This is an important question, because it is thought to be due to temperature, convection and radiative physics. But what about other possible factors? I’ve started gathering a list and some notes for discussion on the excellent and still very active Clive Best lapse rate thread. Regulars know that when a post title contain a question mark, there are going to be demands for help… TB
Dr Tim Ball has a good recent article here, which contains some useful diagrams, such as this one:
Tropopause height at the Poles varies between 7 km in winter and 10 km in summer, at the Equator the range is 17 to 18 km. The difference in seasonal range is because of the difference in seasonal temperature range. How do you build even those simple dynamics into a computer model?
Answer: the best way to make a start is to quantify the various effects which contribute to the height difference in the polar and equatorial tropopause, using the clues offered by seasonal variation as a guide.
So, what else apart from temperature, convection and radiation might affect the height of the tropopause?
Here’s my probably list, I’ll update it with suggestions made in comments.
The Earth is 43.5km bigger in diameter at the equator than it is at the poles. Wikipedia says this is due to rotation; i.e. the centrifugal effect of the spinning mass.
“Without any idea of the Earth’s interior, we can state a “constant density” of 5.515 g/cm³ and, according to theoretical arguments (seeLeonhard Euler, Albert Wangerin, etc.), such a body rotating like the Earth would have a flattening of 1:230.
In fact the measured flattening is 1:298.25, which is more similar to a sphere and a strong argument that the Earth’s core is very compact.”
Of course, for a centrifugal force to be effective, there must be a coupling between the atmosphere and Earth. There clearly is some, because at the surface, the atmosphere goes round at the same speed the Earth does, give or take local variation. If it didn’t, our brollys would be blown inside out by 1000mph winds. However, there is shear as we go up from the surface, and jet streams, geostrophic winds, meridional circulation etc. So how do we quantify the effect?
If the Earth was on it’s own in space, then notwithstanding other effects the atmosphere would be pulled towards sphericity. However, Earth lies in a planetary plane orbiting the Sun, with a close by moon 1/6th its mass. About.com has this to say:
“The gravitational pull of the moon and the sun creates tides on the earth. While tides are most commonly associated with oceans and large bodies of water, gravity creates tides in the atmosphere and even the lithosphere (the surface of the earth). The atmospheric tidal bulge extends far into space but the tidal bulge of the lithosphere is limited to approximately 12 inches (30 cm) twice a day.
The moon, which is approximately 240,000 miles (386,240 km) from the earth, exerts a greater influence on the tides then does the sun, which sits 93 million miles (150 million km) from the earth. The strength of the sun’s gravity is 179 times that of the moon’s but the moon is responsible for 56% of the earth’s tidal energy while the sun claims responsibility for a mere 44% (due to the moon’s proximity but the sun’s much larger size).”
But we would expect the atmosphere to relax back towards sphericity (notwithstanding other factors) where it is not in line with these heavenly bodies. So how fast does it relax? How far is “far out into space”?
Clearly the Earth’s topography has a hand in keeping the near surface atmosphere moving round with the surface, indeed there are clear inverse relationships between changes in atmospheric angular momentum (AAM), zonal Atmospheric Circulation Index ACI and the Earth’s length of day (LOD). So large amounts of coupled energy are evidence of the coupling between the atmosphere and solid Earth. What effect does this have on the difference in height of the troposphere between equator and poles? The north pole is pretty flat, the northern hemisphere is pretty mountainous and forested in places. The southern hemisphere is mostly ocean, but big waves will also couple the air and sea. Is there a difference in tropopause profile between the hemispheres which offers any clues here? Or would its signal be lost in profile differences due to larger effects such as general circulation?
Over to you.