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SONG-LAK KANG: ATMOSPHERIC BOUNDARY LAYER STRUCTURES AND PROCESSES OVER HETEROGENEOUS SURFACES

The ABL is an important component of the weather and climate system. In a numerical model, slight differences of ABL parameterization could result in significant cloud/precipitation differences. In addition, the ABL itself is of great importance to weather-related industry such as air pollution control and wind power development. For example, wind turbine is usually installed at a lower ABL height. Thus, the potential benefits of precise ABL wind forecast are obvious in wind power generation.

The assumption of horizontal homogeneity has been used for ABL parameterization in a mesoscale model (MM). In addition, observational studies on ABL structures and processes have commonly filtered out mesoscale fluctuations, which are on the scale larger than the characteristic length scale of the ABL turbulence. The Atmospheric Boundary Layer (ABL) is by definition directly influenced by the Earth’s surface. Given that the earth’s surface is heterogeneous on various spatial and temporal scales, the assumption of horizontal homogeneity has limitation in describing and predicting ABL structures and processes. Although numerous studies on surface-atmosphere interaction have been performed, there has been less investigation into ABL modification by heterogeneous surfaces on various scales.

By using a large eddy simulation (LES), Song-Lak Kang investigated ABL responses to surface heterogeneity on the scales ranging from a few kilometers to a few hundred kilometers. He found that ABL changed from in a quasi stationary state to in a non-quasi stationary state due to the temporal oscillation onset of surface-heterogeneity-induced horizontal flows (Fig. 1).

Figure 1
Figure 1: The time evolution of the magnitude of the generated horizontal velocity at 95 m above ground level for cases with the surface sensible heat flux variation amplitudes of 50, 125, and 200 Wm-2 at the variation wavelength of 32 km.

Theoretically, with the Navier-Stokes equation and potential temperature equation that are low-pass filtered with a mesoscale wave cutoff filter, the temporal oscillation onset of the generated horizontal flows was explained. Also he found that, in the ABL transition, energy cascades from the surface heterogeneity scale to smaller scales (Fig. 2).

Figure 2
Figure 2: One-dimensional horizontal velocity spectra at 95 m above the ground level at the times of 70, 115, 160, 205, 250, and 295 min for case with the surface sensible heat flux variation amplitudes of 200 Wm-2 at the variation wavelength of 32 km. The temporal oscillation onset occurs at 160 min.

The surface heterogeneity on a scale of tens of kilometers more likely generates mesoscale fluctuations that are temporally oscillating and cascade from the heterogeneity scale down to the ABL turbulence. These results raise a question about the feasibility of mesoscale model (MM) using an ABL parameterization, which is built on the horizontal homogeneity assumption, in investigating the atmosphere over extremely heterogeneous land surfaces.

Song-Lak Kang focuses on the generalization of the previous research results: the temporal oscillation onset of the generated horizontal flows and the downscale energy cascade in the ABL over heterogeneous surfaces. In the LES experiment, the diurnal cycle of surface flux variation (Fig. 3) and the Coriolis force are considered. Also, multiple wavelengths of surface flux variation are used. The LES results are compared with the MM results with the same surface and atmospheric conditions. The practical purpose of this research is to develop a better methodology to deal with surface-atmosphere interactions in an MM system.

Figure 3
Figure 3: Sensible heat flux variation prescribed in the LES experiment.

ASP Spotlight February 2009
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