Search Results

You are looking at 1 - 10 of 21 items for

  • Author or Editor: Mark R. Sinclair x
  • Refine by Access: All Content x
Clear All Modify Search
Mark R. Sinclair

Abstract

No abstract

Full access
Mark R. Sinclair

Abstract

This is the second of two papers on tropical storms entering middle latitudes in the southwest Pacific. This study focuses on the heavy rainfall associated with Tropical Cyclone Bola during 6–8 March 1988 following its passage from the tropics to a position new northern New Zealand. The heaviest precipitation fell on the upwind side of the Gisborne Ranges, where 5-day rainfall totals exceeding 800 mm were recorded.

Precipitation diagnoses from the 2.51 × 2.5° ECMWF dataset seriously underestimated the intensity of observed rainfall over the mountainous terrain. This failure resulted primarily from an inability to resolve the large orographic component of this rainfall. A simple scheme, based on ascent caused by flow over topography, was able to replicate the huge rainfall amounts on the upwind side of the mountain only when details of the local topography to approximately 10 km were included.

A quasigeostrophic ω diagnosis from the ECMWF data showed that large-scale ascent occurred where upper-level vorticity advection and increasing low-level thermal advection acted in phase. Strongest ascent occurred in the region between the difluent exit region of a westerly jet to the north of Bola and the confluent entrance to a second jet southeast of the South Island of New Zealand. A cross section revealed an associated two-cell vertical circulation pattern similar to that observed in other parts of the world. The heavy rain occurred just downstream from a low-level confluence where tropical air from Bola's eastern flank met cooler air from the southeast. Frontogenesis and moisture convergence associated with this confluence possibly helped to focus ascent near the Gisborne Ranges, although this could not be confirmed, because of the lack of observations.

Full access
Mark R. Sinclair

Abstract

During the period 23–29 December 1978 abnormally high temperatures were recorded at several Antarctic stations. New records were set at South Pole, MeMurdo. and Vostok, and near record high temperatures were reached at coastal stations in East Antarctica. Synoptic scale events accompanying these unprecedented high temperatures were investigated using surface, satellite and radiosonde data. The high temperatures were associated with two intrusions of warm air which penetrated the continent from the Atlantic and Indian Ocean sectors. Warming at South Pole occurred with the passage of a jet-like feature and its accompanying baroclinic zone.

Full access
Mark R. Sinclair

Abstract

This second of two papers on extratropical transition (ET) over the southwest Pacific Ocean focuses on the variability of ET. A climatology of ET onset based on a previously described objective technique shows that ET commences 15° of latitude nearer the equator on average than similar cases from the Northern Hemisphere. Characteristic midlatitude circulation patterns accompanying ET near 30°S are identified by means of empirical orthogonal function (EOF) analysis of 50 storms. The first eigenvector pattern, explaining nearly half the circulation variability, expresses relaxed and enhanced pressure gradients south of the storm that define composites similar to “cradled” and “captured” classifications previously described for the southeast Indian Ocean. The second EOF distinguishes redeveloping from weakening storms. Reintensifying storms were located beneath strong cyclonic vorticity advection (CVA) near the equatorward entrance region of an upper jet whereas the upper jet was well to the west of weakening storms.

A survey of factors responsible for modulating central sea level pressure change during ET was conducted for the 50 storms. The quantity most strongly correlated with surface development was found to be CVA at the jet level. Extratropical reintensification occurred when the surface cyclone was located beneath the equatorward entrance region of an upper jet for storms between 28° and 34°S, and beneath the poleward exit jet region for storms farther south. Strongest examples of redevelopment each occurred beneath a potent double jet signature aloft, with maximum storm-relative upper-level CVA located directly above the surface low and net vorticity fluxes reflecting amplification of the upper wave. Weakening storms featured a weakening upper trough directly above the storm, with CVA to the east.

Full access
Mark R. Sinclair

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 Australia, NZ, South America, and Africa are especially prone to rapid anticyclogenesis.

Blocking episodes were objectively identified both from the anticyclone track data as persistent highs having central pressure exceeding the time-averaged mean sea level pressure (MSLP) by more than 20 hpa and as persistent positive MSLP anomalies. Weaker or more mobile blocks were found throughout the South Pacific east of NZ, with secondary maxima east of South America and southeast of Africa, as in previous studies. However, more persistent and intense blocks were largely confined to two regions of the South Pacific—south-east of NZ and near 55°S, 110°W, west of South America.

Full access
Mark R. Sinclair

Abstract

This is the first of two case studies on the transformation of tropical storms in the southwest Pacific. This study uses ECMWF analyses and observations to examine the behavior of Tropical Cyclone Patsy in December 1986 prior to and during its transit to midlatitudes. These analyses adequately depicted synoptic-scale features during the extratropical phase but underestimated the inner core of strong wind during the hurricane phase. Interactions with troughs in the subtropical westerlies provided the impetus for the major motion and structure changes during Patsy's life. During the hurricane phase, Patsy was steered by the flow above 400 hPa. The passage of a first trough to poleward led to disintegration of the characteristic tropical cyclone structure. Decay was due to shearing effects, loss of upper-level outflow, and the transient incorporation of cooler air. into the low-level circulation. A few days later, Patsy commenced moving rapidly south as a single center as ascent redeveloped in the poleward sector ahead of the next approaching trough. During this extratropical transition stage, cyclone motion was controlled by the mean flow below 600 hPa and was directed slightly to the riot of the ascent maximum. This ascent was triggered by cyclonic vorticity advection in the equatorward entrance region of a propagating subtropical jet that was able to tap Patsy's moisture supply. It was prolonged by a long period of coupling with this jet, which was in turn sustained by outflow from the heating within the baroclinic zone beneath it.

Full access
Mark R. Sinclair

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 Zealand. Large differences were found between the geographical distribution of developing cyclones as inferred from ζg, tendencies and those obtained from central pressure change. The latter were found to be highly dependent on cyclone motion relative to the background pressure field.

Full access
Mark R. Sinclair

Abstract

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 35°S, 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 Niño years, tropical cyclones that move into middle latitudes exhibit stronger zonal motion and occur over a wider range of longitudes than during La Niña 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 20°S, 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 25°S, 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 30°S 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.

Full access
Mark R. Sinclair

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·∇z s, where z s 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 r g 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 r g. Best results were obtained at a resolution near 10 km, for which an r g of 0.84 was obtained. However, ignoring drift effects at 25-km resolution resulted in model skill nearly equivalent to the full model at 10 km, highlighting the need to demonstrate that supposed model improvements do, in fact, lead to significant increases in skill.

Full access
Mark R. Sinclair

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 maxima around Antarctica may include many spurious centers that are artifacts of a fictitious surface anticyclone over the elevated continent. Maxima around East Antarctica were located near katabatic-prone stations. Large counts near the three midiatitude landmasses were mostly heat lows and lee troughs. These stationary orographic features were eliminated to retain just the traveling disturbances that dominate the weather and climate of the region. These mobile centers were distributed much more uniformly. When centers were counted just once per grid square per cyclone, the resulting “track density” maximized year-round in a belt near 50°S rather than within the cyclone “graveyard” of the circumpolar trough, in good agreement with baroclinic storm tracks obtained elsewhere from eddy statistics. A second maximum associated with the subtropical jet was found during winter and spring near 40°S in the New Zealand-Pacific sector. Intense cyclones, stratified by ζg, occurred most frequently in winter near New Zealand, east of South America, and in the southern Indian Ocean. An apparent increase in cyclone numbers in early 1933 coincided with the introduction of envelope orography at the ECMWF.

Full access