摘要:

AbstractMesoscale convective cloud systems that produce large amounts of rain in mid‐latitudes and most of the rain in the tropics consist of a combination of convective and stratiform cloud. The convective regions contain numerous deep cells that are often but not always arranged in lines. The stratiform region is an outgrowth of the convective ensemble; it lies adjacent to the convective region and is seeded by ice particles detrained from convective towers. Sometimes it lies to the rear of a propagating convective line, while on other occasions it surrounds the convection.The heating of the large‐scale environment by a mesoscale convective system is affected by both the convective and stratiform regions. Although processes such as melting and radiation are important, the net heating by a system is dominated by condensation and evaporation associated with vertical air motions. This paper reviews recent observational evidence regarding the mean vertical motion profiles in the convective and stratiform regions of mesoscale convective systems and the implications of these mean motions for the vertical distribution of heating of the large‐scale environment.In both the convective and the stratiform regions, vertical motions have been determined by various techniques, including composites of rawinsonde and aircraft wind data, single‐ and dual‐Doppler precipitation radar analyses, and wind‐profiler observations. In stratiform regions, these data consistently show mean vertical velocity that is upward in the upper troposphere and downward in the lower troposphere. The level separating upward from downward motion is located from 0 to 2 km above the 0°C level, depending on location within the stratiform region. Diagnostic calculations show that these vertical motion profiles imply heating of the upper troposphere and cooling of lower levels by stratiform‐region processes.Data on vertical motions in the convective regions are less consistent from case to case. These data sometimes indicate that the mean vertical velocity in convective regions is maximum in the lower troposphere. In other cases, the data indicate a maximum in the high troposphere. Diagnostic calculations show that heating profiles diagnosed from these two types of profile are quite different, the first having a maximum of heating in the lower troposphere, while the second has a maximum in the middle troposphere. Although it is difficult to determine whether or not the differences in estimates arise from different types of observations, analysis methods or sampling strategies, it seems likely that they stem from differences in the large‐scale environment of the different mesoscale systems.The ubiquitous occurrence of stratiform regions in mesoscale convective systems and hurricanes together with their consistent heating profiles, which systematically concentrate heating in upper levels while cooling lower levels, are a major consideration in evaluating the interaction of mesoscale systems with the large‐scale environment. However, the consistency of the stratiform profiles from case to case indicates that the variability of net (convective plus stratiform) heating profiles from case to case lies primarily in the variation of the convective‐region profiles from one case to another. It is suggested that future work should focus on the convective‐region vertical profiles of vertical motion and heating and on the large‐scale environmental factors that may control the variation of these pr

ISSN:0035-9009    

DOI:10.1002/qj.49711548702

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractFollowing the recent trend for composite analysis of the energetics of cyclonic systems, a rapidly deepening Saharan depression is studied within this framework. This case refers to a cyclone which was the prevailing feature in the synoptic analyses over the Mediterranean for several days in mid March 1981. This system was marked with a range of interesting phenomena over the central and eastern Mediterranean, such as very strong surface winds, rising and transportation of desert dust to large distances, and coloured precipitation. The synoptic dynamics of the evolution of the system is briefly presented and a reasoning for its unusually rapid deepening is provided.The forms of energy considered here are kinetic energy, and sensible and latent heats. The diagnostic analysis of the budgets of these forms of energy has been carried out using the sigma coordinate system. Bearing in mind the explosive nature of the cyclogenesis studied here, the analysis is separated into two parts: one associated with the deepening phase of the cyclone and the other with its filling phase. The results are presented and discussed as time‐height cross‐sections of the energy contents; as time‐averaged vertical distributions of budget components; and as vertically averaged budgets. The analysis revealed contrasting differences in the energy budgets in each phase of the development of the s

ISSN:0035-9009    

DOI:10.1002/qj.49711548703

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractData from a series of five research flights in marine stratocumulus are used to investigate the structure of radiatively driven, free convective layers. Conditional sampling methods are used to determine the properties of the primary convective elements: negatively buoyant downdraughts. The turbulent velocity data are also subjected to spectral analysis and the turbulent kinetic energy (TKE) balance is evaluated for each case. Results from each case are found to be very similar if scaled using appropriate mixed layer quantities.Downdraughts are selected using a criterion based on vertical velocity. The distributions of their intersected widths at cloud top, and the velocity spectra, are consistent with downdraughts occupying narrow regions (∼0.1‐0.15hwide,hbeing the mean mixed layer depth) around the periphery of larger, regular (∼0.5h‐0.75hdiameter) updraughts as suggested by cellular patterns observed in the cloud tops. Downdraughts are found to carry over half of the total turbulent fluxes of heat and water substance within cloud, and also transport variance down the mean gradients. Their thermodynamic properties are consistent with their containing a small fraction of air from above cloud top although their negative buoyancy is almost entirely due to the incorporation of radiatively cooled air. The properties of the downdraughts near cloud top suggest they are formed primarily as a result of the local horizontal convergence of upwelling motions constrained by the inversion. While being cooled radiatively, these quasi‐horizontal motions scour the base of the inversion, incorporating some drier air before being forced downwards in relatively narrow zones. Comparisons with other convective layers over land or sea suggest that convection in stratocumulus proceeds with a greater mass flux and correspondingly reduced differences between convective elements and the environment, possibly reflecting the improved ventilation possible at an inversion interface compared with much rougher surfaces.The TKE balance divides into three main regions and is consistent with the interpretations given above. The pressure transport is implied to be the largest source term close to cloud top. Comparisons with model results reveal some important di

ISSN:0035-9009    

DOI:10.1002/qj.49711548704

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractThe two‐dimensional Eady mode of baroclinic instability exhibits a strong cold front structure after a period of development using semi‐geostrophic equations excluding moist processes. Indeed the surface wave has a single centre of cyclonic vorticity on the cold side of the warm air anomaly. Here results are presented from a semi‐geostrophic numerical model including moist effects which predicts an Eady wave with a coldanda warm front associated with two centres of surface vorticity located on either side of the warm anomaly. The ascent regions are taken to be saturated and close to moist neutral which in this baroclinic environment corresponds to a condition of zero equivalent potential vorticity; the descent is taken to be unsaturated.This development of a structure resembling a warm front occursonlywhen the moist effects become dominant within a pre‐existing, weak, dry Eady mode. Using the same condensation representation starting from an extremely small, and random, amplitude disturbance a ‘pure’ moist Eady mode develops with a single vorticity maximum in the form of a cold front. In nature it is likely that often the moisture becomes dominant after a period, albeit sometimes a short period, of essentially unsaturated growth, although observational evidence is limited. Therefore the process of warm frontogenesis described here may be a contributory factor to those minority of mid‐latitude cyclones which exhibit a strong warm front. The model indicates that the warm front moves more slowly than the cold front, resulting in a cold occlusion process.These investigations require a detailed examination of the initial growth of small amplitude disturbances near the limit of neutral moist ascent, i.e. pure moist modes. Results of previous such calculations using an analytical 2‐level model are confirmed here using a numerical multi‐level model. The more accurate estimates obtained here show that the finite growth rate in the limit of zero equivalent potential vorticity has a value of 1.9 times the dry value compared with the 2.4‐fold increase given b

ISSN:0035-9009    

DOI:10.1002/qj.49711548705

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractIt is shown that the standard normal mode initialization scheme mishandles atmospheric tides. A suitable modification is proposed. The performance of the revised scheme is discussed.

ISSN:0035-9009    

DOI:10.1002/qj.49711548706

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractTwo‐ and three‐dimensional versions of a nonlinear atmospheric model were used to investigate environmental controls on the offshore influence of sea‐breeze circulations. The temperature of the water surface relative to the atmospheric temperature had a small effect when the water was colder than the overlying air, and a greater effect when the water temperature was high enough to create convective mixing in the planetary boundary layer over the water. Ambient thermal stratification, including elevated inversions with thermal stratification similar to trade wind inversions, had a small influence. The predicted offshore extent of the sea breeze was substantially affected by latitude. The effects of both ambient stratification and latitude were generally consistent with predictions of linear theory. Prevailing synoptic winds were also found to have a significant effect on the sea breeze offshore, with offshore influence greatly suppressed by onshore large‐scale flow. Curvature of the coastline produces slightly stronger winds offshore for a concave coast than for a convex coast. The subsidence offshore also extends further over the water for a concave coast than for a convex coast.The model predictions were compared with previous investigations which used linear theory. It was found that the present results agreed with the linear theory of R. Rotunno with regard to the effects of latitude and ambient thermal stability. Linear theory is less suitable for explaining the effects of water temperature, owing to the simplified treatment of turbulent exchange necessary for a tractable linea

ISSN:0035-9009    

DOI:10.1002/qj.49711548707

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractObservations from a parachute‐borne omegasonde that executed a trajectory through, along, and up and over the head of a sea breeze are analysed. This sea‐breeze head is shown to have similar characteristics to those of a gravity current. A deep lobe/cleft combination was observed at the front where continental air was entrained into the sea‐breeze air. Observed vertical motion near the sea‐breeze front exceeded 5 ms−1and the sonde rose into a cloud that had formed in rising continental air over this front. An evaporatively cooled downdraught from this cloud was observed to be entrained through the turbulent wake region and deep into the sea‐

ISSN:0035-9009    

DOI:10.1002/qj.49711548708

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractCase studies of three sea fog (haar) situations off the coast of NE Scotland are presented. The studies are based on aircraft and mini‐sonde observations and satellite imagery, and are used to assess the roles of physical and dynamical factors in developing and advecting sea fog in order to improve guidance to forecasters at coastal outstations. Two fogs had formed 2–3 days previously west of Ireland and had been advected round the N coast of Scotland, but the second fog had lifted into stratus by the time the aircraft had reached it. The third fog was warmer than the sea and appeared to be in the development/advection stage characterized by much stronger winds and lower liquid water contents.Analysis and some modelling of the one‐dimensional heat and water budgets of sea fog suggest that after a period of development during transport over a relatively cold sea, initially ‘warm’ advection fog tends to become a diurnally modulated, self‐maintaining ‘cold’ radiation fog held at a temperature 1–2 degC below sea surface temperature. In other words, radiative cooling has depressed the temperature of the fog far enough below sea surface temperature for this cooling to become balanced by latent and sensible convective heat input from the sea surface.The aircraft observations demonstrated the convective control of the vertical structure of ‘cold’ sea fog, and the influence of diurnally forced wind systems (in this case, a heat low formed over the Scottish mainland) in moving

ISSN:0035-9009    

DOI:10.1002/qj.49711548709

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractThe transfer of scalar atmospheric constituents within vegetation canopies cannot be predicted from simple gradient‐diffusion theory. To replace gradient‐diffusion theory in this context, an analytic Lagrangian theory is developed which predicts the concentration field of a scalar atmospheric constituent emanating from a spatially extensive source in an inhomogeneous turbulent flow. In this ‘localized near‐field’ theory, the mean scalar concentrationCis expressed as the sum of a diffusive far‐field contributionCfwhich obeys gradient‐diffusion theory, and a non‐diffusive near‐field contributionCnwhich is calculated for each source element by assuming the turbulence to be locally homogeneous. It follows thatCfprovides the large‐scale background variation andCnthe detailed local structure of theCprofile. The theory applies to both non‐advective and advective situations. Comparison with random‐flight predictions ofCshows that the assumptions of the theory (which are exact in homogeneous turbulence) are adequate for inhomogeneous turbulence typical of real canopies. The random‐flight predictions also show that vertical velocity skewness (not accounted for in the localized near‐field theory but typically of order −0.5 to −1 in canopi

ISSN:0035-9009    

DOI:10.1002/qj.49711548710

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractA simple model of the influence of small‐scale topography on precipitation via the ‘seeder‐feeder’ mechanism is presented. In the model the large‐scale stratiform seeder cloud and the subcloud layer are modified by the airflow perturbations caused by small‐scale topography. Two separate scales of motion are considered; the large scale, with the updraught maintaining the seeder cloud, and the small scale tied to topography. In the diagnostic numerical model the interaction of these two scales is simulated. Two basic situations are considered: rain with a ground temperature of 12°C and snow with –5°C. Results of the model application to a bell‐shaped solitary hill are presented and compared with previous theoretical studies and field observations in the Welsh hills. The influence of orographic airflow perturbations on snow growth above the melting level may explain the observed strong dependence of rain enhancement on the horizontal wind speed. Simple considerations for the complex topography are also presented with reference to observations made near Uppsala, Sweden. It is suggested that the observed rain distribution may again result from influences on snow growth, with both the amplitude of airflow perturbations near the melting level and its phase relative to the topography beneath bei

ISSN:0035-9009    

DOI:10.1002/qj.49711548711

出版商:John Wiley&Sons, Ltd    

年代:1989

数据来源: WILEY