摘要:

AbstractDetailed diagnostic analyses are performed upon a mesoscale numerical simulation of a well‐observed gravity‐wave event that occurred on 4 January 1994 along the East Coast of the United States. The value of using wavelet analysis to investigate the evolving gravity‐wave structure and of using potential vorticity (PV) inversion to study the nature of the flow imbalance in the wave generation region is demonstrated. The cross‐stream Lagrangian Rossby number, the residual in the nonlinear balance equation, and the unbalanced geopotential‐height field obtained from PV inversion are each evaluated for their usefulness in diagnosing the flow imbalance. All of these fields showed clear evidence of strong imbalance associated with a middle‐to‐upper tropospheric jet streak, and tropopause fold upstream of the large‐amplitude gravity wave several hours before the wave became apparent at the surface.Analysis indicates that a train of gravity waves was continuously generated by geostrophic adjustment in the exit region of the unbalanced upper‐level jet streak as it approached the inflection axis in the height field immediately downstream of the maximum imbalance associated with the tropopause fold. A split front in the middle troposphere, characterized by the advance of the dry conveyor belt above the warm front, was overtaken by one of these propagating waves. During this merger process, a resonant interaction resulted, which promoted the rapid amplification and scale contraction of both the incipient wave (nonlinear wave development) and the split front (frontogenesis). The gravity wave and front aloft became inseparable following this merger. The situation became even more complex within a few hours as the vertical motion enhanced by this front‐wave interaction acted upon a saturated, potentially unstable layer to produce elevated moist convection. An analysis of the temporal changes in the vertical profile of wave energy flux suggests that moist convective downdraughts efficiently transported the wave energy from the midlevels downward beneath the warm‐front surface, where the wave became ducted. However, pure ducting was not sufficient for maintaining and amplifying the waves; rather, wave‐CISK (Conditional Instability of the Second Kind) was crucial.This complex sequence of nonlinear interactions produced a long‐lived, large‐amplitude gravity wave that created hazardous winter weather and disrupted society over a broad and highly populated area. Although gravity waves with similar appearance to this large‐amplitude wave of depression occasionally have been seen in other strong cyclogenesis cases involving a jet streak ahead of the upper‐level trough axis, it is unknown whether other such events share thi

ISSN:0035-9009    

DOI:10.1002/qj.49712757702

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe life cycle of a very deep tropopause fold (820 hPa) is documented with aircraft and ship observations during the Intensive Observing Period 18 of the Fronts and Atlantic Storm‐Track EXperiment (FASTEX). The initial setting involves a coherent tropopause disturbance and an associated Arctic tropopause fold. The confluence episode that results from the phasing up of the tropopause disturbance and a southern ridge, ends in the formation of an intense jet streak, the dynamics of which are associated with the development of a polar tropopause fold. A diagnostic analysis suggests that the final dramatic stratospheric intrusion is the consequence of the vertical superposition of the Arctic and polar tropopause folds. The Mesoscale Non‐Hydrostatic (Meso‐NH) model is used to discuss this hypothesis. Mixing of the passive stratospheric tracer within the marine boundary layer is investigated with sensitivity tests which unplug, in turn, the model physical parametrizations. Finally, upper‐level forcings associated with the development of the tropopause fold are investigated in detail in a companio

ISSN:0035-9009    

DOI:10.1002/qj.49712757703

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractA set of two companion papers is dedicated to the documentation of the life cycle of a very deep tropopause fold (820 hPa) during the Intensive Observing Period 18 of the Fronts and Atlantic Storm‐Track EXperiment (FASTEX). In this second part, diagnoses of vertical motion using a Q‐vector partitioning in the natural coordinate system that follows the geostrophic wind are analysed. The partitioning allows the evaluation of vertical motions associated with forcing mechanisms such as confluence and diffluence, thermal advection by the horizontal geostrophic shear (shear advection) and curvature of the flow. The synoptic situation involves the formation of an intense upper‐level jet streak when an Arctic trough and a southern ridge move into phase with each other. Results assessed in the course of the tropopause‐fold life cycle show that subsiding vertical motions associated with each of the forcing mechanisms (confluence, shear advection and curvature) overlap on the cyclonic‐shear side of the entrance region of the jet streak. It is shown that an additional effect of shear advection over the confluence, a necessary ingredient for the development of deep tropopause folds in two‐dimensional contexts, takes place in the present case‐study. However, the forcing mechanisms that contribute mostly to subsiding vertical motions over the warm side of the upper‐level frontal zone (i.e. with a maximum frontogenetic effect) are, in order of importance, the shear advection an

ISSN:0035-9009    

DOI:10.1002/qj.49712757704

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractIt is important to understand the variability of results which can be produced in a cloud‐resolving model (CRM) before using its output to develop parametrizations. In the work presented here a broad range of sensitivity experiments are carried out so that the significance of each change to the model can be put into context. Changes to the parametrization of microphysics and radiation in the CRM are considered along with the influence of dimensionality, horizontal resolution and domain size. Further detailed sensitivity studies are then carried out to investigate the impact of the spatial and temporal distribution of the radiative heating.The results presented show that, out of all the sensitivity studies considered, no particular feature or physical parametrization in a CRM dominates. The domain size, horizontal grid length, use of a third dimension and parametrization of microphysics all significantly impact the simulation in some way. Radiative heating has a large impact on the simulation because it dominates over the large‐scale forcing above 8 km. However, the magnitude of this impact is strongly dependent on other aspects of the model, such as the choice of microphysical scheme. A diurnal cycle, which is inherent in the large‐scale forcing, is not significantly enhanced by radiation in the CRM. However, accounting for the temporal and spatial variability in the radiative heating is important because neglecting this is shown to overestimate the strength of conve

ISSN:0035-9009    

DOI:10.1002/qj.49712757705

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThis paper is a contribution towards better understanding of the continually occurring symmetrization processes in tropical cyclones. Before a tropical cyclone develops an eye, it can often possess an asymmetric monopolar vorticity distribution. As an idealized initial condition in a non‐divergent barotropic model, we use a lopsided monopole with different degrees of asymmetry of the monotonic vorticity field. We then study the axi‐symmetrization process which involves the ejection of a winding spiral band. For extreme asymmetric initial conditions, the band can produce regions of barotropic instability, resulting in nonlinear mixing of vorticity and the formation of polygonal structures. In a second series of experiments, we study the case of a tropical cyclone with a developed eye, modelled as a hollow‐tower vorticity distribution, i.e. an annular region of elevated vorticity, with low vorticity in the eye. If the eye is offset and the annular region of elevated vorticity is not of uniform width, complex symmetrization processes can occur, sometimes leading to a tripole structure of the hurricane's vorticity field. Long‐lived hurricane eyes are found for initial conditions with a slight offset. For such initial conditions, passive tracers can remain in the eye for as long as 72 hours, showing that in this model it is possible for air inside the eye to remain there for long periods of time, while moving coherently with the storm. Predictions of the axi‐symmetric final equilibrium states of the flow are obtained using the statistical mechanics theory of maximum Boltzmann mixing entropy. These predictions are then compared with results from the direct numerical integrations for both the lopsided monopole and the offset hurricane eye. The distribution of air‐parcel tracers initially placed in selective regions of vorticity for the direct numerical integrations are compared with results from the statist

ISSN:0035-9009    

DOI:10.1002/qj.49712757706

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe daily budget of zonal and eddy components of kinetic and available potential energy (APE) including boundary and generation terms are computed for a limited area from 52.5 to 22.5°W and from 20°S to 2.5°N and extending in the vertical from 1000 to 50 hPa, during the period 1–10 January 1993. During the vortex period, 5–10 January, a cyclonic vortex in the upper troposphere with its mean centre at 10°S, 35°W was present. The combined boundary pressure work and dissipation terms of the kinetic energy budget equations, as well as APE generation terms, are computed as residuals. The meridional transports of eddy momentum and sensible heat are also computed.The time‐pressure distributions of the zonal and eddy components of kinetic energy (KZ and KE, respectively) and APE are examined. The upper layer 500–100 hPa is identified as the vertical layer which contains the vortex and its interaction with the ambient flow. A sharp increase of KZ in the upper layer is seen as an important feature of the pre‐vortex period 1–4 January. The role of boundary pressure work in the sharp increase of KZ associated with the development of a strong shear zone before the formation of the vortex is discussed. The vortex formation in the upper layer is associated with a rapid decrease of KZ, a sharp increase of KE and barotropic energy conversion from KZ to KE, the commencement of eddy APE (AE) to KE conversion and the generation of AE. These are identified as the main characteristics of the vortex formation from the time variation of energy variables. The vortex formation is also associated with a large increase of down‐the‐gradient eddy momentum transport.It is found from the vertically integrated mean energy cycle in the upper layer that both the barotropic and AE to KE energy conversions maintain the vortex. However, the former is found to be 0.63 W m−2and dominates over the latter by a factor of 3.5. A balance between the baroclinic energy conversion and the generation of AE is noted. The system is dynamically (thermally) more active in the vertical layer 350–100 hPa (500–100 hPa) as revealed by the vertical variation of energy variables. The reasonableness of computed energy variables is discussed qualitatively and quanti

ISSN:0035-9009    

DOI:10.1002/qj.49712757707

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe temperature structure of the tropical troposphere resembles a moist adiabat, with a lapse‐rate transition toward dry adiabatic where water becomes scarce at an altitudeHma∼ 8 km (350 hPa). Infrared emission by water vapour cools a deeper layer, extending up toHrad∼ 14 km (160 hPa). Five consequences of these unequal heights are reviewed.1. Upper‐tropospheric relative humidity is often low, highly variable, and bimodal, due to the rapidity of drying by radiative subsidence.2. Large‐scale divergent circulations (e.g. equatorialvwind) exhibit a two‐celled vertical structure, with an elevated convergence layer near 8‐10 km in the rising branch.3. The dominant deep convective heating process changes from latent heating at low levels to eddy heat‐flux convergence in the upper troposphere. This requires a substantial updraught‐environment temperature difference, which leads to large entrainment nearHma, yielding stratiform anvil clouds which also contribute radiative heating.4. The rising branches of deep (∼Hrad) vertical circulations export more heat than they import as moisture, so that large‐scale tropical dynamics can be characterized by a ‘gross moist stability’.5. Divergent motions with a vertical wavelength ∼8 km, corresponding to Kelvin or gravity wave speeds of ∼15 m s−1, are excited by simple (e.g. uniform) heating profiles extending throug

ISSN:0035-9009    

DOI:10.1002/qj.49712757708

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractThe theoretical relationship between cloud optical depth and effective radius is examined. The relationship between cloud optical depth and effective radius is shown to depend on (a) the relative variability of droplet concentration, cloud depth, mixing and precipitation, and (b) the correlation between the two most important parameters, namely cloud depth and droplet concentration. It is possible to obtain positive, negative or zero correlation between optical depth and effective radius for different values of (a) and (b). Of all these parameters, mixing appears to be the least important. If clouds are not affected by precipitation, negative correlations between optical depth and effective radius (the expected signature of the first indirect aerosol effect) can be caused by a small spread in cloud depths relative to that in droplet concentration, or (more importantly) by a positive correlation between droplet concentration and cloud depth.Precipitation tends to reduce the correlation between optical depth and effective radius, because one of the main effects of precipitation is to increase the spread in droplet effective radius. If the effect of increases in aerosols is a reduction in precipitation, our results indicate that the correlation coefficient between optical depth and effective radius should increase, an effect that should be most clearly visible over the ocean. These findings illustrate the complexity of the physical processes that underpin the linkage of optical depth and effective radius.

ISSN:0035-9009    

DOI:10.1002/qj.49712757709

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractCloud‐resolving model simulations over a tropical ocean and a mid‐latitude continental region have been used to investigate the influence of subgrid‐scale variations of relative humidity on the direct radiative forcing of sulphate aerosols. Offline radiation calculations based on output from a cloud‐resolving model, with and without sulphate aerosol included, are used to calculate the direct radiative forcing of the aerosol. This forcing is compared with results from single‐column radiation calculations typical of those produced by a climate model.The results from this idealized study show that a typical climate model can underestimate the direct radiative forcing of aerosols by up to 80%. The errors in a climate‐model calculation are largest when the mean relative humidity is high or there are moist regions within a drier domain; this is usually reflected in a larger standard deviation of the relative humidity. Over the more humid tropical ocean, a climate model may underestimate the direct radiative forcing of sulphate aerosols by 43%, on average (ranging between 30% and 80%). Over the drier continental mid‐latitude region the average error is only 10%, but instantaneous values can exceed 50% during times when both the mean relative humidity and its standard deviation are large; this is typically close to conv

ISSN:0035-9009    

DOI:10.1002/qj.49712757710

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY

摘要:

AbstractA parametrization of the Henyey‐Greenstein scattering phase function is presented for application to aircraft and satellite remote sensing of cirrus‐cloud bulk and microphysical properties at non‐absorbing and absorbing wavelengths. The phase function is based initially around a non‐absorbing laboratory‐measured phase function between scattering angles of 30° and 180°, although at scattering angles greater than 95° the phase function is made constant with scattering angle. This ‘analytic’ phase function has an asymmetry parameter value of 0.80 at non‐absorbing wavelengths. This analytic phase function is tested using aircraft‐based transmission radiance measurements at the wavelengths of 0.55 μm, 0.87 μm, 1.6 μm and 3.7 μm between scattering angles of 10° and 120°, and satellite retrievals of cirrus‐cloud spherical albedo at the wavelength of 0.67 μm between scattering angles of 60° and 180°. The analytic phase function reproduces both aircraft and satellite measurements well, and is shown to be a distinct improvement over single‐crystal model phase functions representing both small and large crystals in terms of angular scattering patter

ISSN:0035-9009    

DOI:10.1002/qj.49712757711

出版商:John Wiley&Sons, Ltd    

年代:2001

数据来源: WILEY