Sensitivity of tropical-cyclone models to the surface drag coefficient in different boundary-layer schemes

2013 ◽  
Vol 140 (680) ◽  
pp. 792-804 ◽  
Author(s):  
Roger K. Smith ◽  
Michael T. Montgomery ◽  
Gerald L. Thomsen
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Drag Coefficient ◽  
Surface Drag

scholarly journals Dynamics of Lower-Tropospheric Vorticity in Idealized Simulations of Tropical Cyclone Formation

2019 ◽  
Vol 76 (3) ◽  
pp. 707-727 ◽  
Author(s):  
Yaping Wang ◽  
Christopher A. Davis ◽  
Yongjie Huang
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Mass Flux ◽  
Cloud Model ◽  
Vertical Wind Shear ◽  
Lower Troposphere ◽  
Cold Pool ◽  
Surface Drag ◽  
Near Surface ◽  
Cold Pools

Abstract Idealized simulations are conducted using the Cloud Model version 1 (CM1) to explore the mechanism of tropical cyclone (TC) genesis from a preexisting midtropospheric vortex that forms in radiative–convective equilibrium. With lower-tropospheric air approaching near saturation during TC genesis, convective cells become stronger, along with the intensifying updrafts and downdrafts and the larger area coverage of updrafts relative to downdrafts. Consequently, the low-level vertical mass flux increases, inducing vorticity amplification above the boundary layer. Of interest is that while surface cold pools help organize lower-tropospheric updrafts, genesis still proceeds, only slightly delayed, if subcloud evaporation cooling and cold pool intensity are drastically reduced. More detrimental is the disruption of near saturation through the introduction of weak vertical wind shear. The lower-tropospheric dry air suppresses the strengthening of convection, leading to weaker upward mass flux and much slower near-surface vortex spinup. We also find that surface spinup is similarly inhibited by decreasing surface drag despite the existence of a nearly saturated column, whereas larger drag accelerates spinup. Increased vorticity above the boundary layer is followed by the emergence of a horizontal pressure gradient through the depth of the boundary layer. Then the corresponding convergence resulting from the gradient imbalance in the frictional boundary layer causes vorticity amplification near the surface. It is suggested that near saturation in the lower troposphere is critical for increasing the mass flux and vorticity just above the boundary layer, but it is necessary yet insufficient because the spinup is strongly governed by boundary layer dynamics.

Idealized Large-Eddy Simulations of a Tropical Cyclone–like Boundary Layer

2015 ◽  
Vol 72 (5) ◽  
pp. 1743-1764 ◽  
Author(s):  
Benjamin W. Green ◽  
Fuqing Zhang
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Partial Slip ◽  
Inertial Oscillation ◽  
Surface Drag ◽  
Coriolis Parameter ◽  
Centripetal Acceleration ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Extreme Winds

Abstract The tropical cyclone (TC) boundary layer (TCBL)—featuring extreme winds over a rough ocean—is difficult to study observationally. With increasing computational power, high-resolution large-eddy simulation (LES) has become an attractive tool to advance understanding of the TCBL. Here, an idealized Cartesian-based LES is employed to investigate boundary layers driven by extreme TC-like winds. The LES includes the effects of centripetal acceleration through an “effective” Coriolis parameter f* = f + 2Vg/R, with the Earth Coriolis parameter f, gradient wind Vg, and (fixed) radius R. Multiple LES experiments are conducted to elucidate how the boundary layer develops and persists in the strongly rotating TC environment. In all simulations, an overshooting jet develops, the height of which increases with Vg, R, and surface drag. Normalized jet strength also increases with R and drag but decreases with Vg. Turbulent diffusivity Km—which must be parameterized in mesoscale and global models but can be diagnosed by LES—varies considerably both within and among simulations. Also evident is a pseudo-inertial oscillation with a period close to the theoretical 2π/f* and an amplitude that decreases exponentially with time. The LES simulations agree with the linear theory for partial-slip Ekman spirals, except when the effects of Km overwhelmingly counter the effects of Vg.

scholarly journals Impact of Physics Representations in the HWRFX on Simulated Hurricane Structure and Pressure–Wind Relationships

2012 ◽  
Vol 140 (10) ◽  
pp. 3278-3299 ◽  
Author(s):  
J.-W. Bao ◽  
S. G. Gopalakrishnan ◽  
S. A. Michelson ◽  
F. D. Marks ◽  
M. T. Montgomery
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Structural Characteristics ◽  
Structural Parameters ◽  
Cloud Microphysics ◽  
Surface Drag ◽  
Parameterization Schemes ◽  
Tangential Wind ◽  
Turbulence Mixing ◽  
Convection Parameterization

Abstract A series of idealized experiments with the NOAA Experimental Hurricane Weather Research and Forecasting Model (HWRFX) are performed to examine the sensitivity of idealized tropical cyclone (TC) intensification to various parameterization schemes of the boundary layer (BL), subgrid convection, cloud microphysics, and radiation. Results from all the experiments are compared in terms of the maximum surface 10-m wind (VMAX) and minimum sea level pressure (PMIN)—operational metrics of TC intensity—as well as the azimuthally averaged temporal and spatial structure of the tangential wind and its material acceleration. The conventional metrics of TC intensity (VMAX and PMIN) are found to be insufficient to reveal the sensitivity of the simulated TC to variations in model physics. Comparisons of the sensitivity runs indicate that (i) different boundary layer physics parameterization schemes for vertical subgrid turbulence mixing lead to differences not only in the intensity evolution in terms of VMAX and PMIN, but also in the structural characteristics of the simulated tropical cyclone; (ii) the surface drag coefficient is a key parameter that controls the VMAX–PMIN relationship near the surface; and (iii) different microphysics and subgrid convection parameterization schemes, because of their different realizations of diabatic heating distribution, lead to significant variations in the vortex structure. The quantitative aspects of these results indicate that the current uncertainties in the BL mixing, surface drag, and microphysics parameterization schemes have comparable impacts on the intensity and structure of simulated TCs. The results also indicate that there is a need to include structural parameters in the HWRFX evaluation.

scholarly journals Sensitivity of tropical-cyclone models to the surface drag coefficient

2010 ◽  
Vol 136 (653) ◽  
pp. 1945-1953 ◽  
Author(s):  
Michael T. Montgomery ◽  
Roger K. Smith ◽  
Sang V. Nguyen
Keyword(s):  
Tropical Cyclone ◽  
Drag Coefficient ◽  
Surface Drag

scholarly journals Parameterization of a surface drag coefficient in conventionally neutral planetary boundary layer

2004 ◽  
Vol 22 (10) ◽  
pp. 3353-3362 ◽  
Author(s):  
I. N. Esau
Keyword(s):  
Boundary Layer ◽  
Drag Coefficient ◽  
Planetary Boundary Layer ◽  
Large Scale ◽  
Surface Drag ◽  
Free Atmosphere ◽  
Mean Velocity ◽  
Dimensional Parameter ◽  
Drag Coefficients ◽  
Computational Level

Abstract. Modern large-scale models (LSMs) rely on surface drag coefficients to parameterize turbulent exchange between surface and the first computational level in the atmosphere. A classical parameterization in an Ekman boundary layer is rather simple. It is based on a robust concept of a layer of constant fluxes. In such a layer (log-layer), the mean velocity profile is logarithmic. It results in an universal dependence of the surface drag coefficient on a single internal non-dimensional parameter, namely the ratio of a height within this layer to a surface roughness length scale. A realistic near-neutral planetary boundary layer (PBL) is usually much more shallow than the idealized Ekman layer. The reason is that the PBL is developing against a stably stratified free atmosphere. The ambient atmospheric stratification reduces the PBL depth and simultaneously the depth of the log-layer. Therefore, the first computational level in the LSMs may be placed above the log-layer. In such a case, the classical parameterization is unjustified and inaccurate. The paper proposes several ways to improve the classical parameterization of the surface drag coefficient for momentum. The discussion is focused on a conventionally neutral PBL, i.e. on the neutrally stratified PBL under the stably stratified free atmosphere. The analysis is based on large eddy simulation (LES) data. This data reveals that discrepancy between drag coefficients predicted by the classical parameterization and the actual drag coefficients can be very large in the shallow PBL. The improved parameterizations provide a more accurate prediction. The inaccuracy is reduced to one-tenth of the actual values of the coefficients.

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