ASOS (left) AWOS (right) – both at airports click for larger images
People send me stuff. Last night I got an email from reader Andrew Schut that said:
See public information statement below. I’m perplexed.
ASOS was put in for “aviation purposes” given its tolerances, yet we use it for climate purposes, why should AWOS be any different?
Not to mention the sensor that AWOS uses is a Vaisala sensor and at least a decade ahead in terms of sensor technology compared to the prehistoric 1088 RTD thermistor that the NWS has been using since the mid 80’s.
What Andrew was referring to was this unusual public information statement from the National Weather Service in Chicago, nullifying an apparently new low statewide temperature record from Rochelle Illinois:
PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE CHICAGO IL
432 PM CST FRI JAN 16 2009
…REGARDING ROCHELLE`S LOW TEMPERATURE THIS MORNING…
THE AUTOMATED WEATHER OBSERVING SYSTEM (AWOS) AT THE ROCHELLE
AIRPORT RECORDED A TEMPERATURE OF -36F AT 745 AM THIS MORNING.
WHILE THE THERMOMETER ON THE AWOS WAS RE-CALIBRATED YESTERDAY
AND MAY INDEED BE ACCURATE…AWOS OBSERVATIONS ARE NOT QUALITY
CONTROLLED OR CALIBRATED BY THE NATIONAL WEATHER SERVICE AND
ARE ALSO NOT DESIGNED FOR CLIMATE PURPOSES.
THEREFORE…THE STATE CLIMATOLOGIST DOES NOT CONSIDER THIS
TEMPERATURE AN OFFICIAL MEASUREMENT FOR THE PURPOSE OF
DETERMINING WHETHER OR NOT AN ALL TIME RECORD LOW FOR THE
STATE WAS REACHED. FOR THE PURPOSE OF DETERMINING RECORD
TEMPERATURES FOR THE STATE…ONLY ASOS AND
COOPERATIVE OBSERVER OBSERVATIONS WILL BE USED SINCE BOTH OF
THESE OBSERVATIONS ARE QUALITY CONTROLLED BY THE
NATIONAL WEATHER SERVICE.
See the original here. Personally, I don’t think airports are a suitable place for ANY climate measurements to be made. Here is why.
Airports are dynamic environments, with changes in air traffic, runway upgrades, new runways, new terminals, more tarmac/access roads, and increased infrastructure in general over time. For the NWS in Chicago to say that one ASOS at one airport is somehow better that an AWOS at another, particularly one calibrated the day before, is simply disingenuous. Throw them both out I say. Airports aren’t “quality controlled” for station siting changes. The claim that ASOS is somehow thus better than AWOS is simply ludicrous. There is no basis for this claim.
Let’s look at some examples of ASOS climate stations and the type of quality control that goes on:
First the issue of encroachment by infrastructure, here’s a new fire station going up next to an ASOS station (which is a USHCN climate station) in Lafayette, LA, I believe the station to the lower right is an older AWOS station:
Click for a larger interactive image
Of course it didn’t always used to be this way, the Google Earth image, which is a bit older, shows the area before the construction started:
Click for a larger interactive image
A call to the LFT airport authority at this contact from their website told me that the contract for the new fire station facility was awarded in July of 2005 and that construction started shortly after that. The new fire station, show being constructed in the top photo is now complete.
Reader Davis Smith writes:
Fifteen miles north of the Lafayette airport is another temperature site named Grand Coteau. It seems reasonable to expect the two to have similar trends (using GISS adjusted data) due to their proximity. A comparison of the two for recent years is here :
Click for larger image
Looks like a divergence circa 2004.
Ok that’s just one example. How about then the issues with the ASOS station in Reno, NV, that the NWS had to move because they didn’t agree with the readings it gave? NOAA uses Reno’s placement problems as an example in a training manual for climate monitoring COOP managers.
See NOAA Professional Competency Unit 6
What was amazing is that the NWS determined that there were significant problems with this USHCN ASOS station placement at the Reno Airport that contributed a significant warming bias to the record.
From that manual:
Reno’s busy urban airport has seen the growth of an urban heat bubble on its north end.
The corresponding graph of mean annual minimum temperature (average of 365 nighttime
minimums each year) has as a consequence been steadily rising. When the new
ASOS sensor was installed, the site was moved to the much cooler south end of the
runway. Nearby records indicate that the two cool post-ASOS years should have been
warmer rather than cooler. When air traffic controllers asked for a location not so close
to nearby trees (for better wind readings), the station was moved back. The first move
was documented, the second was not. The climate record shows both the steady warming
of the site, as well as the big difference in overnight temperature between one end of this
flat and seemingly homogeneous setting, an observation borne out by automobile
traverses around the airport at night.
They were also kind enough to provide a photo essay of their own as well as a graph. You can click the aerial photo to get a Google Earth interactive view of the area.
This is NOAA’s graph showing the changes to the official climate record when they made station moves:
Here is what a surface temperature transect of Reno looks like, I did this one myself:
Click for larger image, note that the airport is in the middle of the UHI bubble.
Russ Steele did a comparison as a guest post here of the data from the Reno ASOS USHCN station to a RAWS station run by the Forest Service a few miles away and writes:
Last year, I found a Remote Automated Weather Station operated by the Forest Service at Desert Springs that is 11.28 miles due north of the Reno Airport, in a remote area well away from urban influences. The annual temperature in desert far from urban influence in 2007 was 52.54 F, which was 2.8 F below the Airport ASOS just eleven miles away. As you can see this site is quite remote.
Desert Springs, click for larger image.
Here is a plot from last year comparing the Desert Springs and Reno ASOS.
But location and encroachments within the airport aren’t the only issues with ASOS, there is the ASOS temperature sensor itself, which has been shown to be inaccurate. There’s the famous HO83 temperature-dewpoint sensor, a product of “lowest bidder” engineering.
|HO83 ASOS Hygrothermometer
The HO-83 is know to have a warm bias between 0.5C to 0.7C. as shown here.
The most famous problem occurred in Tucson, AZ in the mid 1980’s where a malfunctioning HO83 unit created dozens of new high temperature records for the city, even though surrounding areas had no such measured extremes. Unfortunately those new high temperature records including the all time high of 117 degrees F, became part of the official climate record and still stand today. Here is a New York Times article that highlights the problem and a research paper from Kessler et al outlining similar problems in Albany New York as well as Tucson.
One of the biggest problems was that the early design of the HO83 allowed exhaust air (warmed by the warm side of Peltier chip) to recirculate from the mushroom shaped cap down the sides of the chamber, and back into the air inlet at the bottom. The problem was solved a few years later by the addition of a metal skirt which deflects the exhaust air.
Unfortunately, even though NOAA has modernization plans in place for the ASOS network, there are still some of the original designs that remain in operation today, such as this USHCN station which is the official climate station of record for New Orleans:
Photo from sufracestations.org volunteer Fred Perkins 8/25/07 click for larger photo.
Thus, the HO83 induced bias first noted in the mid 1980’s continues in the surface temperature record even today.
While only 5% of the USHCN network is ASOS, the biases produced by the HO83 are quite large, and there appears to be no adjustments to remove the bias. Since determining the individual maintenance records and biases of each ASOS station would be a significant task, the simplest solution would be to remove all ASOS stations from the USHCN record set.
But the most damning evidence that ASOS stations are probably lwarmer biased than AWOS stations comes from this internal NOAA technical paper on May 29th, 2001, from Brian Fehrn of the NWS office in Elko, NV who did a year long side by side comparison of an ASOS station being installed just 500 feet away from an AWOS station being decommissioned.
Here, courtesy Russ Steele, is a photo of the Elko ASOS station, which happens also to be a USHCN climate station of record:
Fehrn’s table of monthly data tells the story pretty well:
**ASOS Data from the 19th are missing
***ASOS Data from the 9th, 10th, 14th, 23rd, and 24th are missing
His conclusion says it all, emphasis mine:
While this study encompasses only a year, the data seem to indicate the uncommissioned ASOS records warmer temperatures than the AWOS. With recent reports and concerns about global warming, it is of note that this comparison of “unofficial” data to the official observations show that instrumentation located only a couple hundred yards apart can give a notable temperature discrepance. It is not the purpose of this study to determine what causes the Elko ASOS to record warmer temperatures than the AWOS. The main message is it appears the ASOS will probably record higher temperature values than the AWOS once the ASOS is commissioned and becomes the official site. This study should prove beneficial to forecasters once the ASOS becomes the official temperature site. Forecasters will be aware of the average temperature discrepancy and, if all other factors remain equal, it is quite possible that Elko may experience a rise in the overall temperature over time when the ASOS is used as the official data for Elko, Nevada.
So when we see public information statements like the one yesterday from the National Weather Service telling us that the ASOS system is more acceptable that an AWOS system calibrated just the day before, I’m quite comfortable in calling BS on that statement.
We shouldn’t measure climate data at airports with aviation instruments, period.