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Tuck, A. F.; Baumgardner, D.; Chan, K. R.; Dye, J. E.; Elkins, J. W.; Hovde, S. J.; Kelly, K. K.; Loewenstein, M.; Margitan, J. J.; May, R. D.; Podolske, J. R.; Proffitt, M. H.; Rosenlof, K. H.; Smith, W. L.; Webster, C. R.; Wilson, J. C.
doi: 10.1002/qj.49712353702pmid: N/A
Fast response in situ measurements of a suite of chemical species made from the NASA ER2 high altitude aircraft, between 60°N and 70°S at potential temperatures up to 530 K from March to November 1994 at longitudes 115° W to 150°E, are considered for the view they offer of the Brewer‐Dobson circulation in the lower stratosphere and upper troposphere. In the southern hemisphere, where most of the flights occurred, comparisons are made with measurements taken in August/September 1987 at longitudes 120° W to 60° W to examine temporal and longitudinal differences. Interpretations made suggest conceptual modifications to the simple construct of advection in a two dimensional long‐term mean.
doi: 10.1002/qj.49712353703pmid: N/A
The mean meridional circulation in the troposphere is driven by eddy momentum fluxes associated with baroclinic waves. the dynamics of these waves can be described qualitatively by quasi‐geostrophic theory. Here, the flow in the vicinity of the storm tracks is modelled using f‐plane quasi‐geostrophic theory with the tropopause represented as a nearly horizontal surface separating a troposphere of uniform potential vorticity from a similarly uniform stratosphere. Combining an analysis of the mean meridional mass‐flux associated with decaying eddies and the cross‐tropopause mass‐flux generated in the same process, it is possible to estimate the net flux both in the zonal mean and in a quasi‐Lagrangian mean framework. the quasi‐Lagrangian mean is here defined as an average along potential temperature contours on the tropopause. Consistent with earlier numerical and observational studies, it is found that the zonal mean mass‐flux is upwards poleward of the jet, whereas the quasi‐Lagrangian mean is downwards.
Lott, François; Miller, Martin J.
doi: 10.1002/qj.49712353704pmid: N/A
A scheme for the representation of subgrid‐scale orography (SSO) in numerical weather prediction and climate models is presented. the new scheme arose in part from a desire to represent nonlinear low‐level mountain drag effects not currently parametrized. an important feature of the scheme is that it deals explicitly with a low‐level flow which is ‘blocked’, when the effective height of the subgrid‐scale orography is sufficiently high. In this new scheme, it is assumed that, for this ‘blocked’ flow, separation occurs at the mountain flanks, resulting in a form drag. This drag is parametrized on model levels which are intersected by the SSO, and provides a dynamically based replacement for envelope orography. the upper part of the low‐level flow goes over the orography and generates gravity waves. At the model resolutions considered (T106 and T213) it is assumed that the length scales characteristic of the SSO are sufficiently small for the Coriolis force to be neglected. the various parameters of the scheme are adjusted using an off‐line procedure in which the scheme is used to estimate the mountain drag and the momentum profiles above the Pyrenees; and these estimates are validated with the PYREX data. Forecasts using T106 and T213 resolutions with this new scheme, and with mean orography, show that the forecast mountain drag consistently reproduces the drag measured during PYREX whenever the flow component normal to the ridge is large. Isentropic flow diagnostics, further, show that the new scheme has a realistic impact on the flow dynamics, reinforcing the low‐level wake observed in mesoscale analyses of the flow. With this new scheme and a mean orography, the ECMWF model outperformed, in forecast skill, a version of the model which had an envelope orography and the old gravity‐wave‐drag scheme, while no longer suffering any disadvantages of envelope orography. the proposed low‐level drag parametrization should also be relevant at model horizontal resolutions much higher than T213.
doi: 10.1002/qj.49712353705pmid: N/A
Four microbarographs, accompanied by wind vanes and anemometers, were deployed on a mountain called Black Combe (height 600 m) in Cumbria during a field experiment that took place in Novemberer 1991. the aims of the experiment were to measure the small flow‐induced pressure differences across the mountain and relate these to the local wind. These pressure differences may be used to calculate directly the drag exerted by the atmosphere on Black Combe.
doi: 10.1002/qj.49712353706pmid: N/A
A method has been developed for estimating surface precipitation rates using reflectivities measured with a radar scanning at several different angles of elevation. In each pixel of a radar image, an idealized profile of the reflectivity factor is constructed. Each profile is defined in terms of three unknowns: the reflectivity in the rain beneath the melting layer, and the slope of the profile of the reflectivity in each of two layers above the melting layer. In these layers above the melting layer, it is necessary to invoke assumptions of horizontal homogeneity in the shape of the profile. Simple parametrizations are used for low‐level orographic growth and for the bright band, and the profile is diagnosed with the help of non‐radar information. the difference between the idealized and observed profiles is expressed as a single value for an entire radar‐image; the difference is minimized by iteration and this avoids using complicated methods of inversion. A simulation experiment was carried out for 15 different cases; in most of them, the incorporation of data from several elevation angles reduced bias errors at long range.
doi: 10.1002/qj.49712353707pmid: N/A
The mechanisms of drop‐concentration inhomogeneity formation are studied using both a numerical simulation with a model of isotropic and homogeneous turbulence, and analytical methods. It is shown that atmospheric turbulence can create a significant drop‐concentration inhomogeneity due to the effects of drop inertia. Two types of area in the turbulent flow are revealed. Drops tend to leave the areas of ‘drop vortices’ and collect within the zones out of the vortices. As a result, the zones of drop‐track collection turn out to be the zones of enhanced drop concentration. the rate of concentration enhancement is studied for drops of different sizes using the Monte Carlo method. It is shown that the drop flux velocity divergence and droplet‐concentration variations reach their maximum at 100 μm drop radius. Zones of enhanced drop concentrations are stretched along the drop tracks. Characteristic scales of drop‐concentration fluctuations along the drop tracks are of the order of several metres or even a few tens of metres. Across the drop tracks, the characteristic scale of concentration pulsations is of the order of a few centimetres.
Dharssi, I.; Kershaw, R.; Tao, W. K.
doi: 10.1002/qj.49712353708pmid: N/A
A two‐dimensional cloud‐resolving model is used to simulate a tropical squall line and to examine its sensitivity to long‐wave radiation. Model results show that the magnitude of the sensitivity to long‐wave radiation depend on the organization and structure of the squall line. For one simulation, the inclusion of long‐wave radiation increased surface precipitation by nearly a third. Long‐wave radiation is also found to enhance the mid‐level rear inflow and upper‐level outflow of the squall line. Sensitivity experiments are performed to examine the role of cloud‐top cooling and cloud‐base warming, differential cooling between clear and cloudy regions, and domain‐wide cooling.
Boers, R.; Jensen, J. B.; Krummel, P. B.
doi: 10.1002/qj.49712353709pmid: N/A
During the Southern Ocean Cloud Experiment a line of convection embedded in an otherwise homogeneous stratocumulus layer was sampled at four levels by means of an instrumented aircraft. At the lower levels there was an abrupt change in microphysical and thermodynamic properties from the updraught region to the environment. At the upper level the change was more gradual and extended over a larger region, indicating the spreading out of the convective region underneath the strong inversion capping the boundary layer. Precipitation was observed at all levels, even in the convective region except near its centre core. Vertical‐velocity fluctuations in the convective region near the inversion were indistinguishable from the surroundings. At these high levels the horizontal wind in the convective region was from the same direction as the air close to the surface, but was at an angle of up to 60° with the horizontal wind from the surroundings. the convective region was characterized by cloud droplet concentrations that were often more than double those recorded in the surrounding cloud. This indicates that the modifications to the cloud droplet spectra associated with the onset of precipitation starts in the convective region and is enhanced in the stratiform region.
Galmarini, S.; Vilà‐Guerau De Arellano, J.; Duynkerke, P. G.
doi: 10.1002/qj.49712353710pmid: N/A
A model of the vertical transport of chemically reactive species is presented which is applicable to all possible stability conditions in the atmospheric surface‐layer. the non‐dimensional formulation adopted allows any chemical scheme to be included. an equation hierarchy is presented for a fully second‐order closure description of the process. the model describes comprehensively the turbulence/chemistry process by calculating the mean concentrations, the turbulent fluxes and concentration covariances of the chemical species. It explicitly takes into account the chemical transformation in first‐ and second‐order variable equations. the model is applied to the NO‐NO2‐O3 cycle and to the so‐called ‘night‐time’ chemical scheme of nitrogen oxides. an analysis of the flux budget indicates that the relative importance of the chemistry term in the flux equation varies with distance from the surface. For atmospheric neutral conditions, it is suggested that the chemistry term in the flux equation can be used to make an a priori evaluation of the possible effect of chemistry upon the flux of a chemical species. the model also shows that under stable conditions, despite the greatly reduced activity of turbulence, differences can occur between the turbulent transport of an inert tracer and that of a chemically reactive species. Under any stratification and for typical surface‐layer fluxes of nitrogen oxides, the intensity of segregation is very low and can in general be neglected in model calculations.
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