NSA/VCEIL/C1 - Reprocess: Intermittent data and clock drift

The available processed data for the NSA.C1 VCEIL for the time period indicated are 
incomplete.  The raw data is largely complete but pre-processing is required before these data 
can be reprocessed to produce a complete netcdf data set.

While awaiting the reprocessed data set, the intermittent data are available from the ARM 
Archive.  Users should be aware that the data are additionally compromised by a clock 
drift of at least 10 minutes, though this likely has little affect on cloud detection 
results.  The clock drift will not be addressed by the reprocessing.

Note also, that the clock drift was corrected at the end of the DQR period, but it may 
have existed before the beginning of the DQR period.

The data for this period should be used with caution.

• Third cloud base height(third_cbh)
• alt_highest_signal
• receiver_sensitivity
• laser_temperature
• Second lowest cloud base height(second_cbh)
• Lowest cloud base height detected.(first_cbh)
• filename
• Vertical visibility(vertical_visibility)
• tilt_angle
• time_offset
• measurement_parameters
• lat
• status_flag
• sum_backscatter
• lon
• range
• base_time
• alt
• Backscatter(backscatter)
• background_light
• status_string
• window_contamination
• laser_pulse_energy
• detection_status

NSA/VCEIL/C1 - poor sensitivity at low altitude

The ceilometer has an internal amplifier that it sets to low or high depending on 
prevalant conditions. Normally, the gain is automatically set high during clear skies. It appears 
that during these times, a slight degradation in either the transmitter or receiver is 
being amplified and produces a "ripple" at low altitudes in the backscatter plot. Cloud 
base height detection is not affected.

• receiver_sensitivity
• Backscatter(backscatter)

NSA/VCEIL/C1 - Degraded window

The ceilometer continuously gave warnings for window contamination because the window had 
deteriorated. Cloud base height detection did not appear to be significantly affected. 
The window conditioner was replaced on 8/11/09 but this did not clear the warning. The 
instrument was replaced.

• Third cloud base height(third_cbh)
• alt_highest_signal
• Second lowest cloud base height(second_cbh)
• status_flag
• sum_backscatter
• Lowest cloud base height detected.(first_cbh)
• Backscatter(backscatter)
• background_light
• status_string
• Vertical visibility(vertical_visibility)
• window_contamination

NSA/VCEIL/C1 - Missing data

Data are missing and unrecoverable.

• Third cloud base height(third_cbh)
• alt_highest_signal
• receiver_sensitivity
• laser_temperature
• Second lowest cloud base height(second_cbh)
• Lowest cloud base height detected.(first_cbh)
• filename
• Vertical visibility(vertical_visibility)
• tilt_angle
• time_offset
• measurement_parameters
• lat
• status_flag
• sum_backscatter
• lon
• range
• base_time
• alt
• Backscatter(backscatter)
• background_light
• status_string
• window_contamination
• laser_pulse_energy
• detection_status

NSA/VCEIL/C1/C2 - Reprocess: Incorrect serial_number and sum_backscatter fields

Attribute serial_number: In most cases, this field is not representative of the actual 
serial number of the instrument because the value did not change when an instrument changed 
locations or was replaced with a spare.
   
Variable sum_backscatter: An out-of-range condition for the "sum of detected and 
normalized backscatter" was reported as 0 instead of missing (-9999). The valid range is 0 - 
0.0999 /srad.
   
This DQR also applies to a1 and b1 level data collected prior to the begin date.

• sum_backscatter

• sum_backscatter

NSA/VCEIL/C1 - Small gaps in data

When the ceilometer was replaced, small gaps in the netCDF files occurred due to the 
presence of '?' characters in the backscatter and header records of the raw data that could 
not be ingested. It is unknown whether this was caused by a poor CPU board or 
incompatibilities with the computer configuration (hyper-threading). The problem was corrected when 
the instrument and computer were both replaced. 

During this period, approximately 3% of data are missing each hour due to this problem.

• Third cloud base height(third_cbh)
• alt_highest_signal
• receiver_sensitivity
• laser_temperature
• Second lowest cloud base height(second_cbh)
• Lowest cloud base height detected.(first_cbh)
• Vertical visibility(vertical_visibility)
• tilt_angle
• time_offset
• measurement_parameters
• status_flag
• sum_backscatter
• range
• base_time
• Backscatter(backscatter)
• status_string
• background_light
• window_contamination
• laser_pulse_energy
• detection_status

NSA/VCEIL/C1 - Gaps in data

When the CT25K ceilometer was replaced with a new CL31, checksum errors began occurring at 
regular 6-min intervals and several things were done to diagnose the problem, including 
changing the serial cable and trying different computer types. It was finally corrected 
by replacing the computer with one in which hyperthreading was disabled by default in the 
BIOS.

• Third cloud base height(third_cbh)
• alt_highest_signal
• receiver_sensitivity
• laser_temperature
• Second lowest cloud base height(second_cbh)
• Lowest cloud base height detected.(first_cbh)
• Vertical visibility(vertical_visibility)
• tilt_angle
• time_offset
• measurement_parameters
• status_flag
• sum_backscatter
• range
• base_time
• Backscatter(backscatter)
• status_string
• background_light
• window_contamination
• laser_pulse_energy
• detection_status