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The Wind Blowing

Our good friends at Harvard Forest are working on the 1938 hurricane, its wind fields and the blow downs it produced. Emery Boose has built a nice model that brings ashore hurricanes and puts out wind fields (direction and speed). His focus right now is Harvard Forest, Luquillo and North Inlet LTERs. This is intersite stuff. Recently he put in topography to produce a high resolution, site-specific wind fields and then compare them to the blow down records. The next step is to have a feedback from the vegetation to the wind field. He asked how I viewed the problem and so I will core dump here.

First, let me say that the sun builds the solenoids that accelerate the winds. So when sonny asks daddy, "Why do the winds blow?" you will have a cute answer. But, what are you going to do when he asks, "What stops the winds?" The answer is grass, flowers, shrubs and trees. On big islands, like England, wind speeds inland are about half what they are over smoother surface of the ocean (R. W. Gloyne. 1964. Sci. Hort. 17:7-19). Mostly the vegetation on the land and the waves dissipates the fury of the winds at sea. That is about 2.3 watts per meter squared frictional work. That is the average for the Earth. That is about the same as how much a 2XCO2 change will alter the earth’s radiation budget!

The vegetation slows down the winds but also makes the wind turn to the left in the northern hemisphere. Why left you say. Well friction slows down the wind. The Coriolis acceleration to the right is the Coriolis parameter times the velocity. So the Coriolis acceleration gets less when friction slows the wind. The normal Coriolis acceleration to the right of the wind is then smaller than the acceleration do to the pressure gradient and the wind turns left toward low pressure (down hill). Low pressure is to the left of the wind in our hemisphere!

Well how many degree leftward does it turn. That is what Emery wants to put in his model to predict blow-downs. Well it depends on how rough the vegetations surface is. A measure of such roughness is surface roughness (Zo). The maximum possible leftward turn is 90 degrees. For this to happen the trees must have brought the wind to a stop! Friction this great can be found inside a forest. The winds inside the forest (usually just light merry little breezes) tend to flow to low pressure. Find one of your colleagues who still smokes [you might have to go to the psychology department to find one or to central administration] take them out to your local forest and turn the smoking light on. Get her or him to puff away. Watch the smoke. Which way does it drift? Then look at the direction of movement of some low clouds. They should be about 90 to the smoke. These to low pressure winds in forests are called antitriptic winds. This is maxed out leftward deflection of the winds due to the total friction offered by the vegetation. If you looked at the leaves and twigs at the top of the canopy, you would see a wind direction about 45 degrees left of the free airflow above the trees.

If you are testing out all these things right after Christmas you are likely to have lots of Christmas tree rain (plastic "tin" foil strips) or you can watch for your neighbor to throw out his tree for the trash men. Neighbors rarely pick off all this scientific equipment before disposing. Take these little wind vanes and drape them (or tie them) to the branches of the trees from the top of the canopy (use one of Franklin's canopy
insertion devices if you have one) branch by branch down through the canopy. Now you will be able to see the direction of airflow at all heights within the forest. You might even see (or photograph) the eddies of airflow in the trees. How big are those eddies. Let me know. If you have to put the cost of these little wind vanes into your LTER renewal budget you should plan on about $1.49 per box of 60 vanes.

Well, how many degrees of leftward deflection should there be per unit of surface roughness. I have looked for the key regression equation for this and I can't find it. But here are some numbers I found and some that are stuck in between the found numbers based on published surface roughness numbers.

Surface Degrees of Leftward Deflection

Ocean

20*

Grass

20*

City Canopy

30*

Orchard

30/45*#

Paris France

45*#

20 m Deciduous Forest

50

30 m Deciduous Forest

60

Inside a forest

90

* from Byers' General Meteorology, 1959
# from Landsberg (1970)
other numbers are based on published surface roughness and Byers' data.

Byers got his estimates by measuring the difference in angle between the winds and the isobars of the pressure field. Without friction, the winds should go parallel to the isobars. With friction, they cross the isobars to the left. With friction winds flow out of a high-pressure system and into a low-pressure system. This means that air must sink into high pressure cells and rise up in low pressure cells. To be otherwise would be a mass imbalance.

I took a look at winter 1967 weather maps and looked for this angle at three LTER locations Coweeta (deciduous forest), Konza (grassland) and Sevilleta (desert). I picked days with straight isobars and no front or storm at the sites. I measured 12 angles for each site. Tossed out the highest and lowest values (This is Olympics diving methodology.) and found the average.

LTER Site

Vegetation

Leftward Deflection Degrees

Coweeta

Deciduous Forest

52.2

Sevilleta

Scrubby Desert

43.5

Konza

Grassland

34.1

So, I found Sevilleta to be as rough as Paris, France. This is a unidirectional complement to our neighbors with broad horizontal black and white striped T-shirts. Konza is like a smooth city. It just goes to show that vegetation is a better "baffle" for the wind that the unbending buildings of the city. Grasslands in the wind look like “amber waves of gain.” Grasslands are self-streamlining. So the surface roughness is less and frictional deflection to the left is small.

Coweeta came out about as expected given the first table presented. Emery now needs to use his GIS and assign deflection angles to the pre-hurricane 1930 landscape at Petersham. He had forests and fields. So he should find places where the winds converged (here they must speed up) and other places where the winds diverge (here they must slow down). Landscape heterogeneity due to vegetation gives rise to velocity maxima here and minima there.

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