From Dr. Roger Pielke Sr. comes word of an important new paper that shows how the air near the ground (boundary layer) is highly affected by sensitive nighttime dynamics, which show up in the Tmin of weather station data (GHCN stations in this case) but are not captured by climate models. The paper also showed that the stable nocturnal boundary layer was very sensitive to the turbulent parameterization and surface characteristics such as roughness, and land surface heat capacity and conductivity. That (bold mine) is the sorts of heat sinks/siting issues pointed out in the surfacestations project which showed that 90% of the stations in the USHCN (many of which are also part of GHCN) don’t meet siting specifications (NOAA’s 100 foot rule for example) and have been compromised by urbanization. There will be more coming on this issue in the future.
The authors also found that mixing of air aloft was the biggest contributor to nightime warming, and this may be due to surface roughness causing increased turbulence, and thus vertical mixing. I’ve added this graphic to give you an idea of how this works. A weather station downwind of the city, even in rural areas, would get more mixed air that is a composite of naturally striated warmer air aloft as well as air that was warmed from the heat sink effect of the city releasing LWIR at night into the boundary layer.
This is a very significant finding and may explain why we see things like the UHI temperature bubble in Reno Nevada, where there is a strong measured nighttime UHI effect at the surface, and it manifests itself in the USHCN/GHCN weather station at the airport. The Tmin at that station is rising faster than the Tmax. – Anthony
New JGR – Atmosphere Article “Response And Sensitivity Of The Nocturnal Boundary Layer Over Land To Added Longwave Radiative Forcing”
By Richard McNider, University of Alabama in Huntsville
We have just had a paper published in JGR entitled
McNider, R. T., G.J. Steeneveld, B. Holtslag, R. Pielke Sr, S. Mackaro, A. Pour Biazar, J. T. Walters, U. S. Nair, and J. R. Christy (2012). Response and sensitivity of the nocturnal boundary layer over land to added longwave radiative forcing, J. Geophys. Res.,doi:10.1029/2012JD017578, in press. [for the complete paper, click here]
The paper addresses the diurnal asymmetry in warming that has occurred in the observed temperature trends in the last century in which minimum temperatures have warmed at a substantially greater rate than maximum temperatures. While the paper goes into considerable detail on the response of the stable boundary layer to radiative forcing that perhaps only a stable boundary layer junkie can appreciate, the implications of the paper ,I believe, are critical to interpreting both the historical temperature data set and global modeling over the last century. For those who do not want to be overwhelmed with details, I believe the introduction and conclusions are tractable for non-boundary layer specialists.
Here let me summarize and at the end editorialize on the key points of the paper. In the last century minimum temperatures have warmed nearly three times more than maximum temperatures as captured by the NOAA Global Historical Climate Network. In fact this asymmetry is one of the most significant signals in the climate record and has been the subject of many papers.
Our paper shows that the CMIP3 climate models only capture about 20% of this trend difference.
This is consistent with other studies. Because climate models have not captured this asymmetry, many investigators have looked to forcing or processes that models have not included such as jet contrails, cloud trends, aerosols, and land use change to explain the lack of fidelity of models. However, our paper takes an alternative approach that explores the role of nonlinear dynamics of the stable nocturnal boundary layer that may provide a general explanation of the asymmetry.
This was first postulated in a nonlinear analysis of a simple two layer model we carried out a few years ago (Walters et al. 2007) that indicated that slight changes in incoming longwave radiation from greenhouse gases might result in large changes in the near surface temperature as the boundary is destabilized slightly due to the added downward radiation. This produced a mixing of warmer temperatures from aloft to the surface as the turbulent mixing was enhanced just as an increase in wind speed can destabilize the nighttime boundary and mix warm air from aloft to the surface.
…
Most of the warming at shelter height was due to the warm air mixed from aloft. This is illustrated in figure 10 in the paper. Thus, this process is a highly sensitive positive feedback to surface warming.
Figure 10: (top) Expanded view of the difference in potential temperature profile between the case of added GHG energy and base case for a geostrophic wind of 8 m s-1(top). (bottom) Expanded view of profile difference.
Read the entire post here at Dr. Roger Pielke Sr.’s website: “Response And Sensitivity Of The Nocturnal Boundary Layer Over Land To Added Longwave Radiative Forcing”
The full paper is available here: http://pielkeclimatesci.files.wordpress.com/2012/07/r-371.pdf
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From a quick read it seems the authors mean well but there are many wrong assumptions and calculations. For a start mentioning GHG forcing would turn off any engineer who understands heat and mass transfer. There are drivers such as temperature difference, pressure differences, concentration differences, applied forces, gravity, etc but a gas such as CO2 can not force anything. They (and I assume all so-called climate scientists) seem not to have heard of natural and forced convection., there is no mention of dimensionless numbers such as the Nusselt No, , Prandtl No or the Schmidt No, (which Gavan Schmidt does not understand). Sure UHI exits, I see it every day and night driving in my SUV which has a outside thermometer. The temperature can rise be as much as 3C within 1 or 2 km driving from rural to an urban centre. However, UHI has nothing to do with so-called greenhouse gases.
In consideration of Mosher above, curiosity gets the best of me. I wonder if anyone has filtered out urban data as best it can be, and focused only on rural station data, in its many flavors (T-whatever). Papers, charts etc.?
Dr Roy Spencer has a population adjusted US temperature dataset. He has several posts about it at his site.
He uses hourly temperature measurements and avoids the issues present in the Tmin – Tmax data.
One thing his data shows (my interpretation, not his) is the Oasis Effect in the US southwest. The Oasis effect is where urban irrigation increases evapotranspiration and decreases temperatures. This effect is particularly large here in Perth where everyone irrigates their garden.
Not all urban effects cause warmer temperatures.
Heat loss from the surface on clear nights is by direct IR to space. This disruption of the boundary layer as described results in an increase in night time surface temperatures and corresponding reduction of air temperatures.
The IR to space is proportional to 4th power of surface temperatures, so this increased surface minima should produce a significant increase in heat loss from the Earth.
As we all know, most of the warming over the past century had occurred as raised minima rather than raised maxima.
Thus what is being read global warming could in fact be due to increased heat loss from the entire global – i.e. surface warming == global cooling.
News flash: downwind of cities, the nights are warmer.
File under “The Bleeding Blinding Obvious”.
Thank you.
Mark says:
July 12, 2012 at 2:19 pm
There appears to be an important environment missing from the diagram. That being “airport”. Where you’d need to factor in the effect of the planes moving air around.
If a butterfly flapping its wings on one side of the planet can cause a hurricane on the other side, just think what a jet engine can do!
Brian
I think, although it might be bleeding obvious, I would have just assumed that it might be the result of warmer air from the city moving into the rural area. What is described above, again is the bleeding obvious once it’s pointed out to you. I guess the whole thing become complicated by wind strength, atmospheric temperature, physical geography etc.
It’d be realy interesting to see a “thermal” map of a region around a city with prevailing wind direction (for each of the four seasons).
I was in error on one of my post last evening. At 6:45 pm last evening the differential between the two bank thermometers 4 blocks apart in Hardin, MT was 7 degrees fahrenheit. Take the higher of the two for calculating global warming.
DocMartyn says:
July 12, 2012 at 6:24 pm
Thanks. I’d forgotten about Dutch Elm disease.
davidmhoffer says:
July 12, 2012 at 9:05 pm
I initially thought you might be on to something significant. But science is a process of discovering where you are wrong, and discovering you are wrong is more important than thinking you are right.
While I appreciate the comments, it seems there is some confusion on our paper, perhaps because Anthony only put part of my blog on his site. I would suggest you go to Roger Pielke’s site to read the full blog. Our paper was not meant as another CO2 contributes to global warming paper – in fact a reviewer said the global warming community would not like our results. It was also not meant to be a paper on the UHI but our results do show the nightime bounadry layer is sensitive to surface parameters . I believe strongly that land use changes have a profound effect on temperatures especially nighttime temps. Rather, our paper was pointing out that even in areas where land use has not impacted temperatures that there is a boundary layer process where added downward radiation (from CO2 or aerosols) can destabilize the boundary layer and produce a redistribution of heat (just as land surface changes do) warming the shelter level temperature. Thus, this process is not about an accumulation of heat (the real essense of global warming theory) but about a warming of a thin layer at the surface 20-50 meters thick due largely to a redistribution of heat in the nocturnal boundary layer.
The asymmetry in warming of Tmin by this process (and land use) and the lack of GCM’s to replicate this Tmin warming (and land use Tmin warming) means that when models match the global Tmean (which is what modelers claim as proof of model fidelity) but not Tmin then they are warming Tmax too much. Because Tmax is tied to a deeper boundary layer it means that they are likely warming a much deeper layer than the thin warming in observed Tmin. Water vapor feedbacks require a deep warming. I think our paper indicates that observed temperatures are only showing a shallow warming, so models are likely overstating the deep troposphere warming (which is consistent with the Christy-Spencer satellite and model discrepancy). Because models are over-warming a deeper layer, water vapor feedbacks are likely being overstated. I think this is consistent with Roy Spencer’s hypothesis that models are too sensitive to feedbacks.
No Mosher delusionally believes he is more objective than everyone else – the one true rational one, the independent thinker, the moderate or whatever. It gets old fast.
Gee… Most of the GHCN thermometers are at airports. Vertical mixing causes increased readings (despite no real increase in heat arrival or retention) as a result of mixing. Airplanes rise and descend through the air mass at airports. Wonder if having mega tons of aircraft doing vertical maneuvers (not to mention burning all that kerosene in the air column) and blasting jet exhaust plumes at various angles just might cause some “vertical mixing”…
Spot on Poptech. Mosher is too much in love wit himself and his own perception of his knowledge. He as long ceased to be an objective arbitrator on anything related to temperature records and models. He reacts exactly like what the AGW crowd do to any paper which challenges their position, which is by irrationally striking out aggressively and foolishly.
Dick McNider says:
July 13, 2012 at 8:45 pm
Welcome! We’re always glad when a paper’s authors stop by.
Yeah, I suppose I should. … Oh, I like this comment:
It wasn’t until I got my Vantage Pro weather station and having logging data every 10 minutes that I really began to appreciate the nighttime boundary layer (what happened to air inversion?)
I live in a residential area, sort of in a river valley, and it doesn’t take long to realize just how unrepresentative nighttime near ground temperatures are of, well, much of anything other than heating bills. I was in a weather mail list where one of the contributors often reported his “garden” temperature a few inches above ground, and often several degrees cooler than is “real” setting.
If there’s any wind at all, we have little cooling, even a little gust in the middle of the night can scour out the inversion. In the summer I can see temperature fall to the dew point, then decline with a longer time constant as dew formation retards cooling. On really cold nights, I can see the temperature hit the dew point and barely slow down because the absolute humidity is so low. Averaging Tmax and Tmin is like averaging apples and oranges, (whatever that means).
During the day temperature climbs starting at dawn, but when the inversion warms up, then convection and mixing with the entire air column slows further warming way down because now the whole air column needs to be warmed.
This shows up well at Mt Washington. The weather observatory on top is always in air flow and there’s very little nighttime drop. Their temperature plus 30°F (for adiabatic expansion) is often a good prediction of what my Tmax will be that day.
You continue with:
Tmin is certainly important, both for human impact and surface radiation. For climate modeling, it’s mostly another confounding variable. That’s one reason I like the satellite record.
Refs:
http://home.comcast.net/~ewerme/wx/current.htm – look at the amazingly flat top of today’s temperature curve.
http://vortex.plymouth.edu/mwn24.gif – Note Washington’s temperature today was 62-63°F, mine was 88-92°F, just about 30°F warmer.
Philip Bradley says:
The answer appears (from some google searches) to be neither. Because airport runways are oriented to the prevailing winds, in order to avoid airplanes overflying the nearby city, airports are located at right angles to the prevailing wind. If the prevailing winds are from the west, the airport would generally, be north or south of the city.
Not always the case. e.g. LHR/EGLL is west of London with east/west runways. It also isn’t uncommon for cities to expand to surround airports which serve them. Factors such as where the cheap land was when the airport was built may also be relevent.
EMS;
vertical mixing at airports? Since the post suggests that low mixing is causing artificially high TMin readings, isn’t it a good thing to bust up the boundary layer at night around the weather stations?
😉