Evaluating the Frontal Influence on Convection Using Parcel Trajectories

Record-breaking rainfall events occurred along the East Coast in September 2011 and October 2015 in association with a coastal front and elevated convection. In both events, the synoptic environment consisted of a tropical cyclone in the Atlantic and an extratropical cyclone over the Eastern United States, resulting in a deformation zone and concentrated flow of moisture over the stationary front along the East Coast. Significant elevated convection brought more than 250 mm (10 inches) of precipitation in 24 hours to the coastal states. Preliminary research indicated that the cyclones contributed to the location of heavy precipitation, while the coastal front was the main contributor in the initialization and maintenance of convection in both cases. In either case, the tropical cyclone never made landfall and the coastal states still received life-threatening flooding. Thus, an investigation into the coastal front is necessary to further understand the cause of the copious amounts of rainfall. The Weather Research and Forecasting (WRF) – Advanced Research WRF (ARW) model was used to create control simulations of both events. Parcel trajectories were created using Read/Interpolate/Plot (RIP), which allows for further evaluation of the parcel environment along the trajectory. Backward trajectories beginning in the region of heaviest precipitation display the moisture flow in the synoptic environment originating in the mid-Atlantic. Cross-sections along the trajectory give the location of the coastal front, while thermodynamic profiles and diagnostics show the environmental changes for the parcel as it approaches and crosses the frontal boundary to evaluate the strength of the front and elevated convection. This methodology allows for the assessment of the region where convection initiates to verify the dominant forcing mechanism, which for both cases is the frontal boundary lifting parcels to the level of free convection (LFC).