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

Professor Robert Houze Jr. pioneered work during the 1970s-80s investigating precipitation bands within extratropical cyclones using aircraft and radar data along the Washington coast. This helped lead to a conceptual model of a warm frontal precipitation band that is nearly parallel to the warm front, which is forced by low-level warm advection and frontogenesis and there a strong connection with this band to generating cells aloft. Warm frontal bands are generally fairly weak, but on 18 February 2012 an intense precipitation band associated with a relatively weak warm front occurred during the Global Precipitation Measurement (GPM) Mission Cold-season Precipitation Experiment (GCPEx) over southern Ontario. The warm frontal precipitation band went through genesis, maturity, and decay over a 5-6 hour period. The Weather Research and Forecasting (WRF) model nested down to 1-km grid spacing was able to realistically predict the precipitation band evolution, albeit somewhat weaker and slightly further south than observed. Band genesis began in an area of precipitation with embedded convection to the north of the warm front in a region of weak frontogenetical forcing at low-levels and a weakly positive to slightly negative moist potential vorticity (MPV*) from 900-650 hPa. A mid-level dry intrusion helped reduce the mid-level stability, while the precipitation band intensified as the low-level frontogenesis intensified in a sloping layer with the warm front. Aggregates of unrimed snow occurred within the band during early maturity, while more supercooled water and graupel occurred as the upward motion increased due to the frontogenetical circulation. As the low-level cyclone moved east, the low-level deformation decreased and the column stabilized for vertical and slantwise ascent, and the warm frontal band weakened. A WRF experiment turning off latent heating resulted in limited precipitation band development and a weaker warm front, while turning off latent cooling only intensified the frontal precipitation band as additional mid-level instability compensated for the small decrease in frontogenetical forcing.