Does the Moon's Gravity Affect Atmospheric Tides?

[Nidum]

[Mentor's Note: Post moved to it's own thread]
I there a tidal swell or circulatory effect in the atmosphere caused by the moons gravity ?

 

[Dr. Courtney]

That's a great question. I think the easiest way to detect it would be to take a Fourier transform of a barometric pressure measurement over the course of a year or so and see of all the well-known tidal frequencies are present. I bet they are. See:

https://en.wikipedia.org/wiki/Theory_of_tides

 

[D H]

Is there a tidal swell or circulatory effect in the atmosphere caused by the moons gravity ?

Yes, but it's very small.

People have been measuring barometric pressure around the world for hundreds of years, so there's lots of data to see tidal signatures in those readings. And they certainly are present. Surprisingly, people found the frequency response to match that of the solar day rather than the lunar day. It took quite a while to see that there was a lunar contribution; it's about 1/20th of that from the Sun. That's inconsistent with a gravitationally driven tides.

The solar atmospheric tides are driven primarily by sunlight, and primarily by absorption of ultraviolet by ozone in the stratosphere and absorption of visible and infrared by water vapor in the troposphere. The lunar atmospheric tides apparently are gravitational.

 

[Dr. Courtney]

Surprisingly, people found the frequency response to match that of the solar day rather than the lunar day. It took quite a while to see that there was a lunar contribution; it's about 1/20th of that from the Sun. That's inconsistent with a gravitationally driven tides.

The atmospheric tides are not dominated by gravitational driving, but they are gravitationally driven. It's just that the solar radiation driving produces a larger effect.

This is easy to distinguish with Fourier analysis, because of the varying driving frequencies and phases. In a single year of data, it is not hard to pick out driving forces that are only 1-2% of the dominant ones. Looking in the time domain is a weak technique for effects that are most easily discerned in the frequency domain.

To see this effect for yourself, download a reasonably accurate barometric pressure sampled every 15 minutes or so for a whole year and compute the Fourier transform. The largest amplitude is the solar heating term, with a frequency of 1.000 (in units of 1/day). But you will also see the lunar diurnal and semidiurnal peaks also, as well as the solar semidiurnal with a frequency of 2.000. These peaks are evidence of gravitational driving.

 

[D H]

The largest amplitude is the solar heating term, with a frequency of 1.000 (in units of 1/day). But you will also see the lunar diurnal and semidiurnal peaks also, as well as the solar semidiurnal with a frequency of 2.000. These peaks are evidence of gravitational driving.

The largest is the solar semidiurnal, not the solar diurnal. It's primarily a heating effect. For more, see chapter 9 of the lecture notes of the MIT OpenCourseWare "Dynamics of the Atmosphere" at http://ocw.mit.edu/courses/earth-at...-of-the-atmosphere-spring-2008/lecture-notes/