| Description: | DQR DESCRIPTION:
DQR No: Platform: BBSS
Subject: Sonde RESEARCH mode altitudes biased low
Date Submitted: 8/15/95
Submitted By: Barry Lesht _X_ Instrument Mentor
___ EST Member
___ Science Team Member
___ Other _____________________________
For questions or problems, please contact the ARM Experiment Center at
509-375-6898 or via email at dqr@arm.gov.
Platform/Measurement:
What level data: (raw,a0,a1,b1,c1 etc): All
What location was the data collected at: CF, all BFs
Period of time in question
Begin Date 4/ 7/94 Time : (GMT)
End Date / / Time : (GMT)
Data should be labeled:
___ questionable ___ All data fields affected
_X_ incorrect _X_ Only some data fields affected
___ wrong calibration
___ others
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. My
simulation experiments have not yet progressed to the point where I can
confirm this independently.
Other observations/measurements impacted by this problem: Unknown.
Suggested Corrections of the Problem: (e.g. change calibration factor and
recompute, flag data with this comment, etc.)
Users of the data should be informed of the problem. I have developed software
that will reproduce the standard output stream with correct altitudes from
the raw binary data and can provide it to interested users. Boundary facility
soundings can also be reprocessed through the digi-CORA systems if need be.
Data Processing Notes Date |