**Suggested Procedures for Creating Sea & Land Breezes**\
**- 6/17/03**\
**UPDATED 9/25/03**

By Andy Nash

I\'ve already discussed the option of using the Pencil Tool on the Wind
Grid with the  \"direction only\" attribute set to result in the Pencil
Tool behaving like a streamline tool. Although that is quicker than
other ways of creating land/sea breezes, it can be tedious. Thus we have
installed a couple of SmartTools that operate on the wind grids. They
arethe **\"SeaBreezes\"** and **\"LandBreezes\"** tools.

**[UPDATE:]{.underline}**

These tools also comes in a couple of different \"flavors\". The
combined \"**Land_Sea_Breeze**\" smart tool which works in the same
simple way as discussed below, but allows you to select which type of
flow you want to compute.

The other tool is the \"**Land_Sea_Breeze_Vect_Avg**\".  When run, you
are prompted with a choice of a land or sea breeze, as well as the
strength in knots \-- in increments of 5 (5, 10, 15, 20).  This tool
will then calculate what each grid point would have for a land or sea
breeze of the assigned strength. This is done by just calculating a
vector along the gradient of topography. It then vector averages this
value in with the value that was previously in place at that grid point.

For example, over HNL Airport, a pure sea breeze based upon gradient of
the nearby topography gives a  south/southwest (210 deg) wind.\
Let\'s say you had winds of E-12 kts at that grid point and picked a sea
breeze strength of 10 kts. The resultant would give you\
something near SE-10 kts of wind.

[PROS:]{.underline}\
This can provide a decent estimate of winds during lighter wind regimes,
especially on the smaller islands or on parts of the smaller\
islands.

[CONS:]{.underline}\
Doesn\'t do a great job with the Kona sea breeze, which has a direct
relationship with the strength of the trades. The stronger the trades,
the stronger the sea breeze.  This smart tool does not recognize that.

You may have to test it a couple of times with different sea/land breeze
\"strengths\" selected to see if it gives something you like

 

They are actually pretty simple to use. Let\'s step through the process:

**[Sea Breezes:]{.underline}**

Here\'s our basic wind pattern\....light trades. As we\'ve seen,
populating the GFE Forecast Database from the higher resolution models
(eg: RSM, ETA) does give us a good first guess at the Big Island
circulations, however no local circulations show up for the rest of the
islands.

![](media/image1.png){width="5.997916666666667in"
height="4.771527777777778in"}

**First step is to define the area(s) we want to show the sea breeze.**
Just a **couple of words of caution** on this\.....

1\. Do not extend your Edit Area too far from the islands \-- roughly 10
grid points or 25km. If you do this, you will end up with a default wind
direction of either north or south.\
2. These SmartTools do not change the wind speed at all, so if you want
to reduce/increase the wind speeds you\'ll have to do that by the
Taper_Adjust tool.

Here are the areas I\'m forecasting sea breezes:

![](media/image2.png){width="6.31875in" height="5.027083333333334in"}

Now just select the \"**SeaBreezes**\" SmartTool and you\'ll end up
with\....

![](media/image3.png){width="6.498611111111111in"
height="5.170138888888889in"}

Notice how the wind directions changed.

Let\'s look more closely on Oahu\....

![](media/image4.png){width="6.690972222222222in"
height="5.303472222222222in"}

T**he SmartTool is simply changing the wind direction to go parallel
with the slope of the terrain**.  IFor example, look at the wind
direction at HNL.  Typically in a light trade flow, the winds at HNL are
more SE or S.  If we superimposed this circulation over the light trades
we would indeed have a wind from the SSE rather than the SSW flow we get
due to the terrain slope. Using the \"Land_Sea_Breeze_Vect_Avg\" smart
tool would give you something along these lines.

On a related note, **this (and the LandBreezes version) do not assume
any wind criteria for when a sea/land breeze can\'t occur**. In other
words, if you have an overall grid of 30 kt trades and run this tool
over Oahu, you will get 30 kt onshore flow on all coasts. It is up to us
as the forecaster to decide/know when to use a particular SmartTool.

**[Land Breeze Tool:]{.underline}**

We follow the same procedure for the Land Breeze. First define the
area(s) you want. I\'ll do the Big Island downslope flow at night. Note
how I extend the area offshore, but not too far.

![](media/image5.png){width="6.498611111111111in"
height="5.190972222222222in"}

Run the SmartTool and get\....

![](media/image6.png){width="6.5625in" height="5.211111111111111in"}

There still may be some clean up to do here or there.

Lastly, the suggestion is that you would run these Tools for the time
that you expect the circulations to be fully developed, and then
interpolate to fill in the blanks. For example, you may decide to run
the Sea Breeze tool for a grid valid at 22Z-02Z and then interpolate
grids at 2 hour time steps between 16Z and 22Z to show the change from
land breeze to sea breeze.\
 \