Forecasting and Ground-based Remote Icing Detection System (GRIDS) documentation of the transition from a glaciated environment to an SLD icing environment

On 11 November 2003, a striking transition in cloud composition moved across the AIRS II field program suite of instruments, based at Montreal's Mirabel Airport. Deep, glaciated clouds present during the morning hours provided little, if any icing threat. Later in the day, dry conditions advected in from the west, separating the upper-level clouds responsible for significant snow production from a lower layer of clouds. The lower clouds were being lifted over a warm frontal surface, resulting in warm-topped, stably-stratified, water dominated cloud at subfreezing temperatures. These clouds featured significant water contents and supercooled large drops (SLD) with diameters exceeding 300 microns, resulting in moderate and even moderate-to-severe icing conditions. This paper presents the meteorological conditions and the forecast that accurately predicted the transition from glaciated to SLD conditions. The Ground-based Remote Icing Detection System (GRIDS) documented the fine-scale structure of the clouds over the Mirabel field site throughout this period, and found telltale signatures of SLD after the passage of the dry slot. The NASA-Glenn Twin Otter research aircraft sampled both the glaciated and icing environments above GRIDS during two flights, while the NRC Convair and NCAR C-130 research aircraft completed coordinated samples of the icing environment with the Twin Otter, flying in a stacked formation.