Category Archives: Atmospheric Physics

Cloud-loss-could-add-8-degrees-to-global-warming

A recent article in Quanta Magazine reports that simulations indicate some cloud species may not form,  causing a positive feedback in global warming,  giving rise to added global temperature rise.

https://www.quantamagazine.org/cloud-loss-could-add-8-degrees-to-global-warming-20190225/

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Ceilometers vs MPL

There is a decision to be made when deciding between these 2 forms of instrument when considering a monitoring program.

Firstly both Ceilometers and MPL ( Micro pulsed Lidar)  are  instruments based on the LIDAR principle.     MPL uses photon counting detectors,  while Ceilometers use avalanche photodiodes in analog mode.   Beyond that all sampling and data conversion are digital.

  • Cost

Ceilometers are significantly lower in cost than MPL  This is mainly due to the laser technology used.    The lasers in Ceilometers can last up to 6-8 years and the laser replacement cost is  relatively low.

  • Laser Pulse Source.

Most Ceilometers use  a low cost solid-state pulsed laser diode,  while the MPL  uses a frequency doubled NdYag laser.

  • Wavelength

Most Ceilometers emit in the range 905 to 920 nm,  which is not  visible and the launch pulses are reduced in energy so that the sensor is eye safe.   The MPL emits at 532 nm and is able to launch higher energy pulses meaning the range is greater,  or conversely the ability to detect low levels of scattering ( eg MIE )  at low levels is enhanced.   Potential users need to look at the differences between scattering properties of the target at the 2 wavelengths.

  • Polarisation

Ceilometers do not have dual polarisation channels and the discrimination capabilities of MPL may make them more suitable for the planned studies

  • Deployment Costs

Ceilometers are designed to be operated long term in a wide range of environments and are more suited to remote deployment where there is less maintenance available.

Ceilometers are widely used for PBL studies.   Because of increased range/sensitivity MPL is favoured in high cloud studies,  but high range ceilometers are available and might be selected if a multi sensor long term study is being undertaken.

 

Ceilometers for Cloud Amount

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.

https://journals.ametsoc.org/doi/pdf/10.1175/JTECH-D-15-0258.1

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

 

 

ToPROF: Ceilometers and Lidar Applications

 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.
For overall details refer to the ToPROF Home Page
Within the To PROF group,  a Ceilometer Working group has been established.
Here are the details of this working group  This group would be of interest to  those within the following End-Users
  • 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
    dilution effects.
  • 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.

International Commission on Clouds and Precipitation (ICCP)

The International Commission on Clouds and Precipitation (ICCP) is a Commission of the International Association of Meteorology and Atmospheric Sciences (IAMAS)
The IAMAS is one of the associations of the International Union of Geodesy and Geophysics (IUGG)

The ICCP holds a conference every 4 years.   The last conference was at Manchester University in 2016, The next is due in 2020.

Typical subjects in calls for papers are theoretical, observational and numerical modelling studies of cloud and precipitation physics, cloud chemistry and cloud dynamics.  

For instance the following subjects are commonly covered at the conferences

  • Basic cloud and precipitation physics
  • Warm boundary layer clouds
  • Convective clouds (including cloud electrification)
  • Mixed phase clouds (including Arctic/Antarctic stratus, mid-level clouds)
  • Cirrus clouds
  • Orographic clouds
  • Fog and fog layers
  • Mesoscale cloud systems (including severe storms)
  • Tropical clouds
  • Southern Ocean clouds
  • Polar stratospheric clouds and noctilucent clouds
  • Aerosol-cloud-precipitation-interactions and processing
  • Clouds and climate (including radiative properties of clouds)
  • Ice nuclei and cloud condensation nuclei
  • Cloud and precipitation chemistry
  • Measurement techniques (of cloud and precipitation properties) and uncertainties
  • Applications of cloud and precipitation physics

Ceilometers and Snow

Like Rain, Snow produces quite high levels of backscatter.   But the raindrops and snow flakes have very different shapes, velocities and surface area to mass ratios.  More expensive ceilometers may have the ability to discriminate between snow and rain.

A typical LIDAR curtain plot for cloud appears below:

snow plot

Ref: University of Utah Atmospheric Science

The snow is coming from a cloud at around 500m .  The cloud and snow appear to extinguish the returns from higher layers,  if any.   Some of the snow is light and evaporates before it gets to ground level.  ( low level green return )

Work has been done to try to determine snowfall rate from Lidar returns.  According to Ed Eloranta of the University of Wisconsin Madison,  the technique requires radar and  does not require any knowledge of the snowflake shape.

Download Poster

http://lidar.ssec.wisc.edu/papers/conferences/arm2009/arm_09.pdf