Cloud Detection Algorithm for Hydrometeorological Applications Using
Computer Vision Techniques and a Ground-Based Whole-Sky Camera Network
Abstract
The Aburrá Valley is a narrow highly complex mountainous terrain located
in the Colombian Andes. Due to topographical features of the region, and
the tropical setting, the meteorological variability is very high and in
specific periods of the year limit the atmospheric pollutant vertical
dispersion, resulting in high concentrations within the valley. The
presence of prevalent low-level clouds in these periods reduce incoming
solar radiation to the surface thus diminishing surface sensible heat
flux to the lower atmosphere. Therefore, the spatial distribution and
temporal variability of cloud coverage play a crucial role in the
surface energy balance in the region. Cloud variability is also relevant
to study the local hydrological cycle and long-term climate variability
and change. The most widespread techniques for cloud observations are
human observations, which strongly depends on the objectivity and
commitment of the observer, and satellite observations, that has the
disadvantage of having a relatively low temporal and spatial resolution
for some applications. This research focuses on the implementation of an
operational system for in-situ clouds detection based on a ground-based
network of whole-sky visible cameras. The operational system uses
computer vision techniques and image classification algorithms such as
Support Vector Machine (SVM), K-Nearest Neighbors (KNN) among others to
obtain. The algorithm starts with a group of training images with set
attribute tags such as cloudy and cloud-free skies, to identify and
calculate the percentage of clouds coverage in untagged images. The
methodology then projects the images from the natural polar coordinates
system of whole-sky cameras to 2D cartesian coordinates. Following the
reprojection, overlapping images from each camera are combined using a
panorama stitching technique to generate a single regional cloud
fraction map. Clear-sky combined direct and indirect incoming solar
radiation are adjusted using the regional cloud map to generate an
estimate of the spatial distribution of cloud-forced incoming solar
radiation. Cloud height from ceilometers and in-situ pyranometer
measurements of incoming solar radiation provide the additional required
information to generate the radiation maps.