Example of a notification email
The following is an example of a notification email
At the start of the message, date and time of the analysis process are
given, as well as the instrument that made the observations, and the
number of notices (alerts) that are in the message -- since several
segments are analysis in one process, there can be more than one alert
per process.
For each alert the message gives the URL of a dedicated web page about the
alert (with info and maps), followed by a few lines of info on the orbit
segment that triggered the alert:
- The date and time of the first measurement of the segment.
- The average longitude and latitude of the segment, averaged over all
measurements in the segment; this more or less corresponds to the centre
of the state.
- The average solar zenith angle ("sza"), averaged over all measurements
in the segment.
- The peak value of the SO2 total (vertical) column density ("vcd")
somewhere within the segment, in Dobson Units.
If the average solar zenith angle ("sza"), averaged over all measurements
in the segment, is very high
(SZA > 75 degrees),
a 'Remark' line is added:
It is relevant to know this, because measurements of SO2 at high SZA are
notoriously difficult and this leads to
artifacts in the data and "false alerts" to be issued. To avoid most of
these "false alerts", no email message is send for events with an average
SZA > 80 degrees.
These messages are made, though, and available via the web site.
(Note that only data with SZA < 85 degrees is processed.)
Measurements taken over the South Atlantic Anomaly (SAA; see the product
information for details) can give rise to artifacts in the SO2 data and
therefore to false alerts. If the orbit segment that triggers an alert lies
partly in a region defined for the SAA, this remark is added:
Note:
Below the line with "[end of file]" follows a code given between angle
brackets; this code is for internal use to check the email service.
Info on the dedicated web page
The dedicated web page of a certain alert starts with giving the same info
on the location of the segment that triggered the alert (see above),
with two additional lines:
- The peak value of the SO2 slant column density ("scd")
somewhere within the segment, in Dobson Units.
- Information on the cloud data used for the SO2 total column -- see
note below.
Then follow images of SO2 and cloud cover in a 30 by 30 degree region
around the segment that issued the alert, plus a plot of the location of the
volcanoes in that region (if any). The web page further provides links to
the near-real time data pages for the pre-defined geographic regions the
segment (partly) lies in.
If no cloud cover information was available, the map of the SO2 shows
the values assuming clear-sky conditions and the cloud cover map is
empty.
Some notes on cloud data usage
The product information (see the menu at the top of the page) provided
background information on the retrieval of the SO2 data and how the
total (vertical) column density (VCD) is derived. For that derivation
it is necessary to have information on the (effective) cloud cover fraction
for that pixel.
In can happen that the cloud cover information is not available in the
near-real time processing, e.g. because it is retrieved from an external
data source. In case cloud info is absent one can assume that the pixel is
cloud-free and compute a VCD on the bases of that -- this is probably not
accurate but gives at least some information.
For this reason the alert info on the web pages mention briefly something
about the cloud data. The following messages can appear:
-
used for vcd
This means that valid cloud cover data was available and used
to computed the VCD.
-
assuming clear-sky
This means that valid cloud cover data was not available and
that the VCD is computed assuming the pixel is free of clouds.
Assuming that pixels are cloud-free in the cases where no cloud cover
information is available, is done only in the near-real time processing,
not in the off-line archive processing.
The "full text" of an alert in the archive
In the overview per month of the notifications sent each day, sorted by the
date of the processing, there is a link called "full text" for each process.
The link lead to a text page with the alert notifications of that process.
For most users of the notification service this "full text" is not really
relevant.
For each alert the page provides the same info as the email alert and
dedicated web page -- as described above -- plus some process details:
the numbers of the pre-defined regions involved in the alert, the name of
the data file with the segment that triggered the alert, and the process
version. This additional information is only important for expert users.
