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K. J. Tory and J. D. Kepert

Abstract

Pyrocumulonimbus (pyroCb) clouds are difficult to predict and can produce extreme and unexpected wildfire behaviour that can be very hazardous to fire crews. Many forecasters modify conventional thunderstorm diagnostics to predict pyroCb potential, by adding temperature (Δθ) and moisture increments (Δq) to represent smoke plume thermodynamics near the expected plume condensation level. However, estimating these Δθ and Δq increments is a highly subjective process that requires expert knowledge of all factors that might influence future fire size and intensity. In this paper, instead of trying to anticipate these Δθ and Δq increments for a particular fire, the minimum firepower required to generate pyroCb for a given atmospheric environment is considered. This concept, termed the PyroCb Firepower Threshold (PFT) requires only atmospheric information, removing the need for subjective estimates of the fire contribution.

A simple approach to calculating PFT is presented that incorporates only basic plume-rise physics, yielding an analytic solution that offers important insight into plume behaviour and pyroCb formation. Minimum increments of Δθ and Δq required for deep, moist convection, plus a minimum cloud-base height (Z fc), are diagnosed on a thermodynamic diagram. Briggs plume rise equations are used to convert Δθ, Z fc, and a mean horizontal windspeed (U) to a measure of the PFT: the minimum heat flux entering the base of the plume. This PFT is proportional to the product of U, Δθ, and the square of Z fc. Plume behaviour insights provided by the Briggs equations are discussed, and a selection of PFT examples presented.

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K. J. Tory and R. A. Dare

Abstract

Almost 70 years ago a sea surface temperature (SST) threshold of 26°–27°C, below which tropical cyclones (TCs) did not form, was proposed, based on a qualitative assessment of warm-season global SST and known TC formation regions. This threshold was widely accepted without further testing, until a recent study suggested a threshold of 25.5°C. That study is revisited here by reexamining the SST for all global TC formations from 1981 to 2008 using (i) a broader range of SST threshold values, (ii) an improved method for identifying subtropical storms—any storm that forms poleward of the subtropical jet (STJ), and (iii) a range of TC formation gestation periods, which refers to a time interval prior to formation in which the SST threshold is exceeded for at least one 6-h period. Consequently, thresholds reported in this paper are expressed as a combination of SST and gestation period.

Using the STJ position to identify and exclude subtropical storms, the threshold of 25.5°C SST–48-h gestation period was found to be robust, but conservative. An examination of TCs of questionable validity (e.g., weak, short lived, and/or storms that formed with baroclinic influences) revealed a further 26 storms (1.2%) that could arguably be excluded from the analysis. With these storms removed, several SST–gestation period threshold combinations were found to be valid, including 25.5°C–18 h and 26.5°C–36 h. A practical threshold combination of 26.5°C–24 h is proposed as only two additional storms failed to meet this threshold, which supports the often-quoted 26.5°C SST necessary condition for TC formation.

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K. J. Tory, P. L. Jackson, and C. J. C. Reason

Abstract

The sensitivity of a coastally trapped disturbance (CTD) to topographic height is examined using simulations of the 15–18 May 1985 CTD. These simulations include three with uniform topography, in which the North American west coast mountains are represented by a three-piece uniform ramp at the coast leading to a constant plateau inland, and three with realistic topography. In each trio of uniform and realistic topography simulations, there is a control case in which the terrain height closely approximates reality, and two variations in which the topography is multiplied everywhere by a topographic multiplication factor (TMF) of 1.5 and 0.5 to assess the sensitivity of the simulation to the barrier height. Average propagation speeds increased (decreased) by 15%–20% with the increased (decreased) TMF.

It was found that the position of the CTD leading edge generally followed close behind a leading pressure trough minimum, which propagated northward along the coast in a manner similar to a topographically trapped Rossby wave (TTRW). The propagation of both the CTD and TTRW was diurnally modulated, with slowing during the day. The diurnal effects were stronger on the CTD propagation, which led to a CTD lag after the heating period followed by an acceleration back toward the position of the trough minimum. Further variability in the CTD propagation was present in the more realistic topography simulations caused by pressure variations ahead of the CTD related to alongshore differences in marine boundary layer (MBL) structure.

The average propagation speed of the leading coastal trough was proportional to barrier height and not barrier slope, which is consistent with TTRW theory applied to the model barrier structure. Due to the dominant influence of the coastal trough on CTD propagation this led to an average CTD propagation speed proportional to TMF.

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K. J. Tory, C. J. C. Reason, and P. L. Jackson

Abstract

A numerical study of the 9–11 November 1982 southeast Australian coastal ridging event is presented. The mesoscale coastal features of this event have been previously described as a coastally trapped disturbance (CTD). However, the analysis presented in this paper shows that the model coastal trapping is of secondary importance in generating the coastal ridging. Given the potential controversy, particular emphasis in this paper is placed on identifying the causes of the ridging. The paper follows a previous study by the authors in which a Colorado State University Regional Atmospheric Modeling System simulation of this event was validated.

In this event, contributions to ridging along the southeastern Australian coast came from both synoptic and mesoscale phenomena in the model. A Southern Ocean anticyclone that tracked east from the Great Australian Bight toward the southern Tasman Sea led to a large-scale steady pressure increase over the entire southeast Australian region. Ridging at the Victorian coastal stations developed with the passage of a synoptic-scale wave of cooler denser air at midlevels, and the arrival of low-level cooler marine air with a wind shift from the southwest to a more southerly direction. On the east coast, the pressure change was most abrupt in the south as warm continental air was replaced with cooler marine air after strong winds passed through Bass Strait and turned left around the southeast continental corner. The flow closest to the coast continued turning left, enhanced by the daytime lowering of pressure over the continent, and pushed inland to the mountain barrier.

The most intense part of the wind surge crossed the coast during the evening near the Hunter Valley (a significant gap in the east coast mountain barrier). This led to flow splitting inland up the Hunter Valley and northward along the east coast. The coastal component developed into a CTD in the form of a wind surge that accelerated ahead of the region of onshore forcing until it encountered a reduced mountain barrier farther north, and the flow spilled inland signaling the end of the model event.

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C. J. C. Reason, K. J. Tory, and P. L. Jackson

Abstract

Numerical simulations of the 15–17 May 1985 coastally trapped disturbance (CTD) event along the west coast of North America are compared with the schematic model of CTD evolution developed by (SRK), which was based upon more idealized simulations. It is shown that the general evolution of the simulated May 1985 CTD is consistent with the SRK schematic model. It is further shown that secondary effects not contained in the SRK simulations, such as diurnal radiation variations and mesoscale topographic variations, can account for the variable CTD initiation and propagation observed both in nature and in the present numerical simulations. Diurnal radiation variations, coupled with differential heating of land and ocean, appear to play an important role in setting up the alongshore temperature gradient necessary for CTD formation and evolution. The modeled CTD is found to change dynamical characteristics from an initial Kelvin wave/bore similar to that discussed by Ralph, Nieman, Persson, Bane, Cancillo, and Wilczak to a gravity current, and this change is consistent and coincident with a sharp change in translation speed of the disturbance.

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K. J. Tory, N. E. Davidson, and M. T. Montgomery

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 diabatic heating, in the updraft cores (secondary mechanism). The nondeveloping cases illustrated that genesis can be hampered by (i) vertical wind shear, which may tilt and tear apart the PV cores as they develop, and (ii) an insufficient large-scale cyclonic environment, which may fail to sufficiently confine the warming and enhanced cyclonic winds, associated with the atmospheric adjustment to the convective updrafts.

The exact detail of the vortex interactions was found to be unimportant for qualitative genesis forecast success. Instead the critical ingredients were found to be sufficient net deep convection in a sufficiently cyclonic environment in which vertical shear was less than some destructive limit. The often-observed TC genesis pattern of convection convergence, where the active convective regions converge into a 100-km-diameter center, prior to an intense convective burst and development to tropical storm intensity is evident in the developing TC-LAPS simulations. The simulations presented in this study and numerous other simulations not yet reported on have shown good qualitative forecast success. Assuming such success continues in a more rigorous study (currently under way) it could be argued that TC genesis is largely predictable provided the large-scale environment (vorticity, vertical shear, and convective forcing) is sufficiently resolved and initialized.

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K. J. Tory, M. T. Montgomery, and N. E. Davidson

Abstract

This is the first 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. The primary TC-LAPS vortex enhancement mechanism is presented in Part I, the entire genesis process is illustrated in Part II using a single TC-LAPS simulation, and in Part III a number of simulations are presented exploring the sensitivity and variability of genesis forecasts in TC-LAPS.

The primary vortex enhancement mechanism in TC-LAPS is found to be convergence/stretching and vertical advection of absolute vorticity in deep intense updrafts, which result in deep vortex cores of 60–100 km in diameter (the minimum resolvable scale is limited by the 0.15° horizontal grid spacing). On the basis of the results presented, it is hypothesized that updrafts of this scale adequately represent mean vertical motions in real TC genesis convective regions, and perhaps that explicitly resolving the individual convective processes may not be necessary for qualitative TC genesis forecasts. Although observations of sufficient spatial and temporal resolution do not currently exist to support or refute this proposition, relatively large-scale (30 km and greater), lower- to midlevel tropospheric convergent regions have been observed in tropical oceanic environments during the Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE), the Equatorial Mesoscale Experiment (EMEX), and the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE), and regions of extreme convection of the order of 50 km are often (remotely) observed in TC genesis environments. These vortex cores are fundamental for genesis in TC-LAPS. They interact to form larger cores, and provide net heating that drives the system-scale secondary circulation, which enhances vorticity on the system scale akin to the classical Eliassen problem of a balanced vortex driven by heat sources. These secondary vortex enhancement mechanisms are documented in Part II.

In some recent TC genesis theories featured in the literature, vortex enhancement in deep convective regions of mesoscale convective systems (MCSs) has largely been ignored. Instead, they focus on the stratiform regions. While it is recognized that vortex enhancement through midlevel convergence into the stratiform precipitation deck can greatly enhance midtropospheric cyclonic vorticity, it is suggested here that this mechanism only increases the potential for genesis, whereas vortex enhancement through low- to midlevel convergence into deep convective regions is necessary for genesis.

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K. J. Tory, J. D. Kepert, J. A. Sippel, and C. M. Nguyen

Abstract

This 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 for many convective system applications, there are likely to be some applications in which the formulation is difficult to apply or is not ideal.

This study introduces a family of PV tendency equations in geometric coordinates that is free from the deficiencies of the above formulations. Simpler forms are complemented by more complex forms that expand the vorticity tendency term to offer additional insight into flow dynamics. The more complex forms provide insight similar to the influential Haynes and McIntyre isentropic formulation.

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K. J. Tory, S. S. Chand, R. A. Dare, and J. L. McBride

Abstract

A 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 underdetection was present in all models. The GFDL CM2.0 model was excluded from the projection study because of a systematic underdetection in all basins. The above detection problems have been reported in other published studies, which suggests model rather than detector limitations are mostly responsible. This study demonstrates that coarse-resolution climate models do in general produce TC-like circulations with realistic geographical and seasonal distributions detectable by the OWZP TC detector. The preliminary projection results are consistent with the published literature, based on higher-resolution studies, of a global reduction of TCs between about 6% and 20%, with a much larger spread of results (about +20% to −50%) in individual basins.

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K. J. Tory, S. S. Chand, R. A. Dare, and J. L. McBride

Abstract

A novel approach to tropical cyclone (TC) detection in coarse-resolution numerical model data is introduced and assessed. This approach differs from traditional detectors in two main ways. First, it was developed and tuned using 20 yr of ECMWF Interim Re-Analysis (ERA-Interim) data, rather than using climate model data. This ensures that the detector is independent of any climate models to which it will later be applied. Second, only relatively large-scale parameters resolvable in climate models are included, in order to minimize any grid-resolution dependence on parameter thresholds. This approach is taken in an attempt to construct a unified TC detection procedure applicable to all climate models without the need for any further tuning or adjustment.

Unlike traditional detectors that seek to identify TCs directly, the authors' method seeks to identify conditions favorable for TC formation. Favorable TC formation regions at the center of closed circulations in the lower troposphere to the midtroposphere are identified using a low-deformation vorticity parameter. Additional relative and specific humidity thresholds are applied to ensure the thermodynamic environment is favorable, and a vertical wind shear threshold is applied to eliminate storms in a destructive shear environment. A further requirement is that thresholds for all parameters must be satisfied for at least 48 h before a TC is deemed to have developed.

A thorough assessment of the detector performance is provided. It is demonstrated that the method reproduces realistic TC genesis frequency and spatial distributions in the ERA-Interim data. Application of the detector to four climate models is presented in a companion paper.

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