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Basic Measurements in Progressive Science

Al Cooper 21 June 2006. Basic Measurements in Progressive Science. Derived Variables GPS and D-values (Jensen) Dropsondes. Measurements to Discuss. Temperature Pressure Wind (Friesen) horizontal vertical Humidity. Temperature. Five sensors available: TT_A, TTH{L,R}{1,2}

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Basic Measurements in Progressive Science

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  1. Al Cooper 21 June 2006 Basic MeasurementsinProgressive Science

  2. Derived Variables GPS and D-values (Jensen) Dropsondes Measurements to Discuss • Temperature • Pressure • Wind (Friesen) • horizontal • vertical • Humidity

  3. Temperature • Five sensors available: TT_A, TTH{L,R}{1,2} • TT_A comes from Gulfstream avionics (with their processing) • TTH are heated sensors, hence slow-responding (>second) • TTX is selected for use in derived quantities like potential temperature. For PS, ==TT_A

  4. Processing Uncertainties • Calibration! Based on bath calibration, but there were inconsistencies still not understood and problems with TTH measurements • Mach Number dependence: NOT used, but expected. See variables TTH{L,R}{1,2}C • Recovery factor: Used 1.0, but still fine-tuning

  5. Suggestions, Temperature (for now): • Use TT_A and associated variables. (Ambient T is AT_A, and this is also ATX). No evidence of delay or filtering in AT_A; seems as good as ATH for response (unlike avionics pressure) • Avoid ATHL1: many bad flights. • Use caution for values <-60C for now. There is an apparent calibration difference at T<-60., with AT_A about 5C above others. There is also uncertainty in the recovery factor, not yet resolved.

  6. Time response of heated sensors (here TTHR3) is obviously slow vs this unheated (TTRL) one from TREX.

  7. Pressure: • As for T, there is an avionics-supplied static pressure: PS_A. The aircraft is “RVSM” certified, which needs altitude accuracy of ca. 80 ft or, at 40Kft, pressure accuracy of ca. 0.8 mb. • Pressure is measured independently as PSFC (after corrections are applied). This is the reference pressure used for Prog. Sci.

  8. Other aspects of pressure: • The avionics pressure PS_A is inherently slow and has an evident lag, so it is best avoided except as a reference for calibration. • Beware of regular several-minute oscillations created by the auto-throttles, which produce oscillations in P, Z, TAS, and sometimes other variables. (These are filtered from PS_A.)

  9. Recommendation, Static P • Avoid PS_A except for calibration: It has slow time response and an inherent lag. • Use PSFC (or PSX) for moderate accuracy. • See the study to be presented by Jorgen Jensen for higher accuracy, as needed for combining pressure with GPS-altitude to study pressure fields.

  10. Wind Measurements • Dick Friesen will discuss in some detail • Measurements look reasonable as first estimates, but need (and are undergoing) fine-tuning for such things as radome calibration, removal of airspeed offset, and an apparent but variable bias in vertical wind. • Important qualification: Sensors are slower than expected, don't provide >~5Hz

  11. Humidity • Three measurements: • DPLC and DPRC: dew point from chilled-mirror instruments. 1/s samples used (serial); also sampled at higher rate as analog output. “C” refers to correction from measured frost point to calculated dewpoint. • TDL, provided in two channels with different sensitivity. (archived as ppmv)

  12. DP sensors: * slow response * searching leads to errors * reasonable match * lower DP than prev. * P correction? * still, RMS<1.5C

  13. Beware of MR fluctuations: • frost-pt values both have floors at -74C, which is d.p. of about -87C • As T changes, MR changes, giving false signal not representing humidity changes.

  14. TDL: • Does not have this lower-limit problem • Calibration is independent of DPs • Much more sensitive at low humidity • Archived value is ppmv. Convert to MR in g/kg by multiplying by 0.622E-3. • vs DPs, values appear to be biased high?

  15. Comparison, MR from TDL and from DP

  16. Derived Variables: Some traps • Be wary of MR fluctuations; they may not be real for low values. Similar: q, RH, etc. • Potential temperature is calculated from AT_A and PSFC as in data files, and these are best values. ATHxx and PS_A are less suited to this calculation. Be aware of a remaining calibration issue with T, though, at low values: This still needs to be resolved.

  17. GPS position and altitude • Basic aircraft system provides high accuracy • For best accuracy, differential GPS data are available, but recorded separately and not yet part of the processed files. • There is high potential to use these measurements to study pressure fields with good precision and spatial resolution • Errors are now such that the error in pressure is the dominant source of uncertainty. See Jorgen Jensen's talk for ongoing work on this source of error.

  18. Example • Geostrophic wind Wg=-(g/f) (dz/dn): • measured D-value slope indicates about 80 m/s wind at left, decreasing to right

  19. Dropsondes • New installation encountered some problems with sondes sticking in the exit tube and with some striking the aircraft on release. • System was modified, and after modification mostly worked satisfactorily.

  20. see ftp://ftp.eol.ucar.edu/pub/temp/users/kbeierle/progressive_sci/

  21. Expected Revisions: • Wind: refinements underway (Friesen) • Pressure: refinements underway (Jensen) • Temperature: still need to resolve calibration, but may not revise unless ATX=AT_A needs revision • Higher-rate processing being implemented • Differential GPS handling: may be merged to primary dataset?

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