Category Archives: Atmospheric 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

 

 

Ceilometers for Dust Storm Profiling

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.

dust-storm

(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

High Altitude Clouds

High Altitude clouds  fall into 2 categories,

  1. Those with  a low base and vertical development are Cumulonimbus and Towering Cumulus.     At the base these are water clouds,    so the ceilometer only “sees”  a few hundred feet into the cloud.
  2. Those with a high base,  including Cirrus, Cirrostratus and Cirrocumulus and Altostratus

The Cirrus cloud family are composed of Ice Crystals,  and are very often “optically thin”and they have low backscatter coefficients,  so are difficult to detect with ceilometers,  because the laser pulse energy is limited to eye safe levels.

Altostratus may be composed of ice crystals. In some ice crystal altostratus, very thin, rapidly disappearing horizontal sheets of water droplets appear at random. The sizes of the ice crystals in the cloud tended to increase as altitude decreased. However, close to the bottom of the cloud, the particles decreased in size again

Altostratus cloud with a water phase  may have a strong backscatter signal and can be picked up as in the case below

17000 FT 26-11 TREVISO

Alto stratus Cloud at 17,000 ft

Ref 1 : Wikipedia : Alto Stratus entry.

Ref 2.  Wikipedia Cirrus Cloud entry.

Cloud Atlas

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

Clouds Online

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.

Sky Condition, How is it assessed ?

A human observer looks at the sky and estimates the coverage in 8ths ,  0 being clear sky and 8 being overcast.   The human observer then estimates cloud height and applies these estimates of cover for each layer.    It is quite obvious that if there are no breaks in the sky,  any higher layers present cannot be estimated.    The human observer also suffers from the “packing” effect of an oblique line of sight , and usually tends to overestimate cover.

For each layer the human observer will give the condition FEW,  SCATTER,  BROKEN AND overcast.

A ceilometer can only “see” cloud above it,  so can only estimate the sky condition by analysing heights over a time period.

The Sky Condition Algorithm in the 8200-CHS  is based on that developed by the US National Weather Service and used in their automated surface observing system (ASOS) units and  guidelines published by the World Meteorological Organization.

A study by the Hughes STX Corp. found that when ceilings were under 5,000 feet, this algorithm agreed with the human observer 78% of the time. With fog, the comparability was 84%, with rain it was 69%, and when snowing 74%. During rain, the NWS Algorithm reported more changes than the human observer.

However at the transition between scattered and broken cloud coverage 4 oktas humans often report too much cloud coverage. This is attributed to the “packing effect;” a condition where an observer does not see the openings in the cloud decks near the horizon due to the viewing angle.   Pilots tend to overestimate the coverage even more than ground observers because of visual compression.

The 8200-CHS algorithm is not biased by the “packing effect” because it measures only the sky conditions passing over the sensor

Details of the 8200-CHS specifications can be found here 8200-CHS Page

Why tilt a ceilometer?

A slight tilting of the ceilometer gives  better performance in rain.

Rain drops tend to flatten as they fall.  See the NASA explanation for this 

Consequently, when aligned vertically the backscatter from raindrops may be sufficiently high to cause difficult in resolving the cloud base above the rain.

However when tilted, the backscatter of laser pulses by the raindrops is reduced .

In heavy rain even a tilted ceilometer cannot resolve the cloud base since the integrated backscatter quickly dominates and prevents further penetration up to the cloudbase,  while extinction in the return path also starts to extinguish the return signal..

Please refer to the screen below.

8200-tilted-untilted

These tests were carried out during light rain.  The tilted unit shows resolvable cloud base while the untilted unit reverts to vertical visibility.