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HOW ROUGH IS ROUGH ENOUGH?

A Fanciful Day at the Beach

The earth is a dissipative system (Paltridge [1979] "Climate and Thermodynamic Systems of Maximum Dissipation" Nature 279:630-631). About 2.3 watts per meter squared of the 230 watts per meter squared available each average day from the sun goes into making the wind blow (kinetic energy) and the rest goes into our internal energy, earth is a nice warm place. Eventually this 2.3 watts per meter squared gets converted to heat by means of friction. Most of this dissipation is into the vegetation on the land surface and into the oceans making waves and currents.

The oceans are, 70% of earth, so we will say it does 70% of the 2.3 watts per meter squared of dissipation. This raises the average wave height at sea to some value, its global climatic mean wave height. The vegetation-covered land, 30% by area, dissipates 30% of the 2.3 watts per meter squared. A surface of "climatically average" surface roughness is needed to do all the dissipation work required, i.e. a vegetation height patchwork that gives that global land mean surface roughness. Now what would happen if we had some strange sort of climate change that reduced the average planetary wind speed? Well, waves at sea might, on average, be a little less high or maybe be a little shorter in period (choppier). The other thing that could happen is to have less rough vegetation on the land surface. How do we do that? Well, we could have a greater abundance of shorter vegetation. We would need fewer forests. However, slower winds usually (e.g. greenhouse studies) mean taller less stiff plants. Taller would increase surface roughness. Less stiff would lower surface roughness because the canopy would then be more wave-form and its streamlined form would be less rough. (Grace and Russell, 1977. J. Exp. Bot. 28:268-274. and Plant Response to Wind by J. Grace, Academic Press 1977).

With a less windy world we could have a more graceful world with lots of amber waves of grain. Now, what if people (men and women) went around with his trusty axe with a bent for hewing trees for the good of person-kind. The land would be less rough to the wind and the land area would do less of the total 2.3 watts per meter squared of work that just has to be done. Either the non-hewed land would have to get taller vegetation or stiffer vegetation or the oceans would have to do this work and build higher waves and put at risk your beach cottage and make sailing ships go faster. Lets see -- cut down the trees to get the sailing ships to go faster. That is what they did (not for that reason) in New England when they cut down the tall straight white pines for sailing ship masts. The more they cut down the faster the ships could go. Trade goods faster. Finance more ships that could go still faster. Now well before Emery's 1933 hurricane blew down the trees the New Englanders had left standing, most of New England had been carefully devegetated. The less rough surface permitted higher wind speeds close to the ground. In 1938, New England was ripe for Andrew-like picking. Feedbacks are fun to run to doomsday scenarios. If you don't think that the surface roughness due to plants plays a global role, you need to read the stuff of Sud. [J. of Clim. 4:383-398, 1984; Monthly Weather Review 116:2388-2400, 1988; J. Appl. Metero. 27:1036-1054, 1988; J. of Clim. & Appl. Metero. 24:1015-1036, 1985 and Boundary-Layer Metero. 33:15-49, 1985]

This last piece has been off the top of my head. True a flight of fancy but it does set one to thinking of what good the vegetation is anyway in a global sense. Just remember the sun speeds things up to the tune of 2.3 watts per meter squared and the vegetation must do its part in slowing things down the requisite 2.3 watts per meter squared.

Photo: © Copyright Steve Daniels and licensed for reuse under this Creative Commons Licence.

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