Guide to interpreting laser ceilometer data

Laser ceilometers are vertically pointing lasers developed to report the heights of cloud bases or cloud "ceilings". The lasers we are using do not produce visible light and are eye-safe. Similar ceilometers are used at airports throughout the world.

Some of the light from the vertically pointing laser ceilometer beam is scattered back to the instrument from cloud bases and atmospheric aerosols. The fraction of the light backscattered from aerosol layers is termed the backscatter coefficient. The backscattering is range-corrected, as returns from higher in the amosphere are greatly weakened by losses in traversing a deeper layer of atmosphere. We are producing constantly updated graphs that use false color to show the laser backscatter coefficient as a function of height and time. Clouds, when present, produce large backscatter coefficients that are generally indicated by a red color. Red colors in the upper parts of the figures are clouds and the heights of the cloud bases in meters can be determined by looking at the height scale on the y-axis of the graph. It is surprising how many of our clouds are producing shafts of precipitation that dissipate before reaching the ground. These streaks of falling precipitation or "virga" are often seen below cloud base, sometimes making it difficult to determine cloud base height. Red colors near the ground are fogs. If fog, cloud or aerosol layers are thick, the laser will be unable to "see through" the cloud, so that there is then no information on the height of the top of the cloud or what aerosols or additional clouds are present above the cloud.

When the amount of light scattered back to the ceilometer is extremely weak, we designate that by a dark blue color. If the laser return is weak but variable, the figures take on a mottled or 'noisy' appearance that is often seen at the upper elevations of the plots.

Smaller backscatter coefficients indicated by the cooler colors (i.e., blues, yellows, and greens) indicate the presence of aerosols in the atmosphere. Gradations in color, which are sometimes rather subtle, are indicative of the relative concentration or aerosols. Unfortunately, actual aerosol concentrations cannot be determined from backscatter coefficients because aerosols of different shapes and sizes produce different amounts of backscatter and we do not have independent information about the sizes and shapes of the aerosols in our valley's atmosphere. Sometimes our aerosol layers are elevated, but most frequently during cold-air pools we see a surface-based layer of aerosols that typically extend up to 400 m or so above the surface.

Some example images are shown below from our ceilometer at the SW Valley site. Note that the ceilometer at the airport at the north end of the valley is a different model, with a different sensitivity and operating characteristics. Times on the x-axis are in the time zone of the computer from which the site is accessed, shown in parentheses above the figure.

16_Jan_2013

Precipitating cloud (nimbostratus) with base at about 500 m. The preciptation is snow. The cloud is much thicker than indicated, because the laser light is fully attenuated by the cloud and cannot penetrate it to reach the cloud top.


19_Jan_2012

Ground-based aerosol layer, with lots of aerosol fine structure within it. Elevated aerosol filaments are seen near the top of the layer at about 400-500 m above ground. Fog forms at about 0500 MST.


5_Feb_2013

The ground-based aerosol layer here extends to about 400 m, but with the bulk of it below 250 m. Clouds of high aerosol concentration come and go, with relatively clean air from 2230 to 0130 MST. A cloud drifts over the site late in the period, with a cloud base at 2700 m. The Oquirrh Mountain ridgeline is about 2500 m above the valley floor, so this cloud (probably altocumulus) is just above the Oquirrh Mountain ridgetop.



Acknowledgments: Prof. John Horel, graduate student Joe Young, and others in the University of Utah Atmospheric Sciences Department's Mountain Meteorology and MesoWest groups have worked together to get the laser ceilometers and the MesoWest webpages set up to provide real-time ceilometer data.