摘要:

AbstractIt has recently become common practice to parametrize vertical momentum transfer, due to subgridscale motions over inhomogeneous areas, by a grid‐cell averaged effective roughness length, z 0eff, based on the concept of a blending height. Here, it is argued that the requirement that z 0effbe determined above a blending height is not a sufficient condition for its parametrization. Results from a numerical experiment are presented which suggest that z 0effis strongly modulated by the texture of surface variability. These results demonstrate that the neglect of subgridscale surface texture may lead to an underestimation of z 0effby up to an order of magnitude. Surface texture, or the spatial relationship between different roughness patches, is interpreted as being of a ‘second‐order roughness’, by which the regular ‘first‐order roughness’ is enhanced owing to the asymmetry of local advection across surface‐type transitions. It is submitted for further discussion that texture analysis provides a framework for the objective quantification of this net effect of subgridscale processes or ‘second‐order roughness’. In the appendix, the flow models used in this study are verified by comparing them with the micrometeorolo

ISSN:0035-9009    

DOI:10.1002/qj.49712152102

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA simple one‐dimensional microphysical retrieval model is developed for estimating vertical profiles of liquid and frozen hydrometeor mixing ratios from observed vertical profiles of area‐mean vertical velocity in regions of convective and/or stratiform precipitation. The mean vertical‐velocity profiles can be obtained from Doppler radar (single and dual) or other means. The one‐dimensional results are shown to be in good agreement with two‐dimensional microphysical fields from a previous study. Sensitivity tests are

ISSN:0035-9009    

DOI:10.1002/qj.49712152103

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractMicrophysical and thermodynamic fields retrieved from dual‐Doppler synthesized winds are used to examine the distribution of melting and freezing rates within a midlatitude squall line with trailing stratiform precipitation. Previously, the effect of the heating/cooling processes associated with the release/consumption of the latent heat of fusion have emphasized the melting layer in the stratiform precipitation region, where the layer is typically marked by a radar bright band. The retrieved fields reveal the melting and freezing in the convective as well as stratiform region. Two‐dimensional retrieved fields indicate that the strong melting effect is not confined to the stratiform region, but extends across the entire breadth of the storm. The peak rates of cooling in the stratiform precipitation region are −2 to −3 K h−1, in general agreement with previous studies. In contrast, cooling rates in the convective precipitation region reach −14 K h−1. The heating by freezing is concentrated in a vertical column associated with the convective updraughts, with peak heating rates of 8 K h−1just above the 0 °C level. For the mature stage of the storm, the convective region is found to account for 56% of the total cooling by melting, and 87% of the total heating by freezing within the squall line.A one‐dimensional retrieval model is applied to mean vertical‐velocity profiles to retrieve area‐mean profiles of the melting and freezing rates. In the convective region, average cooling rates of −10 K h−1are found. The strong cooling by melting is expected to exert a strong influence on the development of convective downdraughts, the cold pool, and the gust front. The heating by freezing occurs through a deep layer, with a peak average heating rate of about 4 K h−1just above the 0 °C level. In the stratiform precipitation region, a melting rate of about −2 K h−1produced a near 0 °C isothermal layer approximately 500 m deep, similar to isothermal layers observed in stratiform precipit

ISSN:0035-9009    

DOI:10.1002/qj.49712152104

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractIt is found that, in axisymmetric simulations of a convective polar low, long‐wave radiative cooling increases both growth rate and maximum intensity. This result is due to cooling of the system's environment relative to its warm core. The magnitude of the increase in growth rate is approximately 30%, but is sensitive to the parametrization of cloud microphysics and radiative properties. A steady‐state Carnot cycle was not obtai

ISSN:0035-9009    

DOI:10.1002/qj.49712152105

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe binary interaction of tropical cyclones is investigated using a three‐dimensional primitive‐equation model. The extended anticyclonic circulations in the upper troposphere merge at very large vortex‐separation distances. For the cyclonic component in the lower troposphere, we find three fundamental modes of interaction separated by two critical separation distances: a mutual approach separation (MAS) and a mutual merger separation (MMS). We suggest that failure to identify these modes may have caused some confusion in interpreting previous baroclinic interaction studies.The MAS delineates vortices that approach each other from those that move on divergent orbits. The approach phase consists of steady radial movement and gradual acceleration, with deformation of the outer vorticity structure of each vortex but little change to their cores. In contrast to barotropic studies, the MAS is much larger than the radius at which the potential‐vorticity gradient of each vortex changes sign. Vortex tilting associated with the vertical shear of the azimuthal winds from the opposing vortex and secondary circulations associated with diabatic heating increases the mutual vortex attraction. The presence of an earth‐vorticity gradient reduces this attraction slightly, but also introduces considerable sensitivity to vortex orientation. When all physical processes are included, we find an MAS of around 1000 km with a scatter of several hundred km, which agrees well with observational studies.Approach occurs with little change in the vortex cores until they reach the MMS. Merger then occurs very rapidly, usually within several hours, and follows that described in parts II and III for barotropic vortices. The MMS is approximately three times the equivalent vortex‐patch radius for the cyclones; it is slightly reduced by diabatic heating, but it is largely independent of the earth‐vorticity gradient. The vortices experience a slight weakening during the approach and initial merger stages. However, with diabatic heating, rapid intensification follows merger; such intensification may have implications for rapid development of trop

ISSN:0035-9009    

DOI:10.1002/qj.49712152106

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA mechanistic axisymmetric Eliassen balanced vortex model is presented for the investigation of the role of diabatic heating in the dynamic evolution of a cut‐off cyclone and the related stratosphere‐troposphere exchange. As an initial state, a balanced upper‐level vortex is chosen representing an idealized cut‐off cyclone. After specifying a diabatic heating field, which is supposed to simulate latent heat release in deep convection, one can diagnose the induced cross‐vortex circulation and calculate the ensuing vortex evolution. Tropospheric heating with a maximum of some 10 K d−1underneath the lowered tropopause leads to the decay of the upper‐level vortex within a few days. During the decay there is a sizeable, diabatically induced mass flux across the PV‐defined tropopause: air in the vortex centre which initially was part of the lower stratosphere gradually turns into tropospheric air. This stratosphere‐troposphere exchange depends sensitively on the heating profile around the tropopause level and, hence, on the maximum height reached by the convection. The inclusion of diabatic cooling to account for the effect of radiation at cloud tops leaves the vortex evolution almost unchanged but increases the rate of stratosphere‐troposphere exchange dramatically. Cross‐isentropic mixing alone may result in strengthening rather than in weakening the vortex thus re‐emphasizing the role of diabat

ISSN:0035-9009    

DOI:10.1002/qj.49712152107

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractFluctuations in vertical profiles of atmospheric temperature and horizontal wind in the 20–60 km altitude range have been isolated from meteorological rocket measurements during 1977–87 at 15 widely separated sites. The seasonal, geographical, and vertical variability of the variance of horizontal velocities,u′2+v′2, and relative‐temperature perturbations,T′2, were studied. The bulk of the variance of both quantities in the 2–10 km and 2–20 km vertical‐wavelength bands was associated with gravity‐wave motions, although in‐depth study of the wave polarization shows that planetary‐scale equatorial wave modes contribute to the variance at equatorial sites. Annual mean variances varied widely among the 15 stations, suggesting appreciable geographical variability in stratospheric wave activity. Whereasu′2+v′2values generally increased significantly with altitude throughout the stratosphere,T′2values grew less substantially and often decreased with altitude at upper heights. Rotations of wave‐velocity phasors with height were always more frequently clockwise than anticlockwise in the northern hemisphere, consistent with upward‐propagating wave energy, yet these percentages (>50%) showed a marked semi‐annual variation, with equinoctial maxima and minima at the solstices. At high latitudes (∼50°N–80°N) variances exhibited a strong annual variation, with the minimum in summer and a strong peak during winter at both lower (20–40 km) and upper (40–60 km) heights. The annual variance cycle attenuated somewhat at mid‐latitudes (∼25°N–40°N), and a strong peak in August dominated theu′2+v′2variations at 40–60 km. The peak was also evident inT′2, but was smaller relative to the winter peak. At low latitudes (∼15°N–25°N) the wave morphology was broadly similar to that at mid‐latitudes, apart from an additional upper‐level peak in the variance in May. This peak in May occurred in some years but not in others at mid‐latitude stations. At the equatorial stations (∼10°N–10°S) the low‐level variance showed little systematic seasonal variability, but exhibited clear modulation over a quasi‐two‐year period. Much of this variance was consistent with the Kelvin modes thought to drive the eastward phase of the stratospheric quasi‐biennial oscillation (QBO). However, the uniform east‐west alignment of waves was inconsistent with the expected polarization of the mixed Rossby‐gravity wave mode which is believed to drive the westward phase of the QBO. At 40–60 km, the variance was strongly attenuated around April‐May and November, when bothu′2+v′2andT′2decreased with height around the 40–45 km range, indicating that wave dissipation occurs here. This produced a semi‐annual variation at upper heights, with maxima around January and July, which may contribute significantly to the semi‐annual wave driving of the equatorial upper stratosphere. Polarization studies showed that this variance in the 2

ISSN:0035-9009    

DOI:10.1002/qj.49712152108

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThis paper describes a simple, but effective, procedure for producing three‐dimensional tracer fields and general diagnostics based on an equivalent PV latitude–potential‐temperature coordinate system. The tracer fields generated by this method are shown to agree quantitatively with independently measured profiles and total columns. Using this initialization makes short model simulations of around a week or so much more realistic. It can be used to assimilate non‐global measurements, such as sonde or solar‐occultation satellite data, or to transform two‐dimensional model data into three‐dimen

ISSN:0035-9009    

DOI:10.1002/qj.49712152109

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA general circulation model was used in perpetual January mode, to study the wintertime climatological sensitivity to changes in the horizontal hyper‐Laplacian diffusion scheme at both spectral T21 and T42 horizontal resolutions.Analysis of the global energy cycle revealed a large impact especially on the conversion of eddy to zonal kinetic energy. Less diffusion resulted in less zonal available potential energy, weaker baroclinic conversions and caused the barotropic conversion of eddy to zonal kinetic energy to become significantly weaker. It was found that by weakening the diffusion in the T21 model, the eddy kinetic energy increased and became more realistic yet at the same time the conversion of eddy to zonal kinetic energy decreased and became less realistic.Weakening the horizontal diffusion caused a northern hemisphere tropospheric response similar to that obtained by increasing the upper‐level orographic gravity wave drag in the model. The zonal‐mean zonal winds weakened poleward of the subtropical jets, the Ferrel cells weakened and the troposphere warmed up at high latitudes. The weakening of the zonal wind and the Ferrel cells was associated with a weakening of the upper‐level convergence of the poleward eddy momentum flux and the wave‐mean flow interaction was demonstrated using Eliassen–Palm diagnostics. A substantial part of the change in the poleward eddy momentum flux was due to large changes in the poleward momentum flux of the transient eddies in the mid‐latitude stormtracks and changes in their horizontal phase tilt. A large northern hemisphere stationary‐wave change appeared in these regions and had a similar structure in the Atlantic sector to that seen when strengthening upper‐level

ISSN:0035-9009    

DOI:10.1002/qj.49712152110

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

ISSN:0035-9009    

DOI:10.1002/qj.49712152111

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY