During the past month there have been repeated "vog" episodes impacting all of the main Hawaiian Islands. The photo above shows the reduction in visibility above Diamond Head on Oahu.
Below are two photos taken from my house that contrast a voggy day versus a clear day. Notice how the ocean is almost completely obscured in the first image taken today (2/4/17). In this post I would like to explain the weather pattern that contributes to vog episodes in Hawaii. In a future post I will describe how my group models and forecasts vog trajectories and dispersion to mitigate the hazards.
What is vog?Sulfur-dioxide gas emitted by Kilauea volcano on the Island of Hawaii coverts to acid sulfate aerosol particles in the clear air and in clouds, forming volcanic smog known as vog.
Where does vog come from?Kilauea volcano is the most active volcano on earth. The current eruption has been ongoing since 1983, while a new summit eruption began in 2008. The photo below shows the summit emission plume. The most significant effect of this new eruption vent has been a 50% increase the amount of sulfur dioxide (SO2) gas that is emitted into Hawaii’s atmosphere. The current summit eruption has been ongoing for six years and shows no signs of abating. Higher gas fluxes from Kilauea appear to be the new norm.
What hazards are associated with vog?Vog poses a threat to the health of Hawaii’s people as well as being harmful to the state’s ecosystems, and agriculture, and reductions in visibility can be a hazard to general aviation (see first photo above). The presence of vog has been linked to asthma, sinusitis, respiratory disease, lung cancer and/or chronic obstructive pulmonary disease.
Vog EpisodesVog episodes occur when the winds turn from a northeast trade-wind direction, to southeasterly. This happens when the prevailing high pressure center (red "H" below) to the north of Hawaii is pushed eastward by an approaching cold front (winter storm) or kona low. The shading in the map below shows forecast precipitation valid at 10 AM tomorrow (2/4/17) northwest of Hawaii associated with an approaching cold front. The solid yellow contours are the sea-level pressure lines (isobars) and show that the ridge axis (red dashed line) has shifted south of the Islands.
Shifting of the ridge axis southward has three important consequences that create conditions conducive to bad vog episodes, i) the winds turn southeast bringing vog from Kilauea across the Island chain (see wind barbs below), ii) the wind speeds decrease increasing the emission load from Kilauea into the air mass, and
iii) the sinking motion (subsidence) associated with the ridge reduces vertical mixing in the air and traps the vog in a shallow layer (~3-6,000' deep) further enhancing the vog concentrations. The radiosonde sounding from Hilo at 2 PM today shows a sharp increase in temperature with height (temperature inversion) starting at about 900 mb (~3000'). Vog is essentially trapped below this temperature inversion by the warmer air above (cold air sinks).
As the cold front continues to approach, the winds will turn southerly and then westerly, blowing the vog plume across the Island chain and then off to the northeast. Eventually, when the northeast trades return, a remnant of the vog plume will be advected back over the Hawaiian Islands and on to the southwest. Below is the forecast by the UHM Vog Model for 11 PM on Saturday 2/4/17 and at 9 AM on Sunday 2/5/17.
I will discuss the workings of the Vog Model in a future post. In the mean time you can see the vog model output daily here. Southeast winds and vog episodes are more common in winter than in summer owing to the greater strength of the Hawaiian high and greater number of trade wind days in summer than in winter (see graph below). Therefore, expect more vog episodes this winter as cold fronts and low pressure troughs approach Hawaii.