Case studies of water supply in Oceanic extratropical cyclones using an Eulerian-Lagrangian Method
Authors:   , Liu Gongbo 
Published: July 2024

An Eulerian-Lagrangian method has been developed to study the transport of water supply to precipitation in a rainstorm. Water mass in an air parcel is not conservative so the trajectory of the air parcel cannot describe the complete motion of the moisture. This method represents a rainstorm, a limited area of heavy precipitation in a three-hour period in a model, with several thousands air parcels. The water contributing to the rainstorm's precipitation from these parcels is tracked backward to recover its transport and its source regions. Exchange of moisture among these parcel by diffusion, convection and stable precipitation is computed. Water from surface evaporation that is ingested into parcels in the planetary boundary layer is computed. The moisture exchange between these parcels and other portions of the atmosphere is excluded from consideration. Water supply of two oceanic extratropical cyclones has been simulated. Fields of wind velocity, moisture and moisture sources/sinks are provided on the Eulerian grid using the Limited Area Mesoscale Prediction System for 90s (Kreitzberg and Perkey 1976, Cohen 1994). Simulations are performed over a period of 24 hours for one cyclone and 36 hours for the other. Seven rainstorms are selected during the life cycles of the two cyclones to represent different developing stages and different precipitation systems. Results show that about 80% of the water supply for identified precipitation can be tracked back 24 hours for some rainstorms and 36 hours for others. Tests on water budget computations show other errors are negligible. Transport tracks are established between rainstorm precipitation and the water source regions. They are roughly categorized as anticyclonic, cyclonic, sharp-turnings and complex motion patterns associated with horizontal origins and initial altitudes. Comparison of these tracks with the warm conveyor belt (Browning 1971, Harrold 1973) and the cold conveyor belt (Carlson 1980) conceptual models indicates similarities and differences. For the precipitation near the center of a cyclone the warm conveyor belt may exist but its quantitative contribution has been questioned. Source region analysis show up to 92% of identified precipitation of a new born cyclone originate from the northeast of the rainstorm and is transported anticyclonically. For another cyclone, 70 to 77% of identified moisture supply to the rainstorm near the low pressure center came from the south and 80% of identified moisture supply to the cold front precipitation came from the far southwest. Analyses indicate that 12 hours before the rainstorm, 73 to 93% of identified moisture supply was below 2 km, including 14 to 29% that came from the surface moisture flux. The evolution of thermal properties of the water supply to a cyclone's precipitation show that the air from the northeast of the rainstorm was in a colder and drier area and warmed and moistened when moving to the south. Diagnostic analyses show impressively high precipitation efficiencies and short time scales during intense precipitation periods.