The conclusion of our Pielke and Matsui (2005), Lin et al (2007), Klotzbach et al (2009) and Mahmood et al (2009) papers that the use of minimum temperatures over land to diagnose climate system heat changes (i.e. global warming) introduces a bias is further substantiated by the paper
Sun, J-L et al, 2003: Heat balance in the nocturnal boundary layer during CASES-99 J. Appl. Meterologogy. 42, 1649-1666.
The abstract reads
“A unique set of nocturnal longwave radiative and sensible heat flux divergences was obtained during the 1999 Cooperative Atmosphere–Surface Exchange Study (CASES-99). These divergences are based on upward and downward longwave radiation measurements at two levels and turbulent eddy correlation measurements at eight levels. In contrast to previous radiation divergence measurements obtained within 10 m above the ground, radiative flux divergence was measured within a deeper layer—between 2 and 48 m. Within the layer, the radiative flux divergence is, on average, comparable to or smaller than the sensible heat flux divergence. The horizontal and vertical temperature advection, derived as the residual in the heat balance using observed sensible heat and radiative fluxes, are found to be significant terms in the heat balance at night. The observations also indicate that the radiative flux divergence between 2 and 48 m was typically largest in the early evening. Its magnitude depends on how fast the ground cools and on how large the vertical temperature gradient is within the layer. A radiative flux difference of more than 10 W per meter squared over 46 m of height was observed under weak wind and clear-sky conditions after hot days. Wind speed variation can change not only the sensible heat transfer but also the surface longwave radiation because of variations of the area exposure of the warmer grass stems and soil surfaces versus the cooler grass blade tips, leading to fluctuations of the radiative flux divergence throughout the night.”
This excellent paper includes the following findings:
1. In Figure 3, the CASES-99 field campaign (October 1999) average nightime radiative flux difference between 48m and 2m differs by up to 20 Watts per meter squared depending on the location including an effect from landscape type as given in Table 1:
2. As seen in Figure 5 for the night of October 21, the net radiative flux difference (the flux divergence over the layer 2m to 48 m) within the nocturnal boundary layer, is on the order of 5 Watts per meter squared.
3. As evident in Figure 6, the temperature difference between 2m and 48 m becomes larger through the night as the stable stratification increases; a typical occurrence of the light wind nocturnal boundary layer as discussed in Pielke and Matsui (2005). The increased nonlinear shape to the temperature profile illustrates that the radiative flux divergence with light winds is a function of height in the layer from 2m to 48m. From Figure 7, the sensible turbulent heat flux divergence is also a function of height within this layer.
4. As seen in Figure 9, when the winds are light at night, there are significant differences in 2m temperatures depending on the landscape and terrain. The authors wrote
“ The elevation-dependent 2-m air temperature is clearly demonstrated in the air temperature difference between the lowest station (station 3) and the other stations (Fig. 9). The dependence of the air temperature on the surface elevation occurred only at night and was inversely related to the wind speed. When the wind speed is stronger than 5 meters per second, strong turbulent mixing and advection eliminate temperature differences at various elevations and lead to a homogeneous temperature field. Similar results were also found by Harrison (1971), Geiger et al. (1995), LeMone and Grossman (2000), and Acevedo and Fitzjarrald (2001). This implies that to capture spatial variations of synoptic or ambient temperature, the temperature measurement height needs to be above the influence of surface drainage flows.”
The differences were on the order of 2-3 degrees Celsius.
In the conclusion, the authors state
“Our study indicates that the radiative flux divergence between 2 and 48 m is close to zero except in the early evening. However, it can be significant close to the ground based on the radiative flux divergence below 10 m in the earlier studies. On average, the radiative flux divergence increases with stability. The vertical variation of the sensible heat flux is found to be more than 10% of its mean below 20 m. Therefore, the conditions for application of M–O similarity theory are often violated in the surface layer in the early evening and below 10 m above the ground at night because of the large radiative and sensible heat flux divergences.”
This paper further documents the large variation of 2m temperatures at night under light winds due to landscape and terrain influences. As the abstract states
“A radiative flux difference of more than 10 W per meter squared over 46 m of height was observed under weak wind and clear-sky conditions after hot days. Wind speed variation can change not only the sensible heat transfer but also the surface longwave radiation because of variations of the area exposure of the warmer grass stems and soil surfaces versus the cooler grass blade tips, leading to fluctuations of the radiative flux divergence throughout the night.”
Those who claim that the minimum 2m temperatures are appropriate values to include in a diagnosis of global warming are in error. As we summarize in Mahmood et al (2009) in the post on August 24 2009
“The stable nocturnal boundary layer does not measure the heat content in a large part of the atmosphere where the greenhouse signal should be the largest (Lin et al. 2007; Pielke et al. 2007a). Because of nonlinearities in some parameters of the stable boundary layer (McNider et al. 1995), minimum temperature is highly sensitive to slight changes in cloud cover, greenhouse gases, and other radiative forcings. However, this sensitivity is reflective of a change in the turbulent state of the atmosphere and a redistribution of heat not a change in the heat content of the atmosphere (Walters et al. 2007). Using the Lin et al. (2007) observational results, a conservative estimate of the warm bias resulting from measuring the temperature from a single level near the ground is around 0.21°C per decade (with the nighttime minimum temperature contributing a large part of this bias). Since land covers about 29% of the Earth.s surface, extrapolating this warm bias could explain about 30% of the IPCC estimate of global warming. In other words, consideration of the bias in temperature could reduce the IPCC trend to about 0.14°C per decade; still a warming, but not as large as indicated by the IPCC.”