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John L. McBride

Abstract

Vertically integrated budgets of moisture, heat, angular momentum and kinetic energy are calculated from the composite data sets of Part I (McBride, 1981).

The transition from cloud cluster to typhoon/hurricane is characterized by a warming of the troposphere and increase of tangential wind. Observations are presented to show that these effects are not restricted to the system's inner core region, but rather take place over a volume extending out to at least 6° latitude radius from the system center. Accordingly, in this paper cyclogenesis is investigated by analyzing budgets over this large scale.

The heat budget calculations show that the observed warming of the troposphere is an order of magnitude smaller than the other terms in the budget equation. Most of the released latent heat LP 0 is exported laterally through the boundaries of the region through conversion to the term ∇·V s . The portion of LP 0 which is released within the volume acts to counter the net radiative cooling Q R .

All the composite weather systems export moist static energy h through their transverse circulation. This means that intensification cannot be brought about simply in response to increased cumulus heating due to increased mass circulation. To bring about an increase in h, any change must be such that the quantity (E 0 − ∇·V h) is increased, where E 0 is the surface evaporation.

All the composite weather systems export kinetic energy. The export takes place completely in the upper tropospheric outflow layer.

The kinetic energy budgets show a residual requirement for a generation of kinetic energy by subgrid-scale processes. This eddy generation appears to be of the same magnitude as the generation by the mean radial flow, −·∇ϕ¯.

Compared to non-developing systems, developing cloud clusters have twice to three times as much import of relative angular momentum through their lateral boundaries. This is related to the developing system having greater outer radius low-level positive and upper level negative surrounding tangential wind fields.

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John L. McBride

Abstract

Twelve composite data sets are constructed from rawinsonde data in the tropical northwest Pacific and tropical northwest Atlantic Oceans. Each data set is a composite average of approximately 80 individual disturbances. Four different types of non-developing oceanic tropical disturbance are composited. For comparison, disturbances in each ocean are composited at four different stages of intensification from pre-hurricane to hurricane and pre-typhoon to typhoon. In total, 912 different tropical weather systems go into the composites and approximately 40 000 rawinsonde observations are used.

Details are presented on data density, number of individual weather systems averaged and mean position for each composite system. The basic thermodynamic and dynamic properties of the systems are discussed as well as regional differences between the Pacific and the Atlantic. The analyses presented here form a framework for Part II and subsequent papers which use these composite data sets to investigate the genesis and intensification of tropical cyclones.

The composited systems in both oceans exist in easterly winds at low and middle tropospheric levels. The Pacific systems have westerlies close to the south, whereas the Atlantic systems are completely embedded in the easterly trade winds. At the 200 mb level, systems in both oceans are in the vicinity of the subtropical ridge with easterlies to the south and westerlies to the north.

The cloud clusters and tropical cyclones in both oceans are characterized by an upper level warm core and a local positive moisture anomaly. They have a two-layer wind structure with convergence and cyclonic vorticity between the surface and 350 mb and divergence and anticyclonic vorticity above that level. Most of the inflow occurs above the frictional boundary layer and is subsequently believed to be flow down the pressure gradient.

Regional differences between systems in the two oceans are discussed in terms of Gray's seasonal genesis parameter and it is shown that the background surface evaporation rate is much greater in the Pacific than in the Atlantic.

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John L. McBride
and
Raymond Zehr

Abstract

The thermodynamic and dynamic fields surrounding the composite tropical weather systems described in Part I (McBride, 1981a) are examined for differences between non-developing and developing systems. The main findings are as follows: (i) Both non-developing and developing systems are warm core in the upper levels. The temperature (and height) gradients are more pronounced in the developing system, but the magnitudes are so small that the differences would be difficult to measure for individual systems. (ii) The developing or pre-typhoon cloud cluster exists in a warmer atmosphere over a large horizontal scale, for example, out to 8° latitude radius in all directions. (iii) There is no obvious difference in vertical stability for moist convection between the systems. (iv) There is no obvious difference in moisture content or moisture gradient. (v) Pre-typhoon and pre-hurricane systems are located in large areas of high values of low-level relative vorticity. The low-level vorticity in the vicinity of a developing cloud cluster is approximately twice as large as that observed with non-developing cloud clusters. (vi) Mean divergence and vertical motion for the typical western Atlantic weather system are well below the magnitudes found in pre-tropical storm systems. (vii) Once a system has sufficient divergence to maintain 100 mb or more per day upward vertical motion over a 4° radius area, there appears to be no relationship between the amount of upward vertical velocity and the potential of the system for development. (viii) Cyclogenesis takes place under conditions of zero vertical wind shear near the system center. (ix) There is a requirement for large positive zonal shear to the north and negative zonal shear close to the south of a developing system. There is also a requirement for southerly shear to the west and northerly shear to the east. The scale of this shear pattern is over a 10° latitude radius circle with maximum amplitude at ∼6° radius.

Under the assumption of a symmetric disturbance, these findings can be synthesized into one parameter for the potential of a system for development into a hurricane or typhoon: Daily Genesis Potential (DGP) = ζ900 mb − ζ200 mb, when applied over 0-6° radius.

Wind fields are examined surrounding 79 individual weather systems in the tropical Atlantic and it is shown that the composite findings are present on a case by case basis. The individual case analysis also reveals that the high values of DGP must be made up of fairly equal contributions from all directions around the disturbance. This is consistent with the requirement for the existence of zero lines in both zonal and meridional vertical shear.

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Klaus Fraedrich
and
John L. McBride

Abstract

Linear convective instability is revisited to demonstrate the structurally different growth rates of disturbances in balanced and unbalanced models where diabatic heating is parameterized to be proportional to the vertical mass flux, and Ekman-type lower boundary conditions are introduced. The heating parameterization leads to an “effective static stability,” which is negative when the vertical cumulus mass flux exceeds the total mass flux. This results in large-scale convective overturning. The appropriate horizontal scale is the usual Rossby deformation radius modified by the parameter γ−7, where γ is the ratio of cumulus to total mass flux. The unbalanced flow instability varies from zero growth (σ=0) at finite horizontal scale (corresponding to twice the modified deformation radius L=2R) to infinitely large values (σ→∞) at smallest scales (L→0). The growth of the related balanced model commences at the same scale (L=2R) but attains infinitely large values on approaching the scale of the modified deformation radius L=R. This short-wave cutoff appears as a result of the changing vertical mass flux-heating profile associated with the Ekman boundary condition. Growth rates, horizontal length scales, and associated mass flux profiles am qualitatively supported by observations.

A feature of the solution is its dependence on vertical structure. Specifically, for each imposed vertical structure there are two solutions: one unbalanced corresponding to the cloud scale, and one balanced corresponding to the scale of the modified deformation radius. It is the thesis of this paper that the latter (large scale) solution represents a viable mechanism for the initial growth of either cloud clusters or tropical cyclones in nature.

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Klaus Fraedrich
and
John L. McBride

Abstract

The properties of the “free-ride” assumption of balance between diabatic heating and adiabatic cooling are investigated by incorporating it into the classical two-level CISK (Conditional Instability of the Second Kind) model of Charney and Eliassen. The free-ride model is found to give a CISK-type instability when the heating amplitude exceeds the modified static stability (saξ). The free-ride solution is very similar in structure to the CISK solution, except that the free-ride growth rate is independent of scale.

Inspection of the classical CISK model reveals that its growth rate is also independent of scale over the range of scales for which the instability is efficient, and over this range of scales the free-ride and the classical models are essentially identical.

This leads to a new physical interpretation of CISK. Given that the cumulonimbus heating rate is proportional (through the Ekman pumping effect) to the low-level vorticity, the CISK mechanism is interpreted in terms of a balance and a feedback. The balance is the free-ride balance between the (Ekman) heating and the adiabatic cooling by the divergent circulation. The feedback is through increase of the vorticity by the inward advection or angular momentum by the divergent circulation.

This interpretation gives insight into the nature of the CISK mechanism. It explains why the characteristic CISK time scale is half the Ekman spin-down time, and why the space scale is an order of magnitude below the deformation radius. It also reveals that the CISK feedback is through the spinup brought about by the divergent circulation above the boundary layer.

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John L. McBride
and
Kathleen L. McInnes

Abstract

A methodology is presented for comparing nonlinear 3D model output to the physical principles embodied in idealized and analytical models, through the use of the transformation of wind components (v, v g,v a) to accelerations k/f×(−fk×v,∇pd v/dt. This methodology is applied to numerical simulations of the interaction of a cold front with coastal topography, leading to the formation of a “southerly busier” current along Australia's east coast.

It is found that while the front is moving along the south coast mountain chain, the dynamics of the perturbation are similar in character to an orographically trapped density-current-type perturbation on the background front. Along the east coast, however, the southerly busier current bears no dynamical relationship to either an orographically trapped density current or a Kelvin wave, the balance of forces here being dominated by inertial terms.

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Kathleen L. McInnes
and
John L. McBride

Abstract

A diagnostic study Of the Australian southerly buster is performed using a numerical weather prediction model. It is demonstrated that the model, run at 50-km resolution, is capable of capturing the transition of a cold front into a southerly buster as it traverses the cut coast of the Australian continent. The main features of the modified front am identified through comparison of the simulation with an “adiabatic simulation” from which precipitation and surface processes including orography have been removed. From this comparison, the southerly buster effect is defined in terms of the deformation of the front from its adiabatic ostentation and in terms of enhancement of temperature gradients and southerly winds along the coastal strip. Sensitivity experiments are then analyzed with a view to objectively determining the relative roles of mountains, precipitation, and land-sea contrasts of heating and friction in bringing about these perturbations.

The major conclusions are that (i) the primary effect of northward movement of southerly winds along the Australian east coast is brought about by synoptic-scale frontal dynamics, and that (ii) the enhanced northward movement associated with the 5-shaped deformation of the frontal continuity is brought about solely by the interaction of the eastward-moving front with topography. The effects of surface heating and friction in modifying the front in the vicinity of the coast are also discussed.

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Jun-Ichi Yano
and
John L. McBride

Abstract

A hypothesis is proposed that the seasonal evolution of sea surface temperature (SST) is the major forcing to control both the onset and the life cycle of the monsoon. A sensitive coupling of surface heat flux and cumulus convection is the central process and, in the current model, is realized by wind-induced surface-heat exchange. The model adopted is a shallow water analog in dynamics with two vertical levels for thermodynamics. The land forcing effect is neglected as a crucial simplification of the model experiments, along with the absence of the dynamical feedback to the SST in the model.

Experiments with steady SST forcing reveal the presence of three regimes of response. Weak SST forcing realizes two unsteady regimes, depending on the latitude of the forcing: (i) the supercluster regime, characterized by equatorially trapped eastward propagating convective coherencies akin to the Madden–Julian waves, and (ii) the monsoon regime, characterized by an intermittent planetary-scale standing convective oscillation at the subtropics. For a large SST forcing, a steady response is found similar to the earlier solutions of Matsuno, Webster, and Gill.

Experiments with a seasonally varying SST anomaly simulate both the sudden onset and the active–break cycle of the monsoon. In particular the onset is interpreted as the atmosphere undergoing a sudden switch from one dynamical regime to the other. The two unsteady regimes are seen to be in competition, as in the model they cannot coexist. Implications for the atmospheric monsoon are (i) that the SST forcing away from the equator should precede monsoon onset and (ii) that equatorial intraseasonal convective activity should be less active at the time of year when monsoon activity occurs away from the equator.

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Richard A. Dare
and
John L. McBride

Abstract

The response of sea surface temperature (SST) to tropical cyclones is studied using gridded SST data and global cyclone tracks from the period 1981–2008. A compositing approach is used whereby temperature time series before and after cyclone occurrence at individual cyclone track positions are averaged together.

Results reveal a variability of several days in the time of maximum cooling with respect to cyclone passage, with the most common occurrence 1 day after cyclone passage. When compositing is carried out relative to the day of maximum cooling, the global average response to cyclone passage is a local minimum SST anomaly of −0.9°C. The recovery of the ocean to cyclone passage is generally quite rapid with 44% of the data points recovering to climatological SST within 5 days, and 88% of the data points recovering within 30 days. Although differences exist between the mean results from the separate tropical cyclone basins, they are in broad agreement with the global mean results. Storm intensity and translation speed affect both the size of the SST response and the recovery time.

Cyclones occurring in the first half of the cyclone season disrupt the seasonal warming trend, which is not resumed until 20–30 days after cyclone passage. Conversely, cyclone occurrences in the later half of the season bring about a 0.5°C temperature drop from which the ocean does not recover due to the seasonal cooling cycle.

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John L. Mcbride
and
Hugh E. Willoughby

Abstract

This comment presents a detailed examination of the published model results of Kurihara and Kawase in an attempt to clarify the role of wave-CISK in the development of tropical cyclones. Kurihara and Kawase's model simulates the development of a tropical depression, although the vertical structure differs significantly from observations. The physical roles of vertical shear and nonlinear dynamics in the development in this model are unclear. The authors propose that the nonlinear terms in the equations promote rapid growth by increasing the “inertial stiffness”. A major concern, however, is that the enhanced development may occur because the nonlinear terms excite modes with high horizontal wavenumbers. These modes grow rapidly through wave-CISK. From considerations of the climatological importance of horizontal shear to tropical-cyclone development in nature, this model may be less relevant to tropical cyclogenesis than one that allows horizontal sheer of the environmental flow. The authors discuss the model's response to changes in the vertical shear of the basic state, which appears to have the opposite effect in the model from what it has in nature.

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