bluuebunny/crossref_metadata_2025 huggingface dataset:
Stratospheric transport by stationary planetary waves ‐ the importance of chemical processes
Authors: Pyle J. A. , Rogers C. F.
Published: February 2007

The transport of reactive trace gases by stationary planetary scale waves has been investigated with the aid of Matsuno's stationary planetary wave model (1970) and the linear eddy equation derived from the tracer continuity equation. It is shown that, for stationary fluxes, K‐theory fields, as introduced by Reed and German (1965), may be defined. These become matrix fields when photochemical coupling effects are considered. The eddy flux of a particular trace gas is related to the gradients of the mean concentrations of the other chemically coupled trace gases, as well as to the gradients of its own mean concentration. In the special case of a chemically inert tracer our treatment indicates Kyy = Kxx = 0 and Kyx = – Kxy in contradiction to the results obtained by Reed and German (1965) but in agreement with recent results of Clark and Rogers (1978) and Matsuno (1980). The consequences of photochemical coupling effects for eddy fluxes are investigated quantitatively for two simple chemical schemes involving the pairs O3 and NOx, and NO and NO2. These effects are shown to be significant especially for NO and NO2 whose photochemical time‐constants are short. Furthermore, even in regions of the atmosphere where coupling effects become small, such as the lower stratosphere for O3 and NOx, the uncoupled K‐fields are quite different for various species indicating a source of error in current 1‐ and 2‐dimensional photochemical models. The relative importance of dynamical and chemical contributions to the uncoupled K‐fields is dependent on the characteristic time‐scales for photochemical and zonal advection processes. The relation of our work to a Lagrangian description of the transport process is discussed.
Dynamical models of two‐dimensional downdraughts
Authors: Thorpe A. J. , Miller M. J. , Moncrieff M. W.
Published: February 2007

Observational and numerical model data have indicated the crucial role of the downdraught in the motion, structure and regeneration of severe storms. This paper considers downdraught dynamics in two dimensions, using both a numerical and an analytical model. In the numerical model the downdraught is produced and maintained by a constant heat sink. The adjustment of an ambient inflow, and the resultant downdraught and potential regeneration of storm cells, is simulated.

To elucidate physical principles, steady analytical models are also described in which a simple parametrization of downdraught thermodynamics is used. Two distinct types of individual draught dynamics, labelled symmetric and jump, have been identified allowing a qualitative description of six dynamically consistent updraughts and downdraughts. In particular, a cold symmetric downdraught and a cold jumptype updraught are analysed, the former having negative, the latter positive outflow shear. These analytical solutions agree well with steady states developed by the numerical model.

The numerical model is also used to describe the propagation of downstream and upstream gust fronts, and these results are in good agreement with atmospheric and tank data.
A laboratory model of an atmospheric mesofront
Authors: Simpson J. E. , Britter R. E.
Published: February 2007

Consideration is given to the leading edge of a well‐defined flow of dense air without significant condensation, such as occurs at a thunderstorm outflow or a sea‐breeze front. Explanation of the front is proposed in terms of a gravity (density) current formed by the denser air. Observations of such flows are related both to laboratory gravity current heads in which head, tail and calm wind conditions are simulated and to a simple, theoretical gravity current model based primarily on a momentum balance. Good agreement is found between the atmosphere, laboratory and analytical results.

The mean flow in the gravity current behind the head is greater than the velocity of the head, a result of mixing between the two fluids. This mixing nearly all occurs at the leading edge of the current. The excess height of the head of the gravity current is interpreted as the mixing region at the leading edge, and this mixed fluid lays down a stable layer above the following gravity current. In the laboratory the effect of a headwind is to flatten the profile of the raised head at the front and to reduce its rate of advance by about three‐fifths of the value of the opposing flow.
The influence of entrainment on the evolution of cloud droplet spectra: I. A model of inhomogeneous mixing
Authors: Baker M. B. , Corbin R. G. , Latham J.
Published: February 2007

In this, the first of two related papers, we present calculations of the growth of a population of condensate droplets rising above cloud base within small cumuli which are entraining undersaturated environmental air. It is assumed, on the basis of dimensional arguments and laboratory experiments on entrainment, conducted within a cloud droplet evolution tunnel, that this mixing process is inhomogeneous.

In the extreme situation to which the calculations apply undersaturated air is entrained in a stream, or in blobs, and some droplets of all sizes are completely removed from the condensate spectrum by evaporation, while others do not change in size. This is equivalent to assuming that the time constant for turbulent mixing (ττ) is large relative to that for droplet evaporation (τr), and is thus the antithesis to the homogeneous model utilized by other workers, which assumes implicitly that ττ/τr = 0.

The calculations based on the extreme inhomogeneous model produce spectral shapes which agree well with those reported in cumulus by Warner (1969a) and indicate that a small proportion of the droplets can grow several times faster through the condensate spectrum than classical theory predicts.
Destruction of ozone at the earth's surface
Authors: Galbally I. E. , Roy C. R.
Published: February 2007

Several hundred new measurements of the surface resistance to ozone uptake of grass, soil, sand, fresh water, sea water and snow are presented. Grass and soil show, in agreement with previous work, a daytime surface resistance of median value 100 sm−1. The first direct evidence is presented of a nocturnal increase in surface resistance for grass and soil, with night‐time values of surface resistance of about 300 sm−1. The surface resistances of sea water and snow are considerably larger, about 1000 to 2000 sm−1. A representative global ozone destruction rate for the earth's surface is derived using the above information and other published ozone data. Allowance is made for the latitudinal variation of the different types of surface, their various ozone destruction constants, the latitudinal and diurnal variation of ozone in the surface air and the diurnal variation of eddy transfer near the earth's surface. The global average ozone destruction rate is estimated to be in the range 2 to 6 × 1029 molecules s−1 (0.5 to 1.5 × 1012kg yr−1). This range extends to slightly higher values than previous estimates. The ratio of ozone destruction rate between the southern and northern hemispheres is about 2:3. The implications for the tropospheric ozone cycle are discussed.
Surface influence upon vertical profiles in the atmospheric near‐surface layer
Authors: Garratt J. R.
Published: February 2007

Observations from two towers situated in flat, tree‐covered terrain (z0 lying between 0.4 and 0.9 m) have been used to investigate the
flux‐profile relations in the height range z/z0 from 5 to 85, where z is the height above the zero‐plane displacement. The analysis confirms a lower height limit (at z = z*) to the validity of the Monin and Obukhov functions ϕM, H(z/L) in unstable conditions and, by implication, of the logarithmic wind law in neutral conditions. We find z*/z0 ≃ 35 and 150 for wind at the denser and less dense (lower z0) surfaces, whilst for temperature z*/z0 ≃ 100.

The level z* corresponds with the top of the transition layer, within which it is assumed the profiles depend additionally upon a length scale zs, related to surface wake generation. On the assumption that z* α zs, modification of the profiles in the transition layer is then described through a function ø(z/z*) whose explicit form is derived from length‐scale considerations in a region of wake‐shear interaction. The observed non‐dimensional profiles ϕ° are well represented by

ϕ° ≃ 0.5ϕ(z/L)exp(0.7z/z*)

Both for wind and temperature. For wind at both surfaces, the depth z* is approximately constant in unstable conditions and equal to 3δ, δ being the tree spacing. We tentatively conclude that δ is the relevant surface length scale zs characterizing the wake field and depth of penetration z*.
The influence of entrainment on the evolution of cloud droplet spectra: II. Field experiments at Great Dun Fell
Authors: Blyth A. M. , Choularton T. W. , Fullarton G. , Latham J. , Mill C. S. , Smith M. H. , Stromberg I. M.
Published: February 2007

Measurements have been made of the liquid water content and droplet‐size distributions within clouds enveloping the summit of Great Dun Fell, Cumbria. These covered extensive periods and a variety of meteorological situations.

It was found that in highly stable atmospheric conditions cap clouds contain narrow drop‐size distributions and near adiabatic liquid water content. In less stable conditions mixing with the cloud environment was found to have a substantial effect on the microphysical properties of the cloud.

In the cases studied the admixture of undersaturated air was found to produce a substantially subadiabatic liquid water content and the spectral shape experienced broadening in a way more consistent with the extreme inhomogeneous model of mixing described in Paper I* than with the classical treatment. In particular, it was found that a small number of large drops experience a greatly enhanced growth rate.

In a particular study, it was observed that the interaction of the cap cloud with a pre‐existing strato‐cumulus deck produced a broad drop‐size distribution and an enhanced liquid water content.
An analysis of the large‐scale features of the upper troposphere and the stratosphere in a global, three‐dimensional, general circulation model
Authors: O'neill A. , Newson R. L. , Murgatroyd R. J.
Published: February 2007

The formulation of a 13–Ievel general circulation model of the troposphere and stratosphere is described and an account given of the climatology produced from an annual integration. Emphasis is placed on the interhemispheric differences in the stratosphere and the seasonal variability exhibited by its circulation. Overall, a realistic simulation of various features of the large‐scale tropospheric and stratospheric flow patterns was achieved, although a number of shortcomings are noted.

The atmospheric state of the model is resolved into zonal mean and eddy components by Fourier analysis around latitude circles, while the latter are further divided into time‐averaged and transient modes. The structure of these aspects of the flow is described and the influence of the eddies on the mean flow in the stratosphere is discussed from a dynamical point of view by considering momentum and heat budgets. An attempt is made to identify the various factors governing the seasonal cycle of zonal mean wind and temperature in the stratosphere, and it is suggested that wave transience initiated in the troposphere may play a role in this respect, in addition to the evolving field of net radiative heating.

The apparent success of the model in spontaneously generating events which resemble stratospheric sudden warmings is also reported.
A field study of nocturnal stratocumulus: I. Mean structure and budgets
Authors: Roach W. T. , Brown R. , Caughey S. J. , Crease B. A. , Slingo A.
Published: February 2007

Observations of anticyclonic stratocumulus obtained with ground‐based and balloon‐borne equipment during the night of 19/20 November 1976 at Cardington, Bedford, UK, are described in relation to the synoptic situation.

The mean structure of the atmosphere from ground to about 300m above cloud top is discussed. Of particular interest are the large temperature and humidity ‘jumps’ observed within a layer 1‐10m thick at cloud top, and the thermally stable surface layer about 200m thick in the presence of an overcast sky and surface wind speed of 3–4 ms−1. The mean liquid water content profile is accounted for in terms of turbulence theory, but the cause of observed large variations in total cloud water content is not identified.

The locally steady conditions allowed estimates to be made of the local budgets of heat and water for the boundary layer (cloud top to ground) as a whole. In particular, the shape of the temperature inversion above cloud top allowed a quantitative estimate of cloud top entrainment rate to be made.

Although the local water budget appears to be in approximate balance, only about one‐third of the radiative loss from cloud top is accounted for locally. It is not possible to account for this deficit in terms of the observations; it is suggested it could result in a downstream mean cooling of the boundary layer of about 0.5K per 100km.

The observed dispersal of cloud downstream cannot be explained by the local budget deficit (which would tend to increase cloud). It is estimated that a local subsidence rate of 2–4cms−1 through the cloud would be required to disperse it on the observed time scale. Potential sources of evidence for this are discussed.
Lagrangian Monte Carlo simulation of the turbulent motion of a pair of particles
Authors: Sawford B. L.
Published: February 2007

A new scheme for simulating the turbulent motion of a pair of particles is presented. Realistic and self‐consistent inter‐particle correlations are introduced at a fundamental level using a simple parametrization of the two‐particle Lagrangian velocity correlation function. For the simple case of stationary homogeneous turbulence in one‐dimension, the simulation procedure is in excellent agreement with exact theoretical results. Extension of the method to include more general circumstances is straightforward.
An economical method for computing the radiative energy transfer in circulation models
Authors: Hense A. , Raschke E. , Kerschgens M.
Published: February 2007

Based on the methodology of a two‐stream approximation (Kerschgenset al.1978) the transfer of radiative energy in model atmospheres can now be calculated with high economy and sufficient accuracy. In this model the number of spectral intervals has been minimized to four (solar spectrum) and six (infrared spectrum) for which new effective transmission functions have been computed, where standard aerosol profiles and absorption and scattering coefficients are incorporated. The concentrations of major atmospheric gases (water vapour, carbon dioxide, ozone), the cloud cover and cloud liquid water content of each layer can be changed freely. Thus this model could be used in any numerical circulation model.

In the first part of this paper we discuss the basic principles of the method and results obtained for cloudless model atmospheres. The accuracy of computed flux densities is better than 5% and of flux divergences better than 20% in the worst cases.

In the second part this method will be applied to total and partial cloud cover in each layer.
The addition of heat to a stratified airstream with application to the dynamics of orographic rain
Authors: Smith Ronald B. , Lin Yuh‐Lang
Published: February 2007

The response of a stratified airstream to combined thermal and orographic forcing is investigated theoretically using the linearized hydrostatic equations of motion. The magnitude of the heating aloft is computed from observed rainfall rates. The elevated heating is shown to produce vertically propagating waves whose amplitude, relative to the mountain waves, is determined by the parameter gQb/apTU‐2Nh (symbols Appendix I). For typical wind speeds and rainfall rates the thermally generated waves will equal or exceed the oro‐graphically generated waves. These waves produce large vertical variations in the wave momentum flux at the levels where heat is being added and alter, or even reverse, the mountain drag. In hydrostatic flow, the phase relationship between the heating rate and the induced vertical displacements is found, unexpectedly, to be negative. In the vicinity of the heating, air parcels are displaced downwards. This result can be explained by wave propagation ideas but not by parcel arguments. This helps to explain why mountain wave amplitudes are sometimes reduced in moist atmospheres. The broad heat‐induced descent may act to limit the amount of condensation‐precipitation occurring in a stable middle‐level orographic cloud. Other types of orographic precipitation involving embedded convection in stratus layers, low level feeder clouds, or deep cumulus convection may not be adversely effected by the neat‐induced descent.
The forcing of stationary wave motion by tropical diabatic heating
Authors: Simmons A. J.
Published: February 2007

The forcing of stationary wave motion by tropical diabatic heating is examined using idealized models. A steady, linear, primitive equation model is used to study both the extratropical and the tropical response to tropical forcing. In the former case an isolated region of heating may excite a wavetrain with a substantial poleward direction of propagation. The sensitivity of this response is discussed. The tropical response is largely in agreement with a simpler model proposed by Gill, but some significant modifications due to the presence of a zonal‐mean flow and more detailed vertical structure are found. Specifying the tropical forcing by means of a relaxation towards climatology results in a large extratropical response.

Barotropic experiments are discussed in which an isolated tropical forcing is superimposed on a constant forcing which maintains a climatological basic flow with zonal variation. In these cases a large response is found in the North Pacific region, particularly when the forcing is located to the south of the region of strongest flow. Non‐linearity amplifies or damps this response depending on the sign of the forcing. Maintenance of a region of tropical westerly wind of limited zonal extent results in a substantial cross‐equatorial propagation.
Extremal principles for global climate models
Authors: Mobbs S. D.
Published: February 2007

Paltridge has recently hypothesized that the mean global climate operates near a state of maximum entropy production. In this paper an attempt is made to justify this hypothesis by parametrizing the eddy heat fluxes in terms of eddy diffusivities and using the local potential method of Glansdorff and Prigogine. With eddy diffusivities independent of the temperature and its gradient a principle of minimum entropy production is in fact found. However, by using a more realistic parametrization, a quantity closely related to entropy production is found to be maximized. The relevance of this to global climate models is discussed.
An experiment on the initial conditions for a mesoscale forecast
Authors: Carpenter K. M. , Lowther L. R.
Published: February 2007

A 6–h forecast starting at midday on a good sea breeze day has been repeated for seven different sets of initial conditions. The initial values for each field could either be given by an 8–h mesoscale forecast valid at 1200GMT or interpolated from a synoptic scale forecast valid at the same time. It was found that changing the initial temperature field had an important effect on the subsequent forecast, but the impact of changing the initial wind field was relatively small. Changing the initial surface pressures had no visible effect on any aspect of the subsequent forecast other than the forecast of surface pressure itself.
The analysis of umkehr observations of stratospheric ozone by a ‘Maximum Entropy’ method
Authors: Read P. L.
Published: February 2007

An application is described of the ‘Maximum Entropy’ (ME) principle to the analysis of umkehr observations of atmospheric ozone. The method is shown to produce results which are comparable with more conventional analyses, and which show a marginally higher correlation with simultaneous ozonesonde measurements than the standard evaluation method. Differences between conventional and ME solutions are attributable to the low intrinsic information content of umkehr observations, and the use of climatological information in addition to observational data in the standard method of analysis. This additional information may also be included in ME solutions, reducing the systematic errors of the simple ME solutions, but possibly degrading that part of the information content of the solution which is due to the observations. Some further meteorological applications of the method are discussed.
Diurnal variation of pressure‐heights in the Eastern Atlantic (GATE)
Authors: Riehl H. , Haurwitz B.
Published: February 2007

At first, the daily variation of pressure‐heights in the troposphere and low stratosphere is determined with observations from GATE. Resulting values are compared with those obtained in the Caribbean in the 1940's. One finds the semi‐diurnal cycle well maintained up to 500mb in both samples, as well as the transition to a single asymmetric wave in the high troposphere and low stratosphere. The amplitude is much reduced in GATE, suggesting that the temperature corrections applied in the Caribbean were too small. Nevertheless, the range of 60m at 100mb still corresponds to the mean latitudinal pressure‐height difference over 20° latitude across the GATE area in the Northern Hemisphere summer.

The 24‐ and 12‐hourly oscillations of the standard pressure surfaces have been determined by harmonic analysis up to 100mb, the highest level for which pressure‐height data were available. The results are given in Table 3 and they show more precisely than Fig. 1 the vertical distribution of the first and second harmonics and the transition from a daily pressure variation with two maxima and minima to one where only one predominant maximum and one predominant minimum appears each day.
Planetary wave modelling of the Middle Atmosphere: The importance of travelling wave components
Authors: Austin John
Published: February 2007

A linerized stationary planetary wave model is applied to mean data for a chosen quiescent winter month (December 1979). Comparison with observations from the Stratosperic Sounding Unit on board NOAA‐6 shows that both the amplitude and height of propagation of wavenumber one are significantly underpredicted. This behaviour is not substantially modified by extension to more sophisticated model formulation and representation of dissipative processes. It is shown that the response to forcing varies appreciably with travelling wave frequency, even at periods greater than one month and it is suggested that this sensitivity is a significant limitation on the applicability of stationary wave models based only on time mean behaviour.
A two‐dimensional numerical study of horizontal roll vortices in an inversion capped planetary boundary layer
Authors: Mason P. J. , Sykes R. I.
Published: February 2007

The dynamics of large‐scale horizontal roll vortices in an inversion‐capped planetary boundary layer subject to surface heating are investigated by means of a numerical model. The rolls are assumed to be twodimensional and calculated explicity, while small scale turbulence is parametrized using a buoyancy‐dependent mixing length hypothesis. The assumptions of the model are based upon atmospheric observations which show the occurrence of large eddies highly elongated in a direction close to that of the geosstrophic wind and a partitioning of turbulence energy between these large eddies and smaller scales. The model produces realistic turbulence statistics and provides information on the sensitivity to various assumptions and parameters. Apart from the generation of internal gravity waves in the stable region above the boundary layer the results are insensitive to the details of the parametization. The internal gravity wave generation is sensitive to most parameterss and in particular to the roll orientation.
A diffusive model of the turbulent mixing of dry and cloudy air
Authors: Baker M. B. , Latham J.
Published: February 2007

A model is presented of the turbulent mixing between a shere of droplet‐free air (saturated or under‐saturated) and a spherical volume of cloud within which it is embedded. The rates of equalization of super‐saturation, temperature and liquid water are examined as functions of the turbulent energy dissipation rate, the dimensions of the interacting volumes, the drop‐size distribution in the cloud and other parameters. Spectral changes resulting from the mixing conform closely with those predicted by the recent inhomogeneous or dilution‐only descriptions of mixing (Baker et al. 1980; Telford and Chai 1980).

It is shown that spectral changes observed during entainment into shallow cumulus, stratocumulus and cap clouds are good agreement with those predicted on the basis of this diffusive model.

Calculations of mixing in the presence of an updraught yield size‐distributions (including bimodal spectra) similar to those oberved in natural clouds.

Calculations of mixing during descent from cloud‐top suggest that regions of substantial under‐saturation may exist in the deep interiors of clouds which are entraining environmental air at their summits. Crude estimates of the degree of lateral spreading produced by turbulence as this descent proceeds indicates that significant horizontal structure in the water properties may exist throughout a substantial fraction of the cloud‐depth.
On the droplet distribution near the base of cumulus clouds
Authors: Manton M. J. , Warner J.
Published: February 2007

Lateral entrainment in cumulus clouds is usually assumed to occur continuously as a cloud parcel rises. Computations of the droplet distribution using that assumption are compared with those in which entrainment occurs intermittently, and little difference is found. The width of the droplet distribution is shown to increase as the relative humidity of the entrained air is reduced. The inclusion of the effects of molecular accommodation at the surface of droplets also leads to substantial broadening of the droplet distribution.
Kinematic and thermal structures of two surges of flow in the Northern Mozambique Channel Area
Authors: Rao Gandikota V. , Haney James L.
Published: February 2007

Two atmospheric surges near the northern Mozambique Channel were explored by NCAR's Electra aircraft on 17 June 1977 and 3 July 1977 during MONSOON–77 field experiment. The surges were probed from Nairobi eastwards to the coast and then south from 1°S to 8°S along 41°E. Offshore the former day was characterized by deep south‐south‐easterlies and south‐easterlies, squalls, and rain and the latter day by shallow south‐south‐easterlies overlain by south‐westerlies and fair weather cumulus.

Vertical structures of u, v, T and q for both days are discussed. Early on the disturbed day southerlies reached their maximum over land at 880 mb on a cross‐section approximately along 2°S. By noon diurnal heating reduced the overland winds, resulting in an apparent ocean‐ward shift of the jet axis. On the undisturbed day the southerly maximum was over the ocean near the coast.

On both days two southerly maxima were found on longitudinal coross‐sections along 41°E: an elevated and stronger maximum near the equator; a depressed and weaker one near 8°S. On the disturbed day the equatorial maximum was located in the south‐south‐easterlies and on the undisturbed day was sandwiched between south‐south‐easterlies in the low levels and south‐south‐westerlies at 770 mb. The southern maximum was from south‐south‐east on both days.

During these two periods meridional mass fluxes from two stations, Port Elizabeth and Dar‐es‐Salaam, were computed. They showed that the latter station lags the former by as much as four days; however, its meridional mass flux is double the former.
The seasonal response of a general circulation model to changes in CO2 and sea temperatures
Authors: Mitchell John F. B.
Published: February 2007

The seasonal response of an atmospheric general circulation model to changes in atmospheric carbon dioxide concentrations and sea surface temperatures is discussed. The model has five layers and a quasi‐uniform 330km horizontal grid. Sea surface temperatures, sea ice extents, and zonally mean cloud amounts are prescribed from climatology, so that feedbacks between these variables and the rest of the model are ignored. Soil moisture, snow depth and boundary layer height are modelled explicitly, and both diurnal and seasonal variations of solar zenith angle are included. Two experiments are carried out, and compared with a three‐year control integration. In each case, the model's response varies with season and location. In the first experiment the effect of increasing atmospheric carbon dioxide concentrations with prescribed present day sea surface temperatures is examined. The model's troposphere becomes warmer, thereby increasing the low level static stability over the ocean and reducing evaporation and precipitation. The warming is larger over land than over the oceans. In summer, this results in an increase in precipitation along the eastern coasts of continents. In the second experiment, the sea surface temperatures are increased by 2 K and the carbon dioxide concentration is doubled. The land surface temperature rises by 3 K. Evaporation increases markedly over the oceans. Precipitation increases in the main regions of atmospheric convergence and decreases in some regions of the subtropics. The magnitude of the model's response is shown to be reasonably consistent with that found in other three‐dimensional climate models.
Variations of zonal mean sea surface temperature and large‐scale air‐sea interaction
Authors: Chiu Long S. , Newell Reginald E.
Published: February 2007

Twenty‐four years data of zonal mean sea surface temperature (SST) were assembled to examine the seasonal and nonseasonal variations of SST. Outside the tropics, annual and semi‐annual variations of SST dominate, accounting for over 90% of the variance. In the tropics, the nonseasonal variation of SST is dominated by low‐frequency variations with periods longer than 3 years in the Pacific. Variations with periods of 24–32 months account for more than 10% of the variance in the tropical Atlantic from 25 £N to 10 £S and in the equatorial Pacific from 0–10 £S. An empirical orthogonal function (EOF) analysis showed that the major global pattern of zonal mean SST variations is dominated by variations in the equatorial eastern Pacific which precede zonal mean SST variations elsewhere. The time series of this EOF is found to be correlated with an index of the Southern Oscillation. The maximum correlation coefficient is −0£61, with the index leading by 2 months. Finally, the relation between SST and tropical tropospheric temperature (TTT) is examined. SST variations in the Atlantic and in the Indian Ocean are almost contemporary with variations in TTT. Possible explanations for the 1–2 seasons lead time between SST and TTT as found in earlier studies are suggested.
Simulation of storm surges using a three‐dimensional numerical model: An application to the 1977 Andhra cyclone
Authors: Johns B. , Sinha P. C. , Dube S. K. , Mohanty U. C. , Rao A. D.
Published: February 2007

A three‐dimensional numerical model is developed for the simulation of storm surges generated by tropical cyclones off the east coast of India. Experiments are performed using wind‐stress forcing data representative of the 1977 Andhra cyclone and the results are compared with earlier simulations using a depth‐averaged model. The three‐dimensional model incorporates a turbulence energy closure scheme and is fairly sophisticated in comparison with the depth‐averaged approach. Even with the apparently more refined treatment of the frictional processes, there is no substantial difference between simulations performed with the two models.
An accurate variation of the two‐step lax‐wendroff integration of horizontal advection
Authors: Collins William G.
Published: February 2007

The split technique is one method used to integrate the primitive equations of motion efficiently. The horizontal advection is commonly integrated by the two‐step Lax‐Wendroff scheme. Both Gadd and Carpenter have discussed the design of operators to give either accurate amplitude or phase speed. This paper extends their analysis to yield a method which has both accurate amplitude and phase speed.
Mountain torque and angular momentum in barotropic planetary flows: Equilibrium solutions
Authors: Sawford B. L. , Frederiksen J. S.
Published: February 2007

The climatic‐mean‐value of the total zonal relative angular momentum of the atmosphere in topographically forced inviscid barotropic models on a sphere is examined using the methods of equilibrium statistical mechanics. Models incorporating wave–wave interactions as well as simplified systems in which only wave–zonal‐flow interactions are allowed are considered. It is shown analytically that for a given truncation the equilibrium state of the system does not depend on the presence of wave–wave interactions although the detailed evolution towards it does.

We interpret our results in terms of a generalization of the properties of the equilibrium solutions for unforced barotropic flow. For realistic resolution (with truncation wavenumber ° 15) most flows evolve towards an equilibrium with westward angular momentum, and the solutions are strongly dependent on the initial conditions. An important exception is the case when the flow initially has westward angular momentum, when the equilibrium state remains close to the initial state regardless of resolution. An upper bound on the climatic‐mean‐value of the solid‐body rotation component of the flow is derived. In general, highly truncated systems are insensitive to the initial conditions and evolve towards a state in which the angular momentum is eastward and close to the upper bound.

Reasonable rescating of the global topography used has little influence on the climatic‐mean angular momentum, but the case of no topography represents a singular limit in which angular momentum is conserved.
Dual channel satellite measurements of sea surface temperature
Authors: Barton I. J.
Published: February 2007

Results from the 11.6γm channel on the Nimbus 5 selective chopper radiometer are presented that show ambiguity in the temperature deficit obtained for any tropical sea surface temperature (s.s.t.). The analysis indicates that to gain an accurate measurement of s.s.t. in the 10–13γm band further information on the water vapour content of the overlying atmosphere is required. A transmission model is used to show that for two channels a linear relation exists between the two apparent surface temperatures and simple algorithms are derived that give s.s.t.s with an accuracy better than 0.3K for vertically‐viewing satellite radiometers. Algorithms giving s.s.t. are derived for both dual wavelength and dual angle‐of‐view techniques and these are compared with those derived by other authors.
Observed inter‐relations between 10m winds, ocean whitecaps and marine aerosols
Authors: Monahan Edward C. , Fairall Christopher W. , Davidson Kenneth L. , Boyle Patricia Jones
Published: February 2007

Low elevation aerosol spectra, ocean whitecap cover and 10m wind speeds measured during the 1978 JASIN experiment have been inter‐related and compared with previously published observations. The positive dependence of aerosol concentration upon whitecap cover was found to increase with droplet radius reflecting the expected higher correlation of the concentration of larger droplets, which have shorter effective residence time in the marine atmospheric boundary layer, with the immediate whitecap cover, which reflects the instantaneous rate of aerosol generation at the sea surface. The power‐law wind dependence, Uγ of the low elevation concentration of droplets larger than 8γm radius was determined to be similar to the wind dependence of whitecap cover, with γ values of 3.23 and 3.31 resulting from the respective application of the robust bi‐weight fitting technique. This observation is consonant with an aerosol generation model in which the instantaneous rate of production is simply proportional to the immediate whitecap cover.

The large droplet end of the JASIN low elevation aerosol spectrum is seen to undergo a marked enhancement when the wind speed exceeds 10ms−1. This is a consequence of the onset at that speed of supplementary droplet production via the mechanical disruption of wave crests. The observed growth, with increasing wind speed, in the disparity in amplitude of near‐sea‐surface and near‐cloud‐base aerosol spectra is in part a consequence of the fact that the larger droplets, produced in relative abundance at the higher wind speeds, fall out before they can be mixed effectively through the boundary layer.
A wave prediction system for real‐time sea state forecasting
Authors: Golding B.
Published: February 2007

The considerable increase in requirements for sea state forecasts in recent years has led to development of a numerical wave forecasting system in the Meteorological Office. This is based on a wave prediction model which combines the advantages of the parametric technique in predicting a growing wind‐sea with those of a discrete spectral model in the swell regime. This is done using a discrete model by parametrizing the nonlinear interactions term in the energy balance equation in a way that reproduces the behaviour of the parametric model. The main difficulty with the method is in the separation of wind‐sea and swell required to do this. Propagation of wave energy is performed using an accurate form of the Lax‐Wendroff integration scheme. A two‐term representation of wave growth is used whilst dissipation is modelled by an explicit whitecapping mechanism. Shallow water effects are included by representations of shoaling, refraction and bottom friction. The operational numerical atmospheric model at the Meteorological Office provides the wind input. An extensive program of evaluation has shown that the results provide high quality guidance with 24‐hour forecasts of wave height having a r.m.s. error ranging from 0·6 m in the southern North Sea to 1·0 m east of the Shetlands.
Application of extreme value analysis to Weibull data
Authors: Carter D. J. T. , Challenor P. G.
Published: February 2007

In the application of extreme value analysis it is usually assumed that the size of the samples from which the extreme values are obtained is sufficiently large for the asymptotic extreme value distribution to be used. The necessary sample size depends upon the population distribution and this is generally not known; but assuming a Weibull distribution, which is often fitted to wind speed and wave height data, it is shown that the rate of convergence is rapid and that the asymptotic distribution may be used for a sample size as small as ten. An ‘exponential approximation’ for the distribution of maxima is sometimes confused with the extreme value distribution. This approximate form is derived for the Weibull distribution and the essential difference between it and the asymptotic extreme value distribution is explained.
Upper stratospheric circulations in the two hemispheres observed by satellites
Authors: Hirota I. , Hirooka T. , Shiotani M.
Published: February 2007

A global analysis is made of upper stratospheric circulations with the aid of Nimbus 5 and Tiros N satellite observations in order to make a comparison of characteristics between the northern hemisphere (NH) and the southern hemisphere (SH).

It is shown that the reversal of north‐south gradient of the zonal mean temperature at the stratopause level is always observed in higher latitudes of the SH winter, while the temperature gradient of the NH winter is variable because of the occurrence of sudden warmings. Consequently, due to the thermal wind relation, the polar night westerlies of the SH stratosphere are generally stronger than those of the NH stratosphere. In association with this difference, the activity of transient planetary waves of wavenumber 2 is striking in the SH, in contrast to the predominance of steady waves in the NH.

Finally, the interrelation between the zonal mean field and planetary waves in the upper stratosphere is briefly discussed.
Reduction of systematic forecast errors in the ECMWF model through the introduction of an envelope orography
Authors: Wallace John M. , Tibaldi Stefano , Simmons Adrian J.
Published: February 2007

Day 1 forecast errors in the ECMWF model geopotential height fields during wintertime show a distinctive and highly reproducible signature with negative biases over the major mountain barriers. These biases are largest on days when the 500mb flow over the mountains is strong and they tend to shift position from day to day so as to remain close to the region where the jet stream crosses the mountain barrier. These systematic errors evolve through the forecast interval until by day 10 they assume an equivalent barotropic structure which strongly resembles the upper‐level stationary wave pattern, but has the opposite sign, which indicates that the model does not have sufficient (presumably orographic) forcing to maintain the stationary waves.

The time evolution of the systematic error pattern is investigated by means of a series of experiments with a barotropic model, in which the observed mean wintertime 300 mb flow is perturbed by a steady forcing derived from the day 1 forecast error pattern. The day 10 error patterns, as simulated by the barotropic model, bear a strong similarity to the observed ones. The forcing in the vicinity of the northern Rockies makes a particularly large contribution to the simulated day 10 error pattern whereas that in the Himalaya region appears to be relatively less important.

The impact of an enhanced orography upon the climate of the ECMWF model is investigated by carrying out a pair of extended integrations out to 50 days. The control run is based on the conventional average‐type orography used for operational forecasting; the other run is based on an ‘envelope orography’ constructed by adding to the conventional, grid‐square averaged, orography an increment proportional to the standard deviation of the sub‐grid scale variance. Results for this one, rather short pair of integrations suggest that the envelope orography may be capable of yielding a more realistic simulation of the observed wintertime flow pattern, particularly with respect to features in the Pacific and western North American sectors. Certain aspects of the zonally averaged circulation are also more realistic in this envelope simulation.

A series of 21 successive ten‐day forecast integrations has also been carried out with this envelope orography from February 1982 data. Results indicate that the introduction of the envelope orography results in a slight degradation of the short‐range forecasts together with a distinct improvement of the forecast beyond four days. The beneficial impact towards the end of the forecast interval appears to be large enough to increase the forecast usefulness by perhaps as much as half a day.
Planetary wave modelling of the middle atmosphere: The importance of transients
Authors: Austin John
Published: February 2007

A linear planetary wave model of the middle atmosphere is used to diagnose the wavenumber‐1 structure for a day during December 1979 given the wave forcing and the time‐derivative terms. The time derivatives are calculated using data from the stratospheric sounding unit (SSU) on NOAA 6 for days on either side of the principal day of interest. The effect of the time‐derivative terms is significant and the model is found to be in fair agreement with the data in high latitudes although low latitude features are poorly reproduced. The influence of the transient wave components is discussed and in particular some dissipation‐like effects are demonstrated which might clarify the role of Rayleigh friction in planetary wave models.
Statistical mechanics of a finite difference approximation to the barotropic vorticity equation
Authors: Bennett A. F. , Middleton J. F.
Published: February 2007

A conventional finite‐difference representation of the barotropic vorticity equation is shown to possess a statistical mechanical equilibrium state, which is attained as a quasi‐equilibrium in the presence of weak dissipation. The state is isotropic only in suitably chosen wavenumber coordinates. The autocorrelation functions of the discrete complex Fourier amplitudes have integral time scales which are O(wave number)−1 at middle wavenumber, but which are infinite at the highest wavenumbers. This is demonstrated experimentally and confirmed by inspection of the interaction coefficients for the finite‐difference Jacobian. The results emphasize the dependence of predictability upon the choice of numerical method.
Aircraft measurements of mountain waves and their associated momentum flux over the British Isles
Authors: Brown Philip R. A.
Published: February 2007

The Hercules and Canberra aircraft of the Meteorological Research Flight have made a number of flights to investigate mountain waves over different mountainous areas of the British Isles. Results from five flight days are presented in this study. For each flight day the observed values of the dominant wavelength and the wave amplitude as a function of height were compared with the values predicted by two‐dimensional linear lee wave theory. Good agreement was obtained in four of the five cases. Measurements of the vertical flux of horizontal momentum show that the largest values were obtained when the wavelength was 20·9 km and the waves were only weakly trapped. On this occasion, the flux was observed to be constant with height in the range over which measurements were made, supporting a theoretical result of Eliassen and Palm. The value –0·35Nm−2 is comparable with some aircraft measurements made over the Rocky Mountains.
Ice nucleus measurement with a continuous flow chamber
Authors: Hussain K. , Saunders C. P. R.
Published: February 2007

A continuous flow vapour diffusion chamber has been designed and constructed so that ice nuclei can be activated over ranges of temperature and supersaturation similar to those found in natural clouds. A conventional filter‐processing chamber was also built for carrying out comparative measurements. Simultaneous use of the two chambers under similar conditions of temperature and supersaturation has shown that the filter method leads to an underestimate of the concentration of ice nuclei in an air sample. The ratio of the two simultaneous measurements is strongly dependent on the concentration of Aitken nuclei in the air sample and is also dependent on the chamber supersaturation. Some tentative evidence of the presence of condensation‐freezing nuclei active above water saturation is presented.
A study of mesoscale convective bands behind cold fronts. Part I: Mesoscale organization
Authors: Bennetts D. A. , Ryder P.
Published: February 2007

This paper reports observations made over the North Atlantic of a system of convective cloud bands found within the cold air behind an occluded front. The observations were made from an instrumented aircraft which additionally released a pattern of dropsondes, measuring temperature, relative humidity and wind, as functions of pressure. These data are analysed to establish some of the mesoscale, cloud‐scale and microphysical properties of the system.

Here, in part I, the mesoscale structure is described and compared with the predictions of various theoretical models. The convective cloud structure emerges as a consequence of the mesoscale organization but appears to play only a minor role in its creation and maintenance. One of the theoretical models considered, conditional symmetric instability, provides a piausible explanation of the mesoscale observations.