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Novel Fog Research Method Could Help Predict Forests’ Drought Risk

Spatial and Temporal Patterns of Cloud Cover and Fog Inundation in Coastal California: Ecological Implications

A new way to track clouds over space and time could be a valuable tool in researching water availability in redwood forests, which rely on fog. Photo by Stephen Sillett
A new way to track clouds over space and time could be a valuable tool in researching water availability in redwood forests, which rely on fog. Photo by Stephen Sillett

Fog along the California coast helps redwood forests thrive. It lowers temperatures and reduces evaporation. Moisture from fog droplets coalesce on plant leaves and eventually drips to the soil, becoming an important source of water for young trees and other understory plants. Some plants can even absorb water directly through their leaves in a process called foliar uptake.

Across coastal California, a thick layer of fog blankets the coast in early mornings. It rolls in from the ocean at night, and usually dissipates each day by early afternoon. Fog is defined as a cloud at ground level. Low-hanging stratocumulus clouds, characterized by many dark, round masses, also provide relief from drought and temperature stress to coastal forests.

In a paper published in January 2016, researchers at the University of California, Santa Barbara, and Oregon State University outline a new way to track clouds (and fog) over space and time. They carried out this technique over California’s northern Channel Islands, an archipelago near Santa Barbara. Similar to the coast redwood ecosystem, many species on the Channel Islands once had geographic ranges that were widespread in the past, when the planet was cooler and wetter than it is today. On the islands, clouds are assumed to be ecologically significant; according to previous research, for example, summer cloud cover was the most important factor in determining bishop pine deaths during a drought. In fact, this southernmost population of bishop pine is considerably more vulnerable to climate change than populations in northern California, and thus more reliant on cloud cover and fog immersion to mitigate water stress.

Often, to study fog and clouds, researchers use images taken by a NASA satellite-based sensor named the Moderate Resolution Imaging Spectroradiometer, or MODIS. The problem, however, is that MODIS only collects two images during the daytime, which isn’t that helpful for tracking clouds’ movements, especially since clouds formed in the evening and overnight can often burn off before the MODIS images are captured. Alternatively, NOAA’s Geostationary Operational Environmental Satellite, or GOES, takes pictures every half hour. But the quality of each image is rough, and the spatial resolution is coarser than MODIS. Importantly, neither GOES nor MODIS can measure the height of clouds either, which would help indicate whether low-lying clouds are above the land or are in contact with the surface, i.e., they are fog. It’s critical to know whether and how often clouds are in contact with vegetation to fully understand questions like whether fog drip or foliar uptake might be occurring.

To work around these limitations, the researchers took a novel approach, combining the GOES satellite imagery with three other elements: First, they measured cloud ceiling height at nearby airports, and deduced cloud top heights from weather balloons called radiosondes that are launched daily by the National Weather Service. They then combined these datasets with a digital elevation model (a 3-D map of the landscape) to deduce where clouds actually swept over the land as fog. Combining the information gathered from these three techniques, the researchers were able to track clouds over shorter time periods and at much higher spatial resolution.

The team used GOES images captured between 2001 and 2012. From each image they extracted albedo (or brightness) values, calculated the frequency of cloud cover for each day, and eventually turned those numbers into monthly averages. They then used the airport and weather balloon data to determine how frequently that cloud cover touched land and could be considered fog.

Although this particular study focused on fog patterns on the Channel Islands, the method could be extended elsewhere, including coast redwood forests, which rely on fog during the dry summer months. In general, by seeing when and where fog comes into contact with an ecosystem, researchers can begin to uncover meaningful patterns between fog and flora. By knowing exactly where fog hits the ground, for example, researchers might estimate how much fog drip is entering the ecosystem and which areas of the forest are receiving sufficient water. Since this method yields information at a fine scale, it could be enormously helpful in predicting which forests are most at risk during a drought and in the face of climate change.

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