The Impact of Horizontal Grid Spacing on the Microphysical and Kinematic Structures of Strong Tropical Cyclones Simulated with the WRF-ARW Model

Abstract

Using the Advanced Weather Research and Forecasting numerical model, the impact of horizontal grid spacing on the microphysical and kinematic structure of a numerically simulated tropical cyclone (TC), and their relationship to storm intensity was investigated with a set of five numerical simulations using input data for the case of Hurricane Rita (2005). The horizontal grid spacing of the parent domain was systematically changed such that the horizontal grid spacing of the inner nest varied from 1 to 5 km by an increment of 1 km, this while keeping geographical dimensions of the domains identical.Within this small range of horizontal grid spacing, the morphology of the simulated storms and the evolution of the kinematic and microphysics field showed noteworthy differences. As grid spacing increased, the model produced a wider, more tilted eyewall, a larger radius of maximum winds, and higher-amplitude, low wavenumber eyewall asymmetries. The coarser-resolution simulations also produced larger volume, areal coverage, and mass flux of updraft speeds $5 m s 21 ; larger volumes of condensate and ice-phase particles aloft; larger boundary layer kinetic energy; and a stronger secondary circulation. While the contribution of updrafts $5 m s 21 to the total updraft mass flux varied little between the five cases, the contribution of downdrafts #22 m s 21 to the total downdraft mass flux was by far the largest in the finest-resolution simulation.Despite these structural differences, all of the simulations produced storms of similar intensity, as measured by peak 10-m wind speed and minimum surface pressure, suggesting that features in the higher-resolution simulations that tend to weaken TCs (i.e., smaller area of high surface fluxes and weaker total updraft mass flux) compensate for features that favor TC intensity (i.e., smaller-amplitude eyewall asymmetries and larger radial gradients). This raises the possibility that resolution increases in this range may not be as important as other model features (e.g., physical parameterization and initial condition improvements) for improving TC intensity forecasts.

Impact of local data assimilation on tropical cyclone predictions over the Bay of Bengal using the ARW model

“…With the advent of high-performance computing, advanced highresolution mesoscale models are employed to simulate the above processes for better prediction of tropical cyclones (e.g. Chen et al, 1995;Liu et al, 1997;Kurihara et al, 1998;Aberson, 2001;Wang, 2001;Krishnamurti, 2005;Braun et al, 2006;Fierro et al, 2009;Smith and Thomsen, 2010;Nolan et al, 2009;Gentry and Lackmann, 2010;among others).…”

“…The errors in tropical cyclone track prediction have steadily decreased over the past several decades due to the use of increasing horizontal and vertical model resolution, improvements in model physics and advanced data assimilation (Rogers et al, 2003;Gentry and Lackmann, 2010). Several model performance evaluation studies have been conducted for TC predictions in recent times (e.g.…”

Vacillation Cycles in WRF Simulations of Hurricane Katrina

“…The non-hydrostatic WRF model The Weather Research and Forecast model (WRF) is an atmospheric simulation system, which has been previously applied to a wide range of meteorological problems, including tropical cyclone intensification (Rotunno et al 2009, Li and Pu 2008, Nolan et al2009, Fierro et al 2009). The Advanced Research WRF (ARW) dynamic core, chosen here, uses the non-hydrostatic perturbation form of the Euler equations written in flux-form (Skamarock 2008).…”

“…Eventually the vortex deepens as the PV anomalies propagate towards the centre and merge. Fierro et al (2009) documented changes in the structure of hurricane Rita (2005) in simulations with WRF as the resolution increases from 1 to 5 km. Although the intensity did not change appreciably, they found the energy at low wave numbers (2-4) decreased with increasing resolution, while the energy of the symmetric flow and that at higher wave numbers increased with higher resolution.…”

Mechanisms Governing the Eyewall Replacement Cycle in Numerical Simulations of Tropical Cyclones

“…Observations show that the hydrometeors in the TC core are dominated by ice crystals and snow aggregates (Black and Hallett, 1986), and therefore, 1 Since simulations are performed on a f-plane and there is no environmental flow, the simulated vortex does not drift far away from its initial position. Because of this, the innermost domain is able to cover the entire inner core region throughout the simulation even though the nest does not follow the vortex.2 Full levels are where the vertical velocity and geopotential are predicted, whereas half levels are where the horizontal velocities, temperature, moisture, and pressure are computed and stored.3 A TC is fueled by the air-sea surface flux exchanges of moist enthalpy via surface latent heat and sensible heat fluxes (e.g., Emanuel 1986), while the maintenance of the secondary circulation of a TC requires an uninterrupted anticyclonic ventilation flow in the upper troposphere (Zhang et al 2002;Fierro et al 2009), and hence, a better representation of these two regions can lead to a more realistic simulation of TCs.15 the ability to predict ice number concentration is desirable for a realistic TC simulation.Moreover, the treatment of snow growth in this scheme is designed to prevent or alleviate the problem of over-prediction of graupel, a problem that is shared by many singlemomentum microphysics schemes (Hong et al 2004). These advantages make the and moisture from the predicted TKE to determine the SGS tendencies of these quantities.…”