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N. E. Davidson

Abstract

Satellite cloud imagery and single station rainfall data are used to define large-scale cloudy and clear phases of the Australian monsoon for the Winter (W) Monex period 20 December 1978 to 31 January 1979. To examine the flow characteristics and possible forcing mechanisms of these phases, large-wale, objective wind and mean sea level pressure analyses have been produced using W Monex IIB data. Composited Row configurations for each phase show marked differences in areal mean divergence and vertical motion over the monsoon region. The other main distinguishing features are the strength of the northeas trades, the strength of the southeast trades over the Indian Ocean, the amplitude of Southern Hemisphere midlatitude upper troughs and the location of the Northern Hemisphere jet maximum. No obvious differences in the wind field are evident over the deep tropics. For each mean phase, diagnosed divergent wind analyses indicate that the ITCZ is farther south and the Southern Hemisphere Hadley cell more organized for the cloudy phase.

The effect on the individual cloudy and clear events of Northern Hemisphere cold surge events, cross-equatorial flow, Southern Hemisphere trade-wind changes and changes in the monsoonal westerlies is examined. For this season it is suggested that large-scale convective changes were associated with local Hadley cell variations in the Southern Hemisphere. An important part of the Hadley cell intensification for convectively active periods seemed to be the strengthening of the southeast trade-wind maximum off the West Australian cost. The Northern Hemisphere circulation generally played a relatively passive but co- operative role in the short-term variations of the monsoon.

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N. E. Davidson and G. J. Holland

Abstract

Analyses of mean sea level pressure, wind, temperature and dewpoint am used to study the life cycles of two intense, heavy-rain-producing monsoon depressions over northern Australia. Two aspects are considered: (a) the large forcing, using both synoptic flow field changes and angular momentum budgets, and (b) the role of convective and stratiform clouds using kinematic and thermodynamic budgets.

For each situation, the Northern Hemisphere circulation becomes favorable well prior to genesis. The short-term trigger for development is the strengthening of the Southern Hemisphere subtropical ridge at the surface and an amplifying upper-level trough and subtropical jetstreak to the southwest of the formation point.

The outer region structure of these monsoon depressions is remarkably similar to that of a tropical cyclone, even though the systems develop over land. During development, maximum convective heating occurs at middle levels and within a region of already high cyclonic vorticity. Evidence suggests that the cloud population is mostly comprised of deep cumulonimbus clouds, middle-level stratiform cloud and shallow cumulus. The physical significance of these findings is discussed.

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N. E. Davidson, J. L. Mcbride, and B. J. McAvaney

Abstract

Large scale numerical analyses of divergence and the divergent component of wind are examined at two levels in the lower and upper troposphere. The synoptic sequence studied includes the onset of the Southern Hemisphere summer monsoon. Comparison with satellite-observed cloudiness leads to the conclusion that the analyzed patterns of divergence contain synoptically realistic meteorological information. Them seems to be virtually no information, however, in the day-to-day changes in magnitude of analyzed divergence in the lower troposphere, and only a weak signal in the upper troposphere.

The divergent wind analyses reveal the Intertropical Convergence Zone (ITCZ) to be a readily identifiable feature on individual days, and its location to he both vertically consistent and coincident with the satellite-observed cloud. Two days prior to monsoon onset the analyzed ITCZ moves poleward by 8° latitude. Monsoon convection exists at the intersection of Northern and Southern Hemisphere Hadley cells; it is well removed from the upward branch of any east-west Walker circulations in this situation.

The concept of a divergent surge is introduced to denote vertically consistent divergent circulations extending over distances greater than 20° latitude. This concept is shown to be useful in the physical interpretation of the role of the Southern Hemisphere subtropics in the triggering of monsoon onset. Use of the concept is also helpful in relating the day-to-day changes in tropical convection to simultaneous changes in location and intensity of (mean sea level) subtropical high pressure cells in both hemispheres.

In addition, solutions for the divergent component of wind calculated over a limited domain are compared with solutions calculated over a sphere.

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N. E. Davidson, J. L. McBride, and B. J. McAvaney

Abstract

A case study is presented of the onset of the Southern Hemisphere summer monsoon at longitudes near Australia during December 1978. The numerical analyses comprising this case study are used in conjunction with station data and operational manually derived analyses for other years to investigate the following: 1) the case or definition of monsoon onset; 2) the three-dimensional structure of the troposphere during an active monsoon situation; and 3) the flow changes preceding and during the transition from a period of suppressed to a period of enhanced cumulonimbus activity over tropical Australia.

A well-defined onset occurs in six of the seven years considered. Onset, defined as a satellite-observed, large-scale increase in tropical convection, is consistent with that determined by the wind criterion of Troup (1961).

In 1978 onset occurs in two stages: an increase in convergence, followed by an increase in convection. The monsoon cloudiness exists entirely in the region of low-level westerly wind. The convergence extends through a deep layer from the surface to 400 mb and exists in the upward branch of two linked Hadley cells, one from each of the Northern and Southern Hemispheres.

Observations of the flow changes prior to onset lead to the hypothesis that the trigger mechanism lies in the Southern Hemisphere subtropics. It is conjectured that the seasonal buildup of planetary-scale land-sea temperature gradients has reached a critical stage such that the troposphere is in a state of readiness for the monsoon. Before the onset can take place, however, it must wait for the Southern Hemisphere midlatitude synoptic systems to be in such a configuration that low-level trade wind easterlies are prevalent across the Australian continent.

The evidence is discussed also in favor of various alternative triggering mechanisms such as the influence of a Northern Hemisphere cold surge in the South China Sea and the westward propagation of equatorward westerlies from the Pacific Ocean near the international date line.

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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, 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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J. L. McBride, N. E. Davidson, K. Puri, and G. C. Tyrell

Abstract

The evolution of the large-scale flow through the four-month intensive observing period of TOGA COARE is documented from large-scale numerical analyses and GMS cloud imagery produced by the Australian Bureau of Meteorology and transmitted to the field stations during the experiment. The evolution of the flow is dominated by the following phenomena:

1) the normal seasonal evolution of the tropical flow over this region, including a southward and eastward progression of the tropical convective heat source as the Southern Hemisphere monsoon developed and matured;

2) a more eastward than normal progression of this monsoon circulation, associated with a warm event of the ENSO phenomenon;

3) the existence of a major westerly–easterly–westerly cycle of the Madden–Julian low-frequency wave occurring during the latter half of the experimental period, and

4) the development and subsequent movement of tropical cyclones in both (northern and southern) hemispheres.

The Madden–Julian event consisted of two eastward progressions across the domain of satellite-observed cloud, south of the equator. The horizontal scale of the cloud regions is approximately 10° latitude × 40° longitude and the eastward phase speed is approximately 3.7 m s−1. Linear correlation studies substantiate the eastward movement of both cloud and zonal wind across the domain. The correlation analysis reveals a strong relationship between cloud and low-level zonal wind, with the cloud variations leading those in wind by approximately five days.

Time-longitude sections of relative vorticity show that the synoptic activity also progressed eastward with the cloud, and its structure is suggestive that the controlling dynamics (for the synoptic activity) may be the energy dispersion mechanism of Davidson and Hendon. The development of each westerly event was accompanied by a major change in the Southern Hemisphere deep-layer mean flow from easterly to westerly.

Examination of flow fields and satellite imagery for individual days shows that the peak of the first westerly event is associated with the flow patterns surrounding two Southern Hemisphere tropical cyclones. The subsequent rapid evolution to an easterly state occurs as the cyclones move eastward and southward, and the monsoon flow collapses in their wake. There is an accompanying ridging at low levels in the subtropics and the establishment of the Southern Hemisphere subtropical jet. The subsequent reestablishment of the monsoon (the second westerly event) occurs from west to east with the eastward moving cloud bands. There is also a suggestion that an equatorward extension of a Southern Hemisphere upper-level trough may have played a role.

Major active and break periods are identified over four tropical subdomains over the TOGA COARE region. These are most easily defined in the Southern Hemisphere subdomains. They are characterized by a slowly ,varying signal in the satellite-observed average cloud-top temperature. Superimposed on this is a rapid transition between the active and break states.

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N. E. Davidson, G. J. Holland, J. L. McBride, and T. D. Keenan

Abstract

The AMEX observational dataset, with its high temporal and spatial resolution, has been used to study the formation and structure of Tropical Cyclones Irma and Jason. These systems developed and evolved entirely within the experiment's special observing network. The study is mostly based upon six hourly numerical analyses of the mass and wind fields, on 11 vertical levels over a 1.25 lat/long grid. The systems are traced from prior to the formation of a resolvable closed surface circulation to when they were operationally classified as tropical cyclones.

The discussion focuses on the synoptic to cyclone scale changes during formation. Time sections of various kinematic variables, together with an index of deep convection obtained from digital satellite cloud imagery, are used to trace the development.

Both systems developed during active phases of the monsoon and initially were of maximum intensity in the middle troposphere. Low level spinup occurred in three stages. The first stage was associated with the establishment of a favorable large-scale environment and the development of a closed, low-level circulation. The second stage was marked by a strengthening in the low-level outer circulation and the development of a deep vortex. The final stage was the transformation of the tropical depressions into tropical cyclones, and was indicated by a large increase in low-level convergence, a burst in inner core convection, and intensification of the low-level inner circulation.

The evolution of the flow during development agrees well with the results of earlier tropical cyclogenesis studies. Large scale spinup appears to be at least partly associated with downstream Rossby-wave dispersion leading to increases in low-level horizontal wind shear and eventually the formation and strengthening of the low-level outer circulation. For the final transformation to cyclone status we suggest that the establishment of favorable patterns of vertical wind shear and inward propagation of eddy angular momentum flux convergence in the upper troposphere were important for intensification. Thermodynamic structure changes suggest that maintenance, rather than triggering of core convection, was dependent on surface evaporation.

The role of the observed structure changes, together with the processes operating during each phase of development are documented and discussed.

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

Abstract

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.

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Linda A. Paterson, Barry N. Hanstrum, Noel E. Davidson, and Harry C. Weber

Abstract

NCEP–NCAR reanalyses have been used to investigate the impact of environmental wind shear on the intensity change of hurricane-strength tropical cyclones in the Australian region. A method of removing a symmetric vortex from objective analyses is used to isolate the environmental flow. A relationship between wind shear and intensity change is documented. Correlations between wind shear and intensity change to 36 h are of the order of 0.4.

Typically a critical wind shear value of ∼10 m s−1 represents a change from intensification to dissipation. Wind shear values of less than ∼10 m s−1 favor intensification, with values between ∼2 and 4 m s−1 favoring rapid intensification. Shear values greater than ∼10 m s−1 are associated with weakening, with values greater than 12 m s−1 favoring rapid weakening. There appears to be a time lag between the onset of increased vertical wind shear and the onset of weakening, typically between 12 and 36 h.

A review of synoptic patterns during intensification-weakening cycles revealed the juxtaposition of a low-level anticyclone on the poleward side of the storm and an approaching 200-hPa trough to the west. In most cases, intensification commences under weak shear with the approach of the trough, but just prior to the onset of high shear. Further, based on described cases when wind shear was weak but no intensification occurred, it is suggested that weak shear is a necessary but not a sufficient condition for intensification. It is illustrated here that the remote dynamical influence of upper-level potential vorticity anomalies may offset the negative effects of environmental shear.

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