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Daniel P. Stern
and
George H. Bryan

Abstract

Extreme updrafts (≥10 m s−1) and wind gusts (≥90 m s−1) are ubiquitous within the low-level eyewall of intense tropical cyclones (TCs). Previous studies suggest that both of these features are associated with coherent subkilometer-scale vortices. Here, over 100 000 “virtual” dropsonde trajectories are examined within a large-eddy simulation (31.25-m horizontal grid spacing) of a category 5 hurricane in order to gain insight into the nature of these features and to better understand and interpret dropsonde observations. At such a high resolution, profiles of wind speed and vertical velocity from the virtual sondes are difficult to distinguish from those of real dropsondes. PDFs of the strength of updrafts and wind gusts compare well between the simulated and observed dropsondes, as do the respective range of heights over which these features are found. Individual simulated updrafts can be tracked for periods of up to several minutes, revealing structures that are both coherent and rapidly evolving. It appears that the updrafts are closely associated with vortices and wind speed maxima, consistent with previous studies. The peak instantaneous wind gusts in the simulations (up to 150 m s−1) are substantially stronger than have ever been observed. Using the virtual sondes, it is demonstrated that the probability of sampling such extremes is vanishingly small, and it is argued that actual intense TCs might also be characterized by gusts of these magnitudes.

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Falko Judt
,
Rosimar Rios-Berrios
, and
George H. Bryan

Abstract

Tropical cyclones that intensify abruptly experience “rapid intensification.” Rapid intensification remains a formidable forecast challenge, in part because the underlying science has not been settled. One way to reconcile the debates and inconsistencies in the literature is to presume that different forms (or modes) of rapid intensification exist. The present study provides evidence in support of this hypothesis by documenting two modes of rapid intensification in a global convection-permitting simulation and the HURDAT2 database. The “marathon mode” is characterized by a moderately paced and long-lived intensification period, whereas the “sprint mode” is characterized by explosive and short-lived intensification bursts. Differences between the modes were also found in initial vortex structure (well defined versus poorly defined), nature of intensification (symmetric versus asymmetric), and environmental conditions (weak shear versus strong shear). Collectively, these differences indicate that the two modes involve distinct intensification mechanisms. Recognizing the existence of multiple intensification modes may help to better understand and predict rapid intensification by, for example, explaining the lack of consensus in the literature, or by raising awareness that rapid intensification in strongly sheared cyclones is not just an exception to a rule, but a typical process.

Significance Statement

Hurricanes are serious threats to society—in particular those that suddenly and quickly intensify before striking land. Forecasting these “rapid intensification” events is a challenge, in part because we do not fully understand the science behind rapid intensification. This study furthers our understanding of hurricane rapid intensification by documenting that rapid intensification comes in different types. Specifically, we show that one type of rapid intensification happens under conditions that meteorologists have thought would lessen the chances of intensification. Awareness of such a type of rapid intensification could lead to better predictions of hurricane intensity because forecasters are more cognizant of this type of event and the conditions in which they occur.

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George H. Bryan
,
Jason C. Knievel
, and
Matthew D. Parker

Abstract

The authors evaluate whether the structure and intensity of simulated squall lines can be explained by “RKW theory,” which most specifically addresses how density currents evolve in sheared environments. In contrast to earlier studies, this study compares output from four numerical models, rather than from just one. All of the authors’ simulations support the qualitative application of RKW theory, whereby squall-line structure is primarily governed by two effects: the intensity of the squall line’s surface-based cold pool, and the low- to midlevel environmental vertical wind shear. The simulations using newly developed models generally support the theory’s quantitative application, whereby an optimal state for system structure also optimizes system intensity. However, there are significant systematic differences between the newer numerical models and the older model that was originally used to develop RKW theory. Two systematic differences are analyzed in detail, and causes for these differences are proposed.

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Jason C. Knievel
,
George H. Bryan
, and
Joshua P. Hacker

Abstract

Diffusion that is implicit in the odd-ordered advection schemes in early versions of the Advanced Research core of the Weather Research and Forecasting (WRF) model is sometimes insufficient to remove noise from kinematical fields. The problem is worst when grid-relative wind speeds are low and when stratification is nearly neutral or unstable, such as in weakly forced daytime boundary layers, where noise can grow until it competes with the physical phenomena being simulated. One solution to this problem is an explicit, sixth-order numerical diffusion scheme that preserves the WRF model’s high effective resolution and uses a flux limiter to ensure monotonicity. The scheme, and how it was added to the WRF model, are explained. The scheme is then demonstrated in an idealized framework and in simulations of salt breezes and lake breezes in northwestern Utah.

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Daniel P. Stern
,
George H. Bryan
, and
Sim D. Aberson

Abstract

Previous studies have found surprisingly strong vertical motions in low levels of some tropical cyclones. In this study, all available dropsondes ( 12 000) within tropical cyclones during 1997–2013 are examined, in order to create a dataset of the most extreme updrafts ( 10 m s−1; 169 sondes) and wind speeds ( 90 m s−1; 64 sondes). It is shown that extreme low-level (0–3 km) updrafts are ubiquitous within intense (category 4 and 5) tropical cyclones, and that few such updrafts have been observed within weaker storms. These extreme updrafts, which are almost exclusively found within the eyewall just inward of the radius of maximum winds, sometimes occur in close association with extreme horizontal wind speeds. Consistent with previous studies, it is suggested that both the extremes in vertical velocity and wind speed are associated with small-scale ( 1 km) vortices that exist along the eye–eyewall interface. As a substantial number of updrafts are found within a kilometer of the surface, it can be shown that it is implausible for buoyancy to be the primary mechanism for vertical acceleration. Additionally, the azimuthal distribution of both the extreme updrafts and wind speeds is strongly associated with the orientation of the environmental vertical wind shear.

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George H. Bryan
,
John C. Wyngaard
, and
J. Michael Fritsch

Abstract

The spatial resolution appropriate for the simulation of deep moist convection is addressed from a turbulence perspective. To provide a clear theoretical framework for the problem, techniques for simulating turbulent flows are reviewed, and the source of the subgrid terms in the Navier–Stokes equation is clarified.

For decades, cloud-resolving models have used large-eddy simulation (LES) techniques to parameterize the subgrid terms. A literature review suggests that the appropriateness of using traditional LES closures for this purpose has never been established. Furthermore, examination of the assumptions inherent in these closures suggests that grid spacing on the order of 100 m may be required for the performance of cloud models to be consistent with their design.

Based on these arguments, numerical simulations of squall lines were conducted with grid spacings between 1 km and 125 m. The results reveal that simulations with 1-km grid spacing do not produce equivalent squall-line structure and evolution as compared to the higher-resolution simulations. Details of the simulated squall lines that change as resolution is increased include precipitation amount, system phase speed, cloud depth, static stability values, the size of thunderstorm cells, and the organizational mode of convective overturning (e.g., upright towers versus sloped plumes). It is argued that the ability of the higher-resolution runs to become turbulent leads directly to the differences in evolution.

There appear to be no systematic trends in specific fields as resolution is increased. For example, mean vertical velocity and rainwater values increase in magnitude with increasing resolution in some environments, but decrease with increasing resolution in other environments. The statistical properties of the simulated squall lines are still not converged between the 250- and 125-m runs. Several possible explanations for the lack of convergence are offered. Nevertheless, it is clear that simulations with O(1 km) grid spacing should not be used as benchmark or control solutions for resolution sensitivity studies.

The simulations also support the contention that a minimum grid spacing of O(100 m) is required for traditional LES closures to perform appropriately for their design. Specifically, only simulations with 250- and 125-m grid spacing resolve an inertial subrange. In contrast, the 1-km simulations do not even reproduce the correct magnitude or scale of the spectral kinetic energy maximum. Furthermore, the 1-km simulations contain an unacceptably large amount of subgrid turbulence kinetic energy, and do not adequately resolve turbulent fluxes of total water.

A guide to resolution requirements for the operational and research communities is proposed. The proposal is based primarily on the intended use of the model output. Even though simulations with O(1 km) grid spacing display behavior that is unacceptable for the model design, it is argued that these simulations can still provide valuable information to operational forecasters. For the research community, O(100 m) grid spacing is recommended for most applications, because a modeling system that is well founded should be desired for most purposes.

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Aaron Wang
,
Ying Pan
,
George H. Bryan
, and
Paul M. Markowski

Abstract

Unsteadiness and horizontal heterogeneities frequently characterize atmospheric motions, especially within convective storms, which are frequently studied using large-eddy simulations (LES). The models of near-surface turbulence employed by atmospheric LES, however, predominantly assume statistically steady and horizontally homogeneous conditions (known as the equilibrium approach). The primary objective of this work is to investigate the potential consequences of such unrealistic assumptions in simulations of tornadoes. Cloud Model 1 (CM1) LES runs are performed using three approaches to model near-surface turbulence: the “semi-slip” boundary condition (which is the most commonly used equilibrium approach), a recently proposed nonequilibrium approach that accounts for some of the effects of turbulence memory, and a nonequilibrium approach based on thin boundary layer equations (TBLE) originally proposed by the engineering community for smooth-wall boundary layer applications. To be adopted for atmospheric applications, the TBLE approach is modified to account for the surface roughness. The implementation of TBLE into CM1 is evaluated using LES results of an idealized, neutral atmospheric boundary layer. LES runs are then performed for an idealized tornado characterized by rapid evolution, strongly curved air parcel trajectories, and substantial horizontal heterogeneities. The semi-slip boundary condition, by design, always yields a surface shear stress opposite the horizontal wind at the lowest LES grid level. The nonequilibrium approaches of modeling near-surface turbulence allow for a range of surface-shear-stress directions and enhance the resolved turbulence and wind gusts. The TBLE approach even occasionally permits kinetic energy backscatter from unresolved to resolved scales.

Significance Statement

The traditional approach of modeling the near-surface turbulence is not suitable for a tornado characterized by rapid evolution, strongly curved air parcel trajectories, and substantial horizontal heterogeneities. To understand the influence of statistically unsteady and horizontally heterogeneous near-surface conditions on tornadoes, this work adopts a fairly sophisticated approach from the engineering community and implements it into a widely used atmospheric model with necessary modifications. Compared to the traditional approach, the newly implemented approach produces more turbulent near-surface winds, more flexible surface-drag directions, and stronger wind gusts. These findings suggest a simulated tornado is very sensitive to the modeling approach of near-surface turbulence.

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Christopher J. Nowotarski
,
Paul M. Markowski
,
Yvette P. Richardson
, and
George H. Bryan

Abstract

Nearly all previous numerical simulations of supercell thunderstorms have neglected surface fluxes of heat, moisture, and momentum. This choice precludes horizontal inhomogeneities associated with dry boundary layer convection in the near-storm environment. As part of a broader study on how mature supercell thunderstorms are affected by a convective boundary layer (CBL) with quasi-two-dimensional features (i.e., boundary layer rolls), this paper documents the methods used to develop a realistic CBL in an idealized environment supportive of supercells. The evolution and characteristics of the modeled CBL, including the horizontal variability of thermodynamic and kinematic quantities known to affect supercell evolution, are presented. The simulated rolls result in periodic bands of perturbations in temperature, moisture, convective available potential energy (CAPE), vertical wind shear, and storm-relative helicity (SRH). Vertical vorticity is shown to arise within the boundary layer through the tilting of ambient horizontal vorticity associated with the background shear by vertical velocity perturbations in the turbulent CBL. Sensitivity tests suggest that 200-m horizontal grid spacing is adequate to represent rolls using a large-eddy simulation (LES) approach.

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Christopher J. Nowotarski
,
Paul M. Markowski
,
Yvette P. Richardson
, and
George H. Bryan

Abstract

Simulations of supercell thunderstorms in a sheared convective boundary layer (CBL), characterized by quasi-two-dimensional rolls, are compared with simulations having horizontally homogeneous environments. The effects of boundary layer convection on the general characteristics and the low-level mesocyclones of the simulated supercells are investigated for rolls oriented either perpendicular or parallel to storm motion, as well as with and without the effects of cloud shading.

Bulk measures of storm strength are not greatly affected by the presence of rolls in the near-storm environment. Though boundary layer convection diminishes with time under the anvil shadow of the supercells when cloud shading is allowed, simulations without cloud shading suggest that rolls affect the morphology and evolution of supercell low-level mesocyclones. Initially, CBL vertical vorticity perturbations are enhanced along the supercell outflow boundary, resulting in nonnegligible near-ground vertical vorticity regardless of roll orientation. At later times, supercells that move perpendicular to the axes of rolls in their environment have low-level mesocyclones with weaker, less persistent circulation compared to those in a similar horizontally homogeneous environment. For storms moving parallel to rolls, the opposite result is found: that is, low-level mesocyclone circulation is often enhanced relative to that in the corresponding horizontally homogeneous environment.

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George H. Bryan
,
Nathan A. Dahl
,
David S. Nolan
, and
Richard Rotunno

Abstract

The structure and intensity of tornado-like vortices are examined using large-eddy simulations (LES) in an idealized framework. The analysis focuses on whether the simulated boundary layer contains resolved turbulent eddies, and whether most of the vertical component of turbulent momentum flux is resolved rather than parameterized. Initial conditions are first generated numerically using a “precursor simulation” with an axisymmetric model. A three-dimensional “baseline” LES is then integrated using these initial conditions plus random perturbations. With this baseline approach, the inner core of the simulated vortex clearly contains resolved turbulent eddies (as expected); however, the boundary layer inflow has very weak resolved turbulent eddies, and the subgrid model accounts for most of the vertical turbulent momentum flux (contrary to the design of these simulations). To overcome this problem, a second precursor simulation is conducted in which resolved turbulent fluctuations develop within a smaller, doubly periodic LES domain. Perturbation flow fields from this precursor LES are then “injected” into the large-domain LES at a specified radius. With this approach, the boundary layer inflow clearly contains resolved turbulent fluctuations, often organized as quasi-2D rolls, which persist into the inner core of the simulation; thus, the simulated tornado-like vortex and its inflowing boundary layer can be characterized as LES. When turbulence is injected, the inner-core vortex structure is always substantially different, the boundary layer inflow is typically deeper, and in most cases the maximum wind speeds are reduced compared to the baseline simulation.

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