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R. DelBeato and S. L. Barrell

Abstract

A technique is presented which provides estimates of rainfall from extratropical cyclones over an area of 125 000 km2 in southeastern Australia in simulated real time conditions. It utilizes a statistical relation between blackbody temperature of cumuliform cloud and 90 minute rainfall totals to determine estimates of rainfall from cumuliform cloud, and approximates the lesser rainfall amounts from the stratiform pre‐frontal cloud as a fixed proportion of rain from equivalent cumuliform cloud. It is based on the digitized “HR Fax” imagery received at 3 h intervals from the Japanese Geostationary Meteorolgical Satellite (GMS). Five case studies are presented, each for a 24 hour period. Rainfall estimates for rainfall districts within the area vary from the observed district averages, which were calculated from daily gage data, by an average of 22%. The mean absolute error for districts is 4.2 mm.

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K. J. Wilson, S. L. Barrell, and R. Del Beato

Abstract

An approach to verifying position forecasts of line phenomena was developed. The technique, which tests for occurrence/nonoccurrence and the time of arrival of line phenomena, was applied to frontal position prognoses over southeastern Australia. It was found that operational manual prognoses exhibited no skill in cases of frontogenesis or total frontolysis. In other cases, accuracy varied seasonally and geographically, with summer apparently being the most difficult period for frontal prognosis. Frontal position forecasts were successful for 70 percent of frontal crossings in the 27-month trial period, but two of every five forecast events were false alarms. The Hanssen and Kuipers skill discriminant for a 24-hour Yes/No forecast of a frontal crossing was new 0.6.

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B. N. Hanstrum, K. J. Wilson, and S. L. Barrell

Abstract

A climatology of the prefrontal westerly trough of southern Australia is presented, based on data for the 10-yr period 1976–85. Trough formation was confined to the longitudes of the Australian continent in the warmer months of the year. An average of approximately 15 troughs/yr were observed, evenly distributed each month from September to April, with appreciable interannual variability in incidence. Troughs formed in advance of a major Southern Ocean cold front in a region of differential thermal advection. Genesis occurred primarily over the west of the continent in spring and autumn, but over southeastern Australia in summer.

Variability in the location of formation at different times of year was related to the seasonal shift of the subtropical ridge over the continent, the shape of the southern continental coastline, and periods of low zonal-index flow. A class of troughs prone to rapid intensification after formation was also identified.

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B. N. Hanstrum, K. J. Wilson, and S. L. Barrell

Abstract

A case study of frontogenesis within a surface prefrontal trough over southern Australia is presented. The trough developed ahead of a surface cold front and, over a period of approximately 24 h, intensified into a mature summertime frontal system while the original front underwent total frontolysis. Two-hourly rawinsonde ascents at three locations were used to examine the structure of the trough both before and after frontogenesis. Diagnosis of the frontogenetic tendencies showed that horizontal deformation of the potential temperature field was the main forcing mechanism.

The process studied occurs frequently in the warmer months of the year, and a conceptual model is proposed for the process of frontogenesis within the trough and frontolysis of the preexisting midlatitude front. Important components include differential thermal advection, the action of Coriolis turning in association with ageostrophic vertical circulations, and horizontal deformation of the thermal field.

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I. A. Renfrew, R. S. Pickart, K. Våge, G. W. K. Moore, T. J. Bracegirdle, A. D. Elvidge, E. Jeansson, T. Lachlan-Cope, L. T. McRaven, L. Papritz, J. Reuder, H. Sodemann, A. Terpstra, S. Waterman, H. Valdimarsson, A. Weiss, M. Almansi, F. Bahr, A. Brakstad, C. Barrell, J. K. Brooke, B. J. Brooks, I. M. Brooks, M. E. Brooks, E. M. Bruvik, C. Duscha, I. Fer, H. M. Golid, M. Hallerstig, I. Hessevik, J. Huang, L. Houghton, S. Jónsson, M. Jonassen, K. Jackson, K. Kvalsund, E. W. Kolstad, K. Konstali, J. Kristiansen, R. Ladkin, P. Lin, A. Macrander, A. Mitchell, H. Olafsson, A. Pacini, C. Payne, B. Palmason, M. D. Pérez-Hernández, A. K. Peterson, G. N. Petersen, M. N. Pisareva, J. O. Pope, A. Seidl, S. Semper, D. Sergeev, S. Skjelsvik, H. Søiland, D. Smith, M. A. Spall, T. Spengler, A. Touzeau, G. Tupper, Y. Weng, K. D. Williams, X. Yang, and S. Zhou

Abstract

The Iceland Greenland Seas Project (IGP) is a coordinated atmosphere–ocean research program investigating climate processes in the source region of the densest waters of the Atlantic meridional overturning circulation. During February and March 2018, a field campaign was executed over the Iceland and southern Greenland Seas that utilized a range of observing platforms to investigate critical processes in the region, including a research vessel, a research aircraft, moorings, sea gliders, floats, and a meteorological buoy. A remarkable feature of the field campaign was the highly coordinated deployment of the observing platforms, whereby the research vessel and aircraft tracks were planned in concert to allow simultaneous sampling of the atmosphere, the ocean, and their interactions. This joint planning was supported by tailor-made convection-permitting weather forecasts and novel diagnostics from an ensemble prediction system. The scientific aims of the IGP are to characterize the atmospheric forcing and the ocean response of coupled processes; in particular, cold-air outbreaks in the vicinity of the marginal ice zone and their triggering of oceanic heat loss, and the role of freshwater in the generation of dense water masses. The campaign observed the life cycle of a long-lasting cold-air outbreak over the Iceland Sea and the development of a cold-air outbreak over the Greenland Sea. Repeated profiling revealed the immediate impact on the ocean, while a comprehensive hydrographic survey provided a rare picture of these subpolar seas in winter. A joint atmosphere–ocean approach is also being used in the analysis phase, with coupled observational analysis and coordinated numerical modeling activities underway.

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