Abstract
Abstract. The mountain cloud forest of Taiwan can be delimited from other forest types using a map of the ground fog frequency. In order to create such a frequency map from remotely sensed data, an algorithm able to detect ground fog is necessary. Common techniques for ground fog detection based on weather satellite data cannot be applied to fog occurrences in Taiwan as they rely on several assumptions regarding cloud properties. Therefore a new statistical method for the detection of ground fog in mountainous terrain from MODIS Collection 051 data is presented. Due to the sharpening of input data using MODIS bands 1 and 2, the method provides fog masks in a resolution of 250 m per pixel. The new technique is based on negative correlations between optical thickness and terrain height that can be observed if a cloud that is relatively plane-parallel is truncated by the terrain. A validation of the new technique using camera data has shown that the quality of fog detection is comparable to that of another modern fog detection scheme developed and validated for the temperate zones. The method is particularly applicable to optically thinner water clouds. Beyond a cloud optical thickness of ≈ 40, classification errors significantly increase.
Highlights
Cloud forests are tropical and subtropical forest ecosystems characterized by the frequent occurrence of ground fog conditions (Bruijnzeel et al, 2010)
The height of the cloud base is compared to a digital elevation model (DEM)
As they are not suited for low clouds or only work under certain conditions, they cause different problems when applied to ground fog detection over mountainous areas
Summary
Cloud forests are tropical and subtropical forest ecosystems characterized by the frequent occurrence of ground fog conditions (Bruijnzeel et al, 2010) As they intercept water from cloud droplets and, due to their mostly wet canopy, have a decreased rate of transpiration, they play an important role as an ecosystem service provider increasing local water supplies (Mildenberger et al, 2009). As the occurrence of cloud forest depends on the heavy influence of ground fog conditions, it has been shown by Mulligan and Burke (2006) that it can be discriminated from other forest types by the application of a threshold on maps of the ground fog frequency.
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