Tropical Cyclone Track and Structure Sensitivity to Initialization in Idealized Simulations: A Preliminary Study

Abstract

In the absence of environmental steering, tropical cyclone (TC) motion largely reflects "beta drift" owing to differential planetary vorticity advection by the storm's outer circulation. It is known that model physics choices (especially those relating to convection) can significantly alter these outer winds and thus the storm track. Here, semi-idealized simulations are used to explore the influence of the initialization on subsequent vortex evolution and motion. Specifically, TCs bred from a buoyant "bubble" are compared to bogussed vortices having a wide variety of parameterized shapes and sizes matching observations.As expected, the bogussed storms commencing with the strongest outer winds propagated fastest and, as a result, huge structure-dependent position differences quickly appeared. However, the forward speed variation among the initially bogussed TCs subsequently declined as a progressive homogenization harmonized the initially supplied structural differences. The homogenization likely involved model physics such as microphysics. This result casts doubt on the ability of models to retain and propagate forward information supplied at the initialization by advanced data assimilation techniques or parameterized vortex wind profiles.Asymmetries in near-core convective heating emerged as an important structural aspect that survived the homogenization tendency. The bubble and bogussed TCs developed markedly different heating patterns, which appear to help explain why the artificially-established storms tended to move about three times faster than their bubble counterparts. The reasons for this are not presently understood fully.

Prediction and Diagnosis of the Motion and Rapid Intensification of Typhoon Sinlaku during Tropical Cyclone Structure Experiment 2008 (TCS08)

“…5), regardless of the prescribed R34 value, which has no significant impact on the intensity forecast. The results obtained here for a real TC case differ from the semi-idealized simulations of Cao et al (2011). 9) where SM18, the rightmost point, appears as an outlier to the trend.…”

“…As the storm slows down, b propagation becomes gradually significant under weak environmental influences. Such lack of sensitivity to the R34 differs from Cao et al (2011) and suggests a possible case dependency for storms with strong outer wind structures in weak environmental flows. It acts more efficiently when the initial wind distribution is specified with sufficient strength and extent especially at outer radii (larger initial RMW), consistent with Chan and Williams (1987).…”

“…Now that assimilation of bogus data has been shown to have a positive impact on TC intensity prediction for many numerical models in most situations, details in the structure of the idealized bogus need more attention from research and development. Semi-idealized simulations of a suite of modified Rankine vortices defined by RMW and wind profile decay factors suggest that improved initialization in the outer wind region might affect intensity and track forecast (Cao et al 2011). An important result is given by Xu and Wang (2010), who showed that simulated inner-core size is largely determined by initial vortex size.…”

On the Use of Parametric Wind Models for Wind Wave Modeling under Tropical Cyclones

“…where r is the radial distance, and X is the scaling parameter that adjusts the velocity distribution in the radial direction [18]. The theoretical value for X is 0.5 [55,56], and typical values in the literature for X in the Atlantic and Pacific TCs are between 1/3 and 2/3 [57]. For example, Mallen et al [58] found mean values for major TC (V max > 50 m/s) of 0.48 and progressively smaller values for the less intense TCs.…”

The Influences of Boundary Layer Mixing and Cloud-Radiative Forcing on Tropical Cyclone Size

“…ocean model, employing a uniform and constant sea surface temperature (SST) of 302.5 K for the standard runs, and initializing with a horizontally homogeneous tropical sounding [modified from Jordan (1958); see Fovell et al (2010)] without any mean flow. The Cao et al (2011) ''bubble'' procedure was used to initiate the TC and all simulations spanned 4 full days with composite model fields being constructed for the final day in a vortex-following fashion, averaging over one full diurnal cycle. While changes in operational settings (primarily with respect to horizontal smoothing) from the 2013 version used by Bu et al (2014) resulted in somewhat weaker storms for the same experimental design, all of the standard SST HWRF TCs attained major hurricane status (at least category 3 on the Saffir-Simpson scale) for the analysis period.…”