The Air-Land Interface
The ground is heated by the sun during the day and the increase in LWIR flux from the warm surface heats the air. It is important to distinguish clearly between the actual ground surface temperature and meteorological surface air temperature. The surface temperature needed for energy transfer analysis is the ground surface temperature. The meteorological surface air temperature is the air temperature measured in an enclosure placed 1.5 to 2 m above the ground.21 There is no obvious or simple relationship between these two temperatures. Solar radiation is absorbed and reflected by the ground.
The resulting surface temperature depends on the absorption coefficient, the thermal conductivity and heat capacity of the ground, the surface area and angles of incidence, the balance of the upward and downward LWIR flux and the direct air convection. If the ground is moist, latent heat effects also have to be included. The meteorological surface air temperature depends on the origin of the bulk air mass of the local weather system, surface LWIR flux heating, air convection and wind speed. As for the air-ocean interface, the First Law of Thermodynamics imposes energy conservation, but there is no requirement that the flux be conserved on any time scale.
The Second Law of Thermodynamics requires that the heat transfer follow the thermal gradient. Surface heating may be analyzed using a simple thermal conduction model analogous to the ocean heating model. This is illustrated in Figure 5. The results for summer and winter illumination of a concrete surface are shown in Figure 6. The calculated daily temperature excursions for summer and winter solar heating are 40 and 20 C at 35° latitude. A 1% increase in solar flux produces peak summer and winter temperature increases of 0.34 and 0.17 C.
The increases in surface temperature due to a 1.7 W.m-2 increase in LWIR flux vary between 0.14 and 0.36 C, with higher temperature increases observed at lower surface temperatures.
These changes are for ideal ‘clear sky’ conditions over a period of 200 years. Variations in cloud cover, aerosols and humidity will produce fluctuations in LWIR flux that are much larger than 1.7 W.m-2, so the effects of CO2 on ground surface temperature are not measurable. However, no historical record of surface temperature is available, so the meteorological surface air temperature has been substituted for the surface temperature without any consideration of the differences between the two.
Click source to download PDF (paper from Roy Clark)
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