Meteorological Aspects for the Siberian Wildfire Smoke Episode of Southern British Columbia

Meteorological Aspects for the Siberian Wildfire Smoke Episode of Southern British Columbia of July 6-9, 2012. By Dr. Robert Nissen and Dr. Bruce Ainslie Air Quality Science Unit, Meteorological Service of Canada

Wildfires are a common and natural summertime occurrence of boreal forest ecosystems. Extended periods of dry weather coupled with long daylight periods enable sufficient drying of the largely coniferous forest fuels. When the dry weather spell is interrupted by convection associated with a cold upper low system, limited moisture in the lower levels of the atmosphere result in dry lightning strikes during thunderstorm activity. Consequently, wildfires are ignited, and develop rapidly in the unstable conditions as stronger winds from aloft are mixed down. Wildfires typically decrease in activity in the night time hours as nocturnal inversions develop, however in the high latitudes of the boreal forest the summer night time period is short, offering little respite from the fire activity.

In the summer of 2012 a blocking pattern in the long wave circulation resulted in frequently higher than normal temperatures and drier than normal precipitation in the eastern parts of Siberia. This resulted in a very active fire season for eastern Siberia.

Occasionally, Siberian wildfire smoke may be transported in sufficient concentrations to be a factor in air quality in western North America. At least four factors are required for this to happen. Firstly, the fires must be thermally intense enough to inject fine particulates to high elevations (greater than 5 km). At low levels winds are more variable, and mechanical turbulence further disperses smoke emissions quickly. Secondly, winds at the higher levels must be fast enough and not be too horizontally divergent for sufficiently high concentrations to be maintained at large distances downwind. Thirdly, the winds must transport the smoke plume in the correct direction. Fourthly, there must be sufficient subsidence of the smoke plume over western North America from higher to lower elevations. All of these conditions were met during the July 2012 episode.

To diagnose the evolution of the smoke plume, a series of back- and forward trajectory analyses were performed using the HYSPLIT software package for several consecutive daily runs of global numerical model wind fields. For the forward trajectory, the July 1 0000Z release (midnight Universal Coordinated Time) from Yakutsk Airport, Russia (YKS) at 5 km and above is most consistent with the arrival of the smoke plume late on July 6 along the west coast of North America. For the back trajectories, the July 7 0000Z arrival for Vancouver International Airport (YVR) is the best temporal match, consistent with onset observations of smoke aloft. The analyses are reasonably consistent considering that the travel time from YKS to YVR will not be an exact integer number of days. An attempt was made to validate the position of the smoke plume with imagery from three satellite data products. The MODIS Aerosol Optical Depth (AOD) Terra product shows the mean value for a given day, thus approximating the position for 1200Z. The Ozone Mapper Profiler Suite (OMPS) also measures the optical depth averaged for a given day; note that the scale maximum is five times that of the MODIS AOD. The MODIS True Colour AQUA product has a fine horizontal resolution, but smoke detection may be difficult in the presence of clouds. All three products have tracks which will not always overlap well in successive passes.

Coincident with the forward trajectory release time discussed two paragraphs ago, the GEM Global 500 hPa analysis for July 1 at 0000Z shows a slowly decaying cold upper low along the Arctic shoreline of Siberia with strong upper level winds to 80 km/h from the northwest over the YKS wildfire source area southwest of the cold low. The MODIS Aerosol Optical Depth (AOD) imagery shows very high concentrations over the northern Sea of Okhotsh; this product generally does not calculate AODs over continents.

Twenty four hours later, the 500 hPa analysis has the smoke plume in a westerly wind field. From the OMPS imagery, it appears that the rear (west) segment of the observed plume is the portion that will affect the BC coast. The leading edge of the plume is detected on the true colour imagery (full web page with locator map) taken at 0150Z in the upper left portion of the photo.

The upper low starts to open up by the beginning of July 3 with falling 500 hPa heights towards the east of the low and a progressive upper ridge to the south. The smoke plume along the northern shores of the Sea of Okhotsh is in an area of convergent flow aloft. Within the true colour imagery (full web page with locator map) taken at 0050Z the leading parts of the smoke plume may be observed in the cloud hole near the bottom of the image, in contrast to the smoke-free cloud hole at the top. Although there are gaps between successive tracks, the OMPS imagery shows the overall plume stretching towards the east- southeast.

By July 4 0000Z the upper low has accelerated eastward. The smoke plume has been accelerated even more rapidly eastward embedded within winds exceeding 100 km/h to lie south of the Bering Strait and caught up with the cold low center to the north. The OMPS imagery shows the plume progressing rapidly relative to the previous day.

Over the next 24 hours the smoke plume has overtaken the upper low to the northwest, and has crossed the Aleutian Islands south of mainland Alaska. The west-northwesterly flow is starting to become divergent and slow down. The plume optical depth on the subsequent OMPS imagery has decreased, likely due to vertical shear with slower wind speeds at lower elevations. The plume is well placed within a pass of the MODIS AOD product. As July 6 commences the plume has pulled well away from the stalling upper low, and starting to turn more towards the northeast off the BC coast. The plume is even more favourably placed on this day’s MODIS AOD pass.

For July 7 the MODIS true colour imagery clearly shows at 20:05Z (early afternoon local time) the presence of the smoke extending in a southwest- northeast band across the Lower Mainland. The 500 hPa analysis has the upper level winds aligned closely with the orientation of the smoke band. With this orientation, some subsidence in the lee of the Olympic Mountains may be expected, in addition to the general subsidence over this large area of clear skies. The arrival period is in general consistency with the most favourable YVR back trajectory analysis. At the surface, YVR weather observations show typical westerly light sea breeze winds for much of the daytime period.

By the next day the MODIS true colour imagery at 19:10Z shows that for higher elevations the main smoke band has shifted towards the northwest with the southeast edge across Texada Island. The motion of the smoke band is consistent with the trend in the YKS forward trajectories between the July 1 and July 2 7.5 km releases. Also evident is the onset of a marine surge of stratus cloud into the Juan de Fuca Strait. The MODIS AOD product has the smoke plume located in a manner consistent with the true colour product. YVR weather observations show a stronger northwest wind today which is advecting smoke particles descended from the upper level smoke band as it moves away.

For July 9, the MODIS true colour imagery reveals a very faint smoke band aloft visible in a north-south line over Texada Island and undergoing deformation. The imagery also reveals convective clouds over the eastern Lower Fraser Valley where Abbotsford Airport reported a thunderstorm in the early morning hours. YVR observations show gusty outflow winds from the southeast associated with the early morning convection, and the daily sea breeze cycle is largely disrupted. Dew points have started to trend lower late in the day. The Port Hardy sounding shows a destabilizing profile at the mid to upper levels. The MODIS AOD product has the smoke plume continuing to drift towards central and northern BC. With upper source levels of smoke heading away and low levels winds strongly variable, the smoke has significantly dispersed from parts of the South Coast. The Katkam image taken at 21:00 PDT still shows significant smoke to the northwest of Vancouver, and air quality is still compromised in parts of the Central Interior with ozone values to 84 ppb. The situation in the Interior is more complicated as some smoke may have arrived earlier from U.S. wildfires before slowly settling out.

By July 10, YVR observations show dew points dropping to less than 7C. However, the Katkam image taken at 21:00 PDT on July 10 shows better visibility, and air quality readings (ozone 62 ppb) have improved in the BC Interior.

In summary, the Siberian wildfire episode for southern BC occurred due to favourable upper level winds along the south side of a shifting upper low rapidly transporting the smoke plume quickly across the northern Pacific Ocean with little dispersion. Upon reaching coastal British Columbia the plume entered an area of subsidence which assisted in transporting some of the particulates to the lower levels. Sea breezes helped transport the smoke at the low levels into the Lower Fraser Valley. The episode came to a gradual conclusion as the source region of upper level smoke moved away and low level winds became variable under a destabilizing atmosphere, with some scavenging by precipitation in the more eastern sections of the Lower Fraser Valley.