摘要:

AbstractA consensus seems to exist throughout the cloud‐physics community that buoyancy reversal associated with evaporative cooling affects not only the global (cloud‐scale) dynamics of a convective cloud, but also the rate of mixing between the cloud and its environment. The latter effect is associated with the concept of the so‐called ‘cloud‐top entrainment instability’ (CTEI), which assumes a positive feedback between buoyancy reversal and the rate of entrainment. In this paper, effects of buoyancy reversal on cloud dynamics are discussed in the context of an unstratified anelastic two‐fluid system. Convection in this system mimics some essential features of cumulus convection. Two‐dimensional numerical experiments, with and without the effects of buoyancy reversal, have been performed. It was found that buoyancy reversal has a dramatic impact on the overall flow evolution, but that its effect on the rate of mixing between the two fluids is small, i.e. flows which differed dramatically depending on whether there was buoyancy reversal or not still resulted in similar amounts of mass being mixed. This result casts doubt on the concept of CTEI in cumulus dynamics. The distinction between strong global effects of buoyancy reversal and minor effects on the rate of mixing in buoyancy‐reversing convection is supported by laboratory experiments with classical and buoyancy

ISSN:0035-9009    

DOI:10.1002/qj.49712152202

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThis paper investigates the idea of formulating free convection in large‐scale models as a special case of forced convection. In free convection, the wind speed in the surface transfer law is related to the velocity of the large eddies in the mixed layer and is proportional to the convection velocity scale. The empirical coefficient is estimated with the help of large eddy simulation data by Sykeset al.(1993) and the resulting formulation is compared with field data by Stull (1994). This concept is shown to be applicable to smooth ocean surfaces as well as rough land surfaces. It is argued that within this framework, free convection is a natural extension of forced convection and only needs a minor modification of traditional transfer law

ISSN:0035-9009    

DOI:10.1002/qj.49712152203

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractObservations of rotating convective disturbances in the trade‐wind region just east of Hawaii are presented. Rotation results from stretching of vertical ambient vorticity in the convective regions. Surface friction is shown to play a significant dissipative role in these disturbances. Ekman pumping associated with the rotation is probably only of secondary importance. Convection is found to be associated with weak local maxima in the boundary‐layer equivalent potential temperature. This suggests that convection occurs where such anomalies make the buoyancy of surface parcels larger than average. Convective available potential energies are found to be small, which means that such equivalent‐potential‐temperature anomalies are important in determining parcel buoyancy. It is hypothesized that mesoscale circulations induced by the ascending motion of convection reinforce the anomalies, resulting in the intensification of the circulations. The equivalent potential temperature of the boundary layer results from a balance between surface fluxes and entrainment from above. Ascending motion apparently reduces entrainmcnt, thus allowing the equivalent potential temperature to rise, whereas subsidence has the opposite

ISSN:0035-9009    

DOI:10.1002/qj.49712152204

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractMicrophysical processes in a cloud band observed during the MFDP/FRONTS87 experiment are studied. The importance of precipitation evaporation in the downdraught zone of the cloud‐band circulation, due to conditional symmetric instability (CSI), is clearly evidenced thanks to a two‐dimensional (2D) microphysical retrieval model applied to an observed 2D wind field. This study shows the crucial role of snow evaporation in maintaining the descent of the CSI circulation in a nearly saturated state. The evaporation creates a cooling which reduces the resistance of the atmosphere to subsiding motions, and thus reinforces the ascending ones. Comparisons with results on the effect of snow evaporation on mesoscale motions obtained by other authors with a one‐dimensional model clearly show the effect of the horizontal wind comp

ISSN:0035-9009    

DOI:10.1002/qj.49712152205

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractWe describe a physically based empirical technique for correcting historical sea surface temperature measurements for time‐varying biases. The corrections are based on models of heat and moisture transfers from uninsulated (canvas) and partially insulated (wooden) sea temperature buckets exposed on deck. One of the canvas bucket models is tested using measurements on board ship and published wind‐tunnel measurements. The method gives geographically and seasonally varying bias corrections through the period 1856 to 1941. The corrections are fairly insensitive to uncertainties such as the size of the bucket or the details of its exposure on deck. A discussion of the history of sea surface temperature observations provides a background to the procedure.The resulting globally and seasonally averaged sea surface temperature corrections increase from 0.11 degC in 1856 to 0.42 degC by 1940. The corrections are compatible with recent measurements made at sea of the errors of canvas buckets. Global and hemispheric time series of corrected sea surface temperature and night marine air temperature data show good agreement: more detailed verifications of the corrections will be reported elsewh

ISSN:0035-9009    

DOI:10.1002/qj.49712152206

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA 1200‐day perpetual January integration of the Berlin troposphere‐stratosphere‐mesosphere general circulation model (GCM) has been performed with constant boundary conditions. The long‐term mean climatology represents the response to the different forcing terms in the zonal‐mean momentum budget: the winter stratospheric mean climatology is determined by the balance between the mean meridional circulation (m.m.c.), acting via the Coriolis force, and the Eliassen–Palm flux divergence. The winter stratospheric mean state oscillates between periods of strong wind (undisturbed conditions) and weaker winds (following minor warmings) on periods of several hundred days; this is the response to transient eddy forcing from the troposphere which has an almost‐red power spectrum, with maximum power on time‐scales of tens of days. As previously found in observations, there is a near cancellation between the wave‐ and the m.m.c.‐induced forcing of the mean flow, but the acceleration is well correlated with the wave forcing. The dominant spatial modes of variability of the GCM are isolated using principal‐component analysis. The dominant orthogonal modes of wind and temperature extend from the winter into the summer hemisphere. They represent structures close to thermal‐wind balance, with a dipolar structure in the lower stratospheric temperature field associated with variations in the strength of the polar‐night jet and weaker wind anomalies in the summer subtropics (22°S, 5hPa). There is some evidence of long‐period fluctuations in the vertical velocity in the lower tropical stratosphere associated with the high‐latitude velocity variations; these are consistent with the equatorial temperature anomalies. Time‐delayed point correlations with the zonal velocity at 62°N, 1 hPa reveal that the weak anticorrelation of velocity in the northern subtropics (28°N, 1 hPa), which is weak at zero‐lag, increases with time to about 58% at 20‐days lag. Singular‐spectrum analysis of the zonal velocity at the three reference points, which isolates orthogonal modes of temporal variability, reveals that the correlations are increased when only the low‐fr

ISSN:0035-9009    

DOI:10.1002/qj.49712152207

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe spherical polar components of the Coriolis force consist of terms in sin ϕ and terms in cos ϕ, where ϕ is latitude (referred to the frame‐rotation vector as polar axis). The cos ϕ Coriolis terms are not retained in the usual hydrostatic primitive equations of numerical weather prediction and climate simulation, their neglect being consistent with the shallow‐atmosphere approximation and the simultaneous exclusion of various small metric terms. Scale analysis for diabatically driven, synoptic‐scale motion in the tropics, and for planetary‐scale motion, suggests that the cos ϕ Coriolis terms may attain magnitudes of order 10% of those of key terms in the hydrostatic primitive equations. It is argued that the cos ϕ Coriolis terms should be included in global simulation models.A global, quasi‐hydrostatic model having a complete representation of the Coriolis force is proposed. Conservation of axial angular momentum and potential vorticity, as well as energy, is achieved by a formulation in which all metric terms are retained and the shallow‐atmosphere approximation is relaxed. Distance from the centre of the earth is replaced by apseudo‐radiuswhich is a function of pressure only. This model is put forward as a more accurate alternative to the traditional hydrostatic primitive equations; it preserves the desired conservation laws and may be integrated by broadly similar

ISSN:0035-9009    

DOI:10.1002/qj.49712152208

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractA northward component of the earth's rotation (b‐effect) is included in a set of linearized, non‐hydrostatic equations to study symmetric instability. The results show that theb‐effect can be important if the perturbations are functions of the latitude. In comparison with the conventional symmetric instability, results also show that, in the northern hemisphere, when the temperature increases with latitude, the growth rate can be enhanced by theb‐effect, and the growth rate decreases when the temperature decreases with latitude. We also find that when the perturbations are independent of latitude, theb‐effect disappears. The physical explanation is

ISSN:0035-9009    

DOI:10.1002/qj.49712152209

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractIn regional and global numerical weather prediction and climate models, sub‐grid scale effects are often accounted for as part of the surface and boundary‐layer parametrizations. There are several proposals (Wood and Mason 1993; Belcheret al.1993; Tayloret al.1989) regarding the parametrization of drag over small‐scale topography. We review these proposals and make comparisons between them, utilizing results from an efficient numerical model with second‐order turbulence closure (Non‐linear mixed spectral finite difference model—Xuet al.1994). Model results cover an extensive range of parameters, such as hill slope, roughness length, horizontal scale and hill shape. We initially confine ourselves to the neutrally stratified surface boundary‐layer but extensions to the planetary boundary‐layer and for stable stratification

ISSN:0035-9009    

DOI:10.1002/qj.49712152210

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe classical analytical solution for the Ekman layer flow has been generalized by introducing an assigned non‐rapidly‐varying eddy diffusivityK(z). Asymptotic expressions for the wind componentsu,vobtained via the WKB method retain the form and, to a large extent, the simplicity of the classical case in whichKis kept constant. Three examples of idealizedK‐profiles are used to compare the numerical, WKB and constant‐Ksolutions for Ekman p

ISSN:0035-9009    

DOI:10.1002/qj.49712152211

出版商:John Wiley&Sons, Ltd    

年代:1995

数据来源: WILEY