摘要:

AbstractThe development of a sea‐breezecirculationinto a sea‐breezefrontin often observed late in the afternoon. Measurements of horizontal temperature and humidity profiles in a number of sea‐breeze circulation, made using an instrumented light aircraft, are presented. On each of the four days for which data are presented an initial weak horizontal temperature gradient, extending over a distance of 5 to 10 kilometres, developed later in the into a sharp front only a few hundred metres across. The data are discussed in terms of simple gravity‐current frontogenesis models applied to the development of sharp sea‐breeze front from an initial diffuse transition between the air over the land sea. Examination of the equations for frontogenesis, and the result of some previous laboratory experiments on gravity‐current frontogenesis, lead to the proposal that, given sufficient contrast between land and sea temperatures, the formation of a sea‐breeze front depends on the balance between convergent horizontal winds which act to generate a front, and turbulent convective mixing over the land which tends to prevent its formation. the differences in driving force, wind convergence and turbulent intensity are discussed for each day on which data are available and are shown to be consistent with the th

ISSN:0035-9009    

DOI:10.1002/qj.49711950902

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractTwo idealized baroclinic wave‐6 life cycles examined here suggest a framework of opposite extremes (a) in which to view the behaviour of real synoptic‐scale disturbances in middle latitudes, and (b) in which to interrelate the synoptic and wave‐theoretic viewpoints, using the ‘saturation‐propagation‐saturation’ (SPS) picture of wave‐6 life‐cycle behaviour. The two life cycles. dented by LC1 and LC2, are higher‐resolution versions of the Simmons‐Hoskins ‘basic’ and ‘anomalous’ cases (showing strong and weak late decay of eddy kinetic energy, EKE). They illustrate, in varying degrees. Two extreme types of behaviour here disignated ‘anticyclonic’ and ‘cyclonic’, and epitomized by strongly contrasting upper‐air trough behaviour. ‘Anticyclonic’ behaviour dominates the late stages of LC1 and is characterized by backward‐tilted, thinning troughs being advected anticyclonically and equatorward, as in the commoner cases of planetary‐scale mid‐stratospheric ‘Rossby‐wave breaking’. ‘Cyclonic’ behaviour dominates LC2 and is characterized by forward‐tilted, broadening troughs wrapping themselves up cyclonically and poleward. producing major cut‐off cyclones in high latitudes. These morphologies are visualized by upper‐air maps of potential temperature on the nominal tropopause, defined as a constant‐potential‐vorticity surface. Some atmospheric mid‐latitude disturbances examined here, using the same visualization applied to operational analyses, show the same two extreme types of trough behaviour together with intermediate cases.The SPS picture is re‐examined, using Eliassen‐palm and refractive‐index cross‐sections. It is shown, in particular, by reference to a wave‐activity theorem of Haynes, that the late stages of LC2 can be looked upon as a remarkably clear, and morphologically novel, large‐amplitude counterpart of the nonlinear reflection scenario of Rossby‐wave critical‐layer theory. The late stages of LC1, by constrast, look more akin to an nonlinear critical‐layer absorption scenario. LC2 exhibits region of largely undular PV contours adjacent to a nonlinear of irreversiby deformen PV contous. In the latter region PV rearrangement, and hence absorption of Rossby‐wa

ISSN:0035-9009    

DOI:10.1002/qj.49711950903

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe evolution and structure of an indealized low‐pressure system is studied within the framework of the semi‐geostrophic dynamics and in the limit of uniform potential vorticiy. An isolated cyclone is grown from suitably chosen initial conditions, rather than studying the evolution of a longitudinally periodic train of baroclinic systems.It is shown that the resulting development is able to produce a range of flow features that compare favourably with observationally based conceptual models of cyclogenesis. These features include in particular: (i) the simultaneous occurrence of both a cold and a warm front whose alignment shows some of the characteristics of occluded frontal systems and is skin to the notion of a frontal fracture, (ii) a dry descending air‐stream to the rear of the cyclone, (iii) a narrow region of maximum ascent within the warm front an its bent‐back portion, (iv) a poleward traveling air‐stream ahead of the cold front, and (v) a γ‐shaped pattern of vertical lifting comparable with the cloud patterns as commonly observed in satellite pictures.The evolving cyclone is analysed both form Lagrangian and Eulerian viewpoints. It is demonstrated that Lagrangian criteria exist that allow for the objective definition of air‐streams and flow patterns within developing cyclones. The structures of cold and warm fronts at low levels are significantly affected by the different nature of the Lagrangian trajectories within each of these regions. In particular, the air parcels in the warm‐frontal region are transported rapidly towards the centre of the low, resulting in a low‐level warm front with an intrinsically three‐dimensional structure and an associated vorticity gradient in the a

ISSN:0035-9009    

DOI:10.1002/qj.49711950904

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractAn experiment, involving the National Center for Atmospheric Research's Kin Air aeroplane, was conducted in order to measure the microphysical properties of New Mexican summertime cumulus clouds. Since the clouds formed and developed essentially in place, over the mountains, it was possible to make multiple penetrations through a single cloud, therby observing a significant fraction of the cloud's life cycle. In this paper, the questions of primary‐ and secondary‐ice production, and the development of precipitation particles, are addressed.Primary‐ice nucleation was found to occur when the temperature within the cloud reached a value of between −10 and −12°C irrespective of whether this was in the updraught or downdraught. Drops with diameters of about 0.5 mm were often observed in concentrations of about 10L−1before the formation of ice. which suggests a nucleation mechanism involving large drops. The maximum concentrations of ice particles observed in these clouds (up to about 1300L−1) are much greater than typical concentrations of ice particles that can be attributed to primary‐ice nucleation. Evidence suggests that the most likely explanation is the Hallett‐Mossop process of secondary‐ice‐crystal production.Ice particles generally were first observed in the downdraughts. The development of precipitation is often thought to occur via downdraught transport. followed by sedimentation or mixing of ice particles into fresh. liquid‐laden turrets. The multi‐thermal nature of the cloud is considered to

ISSN:0035-9009    

DOI:10.1002/qj.49711950905

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThis paper introduces an approximate method for estimating the global distributions of the entropy fluxed flowing through the upper boundary of the climate systems, and provides an estimate of the entropy exchange between the earth and space and the entropy production of the planet. The entropy fluxes are those associated with the exchange of radiation between the earth and space and a specific method is described to derive these entropy fluxes from available measurements of the earth's radiation budget. The distribution of these entropy fluxes and the entropy production derived from these fluxes differ in significant ways from those reported previously. The major source of these differences arises from the incorrect estimate of entropy exchange associated with reflected solar radiation in these earlier studies. This problem is overcome by the application of a simple physical model for the entropy exchange. Radiative‐transfer simulations of the entropy exchange demonstrate that this model reasonably approximates the entropy exchange associated with reflected solar radiation, and that the long‐wave fluxes of entropy are closely approximated by the equivalent black‐body entropy flux. Entropy fluxes calculated from the Earth Radiation Budget Experiment measurements show how the long‐wave entropy flux densities dominate the total entropy fluxes at all latitudes compared with the entropy flux densities associated with reflected sunlight, although the short‐wave flux densities are important in the context of clear sky‐cloudy sky net entropy flux differences. It appears from the analysis presented in this paper that the entropy production of the planet is both constant for the 36 months of data considered and very near its maximum possible value. The mean value of this production is 0.68 × 1015W K−1and the amplitude of the annual cycle is approximately 1 to 2% of this value, which is within the uncertainty of our estimate. Climatic fluctuations of entropy production are shown to be less than about 2% of the total production and therefore cannot be detected using the method described in this paper. The production of global entropy is dominated entirely by radiative processes. It remains a considerable challenge to detect these climatic fluctuations above such a large background of entr

ISSN:0035-9009    

DOI:10.1002/qj.49711950906

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe paper describes a four‐dimensional variational assimilation using a multilevel global primitive‐equation spectral model. The experiments consist in minimizing the distance between the model solution and conventional observations spread over 24 hours.A first set of experiments was performed at low resolution (spectral truncation T21). The model solution converges to fit the observations satisfactorily over the whole assimilation period. The meteorological quality of the analyses, in terms of the large‐scale part of the flow, is comparable with the ECMWF operational analysis. Confirming early results, the control of gravity waves in the solution can be easily handled using a combination of a penalty term and a normal‐mode initialization scheme within the forward‐backward process of the variational assimilation. Experiments excluding data over an area with a strong baroclinic development show that the information contained in the dynamics of the model is used successfully in the analysis over this area. In addition, the dynamics are able to inter implicitly flow‐dependent structure functions.Higher‐resolution experiments (spectral truncation T42) were performed and compared with optimum interpolation (OI) analyses performed under the same conditions. Although the lack of a first‐guess term in the cost function leads to small‐scale noise generation if the minimization is pursued, the quality of the variational assimilation is comparable with OI, and the error growth of the following 24‐hour forecast is smaller when performed from the variational analysis. Moreover, experiments excluding wind data from aircraft show a clear advantage for the variational approach against OI in using, in a consistent way, the information coming from the model a

ISSN:0035-9009    

DOI:10.1002/qj.49711950907

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe features of the spectra of meteorological fields of wind velocity and temperature observed at Thumba, a site on the western coast of India, are discussed. Data utilized for the study are mostly from an instrumented tower, sodar and long‐period temperature data available for the site. The influence of local circulations like the sea breeze is manifested also in the general spectral characteristics of horizontal wind, as with the spectral gap in the mesoscale region between the synoptic‐scale region and the turbulent‐scale region of the spectra. The width of the spectral gap observed between the wave‐active region and the microscale region in the wind velocity spectra is seen to be dependent on the stability of the medium, and the gap is found to shift to the high‐frequency side with increasing stability. The composite spectrum of temperature also reveals a prominent gap at about an hour. It is seen that with the advection of a sea breeze over the land, a shallow thermal internal boundary layer is formed and regions above one tenth of this depth obey mixed‐layer

ISSN:0035-9009    

DOI:10.1002/qj.49711950908

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractAn integral expression is developed for the mountain wave drag (MWD) associated with linearized, hydrostatic flow over two infinitely long, bell‐shaped ridges. This analysis extends previous studies of airflow over a single ridge. The MWD is expressed in canonical form with three terms: the sole effect of each ridge and a cross‐term. While all three terms tend to zero with the appropriate inverse Rossby number, the cross‐term also tends to zero with large ridge‐ridge separation normalized by the sum of the half‐widths. Asymptotic expansions of the dimensionless integral forms arising from the three terms in the MWD expression are

ISSN:0035-9009    

DOI:10.1002/qj.49711950909

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractRecent attempts to establish a relationship between tropical cyclone motion and the absolute vorticity gradient of the cyclone environment are reviewed. Evidence is presented, both from analytic and numerical calculations, to show that for barotropic vortices there is no unique relationship between vortex motion, c. and the local absolute vorticity gradient of the (imposed) environment, ∇ξa. Our results suggest that it is also unlikely that any similar relationship holds for tropical cyclones.It is shown that conventional methods of averaging over an annular region about tropical cyclone centre to determine the absolute vorticity gradient of the cyclone environment may be very sensitive to the size of the region chosen. For this reason as well as from theoretical considerations our results would indicate that there are certain intrinsic limitations for tropical cyclone track forecasting arising from statistically‐derived relationships between c and

ISSN:0035-9009    

DOI:10.1002/qj.49711950910

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

ISSN:0035-9009    

DOI:10.1002/qj.49711950912

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY