Search Results

You are looking at 1 - 10 of 12 items for

  • Author or Editor: G. Brunet x
  • Refine by Access: All Content x
Clear All Modify Search
G. Brunet and T. Warn

Abstract

It is argued using simple examples that while free, barotropic Rossby wave critical layers on unbounded stable monotone profiles are difficult to excite with smooth initial data, they are easily excited on stable jets when the background absolute-vorticity gradient is sufficiently weak. After sufficient time the critical layer, which is located at the jet maximum, becomes locally nonlinear in the absence of dissipation, regardless of the initial amplitude of the disturbance. Even though the nonlinear terms are smallest at the critical layer, they dominate due to the smallness of the linear terms there. We also speculate on how other nonmonotone profiles might be treated.

Full access
G. Brunet and P. H. Haynes

Abstract

It has been shown that the linearized equations for disturbances to a parabolic jet on a β plane, with curvature Un0(y) such that the basic-state absolute vorticity gradient β − Un 0(y) is zero, ultimately become inconsistent in the neighborhood of the jet axis and that nonlinear effects become important. Numerical solutions of the nonlinear long-time asymptotic form of the equations are presented. The numerical results show that the algebraic decay of the disturbances as t −1/2 predicted by the linear equations is inhibited by the nonlinear formation of coherent vortices new the jet axis. These lead to a disturbance amplitude that decays only through the action of weak numerical diffusion but is otherwise as t 0.

The linear theory is extended to the case when the basic-state absolute vorticity gradient is nonzero but weak. When the gradient is weak and negative the decay is modified and is ultimately as t −3/2. When the gradient is weak and positive, on the other hand, a discrete eigenmode is excited and asymptotic decay is inhibited. In both cases linear theory may give a self-consistent description if the amplitude is small enough. Numerical simulation shows that for both signs of the gradient there is a range of amplitudes for which nonlinear effects become directly important. Coherent vortices may form and either inhibit the decay or disrupt the linear mode. The structure of the nonlinear analog of the linear eigenmode is analyzed and shown to have a propagation speed, relative to the jet axis speed, that is a decreasing function of amplitude, tending to zero as the amplitude approaches a finite limiting value.

Full access
G. Brunet and P. H. Haynes

Abstract

The nonlinear reflection of an isolated Rossby wave train at a low-latitude wave-breaking region is contrasted with the more familiar longitudinally periodic case. General theoretical arguments for nonlinear reflection based on absorptivity bounds do not carry over to the case of an isolated wave train, and detailed investigation is needed to determine the absorption-reflection behavior. Numerical experiments in a single-layer shallow-water model show that wave activity is reflected back into midlatitudes (rather than propagating longitudinally at low latitudes). Finite-amplitudes wave activity diagnostics are used to analyze the nonlinear reflection. Further idealized numerical simulations and simple ideas concerning the propagation of Rossby waves in shear flows are used to give insight into the nonlinear reflection.

Full access
Y. Martinez, G. Brunet, and M. K. Yau

Abstract

Despite the fact that asymmetries in hurricanes, such as spiral rainbands, polygonal eyewalls, and mesovortices, have long been observed in radar and satellite imagery, many aspects of their origin, space–time structure, and dynamics still remain unsolved, particularly their role on the vortex intensification. The underlying inner-core dynamics need to be better understood to improve the science of hurricane intensity forecasting. To fill this gap, a simple 2D barotropic “dry” model is used to perform two experiments starting respectively with a monopole and a ring of enhanced vorticity to mimic hurricane-like vortices during incipient and mature stages of development. The empirical normal mode (ENM) technique, together with the Eliassen–Palm (EP) flux calculations, are used to isolate wave modes from the model datasets to investigate their space–time structure, kinematics, and the impact on the changes in the structure and intensity of the simulated hurricane-like vortices.

From the ENM diagnostics, it is shown in the first experiment how an incipient storm described by a vortex monopole intensifies by “inviscid damping” of a “discrete-like” vortex Rossby wave (VRW) or quasi mode. The critical radius, the structure, and the propagating properties of the quasi mode are found to be consistent with predictions of the linear eigenmode analysis of small perturbations. In the second experiment, the fastest growing wavenumber-4 unstable VRW modes of a vortex ring reminiscent of a mature hurricane are extracted, and their relation with the polygonal eyewalls, mesovortices, and the asymmetric eyewall contraction are established in consistency with results previously obtained from other authors.

Full access
Y. Martinez, G. Brunet, and M. K. Yau

Abstract

Despite the fact that asymmetries in hurricanes (e.g., spiral rainbands, polygonal eyewalls, and mesovortices) have long been observed in radar and satellite imagery, many aspects of their dynamics remain unsolved, particularly in the formation of the secondary eyewall. The underlying associated dynamical processes need to be better understood to advance the science of hurricane intensity forecasting. To fill this gap, a simple 2D barotropic “dry” model is used to simulate a hurricane-like concentric rings vortex. The empirical normal mode (ENM) technique, together with Eliassen–Palm (EP) flux calculations, are used to isolate wave modes from the model datasets to investigate their impact on the changes in the structure and intensity of the simulated hurricane-like vortex.

From the ENM diagnostics, it is shown that asymmetric disturbances outside a strong vortex ring with a large vorticity skirt may relax to form concentric rings of enhanced vorticity that contain a secondary wind maximum. The fact that the critical radius for some of the leading modes is close to the location where the secondary ring of enhanced vorticity develops suggests that a wave–mean flow interaction mechanism based on vortex Rossby wave (VRW) dynamics may explain important dynamical aspects of concentric eyewall genesis (CEG).

Full access
G. Brunet, R. Vautard, B. Legras, and S. Edouard

Abstract

A 25-year dataset of potential vorticity on the 315-K isentropic surface is built from the National Meteorological Center (NMC) final analysis archive. Potential vorticity is calculated from the nonlinear gradient wind balance using temperature and geopotential fields, since the wind field is not available in the early part of the archive.

The validity of this calculation is assessed by comparing the results with potential vorticity obtained directly from European Centre for Medium-Range Weather Forecasts (ECMWF) analyzed winds. The error due to the nonlinear balance approximation turns out to be smaller than the difference between ECMWF and NMC analysis.

The possibility of studying diabatic forcing as a residual of the equation of potential vorticity conservation is examined. The average potential vorticity forcing found in this way is consistent with the authors knowledge of the mean diabatic heating. The amplitude of the residual decreases through the analysis period, reflecting the improvement in the observational network and in the analysis schemes.

Next the authors demonstrate that this dataset can be used for studies of transient-mean flow interactions. The authors present diagnostics of the transient feedback by separating the contribution of vortical and thermal terms on the isentropic surface. Also, the contribution of high-frequency (periods less than 10 days) and low-frequency (periods greater than 10 days) transients is examined. On the 315-K surface, transients act mostly in reducing the potential vorticity gradient through thermal terms and accelerate the zonal flow through vortical forcing.

Finally, these diagnostics are also applied to a long ensemble of blocking events, and the authors study the anomaly of transient feedback during these events. It is found that transients have primarily an advective effect, forcing the dipole structure to retrograde westward. Thermal and vortical terms have a very distinct action on the blocking anomaly. Vortical forcing is constructive and advective, whereas thermal forcing is dissipative and makes the dipole rotate clockwise.

Full access
Y. Martinez, G. Brunet, M. K. Yau, and X. Wang

Abstract

A novel statistical technique called space–time empirical normal mode (ST-ENM) is applied in a diagnostic study of the genesis of a secondary eyewall in a simulated hurricane using the nonhydrostatic, high-resolution fifth-generation Pennsylvania State University (PSU)–National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5). The bases obtained from the ST-ENM technique are nonstationary, dynamically relevant, and orthogonal in the sense of wave activity.

The wave activity spectra of the wavenumber-1 anomalies show that the leading modes (1–6) exhibit mainly characteristics of vortex Rossby waves (VRWs). These modes together explain about 75% of the total wavenumber-1 variance in a period of 24 h.

Analysis of the Eliassen–Palm (EP) flux and its time-mean divergence corresponding to the total contribution from these modes indicated that in the lower troposphere VRWs not only propagate inward (outward) in the primary eyewall region where the radial gradient of the basic-state potential vorticity is large and positive (large and negative), but there is also wave activity propagating outside the primary eyewall. Consequently, maximum cyclonic eddy angular momentum is transported not only inside the radius of maximum wind (RMW) by VRWs in the primary eyewall region, but also close to the location where the secondary eyewall forms by VRWs propagating outside the inner eyewall.

The fact that the critical radius for some of the ST-ENMs is contained inside the region where the secondary eyewall forms and the existence of a signal of maximum eddy cyclonic angular momentum flux propagating outward up to the critical radius suggests that a wave–mean flow interaction mechanism and redistribution of angular momentum may be suitable to explain important dynamical aspects of concentric eyewall genesis.

Full access
Stanley G. Benjamin, John M. Brown, Gilbert Brunet, Peter Lynch, Kazuo Saito, and Thomas W. Schlatter

Abstract

Over the past 100 years, the collaborative effort of the international science community, including government weather services and the media, along with the associated proliferation of environmental observations, improved scientific understanding, and growth of technology, has radically transformed weather forecasting into an effective global and regional environmental prediction capability. This chapter traces the evolution of forecasting, starting in 1919 [when the American Meteorological Society (AMS) was founded], over four eras separated by breakpoints at 1939, 1956, and 1985. The current state of forecasting could not have been achieved without essential collaboration within and among countries in pursuing the common weather and Earth-system prediction challenge. AMS itself has had a strong role in enabling this international collaboration.

Full access
A. J. Dolman, J. Noilhan, P. Durand, C. Sarrat, A. Brut, B. Piguet, A. Butet, N. Jarosz, Y. Brunet, D. Loustau, E. Lamaud, L. Tolk, R. Ronda, F. Miglietta, B. Gioli, V. Magliulo, M. Esposito, C. Gerbig, S. Körner, P. Glademard, M. Ramonet, P. Ciais, B. Neininger, R. W. A. Hutjes, J. A. Elbers, R. Macatangay, O. Schrems, G. Pérez-Landa, M. J. Sanz, Y. Scholz, G. Facon, E. Ceschia, and P. Beziat

The Second Global Soil Wetness Project (GSWP-2) is an initiative to compare and evaluate 10-year simulations by a broad range of land surface models under controlled conditions. A major product of GSWP-2 is the first global gridded multimodel analysis of land surface state variables and fluxes for use by meteorologists, hydrologists, engineers, biogeochemists, agronomists, botanists, ecologists, geographers, climatologists, and educators. Simulations by 13 land models from five nations have gone into production of the analysis. The models are driven by forcing data derived from a combination of gridded atmospheric reanalyses and observations. The resulting analysis consists of multimodel means and standard deviations on the monthly time scale, including profiles of soil moisture and temperature at six levels, as well as daily and climatological (mean annual cycle) fields for over 50 land surface variables. The monthly standard deviations provide a measure of model agreement that may be used as a quality metric. An overview of key characteristics of the analysis is presented here, along with information on obtaining the data.

Full access
D. B. Parsons, M. Beland, D. Burridge, P. Bougeault, G. Brunet, J. Caughey, S. M. Cavallo, M. Charron, H. C. Davies, A. Diongue Niang, V. Ducrocq, P. Gauthier, T. M. Hamill, P. A. Harr, S. C. Jones, R. H. Langland, S. J. Majumdar, B. N. Mills, M. Moncrieff, T. Nakazawa, T. Paccagnella, F. Rabier, J.-L. Redelsperger, C. Riedel, R. W. Saunders, M. A. Shapiro, R. Swinbank, I. Szunyogh, C. Thorncroft, A. J. Thorpe, X. Wang, D. Waliser, H. Wernli, and Z. Toth

Abstract

The Observing System Research and Predictability Experiment (THORPEX) was a 10-yr, international research program organized by the World Meteorological Organization’s World Weather Research Program. THORPEX was motivated by the need to accelerate the rate of improvement in the accuracy of 1-day to 2-week forecasts of high-impact weather for the benefit of society, the economy, and the environment. THORPEX, which took place from 2005 to 2014, was the first major international program focusing on the advancement of global numerical weather prediction systems since the Global Atmospheric Research Program, which took place almost 40 years earlier, from 1967 through 1982. The scientific achievements of THORPEX were accomplished through bringing together scientists from operational centers, research laboratories, and the academic community to collaborate on research that would ultimately advance operational predictive skill. THORPEX included an unprecedented effort to make operational products readily accessible to the broader academic research community, with community efforts focused on problems where challenging science intersected with the potential to accelerate improvements in predictive skill. THORPEX also collaborated with other major programs to identify research areas of mutual interest, such as topics at the intersection of weather and climate. THORPEX research has 1) increased our knowledge of the global-to-regional influences on the initiation, evolution, and predictability of high-impact weather; 2) provided insight into how predictive skill depends on observing strategies and observing systems; 3) improved data assimilation and ensemble forecast systems; 4) advanced knowledge of high-impact weather associated with tropical and polar circulations and their interactions with midlatitude flows; and 5) expanded society’s use of weather information through applied and social science research.

Full access