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Noel E. Davidson

Abstract

Diagnostics from the observational dataset of the Australian Monsoon Experiment (AMEX) have revealed two interesting characteristics of convective systems over the Australian Tropics (Part I of this study). The first is a midlevel convergence maximum in situations of disorganized convection, which implies weak low-level and strong upper-level convective heating. The second is the presence of large apparent vorticity sources during deep convective and stratiform events. It is suggested that these characteristics are related to the ascent and descent motions in tropical cloud systems and to the way in which they redistribute mass and the background vorticity.

To investigate the importance of these features of convection on tropical prediction, a representation of the observed thermodynamic and kinematic effects of clouds has been implemented in the Bureau of Meteorology Research Centre's tropical limited-area model. To represent the observed structure of convective heating in numerical simulation experiments, an analytic heating function is constructed that is similar to the observed heating profile. Satellite cloud imagery is then used to trigger the function in space and time during the model integration. In this way the importance of the convective processes can be assessed, without the uncertainties of incorrect triggering of the parameterization. To represent the observed structure of convective heating in numerical prediction experiments, the Kuo heating profile is modified to reflect the (AMEX) observed profile, with weak low-level heating. To parameterize the kinematic effects, a potential function is introduced. Vorticity tendencies due to clouds are represented by the Laplacian of the function, and the momentum tendencies by the spatial derivatives. Vorticity tendencies are specified according to the observed structure of apparent sources and are dependent on the level of maximum convergence and the ambient vorticity and divergence profiles.

The representation of convection, including both thermodynamic and kinematic effects, has been run in a number of simulations of AMEX weather events. It is shown that, in a limited number of events, realistic forecasts of monsoon onset, midtropospheric lows, and tropical cyclogenesis can be obtained.

For tropical cyclogenesis, it is shown that upper-level convective heating produces the required midlevel convergence and eventually a midtropospheric circulation. The parameterized apparent vorticity sources then act to spin down the midlevels and spin up the upper and lower levels. This, together with the resultant enhanced boundary layer convergence, is sufficient to intensify the low-level circulation without the need for large low-level convective heating. Indeed, simulations with convective heating, and no representation of apparent vorticity sources, cannot accurately reproduce the observed four-dimensional structure of the developing tropical cyclone.

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Noel E. Davidson

Abstract

Six-hourly analyses based on the special observational dataset from the Australian Monsoon Experiment are used to derive vorticity budget diagnostics for a number of tropical weather situations. The quality of the analyses is demonstrated by observation fitting statistics, comparison of digital satellite cloud imagery with diagnosed vertical motion, and by comparison of derived quantities with those obtained directly from the observations using line integral calculations.

For the mean of the entire experimental period, balance generally exists between stretching and horizontal advection, with some contribution from an apparent vorticity source at upper levels. The individual-day behavior, however, is often quite different. Three categories are evident: 1) For weak, low-level cyclonic flows at the time of maximum convection (disorganized, deep convection), an apparent source of cyclonic vorticity is evident at low and high levels, and a sink is indicated through midlevels. These situations are characterized by a midlevel convergence maximum and a related cyclonic vorticity maximum. 2) For strong, low-level vorticity regimes (circulation systems with organized convection), an apparent sink of vorticity is evident everywhere below the convective outflow level, with a source above. These situations are characterized by deep convergence and a low level of maximum vorticity. 3) The third category, which seems to be associated with mostly stratiform regimes, is similar to 1), but a source is diagnosed at mid- to high levels (possibly associated with the stratiform cloud deck) with a sink farther aloft. The author postulates that die vertical structure of apparent vorticity sources is determined by the ascent and descent motions of the dominant cloud forms, which redistribute the background vorticity, and by enhanced boundary layer convergence as circulation systems spin up.

The implications of the (a) midlevel convergence maximum and (b) apparent vorticity sources to the understanding and prediction of monsoon onset, midtropospheric cyclones, and tropical cyclone behavior are discussed. In a companion paper, a representation of the apparent heat and vorticity sources is implemented in a numerical model and used in simulations of the above weather phenomenon.

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Kamal Puri and Noel E. Davidson

Abstract

Geostationary and polar-orbiting satellites can provide useful proxy sources of moisture data and diabatic heating. It is shown that the use of this information during data assimilation leads to improved precipitation in the tropics and has the potential to minimize spinup in the model. Furthermore, the use of moisture initialization leads to improved agreement between the model and observed precipitation during the early stages of model integration.

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Noel E. Davidson and Kamal Puri

Abstract

Some notable problems in tropical prediction have been (i) the sensitivity to, and inaccuracies in, the four-dimensional structure of parameterized convective heating, (ii) the inability of conventional data networks to adequately define tropical cyclone structures, and (iii) the so-called spinup problem of numerical models. To help overcome some of these deficiencies, a diabatic nudging scheme has been developed for the Bureau of Meteorology Research Centre (BMRC) limited-area tropical prediction system.

A target analysis for the nudging is first obtained from statistical interpolation of all observational data, using, as first-guess field, output from a global assimilation and prediction system. Tropical cyclones are optionally inserted via bogus wind observations. From 12 or 24 h prior to the base time of the forecast, the prediction model is nudged toward the target analysis. During nudging the “observationally reliable” rotational wind component is preserved and the heating from the Kuo scheme is replaced by a heating function determined from 6-h satellite-observed cloud-top temperatures. The system introduces realistic tropical cyclone structures into the initial condition, defines a vertical-motion field consistent with the satellite cloud imagery, enhances rainfall rates during the early hours of the forecast, reduces the occurrence of spurious rainfall maxima, and improves mass-wind balance and retention of cyclone circulations during the model integration.

Examples of system performance from enhanced observational datasets and from real-time forecasting are presented. Encouraging results for short-term prediction of both tropical cyclone behavior and rainfall events are documented.

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Noel E. Davidson and Arun Kumar

Abstract

High resolution observational data from the Australian Monsoon Experiment have been used to verify simulations of the development of tropical cyclone Irma.

From a small amplitude, prehurricane cloud cluster, the FSU high resolution regional prediction model quite skillfully simulates the temporal and spatial structure changes during development. The results are, however, sensitive to the initial windfield and somewhat sensitive to both the initial moisture field and the imposed boundary conditions.

Temporal changes in the symmetric and asymmetric structure of the observed and simulated disturbances are compared. Apart from other well-documented necessary conditions, changes in the storm's vertical structure, and the development of horizontal asymmetries appear to be crucial features of the simulated development. The associated physical processes are discussed.

Deficiencies in the simulation are lack of diurnal modulation of the vertical motion field, larger than diagnosed values of vertical motion over the genesis area, and an upper level flow that is too divergent and anticyclonic. We speculate that more accurate representation of diurnal effects, and a parameterization of momentum transports by cumulus clouds will reduce these deficiencies.

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Noel E. Davidson and Harry H. Hendon

Abstract

Evidence is presented of a downstream development mechanism operating across the entire longitudinal span of the 1978/79 Southern Hemisphere monsoon. Observationally it is seen as progressive cyclonic and anticyclonic vorticity increases that develop eastward in the monsoon trough at a speed of approximately 5 m s−1. The process results in many tropical cyclone and tropical depression formations over northern Australia and the South Pacific.

It is shown that the downstream development process is generally consistent with linearized barotropic dynamics, and that the Southern Hemisphere monsoon, because of an intrinsic westerly basic state, is a particularly suitable region for downstream events.

It is also shown that some apparent contradictions in previous observational studies can be rationalized by the theory. The interactions between the regional components of the monsoon (Indonesian, Australian and South Pacific sectors) can also he better understood. We further suggest that the process has implications for other features of the monsoon circulation, namely onset and 40–50 day events.

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Richard A. Dare and Noel E. Davidson

Abstract

Characteristics of 500 tropical cyclones (TCs) in the Australian region and its three individual basins are examined based on 40 yr of satellite-supported observations. While tropical cyclones exhibit highly individual behaviors resulting in significant standard deviations, there are some systematic behaviors, which are documented. Most TCs in the Australian region originate from December to April. About 13 are observed each season, with half occurring in the western basin. Generally, the lifetime of a TC is about 7½ days, during which time it covers over 2500 km at a mean speed of 15 km h−1. Around half of the storms reach a maximum intensity corresponding to category 3 or higher (<970 hPa), as classified using a modified Saffir–Simpson scale. Tropical cyclones in the western and eastern basins have around 25% chance of making landfall, while those in the northern basin have an 80% chance.

There appear to be preferred locations for TC genesis, close to the Australian coastline at around 120°, 135°, and 150°E. Genesis occurs near the mean position of the maximum low-level cyclonic vorticity and coincides with the monsoon trough from December to February, but occurs poleward of the trough in other months. The maximum intensity eventually achieved by TCs varies with genesis locations. For storms that reach category 3 or above, there are more corresponding origin points in the west than in both the gulf and the eastern basin.

Recurvature generally follows attainment of maximum intensity, suggesting the importance of trough interactions on this behavior. The likelihood of extratropical transition of TCs, in the mean, increases to a peak in March, although there is variability across the three basins of the Australian region. Final dissipation has no preferred latitude, with many storms dissipating over the warm tropical oceans equatorward of 20°S.

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Noel E. Davidson and Harry C. Weber

Abstract

A new Tropical Cyclone Limited Area Prediction System has been developed at the Australian Bureau of Meteorology Research Centre. The features of the new prediction system can be summarized as follows: first, a 12-h, coarse-resolution data assimilation is used to define the outer structure and environment of the storm and to provide initial conditions for coarse mesh prediction. Then, synthetic data are generated to define a storm’s circulation, consistent with observed location, size, intensity, and past motion. The method involves a definition of the environmental flow by filtering of the misplaced tropical cyclone circulation in the (old) objective analysis, the generation of a new, correctly located and intense symmetric vortex, and the construction of vortex asymmetries by requiring that the observed motion be the vector sum of the environmental flow and the asymmetric flow. The subsequent initialization for fine-mesh prediction is carried out using 24 h of diabatic, dynamical nudging through 6-hourly, high-resolution objective analyses, which include the synthetic vortex. During this phase the vorticity and surface pressure fields are largely preserved, while infrared satellite cloud imagery is used to reconstruct the vertical motion field. Finally, numerical prediction is carried out with a high-resolution version of the operational limited-area model of the Australian Bureau of Meteorology, which includes high-order numerics and advanced physical parameterizations.

The very encouraging quality of the forecasts is demonstrated in numerous case studies of tropical cyclone events, including improved prediction for some situations when the official forecasts were poor. Average track errors at 24 and 48 h are 115 and 259 km, respectively. These are significantly smaller than corresponding errors in the official and in climatology-persistence forecasts. Given the uncertainties in estimated central pressures, forecasts of intensity are also encouraging. General discussion focuses on system characteristics and some remaining, unresolved issues.

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Xingbao Wang, Yimin Ma, and Noel E. Davidson

Abstract

Multiple secondary eyewall formations (SEFs) and eyewall replacement cycles (ERCs) are simulated with the fifth-generation Pennsylvania State University (PSU)–National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) at horizontal grid spacing of 0.67 km. The simulated hurricane is initialized from a weak, synthetic vortex in a quiescent environment on an f plane. After spinup and rapid intensification, the hurricane enters a mature phase during which the intensity change is relatively slow. Convective clouds then organize into a ring with a secondary tangential wind maximum at radii beyond the hurricane’s primary eyewall. This secondary eyewall (SE) then contracts and strengthens. The primary eyewall weakens and is eventually replaced by the SE. The hurricane grows in size and the radius of maximum wind (RMW) increases as similar ERCs repeat 5 times during the simulation.

Two existing hypotheses on SEF are evaluated using the simulation output. Then, model diagnostics are used to reveal that crucial linked components of SEF are (i) a broadening of the swirling flow, (ii) the structure of the evolving secondary circulation, and (iii) the structure of the net radial force (NRF) in the boundary layer (with largest contributions from the agradient and frictional forces). During SEF, there exists strong positive NRF in the region of the primary eyewall, a secondary positive maximum over the SEF region, and a minimum between the two. As a response of the boundary layer depth–integrated radial flow to the NRF, a secondary maximum convergence zone (SMCZ) in the boundary layer develops at the SEF radii. Eventually moist convection in the SMCZ becomes active as the SEF develops.

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Jianjun Li, Noel E. Davidson, G. Dale Hess, and Graham Mills

Abstract

The landfall on the China coast of two typhoons has been studied using both observational diagnostics and a 30-km, 19-level prediction system. The typhoons vary in size and in the amount of rainfall they produced after landfall. Use is made of a synthetic typhoon insertion scheme and the relatively dense observational network around China to provide objective analyses.

The authors illustrate that, in the forecast model, as the typhoon’s outer circulation impinges on the China coast, an outer cloud band develops offshore, parallel to the coast, extending cyclonically north and east. During this time, the inner circulation weakens. As the outer cloud band strengthens, the center of the circulation approaches it from the east and there is a short-term intensification of both the inner region vertical motion and vorticity. Following landfall rapid dissipation of the core occurs.

Using detailed diagnostics from the model integration, the changing thermal structure of the forecast vortex is illustrated. It is argued that, in the model, over the sea with near-zero sensible heat flux and a diagnosed low-level cold temperature anomaly inside the radius of maximum winds, downward mixing and horizontal, downgradient inflow cooling are the dominant near-surface processes. Over land, similar processes are operating, but diurnally varying sensible heat flux somewhat modifies the balance.

Within the limitations of (a) the quality of the observational network, (b) the ability of the numerical model to represent real atmospheric processes, and (c) the limited number of events analyzed, the following conclusions are drawn. 1) The forecasts of the events, including the rainfall patterns, are quite realistic and provide encouragement for improvements in operational prediction. 2) The time evolution of typhoon structure in the analyses and forecasts shows (a) a weakening of the circulations as they approach land, (b) a slowdown in the weakening (or sometimes strengthening) near landfall, and (c) rapid decay after landfall. 3) As suggested previously, the strengthening seems due to enhanced inflow as the circulations approach the land from the sea. However, for these cases this occurs as the typhoon’s circulation impinges on the coastline and an offshore outer cloud band intensifies and interacts with the approaching vortex center. 4) Weakening after landfall seems related to (a) diurnally modulated cooling of the low levels due to enhanced downgradient inflow, (b) reduced downward mixing of warm air in the lighter wind region near the vortex center, (c) the finite heat capacity of the land and the limit placed on the maximum low-level temperatures by the surrounding sea temperatures, and (d) stabilization following enhanced convection at landfall. These processes act to increase the central pressure and inhibit the development of further deep convection by limiting the heat and moisture supply from the surface and reducing the moist convective instability necessary to grow deep convective clouds. Upper-air soundings areused to validate the evolving structures. 5) Lagrangian trajectory diagnostics suggest that the amount of rainfall at landfall and the subsequent circulation characteristics may in part be determined by the depth and extent of moist inflow from over the sea.

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