INFLUENCE OF SURFACE ENERGY BALANCE ON AIR TEMPERATURE CHANGE IN THE SURFACE BOUNDARY LAYER

Abstract

It is well known that the distribution, of air temperature in time and space near the ground surface is caused by the partitioning of energy at the ground surface. However, the physical relationship between the air temperature change and the surface energy balance is not clear. The purpose of this paper is to describe the influence of surface energy balance on spatial and temporal variation in air temperature using observations of heat balance in the soilatmosphere system near the ground surface. The experimental site was selected at the Environmental Research Center of the University of Tsukuba in Ibaraki Pref., Japan (Fig. 1). Observations were carried out on two occasions. The first was from 04 hr on August 6 to 04 hr on August 7, 1981. The second was from 05 hr on September 13 to 05 hr on September 14, 1981. Elements observed were wet and drybulb temperatures, wind speed, net radiation, soil heat flux, soil temperature and volumetric heat capacity of the soil. The energy balance equation for the earth's surface is customarily expressed in the form: Rn=H+LE+G0(1) where Rn is the net radiation flux density, H. is the sensible heat flux density, LE is the latent heat flux density and G0 is the soil heat flux density into or out of the surface. In this study, Rn was measured directly by a net radiometer. Other components of the surface energy balance were determined by applying the more suitable method available. Daytime H and LE were computed by the Bowen's ratio method, and G0 was calculated with measurements by means of a heat flux plate. On the other hand, nighttime H and LE were computed by the profile method, and G0 was estimated by the energy balance method. As a result of the calculations, it can be seen that in the daytime warming and cooling of the air layer near the surface occurs independently of the sensible heat flux changes (Figs. 4-A and 5). This result suggests that the air temperature changes in the surface boundary layer are usually due to the convergence and divergence of the sensible heat flux within the surface boundary layer. If the air layer near the surface warms and cools as a result of the variation in the sensible heat flux with height, on the assumption that no other processes are involved, rates of air temperature change are expressed by _??_T/_??_t=-(H2-H1)/ρcp(z2-z1) (2) where H2 and H1 are the sensible flux at the height of z2 (100 cm) and z1 (2 cm) respectively, ρ is the air density and cp is the specific heat of air at constant pressure. The vertical differences of sensible heat flux (H2 - H1) are computed by the equation (2). For example, (H2-H1) was approximately 0.92 W m-2 when the maximum cooling rate occurred at 16 hr on August 6, 1981 (Fig. 6). The value of H2 -H1=0.92W m-2 is very small, and a vertical difference of this magnitude is difficult to measure directly. Even though the vertical differences are very small indeed, the convergence and divergence of sensible heat flux plays a very important role in air temperature change in the surface boundary layer. This is particulary important during the daytime hours.