Agronomy 405
Spring 2013
Problem Set 4
Due Tuesday, February 12, 2013.
Assigned February 5, 2013. Updated (Problem 6) February 7, 2013.
1. Write a short story (at least one paragraph) that describes the “adventures” of a helium
balloon named “Roland” after it is accidentally released by a small child at a county
fair. Include in your story descriptions of the wind experienced by the balloon and the
variations in its horizontal and vertical motions as it rises through the surface layer
and into the free atmosphere. Be creative if you would like.
2. 5.1 in Campbell and Norman.
3. 5.2 in Campbell and Norman.
4. 5.3 in Campbell and Norman.
5. If you only had three anemometers and a single 10–m mast or tower, and you wanted
to experimentally determine the friction velocity u∗ and also the roughness length zm
of a bare soil surface, at what heights above the ground would you place/mount your
anemometers to best estimate u∗ and zm ? Why?
6. Write and answer your own question based upon the information presented by Jason
Patton in class on Thursday, February 7.
Example question: In what two ways can sensible heat flux be positive,
taking energy out of a control volume surrounding Earth’s surface?
Answer: Sensible heat flux is positive when w ′ T ′ > 0 which can occur either
when w ′ > 0 and T ′ > 0 (warm air transported up, +ẑ, out of the box,
from the surface to the atmosphere) or when w ′ < 0 and T ′ < 0 (cool air
transported down, −ẑ, into the box, from the atmosphere to the surface)
where w ′ is the fluctuation of vertical wind speed, T ′ is the fluctuation of air
temperature, and ẑ is a unit vector normal to Earth’s surface pointing away
from Earth’s surface.
7. Examine the journal paper “Observed Diurnal Cycle Climatology of Planetary Boundary Layer Height” at doi:10.1175/2010JCLI3552.1. Focus on Figure 12.
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(a) Over which surface (land, ocean, or ice) does the PBL change the most over the
course of a day, on average? Explain why this makes sense.
(b) Over which surface (land, ocean, or ice) does the PBL change the least over the
course of a day, on average? Explain why this makes sense.
(c) What is the maximum and mininum diurnal heights of the PBL over land, and
at what time (LST = local solar time) do they occur, on average?
8. Read “No–Till and Snow Can Help Crops Grow” at http://www.ars.usda.gov/is/
AR/archive/aug12/snow0812.htm.
(a) Standing wheat residue (the dead plant material left after harvest) on a no–till
farm (the residue was not plowed into the soil after harvest, but just left alone)
had two effects on snow cover. What are these two effects?
(b) What is the benefit of no–till in terms of its interaction with snow for ridge tops
(high areas in the field)?
(c) What is the benefit of no–till in terms of its interaction with snow for valleys?
(d) Fundamentally, how does residue effect snow cover? Try to use the words “wind”
and “momentum” in your answer.
9. Read “Wind in the Willows Boosts Biofuel Production...” at http://www.sciencedaily.
com/releases/2013/01/130118125950.htm.
(a) How does wind effect willows?
(b) Why is this good for biofuel production?
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