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da Rocha, Humberto R.; Nobre, Carlos A.; Bonatti, José P.; Wright, Ivan R.; Sellers, Piers J.
doi: 10.1002/qj.49712253102pmid: N/A
The recent high deforestation rates in Amazonia have raised the question of how climate, mainly the precipitation patterns, might be affected as pasture land replaces the primary forest. This work initially shows how the dry season soil moisture and water stress in typical Amazonian pasture modifies the energy partitioning at the surface. Low‐level moisture convergence is a primary mechanism to trigger convective cloud formation and precipitation. The relationship between moisture convergence and local surface evaporation in generating precipitation in Amazonia is studied utilizing a one‐dimensional ‘single column’ model (SiB‐1D). SiB‐1D couples a surface‐vegetation model (SiB) to a physical parametrization of deep convection (Kuo scheme), radiation, turbulent diffusion and large‐scale precipitation. Model simulations for short periods (2 days) show its ability to calculate the Amazonian surface energy‐balance components and boundary‐layer dynamics when compared with field observations from the Rondǒnia Boundary‐Layer Experiment and the Anglo‐Brazilian Amazonian Climate Observation Study. The model was further used to test the sensitivity of the deep convection scheme to a range of typical low‐level moisture convergence situations in a second experiment, and a third test investigated the local convective precipitation generated over forest and grass vegetation as a function of available soil moisture. Results showed that the rainfall over forest vegetation appeared to be almost insensitive to soil water stress whereas reduced precipitation was generated over pasture. When available soil water fell below a threshold of 60% the calculated precipitation over the pasture sites rapidly declined. Although these results are confined by the short integration period and the initial atmospheric profiles, they help to strengthen the notion that deforestation reduces evaporation and convective precipitation, especially during the dry season: a result already indicated by some previous general‐circulation model experiments.
Helmis, C. G.; Papadopoulos, K. H.
doi: 10.1002/qj.49712253103pmid: N/A
Features of the variations in katabatic flow along a simple slope in Athens, Greece are examined both observationally and from a simplified theoretical basis. When undisturbed periods of established flows are selected it is observed that the velocity and temperature fluctuations reveal characteristics which are in agreement with the theory proposed by Fleagle. Interesting implications of the theory are discussed, such as the effects of prescribing a decreasing cooling rate throughout the night or of taking into account the vertical structure of the katabatic layer.
Considine, Geoffrey; Curry, Judith A.
doi: 10.1002/qj.49712253104pmid: N/A
The purpose of this work is to present a simplified model of cloud‐droplet spectra that can be used as a tool for interpreting cloud microphysical observations in boundary‐layer clouds, and as a way to begin quantifying the couplings between cloud microphysics, dynamics and radiative properties for eventual use in cloud parametrizations. The model is steady‐state, ignores nucleation effects, and is formulated to produce horizontally averaged statistics. The major difference between this model and previous work lies in the model assumption that droplet spectra at a given level within a cloud are horizontal averages over a large number of air parcels, each of which can have a different lifting condensation level (LCL). Vertical motions are driven by buoyancy, so that liquid water and vertical velocity are simple functions of height above the LCL. This relationship, treated statistically over a large number of parcels, relates turbulent kinetic energy to horizontally averaged statistics of the droplet spectra. The broadening of droplet spectra is thus directly related to the turbulent kinetic energy within a cloud. In validating the model, results are compared with observations. It is shown that the basic droplet spectra predicted by the model are quite realistic for stratus and stratocumulus. The modelled droplet spectra broaden from cloud base to cloud top as is frequently observed, and the relationship between drop spectrum width and mean radius agrees quite well with observations.
Bott, A.; Trautmann, T.; Zdunkowski, W.
doi: 10.1002/qj.49712253105pmid: N/A
A numerical model of the cloud‐topped planetary boundary‐layer is presented. The model is one‐dimensional with special emphasis on a detailed description of cloud microphysical processes. Aerosols and cloud droplets are treated in a two‐dimensional particle‐distribution whereby the activation of aerosols is calculated explicitly by solving the droplet‐growth equation at all relative humidities. Atmospheric radiation is determined with a δ‐two‐stream radiation scheme. Turbulent fluxes are parametrized as a function of the turbulent kinetic energy. Numerical results are presented which are obtained by utilizing measurements made over the North Sea. The interaction between radiation, turbulence and cloud processes for maintaining the quasi‐steady state of stratiform cloud fields is emphasized.
Matthews, Adrian J.; Hoskins, Brian J.; Slingo, Julia M.; Blackburn, Mike
doi: 10.1002/qj.49712253106pmid: N/A
A subtropical Rossby‐wave propagation mechanism is proposed to account for the poleward and eastward progression of intraseasonal convective anomalies along the South Pacific convergence zone (SPCZ) that is observed in a significant proportion of Madden–Julian oscillations (MJOs). Large‐scale convection, associated with an MJO, is assumed to be already established over the Indonesian region. The latent heating associated with this convection forces an equatorial Rossby‐wave response with an upper‐tropospheric anticyclone centred over, or slightly to the west of, the convection. Large potential‐vorticity (PV) gradients, associated with the subtropical jet and the tropopause, lie just poleward of the anticyclone, and large magnitude PV air is advected equatorwards on the eastern side of the anticyclone. This ‘high’ PV air, or upper‐tropospheric trough, is far enough off the equator that it has associated strong horizontal temperature gradients, and it induces deep ascent on its eastern side, at a latitude of about 15–30°. If this deep ascent is over a region susceptible to deep convection, such as the SPCZ, then convection may be forced or triggered. Hence convection develops along the SPCZ as a forced response to convection over Indonesia. The response mechanism is essentially one of subtropical Rossby‐wave propagation. This hypothesis is based on a case study of a particularly strong MJO in early 1988, and is tested by idealized modelling studies. The mechanism may also be relevant to the existence of the mean SPCZ, as a forced response to mean Indonesian convection.
doi: 10.1002/qj.49712253107pmid: N/A
A comprehensive new radiation code based on the two‐stream equations in both the long‐wave and short‐wave spectral regions is described. The spectral resolution of the code is variable, enabling it to be used in a wide range of applications. Because of its flexibility, the code is well‐suited to the investigation of the sensitivity of radiative calculations to changes in the way in which physical processes are parametrized. The gaseous transmission data are derived from a line‐by‐line model. Particular attention is directed towards the treatment of the water vapour continuum, the overlap between gases, and the sensitivity to changing the carbon dioxide concentrations.
Vaughan, G.; Pepler, S. J.; McKenna, D. S.
doi: 10.1002/qj.49712253108pmid: N/A
This paper describes a novel technique for deriving potential vorticity and Richardson number from aircraft cross‐sections of baroclinic zones. It exploits the fact that gravity waves distort isentropic surfaces, so that an aircraft flying horizontally can measure a vertical profile of temperature and wind under favourable conditions. The technique is applied to two flights of the UK Meteorological Research Flight's C‐130 Hercules aircraft, in April 1983 and June 1991. It is found to work reasonably well only when large‐amplitude gravity waves are present. Values of potential vorticity so derived agree with those calculated in the conventional manner from cross‐sections, and are found to correlate well with ozone mixing ratio.
Purser, R. James; Leslie, Lance M.
doi: 10.1002/qj.49712253109pmid: N/A
We present a generic class of semi‐implicit time‐integration methods, the ‘Generalized Adams–Bashforth’ schemes, for the simultaneous treatment in a semi‐Lagrangian model of the equations of horizontal momentum and kinematics in a rotating environment. The salient feature of the approach is that it deals directly with Lagrangian parcel momentum in terms of the parcel's second time‐derivative of position. The classical Adams–Bashforth methods can be generalized to accommodate equations of second‐derivative form and, as we demonstrate, can be formulated in such a way that the further important refinement of a semi‐implicit handling of the fastest gravity modes follows in a natural way.
doi: 10.1002/qj.49712253110pmid: N/A
The effect of changing the horizontal interpolation for evaluating the quantities at departure points in the global spectral semi‐Lagrangian variable resolution NWP model ARPEGE/IFS is described. The two interpolations involved are conventional cubic Lagrangian and Hermite with estimates of derivatives by fifth‐order compact upwind differencing (CUD‐5). Their properties are compared with examples of different one‐dimensional and two‐dimensional linear‐advection tests. With each kind of horizontal interpolation, twelve 5‐day forecasts are performed, beginning on the fifth day of each month from February 1994 to January 1995, using the same vertical non‐interpolating scheme.
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