Saturday, August 05, 2017

weather ed-smoke

Weather Ed- SMOKED

With widespread smoke across the Pacific Northwest, including the park, here are a few thoughts on the distribution and variability of that smoke.  We have been under the influence of high pressure for sometime now which means that the upper and mid-level winds are pretty light.  Nevertheless smoke from the large fires in BC have been transported south over the course of the week.  It does not take very much wind to transport smoke over long distances.
Looking west from Paradise at 8 AM Saturday Aug 5
Over the last three days we have seen considerable variability in the amount of smoke around the mountain. In general the least amount of smoke at mid and higher elevations occurs around sunrise.  The highest concentrations are occurring in the late afternoon and evening hours. There are a number of possible reasons for this.  The most obvious explanation might be that large quantities of smoke are being transported directly from the fires during the afternoon.  This may be true at times but the significant morning versus evening differences are due to other factors.  One of the important aspects of this type of high pressure is the fact that air in the center of the high tends to sink (subsidence) slowly, nevertheless this means air parcels in the lower atmosphere have a difficult time rising.  In addition, the atmosphere cools during the night; the greatest cooling occurs just above elevated terrain (inversion).  This also generates stable layers in the lower atmosphere.  The net result is that by morning, the bulk of the smoke tends to be trapped below 5-7,000 ft.  Smoke by the way is composed of various hydrocarbon gases and particular matter (minerals found in the during wood) that become attached to other aerosols.
During the day those stable layers weaken or disappear which in turn allows smoke to rise to higher elevations.  Smoke rises due to two external forcings.  First a column or parcel of air may heat enough so that it is warmer than the surrounding air, hence it will rise because it is positively buoyant.  This is most likely a secondary process compared to the following.  Even though winds in the upper and mid-level's are light, lower level winds are generated during the daylight hours over the terrain.  These are valley and slope winds which are typically on the order of 5-10 mph, but nevertheless they can transport smoke back up hill. At night weak mountain or glacier winds are generated on the mountain which transport smoke back down to lower elevations.  [Winds generated by the heating and cooling of the terrain make for a separate Weather Ed topic]
Camp Muir webcam looking south at 8 AM Saturday Aug 5
Unfortunately weather models are showing that there is not going to be any massive changes in the upper level ridge over the next 5 days, with the exception of coastal areas which should see some modest onshore flow later this weekend .  The good news is that upper level westerly will develop Thursday night and slightly stronger southerly winds on Friday.  In short, the smoke is going to be around for some time to come but the concentrations will continue to vary.