DQRID : D950815.1
Start DateStart TimeEnd DateEnd Time Data Quality Metric
04/11/1994143205/31/19942359Suspect
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Subject:
SGP/SONDE/C1 - Sonde RESEARCH mode altitudes biased low
DataStreams:sgpsondewrprB1.a0, sgpsondewrprC1.a0, sgpsondewrprB5.a0, sgpsondewrprB5.a1,
sgpsondewrprB4.a1, sgpsondewrprB1.a1, sgpsondewrprB4.a0, sgpsondewrprC1.a1
Description:
EDITOR'S NOTE:  The C1 begin date is actually 4/7/94.  This DQR applies to
all sgpsondewXprFF.aN data, where X is r or n, FF is B1, B4, B5, C1, and
N is 0 or 1.

Discussion of Problem:  I have discovered a problem with the RESEARCH mode
(for PTU) Vaisala processing that results in a negative bias in the calculation
of sonde altitude.  This bias results from neglecting the sensed relative
humidity when calculating air density when integrating the hydrostatic
equation.  In essence, the Vaisala RESEARCH mode (PTU) processing assumes
a dry atmosphere when calculating sonde altitude.  The magnitude of the bias
is cumulative with height and will depend on the vertical distribution of 
moisture, but I have found that it can be as much as 20 meters at the 
tropopause (the RESEARCH-mode sounding altitudes will be smaller than 
altitudes calculated by using sensed RH).  This problem applies to all 
soundings done in 'WXPR' mode, where X is either R or N.  In particular, 
all soundings done during the April 1994 (RCS) IOP (4/7/94 to 5/21/94) 
are affected as well as those occasional inadvertent WXPR soundings.
Note that the only variable affected is 'alt.'

In response to my queries, Vaisala explained that the dry-air calculation was
selected to increase the processing speed in RESEARCH mode.  They consider
the problem to be a minor one.  Vaisala claims that altitudes in NORMAL
(or NOMINAL) processing are calculated by including the sensed RH.
Suggestions: 
Measurements:sgpsondewrprB5.a0:
  • alt
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sgpsondewrprB5.a1:
  • alt
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sgpsondewrprB4.a0:
  • alt
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sgpsondewrprC1.a0:
  • alt
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sgpsondewrprB4.a1:
  • alt
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sgpsondewrprC1.a1:
  • alt
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sgpsondewrprB1.a0:
  • alt
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sgpsondewrprB1.a1:
  • alt
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