Jun 28 2007
Positive feedback in the Stable Nocturnal Boundary Layer
There’s an interesting paper in the latest issue of Geophysical Research Letters.
Walters, J. T., R. T. McNider, X. Shi, W. B. Norris, and J. R. Christy (2007), Positive surface temperature feedback in the stable nocturnal boundary layer, Geophys. Res. Lett., 34, L12709, doi:10.1029/2007GL029505. [PDF - requires subscription]
Abstract: The techniques of nonlinear analysis are used to examine the behavior of the stable nocturnal boundary layer (SNBL) when it is subjected to changes in incoming radiation or in surface characteristics. A single-column model and nonlinear bifurcation techniques are used to demonstrate that any atmospheric forcing, such as weak radiative forcing from greenhouse gases or cloud cover, can trigger a potentially significant positive feedback. Multiple solutions occur in some parameter spaces. This analysis shows that any forcing that decreases the stability, whether by increasing greenhouse gases or surface heat capacity, can cause large increases in surface temperature as the SNBL shifts from a weak turbulent regime, which allows the surface to cool, to a turbulent regime, which mixes warm air from aloft. Positive feedback may be a key factor in interpreting the long-term observed nocturnal warming trend in the SNBL.
There has been an observed decrease in the surface diurnal temperature range - the difference between the high and low temperature during a 24-hour period. This is been postulated to be caused by the increase in greenhouse gases. And it is widely acknowledged that greenhouse gases have contributed to this changed. However, climate models are unable to accurately reproduce the observed changes. Models have underestimated the observed changes.
This paper uses non-linear techniques (that I’ll admit I don’t totally understand) in an attempt to elucidate this discrepancy. They found that the geostrophic wind can cause a bifurcation and lead to regime changes.
They conclude that “[t]he amount of warming depends on the turbulent regime, being greater in the non-turbulent boundary layer (cold solution) and less in the turbulent boundary layer (warm solution). Of most interest is the fact that increased GHG forcing or increased cloud cover, even if slight, can cause the system to transition from the non-turbulent to the turbulent state and produce large changes in surface temperature. This is the essence of a positive temperature feedback and may be a contributing factor to the observed decrease in DTR.”
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