Extended Abstract (192K)11.2
Heat balance in nocturnal boundary layers
Jielun Sun, NCAR, Boulder, CO; and S. P. Burns, A. C. Delany, S. P. Oncley, T. W. Horst, and D. H. Lenschow
A unique set of nocturnal longwave radiative and sensible heat flux divergences was obtained during the Cooperative Atmosphere-Surface Exchange Study (CASES-99). These divergences are based on upward and downward longwave radiation measurements at 2 levels and turbulence eddy-correlation measurements at 8 levels. In contrast to previous radiation divergence measurements obtained within 10 m above the ground, we measured radiative flux divergence within a deeper layer between 2 m and 48 m. Combining our observations with the earlier studies indicates that the radiative cooling at night is typically stronger than the local cooling close to the ground, but the result is opposite in an overlaying layer. This result confirmed previous numerical calculations that the relative contribution of the radiative flux divergence to the local cooling at night decreases with height. As a result, the relative contribution of the sensible heat flux transport and temperature advection to the local cooling increases with height, although the absolute values of the sensible heat and temperature advection decrease with height following the local cooling. Within our observation layer the radiative flux divergence is, on average, comparable to or smaller than the sensible heat flux divergence. Our unique observations of both radiative flux and sensible heat flux divergence indicate the importance of the temperature advection over an even reasonably flat surface.
Our observations also indicate that the measured radiative flux divergence between 2 m and 48 m was typically largest at the beginning of a night and may fluctuate around zero through out the night due to variations of wind speed, i.e. either convergent or divergent. The wind speed variation can change not only the sensible heat transfer, but also the surface longwave radiation due to variations of the area-exposure of warmer grass stems and soil surfaces. The magnitude of the radiative flux divergence in the early evening depends on how fast the ground cools and on the vertical temperature gradient within the measured layer. The largest radiative flux divergence in the early evening, more than 10 Wm$^{-2}$, was dominated by the largest outgoing longwave radiation under conditions of weak wind and clear sky after a hot day. The air temperature at 2 m, in general, depends on the measurement height when the wind speed is weak.
Session 11, Stable BLs - II
Thursday, 18 July 2002, 10:30 AM-12:30 PM
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