황사발생량 예측을 위한 임계마찰속도 산정 방법 개선 = Improvement of the Method of Determination of Threshold Friction Velocity for Wind-blown Dust Emission Estimation

국문 초록 (Abstract)

먼지발생 모델링은 지표면 근처의 바람의 세기가 임계값을 넘어설 때 먼지가 발생한다고 가정하며, 이 임계값은 흔히 임계마찰속도로 표현된다. 따라서, 황사발생량을 정확히 예측하기 위해서는 임계마찰속도를 정확하게 예측하는 것이 무엇보다도 중요하다. 임계마찰속도는 바람에 의한 먼지 발생을 억제하는 정도를 나타내는 값으로서 토질, 토지피복, 수분함유량과 같은 토양 특성에 따라 달라진다. 황사의 발생과 수송을 모의하는 대기질 모델은 수십 km 크기의 수평 격자들로 이루어지며, 이럴 때 한 지표면 격자에는 여러 가지 토지피복이 혼합돼 있는 것이 일반적이기 때문에, 보통 각토지피복이 차지하는 넓이를 가중인자로 하는 기하평균을 각 격자의 대표 거칠기 길이로 사용하여 마찰속도를 결정하게 된다. 그러나, 본 연구에서 살펴본 바에 따르면, 이렇게 평균 거칠기 길이를 사용하여 모의할 경우, 맨땅이 주요 토지피복이 아닌 격자에 대해서는 지표면 거칠기 길이와 임계마찰속도가 지나치게 크게 계산됨으로써 그 격자 전체에서의 먼지 발생이 불가능한 것으로 나타나는 오류가 있다. 이러한 문제를 해결하기 위해서 본 연구에서는 격자 크기를 무한히 작게 가져가는 이상적인 경우를 살펴보고, 이를 대형 격자 구조에서 구현하는 방법으로 한 격자의 대표 거칠기 길이를 전체 토지피복 범주에 대한 평균이 아닌 맨땅에 대한 거칠기 길이로 정하되 먼지 발생 면적을 그 격자에서 맨땅이 차지하는 분율로 제한하는 방식을 제안하였다. 개선된 알고리즘에 따라 2006년 4월에 발생했던 황사 기간에 대해 평균 지표면 거칠기 길이와 임계마찰속도의 분포를 모의하였다. 임계마찰속도를 구하는 데 필요한 토양 수분함유량을 추정하기 위해 기상 모델 MM5를 사용하였다. 얻어진 평균 지표면 거칠기 길이와 임계마찰속도의 분포는 합리적인 결과를 보였으며, 특히 모의된 임계마찰속도는 관측결과를 통계적으로 분석한 선행 연구의 결과와 비교적 잘 일치하였다.

다국어 초록 (Multilingual Abstract)

Wind-blown dust is emitted when near-surface wind speed exceeds a threshold value, which is often expressed as the threshold friction velocity. Therefore, it is important to estimate the threshold friction velocity accurately for reliable wind-blown dust emission forecast. Threshold friction velocity represents the erodibility of the surface soil and depends on surface characteristics such as land-cover, soil texture, and soil moisture content. Typical air quality models in which a wind-blown dust emission module is incorporated use horizontal grid size of tens of kilometers and each grid consists of several different land-cover categories. Thus, the representative surface roughness length is generally determined as the geometric mean of those land-cover categories for computing the threshold friction velocity. According to the assessment of this study, however, the average surface roughness length leads to too high threshold friction velocity for the entire grid and unreasonably disable the dust emission from barren land portion of the grid. By comparing with an ideal case in which the grid size is infinitesimal, we suggest that the representative surface roughness length should be determined not by average of all land-cover categories but by barren land only and the fraction of barren land should be multiplied when the dust emission amount is calculated in a large grid system. The improved algorithm was applied to modelling of surface roughness length and threshold friction velocity over the wind-blown dust emission areas for an Asian dust storm event that occurred in April 2006. The meteorological model MM5 was used to estimate the soil moisture content required to calculate the threshold friction velocity. The model-estimated surface roughness length and threshold friction velocity were reasonable. In particular, the modelled threshold friction velocity was in good agreement with a previous study that analyzed the observations statistically.

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