Non-diffusive atmospheric flow #4: exploring Z500

1 Sep 2014data-analysis

In the last article, I talked a little about geopotential height and the $Z_{500}$ data we’re going to use for this analysis. Earlier, I talked about how to read data from the NetCDF files that the NCEP reanalysis data comes in. Now we’re going to take a look at some of the features in the data set to get some idea of what we might see in our analysis. In order to do this, we’re going to have to produce some plots. As I’ve said before, I tend not to be very dogmatic about what software to use for plotting–for simple things (scatter plots, line plots, and so on) there are lots of tools that will do the job (including some Haskell tools, like the Chart library), but for more complex things, it tends to be much more efficient to use specialised tools. For example, for 3-D plotting, something like Paraview or Mayavi is a good choice. Here, we’re mostly going to be looking at geospatial data, i.e. maps, and for this there aren’t really any good Haskell tools. Instead, we’re going to use something called NCL (NCAR Command Language). This isn’t by any stretch of the imagination a pretty language from a computer science point of view, but it has a lot of specialised features for plotting climate and meteorological data and is pretty perfect for the needs of this task (the sea level pressure and $Z_{500}$ plots in the last post were made using NCL). I’m not going to talk about the NCL scripts used to produce the plots here, but I might write about NCL a bit more later since it’s a very good tool for this sort of thing.

The first thing we can do is to look at some snapshot views of different patterns of $Z_{500}$. This set of plots shows a sequence of variations in the “normal” pattern of flow over the North Atlantic–contours of constant $Z_{500}$ run more or less east-to-west, representing the prevailing westerly winds we find there:

Normal Z500 snapshots

Remember that because of the rotation of the Earth, the wind blows more or less along lines of constant pressure in the atmosphere, flowing counter-clockwise around areas of low pressure in the Northern hemisphere. The plots here cover a period of nine days, and you can see that the pattern of $Z_{500}$ is pretty consistent over that period. There is some “waviness” in the main east-west pattern of contours over the North Atlantic, but there’s a pretty constant band of gradient in $Z_{500}$ running from Nova Scotia and St. Pierre et Miquelon to the United Kingdom and Scandinavia. There are some other “bulls-eye” features that come and go on timescales of a few days (localised regions of lower or higher surface pressure), but the prevailing east-west pattern is quite clear.

The next plots show another nine-day period from just a few days later than the sequence above:

Blocking Z500 snapshots

Here, the pattern over the North Atlantic is quite different to the first plots. We have a big incursion of higher values of $Z_{500}$ coming in from the southern part of the North Atlantic, eventually forming an isolated region of higher pressure with a distinctive “horseshoe” or “omega” (i.e. Ω) shape around it that persists from about 17 January until about 21 January. This atmospheric flow regime is usually called a “blocking flow”, since the prevailing westerly winds over the North Atlantic are “blocked” by a region of high pressure. Similar patterns occur over the North American continent. Again, these patterns persist for a few days or so, with some smaller scale variation.

The “normal” (first plots) and “blocking” (second plots) flows in the North Atlantic tend to be associated with different kinds of weather over Northern Europe, particularly in the Northern Hemisphere winter. The high pressure systems associated with blocking flow lead to periods of settled fine weather over Northern Europe, while the “normal” east-west pattern of flow is more associated with variable weather and the propagation of low-pressure weather systems from North America across the Atlantic to Europe (along the North Atlantic “storm track”, which is the band of prevailing westerly flow seen in first plots).

We can get some idea of the temporal behaviour of these different regimes from this plot:

Z500 time series

This shows a smoothed time series of the spatial average of $Z_{500}$ over the region 20°W–0°E, 50°N–60°N (an area of the North Atlantic and most of the United Kingdom). The higher pressures (and consequently higher $Z_{500}$) associated with the blocking flow starting around 15 January is quite clear (compare the time series plot with the map views of the blocking conditions above), as is the persistence of these patterns over time–the blocking flow appears to persist until the last week of January in this case. Also visible on the time series plot are fluctuations in the spatial mean of $Z_{500}$ over this region on timescales of a few days or so (this is particularly clear for the period from 1 December 1999 until 15 January 2000). These fluctuations are associated with the passage of weather systems across the Atlantic and with the “waviness” we noted in in the first set of “normal” spatial plots. (These really are waves in the atmosphere, called Rossby waves.)

So there are characteristic spatial patterns in the North Atlantic atmosphere that persist in time and can be associated with different surface weather regimes. What we want to try to do is to disentangle some of this variability, to determine just what these persistent regimes are and to decide whether there is any predictability in the dynamics of transition between these different regimes.

To give you a better feeling for what these variations look like, here is an animation that displays variations in $Z_{500}$ over a few interesting periods:

The values of $Z_{500}$ are shown as coloured regions (warmer colours are larger $Z_{500}$ values) and regions of blocking flow are highlighted with cross-hatching. (The exact criterion used to identify these regions isn’t all that important for our purposes here–it’s just a way of picking out and highlighting these flow regimes for display.) When viewing the animation, notice in particular:

  1. the fairly persistent gradient of $Z_{500}$ from the equatorial regions towards the poles (the orange to yellow or green transition in the mid-latitudes);

  2. the appearance of lower-pressure regions that persist for a day or two (mostly showing up as green circles);

  3. the wave-like disturbances that appear on the main equator-to-pole gradient of $Z_{500}$;

  4. the blocking pattern that appears in the North Atlantic around 14 January 2000 (compare with the blocking maps above), associated with persistent high pressures over North Europe, and its subsequent decay back to the “normal” flow regime;

  5. in the polar view later in the animation, the wave-like patterns of $Z_{500}$ centred on the North Pole and the occurrence of blocking flow over the North Atlantic and North Pacific (highlighted by cross-hatching).

In the next article, we’ll get back to Haskell, and we’ll start with the most basic part of the data analysis that we need to do to examine these persistent flow regimes, by doing some data pre-processing. This is a step that we need to do in almost all scientific data analysis tasks, since it’s rare that the data we get are exactly the data that we need.