Analysis algorithm for sky type and ice halo recognition in all-sky images

Abstract

Abstract. Halo displays, in particular the 22∘ halo, have been captured in long time series of images obtained from total sky imagers (TSIs) at various Atmospheric Radiation Measurement (ARM) sites. Halo displays form if smooth-faced hexagonal ice crystals are present in the optical path. We describe an image analysis algorithm for long time series of TSI images which scores images with respect to the presence of 22∘ halos. Each image is assigned an ice halo score (IHS) for 22∘ halos, as well as a photographic sky type (PST), which differentiates cirrostratus (PST-CS), partially cloudy (PST-PCL), cloudy (PST-CLD), or clear (PST-CLR) within a near-solar image analysis area. The color-resolved radial brightness behavior of the near-solar region is used to define the discriminant properties used to classify photographic sky type and assign an ice halo score. The scoring is based on the tools of multivariate Gaussian analysis applied to a standardized sun-centered image produced from the raw TSI image, following a series of calibrations, rotation, and coordinate transformation. The algorithm is trained based on a training set for each class of images. We present test results on halo observations and photographic sky type for the first 4 months of the year 2018, for TSI images obtained at the Southern Great Plains (SGP) ARM site. A detailed comparison of visual and algorithm scores for the month of March 2018 shows that the algorithm is about 90 % reliable in discriminating the four photographic sky types and identifies 86 % of all visual halos correctly. Numerous instances of halo appearances were identified for the period January through April 2018, with persistence times between 5 and 220 min. Varying by month, we found that between 9 % and 22 % of cirrostratus skies exhibited a full or partial 22∘ halo.

Revealing halos concealed by cirrus clouds

“…An attempt is also being made to achieve automated observations of halos using a sun-tracking camera (Forster et al, 2017) or a total sky imager (Boyd et al, 2019). Image processing techniques can be used to observe even faint halos or other atmospheric optical phenomena in photographs more clearly, which can greatly aid in these types of studies (Riikonen et al, 2000;Moilanen and Gritsevich, 2022;Großmann et al, 2011).…”

Revealing Halos Concealed by Cirrus Clouds

“…Observing halos from the ground is an important way to understand meteorological processes. An attempt is also being made to achieve automated observations of halos using a sun-tracking camera (Forster et al, 2017), or a Total Sky Imager (Boyd et al, 2019). A halo is formed by the interaction of sunlight with the particles in a cloud.…”

Ice crystal characterization in cirrus clouds II: radiometric characterization of HaloCam for the quantitative analysis of halo displays

“…To protect the all-sky imagers from direct sunlight and to reduce stray light effects, a shadow band (WSI) or sun-blocking strip (TSI) is usually employed. These strips, however, cover a significant part of the 22 • halo (Boyd et al, 2019). Forster et al (2017) presented HaloCam, a weather-proof camera system for the automated observation of halo displays, using a sun-tracking mount.…”