16 November 2009

Where is the surface?

I just commented on my facebook status that I'm at a meeting about sea surface temperature.  That part was safe.  Rest of the comment was to observe that I'm now back to wondering whether the sea has a surface, where it is if it does, and if it does, whether it has a temperature.  That prompted a friend to comment 'Great ... this is going to bug me now.'  So for him, here's a longer version.

This sort of question is very common to science.  Of course my musing for facebook is overstated.  But there is usually a real question about what exactly it is you've observed when you take an observation.  When you have very different observing methods, they may well observe things that are different from each other.  There are, let's say 4, different ways of observing the sea surface's temperature.  For a diagram, see the wikipedia article on sea surface temperature

The standard method, and reference for others, is calibrated buoys that carry a thermometer at a known depth, typically 1 meter.  A major drawback to this method (all methods of observing have drawbacks!) is that you need a buoy.  They're not cheap, and it would take several million of them to give us a high resolution data set for global sea surface temperature (acronymed SST).

The longest-used satellite method for obtaining SST is to observe the earth in infrared wavelengths.  Infrared doesn't propagate well through water, so the satellite sees a 'skin' temperature averages over 1 wavelength -- about 10 millionths of a meter (10 microns).  A drawback to this method is that it can't see through clouds.  Clouds also emit infrared, so the satellite tells you the temperature of the clouds, rather than the sea surface.  Weather forecasters use this to improve their forecasts.  But it means there's a lot of the earth these satellites can't see on any given day.

A recent addition to our satellite observing of sea surface temperature is to look at microwave wavelengths.  This usually can see through clouds as the wavelength is carefully chosen to be one that cloud drops don't emit or absorb much.  This gives us a temperature at about 1 mm depth.  The drawback for these is that the satellite averages over a large area -- 25-50 km diameter, as opposed to the 1-4 km for the infrared satellites.

The fourth class is the 'everything else' group: ship 'bucket' temperatures, hull contact temperatures, water intake temperatures, temperature sensors on chains (often from buoys) extending well below the surface (or 1 m depth), or the ARGO floats -- which observe temperatures from 2000 m depth up to close to the surface.  All of these observe temperatures at depths greater, sometimes much greater, than 1 meter below the surface.

If the 10 micron, 1 millimeter, 1 meter, 5-20 meter temperatures reported by the different methods were the same (within observing error), then it'd be a concern for specialists alone.  The reality, however, is that the different temperatures can be substantially different.  Most of the time, over most of the globe, they're close.  But once you have calm winds (less than 5 m/s, 10 mph, roughly) and strong sunlight, you can accumulate skin (that 10 micron temperature the infrared satellites see) heating enough to warm temperatures.  If the winds are very calm, under 2 m/s, it can be by a few degrees -- but only in the skin.  The 1 millimeter ('subskin') warms, but not by as much.  1 meter down the temperature may change only a little.  And at 5-20 meters, almost entirely unchanged.  So ... if you want the 'sea surface temperature', which of the 4 do you want?  And is that 5, or 20 meters for the 'unchanged'?  How close to entirely unchanged is close enough?

Hence my question: Where is the sea surface?

As mentioned by someone else -- under hurricane conditions, are you even sure that there is a sea surface?


quasarpulse said...

Well, why do you want to know? There are a lot of reasons one might want a sea surface temperature, and some of them require different assumptions than others.

Oale said...

For individual molecules the surface could be the energy level needed to evaporate in given conditions... but the hurricane example is a more difficult matter as you'd have to know the salinity in droplets whipped out by the storm... or that's how i see it. Given the energy distribution in the water molecules, there are always some molecules that are on this surface, but they get bogged down very fast, and then you'd have to do some statistics. The 'boundary' would then be the average of the energy levels between gaseous and liquid models of the physical behaviour of h2o. But i guess you meant how to measure it. On that, i have no other solution than the reflective property of the surface that can easily be measured only in calm conditions... if one tries to measure the accurate distance to a wavy surface things get rapidly more difficult as the signal will be very much fainter... so it is a bit of a problem.

Alternatively, one could define it the blog scientist way that the surface is where it splashes when a rock is thrown.

carrot eater said...

A nice post.

I'm having trouble reading this sentence: "so the satellite sees a 'skin' temperature averages over 1 wavelength -- about 10 millionths of a meter"

should that be "averaged", not "averages"?

I assume they use all the IR wavelengths outside the absorption bands of CO2 and methane, and then back out the temperature by comparison with blackbody curves generated by Planck's Law?

Penguindreams said...

quasarpulse: That's the other reason we have the debate.

oale: The rock test is usually pretty good. But come hurricane conditions, and there is so much spray, and the 'ocean' is so mixed with bubbles from the atmosphere, that the rock doesn't so much splash as get progressively wet. If the winds don't flip it right back at you.

You're right, it should be averaged.
You have the right basic idea on how it's done, but there are some interesting wrinkles. I'll write that up separately.

Hank Roberts said...

I'd appreciate a look at this when and if you can:

ISME J. 2009 Sep;3(9):1001-3. Epub 2009 Jun 25.
The sea-surface microlayer is a gelatinous biofilm.

Cunliffe M, Murrell JC.

Department of Biological Sciences, University of Warwick, Coventry, UK.

PMID: 19554040 [PubMed - indexed for MEDLINE]

I don't know how much difference a thin living organized gelatinous layer at the ocean/air interface could make, but given the surface area available, I wonder.

EliRabett said...

What the surface of a liquid is, is by itself an interesting question. With the exception of liquids that have vanishing vapor pressure such as gallium and mercury (at room temperature), it is hard to say, as the molecules in the vapor can interact with the molecules in the bulk over several atomic distances.

Ron Shen did a lot of early work on this with a very imaginative technique. Since the molecules on the surface are in an anisotropic environment, only they can participate in non-linear sum frequency generation.

"Using infrared-visible sum-frequency generation we have obtained the vibrational spectra of CH stretches of methanol molecules at the interface between methanol vapor and liquid. This is the first vibrational spectrum ever observed from a neat liquid surface. The measured polarization dependence and phase of the nonlinear susceptibility allow us to conclude that the surface methanol molecules are polar oriented with the CH3 group pointing away from the liquid with a very broad orientational distribution."