In early September 2020, unusually high winds and continued dry weather caused the rapid expansion of multiple wildfires in Oregon.
09 February 2021
03 February 2021
By Jochen Kerkmann (EUMETSAT) and Ivan Smiljanic (CGI)
It was reported that 1,000,000 acres were burned, and about 40,000 people were evacuated, with a further 500,000 people in evacuation warning areas.
While the impact of the 2020 fires on atmospheric composition is discussed in this California fires case study, this case focuses on the night-time view of the fires and smoke, with emphasis on the interpretation of the Night Microphysics RGB. This RGB gives the best combined view of high, mid and low clouds, cloud microphysics (to a degree), fires, smoke (when the smoke is thick and high enough) and moisture boundaries.
The GOES-17 Night Microphysics RGB with 500 hPa absolute topography overlaid (Figure 1) shows the synoptic situation that led to the strong intensification of the fires on 8 September. A sharp upper level trough (see 500 hPa topography) can be seen over the western US. Related to this trough, a cold front with strong post-frontal winds travelled southward. High surface winds behind the front lifted dust and ash (from still hot burn scars), which can be seen by its magenta colour in the RGB product.
On the GOES-17 Night Microphysics RGB at 09:00 UTC (Figure 2) numerous active (new) fires in Oregon can be seen in vivid magenta colour; old fires and hot spots in Washington and California appear with a dark magenta colour.
A large cyan plume is visible over the Pacific. As this is an unusual colour for this RGB product, we consulted the RGB Colour Interpretation Guide where we found one example for cyan colour, namely a warm, thin dust cloud over cold surfaces (inversion situation).
So, could this be a dust cloud over cold water or is it a smoke plume with large particles? And why is the plume not visible over land? The answer to the latter question is probably the lacking temperature contrast of the plume and the land surface, i.e. a 300 K feature cannot be seen over a 300 K background. The cold water of the Pacific Ocean provides a better contrast to the warm plume. Regarding the first question, looking at the loops of 8 and 9 September, we believe that we are actually looking at a dust plume, mixed with smoke from the Oregon fires.
The animation of the RGB product from 02:00 to 12:30 UTC (Figure 3) visualises the fast movement of the cold front (and related dust plumes) , and it shows the extension of the Oregon dust/smoke plume over the Pacific Ocean. Note that the strong northerly winds also lifted/picked up some ash from California hot spots/burn scars (from fires in August 2020); these plumes appear in bright magenta colour.
These two types of plumes, cyan plumes of smoke from new Oregon fires and magenta plumes from ash from hot spots in California, are still dominating the scene on the following day, 9 September, (see Figure 4 and interpretation of the 09:00 UTC image, Figure 5).
The fires/hot spots in Oregon have become much bigger, and the smoke is visible also over land. In addition, a pyro-Cb can be observed over the Sierra Nevada fire (called the 'Bear Fire', see Northern California’s Bear Fire produces a pyrocumulonimbus cloud). It is better seen in 5-min imagery shown in Figure 6, as a short, explosive pulse, which leaves behind a Cb anvil that travelled southward, similar to a volcanic eruption (see example from Etna eruption case from December 2020).
The day-time Natural Colour RGB image from 9 September 21:00 UTC (Figure 7) confirms the presence of a large dust/smoke plume from Oregon fires over the Pacific Ocean (light cyan colour mixed with brown colour). In contrary, the ash/smoke clouds that cover much of California (Californian coast) appear as brown, indicating a mixture of larger smoke and ash particles.
Figure 8 shows the accumulated concentration of CO from TROPOMI on board Sentinel 5P, from 9-11 September, with an illustrative view from Google Earth. The red area west of California had CO tropospheric concentrations four times higher than those recorded in Amazonia and Central Africa.
More than a month after the fires abated the burn scars were still clearly visible in satellite imagery. On the GOES-17 Natural Color RGB from 29 October (Figure 9), they can be seen as the quite large dark brown regions surrounded by green vegetation.
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