Changes of Water Budget and Precipitation Efficiency of Typhoons Nari (2001) and Morakot (2009) during Landfall on Taiwan

Typhoons Nari (2001) and Morakot (2009) are among the top five typhoons in terms of 24-h accumulated rainfall on Taiwan in the past 50 years. These two tropical cyclones (TCs) had very different characteristics of their rainfall patterns, their tracks relative to Taiwan topography, and their background synoptic-scale environment. If similar physical mechanisms contributing to the heavy orographic rainfall by vapor- and water-budget calculations can be found for the two distinct TCs, results from the water budget and precipitation efficiency (PE) analyses might be general enough to be applied to other landfalling TCs on Taiwan as well. Typhoon Nari (2001) is chosen to represent a small and compact (axisymmetric) TC and Typhoon Morakot (2009) is chosen for a large and asymmetric TC, respectively. The observed 72-h accumulated rainfall reached 1430 mm over northeastern Taiwan for Nari and 2780 mm over southwestern Taiwan for Morakot. Both TCs are simulated with numerical models nested down to cloud-resolving grid sizes (2 km for Nari and 1 km for Morakot). Vapor and water condensate budgets, formulated following our previous studies, are conducted using model output at 2-min time intervals for 2-hour periods (1 hour prior to landfall and 1 hour after landfall on Taiwan). The PE, defined from either a large-scale or microphysics perspective, in Nari increased by 10–20% after landfall, in agreement with the enhanced surface rainfall over the rugged terrain on Taiwan. For Morakot, the PE was highly correlated with the surface rain rate; in particular, the PE reached more than 95% over southern Taiwan when the surface rain rate was greater than 50 mm/h. In a sensitivity experiment with no Taiwan terrain, the PE of Morakot for the same area was reduced to 60–75%. Despite the different tracks and storm structures, a common feature of the water budgets between Nari and Morakot was that after landfall on Taiwan, both TCs had enhanced water-vapor convergence, increased PE and greater surface precipitation. For a local area (with a size of 50 km by 100 km) on the steep terrain over southwestern Taiwan, the PE of Morakot remained 75–100% for 6 hours, producing torrential rainfalls and severe land- and mudslides, and this may explain why the storm produced a near world record of accumulated rainfall on Taiwan.