The Vaisala Ceilometer (CEIL) is a self-contained, ground-based, active, remote-sensing device designed to measure cloud-base height, vertical visibility, and potential backscatter signals by aerosols. It detects up to three cloud layers simultaneously. Model CL31 has a maximum vertical range of 7700 m. The laser ceilometer transmits near-infrared pulses of light and the receiver detects the light scattered back by clouds and precipitation. For more information see the CEIL Instrument web page.
The CEIL does not “see” higher-level clouds. Thus, cloud base heights that exceed approximately 7 km are not normally reported by the CEIL. The CEIL frequently has “seconds-long” data gaps, much like the MPL. However, the frequency of these data gaps is much less, making data availability checks much easier for this instrument. Generally, a data gap over one minute is a data availability issue.
The primary variable for the CEIL is cloud base height for three possible levels. In the CEIL metrics table, it is common for there to be frequent missing data flags for alt_highest_signal, first_cbh, second_cbh, third_cbh, and vertical visibility. This is OK. So long as the diagnostic plots look correct, the instrument is working correctly. This is occurring because the data file needs to report something, and a 0 could be misinterpreted as a cloud base height. So when the instrument does not report a cloud base height it is filled with the missing value indicator, -9999.
It is normal for altitude of highest signal, first cloud base height, second cloud base height, third cloud base height, and vertical visibility to be frequently flagged as missing.
bl_height_1, bl_height_2, and bl_height_3 are new to the ceil metrics tables and represent the first, second, and third boundary layer height candidates. These come from the ceilpblht datastream, and can also frequently be flagged as missing. The ceilpblht datastream contains planetary boundary heights below 4,000 meters, and includes data from the top of the planetary boundary layer; also known as the depth or height of the mixing layer.
Active Alarms or Warning
At times there may be an Alarm or Warning active. This can be due to a dirty window or another issue with the instrument. If there is a Warning active, the status flag will be yellow. If there is an Alarm active, the status flag will be red. Flags only active for a minute at a time do not need to be mentioned, unless specified otherwise by the instrument mentor, but any enduring Warnings and Alarms need to be included in the DQA.
This is an example of a normal VCEIL backscatter plot on a cloudy day.
This plot shows the first, second, and third planetary boundary layer heights.
The first plot shows the detected cloud bases and their heights. The flags are color coded to indicate how many obscurations were detected by the VCEIL. The second plot is the status flag which reports what the instrument is currently reporting about its operation. A Warning level could be an indication of a dirty window or some other possible problem. This may or may not be important and would be additional information if the backscatter plot is showing signs of a dirty window or some other issues. The Alarm level is an indication of a serious problem and should be investigated. If the instrument reports Alarm Level the cloud data will be invalidated. Flags active for only one minute typically do not need to be reported, unless specified by the instrument mentor.
These plots show different diagnostics for the instrument.
This plot shows a comparison between the VCEIL and the IRT IR Sky Temperature.
This plot shows a comparison between the VCEIL and the cloud cover detected by the TSI.
This plot shows a comparison between the VCEIL and other profiling instruments on site. The cloud bases detected by the VCEIL are indicated on the other instruments' plots with a white dot or line for easier comparison.
Problems that do need to be mentioned in DQAs are mentioned below:
Abnormally low backscatter plot amplitudes
Consistently high amplitude backscatter
Wavy backsacatter plots
Little difference between day & night backscatter
Vertical stripes in backscatter
Low amplitude ripple effect
Dirty Window method II
A second way to find if the window is dirty is through the Status Flag. Normal operation has the flag set to OK. When the window begins to show signs of accumulated dirt the Warning status will begin to be set. The change will typically be gradual and may take days or weeks. Although, once the Status Flag is continuously showing Warn or Alarm it's time to notify the appropriate people. See DQPR 1701, DQPR 1762
Intermittent receiver alarms and discontinuities
Tilt Angle Flagged
Occasionally the tilt angle will be flagged due to an incorrect positioning of the instrument. While this should be noted in the DQAs, a DQPR is not necessary unless it persists for several days. As long as the data appears to be good it likely is. This is due to the instrument measurements being compensated for the angle.