Profiling of Winter Storms Project

4/7/2010

Profiling of Winter Storms Project (PLOWS) NCAR C-130 Data Quality Report

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. The first section lists recurring problems, general limitations, and systematic biases in the standard RAF measurements. The second section lists isolated problems occurring on a flight-by-flight basis. A discussion of the performance of the 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. Position Data. Both a Garmin Global Positioning System (_GMN) and a Novatel Global Positioning System (GGPS) were used as more accurate position references during the program. The systems generally performed well. With no real differences between the two sets of data, it was decided to only output the Novatel variables in order to avoid confusion in the data analysis. The algorithm referred to in (3) below also blends the GPS and IRS position to yield a best position (LATC, LONC) that generally removes the GPS spikes.

3. 3D- Wind Data. 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 PLOWS flights TF01, RF08 and RF13 to aid in the performance analysis of the wind gust measurements. The calibration data identified a systematic bias in the pitch and sideslip parameters. 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 for the purpose of data quality review. RAF strongly recommends that the GPS corrected inertial winds be used for all research efforts (WSC,WDC,UXC,VYC,WIC,UIC,VIC).

4. SPECIAL NOTE: 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 two 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 two remote surface temperature sensors (RSTB, RSTB1) generally functioned well but a failure of a sensor head heater in the RSTB system led to excessive drift in these data during the second half of the program. The backup static pressure system showed smaller turbulent fluctuations in the signal (PSFRD) and therefore was selected as the reference pressure (PSXC) used in all of the derived parameters.

5. Ambient Temperature Data. Temperature measurements were made using the standard heated (ATWH) and unheated (ATRR) Rosemount temperature sensors and an OPHIR-III radiometric temperature sensor. Some initial problems with ATWH were tied to a bad data system interface card and were corrected after flight rf03. Performance of all three “insitu” sensors remained stable throughout the rest of the project and showed excellent agreement. Due to its fast response, ATRR was selected as the reference value (ATX) used in calculating the derived parameters for most flights. ATWH was used as the reference temperature on a few selected flights where significant icing conditions adversely affected the response of the ATRR system.

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 hand turns where the ROLL angle exceeds +15 degrees, however, the OPHIR temperature will be influenced by the presence of the surface in the field of view. Typical differences between ATX and OAT during these turns are around +0.1 oC. 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 intermittent signal drop outs. The OPHIR-III sensor has a certain amount of drift, primarily associated with significant and rapid altitude changes. Due to the limited altitude changes during most of the PLOWS flights, this drift is fairly minimal in this data set and the link to the reference temperature (ATX) was not employed. Due the potential for some residual impact for short intervals after significant climbs and descents, however, use of the OPHIR data should be strictly limited to the direct cloud penetrations where the standard sensors have a problem with sensor wetting.

6. Humidity Data. Humidity measurements were made using two collocated thermoelectric dew point sensors and one experimental TDL hygrometer. A comparison of the dew point sensors (DPBC, DPTC) yielded good correlation in instrument signatures during the largest portions of the flights when both instruments were functioning normally. Under conditions where the units had been cold soaked at high altitude or experienced a rapid transition into a moist environment, both units showed a tendency to overshoot. Extended legs in cloud and/or icing conditions showed some degradation in the data. Oscillations in the signals were common under these conditions and restricted sample flows led to unrealistically high dew point temperatures during some of these passes. In general, DPBC seemed less susceptible to these problems and was selected as the reference output (DPXC) for most flights.

The experimental TDL hygrometer has a separate data collection system and requires fairly extensive post processing to produce the final data products. It is not clear what the quality of the data will be at this time.

Note: Even at their best, the response of the thermoelectric dew point sensors is roughly 2 seconds. Response times are dependent upon ambient dew point depression and can exceed 10-15 seconds under very dry conditions.

7. Surface Temperature Data. Heimann radiometric sensors were used to remotely measure surface temperature (RSTB & RSTB1 the surface, RSTT cloud base. A failure of a sensor head heater on the RSTB system led to excessive drift and oscillations in the signal during the early stages of the deployment. The sensor was replaced at the mid-project break. It is recommended that RSTB1 be used as the reference system for this measurement. RSTT also functioned well. Note that when no clouds are present above the aircraft the RSTT signal will be pegged at its maximum “cold” limit of roughly -60 oC.

8. Altitude Data. 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 Atmosphere, which assumes a constant surface pressure of 1013mb and a mean surface temperature of 288 K.

The GPS positioning systems also provide altitude readouts (GGALT & GGALT_GMN). These outputs normally provide a fairly accurate MSL altitude based on a ellipsoid model of the Earth (WGS-84).

A radar altimeter was onboard the aircraft for the project. The unit functioned extremely well. The standard output (HGM232)is in ft AGL. Instrument normally turned on after takeoff and off prior to landing - resulting in short data gaps at both ends of most data files.

9. Liquid Water Content Data. One hot wire liquid water sensor (King Probe: PLWCC1) and one optical (PVM-100: XGLWC, PLWCG) liquid water sensor were mounted on the C-130 for the program. King probe does not function on the ground. Short data gaps in PLWCC1 at the start and end of most data files is normal. Liquid water content is also derived from the concentration and size distributions measured by some of the optical particle probes. Most flight data show good agreement between all of the systems (see exceptions in the CDP probe discussion). Excessive zero drift in XGLWC led to the derivation of a new variable (PLWCG) that baselines the zero value against on of the PMS cloud probes. Note that the PVM-100 also outputs droplet surface area (XGSFC) and an effective droplet radius (XGRFF). These measurements are more problematic in mixed phase clouds and RAF does not recommend using the PVM- 100 particle sizing data from PLOWS to assess particle size.

10. CN Concentration Data (0.01 to 3 um). The calculation of CN sized aerosol particle concentrations (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. The special inlet for this measurement is not susceptible to the normal droplet splashing effects typically noted in all clouds. RAF believes that the in-cloud measurements taken with this system are accurate and represent a good representation on interstitial CN concentrations.

Note: The location of the inlet on the aircraft and length of the tubing connecting the inlet to the counter will induce a lag in the system response to changes in particle concentration.

11. Aerosol & Cloud Droplet Sizing Data. Three PMS 1D particle probes (SPP100-std, SPP100-mod,CDP) were used on the project. Some specific details on each of the probes are summarized below:

SPP100-LWI(FSSP-mod) - The probe is a modified system (SN - #109) physically designed to reduce droplet shattering ahead of the optics. The unit was somewhat noisy in the smaller size bins requiring the 1st 3 bins to be edited out of the overall concentration data. While the data are considered to be acceptable it appears that the probe had difficulty sampling ice particles in the upper end of the sampling range. The probe worked better in mixed phase clouds. Under the conditions targeted by the PLOWS flights, the particle concentration data and calculated liquid water content will under estimate the true values of these variables.

SPP100-LWO(FSSP-std) - The probe is the standard model FSSP with the flow straightening shroud. This unit seemed to function well under all conditions, particularly in the ice clouds encountered at higher altitudes. In liquid precipitation, some droplets shattering likely shifts the size spectra toward higher concentrations of small particles. It is recommended that the cloud size particle data from this system be used as a better representation of the conditions than the other two 1-D probes.

CDP - This probe basically matches the same droplet size distribution as covered by the SPP100 probes but has difficulty in sampling ice particles. Under the conditions targeted by the PLOWS flights, the CDP particle concentration data and calculated liquid water content will under estimate the true values of these variables.

12. Precipitation Sizing Data. Three OAP probes were flown during the project. Unit one was a standard 2D-C probe with 25 um resolution. The second unit was a standard 2D-P probe with 200 um resolution. Both systems functioned well though out the entire project. A newly modified 2D-C with 10 um resolution was added to the payload just prior to departure. The system functioned well but suffered from a problem with the new optical diode array. Most images will have “feathered” boundaries on the back side of the particles. This will be a problem for analysis software packages attempting to automatically produce size distributions from this data set.

13. TECO Ozone Data. This is a dual channel system that cycles between channels every 10 seconds – thus leading to the stair step nature of the data. Both the raw signal (TEO3) and a pressure corrected value (TEO3C) appear in the final data set. The unit was normally turned off prior to landing to avoid contamination. Short data gaps at the end of each flight are normal.

14. SPECIAL NOTE: 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.

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Section II: Flight-by-Flight Summary

RF01 Bad ADS interface channel on heated temperature probe. TTWH & ATWH data bad for entire flight.

Intermittent power drop outs to the OPHIR-III sensor. OAT data bad for entire flight.

PVM-100 probe not responding normally. XGLWC & PLWCG data bad For entire flight.

Excessive drift in one of the down looking remote surface temperature sensors. RSTB data bad for entire flight.

No power to RAF CN Counter system. CONCN data bad for entire flight.

Excessive drift in one of the down looking remote surface temperature sensors. RSTB data bad for entire flight.

Loss of communication between both SPP-100 probes and the ADS system. No SPP-100 data from 1658 to 1729 CUT.

Uncharacteristic response from top dew point sensor. DPTC data bad from 1658 to 1845 and 1939 to 2022 CUT.

Uncharacteristic response from the bottom dew point sensor. DPBC, DPXC and all humidity related variables bad from 1658 to 1745 CUT.

Radome icing affecting 3-D wind data from 1711 to 1750 and 1956 to 2009 CUT.

Power drop out to OPHIR-III temperature sensor. OAT data bad From 213126 to 213245 CUT.

Loss of communication between Inertial Reference Unit (IRU) and ADS data system. No aircraft attitude of 3-D wind data from 1948 to 2114 CUT.

Icing rate indicator heater not functioning. Uncharacteristic RICE response for entire flight.

RF04 Power drop out to OPHIR-III temperature sensor. OAT data bad from 012606 to 012649 CUT.

TECO ozone analyzer failed to initiate correctly during pre- flight. TEO3 & TEO3C data bad for entire flight.

Radome icing affecting 3-D wind data from 030813 to 031445, 040316 to 040807 and 055430 to 061043 CUT.

Uncharacteristic response from bottom dew point sensor likely due to water ingestion or sensor head icing. DPBC data bad from 0022 to 0132 CUT.

Uncharacteristic response from the top dew point sensor. DPTC, DPXC and all humidity related variables bad from 0022 to 0132 & 061447 to 062400 CUT.

RF05 Excessive oscillation in DPBC signal. DPTC selected as reference input (DPXC) to all derived humidity variables.

Intermittent spikes in OPHIR-III temperature data between 1538 and 1648 CUT.

RF06 Power drop out to OPHIR-III temperature sensor. OAT data bad from 2436 to 2438 CUT.

Excessive drift in one of the down looking remote surface temperature sensors. RSTB data bad for entire flight. Radome icing affecting 3-D wind data from 2302 to 2312 CUT.

RF07 Uncharacteristic response from bottom dew point sensor likely due to water ingestion or sensor head icing. DPBC data bad from 0346 to 0429 CUT.

Uncharacteristic response from the top dew point sensor. DPTC, DPXC and all humidity related variables bad from 0336 to 0408 CUT.

Radome icing affecting 3-D wind data from 0336 to 0429 CUT.

RF08 Calibration flight for RAF 3-D winds and Wyoming Cloud Radar (HCR). Lenschow maneuvers and 20 degree circles.

Power drop out to OPHIR-III temperature sensor. OAT data bad from 164034 to 164040 CUT.

RF09 Radome icing affecting 3-D wind data from 1801 to 1813 CUT.

Uncharacteristic response form King Probe. PLWCC1 data bad From 192600 to 192612 and 173417 to 173500 CUT.

RF10 Excessive drift in one of the down looking remote surface temperature sensors. RSTB data bad from 1628 to 2219 CUT.

RF11 Loss of power to heated temperature probe. TTWH & ATWH data Bad from 2150 to 2210 CUT.

Excessive drift in one of the down looking remote surface temperature sensors. RSTB data bad from 1824 to 2210 CUT.

RF12 Uncharacteristic response from top dew point sensor likely due to water ingestion or sensor head icing. DPTC data bad from 025300 to 031238 CUT.

Uncharacteristic response from the bottom dew point sensor. DPBC, DPXC and all humidity related variables bad from 025300 to 030754 CUT.

RF13 Loss of GPS signal to both receivers. No GPS position, ground speed or altitude data from 2014 to 2022 CUT

Intermittent spikes in OPHIR-III temperature data between 2106 and 2132 CUT.

PVM-100 probe not responding normally. XGLWC & PLWCG data bad for entire flight.

TECO ozone analyzer failed to initiate correctly during pre- flight. TEO3 & TEO3C data bad for entire flight.

No power to RAF CN Counter system. CONCN data bad for entire flight.

Excessive oscillation in DPBC signal. DPTC selected as reference input (DPXC) to all derived humidity variables.

RF14 Loss of GPS signal to both receivers. No GPS position, ground speed or altitude data from 230556 to 231848 CUT.

Pump for RAF CN Counter not turned on. CONCN data bad for entire flight.

Excessive oscillation in DPBC signal. DPTC selected as reference input (DPXC) to all derived humidity variables.

Data recording terminated prior to landing.

RF15 Excessive drift in one of the down looking remote surface temperature sensors. RSTB data bad from 1821 to 2218 CUT.

RF16 Intermittent spikes in OPHIR-III temperature data between 1728 and 1837 CUT.

RF17 Intermittent spikes in OPHIR-III temperature data between 1811 and 1824 CUT.

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