Sensitivity of Simulated Tropical Cyclone Structure and Intensity to Horizontal Resolution Speaker: Wang, Jian-Cyuan Advisor: Prof. Yang, Ming-Jen Megan S. Gentry AND Gary M. Lackmann Mon. Wea. Rev (2010) 138, 688–704.

Keyword

PV VRWs

Forecasters for tropical cyclone (TC) : track intensity How the simulation of physical processes, especially those important to TC intensity, are impacted by changes in horizontal grid spacing. This study is to analyze simulations of Hurricane Ivan (2004) to determine the sensitivity of TC intensity and structure to grid spacing for grid lengths between 8 and 1 km. Ivan

Weather Research and Forecasting (WRF) model same physics options with no convective parameterization (CP) used on the inner WSMsix-class (Hong and Lim 2006) microphysics parameterization Mellor–Yamada–Janjic (Janjic´1994, 2002) planetary boundary layer (PBL) scheme One-way nesting; smaller nested domains move with Ivan 46 half-σ levels, with the model top at 50 hPa Simulation characteristics DesignationDomain 1Domain 2Domain 3Characteristic 8 km Grid spacing (km) Time step (s) Domain size (km) 6 km, 2 km Grid spacing (km) Time step (s) Domain size (km) 4 km Grid spacing (km) Time step (s) Domain size (km) 3 km, 1 km Grid spacing (km) Time step (s) Domain size (km) One-way nesting In each experiment, the storm followed a similar track. v.s. Two-way nesting One-way nesting In each experiment, the storm followed a similar track. v.s. Two-way nesting

1° Global Forecast System (GFS) analyses 0.5 ° real-time global sea surface temperature analysis GFS analysis used for initial conditions has a central pressure above 990 hPa(National Hurricane Center(NHC) best track: 926 hPa) All domains are initialized at 00 UTC 11 September 2004 Temporally averaged every hour from forecast hours 60 to 72 9/11_00Z

a. Intensity b. Symmetric eyewall updraft c. Full vertical motion field d. Potential vorticity(PV) features e. Spectral decomposition of PV

a. Intensity 3-Km(913hPa) comes the closest to the best-track data(910hPa) However, some overprediction of the observed intensity can be expected in model simulations with a static sea surface temperature as used here All considerably higher than minimum central pressures in the coarser-resolution( km) runs. Some portion of the reduction of minimum central pressure with decreasing grid spacing is attributable to spatial sampling.

As gridcell area decreases, it is more likely that small-scale pressure minima are resolved. This suggests that differences in the simulations do not result simply from a better-resolved minimum in surface pressure, but that physical processes important to TC intensity are impacted by the resolution changes. 6 -> 4-km drop of 2 hPa 4 -> 2-km drop of 14 hPa

b. Symmetric eyewall updraft Overall, there is an increase in the averaged ascent as resolution increases. 4-km or less: The average of the eyewall updraft is greater than 2 m s -1 3-km ~ 1-km: greater than 2.5 m s -1 1-km: a small visible area of 3.5 m s -1 or greater around the 250-hPa level. The slope of eyewall updraft decreases noticeably as resolution increases. Bryan et al. (2003) suggest that higher resolution could eventually weaken the magnitude of vertical motions as detrimental processes, such as entrainment. An extension of this study to grid spacing below 1 km would be needed to assess this. 8-km4-km 1-km 2-km3-km 6-km 250-hPa

This is consistent with the system-averaged horizontal winds, which show that, in the finer runs, there is a significant increase in the vertical extent of the stronger horizontal winds. Stern and Nolan (2009) founda linear relationship between the outward slope of the radius of maximum wind (RMW) and the size of the RMW. 8-km4-km 1-km 2-km3-km 6-km

The primary circulation is in gradient thermal wind balance. V θ : tangential wind, r : radial distance from the storm center Subscript p denotes: differentiation along a constant pressure surface other terms have their usual meteorological meaning Gradient thermal wind Steady Frictionless cylindrical coordinate Resolution ↗ : RMW ↘ │dT/dr│ ↗ & area ↑ ↓ > < RMW area While not a definitive statement of cause and effect, this analysis demonstrates consistent changes in the thermal and dynamical structure with resolution. 8-km4-km 1-km 2-km3-km 6-km

c. Full vertical motion of filed a~c: weak ascent surrounding the eye, no downdrafts evident within the eyewall. d~f : ascent maxima become more numerous and smaller, and ring of downdrafts appear adjacent to the ring of ascent surrounding the eye. 1-km: vertical velocity is m s -1 ; Magnitude similar to previous observational studies 850-hPa vertical wind at 63h20. The point maxima (minima) of vertical velocity at this time are 4.66 m s -1 for the 3-km run, 5.58 m s -1 for the 2-km run, and m s -1 for the 1-km run. 8-km4-km 1-km 2-km3-km 6-km

Contoured Frequency by Altitude Diagrams(CFADs) (Yuter and Houze 1995)

CFADs of vertical velocity There is a broadening of the range of vertical motions present as grid spacing is decreased. In all simulations, most vertical velocities are less than 2 m s -1. (95%) Extreme W: 0.1% contour - The 2-km strongest extreme updrafts. - The 1-km most intense downdrafts. The presence of stronger updrafts closer to the surface at higher resolution Strong vertical motions of several meters per second below 1-km altitude have also been reported in observational studies (e.g., Marks et al. 2008). 8-km 4-km 1-km2-km 3-km 6-km

d. PV features In the 2- and 1-km runs, some PV anomalies are observed to break off from the high- PV eyewall region and enter the low-PV environment of the eye. As grid spacing is further reduced to 1 km, PV anomalies of this magnitude and greater appear more frequently outside the eyewall. Polygonal → circular 8-km4-km 1-km 2-km3-km 6-km 850-hPa PV at 63 h

8-km4-km 1-km 2-km3-km 6-km As expected, the magnitude of the radial gradient of PV is larger in the finer- resolution simulations with higher values of PV present in the eyewall, both in the full and system-averaged PV fields (Fig. 8).

The composite model-simulated radar reflectivity also illustrates more developed spiral banding in the higher- resolution simulations. 8-km 4-km 2-km 1-km It is hypothesized that these structural changes stem from the ability to resolve a greater range of wavenumbers in the higher-resolution runs.

e. Spectral decomposition of PV some wavenumbers that are not fully resolved contain a significant amount of variance, over 10% in both the 8- and 6-km runs. The ability for a model to resolve a broader spectrum of wavenumbers does not guarantee that the spectral power would become evenly distributed. Investigation of the physical processes that determine this power distribution are beyond the scope of the current paper. Resolve a wave, the wavelength must be 4Dx (Grasso 2000). Fully resolved if their scale is as large as 10Dx (Walters 2000) Ivan’s RMW during 11–14 September 2004 was 27 km (Dx=6.8km ; Dx=2.7) Temporally averaged variance of 850-hPa PV at the radius of maximum PV (RMPV). Dashed lines correspond to wavelengths less than 10Dx.

Therefore, a wider variety of VRW wavenumbers are simulated, and this, in conjunction with stronger lateral PV gradients, leads to an increasing likelihood of wave breaking.

8-km~4-km, TC structure changes little 4 km and below, inner spiral bands appearing more strongly As resolution is increased, CFADs show a broadening of the range of the most intense vertical motions 3-km and below, there is a significant drop in minimum central pressure. Also, polygonal eyewall structures and localized, intense updraft cores appear more frequently 1-km, updrafts on the order of 10 m s -1 are present even at lower altitudes

TC vortex with an eyelike structure, 8-km grid spacing is sufficient. simulate the full TC intensity, grid spacing no greater than 2- or 3-km should be employed. However, only at 1-km grid spacing, are both updraft and downdraft cores within the lower levels of the eyewall partially resolved. As with any single case, additional simulation comparisons of other storms should be made, and simulations with grid lengths below 1 km should be explored.

9/11 00Z 9/14 00Z

Methods Weather Research and Forecasting (WRF) model same physics options with no convective parameterization (CP) used on the inner, higher- resolution nests (with the exception of the 12-km nest present during the 4-km simulation) WSMsix-class (Hong and Lim 2006) microphysics parameterization Mellor–Yamada–Janjic (Janjic´1994, 2002) planetary boundary layer (PBL) scheme One-way nesting; smaller nested domains move with Ivan 123