Search Results

You are looking at 1 - 10 of 19 items for

  • Author or Editor: M. Brunet x
  • All content x
Clear All Modify Search
Ali Asaadi, Gilbert Brunet, and M. K. Yau

Abstract

A shallow-water model is used to study the role of critical layers in tropical cyclogenesis. Forced and unforced problems of disturbances on a parabolic jet associated with weak basic-state meridional potential vorticity (PV) gradients, leading to Kelvin cat’s-eye formation around the jet axis, are first investigated. Numerical simulations with various initial disturbance magnitudes and structures suggest that the results of previous studies can be extended to the next level of complexity toward the more realistic atmosphere. The model is therefore initialized using an observed jet profile obtained from the reanalysis data presented in Part I of this study. For this asymmetric marginally stable basic-state profile, unforced (free) and forced linear integrations show spatial contraction of the perturbation structures in the meridional direction, similar to what occurred in experiments on the parabolic jet. Nonlinear free simulations highlight the role of nonlinear processes in redistributing PV within the critical-layer region. However, they do not yield a realistic time scale for the formation of the cat’s-eye. By including diabatic heating as a mass sink term to represent convective PV generation, the nonlinear forced simulation is found to produce a realistic time scale for cat’s-eye formation, and confirms the analytical solution of τe τQ ~ O(ε −1) obtained in Part I. These results highlight the synergic role of the dynamical mechanisms, including wave breaking and PV redistribution within the nonlinear critical layer characterized by weak PV gradients and the thermodynamical mechanisms such as convectively generated PV anomalies in the cat’s-eye formation in tropical cyclogenesis.

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
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
Ali Asaadi, Gilbert Brunet, and M. K. Yau

Abstract

Motivated by Dunkerton et al., a climatological study of 54 developing easterly waves in 1998–2001 was performed. Time-lagged composites in a translating reference frame following the disturbances indicate a weak meridional potential vorticity (PV) gradient of the easterly jet and a cyclonic critical layer located slightly to the south of the weak PV gradient, consistent with previous findings in the marsupial paradigm. Using a closed PV contour as a criterion for the formation of the cat’s-eye, it was shown that on average it takes ~2.6 days for open PV contours to transform to a closed coherent structure. Bootstrap analysis was then applied to determine the reliability of the easterly wave–like pattern in the composite perturbation PV analysis. It is suggested that the coexistence of a nonlinear critical layer and a region of weak meridional PV gradient over several days, found to occur in only ~25% of the easterly waves, might be a major factor to distinguish developing and nondeveloping disturbances. This finding may explain why only a small fraction of easterly waves contribute to tropical cyclogenesis. Additionally, an analytic time scale of the form was obtained, where Q is the mass sink, ε is the amplitude of the initial disturbance, and τ is the cat’s-eye formation time that governs the onset of nonlinearity for forced disturbances on a parabolic jet critical layer. This time scale is consistent with that found in 54 cases of easterly waves that developed into named storms, highlighting the importance of nonlinear and diabatic processes in cat’s-eye formation.

Full access
Yongsheng Chen, Gilbert Brunet, and M. K. Yau

Abstract

The theory of empirical normal modes (ENMs) was applied in a diagnostic study of the inner spiral bands formed in a simulated hurricane using the high-resolution Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) nonhydrostatic mesoscale model version 5 (MM5). The ENM method has the capability to decompose simultaneously wind and thermal fields into dynamically consistent and orthogonal modes with respect to wave activities.

For wavenumber 1 and 2 anomalies, it was found that the leading modes are vortex Rossby waves. These modes explain 70%–80% of the statistical variances in a 24-h period. Gravity waves have small contribution in terms of wave activities.

Analysis of the Eliassen–Palm (EP) flux and its time-mean divergence shows that vortex Rossby waves are generated in the eyewall region where the radial gradient of the basic-state potential vorticity is large. In general, these waves propagate outward in the lower troposphere and inward in the upper troposphere. Consequently, they transport eddy momentum radially inward and outward, respectively. The wave activities also propagate slowly upward inside the eyewall and downward outside. The associated eddy heat transport tends to warm the air in the eye region. The vortex Rossby waves lead to, through wave–mean flow interaction as indicated by the divergence of the EP flux, an acceleration of the mean tangential wind in the lower and middle troposphere inside and outside the eyewall and a deceleration aloft in the eyewall region.

Full access
Ali Asaadi, Gilbert Brunet, and M. K. Yau

Abstract

Recently Asaadi et al. found that an easterly wave (EW) train over the Atlantic and eastern Pacific is oriented in a southeast–northwest direction because of the observed tilt in the easterly jet. This tilt results in only one out of four (~25%) waves to be located at the cyclonic critical layer south of the jet axis in a comoving frame, and they subsequently developed into named storms. Asaadi et al. suggested a geometrical view for developing disturbances, which is the coexistence of a nonlinear critical layer and a region of weak meridional potential vorticity (PV) gradient over several days. Asaadi et al. focused on the developing waves and did not investigate the nondeveloping ones.

To determine whether the nondeveloping EWs are not associated with a critical layer, a simple objective tracking technique is used to identify EWs. Composite views of the large-scale structure and characteristics of nondeveloping EWs show that ~91% of nondeveloping waves are not located on a critical layer, while the remaining ~9% indicate characteristics similar to the developing waves. Examination of the composite Okubo–Weiss parameter indicates that the nondeveloping waves are characterized by larger negative values, implying that they are dominated by deformation, unlike developing waves, which tend to be more immune from the deformation.

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
C. D. Hewitt, E. Allis, S. J. Mason, M. Muth, R. Pulwarty, J. Shumake-Guillemot, A. Bucher, M. Brunet, A. M. Fischer, A. M. Hama, R. K. Kolli, F. Lucio, O. Ndiaye, and B. Tapia

Abstract

There is growing awareness among governments, businesses, and the general public of risks arising from changes to our climate on time scales from months through to decades. Some climatic changes could be unprecedented in their harmful socioeconomic impacts, while others with adequate forewarning and planning could offer benefits. There is therefore a pressing need for decision-makers, including policy-makers, to have access to and to use high-quality, accessible, relevant, and credible climate information about the past, present, and future to help make better-informed decisions and policies. We refer to the provision and use of such information as climate services. Established programs of research and operational activities are improving observations and climate monitoring, our understanding of climate processes, climate variability and change, and predictions and projections of the future climate. Delivering climate information (including data and knowledge) in a way that is usable and useful for decision-makers has had less attention, and society has yet to optimally benefit from the available information. While weather services routinely help weather-sensitive decision-making, similar services for decisions on longer time scales are less well established. Many organizations are now actively developing climate services, and a growing number of decision-makers are keen to benefit from such services. This article describes progress made over the past decade developing, delivering, and using climate services, in particular from the worldwide effort galvanizing around the Global Framework for Climate Services under the coordination of UN agencies. The article highlights challenges in making further progress and proposes potential new directions to address such challenges.

Free access
Khalid I. El Fadli, Randall S. Cerveny, Christopher C. Burt, Philip Eden, David Parker, Manola Brunet, Thomas C. Peterson, Gianpaolo Mordacchini, Vinicio Pelino, Pierre Bessemoulin, José Luis Stella, Fatima Driouech, M. M Abdel Wahab, and Matthew B. Pace

On 13 September 1922, a temperature of 58°C (136.4°F) was purportedly recorded at El Azizia (approximately 40 km south-southwest of Tripoli) in what is now modern-day Libya. That temperature record of 58°C has been cited by numerous world-record sources as the highest recorded temperature for the planet. During 2010–11, a World Meteorological Organization (WMO) Commission of Climatology (CCl) special international panel of meteorological experts conducted an in-depth investigation of this record temperature for the WMO World Archive of Weather and Climate Extremes (http://wmo.asu.edu/). This committee identified five major concerns with the 1922 El Azizia temperature extreme record, specifically 1) potentially problematical instrumentation, 2) a probable new and inexperienced observer at the time of observation, 3) unrepresentative microclimate of the observation site, 4) poor correspondence of the extreme to other locations, and 5) poor comparison to subsequent temperature values recorded at the site. Based on these concerns, the WMO World Archive of Weather and Climate Extremes rejected this temperature extreme of 58°C as the highest temperature officially recorded on the planet. The WMO assessment is that the highest recorded surface temperature of 56.7°C (134°F) was measured on 10 July 1913 at Greenland Ranch (Death Valley), California.

Full access