Thick clouds of smoke gathered over forests in eastern Canada this month. Within seven days, the smoke reached the Netherlands at an altitude of three to four kilometres, carried along by a strong westerly wind. Scattering of sunlight by the smoke particles in the air coloured the sky bright red-orange during sunset last week.
Forest fires in Canada and the US
Forest fires occur every summer in the vast forests of Canada and the United States. Sometimes as a result of campfires, often spontaneously by lightning combined with parched vegetation and high temperatures. From space, forest fires are closely monitored by satellites and the plumes of smoke are also clearly visible. Figure 1 shows the fires on 12 July this year with orange dots. Over eastern Canada, a large number are visible surrounded by a grey veil of smoke.
Figure 1: Satellite image of fires, smoke and clouds on July 12, 2024. Source: https:// worldview.earthdata.nasa.gov.
Figure 2: Raman lidar Caeli measurements at Cabauw on July 18, 2024 reveal the presence of a smoke veil between 3 and 4 km above the ground (light blue), high clouds above in white and low clouds below (yellow/brown). Source: KNMI.
Figure 3: Result of a calculation of where the air came from that was above Cabauw (NL) at an altitude of 3 kilometres on July 18 at 22:00.
Smoke observed over the Netherlands
On 18 July, using KNMI’s cloud altimeter at measuring station Cabauw, part of Ruisdael Observatory, researchers of KNMI measured not only high and low clouds, but also a veil of smoke at three to four kilometres altitude (see Figure 2). As the evening progresses, the veil of smoke thickens. The red light of the setting sun shines through the veil and is scattered by the smoke particles. The result: the horizon glows with a beautiful red-orange glow. New measurement and lidar instruments played a key role in uncovering this data, which were funded by Ruisdael Observatory. Lidar, which stands for Light Detection and Ranging, is a remote sensing method that uses light in the form of a pulsed laser to determine distances to the Earth’s surface. The smoke plume measured by the large Raman lidar (Caeli) in figure 2 can also be seen in the Windcube200S Doppler lidar at Cabauw, with the Doppler lidar mainly measuring the wind at the height of the smoke plume. This provides additional information about the transport of the smoke plume such as quantity.
Smoke from Canadian forest fires
The researchers investigated the origins of the smoke using the wind during the previous days. They started the calculation at three kilometres above Cabauw at 22:00 and retrieved the wind data at that time at that altitude. This told the researchers at what speed and from what direction the air arrived. With that, they calculated where the air was an hour earlier. Then they retrieved the wind data from that moment at that position and went back in time another hour. Figure 3 shows the result of calculating back over seven days. It turns out that the air came from eastern Canada when, at that time on 12 July, the air there was filled with smoke from the forest fires.
More forest fires expected this summer
Severe forest fires are currently raging in western Canada. Due to continued heat and drought, there is an increased risk of more fires in that region in the coming weeks. Climate change has increased the likelihood of fire-prone weather in Canada. Although fire prevention has reduced the number of fires, the total area of burnt natural areas is increasing.
Ruisdael instruments
The Raman lidar (Caeli) and the new Ruisdael UV-Depolarisation-Raman lidar (UV-lidar for short) have taken more recent measurements in the aftermath of the Canadian forest fires. Figures 4 and 5 are taken from Caeli on the 19th of August and show a much higher aerosol concentration from the fires in Canada. Mostly in the layers between 1.5 and 8 km, the white layers above 8 km are high clouds.
Figure 4: Raman Lidar Caeli measurements on the 19th of August, 2024 show a much higher aerosol concentration from the fires in Canada. Mostly in the layers between 1.5 and 8 km
Figure 5: Raman Lidar Caeli measurements on the 19th of august 2024, later in the evening
Figures 6 and 7 are from the new UV-lidar, which measures the depolarisation of the aerosols, giving us information about the shape and type of aerosols and the phase of cloud particles (liquid or ice). With the depolarisation ratio (figure 7) one can clearly differentiate the aerosols from the boundary lawyer below ˜1.5 km (low depolarisation), the depolarising aerosols from the forest fires (green layers), and the ice particles from the cloud above (higher depolarisation, reddish colour). The UV-lidar will be running 24/7 and allows for these kinds of particle typing. In combination with the dense Dutch ceilometer network and the Cloudnet instruments, these measurements add great value to research and monitoring capabilities.
Figure 6: UV-lidar measurements on the 19th of August 2024
Figure 7: UV-lidar depolarisation ratio on the 19th of August 2024
This news item was created with input from Arnoud Apituley, Diego Alves Gouveia and Frank Selten from KNMI. Photo credit main picture: Arnoud Apituley
