Saturday, February 18, 2017

Cold Front Passing Hawaii

Cold Front Approaching

A cold front moving down the island chain will bring a period of clouds and moderate showers, with cool and breezy trade winds increasing after the front passes. The front will reach Kauai this afternoon.  Note the rope-like cloud band to the NW of Kauai in the MODIS true color image below, valid for 11 AM 18 February 2017.  Near Hawaii cold fronts often take on the appearance of rope clouds.  

The Japanese Himawari satellite gives a broader view of the rope cloud.

The cold front will pass Oahu and Molokai this evening, Maui overnight, and the Big Island Sunday morning, moving south of the Big Island by Sunday night.  See forecast maps below showing precipitation and sea level pressure (labeled contours) for 2 AM Sunday and 2 AM Monday below.  The line of showers is associated with the front. Notice how the pressure gradient tightens with time, increasing the winds.

Heating and moistening of the air over the warm subtropical ocean (surface heat exchange) is greater on the cold side of the front where the air-sea temperature difference is larger.  As a result fronts tend to dissipate as they approach Hawaii, leading to the rope-cloud appearance of the front (see images at the start of this post).  The result of the surface heat exchange the dew point temperature drop is generally greater than the temperature drop after the cold front passes.  At NOAA buoy 51001 located ~195 miles northwest of Kauai, the time-series graph of temperature below shows a drop of 7˚F in ~5 hours, with an overall drop of 8˚F in 24 hours.  Meanwhile the dewpoint temperature drops a full 20˚ F in 24 hours.   

See WRF forecast sea-level temperatures and winds for 8 AM Saturday and 2 AM Monday below.  Increasing winds and cooler temperatures are in store.

The change in dew point temperature is often larger than the change in temperature following a cold front passage in Hawaii.  After the front passes, a mostly dry weather pattern will prevail into Monday, with northeast winds making it feel cool. Note the change in total precipitable water vapor in the images below, valid for 5 AM Saturday 2/18/17 and 2 PM Monday 2/20/17.  The orange colors indicated significantly drier air flowing over the state behind the cold front on Monday.

Storm Force Winds Possible

Following the cold front, the pressure gradient will increase producing 25 to 35 mph winds over the islands on Sunday and Monday.  In the channels between the islands winds will be 20% higher, resulting in storm force gusts in some of these areas.  Breezy trade winds will continue into the middle of next week.  WRF output sea-level winds at 2 PM Sunday 2/19 shows significant enhancement of the winds in the island channels.  In a future post, I will discuss terrain enhancement of the winds in Hawaii and the phenomena of downslope windstorms.

Saturday, February 4, 2017

Vog in the Air

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 one of the most active volcanoes on earth. The current east rift 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 eruptive vent has been a large increase in the amount of sulfur dioxide (SO2) gas that is emitted into Hawaii’s atmosphere. At this time, the current summit eruption 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 some of Hawaii’s people as well as being harmful to agriculture and infrastructure on the Island of Hawai‘i’.  Reductions in visibility can be a hazard to general aviation (see first photo above). Vog can irritate the upper respiratory tract, and  exacerbate pre-existing lung conditions, such as asthma.

Vog Episodes

Vog episodes occur nearly every day, at some location, on the island of Hawai‘i, however, when the winds turn from a northeast trade-wind direction, to southeasterly, the rest of the state can be impacted as well. 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.