Wednesday, February 6, 2008

The Redcliffe Automatic Weather Station

Because of the bad weather we’ve been having in South-East Queensland for the major part of the last two months, I got into the habit of checking the latest weather observations reported by the Automatic Weather Station (AWS) at Redcliffe before going flying. In the US, an AWS is known as an AWOS (Automated Weather Observing System).

On a number of occasion the weather observations showed south-easterly winds at 20 or 25 knots, i.e. almost direct crosswinds for our 07/25 runway, way in excess of the 15 knots crosswind limitation for the C152. However, when I arrived at the club, the windsock would usually show a wind somewhere between 10 and 15 knots.

This was all good news obviously since it made the difference between flying and not flying. However, this prompted me to try to find out how the weather measurements were made at Redcliffe, and why there was such a discrepancy in wind speed.

The information is not easy to find, but if you go to the page for a monthly weather report for Redcliffe, you will find this mention:

This Automatic Weather Station (AWS) is located in Talobilla Park, and sources its wind measurements from an anemometer on Redcliffe jetty.

I went to have a look today and found both the AWS and the anemometer. Apologies for the photos being a tad on the dark side, but today was really one of those days. Redcliffe received 60 mm of rains in about three hours. When I walked into the club I could see Tony and Lee hurriedly pulling planes inside the hangar before the thunderstorm hit. That was the first hint that no flying would happen. Then I looked at the weather radar with Mal and lost all hope of flying today, so we spent half an hour booking lessons for the next weeks and discussing the planning of the rest of the training, and my options for the CPL.

Back to the AWS. The anemometer is actually difficult to miss, it sits on top of a tall metal pole right at the end of the Redcliffe Jetty. The height of the pole is about 10 meters, in line with Bureau Specification 2013 on "the siting and exposure of meteorological instruments and observing facilities" which reads:

[...] ideally anemometers should be exposed over level terrain at a standard height of 10 metres above the mean ground level at locations completely free of all obstructions to the air flow.

Both wind speed and wind direction are measured here. Interestingly, there is another wind direction sensor (wind vane) on a shorter pole just a couple of meters away. On top of the pole also sits a UHF antenna similar to a rooftop TV antenna.

Finding the AWS at Talobilla Park was a bit harder. It is actually located inside a fenced enclosure on top of a hill at the back of a baseball pitch and is hidden from view by a curtain of trees.

A side effect of my search for the Redcliffe AWS was to discover the existence of the Redcliffe Leagues Padres Baseball Club. I didn't even know baseball was played in Australia. Around here we prefer to play that variant of baseball that involves a flat bat and week-long games known as cricket. The Padres' newsletter claims they are “the biggest and some would say best baseball club in Queensland”. Unfortunately I do not know how many baseball clubs there are in Queensland so I can't really put this statement into perspective.

Back to our meteorology topic. The AWS itself is your regular little white hut one meter above ground. It measures temperature, dew point temperature, atmospheric pressure, relative humidity and rainfall. I couldn't see any rain gauge though. Maybe there's an opening at the top of the box to let rain in, or maybe rain is measured somewhere else.

There is a pole nearby with two antennas similar to the one at the Redcliffe Jetty. One of the two antennas points in the direction of the jetty, most likely for a point-to-point radio link to transmit wind measurements to the AWS. The other antenna points in a different direction and is probably used for sending the measurement to the Bureau. It seems odd though that the BOM implemented their own wireless communication network while they could simply hook up the AWS to the cell phone network or a land line.

With all this information in mind, the position of the anemometer on the Redcliffe jetty may explain why the reported wind is stronger there than at the aerodrome. The anemometer is about 3 nautical miles to the east-south-east of the aerodrome. When the wind blows from the East or the South-East, it hits the anemometer first, travels over the ground and finally hits the windsock at the aerodrome. As the wind blows over populated area, it is slowed down by friction and by tumbling over buildings, houses, trees and a few small hills.

According to Bob Tait’s CPL Meteorology, surface wind speed drops by one-third over water and two-thirds over land in comparison with the speed of the gradient wind at altitude. This implies that surface wind over land is half of surface wind over water. I do not know if the distance of three miles from the jetty to the aerodrome is sufficient to slow the wind down to what can be called surface wind speed, but the order of magnitude seems about right: when the wind is blowing at 25 knots at the jetty, we get less than 15 knots at the aerodrome, which makes crosswind landings possible yet challenging.

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