Webinar topics: Basic technology of Lidar Ceilometer, Ceilometer usage in air quality and boundary layer applications, Real-life examples from measurement campaigns
Ceilometers are eye-safe, compact and robust LIDAR systems designed for unattended operation at airfields and meteorological stations. They are primarily used to detect and specify cloud base height and sky cover, but recently their use in the automatic monitoring of boundary layer structures has become more prominent. This is largely a result of the ceilometer’s ability to detect mixing layer height, a key variable in the characterization of air pollution. The planetary boundary layer height is a key parameter for characterization of air pollution together with urban emission source strengths, traffic emissions and weather influences. Vaisala has developed an automatic algorithm for online retrieval of boundary layer depth and additional residual structures that covers not only ideal boundary layer diurnal evolution, but all situations involving clouds, fog, and precipitation.
Cloud base is just the beginning: Modern day use of the laser-ceilometer
Subject matters that are covered in this webinar include:
• Basic technology of Lidar Ceilometer
• Ceilometer usage in air quality and boundary layer applications
• Real-life examples from measurement campaigns
The webinar speaker is Vaisala Research Scientist, Dr Scott Mackaro.
Questions & Answers
Q: What is the range of normal LIDAR? I mean how far can it shoot in the atmosphere?
A: The CL51 range is 15km and the CL31 range is 7.5km.
Q: Doesn't the light scatter in other directions? i.e not all energy is reflected back to the receiver. Doesn’t this mean there needs to be a high sensitivity for the reduced signal level?
A: Indeed the particles will scatter in all directions and only small part of the signal is backscattered. Therefore, the ceilometer requires a highly sensitive receiver.
Q: What model allows the 10m backscatter measurement?
A: Both CL31 and CL51 report backscatter from 10 m distance onwards. Standard range resolution is 10 m.
Q: When we compare ct25k and cl31 there is difference in reading?
A: It is expected that the CT25K and CL31 results differ as they have different resolution (CT25K has 15m and CL31 has 10m) and CL31 SNR is better than CT25K.
Q: Instead of having several ceilometers is it possible to have one ceilometer scanning an arc?
A: The single sensor measurement range is not enough to cover a large area even if it would be scanning.
Q: Will the dust deposited in the window (mirror) of the Ceilometer affects the reading of the instrument?
A: The dust in the window will attenuate the signal and therefore it is important that the window is kept clean as this applies for all optics. The CL31/CL51 enclosure includes a blower to deal with this issue in the field.
Q: Is it possible to determine the mixing layer height on clear days when we get really low backscatter intensity?
A: In practice typically yes, but if there are no scattering particles then there is no signal either that could be used for mixing layer detection.
Q: Was the first visual of the atmosphere retrieved by a ceilometer or by another instrument? (very high altitude)
A: Atmospheric scans have been performed by more sophisticated (and far more expensive) lidars before ceilometers could be used for this.
Q: Actually how high is the ceilometer from Vaisala able to detect the PBL?
A: Up to 4000 meters.
Q: What does Melting Layer refer to?
A: In melting layer the solid precipitation will turn into liquid.
Q: How can you derive the temperature showed in picture on page 27?
A: The temperature profile shown here is not a derived product, but rather radiosonde measurements overlaid on the ceilometer time series plot.
Q: How do you determine the error bars on the aerosol layer tops you showed in your zoomed in example?
A: This is explained in section 2.3 of http://www.esrl.noaa.gov/psd/events/2012/isars/paper_export_pdf.php?paper_id=5&session_id=S06-3
Q: Up to how many layers is the ceilometer able to detect?
A: Up to 3 layers in a single vertical measurement.
Q: How do you discriminate between mixing layer and other aerosol layers using your gradient detection method?
A: The current version of Vaisala BL-VIEW does not discriminate between mixing layer and other aerosol layers.
Q: What degree of human interaction is required to identify aerosols?
A: BL-VIEW identifies aerosol layers automatically, without human interaction.
Q: To what extent can identification be automated?
A: Layer identification is performed automatically. With backscatter signal only (no 2nd wavelength, no depolarization) it is not possible to identify aerosol type.
Q: Will the difference between ash and thin clouds (like cirrus clouds) be as large as the example?
A: Signal from thin clouds is usually larger than that of volcanic ash having travelled already several thousand kms.
Q: Which software can be used to analysis the CL 31 data?
A: Vaisala BL-VIEW software is available for this purpose. Other options include interactive data languages such as NCL, IDL, MatLab, etc.
Addendum to Webinar slide 8: The one over lambda to the fourth relationship for backscattering applies best to molecular scattering. For atmospheric aerosols, the relationship is better represented as one over lambda.