Automatic LIDAR ceilometer
Introduction
The Eyjafjallajökull and the Grimsvötn volcanos in Iceland erupted in April 2010 and in May 2011 respectively causing massive disruption to the European air traffic. These eruptions highlighted the need for Automatic LIDAR (Light Detection And Ranging) Ceilometer (ALC) monitoring stations capable of routinely estimating the vertical profile of aerosols. In May 2011, RMI installed an ALC at Uccle (Belgium). The ALC primarily designed for cloud base height detection (used for air traffic safety and weather forecasting), has greatly improved over the last years, and now offers the opportunity to monitor the vertical profile of aerosols and the mixing layer height (MLH) on a continuous temporal scale. The MLH is an important parameter for several air quality applications. The measurement of MLH can improve the forecasting of the dispersion of trace gases and aerosols in the first layers of the atmosphere.
Fig. 1: The ALC (Vaisala CL51) at Uccle
Principle of an ALC
The operating principle of an ALC is based on the interaction between a short pulse of light (LASER) emitted vertically into the atmosphere by the ALC with the different components of the atmosphere (clouds, fog, particles,...).
Fig. 2: Schematic representation of the operating principle of an ALC. The total time of travel of the light corresponds to the distance of the round-trip between the ALC and the backscattering components.
A part of the light is backscattered (returns to the direction it came from) by these components and is collected by the ALC. The height where the backscattering components are located can be deduced from the measurement of the time needed by the light for the round-trip between the ALC and the backscattering components. The measurement of the magnitude of the backscattering light can provide information on the composition of the atmosphere. In its normal full-range operation, one vertical backscatter profile (with a spatial resolution of 10 m) from ground up to 15 000 meters is measured by the ALC with a period of 6 s.
Fig. 3: Example of one day of ALC backscatter profile measurements at Uccle between 120 m and 3 000 m in function of the time (Coordinated Universal Time). The red points correspond to the detection of the Mixing Layer Height (MLH).
Examples of special events observed with ALCs in Belgium
- 22th April 2020: A major forest fire broke out at the De Liereman estate in the province of Antwerp. The smoke plume released by the fire was visible from several kilometers away. An ALC from our network located in Zeebrugge, about 100 kilometers from the fire, observed the passage of the smoke plume at an altitude of between 600 and 800m. More details about this event are available in French or in Dutch.
- 12th and 13th September 2020: Several major forest fires in the state of California (United States) broke out in early September 2020. These fires were so intense that it was possible to detect with the ALC the smoke plume located at high altitude over Belgium. More details about this event are available in French or in Dutch.
ALC network in Belgium
Over Europe a dense ALC network provide continuous backscatter profiles for the monitoring of vertical profiles of aerosols including volcanic ash. The coordination of this network is ensured by E-PROFILE that is a part of EUMETNET. RMI is part of this network with 4 ALCs (VAISALA CL51) that work in operational way.