Newsgroups: lter.ced Path: LTERnet!news From: "Bruce P. Hayden" Subject: CED 4.7 Message-ID: <1995Aug15.184045.18023@lternet.washington.edu> Sender: news@lternet.washington.edu Organization: Long Term Ecological Research Date: Tue, 15 Aug 1995 18:29:02 GMT ***************************************************************** ***************************************************************** *** *** *** *********** *********** ********** *** *** * * * * *** *** * * * * *** *** * * * * *** *** * ********* * * *** *** * * * * *** *** * * * * *** *** * * * * *** *** * * * * *** *** *********** *********** ********** *** *** *** ***************************************************************** ***************************************************************** Vol.4 No.7 ::: August ::: August 15, 1994 ***************************************************************** ***************************************************************** CED METADATA ---- CED is the Climate/Ecosystem Dynamics bulletin board of the LTER network. In CED, you will find exchanges of ideas, information, data, bibliographies, literature discussions, and a place to find experts within the LTER community. We are interested in both climate controls on ecosystems and ecosystem controls on climate. As this is an inter-disciplin- ary activity, we hope to provide things that you might not come across in your work at your LTER site. CED is a product of the LTER climate committee and contributions to CED for general e-mail release may be sent to either David Greenland of Andrews LTER [Greenlan@oregon.uoregon.edu] or to Bruce Hayden of the Virginia Coast Reserve LTER [bph@Virginia.edu]. We expect that the scope of CED will evolve and reflect the interests of the contributors and users of this service. CED will be issued as the preparation work gets done (usually monthly). Back-issues of CED may be requested from Ray Bero [helper@LTERnet.edu] by the file name given in the masthead.Daniel can also add people to the CED mailing list. Feedback on CED from LTER scientists is welcome (non-$$$$ contributions also welcome.) For example, please forward citations of climate & ecosystem publications on your site. We are keeping a LTER wide bibliography on Climate/Ecosystem Dynamics that we pass on via E-mail. ***************************************************************** ***************************************************************** *** *** *** *** *** FRASER *** *** *** *** *** ***************************************************************** ***************************************************************** Much of this issue of CED comes from a real science hero: Alistair B. Fraser. Alistair teaches, I mean really teaches, at Penn State. In this issue, CED reviews some of his work at the interface between plants and atmospheric optics and rain that doesn't reach the ground. Here is Alistair's short autobiography -- "Alistair B. Fraser is a Professor of Meteorology at the Pennsylvania State University. He started his professional career with the Canadian Government as a weather forecaster in Vancouver before returning to school to gain a Ph.D. in Meteorology from the Imperial College of the University of London. He is an enthusiastic teacher and in his search for more effective ways to communicate to his students has made extensive use of the computer in the classroom. He is the co-author of a textbook and has published research in cloud physics, radiative transfer, artificial intelligence, and social history. He has been awarded a patent. He has prepared scientific exhibits for science museums and a history exhibit for a history museum. His writings for popular consumption have appeared in publications ranging from the Reader's Digest to the Scientific American. His photographs of weather phenomena have appeared in newspapers, magazines, books, exhibits and on television specials." [http://www.ems.psu.edu/~fraser/BadMeteorology.html] is his home page! Power up your NETSCAPE and don't miss a hypertext fork in the superhighway. This one is special. ***************************************************************** ***************************************************************** *** *** *** *** *** VIRGA, aka Fallstriefen *** *** *** *** *** ***************************************************************** ***************************************************************** Streaks or wisps of water or ice particles falling out of a cloud but evaporating before reaching the earth's surface. Definition from the Glossary of Meteorology, an AMS-you-should-have-it-on-your-shelf book for sure. Another definition is "Once in a while, rain is seen to fall out of clouds and evaporate completely before it reaches the ground." If you have not seen virga take in Jim Gosz's lightning talk. SEV is VIRGA-city. When looking across the expanse of Sevilleta National Wildlife Refuge one might see dark streaks from the bottom of such clouds only to seem to disappear, i.e. become brighter before the ground is reached. So Virga is not the dark fall streaks extending from the bottom of the clouds but the termination of said streaks -- the place where there is a sudden change in the brightness shaft of the precipitation. Weather geeks talk so cool. Has the virga explanation ever been put to scientific test or is it just loved natural history. Fraser and Bohren (1992) say it was never tested! See Monthly Weather Review 120:1565-1571. Fraser and Bohren contest the current explanation and offer two new explanations! Good science skeptics are really a lot of fun. If you have doubts about skeptics being fun, antiskeptics do but shouldn't, you really should check into NETSCAPE and tune in to Fraser's homepage. This is a home page worthy of you bookmarking it. Go down every hypertext trail. Don't miss anything in this homepage! Fraser and Bohren's problem with existing natural history theory is that it implies a sharp break in optical thickness from visible rain to invisible water vapor as evaporation proceeds. Measurements indicate that optical thickness on progressing down the rain shaft does not change abruptly but gradually from cloud base to the ground. So new science theory is needed. They offer two. ***************************************************************** ***************************************************************** *** *** *** *** *** MELTING SNOWFLAKES THEORY *** *** *** *** *** ***************************************************************** ***************************************************************** Consider a snowflake falling into warmer air (>0 C). It melts and there is a transition in the precipitation shaft. It switches from a snowshaft to a rainshaft. Optical cross section can be reduced by a factor of about 2. And because drops fall faster than crystals the optical cross section is reduced even more as they speed up on passing from snowshaft to rainshaft. Put the two together and the reduction in optical density is 10X. Dendrite or star flakes have higher optical densities than drops made from the said crystal fall very slowly and so dendritic snow shafts are very dark and when they fall and melt the optical density declines a whole lot, like 20X. You can see right through the rainshaft below the snowhaft. If there were a grauple shaft extending down from a cloud and the grouple melted there would be almost no observed virga because the drop size and fall rate of grouple and the resulting rain drop are nearly the same. ***************************************************************** ***************************************************************** *** *** *** *** *** THE SLOW PRECIPITATION THEORY *** *** *** *** *** ***************************************************************** ***************************************************************** The idea here is that visible precipitation shaft from the bottom of the cloud is getting to the ground but is falling so slow that, well, it just isn't there yet. Wait and watch and maybe it will reach the ground. Consider a 1 mm drop. It falls until it reaches its terminal velocity 400 cm per second (not so fast!) If cloud base is 3000 m, it would take about 750 seconds to get to the ground. Divide by 60 and you get 125 minutes or 2 hours and 5 minutes. Now if the drop is falling through non-saturated air (RH <100%) more water molecules will leave the drop than condense on it and thus there is a net change in favor of evaporation. Smaller drops fall even slower! Will the drop every get to the ground? If RH at the ground = 0% then no drop would make it. SEV can have dry air and so SEV falling drops get small fast and the rain might be visible as a shaft but sort of hang there falling slower than your eyes can see! Now if the downdraft in the shaft is greater than the terminal velocity of the falling drops, drops tend to accumulate at the downdraft leading edge giving a sharp change in optical depth. Multiple causality buffs can go for the melting snowflake and/or slow precipitation combo-theory! The Fraser/Bohren article in MWR is titled Is Virga Rain That Evaporates before Reaching the Ground? Fraser and Bohren use article titles to attract readers. Pick up the next issue of your favorite journal that crosses your desk and see how many articles seem designed to turn you off. The seem to yell "DON'T READ ME!" consider another of Fraser's gems: The sylvanshine: retroreflection from dew-covered trees. Here is a Bohren contribution "Vertical elliptical coronas caused by pollen." Even if you don't know what a vertical elliptical coronas are you still want to read this kind of stuff. Alistair B. Fraser, 1991:A Canadian Flag for Canada. Journal of Canadian Studies, 25(4):64-80. Anyone who can write a 16 page paper on this subject can't be all bad. Alistair B. Fraser, 1995: Transforming chalk dust into mouse droppings. EMS Bulletin, 64(1): (in print). This is about teaching. If you want to know how much work it is to be a great teacher look at Fraser's stuff on his homepage! "Mouse droppings" is a euphemism classroom heroism. ***************************************************************** ***************************************************************** *** *** *** SYLVANSHINE BEFORE DAWN *** *** HEILIGENSCHEIN AFTER DAWN *** *** *** *** *** ***************************************************************** ***************************************************************** Sylvanshine: retroreflection from dew-covered trees. Fraser, A. B. (1994). Applied Optics 33(21):4539-4547. Doing some of my summer work in the Physics library I bumped into this title. I quickly found page 4539 and thought -- what a CED gem! The meaning of sylvanshine should be dopped-out and known to you by now if you can do the transference from sunshine to sylvanshine! Fraser's little poem to start off this paper is so good that he must have made it all up! Get up, sweet-Slug-a-bed, and see The Dew-bespangling Herbe and Tree Robert Herrick (1591-1674) Corinna's Going a Maying Fraser is a lover of meteorological optics. He has been at it a long time and he is very good at it. Fraser discovered sylvanshine at 1:00 AM on 12 August 1984 between Nakusp and New Denver, British Columbia. On that warm summer night he looked through the windshield of his car and saw the "normally Stygian forest began to glow as if snow covered in the moonlight." The sylvanshine continued for mile after mile. In 1989 he drove across Ontario and the trees glowed for hours ended by dawns early light! Jerry Franklin has his new tower towering over one of his old growth spots. He needs to get his camera (he always has it), a car headlight quality torch, a sleeping bag and self-permission to spend a good dark humid night out on the arm of his beloved tower. This is a species specific problem and the arm of the tower should point in the direction of douglas firs, hemlocks or red or yellow cedars and gaze along the path blazed by his light in search of sylvanshine. Norway spruce, even if Jerry had some wouldn't work! Fine plates in Vol. 33 no. 21 Applied Optics to encourage sylvanshine seeker. ***************************************************************** ***************************************************************** *** *** *** *** *** FIRST, HEILIGENSHEIN *** *** *** *** *** ***************************************************************** ***************************************************************** In the "Memoirs of Benvenuto Cellini, 1562," Ben writes "shining light . . . seen over my shadow . . . and it appears to the greatest advantage when the grass is moist with dew." Ben attributed his early morning halo to "the wondrous ways of [God's] providence toward me." Ben then would invite friends for morning walks to show off his god given halo. Fraser comments that "presumably none of those knaves had the gall to point out that each saw it around the head of his own shadow. Ben saw Heiligenshein, aka the light of the holy one. Heiligenshein concerns the anti-solar point. It works like this: the sun, usually at a low angle hits you in the back of the head. Your shadow-head is cast on the ground. The dew drops around your shadow head are not so far off the anti-solar point the photons can enter these drops, reflect of the inside of the drop walls and come back to your eyes. There is no way Ben could see the Heiligenshein of others! and others could not see Ben's. It is one of those Gee!-Did-you-see-that? phenomena. The knaves saw no halo on Ben but on themselves? You bet. In 1874 a chap by the name of Lommel suggested that the dew drop acted like a lens and if the drop was 1 focal length above the leaf the light would come back to the Heiligenshein-seer's eyes. Getting 1 focal length above the leaf required trichomatous leaf surfaces. Later it was discovered that the trichomes were not needed at all. If the drop could sit on the leaf with a contact angle of Pi and the light would enter the drop at an angle of Pi/3 with respect to the leaf, by internal reflections in the drop, the photons would come back to the eye of the beholder. If the leaf is of the right nature and can hold a drop up high on its surface so that it makes contact with the leaf at just the right contact angle then you see God's halo around your shadow head! ***************************************************************** ***************************************************************** *** *** *** *** *** NOW SYLVANSHINE *** *** *** *** *** ***************************************************************** ***************************************************************** Sylvanshine and heiligenshein are nearly the same thing and they would be the same thing if we could put ourselves way on the other side of the sun to do the observing of sylvanshine. In sylvanshine there is no halo. Sorry, Jerry. You look down the bore of your power-flashlight or along the beam of your headlight and the back reflection of light from within the dew drops making just the right contact angle with the leaves illuminates the dew drop festooned trees. Were it not summer you would think the trees all frosted! The best contact angle for drop and leaf in sylvanshine is 140 degrees. The back reflected light is brightest then. This angle depends on the wettability of the leaf and this varies with the genes of the plant. Cuticle and its waxes are in the story now. When the leaves have only a patchy layer of amorphous wax the contact angle is 80 to 90 degrees and you get not so bright sylvanshine. With complete coverage you get a contact angle between 90 and 110 degrees and like skis it depends on the kind of wax. For contact angles higher than 110 degrees you need a crystalline wax on the surface that is known as epicuticular wax. ***************************************************************** ***************************************************************** *** *** *** *** *** WOULD Butch-wax WORK? *** *** *** *** *** ***************************************************************** ***************************************************************** Epicuticular was is often made up of extruded rods a few tenths of a micrometer in diameter and many times that length. Plants with this pure-bred epicuticular rodded wax tend to have a blue cast to them as the rod diameters is less than the wavelength of the light and blue light is scattered. Blue spruce is an example. You don't want that dull sylvanshine you get from a shinny waxed surface. Rodded waxed surfaces vary from species to species and within species from place to place and from time to time. Plant with these rods not only appear to have a blue cast but the also do not absorb UV. Perhaps these waxy rods evolved to abate the UV load and protect the DNA. Isn't this a great story! The right plants are blue spruce, hemlock, juniper, arborvitae, Douglas fir and Fraser fir. Norway spruce doesn't work. Good shrubs are barberry, yew, rhododendron, snowmound and coral berry. The best herbs are crown vetch and dianthus. Remember epicuticular was is easily damaged, i.e. your fingers can mush down all those rods and the sylvanshine goes out! You will be happy to know that NSF supported this work with ATM-8917596. ***************************************************************** ***************************************************************** *** *** *** *** *** ARBOREAL DEW *** *** *** *** *** ***************************************************************** ***************************************************************** Arboreal dew is something new to most of us. Walks on dewy days results in wet shoes but it is rare that that the trees also are dew bedecked. How often can you remember the frozen form of dew (frost) on both the ground and on the canopy? Not often. The well known nocturnal inversion mandates that it is coldest at the ground and it gets warmer with height above the surface. Consider a July night, say 10 PM, with dew on the ground the temperature gradient above the ground from 2.5 to 30 cm is 165 C/100 m and from 30 to 120 cm it is +36 C / 100 m. So if there is dew at 2.5 cm and the temperature there is the same as the dewpoint temperature, then at 30 cm above the temperature is warmer by 0.5 C and the relative humidity is then less than 100% and evaporation exceeds condensation and the dew either will not form or it will quickly evaporate. In the first 20 meters above the ground temperature at night can warm 6 to 7 C. So if dew is so hard to come by in the canopy how do you get it? First it could be pseudo-dew. If a fog or cloud passed through a canopy dew-like drops could be deposited on the leaves. The coastal coniferous forests of the West Coast would be a good place to find this pseudo-dew. To get real dew, drops coming into being right on the leaves, you first cool down the plant surfaces then advect a warmer, moister air mass over the area. Condensation and dew will form everywhere the surface is colder than the dewpoint of the advected air blown in! Sylvanshine is best searched for on herbaceous, small stature plants with the right kind of waxy surfaces. Else, as Fraser suggests, take a sprayer filled with water, select the right plant material, spray away, stand back and turn your headlights on your self-misted sylvanshine creation. This Jerry Franklin could do in his canopy without staying the night! ***************************************************************** ***************************************************************** *** *** *** *** *** WHAT FRASER LIKES *** *** *** *** *** ***************************************************************** ***************************************************************** It has been a pleasure to introduce CED readers to Fraser of Penn State and to egg you on to looking at his home page. Consider the following which Fraser likes. It may amuse you as well. Be very, very careful what you put into that head, because you will never, ever get it out. Cardinal Wolsey (1475?-1530) ********** A University is what a College becomes when it stops paying attention to its students. John Ciardi (1916-1986) ********** Certifications are the devices we employ to convince others of our competence when they otherwise would have little cause to believe in it. A.B.F. ========================================================================= | Bruce P. Hayden | | University of Virginia | | Department of Environmental Sciences | | Charlottesville, VA 22903 | | | | ------------------------------------------------------------------------| | 804-924-0545 (o); 804-924-7761(d); 804-982-2137(FAX) | | bph@virginia.edu; bhayden@lternet.edu | |-----------------------------------------------------------------------| | VIRGINIA COAST RESERVE Long-Term Ecological Research Site | | VCR LTER URL = http://atlantic.evsc.virginia.edu/ | =========================================================================