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