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.

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.

Why is Hawaii the Rainbow Capitol of the World?

Does Hawaii have the Best Rainbows on the Planet?  

The short answer to this question is yes, Hawaii has the most and best rainbows.  Even our sports teams are called the Rainbows. Rainbow in Hawaiian is "ānuenue." In this post, I will try to explain the special circumstances in Hawaii's environment that make rainbows so brilliant and so frequent over our state.  Then I will talk about the Rainbow Stop and the new Rainbow App.

First some basics

To see a rainbow, we need to have sun low in one part of the sky and rain in another.  An observer can then see a rainbow if his/her back is to the sun and facing the rain shower.   A combination of refraction of sunlight as it enters and leaves the drop, and reflection at the back of the drop makes the rainbow visible to the observer, provided the sun is low enough in the sky so that the top of the bow is above the horizon.  The top of the rainbow appears at an angle of ~42 degrees above the head of the observers shadow. 

As the sun moves higher in the sky, the top of the rainbow moves lower.  The photo below is of a shallow rainbow produced by a sun that is rising higher in the morning sky.

Since violet light is refracted more than red light, the top of the bow is red and the bottom of the bow is violet.  See graphic and photo below.

So why is Hawaii the best place on the planet to view rainbows?

Now we can apply the basics above to help answer this question.  Hawaii certainly has bright sunlight, as a result of clean air and blue skies.  Except for an occasional influx of volcanic haze, also known as "vog," the air around Hawaii is exceptionally clean.  See how bright and white the sunlight is in the photo below, even though the sun is touching the horizon.  White light has all the colors that a rainbow needs.

The other ingredient required for a rainbow is rain.  And the rain has to fall from an isolated shower to allow the sunshine to reach under the cloud.  That's why rain from stratiform clouds don't produce many rainbows (see photo below taken in the Pacific Northwest). Blues skies and isolated showers are common in the prevailing trade winds that dominate Hawaii's weather.  See "The Flight of the Laysan Albatross" for more on trade wind weather.  

The Role of Clean Ocean Air

It is the clean marine air mass that surrounds Hawaii that ensures that small clouds produce plenty of rain.  The reason is that small cumulus clouds in clean trade wind air have fewer cloud droplets competing for the available water vapor.  Each cloud droplet forms on a particle in the air that allows the growing droplet to overcome "surface tension," the energy needed to create a new bit of interface between air and water.  Without these particles, also known as aerosol, the saturation in clouds would need to reach ~400% for condensation to take place and a cloud droplet to form. In clean air with fewer aerosol, cloud droplets grow larger than they would in polluted air.  A few large salt aerosol that are pulled up into the cloud from near the sea surface ensures that the growing Hawaiian cloud has everything it needs to produce rain.  The salt aerosol creates an extra large cloud droplet that soon begins to fall and sweep up smaller droplets in its path.  We call this process coalescence and it is particularly efficient in clean marine cumulus clouds (see photo below from Al Riordan).  The resulting rain is referred to as "warm rain."  In mainland air with lots more dust, pollen, and pollution, the small cumulus clouds don't rain because there are very many very small droplets and no large drops to trigger rain.  Under typical mainland conditions, taller clouds that contain ice crystals are needed to produce rain, in a "cold cloud" process that I will explain further in a future post.

The Important Role of Hawaii's Steep Mountains

There is yet another important factor at play in making Hawaii the rainbow capitol of the Planet.  Hawaii has an engine that makes rainbows appear even on drier days, and that is Hawaii's lush green mountains.  In the photo below, notice how the cumulus cloud preferentially forms over the Koolau Mountains.  That is because the mountains force the ocean air to rise and cool, causing cloud formation and rain.  I'll say more about this in a future post as well.

Here is a link to a time-lapse loop of a Koolau Mountain Rainbow 

So what is a Rainbow Stop?

In summary, Hawaii has clean air, bright sunshine, warm rains from isolated showers that are enhanced by our mountainous terrain.  It is a perfect combination to produce lots of spectacular rainbows.  In fact I was approached by a French professor who wanted to come to Hawaii on Sabbatical especially for the purpose of studying rainbows.  He is still working on the funding.  But he has identified the right location for sure.  The best time to find rainbows is early and late in the day for the optimum low sun angles.  In Hawaii mornings tend to have more showers, so early risers get the bow.  Now to answer the question of what is a rainbow stop?  Many people have complained to me that they often see rainbows while driving on the freeway and can't stop to appreciate them without causing a traffic jam.  So my friend Paul Cynn (with me in the photo below) came up with the idea of creating Rainbow Stops where folks can go to get the best view of our rainbows.  

Below is a draft map of Oahu I created that shows red stars for good viewing locations: Rainbow Stops.  The starred locations are lookouts and beach parks with excellent exposure to the sky.  But, when should folks go to the Rainbow Stops?  Anytime is a good time to stop and admire out beautiful skies.  However, that is where the Rainbow App comes in.  

What is the Rainbow App?

The Rainbow App is a smartphone app that uses information from radar and satellite along with your location and the sun's position in the sky to create a short term forecast or "nowcast" of where the most likely places are to see a rainbow during the next hour or so.  The app is currently being coded by scientists in my school, SOEST, at UHM.  If you want to learn more about the Rainbow App and Rainbow Stops go to http://rainbowstop.org.  On this site you can also learn more about double rainbows, moonbows, and fogbows.  The photo of a fogbow below taken by me at the summit of Mauna Kea, could be the highest rainbow in Hawaii.  And please email me your best rainbow photos and we will share them on our site. 

Finally, the proof is in the photos....

Below are a selection of rainbow photographs taken by my colleague at UHM, Minghui Chen.  Not only is Ming an excellent rainbow photographer, he is also the lead programmer for the Rainbow App.  I will announce when the App is available on this blog.  A hui hou.

Below is a rainbow with a reflection.  This is possible to see if you can find still water or in this case wet pavement to reflect the bow.

Below is a rainbow with the Koolau Mountains in the background

Below is a rainbow seen from Diamond Head

Below is the lower part of a rainbow with the Koolau Mountains behind.