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M. E. B. Gray

Abstract

A cloud-resolving model simulation of a convectively active phase of the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) is used to study the properties of the convective and mesoscale updrafts, and precipitating downdrafts of the simulated mesoscale convective systems (MCSs). Analysis of scalar transports confirms the results of previous studies that show that the mesoscale updraft provides an important contribution to the total heat and moisture budget of the MCS in the upper troposphere. The mesoscale updraft is shown to possess little or no positive buoyancy, and it is suggested that the upward vertical motion present is associated with decaying convective cells. Momentum transports are not significant in the mesoscale updraft, but those in the mesoscale downdraft are of the same order as those in the convective downdraft. Horizontal mesoscale circulations, such as rear-to-front inflow, may be important in determining the downdraft momentum transport.

Using the cloud-resolving model results, a mass flux parameterization for the mesoscale updraft is proposed. The mass flux equation is closed by assuming that a fraction of the mass flux detrained from the convective cores is entrained into the mesoscale updraft. Along with a simple evaporation-based parameterization for the mesoscale downdraft, the parameterization is tested in a single-column model for the same TOGA COARE period. Comparisons of the cloud-resolving model and single-column model results show very good agreement between the simulated and parameterized mesoscale scalar transports when the parameterized convection is also well represented. The importance of an accurate convection scheme to drive the mesoscale updraft–downdraft parameterization is stressed.

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G. Vaughan, J. Methven, D. Anderson, B. Antonescu, L. Baker, T. P. Baker, S. P. Ballard, K. N. Bower, P. R. A. Brown, J. Chagnon, T. W. Choularton, J. Chylik, P. J. Connolly, P. A. Cook, R. J. Cotton, J. Crosier, C. Dearden, J. R. Dorsey, T. H. A. Frame, M. W. Gallagher, M. Goodliff, S. L. Gray, B. J. Harvey, P. Knippertz, H. W. Lean, D. Li, G. Lloyd, O. Martínez–Alvarado, J. Nicol, J. Norris, E. Öström, J. Owen, D. J. Parker, R. S. Plant, I. A. Renfrew, N. M. Roberts, P. Rosenberg, A. C. Rudd, D. M. Schultz, J. P. Taylor, T. Trzeciak, R. Tubbs, A. K. Vance, P. J. van Leeuwen, A. Wellpott, and A. Woolley

Abstract

The Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project aims to improve forecasts of high-impact weather in extratropical cyclones through field measurements, high-resolution numerical modeling, and improved design of ensemble forecasting and data assimilation systems. This article introduces DIAMET and presents some of the first results. Four field campaigns were conducted by the project, one of which, in late 2011, coincided with an exceptionally stormy period marked by an unusually strong, zonal North Atlantic jet stream and a succession of severe windstorms in northwest Europe. As a result, December 2011 had the highest monthly North Atlantic Oscillation index (2.52) of any December in the last 60 years. Detailed observations of several of these storms were gathered using the U.K.’s BAe 146 research aircraft and extensive ground-based measurements. As an example of the results obtained during the campaign, observations are presented of Extratropical Cyclone Friedhelm on 8 December 2011, when surface winds with gusts exceeding 30 m s–1 crossed central Scotland, leading to widespread disruption to transportation and electricity supply. Friedhelm deepened 44 hPa in 24 h and developed a pronounced bent-back front wrapping around the storm center. The strongest winds at 850 hPa and the surface occurred in the southern quadrant of the storm, and detailed measurements showed these to be most intense in clear air between bands of showers. High-resolution ensemble forecasts from the Met Office showed similar features, with the strongest winds aligned in linear swaths between the bands, suggesting that there is potential for improved skill in forecasts of damaging winds.

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