Rudolf Husar, rhusar@mecf.wustl.edu, 21 October, 1998

On October 8, 1998 a smoke plume was observed over Alaska stretching from the Canadian boarder through central Alaska to Kodiak Island. Analysis of GOES 10 satellite images and other data has revealed unique features of this unusual plume:

1. The plume showed virtually no lateral dispersion over a 1000 mile transport range;
2. The ground level temperature was several (??) degrees cooler in the plume shadow;
3. The plume has increased the daytime albedo except during noon when it reduced the albedo;
4. Once over the ocean, the plume has evidently inhibited the formation of low clouds.

This is an early account of the analysis of this unusual atmospheric event. Comments are most welcome.

On October 8, 1998 a smoke plume was observed over Alaska stretching from the Canadian boarder through central Alaska to Kodiak Island. The 1000 km long smoke ribbon was clearly visible in the SeaWiFS polar orbiting satellite data as shown in Figure 1.

Figure 1a. True-color (not quite) SeaWiFS image of the 1000 km long Alaskan smoke ribbon Oct 8, 1998 (S1998281224102.L1A_HUAF.HDF). Click on the image for a larger picture.

 The smoke plume was also observed by the GOES 10 geosynchronous satellite (GOES West) as shown in Figure1a. See also a GIF Animation of the plume during the daylight hours on Oct. 8, 98. 

Figure 1. False-color GOES 10 image of the Alaskan smoke ribbon on Oct 8, 1998 (9810090030). Click on the image for a larger picture.

The plume stretches from the north to the south. However, the wind field over central Alaska (Figure 2a) shows an easterly flow field. The animation of the GOE 10 images also indicates that during the day, the smoke plume was transported laterally (not along the plume axis) from the eastern boarders of Alaska to the west. The backtrajectory calculation also indicates that the plume originated over eastern Alaska or in NW Canada. The source of the plume has not been established but it is presumed to be one of the numerous boreal forest fires that occurred throughout the summer season.

Figure 2a and 2b. The plume transport was verified by back-trajectories and wind vectors from the NOAA's READY system.

The smoke plume is unusual in that there was virtually no lateral plume dispersion over the 1000 mile transport range. The narrow plume has the appearance of a contrail (condensation trail) from a high-flying jet aircraft. Usually such plumes are also vertically thin (100-200 meters) and reside in a narrow, stable stratum of the stratified atmosphere. Hence the term 'smoke ribbon'. Also, in hilly terrain the smoke ribbon follows the contours of the terrain at fixed height from the ground..

The plume had a clearly discernable impact on the 3.9 um IR signal. , bet viewed through the animation of the IR images.

Figure 3. Enhanced IR radiation in the shadow of the plume. The outline of the visible plume is highlighted. See a GIF Animation of the IR plume signature over the day

Throughout the day, the smoke plume has a varying influence on the upwelling radiation from the ground. Not only is the magnitude but also the direction of influence changes. In the morning and the afternoon, the brighter aerosol layer adds to the surface albedo, making the plume brighter than the surface. However, at local noon, the aerosol layer actually reduces the upwelling radiation reducing the albedo. The phenomenon is illustrated in Figures  

Figure 4a, 4b, 4c. Visible image of the smoke at 1100, 1200 and 1300 local time. Note the darkening of the plume during the noon hours compared to the adjacent ground. See a GIF Animation of the visible images.

Figure 5a, 5b, 5c. Comparison of the plume brightness to the adjacent smoke-free regions over the ocean (5a), dark land surface (5b) and bright land surface (5c).

Figure 6a, 6b, 6c. Comparison of the daytime brightness of three surfaces, ocean, dark land and bright land. Smoke-free brightness (6a), brightness with smoke (6b) and excess brightness over the smoke plume.

The data in Figure 6 show that over ocean and dark land the aerosol plume contributes excess reflection throughout the day. However, over bright land the noontime upwelling radiation is reduced by the presence of aerosol. This apparent anomaly arises from the two competing roles that aerosols play in radiative transfer: extinction of bright radiation and the addition of backscattered radiation. Over the dark surfaces the backscattering dominates while over bright surfaces extinction prevails. [A more rigorous explanation follows soon]

There is evidence that the presence of the smoke plume inhibited the development of low clouds over the ocean adjacent to Kodiac Island. [This is definitely flaky]

Figure 7. Evidence of smoke-induced inhibition of low cloud formation.

Clearly, aerosols are of interest not only because of their effects but also because they are excellent visualizers of atmospheric phenomena such as poor dispersion during the Arctic autumn.