Monday, January 30, 2012

Mom, where do atmospheric data come from?

The basic sources of data for the upper air - above the surface of the earth - are radiosondes (aka weather balloons) and satellites.  (There are some others, like measurements from commercial aircraft and ground-based remote sensors, but overall these are less important.)

Radiosondes are in some ways still the "best" source of data, because they provide true in situ measurements.  That means one is sticking a probe into the actual system one wants to measure - the atmosphere - at the place one actually wants to measure it.  Radiosondes provide measurements of temperature, humidity, wind, and pressure with (generally) high accuracy at all levels throughout the troposphere and into some of the stratosphere, if nothing goes wrong and causes the balloon to break prematurely.  Over some regions at least - for example, the continental US - there are enough of them to define the horizontal structure of the flow pretty well.  On the other hand, there are large regions of the earth - most of the oceans, many developing countries, Antarctica - where radiosonde launch sites are sparse to nonexistent.  If sondes were all we had, we wouldn't know much about what the atmosphere were doing over these places at any given moment.

Satellites are great this way.  They have a few different types of orbits (geostationary, polar orbiters...) but one way or another they survey large chunks of real estate on a regular basis.  They don't care what the map beneath looks like (except inasmuch as the nature of the earth's surface may affect the measurements themselves, e.g., some instruments work better over ocean vs. land).

The down side of satellites, on the other hand, is that they are remote sensors.  They don't make in situ measurements, which means that they don't directly measure the properties of the atmosphere.  They measure properties of the radiation coming up to space, and infer from that some desired property of the atmosphere below - temperature, rainfall etc.  This can only be done so well.

A particular limitation of satellites is their relative lack of vertical resolution - they can tell you what the temperature is (for example) but they can't tell you with perfect precision at what height is it that temperature.  The radiation coming up from the atmosphere tends can in general have been emitted from more than one particular height in the atmosphere - nothing tags the photons to indicate precisely what altitude they came from.  Two photons coming from different levels look the same as two coming from the same level.  So to the extent that the properties of the photons carry information about the atmosphere, that information is mixed together from different levels.  To get a feeling for this, here's an imperfect analogy:  imagine dropping your keys into a swimming pool (or your other favorite body of water) and watching them as they fall straight down.  How accurately do you think you could tell at any given point how deep they have gone?

There is a whole sophisticated science of remote sensing whose job it is to get around this problem, and due to both improvements in the physical instruments on the satellites and the "retrieval algorithms" (mathematical methods used to turn the measurements of radiation into temperature etc.) it gets better all the time.  But satellites will probably never truly be able to beat radiosondes at their own game, which is high-resolution vertical profiles of atmospheric variables at a single horizontal location.  They don't have to, though;  to the extent that we have both types of measurements, we can use each one to compensate for the weakness of the others.

Tuesday, January 17, 2012

Intensive -> Extended

The "Intensive Observing Period" of DYNAMO has ended, but the experiment is not over.  We are now in the "Extended Observing Period".  What this means is that a lot of the instruments and people are being (or have already been) packed up and shipped home, but some are still out there in the Indian ocean for a few more months.  In particular, we are down to one radiosonde and surface observation site, Gan (in the Indian ocean;  one more, Manus, in the Pacific);  one radar (SMART-R, also at Gan);  no airplanes;  and no ships, if I'm not mistaken.  There are still many more measurements being taken in this spot every day than there would be if DYNAMO weren't still happening, but many fewer than there were for the first couple of months.

Why do we do drag it out like this?  Why are there multiple phases of an experiment?  If we know what we want to measure, why don't we just measure it as long as we need to and then stop when the job is done - or when we can't afford it any more?

Field experiments exist to address several different types of gaps in the regular observational network.  The different phases of the experiment exist because those different needs require different balances between (for lack of a better phrase) the intensity of the observational effort - the frequency, redundancy, density of spatial coverage, and range of different types of observations being taken - and the length of time over which those observations are taken.  In our dreams, we would like all the observations we can imagine to be taken continuously forever, but in reality we have to make compromises.  The different periods reflect an attempt to make different kinds of compromises so as to satisfy different sorts of needs within a single experiment.  To understand this more deeply, it is useful to know a bit about where our more routine observations of the atmosphere - the ones that are used every day for weather prediction and climate monitoring - come from.  I'll write more about this in the next post.

Sunday, January 8, 2012

Observations of the Supressed Stage from Gan

Hi All,

I'm the latest (last?) scientist contributor to this blog to arrive on Gan. I thought I might use one post to give my two cents on the resurrected/reborn debate. It's certainly something that we at S-PolKa have been talking a lot about over the past few weeks. If you look closely at the Hovmoeller diagram that Adam posted a few days ago, you could convince yourself that there is a slight bit of blue slanting up and to the right, connecting the latest MJO with the previous event. If this were a true connection it would imply that the previous MJO had stalled then propagated westward (from the fringe of phase 5 back into phase 4), and finally changed course and began its normal eastward motion. I think the soundings from Gan tell a different story though. If you look at the buildup of moisture and westerly winds that marked the beginning of the most recent MJO, the moisture is clearly building in from the west. As the westerly winds get deeper, so does the moisture.

In my mind, if the previous MJO event was moving from east to west into our domain and restrengthening here, then the winds during the buildup of moisture should have had at least some easterly component. It doesn't make sense to me that the previous MJO feature could be propagating westward while simultaneously generating intensifying moist westerlies. Am I missing some dynamical piece of the MJO that would allow for this to happen? Granted, Gan (longitude ~73E) was on the far left fringe of the latest event, so our soundings may not have been representative of what was happening in the eastern portion of the basin, but to me this looks like a new event.

The downside of having something interesting to talk about is that we now have most definitely transitioned into the suppressed stage over Gan. The time-height plot of moisture and winds over the past month looks like this,

and the daily rainfall estimates from S-PolKa look like this:

With the exception of those three days at the end of 2011, we've now been precip free for the better part of two weeks. Our column-integrated total precipitatable water estimates have been hovering around 40 mm, certainly on the lower side of the low end for this part of the world.

Despite all this dry air limiting our radar to observing nothing but non-precipitating shallow clouds, there has been one benefit. The lack of moisture and pollution in the atmosphere has allowed for some of the bluest skies I've ever seen. Really pretty stuff. The sunsets aren't bad either.


Wednesday, January 4, 2012

Resurrected or reborn?

It's now getting suppressed again over the Indian ocean, as the active phase we had a couple of weeks ago has moved to the Pacific.  The previous event never really got out of the western Pacific.  When it got there, it died, and not much more than a week later, it was the active phase again in the eastern Indian, where it had just been about three weeks ago.   

Here is the RMM phase diagram from the last two months.  Note the loop on the right side, indicating amplitude decrease (line moving in towards the center of the circle) in phases 4-5, followed by an increase again and then further eastward propagation (counterclockwise motion, ending where we were as of a couple days ago, near the vertical line between phases 6 and 7).

Was this a new MJO event, or a resurrection of the previous one?  This was an issue that was discussed in today's episode of the weekly teleconference that the DYNAMO scientists all over the world have been having since the start of the experiment. 

The three-week period separating the two active phases in the Indian ocean was shorter than we usually associate with the MJO.  We typically say the period of the MJO is 30-60 days, but this was only 20.  So does that mean it couldn't be a new event?  Not really.  These numbers are only rough guides.  The spectral peak associated with the MJO (see here for some background) is broad, meaning the period is variable.  Just eyeballing the OLR Hovmoeller plot, the latest event sure looks like the latest one in a series of similar ones (the three blue raggedy stripes going down and to the right in the lower left of the plot);  ok, it's a little cut off on the left (indicating that it started in the eastern IO rather than western) but  it doesn't appear to have grown directly out of the previous event.

While it looks like a new event to me, the honest answer, I think, is that there is no honest answer.  The MJO is a rough, large-scale envelope of weather, rather than a tightly defined weather system whose birth and death we can define precisely.  I don't think we'll ever give MJO events names, like we do with tropical cyclones.  And if we did want to define the start and end of an event with precision, ultimately the way we do that should depend on an understanding of MJO dynamics, not just on statistical or phenomenological features that have been defined purely by subjective convention.  We don't have that dynamical understanding yet.

Another reasonable question to ask might be:  does it matter whether it's a new event or the same old one?  It's pretty clear that it's the MJO one way or another, and the more interesting question is what has made it evolve in this unusual way just now.