projects. These optional functions are not available on HRT data sets. Specifically, the drift-corrected output of the OPHIR-III in-cloud temperature (XOAT) and the altitude-corrected remote surface temperature (XTSURF) fall into this category. Neither of the base sensors for these calculations are particularly fast, anyway, so most of the structure in a high-rate output of these variables would be interpolation. Both variables are available in the standard data archive files.

Last update: Fri Jan 27 16:40:10 MST 2006