The Eyjafjallajökull and the Grimsvötn volcano's in Iceland erupted in April 2010 and in May 2011 respectively causing massive disruption to the European air traffic. More information about the Eyjafjallajökull eruption is available in fr or nl. These eruptions highlighted the need for LIDAR (Light Detection And Ranging) monitoring stations capable of routinely estimating the vertical profile of aerosols. In May 2011, RMI installed a LIDAR ceilometer at Uccle (Belgium) as illustrated in figure 1. More information about the installation of the LIDAR is available in fr or nl. The LIDAR ceilometer 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 LIDAR ceilometer (Vaisala CL51) at Uccle
Principle of a LIDAR
The operating principle of a LIDAR (Light Detection And Ranging, figure 2) is based on the interaction between a short pulse of light (LASER) emitted vertically into the atmosphere by the LIDAR with the different components of the atmosphere (clouds, fog, particles,...). A part of the light is backscattered (returns to the direction it came from) by these components and is collected by the LIDAR. 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 LIDAR 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 LIDAR with a period of 6 s as shown in figure 3.
Fig. 2 Schematic representation of the operating principle of a LIDAR. The total time of travel of the light corresponds to the distance of the round-trip between the LIDAR and the backscattering components.
Fig. 3 Example of one day of LIDAR 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 the LIDAR of Uccle
- 1 February 2012: During the European cold wave in 2012, an invisible (to the eyes) cloud loaded with fine particles was detected by the LIDAR at the ground level. This cold wave started on 27 January in Belgium and affected primarily Eastern Europe. In these regions, in particular in Poland, many wood heating systems were working and have released a high concentration of particles into the atmosphere. The continental circulation associated with the cold wave transported this cloud of particles over Belgium. More details about this event are available in fr or nl.
- 26 June 2013: During the morning, a smoke plume located at an altitude of 3.5 km was observed by the LIDAR. The smoke originated from large wildfires in North America where the meteorological conditions caused the smoke to be lifted rapidly to high altitudes (up to 13,5 km). At high altitudes and under specific meteorological conditions, the smoke was transported to Europe. You can find more details about this event in fr or nl.
LIDAR/ceilometer network in Belgium
During the next years, operational MLH and also particle (ash, sand, smoke...) clouds monitoring networks of LIDAR ceilometers will be established around the world, and in particular in Europe. In Belgium, in 2014, three new ceilometers (Vaisala CL51) are installed by RMI in addition to the one in Uccle.
Enlarge lidar/ceilometer plots by clicking on it. In the plot, the black points correspond to the base of the cloud.