Hewlett Gulch Fire Plume
On 16 May 2012, the Hewlett Gulch fire underwent explosive growth, from ~1000 acres to ~5000 acres in size in one day. During the 1947-2000 UTC period (1:47-2:00pm local) on that day the Colorado Lightning Mapping Array showed several lightning flashes within the plume at high altitude, ~10 km MSL or higher (look for the lightning in northeastern Larimer County).
The CSU-CHILL and Pawnee radars were scanning the plume in a dual-Doppler configuration during this time. Below I show a composite image of reflectivity (DZ), differential reflectivity (DR; a measure of the oblateness of the smoke particles), and vertical velocity (W) at 1935 UTC, about 12 minutes before the lightning began.
The plume was difficult to distinguish from surrounding cloud in reflectivity but featured very high differential reflectivity values, often higher than 5 dB. This is due to the highly oblate smoke and ash particles. Meanwhile, vertical velocity peaked as high as 7.5 m/s (~15 knots), a respectable value for normal convection, during this time. Below I show the same plot as above but now projected in the vertical.
Look at the altitude of that updraft - it goes above 10 km MSL! It is easy to see how this plume became electrified, but what is more interesting is why it took at least 10 minutes to start producing lightning after this growth period. CSU scientists will be collaborating with New Mexico Tech scientists to study the electrification of this smoke plume/pyrocumulus cloud in more detail. Indeed, this is the first time combined polarimetric, multiple Doppler, and lightning observations have ever been made in a fire plume.
UPDATE
I created an animated GIF for the vertical plots, 1905-2130 UTC. The growth above 10 km MSL is clearly evident during the early-to-mid 1900 hour (Click on this plot and others to see original size).
The CSU-CHILL and Pawnee radars were scanning the plume in a dual-Doppler configuration during this time. Below I show a composite image of reflectivity (DZ), differential reflectivity (DR; a measure of the oblateness of the smoke particles), and vertical velocity (W) at 1935 UTC, about 12 minutes before the lightning began.
The plume was difficult to distinguish from surrounding cloud in reflectivity but featured very high differential reflectivity values, often higher than 5 dB. This is due to the highly oblate smoke and ash particles. Meanwhile, vertical velocity peaked as high as 7.5 m/s (~15 knots), a respectable value for normal convection, during this time. Below I show the same plot as above but now projected in the vertical.
Look at the altitude of that updraft - it goes above 10 km MSL! It is easy to see how this plume became electrified, but what is more interesting is why it took at least 10 minutes to start producing lightning after this growth period. CSU scientists will be collaborating with New Mexico Tech scientists to study the electrification of this smoke plume/pyrocumulus cloud in more detail. Indeed, this is the first time combined polarimetric, multiple Doppler, and lightning observations have ever been made in a fire plume.
UPDATE
I created an animated GIF for the vertical plots, 1905-2130 UTC. The growth above 10 km MSL is clearly evident during the early-to-mid 1900 hour (Click on this plot and others to see original size).
Labels: Lightning
posted by Timothy Lang at
1:35 PM
