Reliability of ASOS/AWOS below zero

[Dan Robinson]

With temps near or below zero areawide tonight, some of the stations in southern IL and MO are currently reporting readings well below many of the surrounding stations. Cape Girardeau is at -14/-16 and Paducah -8/-10, while most other surrounding stations are still in the low single digits above/below zero. Dewpoints of surrounding stations are also in the single digits below. There is a snowpack here region-wide.

I suspect the Cape Girardeau and Paducah temps are anomalies. The only thing they have in common is that they are in major river valleys, which I'd think should actually give temps a boost. In my past experience, temps are usually much warmer in the major river valleys due to the large volume of water's "heat reservoir" effect.

Does anyone know of reliability issues with mid-latitude ASOS/AWOS temperature sensors on subzero days? I figure since we only see these extremes once a year or less, the sensors might not be as trustworthy.

 

[Brian G]

That Cape Girardeau reading is impressive. In one hour they dropped from -2F to -12F. They eventually got down to -19F this morning. Cairo was -12F, Metropolis was -13F, and Paducah was -10F. Paducah issued a record report for that reading. This does lend some confidence that the lower readings are more likely to be correct than not. But, yeah, I'd be curious to know if the CGI reading can be validated.

 

[Bob Wagner]

With temps near or below zero areawide tonight, some of the stations in southern IL and MO are currently reporting readings well below many of the surrounding stations. Cape Girardeau is at -14/-16 and Paducah -8/-10, while most other surrounding stations are still in the low single digits above/below zero. Dewpoints of surrounding stations are also in the single digits below. There is a snowpack here region-wide.

I suspect the Cape Girardeau and Paducah temps are anomalies. The only thing they have in common is that they are in major river valleys, which I'd think should actually give temps a boost. In my past experience, temps are usually much warmer in the major river valleys due to the large volume of water's "heat reservoir" effect.

Does anyone know of reliability issues with mid-latitude ASOS/AWOS temperature sensors on subzero days? I figure since we only see these extremes once a year or less, the sensors might not be as trustworthy.

I don't believe it is an ASOS/AWOS issue. It's been 25 years since I took surface observations, predating ASOS. On several occasions during my observing days, I saw similar events occur with significant snowpack. Minor changes in elevation, just a couple of feet over the distance of 100 yards or so, can make a big difference. If the sensor package is at a slightly lower elevation than the surrounding area, you can get a pooling effect of the colder, denser air. Calm winds, the temperature drops like a rock. Even 3-4 knots of wind is enough to mix things up to warm the temperature.

 

[Brian G]

Nice call on the healthy level of skepticism. -14F is still stupid cold for Missouri though.

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE PADUCAH KY
244 PM CST THU FEB 19 2015

...MINIMUM TEMPERATURE ADJUSTED AT CAPE GIRARDEAU MISSOURI DUE TO
SENSOR MALFUNCTION OVERNIGHT...

THE FEBRUARY 19, 2015 LOW TEMPERATURE FOR CAPE GIRARDEAU (AIRPORT)
IS BEING REVISED TO MINUS 14 DEGREES (-14). THE ADJUSTMENT IS
REQUIRED AS A RESULT OF FAULTY SAMPLING IDENTIFIED BY A NWS SENSOR
TECHNICIAN VIA AN ON-SITE INSPECTION AND SUBSEQUENT CORRECTION TO
THE TEMPERATURE SENSING EQUIPMENT. THE RECORDED MINIMUM AS A RESULT
OF THE FAULTY SENSING WAS MINUS NINETEEN DEGREES (-19). THE ADJUSTED
VALUE OF MINUS 14 DEGREES IS BASED UPON SENSOR MAGNITUDE
DETERMINATIONS BY THE NWS TECHNICIAN AND IN CONSIDERATION OF NEARBY
SAMPLING AND METEOROLOGICAL CONDITIONS.

WE ARE SORRY FOR ANY CONFUSION OR INCONVENIENCE THIS MAY HAVE
CAUSED.

 

[Jeff Duda]

While it looks like there were some sensor malfunctions in this particular case, it is not at all unphysical or even that unusual for isolated stations to record temperatures much lower than nearby ones during winter nights with a solid snowpack, clear skies, and light winds. Each station exists in a microclimate where conditions can vary significantly on scales of just a few 10s of meters or so. A station being at a low point makes it even more likely to register surprisingly low temperatures because cold air is heavy and pools in low spots when there is no heating and little wind to keep things stirred up.

One well documented example of this is the EL Reno Oklahoma Mesonet site. There's an AMS journal paper Hunt et al. (2007), JAMC trying to figure out why it seems to disagree so much with other nearby mesonet stations. Topography is believed to play a role, but not significantly enough to cause the differences. It probably has something to do with aspects of the microclimate and light winds.

 

[James Hyde]

Sometimes just the fact that they are located at airports is reason enough to bias either temperature end compared to surroundings. If you think about it, airports need long and flat areas for runways. Often this at the top or bottom of terrain features where the slope is lower. I don't know of any peer reviewed literature to back this up, but it's just something I've noticed.