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Hollingsworth, By A.; Lorenc, A. C.; Tracton, M. S.; Arpe, K.; Cats, G.; Uppala, S.; Kållberg, P.
doi: 10.1002/qj.49711146702pmid: N/A
We present an intercomparison of analyses of the main FGGE level IIb dataset with three advanced analysis systems. the aims of the work are to estimate the extent and magnitude of the differences between the analyses; to identify the reasons for the differences; and, finally, to estimate the significance of the differences for forecast skill. We restrict ourselves primarily to a consideration of the extra‐tropical analyses. the subject of tropical analyses merits separate treatment.
Arpe, By K.; Hollingsworth, A.; Tracton, M. S.; Lorenc, A. C.; Uppala, S.; Kållberg, P.
doi: 10.1002/qj.49711146703pmid: N/A
The purpose of our two‐part study is to assess the importance of the differences between three independent analyses of the same FGGE level IIb observational dataset. Part I is concerned with the nature and origin of the differences among the analyses. Part II of the study is concerned with the implications of the analysis differences for forecast skill, and for estimates of predictability.
doi: 10.1002/qj.49711146704pmid: N/A
An observing systems experiment to assess the impact of aircraft observations on analyses and forecasts from a numerical model is described. Parallel runs with and without aircraft observations show significant analysis differences in the upper troposphere over northern hemisphere oceans, the aircraft giving a better representation of jet strengths and less reliance on poorer quality observations. This positive impact extends to the shape and position of upper tropospheric features in forecasts from these analyses though the magnitude of the improvements is somewhat less.
Hignett, By P.; White, A. A.; Carter, R. D.; Jackson, W. D. N.; Small, R. M.
doi: 10.1002/qj.49711146705pmid: N/A
Quantitative and qualitative comparisons are made between laboratory measurements of rotating annulus flows and corresponding numerical model simulations. Two laboratory annuli, of similar dimensions but differing in instrumentation, are used. One contains a thermocouple array for temperature measurement: the other contains no sensor array but the working fluid is seeded with minute neutrally buoyant beads (600 m̈m diameter) which enable the horizontal velocity field to be measured. Each annulus has a rigid insulating lid in contact with the working fluid. the numerical model is a finite difference formulation based on the Navier‐Stokes equations for baroclinic flow of a Boussinesq liquid. Although the atmosphere and the laboratory annulus are both rotating baroclinic fluid systems, the forcing processes acting in the annulus are much simpler than those acting in the atmosphere, and may be accurately represented by established formulae: under a wide range of conditions no parametrizations of subgrid‐scale dynamical and diabatic processes are required. Comparison of numerical model results with laboratory measurements therefore enables the explicit dynamical formulation of numerical models of rotating, baroclinic flow to be verified to an extent which would be very difficult, if not impossible, to achieve using atmospheric data. Detailed quantitative comparisons for a steady wave flow reveal good agreement for major features of the temperature and horizontal flow fields, although a significant discrepancy in total heat flux is found. Qualitative comparisons are made by investigating the ability of the numerical model to reproduce the main flow types and phenomena of the laboratory system. Numerical simulations of intransitivity, hysteresis, wavenumber transitions, amplitude vacillation and a weak structural vacillation are described. Several suggestions for further comparative studies are made in conclusion.
doi: 10.1002/qj.49711146706pmid: N/A
A diagnosis of the life cycles of three South Pacific cyclones, each tropical in origin, is presented. the study focuses on two of the cyclones which propagate south‐eastward along the western edge of the South Pacific convergence zone (SPCZ) and reach middle latitudes before decaying. Based on enhanced IR satellite imagery from GOES‐West, both cyclones appear to go through stages of development similar to those described by Troup and Streten, even though their classification system generally applies to higher latitude systems. the primary data set used for surface and upper air analyses is based on ECMWF FGGE level III‐b analyses. Variables presented include mean sea level pressure (m.s.l.p.), horizontal divergence, relative vorticity, and winds. A comparison between m.s.l.p. patterns obtained manually from station observations and those produced objectively by ECMWF shows good agreement except when the cyclonic disturbances are subgrid‐scale. Maximum low level convergence occurs in the SPCZ, north and east of the cyclone tracks; however, maxima of low level cyclonic vorticity correspond to the individual cyclones. Cyclone tracks appear to be influenced by the upper level subtropical jet. the two cyclones which propagate into middle latitudes are accompanied by lower tropospheric warm (cold) air advection ahead of (behind) the system. Both cyclones also exhibit upper tropospheric anticyclonic (cyclonic) vorticity ahead of (behind) the system.
doi: 10.1002/qj.49711146707pmid: N/A
Convection in the lower troposphere produces a deficit in the local surface pressure caused by heating of the air in the well‐mixed convection layer. Variations in the surface heat flux can therefore induce horizontal pressure gradients in the convection layer. These pressure gradients can in turn lead to significant geostrophic flows, such as cyclonic ‘heat lows’ in the vicinity of local maxima in the surface heat flux. Turbulent mixing causes the motion in the convection layer to generate vertical velocities, which lead to some geostrophic flow being generated in the middle troposphere above the convection layer.
doi: 10.1002/qj.49711146708pmid: N/A
Microphysical measurements made in small wintertime cumuli on five days in June 1981 are described. On all days an ice crystal multiplication process was active. the main conditions for the operation of the Hallett‐Mossop multiplication process were fulfilled—namely, the presence of both graupel particles and drops ≃ 24m̈m in diameter in the cloud zone between the ‐3 and ‐8°C levels. Shortcomings of the available theoretical models prevented any worth‐while comparisons between their predictions and the observed cloud behaviour. In these clouds, large drops (⩾300 m̈m diameter) grew by coalescence and appeared in a concentration of ≃0.1 per litre at about the same time as ice particles (graupel) of similar size. the drops reached a concentration of ≃5 per litre and then disappeared while the graupel continued to increase to concentrations of ≃10 per litre. There is evidence to indicate that the concentration of ice particles increased exponentially with time, being multiplied by a factor of 10 every 10 min between the values of 1 and 100 per litre. It is likely that the original ice nuclei activated were more numerous than the generally accepted average concentration of about 10m−3 at ‐10°C.
doi: 10.1002/qj.49711146709pmid: N/A
A detailed analysis is presented of the large‐scale, mesoscale and local features of a south‐foehn event in the Alps on 8 November 1982. On this day, with a substantial cross‐mountain flow, instrumented aircraft made programmed flights back and forth across the Alps between southern Germany and northern Italy. Rawinsonde observations were used to complete the data set. A mesoscale double mountain wave with an amplitude of 1 km was found in the upper troposphere. In the mid troposphere above the Inn valley a rather pronounced wave with an amplitude of 2 km and a wavelength of 50 km was analysed. This wave was close to overturning. the foehn turned out to be accompanied by low wave drag at high levels and with strong wave drag at lower levels. Downward southerly (westerly) momentum flux was evaluated to be 0·3 (0·1) Pa. Mountain drag was estimated to be between 1·6 and 6·7 Pa. Light to moderate turbulence was observed immediately to the lee in a low‐level turbulence zone over the region of strong gusty surface winds. Finally, special attention has been devoted to the similarities and possible differences between foehn in the Alps and chinook in the Rocky Mountains.
Crook, Norman A.; Miller, Martin J.
doi: 10.1002/qj.49711146710pmid: N/A
In this paper the formation and structure of an atmospheric undular bore is examined with the aid of a numerical model and theoretical analysis. the numerical model is first used to simulate a density current propagating into an unstratified environment. A detailed comparison of the model results with existing theory shows that for a given pressure head the current can travel at a greater speed than that predicted by irrotational theory.
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