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Michael Lieder and Günther Heinemann

Abstract

The development of three summertime mesoscale cyclones (MCs) over the northern Amundsen and Bellingshausen Seas from 10 to 12 January 1995 (during FROST SOP-3) is studied by means of AVHRR data, ERS and SSM/I retrievals, and mesoscale numerical model data. The most pronounced MC is investigated in detail. It had a diameter of about 800 km, a lifetime of more than 24 h, and reached the intensity of a polar low. The developments take place far away from the sea-ice front or topography. The MCs are detected as cyclonic cloud signatures in the AVHRR imagery, and SSM/I retrievals show a distinct mesoscale signal in the fields of cloud liquid water, wind speed, and integrated water vapor (IWV). The frontal structure of the most intense MC is depicted by high IWV gradients and a large near-surface wind shear. The collocation of ERS- and SSM/I-derived wind speeds shows good agreement (bias, 1.1 m s−1; std dev, 1.2 m s−1). ERS-derived wind vectors give no insight into the structure of the MCs, because of missing direct overpasses over the MCs by the narrow ERS scatterometer swaths, but they are used to validate numerical simulations. The numerical simulations using the mesoscale Norwegian Limited Area Model (NORLAM) show two of the MCs as short-wave baroclinic developments triggered by an upper-level trough, while a less significant third MC is not simulated by the model. In contrast to the satellite retrievals, the simulations give insight into the three-dimensional structure of the MCs. Model results are validated using satellite retrievals and some few available in situ observations. This validation study shows the good quality of the numerical simulations for the IWV and the near-surface wind speed from SSM/I as well as for the near-surface wind vector from ERS over the simulation time of 36 h. The differences between ERS and NORLAM wind vectors are 1.1 ± 2.5 m s−1 (mean bias ± std dev) and −3 ± 25° for wind speed and direction, respectively. The validation using SSM/I retrievals yields a mean bias of 0.3 m s−1 (std dev, 2.9 m s−1) for the wind speed, and of −2.5 kg m−2 (std dev, 2.9 kg m−2) for the IWV.

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John Turner, David Bromwich, Steven Colwell, Stephen Dixon, Tim Gibson, Terry Hart, Günther Heinemann, Hugh Hutchinson, Kieran Jacka, Steven Leonard, Michael Lieder, Lawrie Marsh, Stephen Pendlebury, Henry Phillpot, Mike Pook, and Ian Simmonds

An account is given of the Antarctic First Regional Observing Study of the Troposphere (FROST) project, which has been organized by the Physics and Chemistry of the Atmosphere Group of the Scientific Committee on Antarctic Research. The goals of FROST are to study the meteorology of the Antarctic, to determine the strengths and weaknesses of operational analyses and forecasts over the continent and in the surrounding ocean areas, and to assess the value of new forms of satellite data that are becoming available. FROST is based around three one-month Special Observing Periods (SOPs)—July 1994, 16 October–15 November 1994, and January 1995 for which comprehensive datasets have been established of model fields and in situ and satellite observations. High quality manual surface and upper-air analyses are being prepared for these periods to determine the extent to which non–Global Telecommunications System data can improve the interpretation of the synoptic situation. Over the ocean areas during SOP-1, incorporation of the late data resulted only in a limited improvement in the analyses, indicating that the models are correctly analyzing most of the major weather systems. Over the continent, the production of 500-hPa heights from the automatic weather station data greatly helped in the analysis process. The lack of data around west Antarctica was a major handicap in the analysis process. The rms errors in the forecasts of 500-hPa height for the Antarctic were about 20% greater than those for midlatitude areas. The forecasts from the European Centre for Medium-Range Weather Forecasts were the most accurate of those received.

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