A Comparative Study of Surface Energy Budget on the Grass and the Concrete Surface

Abstract

지표 특성에 따른 기온의 변화는 열적이고 역학적인 에너지 변동의 결과이며, 이는 더욱 세분화된 요소들의 유기적인 관계에 의해서 이루어진다. 본 연구에서는 도심지에서 점유율이 가장 높은 콘크리트와 공원지역을 대표하는 잔디를 대상으로 고정밀 난류계측기기를 이용하여 겨울철의 지표층 내 기온과 더불어 에너지 플럭스 성분을 직접적으로 관측하였다. 난류성분 및 M-O 상사 이론에 의한 미기상학적 변수는 지표특성에 따른 대기상태의 변화를 파악하는데 이용되었다. 그 결과를 통해서 우리는 에너지 플럭스의 특징적인 변동을 규명하고 콘크리트와 잔디 표면 위의 겨울철 에너지 수지 변화에 대한 특성을 세분화하여 정량적으로 분석하였다. 잔디와 콘크리트의 지표 특성차이는 각 지표의 에너지 수지의 변동에 차별적인 영향을 주었다. 주간에 잔디 표면 위의 대기 상태는 불안정하였고 야간에는 중립 혹은 안정한 상태를 보임으로써 주·야간의 안정도 변화가 뚜렷하였다. 주간에 콘크리트 표면 위의 대기 상태는 중립 혹은 안정한 상태에 가까웠고, 야간에 약한 불안정의 대기상태를 확인할 수 있었다. 잔디의 주간 에너지 수지의 50% 이상은 현열 플럭스의 영향이었고, 콘크리트의 야간 에너지수지의 약 80% 이상은 토양열 플럭스의 영향이었다. 즉, 주간에 겨울철 잔디 표면 위의 열적인 에너지 수송은 콘크리트 표면 보다 활발하게 이루어짐으로써 지표면 에너지 수지에서 식생의 효과는 크게 반영되지 못함을 확인할 수 있었다. 그리고 콘크리트 위의 열적 수송의 효과는 주간과 야간에 걸쳐서 지표 특성에 의해 크게 억제되는 경향을 보였고, 주간 열 플럭스의 대류효과는 순복사 에너지의 50% 이상을 차지하는 열 저장량에 의해서 억제되는 경향을 보였다. 지표특성에 따른 에너지 수지의 특징적인 변화에 의해서 콘크리트 위의 기온은 주간과 야간에 각각 약 2와 1로 잔디 위의 기온보다 항상 낮게 분포함을 확인할 수 있었다. 본 연구의 결과는 콘크리트의 토지피복 비중이 높은 도심지의 겨울철 열환경 평가와 개선에 유의한 지표로 이용될 수 있을 것이다.

In orderto analyze the effect of surface characteristics for the grass and concrete on the energy budget, we observed the energy budget for both surfaces reflecting urban opposing surface characteristics. We conducted experiments to understand the characteristics of energy budget on the grass and concrete according to high-resolution observations of energy fluxes and standard meteorological observations. Differences in surface characteristics have a discriminatory effect on variations in the energy budget. From analysis results of micro-meteorological parameters related to turbulence components and M-O similarity parameters, the diurnal and nocturnal atmospheric conditions above the grass are unstable and stable respectively. The diurnal dimensionless wind and temperature gradients for the grass are larger than for concrete, and 직선(w'T')/u_(*) clearly varies with theday and nighttime. The atmospheric condition above the concrete shows characteristics contrary to the grass. The atmospheric condition is nearly stable during the day, but nocturnal atmospheric conditions are weak and unstable. The diurnal dimensionless wind and temperature gradients are stronger for the grass than for the concrete, but the nocturnal gradients occurred more strongly on the concrete due to the unstable atmospheric condition. The variation of 직선(w'T')/u_(*) for the concrete is weaker than for the grass, and u_(*) for the concrete is much higher than for the grass. The results of the parameterized fluxes express the relative importance of each energy flux in the energy budget according to the differences in surface characteristics well. Above 50% of the diurnal energy budget for the grass is affected by the sensible heat flux, while the nocturnal budget is affected by both the soil and sensible heat fluxes. The diurnal sensible and soil heat fluxes for the concrete have equal effects of about 20.30% on the energy budget, and above 80% of nocturnal energy budget is affected by the soil heat flux. From the variation in turbulence components and micro-meteorological parameters due to surface characteristics, we found that the thermal energy transfer from turbulence incurs activity. On the other hand, the thermal energy transfer is mitigated on the concrete and we found that the energy transfer is affected by the dynamic momentum on the concrete. The characteristics of the observed soil heat flux in this study are very significant in the analysis of urban thermal environments related to the surface energy budget of concrete. The diurnal and nocturnal soil heat fluxes shows variations ranging -50 to 50 Wm^(-2) for the grass and greatly varies from -100 to 150 Wm^(-2) for concrete. This result means that the surface thermal energy transported underground by the soil heat flux for the concrete is as much as that for the grass, and the thermal energy gradient decreases rapidly since the thermal energy transfer is strongly restricted. From these results, we can explain that the atmospheric condition is stable over the concrete; we analyze the cause as the sensible heat flux for the concrete decreasing more rapidly than for the grass since H/R_(N) of the concrete is smaller than of the grass. Above 80% of nocturnal energy budget is affected by the soil heat flux for the concrete. This means that the thermal energy transport activity from the underground to the surface is at night. The nocturnal sensible heat flux changes in a negative range as a result of the temperature difference between the surface and air and is caused by surface radiation cooling. However, much of the thermal energy transported from underground to the surface causes a temperature rise in the concrete surface and the temperature gradient between the concrete surface and air strongly decreases. The above results reflect well the fact that the concrete surface and air temperatures are almost the same and explain that the cause is that the sensible heat flux is close to zero on the concrete. We found that the effect of the vegetation canopy is very weak on the surface energy budget, as the diurnal thermal energy transfer on the grass is more active than on the concrete. The effect of thermal transfer on the concrete is mitigated by surface characteristic differences between day and nighttime. The convective heat flux is mitigated by heat storage, which affects over 50% of the net radiation during the day. The air temperatures observed at the same time from September 11 to October 2, 2006 on the grass and concrete reflect the energy budget according to surface characteristics well. We expect that the results of this study can be used in the estimation and improvement of urban thermal environments in winter seasons. An automatic weathers ystem (AWS) of about 500 stations is used nationwide. Among them, about 60% are set up over the concrete surface of building roofs. The U.S.EPA (1976) suggests that the observation height must be 10 m above the concrete surface when the temperature is observed for paved concrete and asphalt areas. The results reflect AWS data observed for grass and concrete and include errors on the differences in surface characteristics. We expect that the results of this study can be used to correct observation errors occurring due to differences in surface characteristics.