RADIOSONDE TEMPERATURE, HUMIDITY, AND PRESSURE RESPONSE AT LOW TEMPERATURES

1
RADIOSONDE TEMPERATURE, HUMIDITY, AND
PRESSURERESPONSE AT LOW TEMPERATURES

• Stephen R. Hudson, Michael S. Town,
• Von P. Walden, and Stephen G. Warren
• 24 June 2003

2
Introduction

• Vaisala RS80 and AIR 4A and 5A radiosondes were
  tested during summer and winter at South Pole.
• Their response to sudden, large changes in
  temperature and humidity was characterized.
• Some data were collected with the RS80s to
  characterize their response to smaller, more
  gradual changes in temperature, humidity and
  pressure.

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Large, Sudden Changes

• Moved sondes from inside to outside and recorded
  response of reported temperature (T), humidity
  (RH) and pressure (P).
• Usually building was heated, with an indoor T of
  5 to 25C.

• A couple tests were done with an unheated
  building, with indoor T between 43 and 56C.
• Outside T ranged from -24 to 71C.
• Range of T differences was 11 to 94 K.

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RS80 Temperature Response
5
RS80 Temperature Response

• Reported temperature responded with a simple
• exponential decay.

Minimum 2.6 s
Median 5.4 s
Mean 5.9 s
Maximum 10.9 s

• Exponential-decay time
• constants ranged from 2.6
• to 10.9 seconds, with a
• mean of 5.9 seconds.

• There was no apparent difference in the
• character or time of the response between
• summer and winter.

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RS80 Humidity Response
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RS80 Humidity Response

• When moved outside, reported RH initially
  decreased, then
• increased with an exponential decay towards
  the outside value.
• The exponential decay began between 5 and 120
  seconds after
• being moved outside.
• E-folding time constants were between 13 and 420
  seconds.
• Response was significantly slower in winter
  (Tlt-40C) than in
• summer (T near 25C).
• Response in unheated winter cases (DT 17 K)
  was similar to
• winter cases using a heated building.

Summer Winter Unheated winter
of Tests 5 24 2
Outside T (C) -24 to -25 -45 to -71 -60
t1 (sec) 5-30 (30) 5-120 (33) 60-90
t (sec) 13-20 (15) 30-420 (146) 50-120
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RS80 Pressure Response
9
RS80 Pressure Response

• When moved outside, the sonde initially reported
  increasing P,
• followed by a slow exponential decay back to
  the correct P.
• Reported P increased by 0.4 to 10.0 mb over a
  period of 175 to
• 450 seconds.
• E-folding time constants were between 230 and
  600 seconds.
• Magnitude of maximum error increased with
  increasing thermal
• shock.

Summer Winter Unheated Winter
of Tests 1 24 2
Outside T (C) -25 -45 to -71 -60
t1 (sec) 450 175 to 450 (300) 275 to 400
dP (mb) 3.3 3.5 to 10.0 (6.0) 0.4 to 1.6
t (sec) N/A 230 to 600 (380) N/A, 1500 to recover
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Small, Gradual Changes

• After equilibrating to conditions outside, sonde
  was raised and lowered on a 22 m tower at speeds
  of 0.4 to 1.0 ms-1.

• At the top of the tower, the temperature was 3
  to 5 K warmer, the relative humidity was 3 to 5
  higher, and the pressure was about 2 mb lower
  than at the surface.

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Tower Tests

• Pressure responded to within the noise level by
  the time descent was completed. Change (about
  2.2 mb) is approximately correct, according to
  hypsometric eqn.
• Temperature responded fully within 8 to 15
  seconds of completion of descent.
• Relative Humidity took 15 to 20 seconds after
  descent to fully equilibrate. The sonde was able
  to correctly report the supersaturation with
  respect to ice.

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Conclusions

• Radiosondes should be stored and prepared at
  ambient temperatures
• If sondes must be prepared inside, they should be
  given at least 30 minutes to equilibrate to cold
  environments before launching
• Problems can arise even from small temperature
  differences, so unheated buildings should be
  avoided unless well ventilated

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Conclusions (continued)

• Tests on the tower indicate that the sondes are
  capable of providing better data in cold
  conditions when given time to equilibrate to
  shock of being moved outside
• Further work should be done in more controlled
  environments, and with proper ventilation
• Radiosonde instrumentation still remains sluggish
  at low temperatures