Stereographic methods for cloud base height determination using two sky imagers

Abstract

Intra-hour solar generation and ramp forecasting with sky imagers benefits from fast, frequent and accurate cloud base height (CBH) calculation. Using two sky imagers, CBH is determined using a two dimensional (2D) method for single homogeneous cloud layers and an enhanced three dimensional (3D) method to provide CBH of the cloud field with high resolution. The 2D method is based on georeferenced projection and statistical correlation of two sky images from the pair of sky cameras. The 3D method utilizes the epipolar technique to determine CBH for each cloud pixel. The 2D method was validated against ceilometer CBH measurements on four days. Several technical considerations for the set up of the sky cameras for CBH calculation, as well as strength and weaknesses of the proposed methods are discussed.