2.7 Monitoring Aral Sea level:

Using altimeter data for enclosed sea level monitoring.

Aral Sea level along the track: need of data editing

We will have a look at what we can see when plotting LLH for (e.g.) cycle 010 with respect to longitude.
If we plot "just" the level wrt longitude, in fact we see more than the Aral sea level. Even using the land/water flag (geo_bad_1.water_land_distribution == 0, which means that the data are considered as taken over open seas), we still have unwanted data -- in part because of the changes of the Aral Sea extend, but not only.

Lake level height for the Aral sea without data editing, T/P cycle 010 (left); The measurements over the sea is the "flat" area in the middle of the curve (between, roughly, 59.1° and 60.3°N). Right, a more problematic cycle, cycle 353, for which no such clear signal can be seen.

We will have a look at some of the data fields that are used in data editing:

RMS of altimetric range, which measure the variability of the altimetry measurement: over an open body of water, this variability should be rather low (between 0 and 0.1 m)
Antenna mispointing, which can come from a change in surface, the radar sensing pulse previously reflected off-nadir
sigma0, the backscatter coefficient, which is the ration of the altimeter radar pulse power that is reflected on the surface and measured by the radar. Abrupt changes in this coefficient can also come from a change in surface reflection.

Several data fields plotted for T/P cycle 10 over the Aral Sea area: RMS of the signal, antenna mispointing, Sigma0. They all show abrupt differences between the measurements outside and over the Sea.

Depending on the water body, fine tuning of the threshold condition used in data editing has to be done.

Aral Sea level along the track at two different time

Here we will have a look at the Aral Sea level along the track in December 1992 (cycle 010) and June 2003 (cycle 360). We compute LLH for both cycle, using the editing given in previous page. In the 'Views' menu, you name your plot, give the plot a title, select both computed LLH fields and click on 'execute' to view them (you can restrict X from 58.5 to 61°N).

Topex/Poseidon along-track LLH over the Aral Sea for cycle 010 (December 1992, red) and cycle 360 (June 2002, green). The slight curvature of the height is due to the geoid model inaccuracies over this area. The level is obviously much lower in 2002, and the extend of the Sea is also much smaller.

Compute the Aral sea level variation over 10 years

Using the dataset with all the pass #107 from cycle 010 to 364, we will be able to plot the evolution of the Aral Sea over the whole period.
By using the time in days ('tim_moy_1') as abscissa, a mean is automatically done for each day for all available data on this day. We will use geographical selection to restrict the data averaged to the Aral Sea (add '&& is_bounded(59.06,lon_tra,60.32) && is_bounded(44.11,lat_tra,45.52)' in the Select expression box)

Aral Sea level variations from Dec 1992 to June 2002 from BRAT. Some more editing should be done to obtain a completely "clean" curve.