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Mitchell, J. F. B.; Senior, C. A.
doi: 10.1002/qj.49711548602pmid: N/A
The southern hemisphere climatology of a recent version of the Meteorological Office 11‐layer general circulation model is presented. The simulated climate is compared with observational data, including Meteorological Office operational analyses for the period 1983‐1987. The surface temperatures and depth and position of the antarctic circumpolar trough are substantially more realistic than found in earlier climate models, but the high latitude upper troposphere is still colder than observed giving an excessively strong westerly jet. The sensitivity of the model to changes in the distribution of sea‐ice is investigated and compared with that found in other models. In a first experiment, all southern hemisphere sea‐ice equatorward of 67.5°S was replaced by sea at 271.2 K, and the surface roughness length was reduced from 10−1m to 10−4m. In a second experiment only the roughness length was changed. It is found that the change in surface roughness contributes substantially to the response to reduced sea‐ice extents. The specification of surface roughness over sea‐ice in numerical models is discussed.
Hsu, Huang‐Hsiung; Hoskins, Brian J.
doi: 10.1002/qj.49711548603pmid: N/A
Diurnal and semi‐diurnal fluctuations in the troposphere and lower stratosphere during the 1986 summer and 1986/87 winter, deduced from data of the European Centre for Medium Range Weather Forecasts, are documented. In general, the tidal fluctuations are consistent with the results of previous investigators based on station data. It thus suggests the adequacy of the data generated by the analysis‐forecast system for presenting the global structure of tidal fluctuations in the troposphere.
doi: 10.1002/qj.49711548604pmid: N/A
Radiative heating rates calculated using monthly mean climatological fields of temperature and ozone are used to diagnose the diabatic circulation in the middle atmosphere. This circulation is then used to infer the zonal force per unit mass.
doi: 10.1002/qj.49711548605pmid: N/A
The effects of the Coriolis force on the hydrostatic flow of a single lower layer over a two‐dimensional ridge are described. For short periods the flows resemble those of the non‐rotating case, but after a few hours (in the atmospheric context) effects of rotation become significant. For steady‐state flow over ridges and plateaux, critical conditions near the crest of the obstacle can control the flow in the same manner as for conventional hydraulics. However, steady disturbances are only manifested a finite distance upstream. For initial Froude number Fo < 1, upstream disturbances decay on the length scale (1 – F o2)1/2L, where L is the Rossby deformation radius. For Fo > 1, upstream bores (where they exist) become arrested at a finite distance ahead of the barrier. When flow on the lee side is supercritical, steady lee‐side disturbances consist of nonlinear inertial oscillations. Obstacle shape is important in determining upstream and downstream flow. The results are useful for understanding atmospheric flows over mesoscale ridges.
doi: 10.1002/qj.49711548606pmid: N/A
Results are presented of thermally forced dry and moist convection and the associated gravity wave fields from three‐dimensional numerical simulations using a non‐hydrostatic anelastic model. This paper extends earlier two‐dimensional simulations to include effects of the third spatial dimension employing a very similar environmental speed‐shear case for the study. The present simulations produce scattered fair weather cumuli in agreement with observations. In many important respects, the physical response is quite similar to that obtained in the earlier two‐dimensional calculations. The near‐uniform surface sensible heat flux results in Rayleigh modes filling the convective boundary layer (CBL) to begin with, whereas later, after convective motions start interacting with the overlying stable layer, larger horizontal scale deep modes become evident and in some cases dominant. The eigenfunction structure of these dominant forced normal modes consists of boundary layer eddies in the lower levels and gravity waves above. They are important organizers of the cumulus convection. As in the earlier two‐dimensional simulations, the efficiency of gravity wave excitation was found to be very sensitive to the mean wind shear in the region spanning the CBL and the overlying stable layer. The dominant horizontal wavelength in the shear direction ranges between 10 and 15 km in the free atmosphere whereas it peaks at about 6 km in the CBL.
Fazu, Chen; Schwerdtfeger, Peter
doi: 10.1002/qj.49711548607pmid: N/A
The flux‐gradient relationships for momentum and heat have been investigated over extensive flat bushland (h = 2.3 m, zo = 0.4 m) with observations being made at several levels extending up to 10 times the canopy height. It has been found that: (1) Eddy diffusivities for momentum and heat, KH and KM, are enhanced by factors of 1.65 and 2.5 respectively under near‐neutral conditions and within the layer close to the canopy. (2) These enhancements distinctly depend on instability, and their variations with height are not simply monotonic, as expected in earlier investigations. (3) Even a counter‐gradient flux of heat could occur under some circumstances. (4) The level of enhancement remains substantially unchanged over different surfaces for heat transfer but decreases as the canopy density increases in the case of momentum transfer. Possible mechanisms are discussed. (5) The length scale of atmospheric turbulence should be taken into account in studies of turbulent transfer over very rough surfaces, where the large‐scale eddies may have a key role.
Dobson, Fred W.; Smith, Stuart D.
doi: 10.1002/qj.49711548608pmid: N/A
In modelling solar radiation from surface weather observations, different approaches have been successful with marine and with land‐based data. We apply a simple model to a marine and a mid‐latitude continental station to illustrate systematic differences in the dependence of the insolation on solar elevation, cloud, and season. Relative to the continental station, the marine station has less insolation at low solar elevations, particularly in winter, but more insolation at high solar elevations. Two mechanisms are discussed to explain these differences.
doi: 10.1002/qj.49711548609pmid: N/A
Measurements of broad‐band radiation at solar (0.3–3.0 μm) and near‐infrared (0.7–3.0 μm) wavelengths are presented for three flights of the Meteorological Research Flight C130 aircraft in broken cloud fields. The solar absorption is determined from observations made above cloud top and below cloud base, using data in the two spectral bands to minimize errors of cloud sampling between different runs and to reduce the scatter due to cloud edge effects. The method assumes that cloud layer absorption in the visible region is negligible compared with that in the near‐infrared. This allows average absorption to be calculated for small sections of the aircraft track, which will generally include both fully clear and totally overcast cases. The smallest absorption measured is about 2%, in agreement with model results for the clear atmosphere. The largest absorption was found to be 8–12% (for a maximum cloud depth of 1 km), which is similar to measurements reported for layer cloud. An additional finding is that absorption by the broken cloud field varies uniformly with albedo, indicating that the parametrization of absorption in numerical models can be adequately described as a simple function of the albedo in partially cloudy areas.
Smith, M. H.; Consterdine, I. E.; Park, P. M.
doi: 10.1002/qj.49711548610pmid: N/A
During a marine aerosol field trial at a coastal site in the Outer Hebrides, a depression formed in the North Atlantic, and moved directly over the site. Wind speeds on site gusted past 100m.p.h., and some data were gathered at very high wind speeds.
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