Friday, November 6, 2015

Precision Barometers on Inland Commercial Vessels by Robert Reeder

The necessity of an accurate barometer for an ocean voyage is well understood. However, even in
inland waterways with modern weather resources, our barometer can be an important tool for
weather routing.

My office is a high-speed passenger ferry which crosses the Strait of Juan de Fuca. Seas there are
seldom monstrously high, but in a gale they can become very steep and confused 6' – 8' waves very
quickly, often with ocean swell underneath them. For passenger comfort and safety we endeavor to
not sail when the seas are expected to be this high, and will opt instead to either reroute for more
protected waters, schedule the sailing for an earlier or later weather window, or in the worst-case
transport our passengers on chartered buses.

Canceling, rerouting or rescheduling a sailing is not a trivial consideration. Passengers need to be
contacted and informed, buses may need to be procured, luggage may need to be rerouted, Customs
agents and shore support personnel may need to be rescheuled, additional fueling may be required if
rerouting adds significantly to the duration of the voyage.

Our best onboard and shoreside weather resources are a combination of NOAA GFS and WRF
modeling, and station reports from NOAA National Data Buoy Center and Environment Canada. It
is our experience that the two of these combined typically give a fairly accurate and robust
representation of the weather we can anticipate. However, in this region much of the modeling can
be in error by several hours time, either early or late. In open ocean this would not be a critical
consideration, however on our vessels and our route we are typically only exposed to the worst of
the weather for about an hour. So a modeling discrepancy of three hours can mean either being
caught in weather unexpectedly, or possibly canceling a sailing needlessly.

Correlating the modeling with buoy and shore station obsevations helps. If a weather system is
modeled to be blowing 40 knots in the central Strait at 0600, and the 0600 station report at
Sheringham Point is still only 4 knots, we have a pretty good idea that the system is moving in more
slowly than anticipated. This informs us of the discrepancy between the models and the
observations, but it does not necessarily give us actionable information beyond the fact that the
weather system will likely arrive some time later than anticipated.

By using a well-calibrated on-board barometer we can compare our actual barometric pressure with
that of the WRF model map at the same time. If the mapped pressure band or isobar which
corresponds to our barometer observation is not in our location, we can use the WRF animations to
estimate how many hours earlier or later than predicted the weather system will move through our
position. The simplest way to do this is to toggle the sequential maps backward or forward until the
modeled pressure at our location matches our actual pressure, and then note the UTC time of that
model image to determine the time error.

With this better time estimate of when a weather system will actually move through the eastern
Strait, we can be much better equipped to make decisions of weather routing, or postponing or
cancelling a trip for passenger comfort and safety.

See Specific Examples


Captain Robert Reeder said...

As a point of clarification, the precision of both computer modeling and affordable shipboard barometers has only become sufficient for this purpose very recently; what is presented in the article is a discussion of how these new resources COULD be used in my industry, not a description of our current operating procedures. At the time of this writing, the user-interface with GFS and WRF data is still barely usable for obtaining barometric pressure to the degree of precision needed for this to be an effective tool for weather routing, but the data is there and can be extrapolated from the WRF graphic with a little effort. With the help of Starpath I have done some very preliminary "proof of principle" testing of these methods, with promising results.

David Burch said...

Thanks for that important clarification. We can be hopeful that this will change shortly with the advent of the new grib formatted HRRR wind and pressure data. The HRRR model predictions are high resolution over these inland waters. and They are updated hourly, and at each update they assimilate all the local buoy and other observations. Thus every hour we get 15 hours of forecasts, with the latest observations folded in. The data are available in grib format from product WeatherNet. For now the wind data are available hourly and we hope to have the pressure data shortly. We will add a blog post in the near future with more details on this exciting new source of marine weather data.