Local Winds

Learning Goal 10a: Explain how the following local winds work and how they apply to sailing: sea breezes, land breezes, katabatic winds, and coastal (barrier) jets.

Sea and Land Breezes

Fundamentals

A sea breeze is a shallow cool wind that blows onshore (from sea to land) during daytime (see figure below). It occurs in large-scale high-pressure regions of weak or calm synoptic-scale wind under mostly clear skies. Similar flows called lake breezes form along lake shorelines, and inland sea breezes form along boundaries between adjacent land regions with different land-use characteristics (e.g., moist irrigated fields of crops adjacent to drier land with less vegetation).

The sea breeze is caused by a 5 °C or greater temperature difference between the sun-heated warm land and the cooler water. There is also a weak return flow aloft from land to sea (see sketch below).  The reason that the ocean warms more slowly than the land during a sunny day is that sunlight is absorbed and spread (diluted with cooler deeper water) by ocean turbulence over several meters or more of ocean depth, but on land all the solar heat is concentrated in the top few centimeters.

Source: R. Stull, 2017: Practical Meteorology. Used with permission.

A sea-breeze front marks the leading edge of the advancing cool marine air over land and behaves similarly to a weak advancing cold front or a thunderstorm gust front. If the updraft ahead of the front is humid enough, a line of cumulus clouds can form along the front, which can grow into a line of thunderstorms if the atmosphere is statically unstable. Kelvin-Helmholtz waves (KHW) can form at the interface between the cool onshore sea breeze and the warm return flow aloft.  The sea-breeze front can advance 10 to 200 km inland by the end of the day, although typical advances are 20 to 60 km unless inhibited by mountains or by opposing synoptic-scale winds. Even without mountain barriers, the sea breeze will eventually turn away from its advance due to Coriolis force.

As the cool marine air flows over the land, a thermal internal boundary layer (TIBL) forms just above the ground (see figure above). The TIBL  is a region of warmed air that grows in depth with increasing distance from the shore as the marine air is heated by the underlying warm ground

In early morning, the sea-breeze circulation does not extend very far from the coast, but spreads out further over land and over water as the day progresses. When fully developed, near-surface wind speeds in the marine, inflow portion of the sea breeze at the coast are 4 to 40 km/h (=1 to 10 m/s) with typical values of about 22 km/h (=6 m/s).

At the end of the day, the sea-breeze circulation dissipates and a weaker, reverse circulation called the land-breeze forms in response to the nighttime infrared cooling of the land surface relative to the sea (assuming clear skies). The reason is that all of the heat loss at night is concentrated in the top few cm of soil over land causing the land surface to get relatively cold, but in the ocean the cooling is spread over tens of meters at night. In the land breeze, low altitude winds flow from land toward ocean, and the return flow aloft is from the ocean toward land. 

In the vertical cross section normal to the coastline (as in the figure above), the surface wind oscillates back and forth between onshore (coming from the sea during the sea breeze) and offshore (coming from the land as a land breeze), reversing directions during the morning and evening hours. The Coriolis effect causes the horizontal wind direction to rotate throughout the course of the day. Rotation is clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. The idealized sea-breeze hodograph has an elliptical shape (see Figure below). For example, along a meridional coastline with the ocean to the west in the Northern Hemisphere, the diurnal component of the surface wind tends to be westerly (onshore) during the mid-day, northerly (alongshore) during the evening, easterly near midnight, and southerly (alongshore) near sunrise.

Source: R. Stull, 2017: Practical Meteorology. Used with permission.

The sea-breeze wind and the mean (24 h average) synoptic-scale surface wind are additive. If the synoptic-scale wind (i.e., wind driven by larger-scale pressure patterns like Lows and Highs) in the above example is blowing from the north, the surface wind speed will tend to be higher around sunset when the mean wind and the diurnal component are in the same direction, than around sunrise when they oppose each other.

Sailing Tips

• Sea breeze is strongest during clear sunny daytimes under High pressure.  Under these conditions, the sea breeze is strongest mid-day through mid-afternoon local time.  Caution, the sea-breeze can die in the late evening BEFORE the sun sets, so plan ahead to return to port before the winds die.
• In synoptic high-pressure regions where the synoptic-scale winds are light to calm (bad sailing), you can still get enough sea-breeze near the coastline to have winds for good sailing.
  
• The surface winds (white arrows in figure below) caused by the sea breeze (cyan color) are in a band (outlined with dashed purple line) parallel to the coastline.  This band of good sailing is roughly 10 to 50 km wide, depending on local conditions.
• The sea-breeze is strongest at the coastline and is weaker the further away from shore. Caution, beware of shallow waters and rocks too close to shore.  But don't venture too far from shore where there is no sea breeze.  So you need to find a compromise sailing distance from shore, roughly within the purple region outlined in the figure below.  (You can anticipate similar good sea-breeze sailing regions parallel to other coastlines.)

Note for non-sailors.  Modern sailboats can travel in almost any direction relative to the wind, except they cannot head directly into the wind (see Sailing Basics).  Thus, for the image below with sea-breeze winds (white arrows) perpendicular to the coastline, sailboats can easily sail parallel to the coastline if they wish.  Thus, the long region of sea-breeze winds outlined in the purple dashed line below means that sailboats within that region can travel long distances parallel to the coastline when a sea-breeze exists.


Source: Google maps, annotated by R. Stull.  Dashed purple area outlines the region of good near-surface sea-breeze winds (white arrows) for sailing along the west coast of Vancouver Island in summer.  The cyan rectangle shows the vertical circulation of the sea-breeze, including the return flow aloft.  But the actual wind directions of the white arrows will change during the day, as suggested in fig 17.14.

Many coasts have complex shaped coastlines with bays or mountains, resulting in a myriad of interactions between local flows that distort the sea breeze and create regions of enhanced convergence and divergence. The sea breeze can also interact with boundary-layer thermals, and urban circulations, causing complex air flows near the shore. If the onshore synoptic-scale (large-scale) wind is too strong, only a TIBL develops of air warming with increasing distance from the coastline (see first figure above) with no sea-breeze circulation.

In regions such as the west coast of the Americas, where major mountain ranges lie within a few hundred kilometers of the coast, sea breezes and terrain-induced winds (such as katabatic winds, described next) appear in combination.

Remember, many winds are named by the direction FROM which they blow, e.g. easterly winds blow from the east. So, sea breezes blow from sea to land, while land breezes blow from land to sea.

Katabatic Winds

When mountain slopes experience surface cooling during cloud-free nights (due to infrared radiation to space), the surface air touching these cold mountain slopes becomes cooler than the air at the same elevation away from the mountain. As the air cools, it becomes denser and sinks due to gravity (buoyancy), creating katabatic winds (recall Learning Goals 6a and 6b). The speed at which these cold winds fall downhill can vary from hurricane force (near the long steep slopes at Greenland and Antarctic coastlines) to a light breeze. Katabatic winds are typical of night-time surface cooling mountainous regions, or can develop at any time of day over bodies of ice or snow on mountain slopes.

• Climbers travel through katabatic winds in Antarctica.  View first 2 minutes of: https://www.youtube.com/watch?v=ZWPo827a4l0

Source: R. Stull, 2017: Practical Meteorology. Used with permission.

If mountain slopes are adjacent to coast lines, then the cold downslope winds from the mountain will descend to the sea surface, where the cold air will spread out and cause a cold wind blowing offshore (away from the mountains).

If the cold downslope winds flow into a fjord, the cold air can pool and become stagnant, causing bad air pollution for towns in that fjord, or can cause ice fog, depending on conditions. 

If the valley floor has some slope to it, then the cold air can flow as a mountain wind down the valley floor in the same direction that streams of water would flow.  If these valleys open into coastlines, then the cold mountain winds can "gush" out over the coastal waters in narrow regions from these valley mouths.  Such an outflow wind can affect shipping along the coast - - outflow winds are discussed as Learning Goal 10b.

Synoptic winds (meaning the larger-scale winds in the region) and pressure gradients can also influence katabatic winds to determine the speed and direction in which these winds will travel (see Katabatic Winds of Greenland video below).

In the image below, you can clearly see the cold air falling off the ice shelf and creating a local wind in the Bellingshausen Sea, Antarctica.

Source: fruchtzwerg's world - Flickr, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=3735232

Sailing Tips

• As katabatic winds are mostly a nighttime phenomenon that occurs very close to the coastline, hopefully few sailors are out at night dangerously close to shore.  But if you are in a larger ship with radar and detailed charts, just be aware that you could experience these cold outflow winds every time you pass in front of a valley mouth or associated inlet.
  

Coastally-trapped low-level (barrier) jet

Sometimes, a Pacific Low with a cold front hits the Coast Mountains of British Columbia from the west.  This creates a shallow layer of cold air trapped against the west side of Coast Mountains.  It also leaves lower pressure along the north coast, and higher pressure along the south coast. 

Source: R. Stull, 2017: Practical Meteorology. Used with permission.

The resulting pressure gradient (change of pressure with distance) along the coast creates a low-altitude jet of fast-moving cold air blowing from south to north over the coast (just west of the mountains).  Similar low-altitude jets of air can occur near other mountainous coastlines, such as portions of the west coast of the USA.

 

Source: R. Stull, 2017: Practical Meteorology. Used with permission.

Sailing Tips

• If you are lucky enough to be sailing northbound when such a low-altitude wind sets up, then you can have a nice brisk tailwind.  Caution, some of these coastally trapped low-level jets can be associated with clouds and rain, while others are associated with fog and low stratus clouds.
  

Additional Resources: (non-required material)

UIUC ww2010 guides: http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/sea/htg.rxml

MBNMS Climate and Meteorology: http://montereybay.noaa.gov/sitechar/clim2.html

Videos: (non-required material)
Animation of sea breeze: http://www.classzone.com/books/earth_science/terc/content/visualizations/es1903/es1903page01.cfm
Katabatic Winds of Greenland: https://www.youtube.com/watch?v=pHYb36LzxnI