A Simple Method for the Assessment of the Cloud Cover State in High-Latitude Regions by a Ground-Based Digital Camera

Abstract

This work describes the development of a simple method of field estimating the sky cloud coverage percentage for several applications at the Brazilian Antarctic Station, Ferraz (62°05′S, 58°23.5′W). The database of this method was acquired by a digital color camera in the visible range of the spectrum. A new algorithm was developed to classify each pixel according to a criteria decision process. The information on the pixel contamination by clouds was obtained from the saturation component of the intensity, hue, and saturation space (IHS). For simplicity, the images were acquired with a limited field of view of 36° pointing to the camera’s zenith to prevent direct sunlight from reaching the internal charge-coupled device (CCD) on the camera. For a priori–classified clear-sky images, the accuracy of the method was superior to 94%. For overcast-sky conditions, the corresponding accuracy was larger than 99%. A comparison test was performed with two human observers and our method. The results for the 29 images collected for several time of days during 50 days in 1999 summer were compared to visual observations of these same digital images by two trained field meteorologists. Correlation coefficients between human observers and the automatic method ranged from 0.84 for clear-sky conditions, and the lowest was 0.09 for undefined-sky conditions.

Coupling sky images with radiative transfer models: a new method to estimate cloud optical depth

“…Other ground-based sky imaging designs have also been developed (Seiz et al, 2007;Souza-Echer et al, 2006;Calbo and Sabburg, 2008;Cazorla et al, 2008;Heinle et al, 2010;Román et al, 2012;Gauchet et al, 2012) with the most dissimilar design consisting of a downward-pointing camera capturing the sky from a reflection off a spherical mirror (Pfister et al, 2003;Kassianov et al, 2005;Long et al, 2006;Mantelli et al, 2010;Martínez-Chico et al, 2011). Most ground imaging devices follow a relationship between the camera's signal and radiance similar to Eq.…”

A importância dos dados de cobertura de nuvens e de sua variabilidade: metodologias para aquisição de dados

“…Essas discrepâncias confirmam que a descrição visual da cobertura de nuvens, embora amplamente aceita,é um método bastante subjetivo. A fração de indeterminação mostrou a melhor correlação entre os resultados apresentados pelos observadores de 0,84 [36]. Alguns fatores que podem ter contribuído para uma boa correlação entre as avaliações dos observadores.…”

“…As nuvens apresentam alta refletância e tons pastéis, o que revela a mistura de vários comprimentos de onda da luz visível. Por outro lado, o céu apresenta uma alta saturação revelando um padrão menos variado de comprimentos de onda na sua composição -"uma cor mais pura" [36,46].…”

Multilayer Perceptron Neural Networks Model for Meteosat Second Generation SEVIRI Daytime Cloud Masking

“…In the literature, various methods have been proposed for the detection and the classification of clouds using visible/infrared (VIS/IR) imaging and most of them rely on a combination of threshold tests applied to different spectral channels [7][8][9][10][11][12], for example multispectral thresholding models [9,[11][12][13][14] or simple intensity threshold analysis and IHS threshold technique [15][16][17] applied to individual pixels. The other models are spatial coherence methods [18,19] or dynamic cloud clustering based on histogram analysis [20,21].…”