TWP/VCEIL/C3 - low receiver sensitivity

The ceilometer reported frequent receiver alarms. There are periods when the receiver 
sensitivity dropped to about 50%. The problem was corrected by replacing the instrument with 
a spare.

• Lowest cloud base height detected.(first_cbh)
• Vertical visibility(vertical_visibility)
• Aerosol backscatter coefficient at 355 nm(backscatter)
• Backscatter sum(sum_backscatter)
• Second lowest cloud base height(second_cbh)
• Third cloud base height(third_cbh)
• Percentage of nominal factory setting (0 to 999%)(receiver_sensitivity)
• Altitude of highest signal(alt_highest_signal)
• Millivolts at internal ADC input (0 to 2500)(background_light)

TWP/MPL/C3 - Temporary laser failure

No backscatter information is in the data. The backscatter data in the files during this 
time is solar background information only. 

The laser turned it's self off, reason: unknown. A power cycle of the entire system 
corrected the problem.

• null(Raw data stream - documentation not supported)

• Preliminary cloud base height(preliminary_cbh)
• Attenuated backscatter(detector_counts)

TWP/MPL/C3 - Backscatter profiles are inaccurate due to computer HD corruption.

The data shows a consistent cloud deck at 7.5km with what appears to be a duplication of 
the data from ground zero. The temperature and energy monitor values are also affected. 
The computer was found with multiple corruption problems. These corruptions may have cause 
the ghost data at 7.5km.  A chkdsk was performed which appears to have corrected the 
problem.

• null(Raw data stream - documentation not supported)

• Detector Temperature(detector_temp)
• Instrument Temperature(instrument_temp)
• Preliminary cloud base height(preliminary_cbh)
• Attenuated backscatter(detector_counts)
• Laser Temperature(laser_temp)
• Background Signal(background_signal)
• Laser output energy per pulse(energy_monitor)

TWP/SMET/C3 - ORG data noisy due to degraded ground

SMET Rain Data began to be noisy during December 2005 and the problem slowly became worse 
over time so that in February 2006 the ORG was reporting with a constant background value 
of around 0.10 mm/hr rain rate.  ORG voltage showed a near constant value above the 85mV 
threshold.  The ground for the ORG had become degraded and was lost due to corrosion.

• Precipitation mean(precip_mean)
• Precipitation maximum(precip_max)
• Precipitation standard deviation(precip_sd)
• precipitation minimum(precip_min)

TWP/MMCR/C3 - Diminished Reflectivity Sensitivity

Radar sensitivity was diminished by approximately 15 - 20 dBZ during
this period.  The result is that upper level cloud tops may be missed
and thin clouds, particularly thin cirrus, may not be detected during
this period.  Reflectivity was correctly calibrated, so values are
correct where signals were detected.  The problem was caused by
lightning damage to the low noise amplifier (LNA).  The LNA was replaced
on 20060718.

• First Order Estimate of Spectral Count Calibration(CalibrationConstant)
• Radar Constant(RadarConstant)
• Receiver Gain(RxGain)
• Receiver Sky Noise(SkyNoiseLevel)
• Minimum detectable reflectivity(MinimumDetectableReflectivity)
• Receiver 290K Level(Rx290KLevel)
• Receiver Cal Check Level(CalCheckLevel)

• Receiver Sky Noise(SkyNoiseLevel)
• Receiver Cal Check Level(CalCheckLevel)
• Minimum detectable reflectivity(MinimumDetectableReflectivity)
• Range Corrected Calibrated Power(RangeCorrectedPower)
• Receiver 290K Level(Rx290KLevel)
• MMCR Reflectivity(Reflectivity)
• Radar Constant(RadarConstant)
• Receiver Gain(RxGain)

TWP/SMET/C3 - Reprocess: Tipping bucket rain gauge added

A tipping bucket rain gauge was added to the SMET suite of instruments effective 20060926. 
 Two new variables (count and rain amount) were added at the end of the twpsmet60sC1.b1 
datastream.  When reprocessing of this datastream occurs these variables will be added 
with -9999 (missing) as the value for dates prior to gauge installation.

• base time(base_time)

TWP/MMCR/C3 - Reprocess:   Height Error

A timing error was found in the PIRAQ MMCR's.  It resulted in recorded heights being too 
high for each radar mode by the following amounts: 
   Mode 1: 24.8756 m too high
   Mode 2: 25.6249 m too high
   Mode 3: 33.1176 m too high
   Mode 4: 33.1176 m too high
Range gate spacing was not affected.

• Height of Center of Each Range Bin(Heights)

• Raw data stream - documentation not supported(raw)

• Array of heights for each power(heights)

TWP/MPL/C3 - Intermittent Double Pulse from laser

When Spectra Physics discontinued support for their laser diode power supply, an 
alternative was need to extend the usefulness of the Spectra Physics laser head inside the MPLs.  
The solution was to use a Coherent laser diode. Unfortunately, the Coherent diode lasers 
do not have the exact same characteristics as the Spectra Physics diode lasers. As a 
result, the output of the Spectra Physics laser head sometimes produces a second pulse close 
in time but lower in intensity from the first main pulse. This is known as ?double pulsing?.

Not all of the Spectra Physics/Coherent combination systems produce a double pulse and the 
systems that do produce a double pulse do not always exhibit this behavior. It can be 
identified by a second peak at ~.2km. At first glance the signal in the data will resemble 
a low aerosol or cloud layer. When the double pulse is weak, the atmospheric signal will 
obscure the double pulse signal. At the ARM sites with a sunshade such as TWP and AMF, 
the double pulse can be seen when the sunshade closes during solar noon. When the sunshade 
closes, the MPL does not see atmospheric signal so the second pulse is evident. At SGP, a 
daily window cleaning is part of the routine. During this time the double pulse can be 
seen. Without a sunshade or window cleaning it is impossible to say definitively if double 
pulse is present. 

The first system to produce a double pulse was MPL s/n 008 located at TWP ? C2 Nauru in 
February 2005. The second system was MPL s/n 004 located at SGP in August 2005. The 
potential for the other systems to produce a double pulse is high. However, as of March 2006, 
the MPLs at NSA, TWP ? C1, TWP ? C3 and AMF have not recorded data with the double pulse 
artifact.  

The ARM program is in the process of replacing all the MPLs with a newer model. After the 
new MPLs are deployed the double pulse problem should not be an issue. The old systems 
will be relegated as spares. The double pulse problem will return if an old system is 
needed to replace a failed new system.

• Attenuated backscatter(detector_counts)

TWP/SMET/C3 - Wind Speed data contain offset due to induced voltage

The marine model of the RM Young wind sensor needs twisted pair shielded cable.  The 
manufacturer did not originally supply the pigtail with twisted pair shielded cable.  Use of 
twisted pair shielded cable for the rest of the cable run to the logger was also not 
specified.  This caused an induced voltage in the wind speed line from the wind direction 
excitation.  The induced voltage varied from sensor to sensor and probably changed anytime 
the cable was replaced or parts of the wind sensor were repaired or replaced. The induced 
voltage has been seen to vary from 0.15 m/s to as much as 1.08 m/s.

• Upper sensor, wind speed maximum(up_spd_max)
• Lower sensor, wind speed minimum(lo_spd_min)
• Lower sensor, wind speed standard deviation(lo_spd_sd)
• Lower sensor, wind speed maximum(lo_spd_max)
• Upper sensor, wind speed arithmetic average(up_spd_arith_avg)
• Lower sensor, wind speed vector average(lo_spd_vec_avg)
• Upper sensor, wind speed minimum(up_spd_min)
• Lower sensor, wind speed arithmetic average(lo_spd_arith_avg)
• Upper sensor, wind speed vector average(up_spd_vec_avg)
• Upper sensor, wind speed standard deviation(up_spd_sd)