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Peter A. G. Watson and Lesley J. Gray

Abstract

The stratospheric polar vortex is weaker in the easterly phase of the quasi-biennial oscillation (QBO-E) than in the westerly phase (QBO-W), but the mechanism behind the QBO's influence is not well understood. The composite difference of the atmospheric state between QBO-E and QBO-W is found to closely resemble the structure of the northern annular mode, the leading empirical orthogonal function of stratospheric variability, including its wave components. Studies of dynamical systems indicate that many different forcings could give rise to this response, and therefore this composite difference does not provide much information about the forcing mechanism. It is argued that the full transient response of a system to an applied forcing is likely to be much more informative about the dynamics of the forcing mechanism, especially the response on time scales shorter than the dynamical time scale, which is about a week for vortex variability. It is shown that the transient response of the vortex to forcing by the QBO in a general circulation model is consistent with the proposed mechanism of Holton and Tan, indicating that this mechanism has a role in the QBO modulation of vortex strength, in contrast to the conclusions of several recent studies. This novel approach of examining the transient response to a forcing on short time scales may be useful in various other outstanding problems.

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D. G. Steyn, J. E. Hay, Ian D. Watson, and Glenn T. Johnson

Abstract

A technique is described whereby sky view-factors may be determined using a video camera equipped with a fish-eye lens. The video image is digitized and then analyzed to distinguish between “sky” and “non-sky” pixels. View-factors are calculated for each pixel and then summed for all “sky” pixels to yield a composite sky view-factor for the image. The technique is illustrated by applying it in three urban locations, all of which are characterized by high building densities (and hence complex skylines). The three images processed have sky, view-factors in the range 0.15 to 0.46 (as independently determined). It is shown that the present technique produces values in close agreement with these and appears quite robust when compared with calculations based on the work of Johnson and Watson.

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R. J. Trapp, E. D. Mitchell, G. A. Tipton, D. W. Effertz, A. I. Watson, D. L. Andra Jr., and M. A. Magsig

Abstract

Tornadic vortex signatures (TVSs) of 52 tornadoes were identified and analyzed, then characterized as either descending or nondescending. This characterization refers to a known tendency of radar-observed tornadic vortices, namely, that of their initial detection aloft and then of their subsequent descent leading to tornadogenesis. Only 52% of the sampled TVSs descended according to this archetypal model. The remaining 48% were detected first near the ground and grew upward or appeared nearly simultaneously over a several kilometer depth; these represent primary modes of tornado development that have been explained theoretically. The descending–nondescending TVSs were stratified according to attributes of the tornado and TVS. Significantly, tornadoes within quasi-linear convective systems tended to be associated with nondescending TVSs, identification of which provided a mean tornado lead time of 5 min.

Two case studies are presented for illustrative purposes. On 1 July 1997 in southern Minnesota, nondescending TVSs and associated tornadogenesis were revealed in the leading edge of a squall line, with a squall line–supercell merger, and later during that day, with the cyclonic bookend vortex of a bow echo. On 22 June 1995 in southern Colorado, a low-topped supercell storm produced a tornado that was associated with a descending TVS.

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T. Wood, A. C. Maycock, P. M. Forster, T. B. Richardson, T. Andrews, O. Boucher, G. Myhre, B. H. Samset, A. Kirkevåg, J.-F. Lamarque, J. Mülmenstädt, D. Olivié, T. Takemura, and D. Watson-Parris

Abstract

Rapid adjustments—the response of meteorology to external forcing while sea surface temperatures (SST) and sea ice are held fixed—can affect the midlatitude circulation and contribute to long-term forced circulation responses in climate simulations. This study examines rapid adjustments in the Southern Hemisphere (SH) circulation using nine models from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP), which perform fixed SST and coupled ocean experiments for five perturbations: a doubling of carbon dioxide (2xCO2), a tripling of methane (3xCH4), a fivefold increase in sulfate aerosol (5xSO4), a tenfold increase in black carbon aerosol (10xBC), and a 2% increase in solar constant (2%Sol). In the coupled experiments, the SH eddy-driven jet shifts poleward and strengthens for forcings that produce global warming (and vice versa for 5xSO4), with the strongest response found in austral summer. In austral winter, the responses project more strongly onto a change in jet strength. For 10xBC, which induces strong shortwave absorption, the multimodel mean (MMM) rapid adjustment in DJF jet latitude is ~75% of the change in the coupled simulations. For the other forcings, which induce larger SST changes, the effect of SST-mediated feedbacks on the SH circulation is larger than the rapid adjustment. Nevertheless, for these perturbations the magnitude of the MMM jet shift due to the rapid adjustment is still around 20%–30% of that in the coupled experiments. The results demonstrate the need to understand the mechanisms for rapid adjustments in the midlatitude circulation, in addition to the effect of changing SSTs.

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J. C. Doran, S. Abbott, J. Archuleta, X. Bian, J. Chow, R. L. Coulter, S. F. J. de Wekker, S. Edgerton, S. Elliott, A. Fernandez, J. D. Fast, J. M. Hubbe, C. King, D. Langley, J. Leach, J. T. Lee, T. J. Martin, D. Martinez, J. L. Martinez, G. Mercado, V. Mora, M. Mulhearn, J. L. Pena, R. Petty, W. Porch, C. Russell, R. Salas, J. D. Shannon, W. J. Shaw, G. Sosa, L. Tellier, B. Templeman, J. G. Watson, R. White, C. D. Whiteman, and D. Wolfe

A boundary layer field experiment in the Mexico City basin during the period 24 February–22 March 1997 is described. A total of six sites were instrumented. At four of the sites, 915-MHz radar wind profilers were deployed and radiosondes were released five times per day. Two of these sites also had sodars collocated with the profilers. Radiosondes were released twice per day at a fifth site to the south of the basin, and rawinsondes were flown from another location to the northeast of the city three times per day. Mixed layers grew to depths of 2500–3500 m, with a rapid period of growth beginning shortly before noon and lasting for several hours. Significant differences between the mixed-layer temperatures in the basin and outside the basin were observed. Three thermally and topographically driven flow patterns were observed that are consistent with previously hypothesized topographical and thermal forcing mechanisms. Despite these features, the circulation patterns in the basin important for the transport and diffusion of air pollutants show less day-to-day regularity than had been anticipated on the basis of Mexico City's tropical location, high altitude and strong insolation, and topographical setting.

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David C. Fritts, Ronald B. Smith, Michael J. Taylor, James D. Doyle, Stephen D. Eckermann, Andreas Dörnbrack, Markus Rapp, Bifford P. Williams, P.-Dominique Pautet, Katrina Bossert, Neal R. Criddle, Carolyn A. Reynolds, P. Alex Reinecke, Michael Uddstrom, Michael J. Revell, Richard Turner, Bernd Kaifler, Johannes S. Wagner, Tyler Mixa, Christopher G. Kruse, Alison D. Nugent, Campbell D. Watson, Sonja Gisinger, Steven M. Smith, Ruth S. Lieberman, Brian Laughman, James J. Moore, William O. Brown, Julie A. Haggerty, Alison Rockwell, Gregory J. Stossmeister, Steven F. Williams, Gonzalo Hernandez, Damian J. Murphy, Andrew R. Klekociuk, Iain M. Reid, and Jun Ma

Abstract

The Deep Propagating Gravity Wave Experiment (DEEPWAVE) was designed to quantify gravity wave (GW) dynamics and effects from orographic and other sources to regions of dissipation at high altitudes. The core DEEPWAVE field phase took place from May through July 2014 using a comprehensive suite of airborne and ground-based instruments providing measurements from Earth’s surface to ∼100 km. Austral winter was chosen to observe deep GW propagation to high altitudes. DEEPWAVE was based on South Island, New Zealand, to provide access to the New Zealand and Tasmanian “hotspots” of GW activity and additional GW sources over the Southern Ocean and Tasman Sea. To observe GWs up to ∼100 km, DEEPWAVE utilized three new instruments built specifically for the National Science Foundation (NSF)/National Center for Atmospheric Research (NCAR) Gulfstream V (GV): a Rayleigh lidar, a sodium resonance lidar, and an advanced mesosphere temperature mapper. These measurements were supplemented by in situ probes, dropsondes, and a microwave temperature profiler on the GV and by in situ probes and a Doppler lidar aboard the German DLR Falcon. Extensive ground-based instrumentation and radiosondes were deployed on South Island, Tasmania, and Southern Ocean islands. Deep orographic GWs were a primary target but multiple flights also observed deep GWs arising from deep convection, jet streams, and frontal systems. Highlights include the following: 1) strong orographic GW forcing accompanying strong cross-mountain flows, 2) strong high-altitude responses even when orographic forcing was weak, 3) large-scale GWs at high altitudes arising from jet stream sources, and 4) significant flight-level energy fluxes and often very large momentum fluxes at high altitudes.

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Keith A. Browning, Alan M. Blyth, Peter A. Clark, Ulrich Corsmeier, Cyril J. Morcrette, Judith L. Agnew, Sue P. Ballard, Dave Bamber, Christian Barthlott, Lindsay J. Bennett, Karl M. Beswick, Mark Bitter, Karen E. Bozier, Barbara J. Brooks, Chris G. Collier, Fay Davies, Bernhard Deny, Mark A. Dixon, Thomas Feuerle, Richard M. Forbes, Catherine Gaffard, Malcolm D. Gray, Rolf Hankers, Tim J. Hewison, Norbert Kalthoff, Samiro Khodayar, Martin Kohler, Christoph Kottmeier, Stephan Kraut, Michael Kunz, Darcy N. Ladd, Humphrey W. Lean, Jürgen Lenfant, Zhihong Li, John Marsham, James McGregor, Stephan D. Mobbs, John Nicol, Emily Norton, Douglas J. Parker, Felicity Perry, Markus Ramatschi, Hugo M. A. Ricketts, Nigel M. Roberts, Andrew Russell, Helmut Schulz, Elizabeth C. Slack, Geraint Vaughan, Joe Waight, David P. Wareing, Robert J. Watson, Ann R. Webb, and Andreas Wieser

The Convective Storm Initiation Project (CSIP) is an international project to understand precisely where, when, and how convective clouds form and develop into showers in the mainly maritime environment of southern England. A major aim of CSIP is to compare the results of the very high resolution Met Office weather forecasting model with detailed observations of the early stages of convective clouds and to use the newly gained understanding to improve the predictions of the model.

A large array of ground-based instruments plus two instrumented aircraft, from the U.K. National Centre for Atmospheric Science (NCAS) and the German Institute for Meteorology and Climate Research (IMK), Karlsruhe, were deployed in southern England, over an area centered on the meteorological radars at Chilbolton, during the summers of 2004 and 2005. In addition to a variety of ground-based remote-sensing instruments, numerous rawinsondes were released at one- to two-hourly intervals from six closely spaced sites. The Met Office weather radar network and Meteosat satellite imagery were used to provide context for the observations made by the instruments deployed during CSIP.

This article presents an overview of the CSIP field campaign and examples from CSIP of the types of convective initiation phenomena that are typical in the United Kingdom. It shows the way in which certain kinds of observational data are able to reveal these phenomena and gives an explanation of how the analyses of data from the field campaign will be used in the development of an improved very high resolution NWP model for operational use.

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