摘要:

AbstractAn analysis is developed for the eddy structure in the convective boundary layer (CBL) both in terms of thermals and downdraughts and in terms of moments, spectra and cross‐correlations of the vertical component of velocity,w. Using previous measurements of the area of thermals, we calculate the contributions from the average velocity 〈WU〉 in updraughts and (wD) in downdraughts to the total variance w̄2and the third moment w̄3. We conclude that the variability in 〈WU〉 (i.e. variance and skewness) produces more than half the contributions to w̄2and w̄3. The mean value of 〈WD〉 is the major contributor to the mode w̄2in the probability distribution of w, but small‐scale turbulence is also important. The results of a statistical theory of eddies near the ground is explained in physical terms, in particular the cross‐correlation ofwat two points (normalized on w̄2at the reference height Zr) in the lower part of the CBL,Rzzr = z/z, where zr>z. The new theory for the cospectrum of w2andwaccounts for the contribution by the thermals at large scales and predicts that the eddies at small scaies are anisotropic, leading to a universal cospectral form that falls off ask−2at the high frequency end (kbeing the wavenumber).Measurements at the Boulder Atmospheric Observatory in convective conditions are presented for a range of zi/L values (zi is depth of the CBL and L the Monin‐Obukhov length). The results for w̄3and w̄3plotted in mixed layer variables, agree with measurements made over the sea and over flat land for z<0.5zi. The mode, w̌ shows some variation with height, and reaches a maximum value of 0.5w*, wherew* is the convection velocity. Rzzr agrees with the theoretical prediction to within ±20%. The cospectra ofwatzandzr also conform to the theoretical form but with large scatter at low frequencies. All these measurements, except for w̄2are effectively functions of z/zi and independent of zi/L, even when |z/L| w̄31, because they are controlled by large‐scale motions. It is shown that the small scales provide an increasing proportion of w̄3as z/zi decreases. This explains why large eddy simulations (LES) give incorrect values of w̄3but correct values of w3/δz. The implications of these results for diffusion calcul

ISSN:0035-9009    

DOI:10.1002/qj.49711448202

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractAn analysis is presented for the mean flow over hills with low slopes, for various kinds of stable stratification in the approach flow. (The Froude numberFbased on the height is always greater than unity.) The analysis is based on dividing up the flow into different regions with different dynamics.We concentrate on the flow in the inviscid outer region of the flow. It is shown that in the lower part of the outer region, the middle layer, where the shear is large, the upwind velocity profile and the shape of the hill, rather than the upwind density profile, largely determine the vertical perturbation velocity. In the ‘upper layer’ the structure of the flow depends largely on theformof the upwind density profile. For example, this largely determines the location of maximum surface wind. New solutions forstableconditions are obtained when there is a constant density gradient, when (N/UB)2∞ z−2and when there is an elevated inversion layer. In the first and third of these cases, we compare the results with the hydrostatic approximation, and also obtain new general formulae for the near‐surface winds. An approximate formula is developed for surface winds at arbitrary values ofF(>1) for uniform stable strat

ISSN:0035-9009    

DOI:10.1002/qj.49711448203

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractAccording to the thermodynamic theory of cloud top evaporative instability, persistent stratocumulus should be observed only when the cloud top jumps in equivalent potential temperature θ and total water mixing ratiorsatisfy the stability condition Δθ>k(L/cp)Δr, where k ± 0.23. Using observations of persistent mid‐latitude and subtropical stratocumulus we find that the above stability condition is violated in many cases. In an attempt to understand how stratocumulus can persist under apparently unstable conditions we first review the thermodynamic instability theory and then develop a dynamical framework using a two‐dimensional Boussinesq moist convection model with spectral discretization and with resolution sufficient to simulate cloud top processes. Idealized initial value experiments confirm that, when the above condition is violated, evaporative instability leads to cloud breakup through sequential dissections of existing cloud. However, initial conditions close to the critical stability line (e.g. Δθ, (L/cp)Δr = –6K, –15 K) lead to cloud breakup with a characteristic cloud half‐life of several hours, while physically realistic initial conditions far from the critical stability line (e.g. Δθ, (L/cp)Δr = –16 K, –25 K) lead to more rapid breakup, with a cloud half‐life on the order of 1/2 hour. When evaporative instability is so weak that the associated cloud half‐life is as long as several hours, other physical processes (such as surface evaporation) can apparently moisten the boundary layer rapidly enough to ma

ISSN:0035-9009    

DOI:10.1002/qj.49711448204

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractRoss Island, Antarctica is located along the Transantarctic Mountains and is subjected to a strong southerly mountain‐parallel wind regime. Despite this, Windless Bight on the island's southern coast is a region of anomalous calm. The atmospheric boundary layer dynamics that gives rise to this phenomenon is analysed both theoretically and observationally. It is the strong static stability of the boundary layer air encountering the high steep topography of Ross Island, that causes a stagnation zone resulting in the calm conditions of Windless Bight.Direct and proxy observations of boundary layer winds provide a detailed description of airflow around the island. The anomalous (with respect to the synoptic pressure field) but persistent north‐easterly winds at Scott Base are due to the deflection of highly stable, low‐level air around Hut Point Peninsula. By contrast, the less frequent, strong southerly winds which override the peninsula are associated with the influx of warm maritime air from cyclonic systems to the east. It is inferred that flow of air around the terrain of Ross Island gives rise to locally strong winds; these are responsible for the ice breakout and polynya occurrences in McMurdo Sound.The surface airflow past Ross Island can be modelled by a two‐dimensional, steady, frictionless, irrotational, incompressible flow past an obstacle, with a shape based on an island height contour. The flow is assumed to separate from the eastern and western sides of the island and form a wake downstream. A solution for this flow is presented, based on potential theory for streaming motion past an obstacle. The streamline and isobar patterns clearly depict the stagnation region in Windless Bight. For approaching winds of 20 ms−1the local pressure field is perturbed by several

ISSN:0035-9009    

DOI:10.1002/qj.49711448205

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractAt Halley Base, which is situated on the Brunt Ice Shelf in Antarctica, internal gravity waves are frequently observed in the stably stratified atmospheric boundary layer. In this paper, time series of wind speed and direction at a height of 8 m, observed from three meteorological masts, are used to determine some of the physical properties of the waves, such as their wavelengths, phase speeds and directions of propagation. The relationship of the results to various theories of the generation of internal gravity waves is considered. It is shown that the waves do not usually propagate in the direction of the surface wind; typically the direction of propagation is rotated about 45° clockwise from the wind vector. The waves most frequently propagate from a direction of about 135°, which is perpendicular both to the Hinge Line (line where the ice shelf meets the land) and to the ridges in the ice shelf, suggesting a topographic influence. It is shown that the waves obey a dispersion relation similar to that for neutral, trapped internal gravity wave

ISSN:0035-9009    

DOI:10.1002/qj.49711448206

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA two‐dimensional model of the seeder–feeder mechanism for the orographic enhancement of rain is presented. It satisfactorily reproduces observed rainfall rate distributions and is helpful in studying some aspects of the physics of orographic rain. The modelling approach is based on the principle of continuity of the water substance and on the two‐dimensional theory of small perturbations of airflow over topographical ridges of modest dimensions. The model is significantly different from those found in the literature and is used to clarify two major points. Firstly, it emphasizes that there is a range of hill dimensions for which the seederfeeder mechanism plays a major role. That is, the model predicts that a change of regime occurs for larger hills (half length>20 km) where the seeder‐feeder mechanism is no longer dominant because of the possibility of rain development within the feeder cloud and also because the horizontal scale of rain drift becomes small compared with that of the topography. Secondly, it is also suggested that the difference of wind curvature below and above a low‐level jet may be important in determining the intensity of orographic rain. The physical process behind the latter finding is closely linked to partial reflection of upward propagating gravity waves which may lead to ‘resonance’ of the orographically induced vert

ISSN:0035-9009    

DOI:10.1002/qj.49711448207

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe performances of grid‐square mean and enhanced ‘envelope’ orographies are compared in a set of 10‐day forecasts performed using the ECMWF spectral model at horizontal resolutions T21, T42, T63 and T106. Twenty‐four cases chosen at monthly intervals from May 1983 to April 1985 are examined, and for diagnosis of results for the northern hemisphere these are divided into 12 ‘winter’ (November to April) and 12 ‘summer’ (May to October) situations.The winter results in many respects confirm those of previous studies. Apart from T21 resolution, use of the envelope orography in this season is clearly beneficial in general, and synoptic assessment illustrates how mountain barrier effects are better represented by the envelope. Conversely, in summer the envelope has a detrimental effect (according to some, though not all measures) at T42 and T63 resolutions, though not at T106. This appears to be at least in part due to the more northerly position of the (northern hemisphere) jet in summer, which interacts with mountains that are less of a barrier, with T106 giving a better separation of the localized orographic features of these regions. Some problems are seen at all seasons with the envelope representation of the Asian relief.Results for the southern hemisphere are, not surprisingly, generally less sensitive to the representation of mountains, the principal impact coming from the southern Andes and Antarctic Peninsula to the south. A small beneficial impact of the envelope orography is found in the tropics.In individual cases, the use of envelope rather than mean orographv gives rise to local forecast differences near mountains which tend to propagate and amplify (principally on synoptic scales), following the upper‐level flow. Much of the hemisphere can be influenced in 7–10 days. Ensemble‐ and time‐averaged errors are generally reduced by use of the envelope, though not dramatically,

ISSN:0035-9009    

DOI:10.1002/qj.49711448208

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractA new version of the NCAR Community Climate Model (CCM1) is used to study the effect of cloud radiative forcing on model simulations. Previous attempts to determine the role played by clouds in climate and the general circulation of the atmosphere are reviewed first. The concept of cloud radiative forcing is discussed and the forcings in the shortwave and longwave spectral regions are contrasted. At low latitudes, the cloud longwave forcing is primarily within the atmosphere, rather than at the surface. It is thus appropriate to study its effect with a model which employs fixed sea surface temperatures (s.s.t.s).The impact of cloud longwave forcing is studied in 510‐day integrations for constant January conditions. The experiments isolate the forcing by tropical and extra‐tropical clouds. Tropical cloud forcing warms the tropical upper troposphere and accelerates the subtropical jets. There are subtle interactions between the forcing, the clear‐sky longwave heating and the latent heating in the tropics. These additional diabatic heating terms oppose the forcing in some regions and enhance it in others.The tropical cloud forcing strengthens the precipitation maxima at low latitudes. There are also changes in the extra‐tropical flow, including the excitation of a pattern in the 200 mb geopotential height differences which is similar to those found in previous studies of the effect of s.s.t. anomalies. This confirms that the forcing may be as important as latent heat release in determining the atmospheric response to such anomalies. The mode is excited with comparable strength by the extra‐tropical cloud forcing. Finally, some concerns regarding the generality of these results and their applicability to other models and the real atmosphere are discussed. These concerns suggest the need for sensitivity studies, some of which are already in

ISSN:0035-9009    

DOI:10.1002/qj.49711448209

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe aim is to study and contrast the dynamics of linear and nonlinear, large‐scale disturbances generated in the stratosphere by a localized perturbation in the troposphere. The perturbation is applied at the lower boundary of a primitive equation model of the stratosphere and mesosphere. The experiments are diagnosed using a range of dynamical quantities; in particular isentropic maps of Ertel's potential vorticity reveal where and how the stratospheric disturbance becomes nonlinear.The simulations are contrasted with an idealized theoretical model of a nonlinear critical layer, and the relevance for our experiments of ‘wave breaking’ and irreversible mixing of potential vorticity is discussed. Apparent polar focusing of Eliassen‐Palm fluxes is related to localized changes in a widening ‘buckling zone’. Wave amplitudes are far too large to apply simple ideas of wave propagation.The notion of pre‐conditioning as it has been applied to the stratosphere is challenged; there are occasions when it is an oversimplification to regard the troposphere as a wavemaker forcing changes in the stratosphere according to the tenets of wave, mean‐flow theory. Preceding a major warming, the flow is asymmetric about any axis (even in the much‐studied case of February 1979), and strong warmings develop through the nonlinear interaction of large, deep vortices associated with a tropospheric asymmetry. The nonlinearity of the flow is also manifested in an anticorrelation in time between zonal harmonics one and two over a deep layer, and by a phase shift of harmonics as amplitudes change. Associated changes in zonal mean winds give the appearance of distinct minor and major warmings, though a synoptic view allows systematic changes to be seen which caution against dividing the evolution int

ISSN:0035-9009    

DOI:10.1002/qj.49711448210

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractAn account is given of the evolution and structure of the circulation in the stratosphere and upper troposphere of the northern hemisphere in the early winter of 1982/83. An early winter warming is described and it is shown to have some features in common with Canadian warmings. A tentative connection is made between the intensification in the troposphere of a trough/ridge pattern associated with the East Asian low and the establishment of the Aleutian high as a persistent feature extending through the whole depth of the stratosphere. Processes leading to the formation of the so‐called surf‐zone are considered, and aspects of zonalmean diagnostics are related to developments on synoptic m

ISSN:0035-9009    

DOI:10.1002/qj.49711448211

出版商:John Wiley&Sons, Ltd    

年代:1988

数据来源: WILEY