Wednesday, December 10, 2014

Sydney-Hobart Climatic Winds

The Rolex Sydney-Hobart Race is notorious for not just strong wind, but strong wind that changes rapidly. We in the Northern Hemisphere are not used to fronts and Lows that move as fast as they do along this route.

You can see the variability in the climatic wind patters at the wonderful COGOW site, from which we have captured the samples below. The COGOW data replace everything in the past as the best source of climatic average winds. They are actual winds measured by the Quikscat satellite. There are 9 years of data, with a pass over a particular region about once a day.  Then the folks at Oregon State University compiled the data and made this super nice presentation.  They have ended up with about 150 observations at each grid point. You can compare this to similar data in the Pilot Charts (much from 19th century logbook data) or other Navy Wind Atlases from the past, but the COGOW data are the best available and the best way to plan a sailing route…  when you might have some option on when and where you sail.

When you must sail on a given date at a specific place, this does not help much. We can then only look at the 5 day forecast at the start compared with latest satellite winds, and then keep updating the knowledge every 6 hr or so.  We get new maps every 6 hr, but we only get new forecasts every 12h.

The Race is on Dec 26th, Boxing Day, and the COGOW winds provide averages for the first and last 2 weeks of each month. Below we show Jan 1-15 and Dec 16 to 31. There is not much difference in these two periods.

COGOW winds overzoomed to compare the change with time.

The compiled picture below showing wind roses are from the Jan 1-15, primarily because it showed  the shear line about 37S, but it turns out we do not see much of this in the individual statistics. At the COGOW site you have several options on how to present the wind statistics. These wind roses are just one option.

You can vary the views on COGOW and get somewhat more precise data by selecting different regions to zoom in on from the base map…. but the general behavior is more or less as indicated in the wind diagrams shown, which are of course just averages.

Look closely at some regions and we do see interesting statistical details, for example how the wind changes near the coast in some regions.  This race is much influenced by strong currents just off shore. I will try to find some good links to these. I believe there is HF radar measurements available.

There are two ends to the wind pole.  The climatic averages of what has taken place at one end (shown below) and the winds at the very moment at the other end (see Sydney-Hobart scatterometer data). In this latter link we have compiled a very convenient way to look at all of the satellite wind data that are available, which is updated each time you look at it. The satellite data are not forecasts or model predictions, they are actual measurements of the wind. With 3 satellites ascending and descending, we end up with 2 or 3 looks per day at the real winds.  These satellite data are used to seed the numerical models, which are consequently pretty good at their zeroth hour report, but we still very often see details in the satellite data that get averaged out by the modeling.

COGOW data. The numbers are the percentage for the direction. The length of the line segments is the percentage wind speeds.  You can get this data in tablular form at the COGOW site. Other than west wind in Bass Strait, there is a lot of variability.  Click this pic to get a better view.

NOTE: sometimes it is tricky at the COGOW site to get to the popup that lets you change between wind rose and table data.  In principle you just left click on the main map and you should see the option, but once you select a choice that window dives to the bottom of all open windows and after that click away and nothing happens.  So if you can't get it, move all the windows around and look underneath for the popup.

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