Abstract
Abstract. An observational and real-time model forecast study of the genesis of Typhoon Nuri during the Tropical Cyclone Structure 2008 (TCS-08) field campaign in the western North Pacific sector is presented. Analysis and observational data show that the surrounding base state is an easterly trade wind flow and the precursor disturbance to Typhoon Nuri is an easterly wave that originates in the ITCZ in the Central Pacific. This disturbance can be tracked more than 10 days prior to tropical storm formation. An overview of the field data is presented here using a newly proposed dynamical framework for tropical cyclone formation within the critical layer of an easterly wave. Despite propagating through a hostile environment ripe with strong vertical wind shear and relatively dry air, the easterly wave critical layer protects the proto-vortex and allows it to gestate until it reaches a more favorable environment. Within this protective "Kelvin cat's eye flow" located within the wave's critical layer existed a sweet spot, defined as the intersection between the wave trough and critical latitude, which is the preferred location for tropical cyclogenesis. Global Forecast System Final Analyses and IR satellite imagery, which shows convective bands wrapping around the sweet spot as genesis nears, confirm that this sweet spot is the location where Typhoon Nuri's dominant low-level circulation emerges. United States Air Force C130 and Naval Research Laboratory P3 research flights on 16 and 17 August collected flight-level, dropwindsonde, and Doppler radar data that allowed an evaluation of the dynamic and thermodynamic processes within the cat's eye circulation. The dropwindsonde analyses identifies the precursor easterly wave disturbance on 16 August and identifies an area of weak low-level cyclonic circulation on 17 August. Real-time forecasts were produced using operational global prediction model data to support scientific missions during TCS-08. These forecasts were found to be useful in flight planning discussions and predicted Typhoon Nuri's eventual genesis latitude within 1.5 degrees 72 h in advance.
Highlights
The western North Pacific region is the most active tropical cyclone (TC) basin on the earth, responsible for, on average, 26 of the 84 named storms annually (Neumann, 1993)
The ultimate origin of the pre-typhoon Nuri disturbance is beyond the scope of this paper, the CIMMS morphed TPW analysis (Fig. 3) suggests that it originated from a “roll-up” episode in the central Pacific Inter-tropical Convergence Zone (ITCZ) on 12:00 Z 7 August
Whether this roll-up is caused by an underlying hydrodynamic instability of the ITCZ (Ferreira and Schubert, 1997; Wang and Magnusdottir, 2006) or a nonlinear evolution of a nearly-neutral moist Rossby wave/vortex on the northern side of the ITCZ (DMW09) is unknown and beyond the scope of the current investigation
Summary
The western North Pacific region is the most active tropical cyclone (TC) basin on the earth, responsible for, on average, 26 of the 84 named storms annually (Neumann, 1993). Numerous synoptic-scale patterns have been identified as favorable to tropical cyclogenesis over the years, including, but not limited to: Tropical Upper Tropospheric Troughs (TUTT) (e.g., Sadler, 1976, 1978), formation within a monsoon trough regime (e.g., Holland, 1995) including the special cases of monsoon depression formation (Harr et al, 1996) and monsoon gyre formation (Lander, 1994), wave energy dispersion from pre-existing depressions (Carr and Elsberry, 1995; Holland, 1995), filamentary intrusions from the subtropics (Deleon, 2008), mixed Rossby-gravity waves (Dickinson and Molinari, 2002), and easterly waves (Ritchie and Holland, 1999; Fu et al, 2007). Using eight years data from the Australian Bureau of Meteorology tropical analysis scheme, they attribute 42% of the western North Pacific tropical cyclogenesis cases to monsoon shear lines, 29% to monsoon confluent regions, 18% to easterly wave disturbances, 8% to wave energy dispersion, and 3% to monsoon gyre formations. Molinari et al (2004) examine genesis cases in the western North Pacific basin and “...believe that a significant fraction, and likely the majority, of western-Pacific tropical cyclones form in association with westward moving disturbances”. Chen et al (2008), for example, have proposed that 80% of the tropical cyclone formations in this basin are due to easterly wave influences either directly or indirectly (through wave-gyre interactions)
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