Retrieving Cloud Characteristics from Ground-Based Daytime Color All-Sky Images

Abstract

A discussion is presented of daytime sky imaging and techniques that may be applied to the analysis of full-color sky images to infer cloud macrophysical properties. Descriptions of two different types of skyimaging systems developed by the authors are presented, one of which has been developed into a commercially available instrument. Retrievals of fractional sky cover from automated processing methods are compared to human retrievals, both from direct observations and visual analyses of sky images. Although some uncertainty exists in fractional sky cover retrievals from sky images, this uncertainty is no greater than that attached to human observations for the commercially available sky-imager retrievals. Thus, the application of automatic digital image processing techniques on sky images is a useful method to complement, or even replace, traditional human observations of sky cover and, potentially, cloud type. Additionally, the possibilities for inferring other cloud parameters such as cloud brokenness and solar obstruction further enhance the usefulness of sky imagers.

ReOBS: a new approach to synthesize long-term multi-variable dataset and application to the SIRTA supersite

“…In the first example, an unphysical change of 2 hPa within 1 min is observed in pressure (larger than 5 hPa during 5 min, see Table 3). In the second example, (Durr and Philipona, 2004; Cloud fraction from sky imager tot_cld_tsi Analysis of color ratio, filtering image into clear or cloudy (Long and DeLuisi, 1998;Long et al, 2006b) Cloud fraction from LW radiation cflw (7) APCADA algorithm (Durr and Philipona, 2004) Cloud fraction from SW radiation cfsw (8) Long et al (2006a) Surface upward sensible, W m −2 hfss (13) (14) Derived from fluctuations of heat and moisture covariances with respect to vertical wind velocity (Brutsaert, 1982;Panofsky and Dutton, 1984) Variances and covariances rotated to streamwise coordinate for flux computation (Kaimal and Finnigan, 1994) Corrections for sonic virtual temperature (Schotanus et al, 1983) and density correction for latent heat flux (Webb et al, 1980) Total GPS water vapor, kg m −2 iwv (19) Businger et al (1996) Liquid water content, g m −2 lwp (20) Brightness temperature at 23.8 and 31.4 GHz + input from temperature and humidity sensors (Bosisio and Mallet, 1998 several temperature spikes (0.6 • C within 1 min for ground at −5 cm) are detected and we reject the data when the increase reaches +3 • C and the decrease −4 • C within 15 min (i.e., +0.2 and −0.27 • C for 1 min resolution). The unphysical persistence in time of the measured values are detected by verifying that the variability within 1 hour is physical, following values in Table 3.…”

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.…”

Correcting for Circumsolar and Near-Horizon Errors in Sky Cover Retrievals from Sky Images

“…This problem has been recognized previously, for instance by Pfister (JAM. 2003) [2], Sabburg (ACP, 2004) [3]; and Long (JTech, 2006) [4] which briefly described the methodology presented here in broad general terms. In this *Address correspondence to this author at the Atmospheric Radiation Measurement Program Pacific Northwest National Laboratory P.O.…”