Kevin J. Tory

Projected Changes in Late-Twenty-First-Century Tropical Cyclone Frequency in 13 Coupled Climate Models from Phase 5 of the Coupled Model Intercomparison Project

AbstractChanges in tropical cyclone (TC) frequency under anthropogenic climate change are examined for 13 global models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), using the Okubo–Weiss–Zeta parameter (OWZP) TC-detection method developed by the authors in earlier papers. The method detects large-scale conditions within which TCs form. It was developed and tuned in atmospheric reanalysis data and then applied without change to the climate models to ensure model and detector independence. Changes in TC frequency are determined by comparing TC detections in the CMIP5 historical runs (1970–2000) with high emission scenario (representative concentration pathway 8.5) future runs (2070–2100). A number of the models project increases in frequency of higher-latitude tropical cyclones in the late twenty-first century. Inspection reveals that these high-latitude systems were subtropical in origin and are thus eliminated from the analysis using an objective classification technique. TC detectio...

Prediction and Diagnosis of Tropical Cyclone Formation in an NWP System. Part II: A Diagnosis of Tropical Cyclone Chris Formation

This is the second of a three-part investigation into tropical cyclone (TC) genesis in the Australian Bureau of Meteorology’s Tropical Cyclone Limited Area Prediction System (TC-LAPS). The primary TC-LAPS vortex enhancement mechanism (convergence/stretching and vertical advection of absolute vorticity in convective updraft regions) was presented in Part I. In this paper (Part II) results from a numerical simulation of TC Chris (western Australia, February 2002) are used to illustrate the primary and two secondary vortex enhancement mechanisms that led to TC genesis. In Part III a number of simulations are presented exploring the sensitivity and variability of genesis forecasts in TC-LAPS. During the first 18 h of the simulation, a mature vortex of TC intensity developed in a monsoon low from a relatively benign initial state. Deep upright vortex cores developed from convergence/stretching and vertical advection of absolute vorticity within the updrafts of intense bursts of cumulus convection. Individual convective bursts lasted for 6–12 h, with a new burst developing as the previous one weakened. The modeled bursts appear as single updrafts, and represent the mean vertical motion in convective regions because the 0.15° grid spacing imposes a minimum updraft scale of about 60 km. This relatively large scale may be unrealistic in the earlier genesis period when multiple smaller-scale, shorter-lived convective regions are often observed, but observational evidence suggests that such scales can be expected later in the process. The large scale may limit the convection to only one or two active bursts at a time, and may have contributed to a more rapid model intensification than that observed. The monsoon low was tilted to the northwest, with convection initiating about 100–200 km west of the low-level center. The convective bursts and associated upright potential vorticity (PV) anomalies were advected cyclonically around the low, weakening as they passed to the north of the circulation center, leaving remnant cyclonic PV anomalies. Strong convergence into the updrafts led to rapid ingestion of nearby cyclonic PV anomalies, including remnant PV cores from decaying convective bursts. Thus convective intensity, rather than the initial vortex size and intensity, determined dominance in vortex interactions. This scavenging of PV by the active convective region, termed diabatic upscale vortex cascade, ensured that PV cores grew successively and contributed to the construction of an upright central monolithic PV core. The system-scale intensification (SSI) process active on the broader scale (300–500-km radius) also contributed. Latent heating slightly dominated adiabatic cooling within the bursts, which enhanced the system-scale secondary circulation. Convergence of low- to midlevel tropospheric absolute vorticity by this enhanced circulation intensified the system-scale vortex. The diabatic upscale vortex cascade and SSI are secondary processes dependent on the locally enhanced vorticity and heat respectively, generated by the primary mechanism.

Prediction and Diagnosis of Tropical Cyclone Formation in an NWP System. Part III: Diagnosis of Developing and Nondeveloping Storms

Abstract This is the third of a three-part investigation into tropical cyclone (TC) genesis in the Australian Bureau of Meteorology’s Tropical Cyclone Limited Area Prediction System (TC-LAPS), an operational numerical weather prediction (NWP) forecast model. In Parts I and II, a primary and two secondary vortex enhancement mechanisms were illustrated, and shown to be responsible for TC genesis in a simulation of TC Chris. In this paper, five more TC-LAPS simulations are investigated: three developing and two nondeveloping. In each developing simulation the pathway to genesis was essentially the same as that reported in Part II. Potential vorticity (PV) cores developed through low- to middle-tropospheric vortex enhancement in model-resolved updraft cores (primary mechanism) and interacted to form larger cores through diabatic upscale vortex cascade (secondary mechanism). On the system scale, vortex intensification resulted from the large-scale mass redistribution forced by the upward mass flux, driven by d...

Impact of Different ENSO Regimes on Southwest Pacific Tropical Cyclones

AbstractThe influence of different types of ENSO on tropical cyclone (TC) interannual variability in the central southwest Pacific region (5°–25°S, 170°E–170°W) is investigated. Using empirical orthogonal function analysis and an agglomerative hierarchical clustering of early tropical cyclone season Pacific sea surface temperature, years are classified into four separate regimes (i.e., canonical El Nino, canonical La Nina, positive-neutral, and negative-neutral) for the period between 1970 and 2009. These regimes are found to have a large impact on TC genesis over the central southwest Pacific region. Both the canonical El Nino and the positive-neutral years have increased numbers of cyclones, with an average of 4.3 yr−1 for positive-neutral and 4 yr−1 for canonical El Nino. In contrast, during a La Nina and negative-neutral events, substantially fewer TCs (averages of ~2.2 and 2.4 yr−1, respectively) are observed in the central southwest Pacific. The enhancement of TC numbers in both canonical El Nino an...

An Assessment of a Model-, Grid-, and Basin-Independent Tropical Cyclone Detection Scheme in Selected CMIP3 Global Climate Models

AbstractA novel tropical cyclone (TC) detection technique designed for coarse-resolution models is tested and evaluated. The detector, based on the Okubo–Weiss–Zeta parameter (OWZP), is applied to a selection of Coupled Model Intercomparison Project, phase 3 (CMIP3), models [Commonwealth Scientific and Industrial Research Organisation Mark, version 3.5 (CSIRO-Mk3.5); Max Planck Institute ECHAM5 (MPI-ECHAM5); and Geophysical Fluid Dynamics Laboratory Climate Model, versions 2.0 (GFDL CM2.0) and 2.1 (GFDL CM2.1)], and the combined performance of the model and detector is assessed by comparison with observed TC climatology for the period 1970–2000. Preliminary TC frequency projections are made using the three better-performing models by comparing the detected TC climatologies between the late twentieth and late twenty-first centuries. Very reasonable TC formation climatologies were detected in CSIRO-Mk3.5, MPI-ECHAM5, and GFDL CM2.1 for most basins, with the exception of the North Atlantic, where a large und...

Extratropical–Tropical Interaction during Onset of the Australian Monsoon: Reanalysis Diagnostics and Idealized Dry Simulations

Abstract The onset of the Australian monsoon is examined using (i) reanalysis data for seasons when enhanced observational networks were available and (ii) a 15-yr onset composite. Similar to previous findings, onset is characterized by a sudden strengthening and deepening in tropical westerly winds, which are overlain with upper-tropospheric easterlies. All onsets are preceded by up to a 7-day preconditioning period of enhanced vertical motion and moistening. During the transition season, the 6 weeks prior to onset, a number of moist westerly events occur. Generally they are only sustained for short periods and overlain by upper-level westerly winds, suggesting an association with midlatitude troughs, which protrude into the deep Tropics. For individual years and for a 15-yr composite, monsoon onset is associated with major cyclogenesis events over the southwest Indian Ocean in the presence of a subtropical jet over the eastern Indian Ocean. The proposed mechanism for extratropical–tropical interaction i...

On the Use of Potential Vorticity Tendency Equations for Diagnosing Atmospheric Dynamics in Numerical Models

AbstractThis study critically assesses potential vorticity (PV) tendency equations used for analyzing atmospheric convective systems. A generic PV tendency format is presented to provide a framework for comparing PV tendency equations, which isolates the contributions to PV tendency from wind and mass field changes. These changes are separated into forcing terms (e.g., diabatic or friction) and flow adjustment and evolution terms (i.e., adiabatic motions).One PV tendency formulation analyzed separates PV tendency into terms representing PV advection and diabatic and frictional PV sources. In this form the PV advection is shown to exhibit large cancellation with the diabatic forcing term when used to analyze deep convective systems, which compromises the dynamical insight that the PV tendency analysis should provide. The isentropic PV substance tendency formulation of Haynes and McIntyre does not suffer from this cancellation problem. However, while the Haynes and McIntyre formulation may be appropriate fo...

The Different Impact of Positive-Neutral and Negative-Neutral ENSO Regimes on Australian Tropical Cyclones

AbstractThe number of tropical cyclones (TCs) in the Australian region exhibits a large variation between different ENSO regimes. While the difference in TC numbers and spatial distribution of genesis locations between the canonical El Nino and La Nina regimes is well known, the authors demonstrate that a statistically significant difference in TC numbers also exists between the recently identified negative-neutral and positive-neutral regimes. Compared to the negative-neutral and La Nina regimes, significantly fewer TCs form in the Australian region during the positive-neutral regime, particularly in the eastern subregion. This difference is attributed to concomitant changes in various large-scale environmental conditions such as sea level pressure, relative vorticity, vertical motion, and sea surface temperature.

The Development of the Australian Air Quality Forecasting System: Current Status

The AAQFS is routinely providing highspatial resolution air quality forecasts for guidance for the EPAs in Melbourne and Sydney. Case studies of photochemical smog events in Melbourne and Sydney have given encouraging agreement with observations. Meteorologically the two airsheds present different challenges: in Melbourne it is important to predict the onset and strength of the Port Phillip Bay breeze and the Bass Strait sea breeze; in Sydney is it important to predict the onset and strength of the Tasman sea breeze, the pollution plume trajectory for flow over complex terrain and the effects of local photochemical smog production and inter-regional transport. In the case studied for Sydney there was an additional complication of a synoptic-scale wind surge called the Southerly Buster. For both airsheds, the interaction between synoptic-scale forcing and mesoscale circulations can strongly influence the characteristics of an air pollution event and thus the meteorological model must be able to accurately simulate these interactions. In general, the LCC photochemical mechanism gave better predictions of the 1-hour ozone peak than the GRS mechanism. Improvements to the GRS mechanism and emissions inventory and online modelling of emissions and photochemistry are being developed and implemented. Work on the meteorological model to improve surface winds, soil moisture analysis and boundary-layer height also continues. We have yet to establish the limits of predictability of the system.

The Australian Air Quality Forecasting System: Modelling of a Severe Smoke Event in Melbourne, Australia

The AAQFS currently issues twice daily 24–36 hour numerical air quality forecast for the Melbourne and Sydney regions for Victoria EPA and NSW EPA in time to provide reference and guidance for air quality forecasting. One of the potential usages of the AAQFS is to provide guidance in the management of prescribed burns and in alerting the public of smoke impacts on air quality.