13A.2
Detecting a greenhouse warming signal using only maximum daily temperatures
Richard T. McNider, University of Alabama in Huntsville, Huntsville, AL; and J. R. Christy and W. B. Norris
The essence of concern in greenhouse gas climate change is that heat will be trapped and accumulate in the earth system, thus altering the climate and altering ecosystems. One of the most significant signals in the thermometer-observed temperature record since 1900 is the decrease in the diurnal temperature range over land, largely due to warming of the minimum temperatures. The cause for this nighttime warming in the observed temperatures has been attributed to a variety of causes including increases in atmospheric water vapor, cloud cover, jet contrails, and changes in surface characteristics, such as land cover and land use. Climate models have in general not replicated the change in diurnal temperature range well.
Here we would like to try to distinguish between warming in the nocturnal boundary layer due to a redistribution of heat and warming due to the accumulation of heat. The temperature at night at shelter height is a result of competition between thermal stability and mechanical shear. If stability wins then turbulence is suppressed and the cooling surface becomes cutoff from the warmer air aloft, which leads to sharp decay in surface air temperature. If shear wins, then turbulence is maintained and warmer air from aloft is continually mixed to the surface, which leads to significantly lower cooling rates and warmer temperatures. This warming occurs due to a redistribution of heat. As will be shown by techniques of nonlinear analysis (Walters et al. 2007 GRL) the winner of the stability and shear contest is very sensitive to changes in greenhouse gas forcing surface roughness, cloudiness, surface heat capacity (including soil moisture). Further, the minimum temperatures measured in the nocturnal boundary layer represent only a very shallow layer of the atmosphere which is usually only a few hundred meters thick. It is likely that the observed warming in minimum temperature, whether caused by additional greenhouse forcing or land use changes or other land surface dynamics, is reflecting a redistribution of heat by turbulence—not an accumulation of heat.
Because of the redistribution phenomena and the shallow layer affected, observed minimum temperatures are a very poor measure of the accumulation of heat in the atmosphere. As will be shown using an analysis of grid size dependence, climate models with their course resolution cannot accurately simulate the physics important to heat redistribution. Thus, their minimum temperatures are suspect. Further, the use of minimum temperatures from climate models as part of information to detect heat accumulation is not warranted.
Surface maximum temperatures would seem to represent a more robust measure of the heat content of the atmosphere since daytime boundary layers connect the surface to a depth of one to two kilometers or more and are well mixed. Climate models more accurately simulate daytime mixing which, by its non-local nature, is not as dependent on grid resolution. Thus, daytime maximum temperatures should be much better for detecting and simulating anthropogenic greenhouse gas heat accumulation in the atmosphere.
Recorded presentationSession 13A, Detection and attribution of climate change: Part II
Thursday, 24 January 2008, 8:30 AM-9:45 AM, 215-216
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