Unfortunately, there is no scale on their figure, but Iâm pretty sure the smoke would be going into the upper troposphere (like Livermore finds). Los Alamos only simulates for a few hours, so that makes sense that hardly any would have gotten to the stratosphere. Typically it takes days to loft to the stratosphere. So I think that would resolve the order of magnitude disagreement on percent of soot making it into the stratosphere for a firestorm.
Actually, I think they only simulate the fires, and therefore soot production, for 40 min:
HIGRAD-FIRETEC simulations for this domain used 5,000 processors and took roughly 96 h to complete for 40 min of simulated time.
So you may well have a good point. I do not know whether it would be a difference by a factor of 10. Figure 6 of Reisner 2018 may be helpful to figure that out, as it contains soot concentration as a function of height after 20 and 40 min of simulation:
Do the green and orange curves look like they are closely approaching stationary state?
I think the short run time could also explain the strange behavior of only a small percent of the material burning at high loading (only ~10%). This was because the oxygen could only penetrate to the outer ring, but if they had run the model longer, most of the fuel would have eventually been consumed. Furthermore, I think a lot of smoke could be produced even without oxygen via pyrolysis because of the high temperatures, but I donât think they model that.
In that slower burn case, would the fuel continue to be consumed in a firestorm regime (which is relevant for the climatic impact)? It looks like the answer is no for the simulation of Reisner 2018:
Note that because of low wind speeds and hence minimal fire spread, the fires are rapidly subsiding at 40 min (not shown) [the time for which they simulated the fire].
For the oxygen to penetrate, I assume the inner and outer radii describing the region on fire would have to be closer, but that would decrease the chance of the firestorm continuing. From Reisner 2019:
It is our contention that the distance between the inner and outer radii is key to the development [and I guess continuation] of a possible firestorm. Obviously, as the inner radius approaches the outer radius, the firestorm probability goes to zero, whereas if the inner radius approaches the center of the detonation, the risk increases due to a large burnable area.
Reisner 2019 also argues most soot is produced in a short time (emphasis mine):
Another important point concerning these simulations is that the rapid burning of the fine fuels leads to both a reduction in oxygen that limits combustion and a large upward transport of heat and mass that stabilizes the upper atmosphere above and downwind of the firestorm. These dynamical and combustion processes help limit fire activity and BC production once the fine material has been consumed (timescale < 30 min [from Reisner 2018, the fire was simulated for â40 minâ]). Hence, the primary time period for BC injection that could impact climate occurs during a relatively short time period compared to the entirety of the fire or the continued burning and/âor smoldering of thicker fuels.
I do not think they model pyrolysis. Do you have a sense of how large would be the area in sufficiently high temperature and low oxygen for pyrolysis to occur, and whether it is an efficient way of producing soot?
I think they ignore secondary ignition, e.g. from broken natural gas lines or existing heating/âcooking fires spreading, the latter of which is all that was required for the San Francisco earthquake firestorm, so I donât think this could be described as âworst case.â
Good point! It is not an absolute worst case. On the other hand, they have more worst case conditions (emphasis mine):
Figure 3 shows the initial vertical profiles of wind speed and potential temperature used for the HIGRAD-FIRETEC simulations. The wind speed profile was chosen to be high enough to maintain fire spread but low enough to keep the plume from tilting too much to prevent significant plume rise (worst case). Wind direction is set as 270° (west-to-east, +x direction) for all heights, with no directional shear, and a weakly stable atmosphere was used below the tropopause to assist plume rise (worst case).
Actually, I think they only simulate the fires, and therefore soot production, for 40 min. So you may well have a good point. I do not know whether it would be a difference by a factor of 10. Figure 6 of Reisner 2018 may be helpful to figure that out, as it contains soot concentration as a function of height after 20 and 40 min of simulation. Do the green and orange curves look like they are closely approaching stationary state?
Wowâonly 40 minutesâmy understanding is actual firestorms take hours. This graph is for the low loading case, which did not produce a firestorm. The lines do look similar for 20 and 40 minutes, but I donât think itâs the case we are interested in. They claim only the fine material that burns rapidly contributes, but I just donât think that is the case with actual firestorms. The 2018 was with low loading, and most of the soot is in the lower troposphere (at least after 40 minutes), so the question is when they actually did find a firestorm, what is the vertical soot distribution? For Livermore, it was mostly upper troposphere. Los Alamos did recognize that they were not doing latent heat release even in the 2019 simulation. I think this is quite important, because itâs the reason that thunderstorms go to the upper troposphere (and sometimes even stratosphere). Itâs been a while since I took geophysical fluid dynamics, but the argument that the initial plume would stabilize the atmosphere seems off to me. If we look at the example of night in the atmospheric boundary layer (lower ~1 km), the surface cools radiatively, so you get stratification (stable). But when the sun comes up, it warms the surface of the earth, and you get thermals, and this upward convection actually destabilizes the boundary layer. Now it is true if you have a fire in a room that the hot gases can go to the ceiling and stabilize the air in the room. But if they are arguing that the plume only goes up a few kilometers (at least for the non-firestorm case), it seems like in those few kilometers, the potential temperature would be more equalized, so overall less stability. Even if thatâs not the case, the plume has hardly even reached the upper troposphere, so there would be hardly any change in stability there. In addition, if the simulation is run over hours, then new atmosphere could come into place that has the same old stability. So I think the Livermore results are more reasonable.
Actually, I think they only simulate the fires, and therefore soot production, for 40 min:
So you may well have a good point. I do not know whether it would be a difference by a factor of 10. Figure 6 of Reisner 2018 may be helpful to figure that out, as it contains soot concentration as a function of height after 20 and 40 min of simulation:
Do the green and orange curves look like they are closely approaching stationary state?
In that slower burn case, would the fuel continue to be consumed in a firestorm regime (which is relevant for the climatic impact)? It looks like the answer is no for the simulation of Reisner 2018:
For the oxygen to penetrate, I assume the inner and outer radii describing the region on fire would have to be closer, but that would decrease the chance of the firestorm continuing. From Reisner 2019:
Reisner 2019 also argues most soot is produced in a short time (emphasis mine):
I do not think they model pyrolysis. Do you have a sense of how large would be the area in sufficiently high temperature and low oxygen for pyrolysis to occur, and whether it is an efficient way of producing soot?
Good point! It is not an absolute worst case. On the other hand, they have more worst case conditions (emphasis mine):
Wowâonly 40 minutesâmy understanding is actual firestorms take hours. This graph is for the low loading case, which did not produce a firestorm. The lines do look similar for 20 and 40 minutes, but I donât think itâs the case we are interested in. They claim only the fine material that burns rapidly contributes, but I just donât think that is the case with actual firestorms. The 2018 was with low loading, and most of the soot is in the lower troposphere (at least after 40 minutes), so the question is when they actually did find a firestorm, what is the vertical soot distribution? For Livermore, it was mostly upper troposphere. Los Alamos did recognize that they were not doing latent heat release even in the 2019 simulation. I think this is quite important, because itâs the reason that thunderstorms go to the upper troposphere (and sometimes even stratosphere). Itâs been a while since I took geophysical fluid dynamics, but the argument that the initial plume would stabilize the atmosphere seems off to me. If we look at the example of night in the atmospheric boundary layer (lower ~1 km), the surface cools radiatively, so you get stratification (stable). But when the sun comes up, it warms the surface of the earth, and you get thermals, and this upward convection actually destabilizes the boundary layer. Now it is true if you have a fire in a room that the hot gases can go to the ceiling and stabilize the air in the room. But if they are arguing that the plume only goes up a few kilometers (at least for the non-firestorm case), it seems like in those few kilometers, the potential temperature would be more equalized, so overall less stability. Even if thatâs not the case, the plume has hardly even reached the upper troposphere, so there would be hardly any change in stability there. In addition, if the simulation is run over hours, then new atmosphere could come into place that has the same old stability. So I think the Livermore results are more reasonable.