Mark R. Sinclair

The Extratropical Transition of Tropical Cyclones: Forecast Challenges, Current Understanding, and Future Directions

Abstract A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that such systems pose a serious threat to land and maritime activities. Changes in the structure of a system as it evolves from a tropical to an extratropical cyclone during ET necessitate changes in forecast strategies. In this paper a brief climatology of ET is given and the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response). The problems associated with the numerical prediction of ET are discussed. A comprehensive review of the current understanding of the processes involved in ET is presented. Classifications of extratropical transition ...

An Objective Cyclone Climatology for the Southern Hemisphere

Abstract An objective method is developed and used to derive a climatology of centers of cyclonic vorticity for the Southern Hemisphere, based on twice-daily European Centre for Medium-Range Weather Forecasts (ECMWF) 1000-hPa analyses during 1980–86. These centers were computed as local minima of geostrophic relative vorticity ζg, extending previous studies based on pressure minima. This use of ζg, avoids a bias favoring slower and/or deeper cyclones that occurs when pressure is used and includes a large number of additional mobile vorticity centers in the 45°–55°S band that are missed where a local pressure minimum cannot be found. An automated tracking algorithm similar to that of Murray and Simmonds is used to match predicted location, pressure, and ζg from previous track history with available centers. Large numbers of ζg centers were found south of 60°S and near the midlatitude continents, as in previous studies. These were a mixture of migratory centers and other fixed topographic features. The maxi...

IMILAST: A Community Effort to Intercompare Extratropical Cyclone Detection and Tracking Algorithms

The variability of results from different automated methods of detection and tracking of extratropical cyclones is assessed in order to identify uncertainties related to the choice of method. Fifteen international teams applied their own algorithms to the same dataset—the period 1989–2009 of interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERAInterim) data. This experiment is part of the community project Intercomparison of Mid Latitude Storm Diagnostics (IMILAST; see www.proclim.ch/imilast/index.html). The spread of results for cyclone frequency, intensity, life cycle, and track location is presented to illustrate the impact of using different methods. Globally, methods agree well for geographical distribution in large oceanic regions, interannual variability of cyclone numbers, geographical patterns of strong trends, and distribution shape for many life cycle characteristics. In contrast, the largest disparities exist for the total numbers of cyclones, the detection of wea...

IMILAST – a community effort to intercompare extratropical cyclone detection and tracking algorithms: assessing method-related uncertainties.

The variability of results from different automated methods of detection and tracking of extratropical cyclones is assessed in order to identify uncertainties related to the choice of method. Fifteen international teams applied their own algorithms to the same dataset—the period 1989–2009 of interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERAInterim) data. This experiment is part of the community project Intercomparison of Mid Latitude Storm Diagnostics (IMILAST; see www.proclim.ch/imilast/index.html). The spread of results for cyclone frequency, intensity, life cycle, and track location is presented to illustrate the impact of using different methods. Globally, methods agree well for geographical distribution in large oceanic regions, interannual variability of cyclone numbers, geographical patterns of strong trends, and distribution shape for many life cycle characteristics. In contrast, the largest disparities exist for the total numbers of cyclones, the detection of wea...

Objective Identification of Cyclones and Their Circulation Intensity, and Climatology

Abstract An updated procedure for objective identification and tracking of surface cyclones from gridded analyses is described. Prior smoothing of the raw data with a constant radius spatial filter is used to remove distortions related to the particular grid configuration used and to consistently admit a known scale of disturbance over the domain. Pitfalls of using central pressure or vorticity to infer cyclone intensity are illustrated, and a procedure for obtaining a more realistic areal measure of circulation is described. An automated selection procedure for storms having specific properties is outlined. Case selection is by computer search of a database of cyclone tracks, obtained from an application of the cyclone finding and tracking procedure to an extended series of gridded mean sea level pressure analyses. A selection of winter season cyclone statistics for both hemispheres is obtained from European Centre for Medium-Range Weather Forecasts analyses. Discrepancies with and between earlier studie...

A Climatology of Anticyclones and Blocking for the Southern Hemisphere

Abstract An automated procedure for locating and tracking high pressure centers is applied to 10 years of twice-daily ECMWF analyses during 1980–89 to survey the behavior of anticyclones in the Southern Hemisphere. Anticyclones are concentrated in the 25°–45°S band, with preferred regions consistent with previous studies except in the New Zealand (NZ)–South Pacific sector, where a double occurrence maximum noted in earlier studies was only weakly evident. Marked interannual variability was found in this region, with the poleward branch of maximum occurrence south of 50°S present in only 3 out of 10 years. Mean central pressure maximized about 10° south of the time-averaged ridge, with highest values in winter. Anticyclones poleward of 50°S involve an anomalous breakdown of the westerlies, with mean central pressures there 20–30 hPa higher than climatology. Highs tend to form and intensify in western regions of ocean basins and weaken and dissipate in the east of these oceans. Regions southeast of Australi...

A Climatology of Cyclogenesis for the Southern Hemisphere

Abstract This study surveys life cycle characteristics of cyclones in the Southern Hemisphere (SH). An objective technique for locating instances of cyclone formation, intensification, maturation, and decay is applied to a 7-year dataset of SH cyclone positions, obtained from an automated finding and tracking scheme based on twice-daily ECMWF analyses during 1980–86. Geostrophic vorticity ζg at 1000 hPa is used to locate cyclones and determine their intensity. Cyclones form and develop within middle latitudes and migrate eastward and poleward during their lives. Preferred genesis and cyclogenesis regions include eastern coasts of Australia and South America in winter, and leeward of the Andes and near the baroclinic zones associated with the principal upper-tropospheric jet streams year round. Over open water, winter cyclones tend to form and intensify near zones of strongest SST gradient. Rapid cyclogenesis is most frequent east of South America, southeast of Africa, south of Australia, and near New Zeal...

A Diagnostic Model for Estimating Orographic Precipitation

Abstract A simple model for estimating detailed orographic precipitation is presented. The model is initialized using coarse-resolution analyses from the European Centre for Medium-Range Weather Forecasts and is applied to the heavy rainfall over northern New Zealand from Tropical Cyclone Bola (March 1988). The model is based on the estimation of topographically forced vertical motion as V·∇zs, where zs is the height of the topography and V is the horizontal flow just upstream of the mountain. Model skill is evaluated by computing the correlation coefficient rg of the model results against 297 rain gauge measurements. Enhancement of the model by adding lee drying, wind drift effects, and modulation by larger-scale humidity and vertical-motion fields each led to a statistically significant increase in rg. Best results were obtained at a resolution near 10 km, for which an rg of 0.84 was obtained. However, ignoring drift effects at 25-km resolution resulted in model skill nearly equivalent to the full model...

Extratropical Transition of Southwest Pacific Tropical Cyclones. Part I: Climatology and Mean Structure Changes

A database of tropical cyclone best track and intensity information for the southwest Pacific Ocean basin is used to construct a 28-year climatology for tropical cyclones that move into middle latitudes. Of the nine or so tropical cyclones that form each year, an average of about three can be expected to migrate south of 358S, with the greatest fraction in March. Storms entering the Tasman Sea west of New Zealand (NZ) move almost due south on average and retain greater intensity than those to the east of NZ, where storms decay quickly while moving rapidly away to the southeast. Storms east of NZ are embedded in a stronger, more zonal flow than those to the west, which move poleward ahead of a larger-amplitude trough. During El Nino years, tropical cyclones that move into middle latitudes exhibit stronger zonal motion and occur over a wider range of longitudes than during La Nina years. Storm intensity is only weakly correlated with concurrent SST anomalies, suggesting that atmospheric circulation is the dominant influence on storm properties. Average structure changes during extratropical transition (ET) are identified using the NCEP-NCAR reanalysis dataset, for a subset of 33 transitioning storms during 1980-97. Composites are used to construct a three- dimensional conceptual model of the transformation from a mature hurricane to an asymmetric baroclinic midlatitude cyclone. Southwest Pacific tropical cyclones encounter the baroclinic westerlies early in their lives, accounting for their average eastward (and poleward) motion. At maximum average intensity near 208S, baroclinic effects are already important, with warm frontogenesis appearing in the southeast quadrant and outflow aloft into a downstream subtropical wind maximum that moves poleward with the storm. By 258S, the average TC has lost the characteristic symmetric anticyclonic outflow aloft and acquired the characteristics of a baroclinic midlatitude storm, including regions of warm and cold frontogenesis, a vertical motion dipole and a westward tilt with height. From about 308S poleward, a second upper-tropospheric wind maximum appears west of the storm, with strengthening cyclonic vorticity advection aloft. Below about 400 hPa, the storm retains the vertical, warm cyclonic core as it migrates poleward.

Objective Assessment of Extratropical Weather Systems in Simulated Climates

An automated weather system identification and tracking scheme is used to appraise the skill of the CSIRO9 GCM in replicating contemporary extratropical cyclone and anticyclone behavior, and to assess possible changes as a result of doubled CO2. Cyclones are identified as centers of cyclonic vorticity rather than pressure minima, which can vanish if the background pressure gradient increases. Comparison with an observational dataset from ECMWF revealed that the GCM control simulation realistically reproduced the present-day storm track locations, but with slightly fewer and generally weaker systems overall. These errors are consistent with the coarser resolution of the GCM and its underestimation of the strength and baroclinicity of the polar vortex in both hemispheres. Comparison between 1 and 2 3 CO2 GCM simulations revealed increases in both 500-hPa geopotential height and 1000‐500-hPa thickness for doubled CO2. As in other studies, these changes are largest near the poles, resulting in weaker westerlies and reduced tropospheric baroclinicity. Decreases of 10%‐15% in both cyclone and anticyclone activity consistent with these circulation changes are found. However, there is some evidence of increased winter cyclone activity near the downstream end of the principal storm tracks. There is also a general reduction in the number and strength of intense storms, despite generally lower central pressures, which arise from global-scale decreases in sea level pressure in the doubled CO 2 atmosphere rather than from greater storm vigor. This underscores the need for GCM projections of midlatitude ‘‘storminess’’ to employ more realistic measures of storm activity and intensity.