The Experimental HWRF System: A Study on the Influence of Horizontal Resolution on the Structure and Intensity Changes in Tropical Cyclones Using an Idealized Framework

Abstract

Forecasting intensity changes in tropical cyclones (TCs) is a complex and challenging multiscale problem. While cloud-resolving numerical models using a horizontal grid resolution of 1-3 km are starting to show some skill in predicting the intensity changes in individual cases, it is not clear at this time what may be a reasonable horizontal resolution for forecasting TC intensity changes on a day-to-day-basis. The Experimental Hurricane Weather Research and Forecasting System (HWRFX) was used within an idealized framework to gain a fundamental understanding of the influence of horizontal grid resolution on the dynamics of TC vortex intensification in three dimensions. HWFRX is a version of the National Centers for Environmental Prediction (NCEP) Hurricane Weather Research and Forecasting (HWRF) model specifically adopted and developed jointly at NOAA's Atlantic Oceanographic and Meteorological Laboratory (AOML) and Earth System Research Laboratory (ESRL) for studying the intensity change problem at a model grid resolution of about 3 km. Based on a series of numerical experiments at the current operating resolution of about 9 km and at a finer resolution of about 3 km, it was found that improved resolution had very little impact on the initial spinup of the vortex. An initial axisymmetric vortex with a maximum wind speed of 20 m s 21 rapidly intensified to 50 m s 21 within about 24 h in either case. During the spinup process, buoyancy appears to have had a pivotal influence on the formation of the warm core and the subsequent rapid intensification of the modeled vortex. The high-resolution simulation at 3 km produced updrafts as large as 48 m s 21 . However, these extreme events were rare, and this study indicated that these events may not contribute significantly to rapid deepening. Additionally, although the structure of the buoyant plumes may differ at 9-and 3-km resolution, interestingly, the axisymmetric structure of the simulated TCs exhibited major similarities. Specifically, the similarities included a deep inflow layer extending up to about 2 km in height with a tangentially averaged maximum inflow velocity of about 12-15 m s 21 , vertical updrafts with an average velocity of about 2 m s 21 , and a very strong outflow produced at both resolutions for a mature storm. It was also found in either case that the spinup of the primary circulation occurred not only due to the weak inflow above the boundary layer but also due to the convergence of vorticity within the boundary layer. Nevertheless, the mature phase of the storm's evolution exhibited significantly different patterns of behavior at 9 and 3 km. While the minimum pressure at the end of 96 h was 934 hPa for the 9-km simulation, it was about 910 hPa for the 3-km run. The maximum tangential wind at that time showed a difference of about 10 m s 21 . Several sensitivity experiments related to the initial vortex intensity, initial radius of the maximum wind, and physics were performed. Based on ensembles of simulations, it appears that radial advection of ...

Paradigms for tropical cyclone intensification

“…The results of Nguyen et al support those of Montgomery et al (2006) in an idealised study of the transformation of a relatively-weak, axisymmetric, middle-level, cold-cored vortex on an f-plane to a warm-cored vortex of tropical cyclone strength and by those of near cloud-resolving numerical simulations of tropical storm Diana (1984) (Davis and Bosart 2002;Hendricks et al 2004), which drew attention to the important role of convectively-amplified vorticity in the spin-up process. In turn, the results are supported by more recent calculations of the intensification process using a range of different cloud-representing models with a refined horizontal resolution (Fang and Zhang 2011, Gopalakrishnan et al 2011, Persing et al 2013. All of these studies indicate that VHTs are the basic coherent structures of the tropical cyclone intensification process.…”

“…The three most established theories are based on axisymmetric considerations and include: Conditional Instability of the Second Kind (CISK); Ooyama's cooperative intensification theory; and Emanuel's air-sea interaction theory (or WISHE 1 ). A fourth theory that has emerged from analyses of more recent cloud-representing numerical model simulations (Nguyen et al 2008, Fang and Zhang 2011, Nguyen et al 2011, Gopalakrishnan et al 2011, Persing et al 2013) highlights the intrinsically non-axisymmetric nature of the spin-up process and suggests a modified view of the axisymmetric aspects of the intensification process.…”

Why Do Model Tropical Cyclones Intensify More Rapidly at Low Latitudes?

“…However, as in previous studies (e.g., Nguyen et al 2008;Fang and Zhang 2011;Gopalakrishnan et al 2011;Persing et al 2013), the flow evolution during the process of intensification is distinctly nonaxisymmetric, with rotating convective structures and their progressive aggregation being a dominant feature. In fact, during the intensification phase, the azimuthally averaged fields of vertical velocity, vertical vorticity, and diabatic heating rate are dominated by these local features.…”

On the Predictability and Error Sources of Tropical Cyclone Intensity Forecasts

“…Within the community of scientists working on tropical cyclones, efforts have been directed toward improving dedicated tropical cyclone models (e.g., Gopalakrishnan et al 2011), real-time in situ and remote sensing observations of storms (e.g., Ruf et al 2016), assimilation of those observations into models (Zhang and Weng 2015;Weng and Zhang 2016;Zhang et al 2016), and statistical forecast models, which are still competitive with deterministic models (Kaplan et al 2015). Numerical modeling of tropical cyclones is especially challenging owing to the very high resolution required to resolve the critical eyewall region (Rotunno et al 2009), to the complex physics of boundary layers and air-sea interaction at high winds speeds (Nolan et al 2009;Green and Zhang 2014;Andreas et al 2015;Green and Zhang 2015), and to the importance of correctly modeling the response of the upper ocean to the storms (Moon et al 2007;Yablonsky and Ginis 2009).…”