Search Results

You are looking at 1 - 5 of 5 items for :

  • Author or Editor: Michael J. Revell x
  • Monthly Weather Review x
  • Refine by Access: All Content x
Clear All Modify Search
Crispin J. Marks
and
Michael J. Revell

Abstract

The authors investigate the processes acting during a cyclonic “blocking” anomaly that dominated the upper troposphere in the New Zealand-Tasman Sea region during the very stormy month of August 1990. The authors use European Centre for Medium-Range Weather Forecasts (ECMWF) analyses and a simple, unambiguous flux form of the isentropic vorticity equation that makes possible a robust physical interpretation of the budget and that does not suffer from the problems of strong cancellation between pairs of terms that mar the traditional, isobaric approach. The simplicity of the equation also makes tractable an analysis of the errors in each of the terms in the monthly mean budget.

On the western and northern flanks of the anomaly at 310 K (≈350 hPa) the tendency of the time-mean flow to advect the anomaly downstream is countered (within the limits imposed by the relatively short averaging period) by mean stretching and the repeated influx of subpolar air by the storms during the month. However, on the eastern side of the anomaly the error analysis gives confidence that the dominant mean stretching term is only partially balanced by the mean and eddy advection terms. Two regions were found, overlying the ends of surface storm tracks, where large-scale and statistically significant residuals are required to balance the isentropic vorticity budget. The authors note that the whole eastern side of the anomaly is characterised by a large-scale drag on the prevailing flow, with isentropic gradients in the drag being responsible for the vorticity residual. Interestingly, embedded within each area of significant residual a smaller region exhibiting a significant acceleration of the zonal flow in the upper troposphere is found.

The authors argue that these features are caused by distinct physical processes and are not the result of systematic errors in vorticity tendencies, diabatic flux divergences, or errors in the analyzed divergent wind field. Instead, the authors conclude that during August 1990 there are several dynamically significant processes operating that are not resolved by 2.5°×2.5° ECMWF analyses: namely, the vertical transport of horizontal momentum associated with convective activity, the quasi-borizontal transport of horizontal momentum by breaking Rossby waves, and the ageostrophic motions associated with frontogenesis.

Full access
James A. Renwick
and
Michael J. Revell

Abstract

Atmospheric blocking events over the South Pacific are investigated using a 39-yr record of 500-hPa height fields from the NCEP–NCAR reanalysis dataset. The analysis extends earlier work using a 16-yr record and confirms that the occurrence of blocking over the southeast Pacific is strongly modulated by the ENSO cycle during austral spring and summer. Comparison of results at 500 hPa with the 300-hPa meridional wind component showed that blocking events are associated with large-scale wave trains lying across the South Pacific from the region of Australia to southern South America. Similar wave trains are evident in both hemispheres in singular value decomposition analyses between 300-hPa meridional wind components and tropical Pacific outgoing longwave radiation (OLR) anomalies.

The hypothesis that the divergence associated with tropical OLR anomalies forces an extratropical wave response that results in enhanced blocking over the southeast Pacific was tested using a linearized, barotropic vorticity equation (BVE) model. Observed 300-hPa mean flow fields and divergence forcing that matched the anomalous OLR were used to drive the BVE model. The resulting pattern of meridional wind and streamfunction anomalies agrees closely with observations. When the tropical OLR anomaly is given an eastward phase speed of 5° per day, the extratropical response agrees even better with observations. This suggests that linear Rossby wave propagation provides an important link between anomalous convection in the Tropics and the occurrence of blocking over the southeast Pacific Ocean.

Full access
Mark R. Sinclair
and
Michael J. Revell

Abstract

Characteristic patterns of cyclogenesis in the southwest Pacific region are identified from a sample of 40 developing cyclones during 1990–94. Cases were chosen objectively to ensure a realistic sampling of typical rather than “ideal” events. A subjective classification based on synoptic-scale upper-tropospheric flow signatures prior to cyclone intensification suggested four classes into which all but three cases fell. Three categories, each containing about a quarter of the population, involved direct coupling with the upper jet. They represent cyclone formation beneath (i) the poleward exit region of a 300-hPa jet upstream from a diffluent trough (class U), (ii) the confluent equatorward entrance region of the upper wind maximum (E), and (iii) the upper jet exit region where the jet is downstream from the upper trough (class D). These are analogous to previously identified categories for the western North Atlantic region. A fourth class involved cyclones forming beneath a preexisting intense upper-level trough (class T) located poleward of the upper-level jet.

Class U cyclones, forming within diffluent airflow, exhibited strong cold fronts, weak warm fronts, and a meridional configuration while class E cyclones forming in confluent flow attained a more zonally elongated structure marked by stronger warm fronts and weak cold fronts. Class U cyclones featured frontal evolution similar to the Norwegian cyclone model while class E and D cyclones exhibited characteristics of the Shapiro–Keyser model. These results provide further observational support for the emerging paradigm of contrasting frontal and cyclone structure resulting from confluent versus diffluent large-scale flow.

Full access
James A. Renwick
and
Michael J. Revell
Full access
Michael J. Revell
,
John W. Kidson
, and
George N. Kiladis

Abstract

The principal modes of Southern Hemisphere low-frequency variability have recently been calculated using a 39-yr record of 300-hPa streamfunction fields from the NCEP–NCAR reanalysis dataset. The authors attempt to interpret these modes as the rotational response to some divergent forcing. For a range of mean states the linearized barotropic vorticity equation (BVE) is used to solve for the divergent wind that would generate (or at least be consistent with) the observed vorticity modes. Several of these low-frequency modes can be generated by forcing the BVE with fairly simple divergent wind fields that could easily be interpreted as resulting from anomalous tropical convection. In particular this is found to be true for streamfunction anomalies with El Niño–Southern Oscillation (ENSO), high-latitude mode, South Pacific wave, and Madden–Julian oscillation structure. The authors speculate that it may be possible to relate these calculated divergent wind fields to recently observed OLR fields and hence explain some of the variance of the next month's 300-hPa streamfunction by solving the inverse problem.

These results are further evidence that linear Rossby wave propagation provides an important link between anomalous convection in the Tropics and the occurrence of circulation anomalies in higher latitudes.

Full access