ECR for Proposed Modification of the BBSS Surface Reference System at SGP
Submitted by:
Larry Miloshevich, Barry Lesht, Mike Ritsche
Suggested Reviewers:
Scott Richardson, Dave Tobin, Dave Turner, Hank Revercomb, Jim Liljegren, Jim Teske, David
Breedlove, Annette Koontz, Karen Creel, Don Bond, Randy Peppler
1. Summary
We propose changes to instrumentation, datastreams, and associated electrical/mechanical
systems relating to the surface meteorological measurements that are made in the ventilated
“mailbox” at the BBSS launch site. The mailbox measurement system currently consists of a
chilled-mirror hygrometer (CM), a Rotronic temperature/humidity (T/RH) sensor, and a
datalogger. These data are ingested as the ‘sgpcm’ datastream. Pictures of the instrumentation
and the mailbox are posted at http:// www.mmm.ucar.edu/science/ecr/ecr.html.
The main purpose of the mailbox measurements is to provide accurate reference data for
improving the accuracy of radiosonde RH and T measurements. By placing the radiosonde
inside the mailbox before launch we can compare the radiosonde readings with the reference
measurements while they are measuring the same air. We hypothesize that these measurements
can be used to improve the accuracy of the radiosonde measurements by allowing us to remove
sonde-specific random error and batch-dependent error that results mainly from production
variability in the accuracy of the radiosonde calibration, provided of course that the reference
measurements are more accurate than the radiosonde measurements. At present, the
CM/Rotronic/mailbox system is not suitable for this purpose, because the measurements are
neither accurate enough nor reliable enough.
The radiosonde RH measurements are important for a wide variety of ARM research,
including radiative transfer calculations, validation of remote sensor and satellite measurements
and retrievals, input to models and evaluation of model results, development of cloud and water
vapor parameterizations, etc. As one example, Dave Tobin has shown that calculations of
downwelling longwave radiation at the target accuracy of 1 W m-2 requires measuring RH in the
lower troposphere to an accuracy of 2 %RH. This target is not achievable without substantial
improvement in the accuracy and reliability of the surface reference measurements that will be
used to correct the radiosonde humidity data for production variability.
We propose to replace the current CM and Rotronic sensors with 6 redundant T/RH sensors,
including 3 Rotronic MP100H-4303-10 and 3 Vaisala HMP-45D T/RH sensors. We also
propose changes in the design of the mailbox that encloses and ventilates the radiosonde and
surface sensors, with the intent that the air inside the mailbox accurately represents the ambient
surface conditions. The co-located Vaisala HMP-233 T/RH sensor that is part of the THWAPS
system and located outside the mailbox will remain as is. The THWAPS system also provides
data for checking the performance of the radiosondes before launch. Although the THWAPS
data are accurate enough to be used for gross performance checks and as continuous time series
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of surface meteorological variables representative of the launch site, they are not sufficiently
accurate for the detailed accuracy improvement we propose.
2. Data Plots that Clarify the Accuracy and Reliability Issues
The CM is not designed for continuous outdoor use with minimal maintenance, which
occasionally leads to inaccurate and even completely erroneous measurements, as well as
periodic downtime. Figure 1 shows daily time series of simultaneous RH measurements from
the CM, Rotronic, THWAPS, and radiosonde sensors, for several situations that illustrate the
inaccuracy and/or unreliability of the CM measurements. In the first four examples the Rotronic
measurements are more accurate and stable than the CM measurements, as judged by
consistency of the Rotronic, THWAPS, and radiosonde measurements. In these cases the CM
measurements either differ substantially or are entirely unreasonable, and instances of missing
data can be seen. Clearly operational CM measurements do not provide a stable and reliable
reference. Also, when the temperature is between 0°C and about –20°C, there is ambiguity as to
whether the CM is measuring the dewpoint or the frostpoint temperature. The fifth example
shows an instance when the CM and Rotronic measurements are in reasonable agreement over a
wide range in RH, and the measurements somewhat reasonably represent the ambient conditions
as judged by consistency with the THWAPS. The sixth example shows that occasionally the
Rotronic humidity measurements can drift upward when the air is at or near 100 %RH, but this is
the only condition of unreliability we have observed with this sensor. We don’t know if this is
an inherent characteristic of this sensor, or if it is caused by wetting of the sensor that points to
an inadequate mailbox. Note that this Rotronic sensor was not designed for continuous outdoor
use, and the manufacturer has suggested a better sensor.
Figure 2 illustrates the technique used to determine the bias in the radiosonde RH
measurements (black) relative to the Rotronic RH measurements (blue), during the time period
prior to launch when the radiosonde is in the mailbox and ventilated by a fan. The mean
difference between the 1-min average points (green vs blue) is the bias used to derive a
radiosonde calibration correction. Figure 3 shows a frequency distribution of the bias for
radiosondes launched during a four month period. The distribution has a range of variability of
about ±5 %RH, which is considerably greater than the target accuracy requirement. This
distribution is consistent with Vaisala’s specification for “random production variability” of ±4
%RH at the 95% confidence level. The tail of the distribution (bias < -5 %RH) occurs when the
surface RH is very high (>90 %RH), and reflects the fact that the calibration inaccuracy
increases with increasing RH. The primary goal of the radiosonde correction procedure is to
remove the variability by “standardizing” the radiosonde measurements to an accurate reference
measurement. With the production variability removed, the corrected radiosonde measurements
will still be uniformly biased from The Truth by some amount that depends mainly on the
absolute accuracy of the reference measurements, and on the degree to which the air in the
mailbox represents the actual ambient air. The two goals of this ECR are to maximize the
accuracy of the reference measurements and to reduce solar radiation and weather impacts on the
air inside the mailbox, thereby improving the accuracy of the corrected radiosonde
measurements.
3. Proposed Changes to Instrumentation
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We recommend that the CM be removed from the mailbox and no longer used for operational
measurements. The Rotronic sensor currently in the mailbox should also be removed because it
was not built for extended outdoor use, and the manufacturer suggests using the MP100H-430310 instead. We feel it is very important to have redundant reference measurements in the
mailbox, partly to allow automated detection of any sensor that is out of calibration or otherwise
not performing well. This requires at least 3 sensors, so that two consistent measurements can
identify the poorly-performing sensor. Redundancy also reduces the random calibration error
through averaging, allowing us to achieve the desired 2 %RH accuracy that is questionable for
any single sensor. The Rotronic sensors are small and relatively inexpensive, which suggests
that we use a great deal of redundancy, both for quality assurance, but more importantly for
reducing the random calibration error. We recommend installing a total of 6 T/RH sensors in the
mailbox, 3 Rotronic sensors and 3 Vaisala HMP-45D sensors, which also allows us to study the
differences between sensors and manufacturers. A different datalogger, the Campbell CR-23X,
will be needed in order to acquire the 6 T/RH measurements using differential connections.
Following are the specifications and costs for the T/RH sensors and the datalogger:
Rotronic MPH100H-4303-10
4 wire RTD Pt-100 ohm 1/10 DIN with Rotronic Hygromer C94 Capacitive humidity sensor.
Output range 0…1V = 0….100% RH, will overrange.
Operating measurment range 0…100% for RH, –40…60 deg C for temp.
Max stdev for 1/10 DIN +/- 0.03 deg C
Optional 4 point calibration for +/- 1.0 %RH and +/- 0.2 deg C accuracy at 25 deg C.
Cost per sensor $835.00
Vaisala HMP-45D
4 wire RTD Pt-100 ohm 1/3 DIN with Humicap 180 Capacitive humidity sensor.
Output range 0….1V = 0….100% RH, will overrange.
Operating measurement range 0….100% for RH, –40…..60 deg C for temp.
Max stdev for 1/3 DIN +/- 0.1 deg C
Cost per sensor $565.00
Campbell Scientific Datalogger CR-23X
CR-23X-4M with 4M extended memory $2570.50
Sealed rechargeable battery base $291.00
Calibration Certificate $150.00
Removable gold plated connector plug (4) $45.20
Header gold plated for connector plug (4) $22.80
Manufacturing cost for special connector $158.00
LoggerNet Datalogger support software $383.15
Cost for sensors and datalogger + one each sensor for spares: $9220.65
(cost does not include GSA pricing except for the datalogger)
4. Proposed Changes to the Mailbox
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We recommend several modifications to the mailbox, in addition to the mechanical/electrical
changes needed to accommodate the new complement of sensors. Two design criteria are
critical: 1) radiative heating/cooling must be minimized, and 2) moisture must not be allowed
into the mailbox for later evaporation. These goals can be accomplished with the current
mailbox by placing it inside a “Stevenson Screen”, similar to the shelters that NWS uses at its
cooperative sites for min and max liquid in glass thermometers. The shelter is a wooden box
painted white with a sloped roof and angled slats on all four sides, which allows free airflow but
protects the mailbox inside the shelter from solar radiation and precipitation. The mailbox will
be placed inside the shelter and will continue to serve the purpose of ventilating the sensors and
radiosonde in close proximity. The shelter pictured on the web page was acquired free of charge,
and will be adapted for our purpose. The foam insulation inside the current mailbox will be
removed, because there is uncertainty as to whether it absorbs moisture during high RH episodes
and then releases this moisture later.
There is uncertainty regarding the time period that a radiosonde is actually inside the mailbox,
and in some cases the correlated time series suggest that the radiosonde was never in the mailbox
at all. Reliable determination of the radiosonde bias requires knowing when the radiosonde and
reference sensors are measuring the same air. For example, the red points in Figure 2 obviously
should not be included in the bias determination, but at present these points are identified based
on several numerical criteria, none of which is actual knowledge that the radiosonde is in the
mailbox. We suggest that a simple mechanism be incorporated into the mailbox that signals the
presence of a radiosonde, and that this signal be recorded and incorporated as a parameter in the
datastream. We suggest a simple contact switch that completes a circuit when a radiosonde is
placed in the proper position, and that a binary 0/1 flag be recorded to indicate the
absence/presence of a radiosonde. This will allow us to define the appropriate time period to use
for the bias determination, with the additional benefit that all operators will automatically place
the radiosondes in the same position relative to the reference sensors, and in the proper
orientation (with the radiosonde battery downwind of the radiosonde and reference sensors).
5. Proposed Changes to the sgpcm Datastream
If the CM is removed, the name of this datastream should be changed. At present, the sgpcm
data files contain the 1 minute average and standard deviation of each parameter. This seems
sufficient, and we recommend that this strategy be continued, except that now there will be
means and standard deviations from more sensors. In addition, there will be the 0/1 parameter
that indicates the presence of a radiosonde in the mailbox.
At present, the datalogger clock is checked against official ARM time daily, and the clock is
reset if the difference exceeds one minute. A one minute offset is quite apparent in the time
series comparisons when the RH is changing on that timescale. We recommend that the
datalogger clock automatically be reset daily regardless of the offset. Note that there is an
additional one minute uncertainty in the radiosonde times because the seconds are not recorded.
We will write an analysis routine that evaluates the 6 redundant measurements and identifies
any outliers that probably indicate a sensor is damaged or going out of calibration. This will
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serve two purposes. First, a mentor QC flag will be set so that the offending sensor can be
investigated. And second, a quality-checked “best estimate” of the surface RH and T will be
calculated from the non-offending sensors, and these estimates will be incorporated as separate
parameters in the datastream.
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