Monday, December 26, 2016

The Flight of the Laysan Albatross

The fresh trades are making the Laysan Albatross particularly happy today. This installment will explore why these elegant birds move so fast through the air, the origin of the strong trade winds today, why the Albatross are so pleased with this weather, how these magnificent birds fly long distances over the ocean with so little effort, and why the geography and meteorology of Kaena Point makes that location such a paradise for a growing colony of Laysan Albatross.

So why do Laysan Albatross fly so fast? Newton's third law of motion tells us that for horizontal flight, the lift on a birds wing must equal the weight of bird.  The lift is proportional to the bird's velocity squared times the area of its wing. Thus, the greater a bird’s weight, the faster a bird must move through the air to have sufficient lift to allow horizontal flight. For the large Laysan Albatross this cruising speed works out to 36 mph relative to the air!  

Enhanced Trade Winds

The Honolulu Airport is reporting winds blowing 15 to 25 mph with higher gusts today, December 26th 2016. These winds are the result of an enhanced pressure gradient, a change in pressure over a distance, between a strong surface high-pressure area to our north and lower pressure nearer the equator in the Intertropical Convergence Zone or ITCZ (area of white clouds at the bottom of the figure below). The image below shows infrared satellite image overlaid with sea-level pressure contours in green for 8 AM this morning. Winds barbs are also plotted in white with the feathers indicating the wind speed and the staff indicating the wind direction, from the northeast.  Strong trade winds are forecast to continue through the week this week.

Return winds aloft are bringing high clouds (yellow and red colors) over Hawaii from the southwest as seen in the colored IR image below. High winds from the southwest and low level winds from the northeast is a hallmark of the Hadley circulation that dominates our weather over the Hawaiian Islands. I will expand on the science behind the Hadley circulation in a future blog.

Dynamic Soaring

Back to the Laysan Albatross. The Laysan albatross glides just above the waves for long distances without flapping its wings. How does it do this? In 1883, Lord Rayleigh first suggested that a bird could continuously soar in nearly-circular flight on an inclined plane that crosses a thin wind–shear layer. He correctly observed that the birds extract energy from the increase in wind speed with height near the surface of the ocean, aka the "wind-shear layer" (see figure below).  In a trick called "dynamic soaring" the albatross turns into the wind to gain height while losing air speed (~30 mph) and turns away from the wind and dives to gain air speed (~60 mph).  This is akin to a bicycle rider gaining speed while coasting down a steep hill and losing speed cycling uphill (converting kinetic energy into gravitational potential energy).  However, the bird gains elevation fast, like taking an elevator, just by turning into the wind shear without having to do work, smart bird!

Kaena Point

Finally, why is Kaena Point such an ideal place for an Albatross colony? Kaena Point is the westernmost tip of land on the island of Oʻahu. 

In 2011, the United States' first predator proof fence was constructed at Ka’ena Point, costing ~ $290,000.[6] The fence is approximately 2,133 feet long, and encompasses 59 acres of land.[7] The total population of Laysan Albatross fledglings, Wedge-tailed Shearwater fledglings, Ohia, Sandalwood trees, and several other species have risen significantly since the fence was installed.[8]  

Aside from the protected habitat, much of the point is comprised of sand dunes that form a low hill that is easy for the wind to traverse, yet to the east the Waianae mountain range forms a formidable barrier to the wind.  In the photo below you can see the west end of the Waianae mountains and if you look close you may be able to spot the fence running along the bottom.  As a result of the orography, or terrain, the wind accelerates across Kaena Point like a wind tunnel.

And so the Laysan Albatross happily soars into the sky on fresh trade winds accelerated by Oahu's steep orography.

Saturday, December 17, 2016

2016 Kona Low

This weekend our weather is dominated by a kona low.

What are Kona Lows?

Kona lows are subtropical cyclones that occur during the cool season in the north central Pacific.  They form as a result of forcing by upper-level lows cut off from the polar westerlies in winter.  Kona lows are cold core with the strongest circulation in middle and upper atmosphere.  They are relatively long lived storms - often affecting Hawaii for a week or more.  Erratic tracks and convective-scale organization of precipitation makes forecasting Kona Low impacts a challenge.

Kona Low Impacts Include:

  • heavy rains, flash floods, and landslides
  • high winds
  • large waves and swell
  • blizzards at higher elevations on Mauna Kea and Mauna Loa
  • severe thunderstorms
  • large hail 
  • tornados 

Below is an F1 tornado over central Oahu.

Blizzard on Mauna Kea came with 130 MPH winds in 2001.

The Kona Low Impacting us This Weekend

A well-developed Kona Low is dominating our weather this weekend.  Cool and showery weather has descended on the Hawaiian Islands as a Kona Low approaches the state from the north.  Here is the IR satellite image for 9 AM on Saturday 12/17/16.

The surface low is about 200 mi north of Kauai as seen in the surface pressure analysis that is overlaid on the IR satellite image for 8 AM today.

Kona Lows are generated by cold air and low pressure aloft, aka a trough aloft.  Below is the 250-mb height field overlaid on the water-vapor image for 8 AM today.

MKWC high-resolution weather model output shows showers circling the low center where the coldest air is aloft.  A vigorous convective band is seen just west of the Big Island.  

IR Satellite image for 8 PM today shows deep convection forming over Kauai and the Big Island, with the threat of heavy rain over Maui indicated.

Heavy rainbands are already impacting the Big Island this evening. See the radar reflectivity for 7 PM this evening below.

As time goes on, the model forecasts the low to slip southwest and for the bands of showers to continue to move over the Big Island.   See the forecast for 8 PM Sunday below.  The smaller islands may escape the worst of the heavy rains, if the model is correct.  However, weather models have difficulty correctly simulating the details of the convective showers and rainbands in Kona Lows.  In addition, the models tend to weaken the lows too quickly.  From past experience I would say we are not out of the woods yet.  So be prepared for the possibility of thunderstorms forming over Oahu during the next couple of days.