Structural Evolution of a Warm Frontal Precipitation Band During the GPM Cold-Season Precipitation Experiment (GCPEx)

Precipitation bands within the comma head of extratropical cyclones are important for quantitative precipitation forecasting, since they results in heterogeneous spatial and temporal precipitation rates. There has been a lot of focus recently on relatively large precipitation bands (snow bands) within the comma head forced by mid-level frontogenesis, as well as cold frontal bands, but there has been much less work on warm frontal precipitation bands. The goal of this study is to document the detailed evolution of a warm frontal band observed during the Global Precipitation Mission Cold-Cold-Season Precipitation Experiment (GCPEx), which occurred from 17 January to 29 February 2012 in Ontario, Canada. The GCPEx experiment used instrumented aircraft (NASA DC-8, University of North Dakota Cessna Citation, and Canadian Convair 580) to collect radar and in situ microphysics, while surface-based horizontal (C and Ka-KU Dual Pol bands) and vertical pointing radars (MRR and X-band) as well as surface distrometers collected high temporal precipitation and microphysical data.

On 18 February 2012 a surface warm front was approaching southern Ontario ahead of a relatively weak cyclone. A well-defined warm frontal precipitation band developed just north of the surface front, which intensified and narrowed rapidly as it moved northward. Initially, the band was fairly broad and was associated with dry snow with little super-cooled water. An hour or two later by its mature phase there was a narrow plume of more aggregated snow, with significant riming on the south side of the band. There were also numerous small-scale gravity wave perturbations near the band and convective cells aloft as revealed by the high-resolution radar data. This presentation will summarize the structural evolution of this band as well as some factors that led to its intensification, which includes the development of a low-level jet, mid-level dry intrusion, and frontogentical forcing. This event was also simulated down to 1-km grid spacing using the Weather Research and Forecasting (WRF) model. The WRF was able to realistically simulate the warm frontal band development and some of its intensification characteristics, so the simulation is used to complement the observations with understanding the evolution of this case.