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doi: 10.1002/qj.49712253602pmid: N/A
Observations of the unstable atmospheric boundary layer were made over the Mediterranean by the airborne backscatter lidar LEANDRE I during a cold air outbreak related to a Tramontane event. These measurements performed during the Pyrénées Experiment show a development of the internal boundary‐layer over the sea influenced by the atmospheric flow‐perturbation caused by the vicinity of the Pyrénées. A one‐dimensional model is used to analyse the internal boundary‐layer growth in relation with mixed‐layer temperature, specific humidity, surface fluxes and vertical velocity. It includes bulk formulae for surface‐flux determination taking into account surface and mixed‐layer values of these parameters. Transfer coefficient values are obtained which are smaller than those computed in the model of Stage and Businger (1981). Results obtained using nomograms of mean column‐heating computed by Chou and Atlas (1982) lead, in the case of the Pyrénées Experiment to overestimated surface‐fluxes. Comparison with our model reveals that the discrepancy originates from the higher heat‐transfer coefficient values used by Chou and Atlas. Model results are compared to lidar and in‐situ data (internal thermal boundary‐layer growth, temperature, specific humidity, average fluxes) and excellent agreement is obtained. The comparison between model results and lidar data shows that the growth rate of the internal thermal‐boundary‐layer is very sensitive to the vertical velocity w̄, which is not the case for surface fluxes. Comparisons between measured and modelled parameters thus allow the derivation of an estimation of the vertical‐velocity field in the vicinity of the Pyrénées. It is finally shown that a simplified equation could be used to estimate surface buoyant heat‐flux far from the shoreline during such cold air outbreaks provided accurate correction of vertical velocity impact on internal thermal‐boundary‐layer growth can be achieved.
doi: 10.1002/qj.49712253603pmid: N/A
Large‐eddy simulations of the non‐entraining dry boundary‐layer, in which the geostrophic wind varies with height, are described. Results are presented for various turbulence statistics, and the large‐eddy model results are used to evaluate the performance of two simple closure‐models. It is shown that the performance of these models is not significantly degraded, in either neutral or convective conditions, by the presence of shear in the geostrophic wind.
O'Dowd, Colin D.; Smith, Michael H.
doi: 10.1002/qj.49712253604pmid: N/A
The vertical structure of aerosol, covering sizes from 0.05‐4 μm radius, was examined under conditions of subsidence during winter and summer over the rural UK. Under well‐mixed boundary layer conditions, dry accumulation mode aerosol was found to be well mixed with height. During the winter campaigns, nocturnal cooling resulted in the development of a stable surface layer, typically 100 m in depth, within which the surface emitted pollutants became trapped leading to concentrations significantly greater than that observed in the mixed boundary layer above it. Under stable boundary‐layer conditions, the aerosol and water vapour vertical profiles exhibited strong negative gradients with height and were indicative of suppressed turbulence associated with stable boundary‐layer conditions. During summer, the boundary layer was normally decoupled and possessed two cloud layers: cumuli forming just below, and penetrating the surface‐layer inversion; and stratocumulus occupying the region under the boundary‐layer capping inversion. Aerosol profiles under decoupled conditions exhibited considerable variability with peak concentrations being observed in the vicinity of cloud edges. Average aerosol concentrations in the main boundary‐layer ranged from 209–651 cm−3 and 0.89–4.3 μm−3 cm−3 for dry number and volume respectively, whilst concentrations and volumetric loadings of 239–2430 cm−3 and 1.1–13.5 μm−3 cm−3 were encountered in surface layers. The majority of the aerosol number and mass concentrations were almost exclusively derived from the size range 0.05‐0.2 μm radius with mode radii often occurring at 0.1 μm or larger. By comparison, free tropospheric aerosol possessed typically an order of magnitude lower concentrations and mass with an associated mode radius of 0.05–0.06 μm or less.
Bower, Keith N.; Moss, S. J.; Johnson, D. W.; Choularton, T. W.; Latham, J.; Brown, P. R. A.; Blyth, A. M.; Cardwell, J.
doi: 10.1002/qj.49712253605pmid: N/A
The properties of the ice phase in a number of cloud types are investigated to improve the ice phase parametrization in atmospheric global‐climate models. Frontal clouds over southern England and the sea areas around the British Isles, maritime convective clouds over the North Atlantic, and continental convective clouds over New Mexico and Montana in the USA are studied.
Browning, K. A.; Roberts, N. M.
doi: 10.1002/qj.49712253606pmid: N/A
A diagnostic study is presented to illustrate the sequence of events in which a warm‐conveyor‐belt (WCB) flow is drawn into the circulation of a developing extratropical cyclone and interacts with dry air aloft to produce distinctive precipitation structures. The poleward boundary of the upstream end of the WCB was associated with a classical (ana) cold‐frontal structure. The other end of the WCB flow, closer to the cyclone centre, was overrun by a ‘dry intrusion’ of low wet‐bulb potential temperature air to give a split (kata) cold front. The orientation of the surface cold front (i.e. the WCB boundary) changed significantly between these two parts but the orientation of the surface geostrophic flow within the WCB changed relatively little between the two parts. The surface cold front was characterized in most places by a narrow cold‐frontal rainband composed of shallow line‐convection elements (precipitation cores) aligned nearly parallel to the surface geostrophic flow in the WCB. The changing relationship between the frontal orientation and the direction of the surface flow accounts for the fact that the line‐convection elements occurred end‐to‐end to give almost two‐dimensional line convection along the classical ana part of the cold front, but were non‐aligned and widely spaced along the split cold‐front part.
doi: 10.1002/qj.49712253607pmid: N/A
Using a detailed line‐by‐line, multiple‐scattering solar radiative‐transfer model, the influences due to cloud internal inhomogeneity in the vertical upon the solar radiative transfer are investigated. In particular, the consequences due to non‐uniform vertical profiles of liquid water and droplet sizes within low clouds are explored in a systematic manner. The fine structure of the spectral overlap between the water droplet and water vapour optical properties, and its effects upon the radiation absorbed within the cloud layer and that reflected at the top of the cloud, are discussed. Without consideration of the in‐cloud water vapour, a vertically inhomogeneous cloud with properties resembling those observed absorbs more solar radiation than an equivalent homogeneous cloud. However, consideration of the effects of the in‐cloud vapour, while still leading to a slightly greater absorption for the inhomogeneous case, partly offsets the difference introduced by the vertical distribution of the drop microphysics. The vertical distribution of cloud heating rate is changed substantially because of the inhomogeneity in the microphysics, with the heating rate in the top region of the cloud nearly 50% more than that due to an equivalent vertically homogeneous cloud. Vertical inhomogeneity of cloud microphysics has little influence on the broadband solar albedo, but can cause significant decreases of the cloud reflectance at specific near‐infrared wavelengths i.e. wavelengths greater than 1 μm, (equivalently, wave numbers less than 10000 cm−1).
Langland, Rolf H.; Elsberry, Russell L.; Errico, Ronald M.
doi: 10.1002/qj.49712253608pmid: N/A
An adjoint model (MAMS1) that includes parametrizations for convective (subgrid‐scale), and non‐convective (grid‐scale) precipitation, and surface latent‐heat flux is used to investigate an idealized extratropical cyclogenesis. The adjoint sensitivity information demonstrates the effects that perturbations of model variables and parameters at various times during the cyclone life cycle have on forecast cyclone intensity. For a nonlinear trajectory that includes precipitation processes and surface latent‐heat flux, the accuracy of the tangent‐linear and adjoint model is much higher when moist physical processes are included. Inclusion of moist processes in the adjoint model increases sensitivity magnitude compared with sensitivity obtained with a dry adjoint model, but does not alter the primary spatial pattern of sensitivity.
Siegmund, P. C.; van Velthoven, P. F. J.; Kelder, H.
doi: 10.1002/qj.49712253609pmid: N/A
The upward and downward mass fluxes of air across the tropopause, defined as the 3.5 potential vorticity unit (PVU) surface, are diagnosed for the extratropical northern hemisphere, using high spatial and temporal resolution (0.5° latitude and longitude, 31 levels, 3‐hourly) circulation data for January 1994 from the European Centre for Medium‐Range Weather Forecasts.
Clough, S. A.; Davitt, C. S. A.; Thorpe, A. J.
doi: 10.1002/qj.49712253610pmid: N/A
Attributing synoptic development and structure to particular atmospheric features is an important practical problem. In this paper, methods which have been proposed for the attribution of quasi‐geostrophic potential vorticity (PV) are extended to the study of sources of vertical motion and the influence of the earth's surface and tropopause.
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