Data Quality Summary
Surface Heat Budget of the Arctic Ocean (SHEBA)
Project No. 8-101
J. Curry, et al.
Prepared by: Krista Laursen (NCAR/RAF)
This document is intended to provide a general overview of problems, limitations, and features to be found in the SHEBA C-130 data set. It should be noted that the quality of the C-130 microphysical (PMS 1-D and 2-D, liquid water content, and CN) data is discussed in a separate document, "Quality Assurance of Cloud and Aerosol Microphysical Measurements SHEBA Project." This document, prepared by Darrel Baumgardner, is included in the data documentation notebook section "Data Quality Reports" and should be consulted by users interested in using any of the SHEBA microphysical data for analytical purposes. Any questions regarding either the SHEBA microphysical data or the microphysical quality summary should be directed to
Investigators should also note that cloud pass microphysical summaries for the SHEBA research flights are available on line by accessing the following RAF Web site: http://www.eol.ucar.edu/raf/Projects/SHEBA/CloudPasses. Any questions regarding the data presented in the cloud pass Web site pages should be direct to Darrel Baumgardner.
The overview information presented in this document is supplemented by a set of Data Quality Overview (DQO) sheets for the sixteen C-130 research flights. Each DQO sheet (one per flight) summarizes the quality of the measurements made in each of several categories (aircraft position and attitude, dewpoint, humidity, etc.) and lists specific problems and bad data time periods as appropriate for that flight. The DQO sheets are not exhaustive; that is, not all variables included in the final low rate netCDF files are listed in the sheets. The variables listed in the DQO sheets were chosen for inclusion because they are believed to be representative of the various categories of measurements and thus provide a good picture of problems detected within the SHEBA C-130 data set.
Users will note during their perusal of the DQO sheets and the microphysical data quality report that the quality of the data collected from some instruments is sometimes listed as "No Good (NG)" or "Bad." In those cases for which the data were not good or bad for an entire flight, the instrument(s) output (i.e., the associated variable or variables) were not output in that flight's netCDF file.
Every attempt has been made to be as thorough and complete as possible during the quality checking of the SHEBA C-130 data set. However, given the significant number of variables in the processed data files, it is possible that some problems within the data set were not detected. Users of these data are encouraged to notify the RAF should they discover additional problems and/or limitations within the data set. Information regarding such problems should be brought to the attention of either Krista Laursen, the SHEBA C-130 project manager,
email or phone: (303)497-1031,
or RAF Data Manager Ron Ruth, email
or phone: (303)497-1084.
General Data Set Quality and Limitations
- In general, measurements of ambient temperature made on the C-130 using the two radome-mounted sensors (ATRL, Ambient Temperature Radome Left; ATRR, Ambient Temperature Radome Right) were in good agreement with each other and with temperature measurements obtained from the heated, wing-mounted sensor (ATWH, Ambient Temperature, Deiced Wing). Temperature measurements obtained from the Ophir radiometric sensor (OAT, Ophir Ambient Temperature) were also, for the most part, in agreement with the ATRL, ATRR, and ATWH data. However, during those times at which the aircraft was flying at high altitudes and/or in colder (less than approximately -5oC) regions of the atmosphere, the Ophir measurements were offset from the other three measurements of ambient temperature. An example of this Ophir response is shown in
While the exact cause of this offset problem has not been identified, it is believed that it may be attributable to over cooling of the Ophir optics and/or electronics by the circulation of very cold air through the Ophir mounting chamber. (An inlet to channel air through the Ophir mounting chamber was installed prior to SHEBA in order to remove unwanted moisture from the chamber environment.)
Following a thorough review of all of the ambient temperature data collected on the C-130 during SHEBA, it was decided that ATRL is the most reliable ambient temperature measurement. Thus, the reference ambient temperature variable ATX has been set to ATRL for all sixteen research flights, and users of the data set should use ATRL in all calculations and studies requiring ambient temperature data.
- Measurements of dewpoint temperature obtained from the two General Eastern hygrometers on the C-130 (DPBC, Dewpoint Temperature, Bottom Corrected; DPTC, Dewpoint Temperature, Top Corrected) were typically in good agreement throughout each of the sixteen SHEBA research flights. The exception to this occurred when the C-130 was flying at high altitudes and in cold temperature regions. During such time periods, DPBC frequently displayed significant oscillatory behavior. (See
for an example of this instrument performance.) Because of this limitation in the DPBC data and the overall good performance of the DPTC sensor, the reference dewpoint temperature variable DPXC has been set to DPTC for all sixteen research flights. Users of the data are therefore advised to use DPTC in all calculations and studies requiring dewpoint temperature data.
- During research flights 1-8 and 14-15, data collected from the UV hygrometer (RAF variable MRUV) were questionable and in some cases no good at high altitudes. This problem, which was basically equivalent to a pressure dependence in the data, may have been attributable to a leak in an O-ring seal in the instrument. For the above-mentioned ten research flights, MRUV data collected at high altitudes should be considered highly questionable, and other measurements of mixing ratio [e.g., MR (Mixing Ratio, Thermo-Electric) or MRLA (Mixing Ratio, Lyman-Alpha)] should be used.
- It should be noted that for research flights 1-9, data collected from the cryogenic dewpoint hygrometer (RAF variables CMRCR, DPCRC, FPCRC, and RHOCR) are no good at low altitudes/warmer temperatures. This is due to a pressure cut-off mechanism built into the instrument that turns off the pump at an altitude of 10,000 feet. This procedure prevents the cryogenic hygrometer from operating at warmer temperatures, an application for which the instrument was not designed. Users should also note that, due to problems with the instrument, data from the cryogenic dewpoint hygrometer were bad during research flights 10-16 and therefore were not output in the final netCDF data files for those flights.
- During several of the C-130 research flights, RSTB (Radiometric Surface Temperature, First Heimann Sensor) data displayed noise (i.e., spiking) for short time periods. An example of this behavior is shown in
During research flights 9-16, an additional problem was discovered in regard to the RSTB data. During these eight flights, the RSTB data were occasionally offset below the RSTB1 (Radiometric Surface Temperature, Second Heimann Sensor) by 0.5-3.5oC. This offset is believed to be due to an inability on the part of RAF staff to obtain a good calibration of the RSTB sensor prior to or following the July SHEBA research flights. Because of this latter limitation, calibration coefficients obtained for the RSTB sensor prior to the May phase of SHEBA had to be used in the final processing of the surface temperature data for flights 9-16. These coefficient values may have been slightly inaccurate. Due to the above-outlined noise and offset problems associated with the RSTB data, users are advised to use RSTB1 as the reference measurement of radiometric surface temperature for all sixteen research flights.
- During processing and quality-checking of the RAF hemispheric radiometer data collected during SHEBA flights 1-8, it was determined that significant offsets (possibly attributable to inaccuracies in the sensor calibration data) existed in the upwelling shortwave radiometer (SWB) and upwelling UV radiometer (UVB) data. It is believed that similar offsets may also have existed in the upwelling IR radiometer data (IRBC) and the downwelling shortwave, UV, and IR radiometer data (SWT, UVT, and IRTC, respectively). However, due to an inability to analytically isolate the offsets in IRBC, SWT, UVT, and IRTC, it was only possible to quantify and remove the offsets in the SWB and UVB data for research flights 1-8. Thus, the absolute magnitudes of the IRBC, SWT, UVT, and IRTC data collected during the first eight flights should be viewed as uncertain. It should be noted that, prior to the C-130 deployment for Phase II of SHEBA, newly-calibrated hemispheric radiometers were installed on the C-130. Thus, offsets in the SWB, SWT, IRBC, IRTC, UVB, and UVT data did not appear in research flights 9-16.
- A modelling study was conducted for each of the sixteen research flights to come up with flight-specific values needed to correct the downwelling shortwave radiometer data (SWT) for the effects of aircraft attitude. A simple radiative transfer model was used to determine shortwave direct and diffuse radiation fractions, both of which are used in the correction algorithm recently introduced into routine processing by the RAF. Unique direct and diffuse fractions were generated for each of the sixteen flights, and these values were then used in the generation of attitude-corrected downwelling shortwave radiometer data (RAF variable XSWTC) for each flight.
- The GPS altitude (GALT) data collected on the C-130 during SHEBA were determined to be of poor quality for each of the sixteen research flights. For this reason, GALT data have been excluded from the final netCDF data files for the project flights.
- Users should be aware that during low-level flight legs, altitude data values derived from pressure calculations (PALT) and IRS altitude data values (ALT) will occasionally fall below zero. (See
In general, PALT and ALT data are not very reliable at low altitudes. Therefore, during time segments corresponding to low-level flight paths, users should rely on the radar altimeter data (HGM232) for information regarding aircraft altitude.
- Steep banked turns (such as those executed during spiral ascents and descents with roll angles greater than 25 degrees) compromise the quality of the C-130 wind data (RAF variables UI, VI, WI, etc.) due to the adverse effects of the prolonged turns on the aircraft's IRS unit. Users are therefore advised to use the processed wind data corresponding to such turn segments with caution.
- It is recommended that investigators use the GPS-corrected wind variables (XUIC, XVIC, XWIC, XWDC, and XWSC) in any studies and/or calculations requiring wind speed and/or direction measurements. These variables are, generally, considered to provide the most reliable wind vector measurements.
- In order to provide users of the SHEBA data set with information regarding the position of the C-130 relative to the SHEBA ship during research flights, four position-related variables were output into each of the final netCDF data files. These variables and their associated definitions are as follows: DEI (Distance East of Fixed Reference, in km); DNI (Distance North of Fixed Reference, in km); FXDIST (Radial Distance from Fixed Reference, in km); FXAZIM (Radial Azimuth from Fixed Reference, in degrees). For all four variables, "Fixed Reference" refers to the SHEBA ship.
- Measurements of particle effective radius (RAF variable XGREFF) obtained from the Gerber PVM-100 probe have been determined to be highly uncertain. Thus, XGREFF data were not output into any of the final netCDF data files for SHEBA. Investigators should also note that the accuracy of the particle surface area measurements (RAF variable XGSFC) obtained from the Gerber PVM-100 probe is also uncertain. While XGSFC data were output in each of the sixteen final data files, users are advised to use these data with caution.
Note: The page
Gerber PVM-100 Probe Corrections for SHEBA
has been added to this site on 26 April 2000.
- The derived 2D-C and 2D-P concentrations (RAF variables CONC2C_IBR and CONC2P_OBR, respectively) generated during processing of the SHEBA data were determined to be uncertain by perhaps as much as a factor of two. For this reason, CONC2C_IBR and CONC2P_OBR data were not included in the final SHEBA data files. The shadow-OR count data for each of the two 2D probes (RAF variables SHDORC_IBR and SHDORP_OBR, respectively) have, however, been output into all of the final data files. Investigators interested in examining 2D probe concentrations will need to use the shadow-OR data to derive their own estimates.
- During several of the SHEBA research flights, mixing ratio and absolute humidity data obtained from the cross-flow Lyman-Alpha hygrometer (RAF variables MRLA1 and RHOLA1, respectively) were offset from the dewpoint hygrometer and second Lyman-Alpha unit measurements of mixing ratio and absolute humidity (RAF variables MR and MRLA for mixing ratio and RHODT and RHOLA for absolute humidity). An example of this offset behavior is shown in
While the exact cause of this problem could not be determined, it was noted (as is illustrated by
Fri Jul 30 12:44:58 MDT 2004
that the offsets and drifts in the MRLA1 and RHOLA1 data tended to be most prevalent during those times at which the C-130 was flying at high altitudes in cold ambient temperatures and/or was changing altitude. MRLA1 and RHOLA1 data are, in general, in good agreement with MR, MRLA, RHODT, and RHOLA data at lower altitudes and in warmer atmospheric regions. Based on the above observation, investigators should use the MRLA1 and RHOLA1 data collected at higher altitudes and/or in colder areas with caution.
- As is noted in the document "Quality Assurance of Cloud and Aerosol Microphysical Measurements SHEBA Project," the manufacturer of the PVM-100 probe notified the RAF following the completion of the SHEBA deployment that the calibration that was supplied with the RAF's PVM unit is incorrect due to problems with a laboratory standard. Thus, the PVM liquid water content data (RAF variable XGLWC) in the final netCDF data files should be considered to be slightly uncertain. The instrument manufacturer will send the RAF new calibration coefficients for the RAF PVM probe after a new calibration standard has been established. At that time, the RAF will determine what type of additional correction needs to be made to the SHEBA PVM data and will notify users of the data set accordingly.