Observations of Boreal Forest Fire Smoke in the Stratosphere
A substantial increase in stratospheric aerosol was recorded between May
and October 1998 at high northern latitudes. This phenomenon was recorded
in the absence of any reported volcanic eruptions with stratospheric
impact potential. The Polar Ozone and Aerosol Measurement III
and the Stratospheric Aerosol and Gas Experiment II
(SAGE II) satellite-based
solar occultation instruments made numerous measurements of aerosol
extinction substantially higher than background values three to five km
above the tropopause. In addition to these satellite-based vertical
profiles, several other detailed measurements by ground-based lidar
and balloon-based backscattersonde observed this unusual summer of
stratospheric aerosol increase. A comparison of these observations with
aerosol index data from NASA's Total Ozone Mapping Spectrometer
forest fire data reveals a strong link between these stratospheric aerosol
and forest fire smoke. Our analysis leads us to conclude that smoke from
boreal forest fires was lofted across the tropopause in substantial amounts
in several episodes occurring in Canada and eastern Russia. The vertical transport
of the smoke through the troposphere is accomplished by convection in
severe (most likely) supercell thunderstorms. Observations throughout the
northern hemisphere summer of 1998 reveal a broad zonal increase in
stratospheric aerosol that persisted for at least three months. These
observations indicate that there were several episodes of intense boreal forest
fire blowups combined with deep convection in 1998.
Boreal forest fires are a routine, natural occurrence that peaks in mid summer. In Canada alone, there are on the order of 30,000 forest fires annually. In some years, a small subset of these fires may develop into the huge wildfires that combine with severe thunderstorms to produce prodigious amounts of smoke and other emissions high in the troposphere and lower stratosphere. In late August 1992 the Upper Atmosphere Research Satellite's (UARS) Cryogenic Limb Array Etalon Spectrometer (CLAES) and Microwave Limb Sounder (MLS) instrument detected anomalous enhancements of stratospheric aerosols and methyl cyanide, respectively. These observations have been traced back to historically intense forest fires near Boise Idaho on August 21 and 22. These and other events in recent history, such as a reported incursion of Canadian forest fire smoke over England in 1950, give a strong indication that long-range, high-altitude, and long-lasting residuals from boreal forest fires are a very real, recurring, and perhaps important phenomenon worthy of continued exploration.
On June 25 2004, in the area of Alaska/Yukon/British Columbia, forest fires
erupted into pyro-convective storms that lofted smoke and carbon monoxide
to the upper troposphere and lower stratosphere. On several days in late
June of 2004 abundant fire, smoke, and convection was found in this region.
A stunning photograph of a pyroCb taken from an airliner on June 27, 2004
is displayed below. The photographer, a passenger on a Japan Airlines flight
over British Columbia, saw what he initially thought was a volcanic plume,
the top of which was at flight level in the UTLS. The pyroCb potential
in June 2004 was watched closely because the fire and meteorological
conditions were suspected in advance to be ripe for extreme blowup.
Figure 1. Photograph of a pyro-convective cloud taken by Mr. Noriyuki Todo
of Japan Airlines International Corporation. This photo is reproduced here
with his permission.
Details of the circumstances are:
ID : JAL009 B747-400
Flying Course : NCA11
Cruising Flight Level : FL340 ( 34,000 feet)
Actual Time of Over BINGA (N57 42.1 W125 00.0) at 20hr 48min 27sec UTC on
Figure 2. Selected aerosol extinction and backscatter ratio profiles showing stratospheric enhancements between July 9 and 18, 1998. Tropopause height collocated with each profile is shown by a matching color-coded horizontal bar.
| Figure 3. Orthographic projection with isentropic back trajectories from the aerosol layers, as color coded in Figure 1. Each trajectory ends on June 27, 1998. Filled diamonds mark observation points. The trajectories were run on the potential temperature surface of the peak backscatter or extinction ratio. The gray shaded portion of each trajectory shows is parcel location between 12 UTC July 4 and 7. The filled triangle marks Korovin volcano, which erupted on June 30 but did not affect the stratosphere.
| Figure 4. Longitude/time series analysis of POAM III 1 micron aerosol optical depth in a 5-km thick column relative to and starting 2 km above the tropopause. The period is May through October 1998. POAM measurement latitude, which changes gradually, is annotated along the right side. The white bar is a 5-day data gap.
| Figure 5. Lidar backscatter ratio profiles taken by Jeff Thayer, SRI International, on August 23, 1998 near Sondrestromfjord, Greenland (67 N, 51 W). The vertical resolution of the lidar is 100 m. Note the three distinct aerosol layers between 13 and 15 km altitude.
| Figure 6. POAM III extinction ratio profile for August 23, 1998, at 63 N, 51 W, which is approximately 450 km south of the Sondrestromfjord lidar observation shown above. Note the aerosol enhancement between 12 and 15 km altitude, in approximate agreement with the lidar-observed layers. POAM's vertical resolution is 1 km. The local tropopause was at approximately 9 km.
| Figure 7. Profiles of balloon measurements taken by James Rosen, Univ of Wyoming, on Aug 23, 1998 near Saskatoon, Saskatchewan, Canada (52 N, 107 W). The ozone and aerosol remote sensing was by backscattersonde. Note the enhancements of aerosol and ozone between 10.5 and 14.5 km altitude. The collocated temperature profile indicates that the local tropopause was at approximately 10 km.
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