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Ian A. Renfrew and G. W. K. Moore

Abstract

Observational data from two research aircraft flights are presented. The flights were planned to investigate the air–sea interaction during an extreme cold-air outbreak, associated with the passage of a synoptic-scale low pressure system over the Labrador Sea during 8 February 1997. This is the first such aircraft-based investigation in this remote region. Both high-level dropsonde and low-level flight-level data were collected. The objectives were twofold: to map out the structure of the roll vortices that cause the ubiquitous cloud streets seen in satellite imagery, and to estimate the sensible and latent heat fluxes between the ocean and atmosphere during the event. The latter was achieved by a Lagrangian analysis of the flight-level data. The flights were part of the Labrador Sea Deep Convection Experiment, investigating deep oceanic convection, and were planned to overpass a research vessel in the area.

The aircraft-observed roll vortices had a characteristic wavelength of 4–5 km, particularly evident in the water vapor signal. Unlike observations of roll vortices in other regions, a roll signature was absent from the temperature data. Analysis of satellite imagery shows the cloud streets had a characteristic wavelength of 7–10 km, indicating a multiscale roll vortex regime. There was a dramatic deepening of the boundary layer with fetch, and also with time. Off the ice edge, surface sensible heat fluxes of 500 W m−2 and surface latent heat fluxes of 100 W m−2 were measured, with uncertainties of ±20%. The very cold air is thought to be responsible for the unusually high Bowen ratio observed.

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Denis Sergeev, Ian A. Renfrew, and Thomas Spengler

ABSTRACT

The life cycles of intense high-latitude mesoscale cyclones and polar lows are strongly shaped by their ambient environments. This study focuses on the influence of the orography of Svalbard and the sea ice cover in the Norwegian and Barents Seas on polar low development. We investigate two typical polar lows that formed near Svalbard during northerly cold-air outbreaks. Each case is simulated using the Met Office Unified Model with convection-permitting grid spacing. A series of sensitivity experiments is conducted with an artificially changed land mask, orography, and sea ice distribution. We find that Svalbard acts to block stably stratified air from the ice-covered Arctic Ocean, and as an additional source of low-level cyclonic vorticity aiding polar low genesis and intensification. A decrease in sea ice cover west of Svalbard results in a moderate intensification of the polar lows, particularly for the more convectively driven case, while an increase in the sea ice cover significantly hinders their development. These experiments exemplify that polar mesoscale cyclones in the northeast Atlantic can withstand large perturbations in the surface conditions (such as the removal of Svalbard) and still develop to sufficient intensity to be labeled as polar lows. However, there is a sensitivity to Svalbard’s orography and surrounding sea ice cover, illustrated by a clear modulation of polar low genesis and development.

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Alan Condron, Grant R. Bigg, and Ian A. Renfrew

Abstract

Polar mesoscale cyclones over the subarctic are thought to be an important component of the coupled atmosphere–ocean climate system. However, the relatively small scale of these features presents some concern as to their representation in the meteorological reanalysis datasets that are commonly used to drive ocean models. Here polar mesocyclones are detected in the 40-Year European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis dataset (ERA-40) in mean sea level pressure and 500-hPa geopotential height, using an automated cyclone detection algorithm. The results are compared to polar mesocyclones detected in satellite imagery over the northeast Atlantic, for the period October 1993–September 1995. Similar trends in monthly cyclone numbers and a similar spatial distribution are found. However, there is a bias in the size of cyclones detected in the reanalysis. Up to 80% of cyclones larger than 500 km are detected in MSL pressure, but this hit rate decreases, approximately linearly, to ∼40% for 250-km-scale cyclones and to ∼20% for 100-km-scale cyclones. Consequently a substantial component of the associated air–sea fluxes may be missing from the reanalysis, presenting a serious shortcoming when using such reanalysis data for ocean modeling simulations. Eight maxima in cyclone density are apparent in the mean sea level pressure, clustered around synoptic observing stations in the northeast Atlantic. They are likely spurious, and a result of unidentified shortcomings in the ERA-40 data assimilation procedure.

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