Thursday, November 27, 2014

Sydney-Hobart Scatterometer Winds

We have set up a convenient way to look at scatterometer winds over the Rolex Sydney-Hobart Yacht Race course that will automatically update on each view.  Unfortunately, the four panels of data available all meet right in the center of the race course, so we had to combine 4 images... otherwise you could just look at the raw data at the Ocean Surface Wind Team web site.  Please have look at compilations we have made. We show ASCAT A and B, both ascending and descending, as well as the WindSat data.


These are large images that you can drag around for best viewing.

To get ship reports in this region, send an email to shipreports@starpath.com with the subject line reading:  38.0S, 150.0E. Send one now.

In preparing this, we realized we needed a bit more instruction on how to figure valid times.  So here are some notes on this, but you can also download a pdf of the information below, which is just captured from that document.





Friday, November 7, 2014

Barometer for Wind Warning

There are numerous tactical uses of barometric pressure for safe efficient navigation, but here we take a quick look at just the basic one of using the barometer to anticipate strong winds.

In the picture below we see a classic case of wind rising with a pressure drop. We show this on the very nice combined display of wind and pressure shown on the NDBC websites, in this case for West Point Lighthouse (WPOW1), which is 1.6 nmi to the SW of Starpath HQ.




Top is wind and pressure from NDBC reports for West Point Lighthouse in Seattle. Bottom is the pressure tendency (mb change over the past 3h) measured with a Mintaka Duo at Starpath HQ, 1.6 nmi away.
We see wind going from 5 kts to 32 kts  in 9h... and in the first 6h of that it only went up to 15 kt, so really this is 15 to 32 in about 3h.  This is something we would like to be prepared for as soon as possible. That 6h of watching the wind go from 5 to 15 is all wasted when we do not have a good barometer that could warn us that this wind increase could keep going on.

But even with a good barometer we still need a guideline to interpret what we observe.  Our guideline is "4-5-6," meaning a drop of 4 or 5 mb in 6h is the sign to pay attention. This implies an average tendency (change in last 3h) of  -2.0 or more.

In this example, we see a drop from 1025  at  12z on 5th to 1008 at 15z on the 6th, or 17 mb in 28h = 0.68 mb/h = 2.0 mb/3h. So the average drop rate in this example is plenty for a good warning, but once it starts down at this rate we still have to see this for 6 hours before the flag goes up.

Shown below the NDBC data, we have plotted the pressure tendency measured here at Starpath with a Mintaka Duo instrument (discussed more at the end of this note).  The scale on the left is pressure change in mb over the previous 3h.  Looking above that to the pressure data, we see a drop from Point A (where the tendency first crossed 0.0) to Point B, marking 4 mb drop. This has dropped enough to keep an eye on, but not enough to conclusively trigger our 4-5-6 guideline because this took about 9h.  After this point, however, you see an average tendency that is close enough to the 2.0 we need for a solid warning.

In other words, by the time of Point B, we should be fairly warned of strong wind potential.  Thus we have (from B to C) about 18h warning that the wind we have watched increase to 15 is likely to increase notably more. Once we get to a tendency near 3.0 for a couple reports, we are beyond the forecasting zone; the strong wind is likely either imminent or present.


Mintaka Duo wind warning arrows




To help mariners take advantage of the accurate pressure measurements using the Mintaka Duo instrument, we show pressure trend arrows next to the digital value of the pressure. These offer a quick graphic view of the trend, which are scaled to match our strong wind warning guideline (4-5-6). The definitions are shown below.


 With these definitions, keeping in mind our 4-5-6 guideline, we can make a shorter summary.


These arrows are updated continuously on the dial face after the unit has been running for 1h, and with these we can anticipate the drop over the next 3h based on a projection of what we learned over the past 1h. (The actual tendency value that is stored in the device with each pressure record is rigorously defined by the WMO and NWS as the pressure difference between now and 3h ago, so this value cannot be stored until the unit has been running for 3h or more.)

The drawing below shows how these arrows would have shown up as the pressure dropped.


Notice that the arrows would change during the short periods when the pressure flattened off during the drop, but the Fast Drop indicating real warning would dominate in this pattern.

Another point to stress is the pressure warning guideline assumes the wind is starting from light air. When the ambient wind is over 15 kt or so, as in the case to the left of this time period (3 days earlier) the guideline is not useful.  In the open ocean, when the wind is above 15 to 20 kts to begin with, a rise in pressure often brings with it an increase of wind—most famously in the "sting of the scorpion's tail," which is the poetic description of a back bent occlusion.  We see this behavior in the left-hand data for completely different reasons, but we are on inland waters here, effectively between two mountain ranges, so any standard pressure-wind forecasting can vary notably.  Our guidelines were developed for the open ocean, but they are nevertheless a good guideline anywhere.  As it turns out, the behavior seen here for this larger pressure drop is a fairly good example of what we might experience at sea.

Mintaka Duo data compared to NDBC data

In the pictures below we compare what we measured at Starpath with the Mintaka Duo instrument compared to what we downloaded from the WPOW1 website.  The agreement is very good. We actually have much more data in finer steps stored in the Duo, since we can only get data hourly from the website.  Thus even with Internet access to this excellent data from the NDBC websites around the country, we still gain more resolution and earlier warning with our own precision instrument in hand. 

In a later article I will discuss how this instrument can be used underway for sailboat racing or cruising to predict wind changes on inland waterways. Then the precision and fine steps stored are especially valuable.



The red curve is from the Mintaka Duo. The blue from WPOW1. If has been randomly offset to show the comparison.  The Starpath instruments are located 5.8 mb above sea level. We see every small variation reproduced, which shows that such fine structure is indeed being created by subtle changes in the atmosphere, that at least cover a 1.6 mi radius, as that is the separation of the two instruments shown above.


We see even more interesting agreement on the tendency measurements at both locations.  Again the red data is from a Mintaka Duo; the blue from WPOW1.




In this case the elevation difference does not matter. Here the scales and measured values are identical. This type of measurement provided by the Mintaka Duo is valuable to ships who must report the pressure tendency with their regular weather reports back to the NWS.  The detailed agreement and reproduced structure of the plot is even better testimony to the precision of the Mintaka Duo.



Wednesday, September 3, 2014

Barometric Pressure During Squall Passage

Last year we posted a note here about an unexpected small rise in barometric pressure as a severe squall passed over a boat in the Caribbean (Local Pressure as a Squall Goes By). The article served two purposes, it documented that unexpected behavior and in doing so demonstrated the value of a high quality barometer, such as the Starpath Mintaka Duo.Today, we get to carry on with that story, but with more data.

It started with a bike ride home from work, with just enough drizzle to put on rain pants. Ten minutes later however, the wind was at least 25 kts, gusting to 30, with rain heavy enough to reduce visibility to about 200 yards for a short time... not to mention an excellent test of good rain gear. This was an honest to goodness tropical squall, which I have seen many times underway... actually it was not at all a "tropical squall;" it just behaved like one.  See Cliff Mass explanation of the system.

After dinner and much banter about the unusual likelihood of such a squall here in Seattle, I happen to notice the trace on one of our Mintaka Duos hanging on the wall. Screen captures are shown below.


Shown on a 6 hr trace, the bump is at about -2.5h
Same data, zoomed to past 3 hr, but captured at a different time.
We can look at real times since the Mintaka Duo has an accurate clock.

The cursor is the close dotted line showing the pressure was 1008.4 at 1802 or so, just before the event.



The peak pressure of 1009.2 occurred at 1813 (PDT).
The pressure continued its pre-bump rise by about 1825.

Thus we see that at about 1813 the pressure rose by about 0.8 mb above its trend over a period of 10 to 15 min.  This instrument was roughly 25 ft above sea level, in a room with open windows, about a mile  or two from the West Point Lighthouse (WPOW1), which recorded hourly (MSL) pressures of 1009.0 and 1009.7 at 1800 and 1900 PDT.

The new data here is some documentation of the squall as it also passed over the Atmospheric Sciences Building at UW, which has several fine instruments on the roof, as well as a precision barograph inside the building.

In the earlier note, linked above, we speculated that maybe there was some brief pressurization of the cabin on the boat where the barometer was located, but now there is clear question about that interpretation. Namely the environment of this instrument was rather different from the boat, and also we now have the UW data (below), from an entirely different environment.


High precision barograph at UW Atmospheric Sciences.

They see the same bump about 5 min later, and about half the intensity, rising about 0.4 mb.  For what it might be worth, my guess is the squall was less severe when it got to UW, which is about 5 mi east of here. I say this because we have the data (below) from their measurements, and I was right in the middle of this one, on a bike, and have some experience in the observations.

Unless their meter is blocked on some level, I know for certain we had much stronger winds at our location.

 But they did indeed have a lot of rain, but this is much harder to gauge by eye to make comparisons. I would guess what we had here for a brief time was violent rain.

They recorded about a quarter inch of rain in a few minutes.

This record of rain gauge bucket tips shows the downpour even more clearly.


And as you would expect with a big squall, there was a fast and notable air temperature drop.
Likewise as we would expect in a fast moving squall, the wind direction was all over the place.

So in conclusion, we have clear proof of a squall and some estimate of its severity (there is likely other data on this as it was an unusual event) and we have two independent measurements of the pressure bump, in rather different measurement environments.

Now we just have to figure out what caused this... though there is no doubt that with a good barometer you learn things you would not otherwise.  For example, we can see very nicely on the Mintaka Duo the local pressure variations that lead to the sea breeze in Puget Sound on a warm clear day. Later I will write up how racing sailors can use this information to predict the wind while underway in a race on Puget Sound.

PS... it is a bit difficult to tell what took place at West Point Lighthouse because they only record data hourly... but there was definitely some larger scale system in place over the region, which we will have to look up.  The Lighthouse data is shown below, with the corresponding pressure trace from a Duo.


Wind and pressure at West Point. Recall wind in kts is about 2 x wind in m/s. Notice that they had more wind here than at UW, and I would guess somewhat less then I experienced for a short period.




This is the corresponding pressure trace from the Duo. We clearly cannot see the details discussed above on this scale.
__________________

Follow up from the next day, Wed Sept 3.

At the office we have many of these instruments running, so I took one to see if we still had detail enough to see the bump, and sure enough in the 3 min data file we see this just fine in all of them, as shown below.

This illustrates three  points.  One is we have still another completely different environment of the instrument during the measurement, so this seems to imply this might not be the factor. 

And second, it seems we may have a real meteorological effect here to understand. We need to rig up our Gill Pressure port, and have this set up outside for the next squall to remove all such influences.... unfortunately (in this regard) Seattle has only some 4 or 5 squall days per year.

The other important point to observe is the great value of the data export function of the Mintaka Duo.  At 24 h ago, this bump was just barely visible on the instrument screen, but we could just connect it to a computer with a USB cable, export the data to a spread sheet and plot it. The whole process took just a few minutes.

These types of details are much easier to see with a good plot. The above picture is from the 3-min table.  I had missed seeing it in the 750 data points of the 1.5 min table and what would have been better  still the 15 second table.