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J. L. McBride

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J. L. McBride

Abstract

The use of the Arakawa-Schubert cloud model to diagnose cloud mass fluxes from the large-scale budgets is becoming an increasingly popular technique in observational tropical meteorology. The results of such studies are now being widely quoted in discussions of convective parameterization and of convective scale-larger scale interactions.

In this paper a concise summary is presented of the technique's methodology and a comparison is made of solutions obtained from different versions of the model (i.e., the basic model, the model with downdrafts, the model with lateral detrainment, etc.) A comparison also is made of solutions obtained on different tropical data sets (cloud clusters from the tropical Northwest Pacific and convective systems from the GARP Atlantic Tropical Experiment).

A simple algebraic analysis on the model equations yields some interesting relationships between the mass flux distribution and the large-scale parameters. In particular:

  1. 1) Once QR h, * are specified, the deep cumulonimbus mass flux is related only to the upper level large-scale vertical velocity.

  2. 2) The lack of convection with tops in the middle troposphere is a result of the shape of the tropical vertical profile of h. The mid-tropospheric minimum means that ωc(hch) for deep clouds is of the same order at middle levels as ′.

  3. 3) For a given amount of upper level divergence and deep convection, the shallow convective activity is inversely related to the magnitude of the low-level convergence.

  4. 4) For the diagnostic cloud model to have a solution, strong low-level convergence requires small low-level values of moist static energy.

These relationships follow mathematically from the insertion of the Arakawa-Schubert cloud model into the large-scale equations, but they are not obvious from the initial formulation of the model. The validity of the original model assumptions is dependent on the validity of these model input-output relationships.

The paper also includes some discussion of the physical interpretation of the large-scale input parameters used in the technique.

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J. L. McBride and N. Nicholls

Abstract

Correlations between indices of the Southern Oscillation (SO) and areal average rainfall for 107 Australian rainfall districts for the period December 1932 to November 1974 have been calculated. Simultaneous correlations between the SO and rainfall show a clear annual cycle with the best relationship occurring in spring (September-November). The season with the weakest relationship is summer (December-February). In all seasons, seasonal rainfalls in some parts of Australia are significantly correlated with the SO in the preceding season. The strongest lag correlations occur with spring rainfall, which for some areas is also significantly correlated with the SO two seasons (six months) earlier.

Correlations were also calculated with the data divided into two subseries from 1932 to 1953 and from 1954 to 1974. These calculations suggest a westward shift with time of the correlation pattern, associated with substantial changes in the magnitude of the correlations in some areas.

Some speculations on the possible causes of certain aspects of the observed seasonal cycle in the correlations are advanced.

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John L. McBride and Greg J. Holland

Questionnaire replies from forecasters in 16 tropical-cyclone warning centers are summarized to provide an overview of the current state of the science in tropical-cyclone analysis and forecasting. Information is tabulated on the data sources and techniques used, on their role and perceived usefulness, and on the levels of verification and accuracy of cyclone forecasting.

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

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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. Nicholls, J. L. McBride, and R. J. Ormerod

Abstract

An index of the date of onset of the North Australian wet season is defined based on rainfall received at a single station (Darwin). It is demonstrated that this index can be predicted some months ahead.

The amount of rain received during the wet season is only weakly related to the date of onset, and the amount of rainfall received in the middle and late portion of the season is totally unrelated to either the date of onset or to the amount of rain received in the early part of the season.

Discussion is presented on the relationship between the wet-season onset as here defined and the Australian monsoon onset as defined by Troup (1961). A distinction is made between the monsoon portion of the season and an earlier transition season which also accounts for a large proportion of the total rainfall.

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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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Murry L. Salby, Patrick J. McBride, and Patrick F. Callaghan

Abstract

Global cloud imagery (GCI) is constructed from multiple satellite platforms that simultaneously monitor the earth. The GCI overcomes sampling limitations that are inherent to measurements from an individual platform, and provides a continuous and high-resolution description of the global convective pattern. However, it must reconcile inconsistencies in the measurements from different platforms. Escaping operational stages of error detection is a spurious brightening (cooling of brightness temperature), which appears sporadically in the composited imagery and must be removed a posteriori. The spurious brightening is shown to follow from a bias between measurements from polar-orbiting platforms and those from geostationary platforms. The bias is related to the zenith angle dependence of geostationary measurements, which enables its efficient removal. GCI is then composited from satellites in which the zenith angle–dependent bias has been removed a priori. The corrected imagery is shown to be virtually free of the systematic error, leaving a more accurate representation of the global convective pattern.

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Greg C. Tyrrell, David J. Karoly, and John L. McBride

Abstract

Data from the Intensive Observation Period of the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (November 1992–February 1993) have been used to investigate the links between intraseasonal variations in tropical convection and those in forcing of upper-tropospheric Rossby waves in the extratropics. The primary databases are Geostationary Meteorological Satellite imagery and tropical wind analyses from the Bureau of Meteorology, Australia. A number of 5-day periods showing convection in different locations were chosen. For each period, mean fields of divergence, cloud-top temperature, and upper-tropospheric Rossby wave source are presented. Vorticity budgets are used to demonstrate the processes responsible for the Rossby wave source patterns. The approach follows earlier studies of links between interannual variations of tropical convection associated with the Southern Oscillation and variations of the extratropical circulation.

It is shown that the regions of tropical convection correspond to longitudinally localized Hadley cells. In the subtropics, at the higher-latitude end of each cell, there is a Rossby wave source dipole with anticyclonic and cyclonic forcing. The anticyclonic forcing of Rossby waves is associated with advection of vorticity by the divergent outflow, while the cyclonic forcing is due to the region of convergence immediately above the down-ward branch of the local Hadley cell. Hence, the authors provide a dynamical basis for tropical-midlatitude interactions associated with intraseasonal variations of tropical convection.

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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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