Lawrence Livermore National Laboratory researchers have identified a mechanism that causes low clouds – and their influence on Earth’s energy balance – to respond differently to global warming depending on their spatial pattern.
The results imply that studies relying solely on recent observed trends are likely to underestimate how much Earth will warm due to increased carbon dioxide. The research appears in the Oct. 31 edition of the journal,Nature Geosciences.
The research focused on clouds, which influence Earth’s climate by reflecting incoming solar radiation and reducing outgoing thermal radiation. As the Earth’s surface warms, the net radiative effect of clouds also changes, contributing a feedback to the climate system. If these cloud changes enhance the radiative cooling of the Earth, they act as a negative, dampening feedback on warming. Otherwise, they act as a positive, amplifying feedback on warming. The amount of global warming due to increased carbon dioxide is critically dependent on the sign and magnitude of the cloud feedback, making it an area of intense research.
This is an area attracting a great deal of research. The ramifications of changes in solar energy reaching the biosphere of earth could be serious. But studies reveal disagreement between a number of authors.
OCTOBER 31, 2016
Chen Zhou et al, Impact of decadal cloud variations on the Earth’s energy budget,Nature Geoscience (2016). DOI: 10.1038/ngeo2828
A recent paper by Wagner and Kleiss compares a Ceilometer with Total Sky Imager and Micropulse lidar looking at suitability of ceilometers for estimation of cloud amount.
In summary they say, in part:
“Ceilometers will be a mainstay of the operational automated weather observation network for years to come.
They are relatively inexpensive and require little ongoing maintenance, and are capable of 24-h observations.
While TSIs produce automated observations that are qualitatively more similar to human observations of sky cover than the ceilometer, ongoing operational
costs like mirror cleaning and reduced hours of operation limit this instrument’s applicability for deployment in unattended environments.
Two significant sources of error are associated with the automated sky cover observations obtained by single ceilometer ASOS installations: the spot view of the instrument renders it unable to see the entire sky, while the 3660-m height limit renders the instrument incapable of observing high clouds. These errors are not insignificant, and their magnitudes vary depending on the actual cloud coverage. The spatiotemporal averaging error is smallest for clear and overcast conditions as the sky exhibits little variability in these conditions. The high cloud error is at its smallest when skies are dominated by low clouds, and it tends to increase as low cloud coverage lessens, allowing high clouds to peek through.”
The ability to reliably detect upper atmosphere cloud is restricted to a subset of available ceilometers which have ranges in excess of the usual 7,500 to 10,000 metre range. Such ceilometers include the a CL51, the 8600-CHS and the CHM15K
As part of the EU “COST” programmes, the ToPROF group is working on operational ground based profiling with ceilometers, doppler lidars and microwave radiometers for improving weather forecasts.
Work started in 2014, and is progressing towards a nominal completion date in 2020.
Within the To PROF group, a Ceilometer Working group has been established.
- National weather services (incl. COPERNICUS/MACC): calibrated attenuated
backscatter profiles to evaluate NWP models (through forward operators); Cloud
base height for NWP evaluation and weather monitoring.
- Agencies in charge of atmospheric surveillance for air traffic: occurrence, height and mass concentrations of ash layers; diagnostic and short-term forecast of fog and other low visibility events.
- Agencies in charge of Air Quality monitoring: boundary layer height; freetropospheric aerosol transport.
- Networks in charge of GHG monitoring: boundary layer height to quantify GHG
- EUMETSAT: European-wide validation of cloud-base height and fog
- Renewable energy industry: Photovoltaic ReN – cloud/fog fraction and evolution for nowcasting applications (combined with geostationary satellite); Concentrated solar power: aerosol vertical distribution; Wind ReN – wind profiles from Doppler Lidars.
On 7 Sep 2015 an unprecedented huge dust plume approached the SE Mediterranean basin from the northeast- Syria region. According to the Israeli meteorological service it is the first time in 75 years of measurements, that a dust storm reaches Israel early September, lasts several days and dust concentrations reach values 100 times the normal (1700µg/m3). Dust storms are normally monitored in the east Mediterranean using satellites and surface PM data. Obviously, these cannot show the vertical evolution of the dust including penetration, sinking and cleaning since vertical profiles are not available. High-resolution, micro Lidar Ceilometer network is gradually established in Israel. A few instruments of this network were already operational during the dust storm. The most crucial vertical information, monitored by these Ceilometers with 10m resolution vertically, every 16s, is analyzed. The difference in the cloud-layers allow the investigation of the high altitude of 1000m dust penetration, its sinking into the complex structured 250-500m mixed layer and the gradual 3D cleaning. This finding contradicts the conventional understanding that cleaning is due to gradual descent and shows not only the vertical fluctuation during the entire event but also the vertical rise to 2000m at the end of the event. The vertical information showed that the actual event period duration was 7 days, compared to only 90 hours based on traditional detectors. Is it a new dust source in the E. Mediterranean-long and short term trends?
There are very few Ceilometer manufacturers in the world. Ceilometers have advanced technical requirements and the cost of development is high. In the production phase ceilometers are assembled from many special optical and electronic parts. During testing they require specialised pulsed laser power meters, spectrophotometers and advanced electronic test equipment, together with a cloudy climate to enable regular testing and continuous product improvement.
Sensor Range Class
- 12500 ft range, ( 3800m) an example of which was the original CT12K. This range is now encompassed by the 25,000 ft range sensors.
- 25,000 ft range ( 7600m) These are the main sensors on the market since in the main application there is little operational need to go beyond even 12500 ft. These sensors also find application in Planetary Boundary Layer ( PBL) studies.
- 50,000 ft range (15,200m) Special instruments that find more application in volcanic ash warning in aviation and upper atmosphere studies in atmospheric science. Although in theory only requiring a modest increase in signal to noise ratio, the cloud species above 20,000 ft are most often comprised of ice crystals and have much lower volume back-scatter coefficients than water cloud so the reliable detection of thin layers of cirrus cloud becomes very difficult while maintaining the laser eye safety mandate. Ceilometers in this range generally achieve the necessary signal to noise ratio improvement by a range of techniques including increasing laser pulse energy ( while still remaining eye safe ), using a different laser wavelength, and or reducing the telescope field of view and laser beam divergence.
25,000 ft range for Aviation and PBL studies
CL31 : http://www.vaisala.com/en/products/ceilometers/Pages/default.aspx
8200-CHS : http://www.ceilometers.com/
CS135 : https://www.campbellsci.com.au/cs135
50000 ft Range for Atmospheric Science
CL51 : http://www.vaisala.com/en/products/ceilometers/Pages/default.aspx
In the past ICAO Annex 3 SARPS recommended that a ceilometer be placed at the middle marker site 900-1200 m from the touch down zone for instrumented runways .
This had an advantage that power and comms were generally already established at that site or were planned at that site.
This gave a reading of cloud base height at a crucial decision height on the glidepath
With addition of ILS and co-located DME, more and more aerodromes have no middle marker. The piece of land located 900 to 1200 m from the landing threshold may be outside the aerodrome airfield and a ceilometer installation may be impracticable, or very costly.
As a consequence in more and more cases an alternative location must be found.
A typical recommendation for siting the ceilometer would be :
“When instrumented systems are used for the measurement of the cloud amount and the height of cloud base, representative observations should be obtained by the use of sensors appropriately sited. For local routine and special reports, in the case of aerodromes with precision approach runways, sensors for cloud amount and height of cloud base should be sited to give the best practicable indications of the height of cloud base and cloud amount at the runway threshold in use. For that purpose, a sensor should be installed at a distance less than 500 m from the threshold. This distance can be extended up to 900-1200 m from the landing threshold in the axis of the approach end of the runway”
The user should refer to the latest edition of Annex 3 to ensure compliance.
Sydney Dust Storm 2009
A wall of dust stretched from northern Queensland to the southern tip of eastern Australia on the morning of September 23, 2009, The storm, the worst in 70 years, led to cancelled or delayed flights, traffic problems, and health issues, The concentration of particles in the air reached 15,000 micrograms per cubic meter in New South Wales during the storm, A normal day sees a particle concentration 10-20 micrograms per cubic meter.
Work on the use of Ceilometers for analysis of that Dust Storm is decribed in the paper:
Laser ceilometer measurements of Australian dust storm highlight need for reassessment of atmospheric dust plume loads By Hamish McGowan and Joshua Soderholm
Among the more interesting information in this paper was the curtain plot showing the increase in backscatter when the wall of the duststorm hit, the very high concentration around ground level and the vertical extent of the dust. The maximum vertical extent of this plot is 1500 metres , or approx 5000 ft.
(Curtain Plot showing onset of the Dust Storm and estimated particle Concentration from paper: Laser ceilometer measurements of Australian dust storm highlight need for reassessment of atmospheric dust plume loads By Hamish McGowan and Joshua Soderholm )
Ceilometers like the 8200-CHS are suitable for this type of work, where dust storms are experienced regularly, such as the Harmattan in sub saharan Africa, the Churgui in Morocco, the Khamasin in Egypt, the Shamal in Iraq or the Kali Andhi in India
The definitive reference to cloud types and Cloud species is the ICAO Cloud Atlas which can be downloaded for free from this site:
WMO Cloud Atlas Vol 1 WMO Cloud Atlas Vol 2
There are a number of less detailed cloud type images on the internet. For example
The Australian Bureau of Meteorology have a good references to cloud observation
Australian Bureau Chapter 13
Cloud Types Graphic
Current ceilometers have a range out to about 25,000 ft, and other models with larger telescopes built in,. can reach up to about 40000 ft. For use in aviation, at airports a range of 12,00 ft is considered adequate.