bluuebunny/crossref_metadata_2025 huggingface dataset:
Tethered‐balloon observations of the structure of small cumulus clouds
Authors: Kitchen M. , Caughey S. J.
Published: July 2010

The turbulence and microphysical structures of small cumulus clouds have been studied in some detail using three turbulence probes and a droplet spectrometer attached to the tethering cable of a large (1300 m3) balloon. the airflow in and around clouds showed some evidence for a ‘P‐shaped’ circulation in the vertical plane and rotation in the horizontal plane. Power spectra of the vertical component of air motion both in and out of cloud and in the mixed layer below cloud base are described and contrasted. Mean turbulence characteristics (i.e. averaged over a few hours) strongly resemble those from the upper regions of the clear convective boundary layer but in the vicinity of clouds the turbulence field is considerably altered. Observations of the cloud droplet field near cloud top and the rate of dissipation of turbulent kinetic energy provide evidence that the entrainment and mixing of dry air proceed essentially inhomogeneously.
On the propagation of cold fronts
Authors: Manton M. J.
Published: July 2010

The assumption of steady two‐dimensional flow imposes constraints on the motion of a layer of cold air propagating through a body of warm air. Some general properties, such as the propagation speed of the front, can be deduced from consideration of the equations of motion away from the frontal region. Thus the differences between anafronts and katafronts are discussed.
Winter depression tracks and climatological jet streams in the Southern Hemisphere during the FGGE year
Authors: Physick W. L.
Published: July 2010

A band‐pass filter transparent to fluctuations in the 2£5‐6 day period range is applied to a set of twice‐daily analyses covering the 1979 (FGGE year) winter of the Southern Hemisphere. Fields analysed include MSL pressure, 500mb height, 300mb wind, and the meridional heat flux due to transient eddies at 850mb. Variance maxima in these filtered fields indicate regions of high cyclonic activity, with favoured cyclolysis areas in the vicinity of the Antarctic coastline also showing up in the MSL pressure statistics. Using these results, supplemented by data on the distribution of cyclogenesis, we are able to identify the major depression tracks. In this analysis, extensive use is made of studies relating satellite‐observed cloud vortices to the various stages of cyclonic development. In general, it appears there was significantly more activity in the eastern hemisphere than in the western during the 1979 winter.

By evaluating the terms in the time‐averaged zonal momentum equation (using the unfiltered data set), it is possible to assess the role of transient eddies in the maintenance of the time‐mean zonal flow. This analysis shows that Coriolis acceleration associated with poleward time‐mean ageostrophic flow at the tropopause level dominates the forcing of the jet centred around 25°S, but that convergence of eddy momentum flux maintains the higher latitude jet around 50°S. However, a significant contribution to the latter jet comes from poleward ageostrophic flow south of Africa.
Numerical simulation of the surge generated by the 1977 Andhra cyclone
Authors: Johns B. , Dube S. K. , Mohanty U. C. , Sinha P. C.
Published: July 2010

Three different numerical models are used to simulate the surge generated by the 1977 Andhra cyclone. In each of these we simulate the effect of three days of wind‐stress forcing before landfall of the cyclone at the Andhra Pradesh coast. the analysis area in the first model includes the entire Bay of Bengal north of 6°N and utilizes a curvilinear boundary treatment to represent the coastlines. the second model is a coastal zone model extending along the east coast of India, again utilizing a curvilinear treatment of the coastline. In the third model, which again covers the entire Bay of Bengal, the coastlines are represented by a conventional stepwise procedure. Using available data on the Andhra cyclone as a guide in representing the effect of wind‐stress forcing in our models, we compare the predicted rise in the sea‐surface elevation obtained from each of these with estimates based on reports received from Andhra Pradesh after the disastrous flooding in that region. Each of the three models produces a qualitatively similar surge response. the slight quantitative differences are explained in terms of the different boundary treatments. the predicted peak surface elevation above mean sea‐level compares well with the estimated 5 m reported along part of the Andhra Pradesh coast south of Masulipatnam.
A comparison of laboratory measurements and numerical simulations of baroclinic wave flows in a rotating cylindrical annulus
Authors: Hignett By P. , White A. A. , Carter R. D. , Jackson W. D. N. , Small R. M.
Published: July 2010

Quantitative and qualitative comparisons are made between laboratory measurements of rotating annulus flows and corresponding numerical model simulations. Two laboratory annuli, of similar dimensions but differing in instrumentation, are used. One contains a thermocouple array for temperature measurement: the other contains no sensor array but the working fluid is seeded with minute neutrally buoyant beads (600 m̈m diameter) which enable the horizontal velocity field to be measured. Each annulus has a rigid insulating lid in contact with the working fluid. the numerical model is a finite difference formulation based on the Navier‐Stokes equations for baroclinic flow of a Boussinesq liquid. Although the atmosphere and the laboratory annulus are both rotating baroclinic fluid systems, the forcing processes acting in the annulus are much simpler than those acting in the atmosphere, and may be accurately represented by established formulae: under a wide range of conditions no parametrizations of subgrid‐scale dynamical and diabatic processes are required. Comparison of numerical model results with laboratory measurements therefore enables the explicit dynamical formulation of numerical models of rotating, baroclinic flow to be verified to an extent which would be very difficult, if not impossible, to achieve using atmospheric data. Detailed quantitative comparisons for a steady wave flow reveal good agreement for major features of the temperature and horizontal flow fields, although a significant discrepancy in total heat flux is found. Qualitative comparisons are made by investigating the ability of the numerical model to reproduce the main flow types and phenomena of the laboratory system. Numerical simulations of intransitivity, hysteresis, wavenumber transitions, amplitude vacillation and a weak structural vacillation are described. Several suggestions for further comparative studies are made in conclusion.
A numerical and analytical study of atmospheric undular bores
Authors: Crook Norman A. , Miller Martin J.
Published: July 2010

In this paper the formation and structure of an atmospheric undular bore is examined with the aid of a numerical model and theoretical analysis. the numerical model is first used to simulate a density current propagating into an unstratified environment. A detailed comparison of the model results with existing theory shows that for a given pressure head the current can travel at a greater speed than that predicted by irrotational theory.

When the density current is allowed to propagate into a low‐level stable layer it is found that for certain parameter regimes an undular bore forms ahead of the current. the results for two different types of stable layers are examined. For a layer with a step profile of potential temperature, comparison is made with both classical bore theory and the theory of density currents in two‐layer fluids and encouraging agreement is found. When a linear profile is specified in the stable layer it is found that energy‐conserving solutions can be obtained. the numerical fields are then compared with measurements of the morning glory, an undular bore observed in north‐eastern Australia, and again agreement is good.

Finally, preliminary results of model simulations with stratification above the stable layer are discussed.
Vorticity balances in the tropics during the 1982‐83 El Niñio‐Southern oscillation event
Authors: Sardeshmukh Prashant D. , Hoskins Brian J.
Published: July 2010

The mean vorticity balances in the tropical atmosphere during December 1982 to February 1983, a season of extraordinary anomalies associated with the warming of the east equatorial Pacific Ocean, are determined using the 6‐hourly initialized and objectively analysed observations archived at the European Centre for Medium Range Weather Forecasts. the budgets are evaluated at one upper (150 mb) and one lower (850 mb) tropospheric level, with particular emphasis on the balance at 150 mb, near the level of maximum outflow associated with organized deep convection in the tropics.

The balances at the upper and lower levels are found to differ qualitatively from one another. At 850 mb, the zonal flow is weak and the relative vorticity and its gradient small compared with that of the Coriolis parameter. the vorticity balance is therefore essentially a Sverdrup balance between the stretching and horizontal advection of planetary vorticity, and an assumption of linearity about a basic state at rest is not unreasonable. None of this applies, however, at 150 mb.

At 150 mb, the balance is essentially nonlinear, and nearly inviscid, the primary balance obtaining between the stretching and advection of absolute vorticity by the time‐mean horizontal flow. Transients play a small though not insignificant role, but terms involving the vertical advection and twisting of vorticity are small enough to be neglected.

The essentially nonlinear character of the upper tropospheric dynamics is highlighted by demonstrating the inability of a diagnostic barotropic model linearized about the observed steady zonal mean flow to simulate the anomalous flow in the central Pacific correctly. the idea of the expansion of material circuits in regions of strong convective outflow, leading to a rapid spin‐down of the local absolute vorticity, and the production of a region of almost zero absolute circulation, is of crucial importance. This is seen to be intimately related to the importance of nonlinearity.
Planetary wave‐mean flow interaction in the stratosphere: A comparison between northern and southern hemispheres
Authors: Shiotani M. , Hirota I.
Published: July 2010

By the use of stratospheric height and temperature field data obtained from the TIROS satellite, dynamical interaction between planetary waves and mean zonal winds in the stratosphere is investigated. Special attention is paid to differences between northern and southern hemispheres. an analysis is made for pressure levels from 70 to 0.4 mb during the period from June 1981 to May 1982. Eliassen‐Palm (E‐P) flux diagnostics are used as a powerful and useful tool to investigate the wave‐mean flow interaction.

From winter to summer the seasonal marches of wave activity (measured by the E‐P flux) and of mean zonal wind are different between the two hemispheres. In the NH, wave activity varies intermittently with a characteristic timescale of about two weeks. Once the core of the stratospheric westerly jet shifts poleward due to a minor warming, the subsequent wave activity breaks the westerlies dramatically to replace them by the easterlies in association with the sudden warming, whilst in the SH wave activity in mid‐winter is quiet, corresponding to small time variations of the maximum westerly speed. In late winter the core of the stratospheric westerly jet suddenly shifts poleward and downward due to a wavenumber‐2 minor warming. After the shifting of the westerly jet the wave activity of wavenumber 1 is enhanced, and continues until early summer.

On the basis of this observational evidence we discuss: (i) the seasonal evolutions of wave activity and the mean zonal wind; (ii) the interannual variability of the stratospheric circulations; and (iii) the time variation of wave activity.
Variance spectra of observed winds and initialized wind fields at the 500 mblevel over europe
Authors: Brown Philip S. , Robinson G. D.
Published: July 2010

Two‐dimensional scale (variance spectrum) analyses were made of the wind field at 500 mb over Europe and western Asia. Seven separate 20‐ to 30‐day periods, four in winter and three in summer, were investigated. On all occasions actual unmodified measured winds were used; on two of them the same techniques were applied to the interpolated and initialized data on a regular grid used by the National Weather Service in prognostic models. Standard analytical spectral analysis cannot be used on the unmodified observations; the method used involved iterative adjustment of a first guess spectrum to fit calculated variance and covariance functions.

Some conclusions are: Our examination of the data does not unambiguously establish an ‘inertial range’ in the spectrum. the Taylor transformation of frequency to wavenumber is not reliable at scales more than about 1000 km. There are significant diurnal changes; in the summer periods they are not confined to the small unresolved scales.

The Weather Service initialization‐interpolation procedure quantitatively removes variance at less than twice the grid scale, but also affects the spectrum in the resolved scales to a degree which might be significant in prognostic models.

On two occasions there was evidence of widespread and persistent variance on a scale of about 350 km.
Measurements and predictions of flow and turbulence over an isolated hill of moderate slope
Authors: Mason P. J. , King J. C.
Published: July 2010

Field measurements of the mean flow and turbulence statistics in neutral static stability flow over an isolated, roughly circular hill, Blashaval, are presented. Blashaval rises approximately 100 m above the surrounding terrain and has a base diameter of about 800 m. the flow speed at 8 m over the summit is found to be increased by a factor of 1.7 over the upstream value and the flow direction reverses in the lee of the hill. the turbulence structure observed above the summit shows marked variations with height. the observed mean flow is compared with results from a model which makes a careful application of linear theory to real terrain. the turbulence measurements are compared with results from available theories.
On the occurrence of cloud streets over Northern Germany
Authors: Müller D. , Etling D. , Kottmeier Ch. , Roth R.
Published: July 2010

Cloud‐street observations over northern Germany are analysed with respect to wind and temperature profiles obtained from routine radiosonde stations in the area. In nearly all cases the wind profiles on cloudstreet days were characterized by an inflection point in the cross‐wind component with unstable stratification of the boundary layer. the height of the inflection point was found to be related to the convection height on those days, which may suggest the importance of both inflection point and convective instability for explanation of cloud‐street development.
Some observations of turbulence structure in stable layers
Authors: Hunt J. C. R. , Kaimal J. C. , Gaynor J. E.
Published: July 2010

Three days of measurements in stable conditions (Monin‐Obukhov length L between 15 and 300 m) at the Boulder Atmospheric Observatory tower are presented. Winds came off the Rocky Mountains on two days and off the plains on the other day. Vertical profiles of the mean horizontal velocity and Brunt‐Väisälä frequency N, and the standard deviation of the vertical component, ow, fluxes, spectra, cospectra, quadspectra, correlations and length scales are examined in this study. the measurements indicate that wave motion was present on all occasions, but which varied in strength, from weak to strong. New theoretical arguments are tested, which suggest that where the Richardson number is less than one, the velocity gradient dU/dz rather than N determines the integral scales Lx(w) of the vertical velocity fluctuations, i.e. Lx(w) ≃ ow/(dU/dz). But theory also suggests that it is not this scale, but the larger buoyancy scale ow/N that determines the temperature variance and thermal diffusivity. On this basis, dimensionless parameters are developed for plotting the measurements of temperature variance and thermal diffusivity. They are found to be in reasonably good agreement with other observations and with theory, even though the relative strength of wave motion to turbulence varied considerably. Low frequency wave motion appeared to be associated with significant heat flux, a possibility that has been excluded or overlooked in some previous studies.
The global atmospheric angular momentum balance inferred from analyses made during the fgge
Authors: Swinbank By R.
Published: July 2010

Using analyses made during the First GARP Global Experiment (FGGE), a study has been made of the angular momentum budget of the atmosphere. the time evolution of atmospheric angular momentum has been compared with the calculated torques acting upon it. This comparison has shown for the first time that the mountain torque is the main contributor to the short‐term variation in the global atmospheric angular momentum. the variation of the torques as functions of latitude has also been compared with the total torque implied by the meridional transport of angular momentum by the atmosphere. the torques calculated agree qualitatively with the angular momentum transport, but there are some significant differences in magnitude; some possible reasons for these differences are discussed in the light of results from a general circulation model and from a second set of analyses made during the FGGE.
The derivation of hydrogen‐containing radical concentrations from satellite data sets
Authors: Pyle J. A. , Zavody A. M.
Published: July 2010

Data from the LIMS and SAMS instruments on Nimbus 7 are used to derive zonal‐mean OH fields from 85°N to 64°s for November 1978 to May 1979. Two methods are used. One relies on the establishment of a photochemical steady state between NO2 and HNO3. In the second method OH is calculated from its sources and sinks. There is substantially good agreement between the two methods. Areas of discrepancy can be explained. Indeed, some differences should be expected if present photochemical theory is correct.
A fast method for calculating scale‐dependent photochemical acceleration in dynamical models of the stratosphere
Authors: Haigh J. D.
Published: July 2010

It is shown that photochemical acceleration of the thermal relaxation rate in the stratosphere may be expressed as a matrix multiplication between a square matrix, which may be calculated for quite general conditions, and a vector representing the deviation of temperature from an equilibrium profile. Such a matrix is presented and its form discussed. Use of this matrix allows for temperature perturbations of any vertical scale and thus provides an accurate, as well as fast, method for calculating photochemical acceleration suitable for use in numerical models of stratospheric dynamics. the inclusion of the ozone 9.6 m̈m band into the heating rate calculations is shown to reduce the photochemical relaxation rate in the upper stratosphere.
The effect of mechanical forcing on the formation of a mesoscale vortex
Authors: Wu Guo‐Xiong , Chen Shou‐Jun
Published: July 2010

The influence of mechanical forcing on the formation of a mesoscale vortex which occurred over the eastern flank of the Qinghai‐Tibetan Plateau (QTP) is investigated by running a series of numerical experiments. Different orography schemes are tested. It is shown that there are three branches of air flow converging towards the vortex: the northerlies associated with the middle latitude westerly currents, the southerlies with the lowlevel monsoon jet, and the easterlies with the west Pacific subtropical high. These flows are very sensitive to mechanical forcing and play very important roles in the formation and location of the vortex.

The model with either the mean or the envelope orography is shown to be capable of predicting the formation of the vortex and the associated torrential rain. However, its location is not predicted precisely. A modified orography, simulating the ‘step’ structure of the eastern flank of the QTP, is then used in an attempt to improve the prediction. Results from this experiment suggest that the weakening of some of the steep slopes of the large‐scale orography in a numerical model, due to excessive smoothing of mountain height and expanding of mountain area, is responsible for the inaccuracy in the forecast of such synoptic systems. Therefore further improvements in the orography scheme are discussed.
Observations of intermittent cumulus convection in the boundary layer
Authors: Brümmer Burghard , Wendel Melchior
Published: July 2010

During the 1978 JASIN experiment radiosonde and tethered balloon measurements were performed from the R. V. Meteor. On 1 September, two cloud layers (cumulus below stratocumulus) were observed below an inversion at about 900m. Such a boundary layer situation occurred relatively often during the experiment. Cumulus clouds were not present all the time but alternately appeared and disappeared. When they occurred on 1 September, a relatively large vertical shear (about 0.006 s−1) together with an inflection point in the profile of the wind component normal to the mean wind direction was observed in the cumulus cloud layer. the airsea temperature difference was small (δT = −0.5 K) and there was a relatively large dynamical contribution to cloud generation. This may suggest the idea of an interaction between large‐scale processes generating a favourable mean wind shear profile and dynamically forced convection.

Significant vertical transport of heat and momentum in the cloud layer occurred only in connection with cumulus clouds. Although in parts of some clouds the transports were locally as large as several 100W m−2 and 1N m−2, there were only a few events with such large excursions so that the transports averaged over the whole field of cumulus clouds were small.

The relative air flow in and around active cumulus clouds was characterized by updraughts within the cloud and downdraughts at the cloud edges. the thermodynamic properties of updraughts and downdraughts are discussed.
Diffusion of gaussian puffs
Authors: Mikkelsen T. , Larsen S. E. , Pécseli H. L.
Published: July 2010

A formula for the relative diffusion of a puff is proposed in which the rate of growth is related to common, one‐dimensional velocity spectra obtainable, for instance, from direct measurements in the atmosphere. Often, an equation of this type is in demand for modelling of instant or short‐term release of potentially harmful gases or smoke, for example, in connection with a puff diffusion model.

A simple expression like Taylor's equation for single‐particle diffusion is not obtainable in general for relative diffusion. Therefore, the present approach is based on a kinematic‐statistical model in which a Gaussian approximation has been applied to the relative displacement process of the fluid particles. This is along lines suggested previously, where the expansion of a Gaussian puff has been related to the kinematic energy spectrum in three‐dimensional, isotropic turbulence. Here, however, we attempt to relate the growth rate directly to the one‐dimensional velocity spectra (or their corresponding correlations) most commonly available in the atmosphere. In particular, this is of value for the surface layer, where diffusions in the horizontal and vertical directions are different (anisotropy) due to the presence of the ground.

Experimental evaluation is based on a series of smoke release experiments carried out in the surface layer over homogeneous terrain in Denmark. Solutions of the puff growth rate equation with the measured correlations are found to compare well with the experimental data of cross‐wind relative diffusion over the limited scale of the experiments.
A comparative study of southern hemisphere blocking during the global weather experiment
Authors: Lejenäs Harald
Published: July 2010

Southern hemisphere blocking during the Global Weather Experiment has been studied. Two different data sets, and two different methods to identify blocking have been used. One data set consists of maps analysed at the Australian Bureau of Meteorology and the other of FGGE level III‐b data. the results show that one method, which tends to emphasize blocks accompanied by cut‐off lows, is more sensitive to data accuracy than the other, which is based on a count of persistent positive anomalies exceeding a certain threshold value.
Convective heating and precipitation estimates for the tropical South Pacific during fgge, 10‐18 january 1979
Authors: Miller Bernard L. , Vincent Dayton G.
Published: July 2010

A modified version of ECMWF analyses is used to compute heat and moisture budgets during part of FGGE SOP‐1 (10‐18 January 1979) for an area which contains the South Pacific convergence zone (SPCZ). Precipitation estimates are derived as residuals and compared with those derived from a GOES IR satellite technique.

The 12‐hour precipitation rates derived from the heat budget appear to be reasonably accurate, whereas those obtained from the moisture budget contain some error due to the contribution from the local rate of change term. Moreover, the heat budget estimates are generally in good agreement with the IR estimates. the axis of maximum precipitation derived from the heat budget is aligned well with the lowest values of outgoing long‐wave radiation (OLR), which represent high, cold cloud tops resulting from deep cumulus convection.

As expected, the vertical advection term is found to be the dominant term in the heat budget which, together with radiative cooling, indicates an approximate balance between adiabatic cooling and diabatic heating. the vertical profiles of convective heating obtained from the heat budget are compared with those derived by other investigators using similar methods for other geographical regions. the present profiles, which show maximum heating near 500 mb, are more comparable to those for the western North Pacific than to those in the eastern Atlantic.
On systematic errors in satellite sounding products and their climatological mean values
Authors: Eyre J. R.
Published: July 2010

Estimates of atmospheric parameters derived from passive measurements of thermal radiation made by satellite‐borne radiometers have rather subtle error characteristics. Moreover, they are sensitive to the ‘first guess’ of the atmospheric state which is used, either explicitly or implicitly, in the inversion process. A proper understanding of these characteristics is desirable when using such data in climate studies or in numerical weather prediction.

Relations describing the error structure of individual retrievals are derived and developed to describe the error characteristics of mean values of retrieved products as typically required in climate studies. It is shown that systematic retrieval errors will occur if the first guess used in the inversion is systematically different from the true atmospheric state. the errors are quantified for one particular inversion method‐a scheme designed to retrieve atmospheric temperature and humidity profiles and surface temperature from TOVS (TIROS‐N Operational Vertical Sounder) data. In this case the biases in retrieved surface temperature are small (but not necessarily negligible). However, for less directly observable quantities such as surface air temperature and surface relative humidity, the problem is acute.

Potential methods for correcting retrieved parameters for such biases, in retrospect, using an improved first guess are discussed. They rely on information concerning the first guess applied in the inversion being available to the user of the retrieval products. This has important implications for the design of operational systems to process, disseminate and archive satellite sounding data.
On the interpretation of the eliassen‐palm flux divergence
Authors: Andrews David G.
Published: July 2010

New expressions are presented for the wave‐activity density and the non‐conservative source of wave activity in the small amplitude Eliassen‐Palm theorem for the primitive equations in log(pressure) and isentropic coordinates. These expressions involve Ertel's potential vorticity, and include versions that are of Eulerian form.

The theoretical results are used as a basis for investigating the interpretation of the Eliassen‐Palm flux divergence in terms of physical wave properties. Implications for modelling and observational studies of planetary waves are discussed and connections with stability theory and the definition of orthogonality of normal modes are mentioned.
Photochemical model comparisons with lims observations in a stratospheric trajectory coordinate system
Authors: Austin J. , Pallister R. C. , Pyle J. A. , Tucks A. F. , Zavody A. M.
Published: July 2010

A stratospheric photochemical model with 32 species and 77 reactions has been integrated along stratospheric air parcel trajectories for both disturbed and quiescent periods in early 1979. the model has fully time‐dependent photodissociation coefficients, allows for scattering and albedo effects and the solar zenith angle is accurately computed. the trajectories were calculated from satellite data using a radiative scheme to compute the vertical displacements of the air parcels relative to isentropic surfaces θ0. The model results are compared with LIMS observations of NO2, HNO3, H2O and O3 at points of near coincidence along trajectories at five levels θ0 = 500 K (∼20km); 650 K (∼25km); 850 K (∼30km); 1100 K (∼35km); and 1400 K (∼40km). JPL‐85 data for rate coefficients were used as standard.

At the upper two levels the amounts of chlorine and methane in the model were found to have an important effect on the results. Using SAMS CH4 and typical but not locally observed mixing ratios of chlorine, agreement between the model and LIMS O3 data could not be obtained using JPL‐85 rate coefficients. This ozone discrepancy could be essentially removed by using the CODATA‐82 rate coefficient data. Large variations in NO2 during the course of a day were successfully modelled at θ0 = 650K, 850K and 1100K. Disagreement between observations and calculations was found for HNO3 at θ0 = 850K and 1100K in the standard case, but agreement could be obtained at θ0 = 850K using CODATA‐82 rates. Sensitivity tests were performed in which molecular oxygen absorption cross‐sections and certain key rate coefficients were varied, among them those for the O + HO2, OH + NO2 + M, NO + O3 and O + O3 reactions. Variations in O3, NO2 and HNO3 during substantial meridional excursions were reproduced by the photochemical model for trajectories where these molecules were expected to be chemically responsive on a time scale of less than 10 days. In other cases, they acted as tracers.
Solar ultraviolet‐B radiation under cloudless skies
Authors: Webb A. R. , Steven M. D.
Published: July 2010

Solar ultraviolet‐B (UVB) irradiance IB (300‐316 nm) and visible radiation IV (300‐700 nm) were measured at Sutton Bonington in the Midlands. On clear days the ratio of irradiance in the two wavebands, IB/IV, plotted against the cosine of the solar zenith angle gave a series of straight lines of the form IB/IV = x + y cos z. the gradients of the lines, y, were consistent from day to day whereas the intercepts, x, changed. Applying a theoretical treatment of aerosol scattering to obtain a theoretical value of IB/IV showed the gradient y to depend only on factors which are constant for Rayleigh scattering, whereas x is a function of other atmospheric variables, including ozone, which change from day to day. A similar procedure for the ratio of UVB to the full solar waveband IF (300‐3000 nm) produced straight lines of the form IB/IF = x′ + y′ cos z on clear days, but both x′ and y′ varied from day to day, possibly because of changes in absorption by water vapour in the infrared region of the spectrum.
Convection waves: Observations of gravity wave systems over convectively active boundary layers
Authors: Kuettner Joachim P. , Hildebrand Peter A. , Clark Terry L.
Published: July 2010

Widespread gravity wave systems have been found to exist over convectively active boundary layers (CBLs) in the presence of vertical wind shear. In contrast to mountain waves, these ‘convection waves’ occur over flat terrain and are ubiquitous over fields of shallow fair weather cumuli or clear air thermals. They extend vertically to at least 9km a.m.s.l., probably to the tropopause. First discovered by glider pilots as ‘thermal waves’, they have now been systematically investigated by research aircraft during the NCAR Convection Wave Project, which also incorporates an effort in numerical simulation. the flight results are reported in this paper, while the numerical simulations are described in a companion paper.

Typical wavelengths of convection waves were found to range from 5 to 15 km, (average 9 km) and typical vertical motion amplitudes from ± 1 to ± 3 m s−1. In all cases vertical wind shear exceeded 3×10−3s−1. Under these conditions cloud tops were measured to move with relative velocities of 8 to 10 m s−1 against their environment, hence they present an obstacle to the surrounding airflow. the analogy to mountainous obstacles launching gravity waves into the atmosphere is suggestive and borne out by the aforementioned numerical simulations, which describe this process step by step.

The interaction between the CBL and the overlying stable layers is studied by spectral and cross‐spectral analysis of aircraft data including coherence and phase relations between wave motions and cloud population underneath. Consideration is given to other potential sources of the observed gravity waves. At this stage the conclusions of our pilot study may be considered tentative. Some implications for further research are also discussed.
Polarization radar studies of precipitation development in convective storms
Authors: Illingworth A. J. , Goddard J. W. F. , Cherry S. M.
Published: July 2010

Radar observations of the differential reflectivity (ZDR) of developing convective clouds are presented. This parameter provides a measure of a mean hydrometeor shape, and when analysed in conjunction with the conventional reflectivity (Z), it gives an indication of whether the precipitation is ice or liquid water; for rain it enables an estimate of raindrop size and concentration to be made. the results suggest that some isolated echoes which subsequently develop into large cumulonimbus clouds initially have very small concentrations of large (diameter greater than 4 mm) supercooled raindrops, a very different size distribution from that normally observed in mature convective clouds. an alternative explanation in terms of an even lower population of much larger ice particles in wet growth is technically possible but does not seem physically realistic.
Data Assimilation: the 1984/85 Revisions of the Ecmwf Mass and Wind Analysis
Authors: Shaw D. B. , Lönnberg P. , Hollingsworth A. , Undén P.
Published: July 2010

A comprehensive evaluation of the ECMWF operational data assimilation system has been made, based largely on statistics of its performance over a long period of time. the evaluation has identified many deficiencies, covering such aspects of the analysis as the optimum interpolation statistics, the data selection algorithms, and the quality control criteria applied within the analysis.

The observation and forecast error variances were seriously in error in the stratosphere and in the tropics. A more faithful model of the forecast error spectrum is provided by the Bessel function expansion than by the Gaussian model. the vertical structure of the height error is assumed to be composed of two components: a ‘barotropic’ part related to a horizontally constant mode and a ‘baroclinic’ structure for the horizontally varying modes. the wind field errors also have a large‐scale component. the extratropical vertical correlation remained similar to the earlier correlations, but the tropical height and wind structures changed dramatically.

Reasonable rejection limits were defined from the distributions of the observation departures from the first guess, as were observation errors for satellite and aircraft wind reports and Australian pseudo‐observations.

A comprehensive and homogeneous data selection algorithm has been devised. However, it has been found necessary to exclude land surface wind reports from the analysis process as the wind departures from a first‐guess field commonly contradict the surface pressure departures due to difficulties in generating a reasonable 10 m first‐guess wind. the omission of surface winds alleviated the data selection problem in data‐dense regions.

The representation of the vertical correlations was changed to a continuous representation in the vertical which enabled a multivariate interpolation from data on standard pressure levels directly to model coordinates. the earlier procedure used a multivariate interpolation to constant pressure surfaces in the model grid, followed by a univariate interpolation to model surfaces in the vertical.

The results of these improvements, in a series of tests covering in total some 30 days of assimilations, are also described. In summary they yield an analysis and a subsequent first guess which are in closer agreement with observations. the analysis scheme yields more balanced fields in the sense that the subsequent initialization produces more modest changes. the major impact on forecasts, in particular in the short range, is also judged to be positive.

These improvements have been incorporated in the ECMWF operational data assimilation system. They have also been incorporated in the reassimilation of the final FGGE level IIb data set at ECMWF.
The Middle Atmosphere In the Absence of Dynamical Heat Fluxes
Authors: Shine K. P.
Published: July 2010

A new radiative transfer scheme, developed for use in a three‐dimensional dynamical model of the stratosphere and mesosphere, is used to consider the hypothetical state of the middle atmosphere in the absence of dynamical heat fluxes. Most previous work in this area has considered radiative equilibrium temperatures using a fixed solar forcing. This equilibrium state is shown to be different from a state calculated using a seasonal cycle of solar forcing; in particular, the winter pole is found to be warmer and the latitudinal temperature gradient across the edge of the polar night is found to be smaller. This difference is due to two processes. Firstly, latitudes just polewards of the edge of the polar night at solstice are in darkness for only a short period. Secondly, radiative time scales lengthen greatly as temperatures decrease, so that equilibrium cannot be reached before the sun reappears.

Recent ozone climatologies are used to impose a realistic annual cycle of ozone on the model. It is found that the simulation of the lower stratosphere is in considerably better agreement with observations than previous radiation‐only calculations. the better simulation is shown to be, in most cases, due to the use of the new ozone climatology. It is also shown that a realistic ozone distribution is necessary for a good simulation of the polar stratopause at the summer solstice.

A comparison of the observed and calculated annual cycle of temperature in the polar regions of the southern hemisphere lower stratosphere indicates that this region is under strong radiative control. This contrasts with an implied strong dynamical control for the same region in the northern hemisphere.
The Formation and Movement In the Stratosphere of Very Dry Air
Authors: Robinson G. D. , Schoen M. G. Atticks
Published: July 2010

Statistics of the saturation mixing ratio of H2 O at the temperature minimum and at 100 mb are extracted from about 16 000 radiosonde ascents between latitudes 20°N and 20°N on 140 days, sampling the year December 1978 through November 1979 (FGGE). ‘Window’ radiance temperatures measured by satellite (TIROS and Japanese GMS) between 15°N and 15°N latitude 85° to 185° longitude, at the highest available resolution (about 5 km × 5 km) on one day (Julian day 28 1979), are analysed to extract frequencies of occurrence of temperatures in ranges below −70°C. (Radiosondes and cloud imagery on this day are representative of conditions in these latitudes in the southern summer.) Low radiance temperatures are rare, the frequency of occurrence in the range −86°C to −90°C is 2.5×10−4, whereas about 1 in 4 of the radiosonde ascents in the area at this time have temperatures below −86°C. It is estimated that the cloud turrets with these low radiance temperatures could provide a sufficient source of the driest stratospheric air, but the temperature structure required, by Danielsen's hypothesis, for subsequent dehydration is not observed by radiosonde.

All published in situ observations of H2 O mixing ratio in the tropical and northern temperate stratosphere are examined in conjunction with the zonal means observed by the LIMS satellite in December 1978 and May 1979. Constraints imposed by the observations on descriptions of air motion within the stratosphere are discussed.
Potential vorticity in models of the ocean circulation
Authors: Bryan Kirk
Published: July 2010

Existing observations suffice to give a qualitative description of the wind‐driven and thermohaline components of the ocean circulation, but a hierarchy of analytical and numerical models is now needed for in coupled ocean‐atmosphere models of the earth's climate. Potentialvorticity is a more appropriate diagnostic field variable than angular momentum for the ocean circulation because of the complicated geometry of basins. Patterns of potential vorticity onsurfaces of constant density help validate ocean circulation models, and give physical insight into how the oceancirculation works.

High resolution models suggest that the lateral mixing of potential vorticity by mesoscale eddies along isopycnal surfaces is of the same order as large‐scale advection, and that the assumption of inviscid, potential‐vorticity‐conserving flow in the thermocline is not appropriate for the real ocean. A satisfactory test of this conjecture will require the extensive measurements planned for the World Ocean Circulation Experiment.

Models also indicate that the transport of water mass properties by mesoscale eddies is largely a mixing along isopycnal surfaces. The wcak temperaturc gradients along isopycnal surfaces in most areas of the ocean limit the effectiveness of the mesoscale eddies in transporting significant amounts of heat across latitude circles.
A Diagnostic Study of the Dynamics of the Northern Hemisphere Winter of 1985‐86
Authors: Hoskins Brian J. , Sardeshmukh Prashant D.
Published: July 2010

February 1986 was notable for a dramatic blocking pattern in the eastern N Atlantic with part of NW Europe having its coldest February in 300 years. However, the previous December and January had been mild in the same region and the hemispheric anomaly field was characterized by a pattern which could be viewed as a wave‐train emanating from the tropical western Pacific. Such an extreme change within a single winter season is unusual and must be understood if reliable monthly and seasonal forecasts are ever to be produced.

The tropospheric flow field, isentropic distributions of potential vorticity, and diabatic heating calculated as a residual in the thermodynamic equation are used to illustrate the similarities and differences in the two parts of the winter and to suggest possible reasons for these differences. the mean vorticity equation is also used to investigate the importance of the change in the zonal flow, the transients and the mean divergence. It is concluded that the change in the fluxes due to the synoptic weather systems was crucial, but that a catalyst for the February block could have been provided by an unusual diabatic forcing in the S American‐Caribbean region. Implications are drawn for the ingredients which must be included in dynamical models for predicting on the monthly to seasonal time‐scales.
The Effect of the Hadley Circulation On the Meridional Propagation of Stationary Waves
Authors: Watterson Ian G. , Schneider Edwin K.
Published: July 2010

The effect on the meridional propagation of stationary planetary waves of the inclusion of the zonal mean Hadley circulation in linearized models of the atmosphere is examined. In a forced barotropic vorticity equation model a meridional wind in the basic state enables wave propagation through tropical easterlies in the direction of the wind. This is shown to be consistent with the WKB solution.

In a multi‐level linearized steady primitive equation model with an idealized meridional circulation tropical forcing can still produce response through easterlies although propagation from one mid‐latitude to the other is much less than in the barotropic result. This is explained through WKB and wave‐tracing methods for a simplified three‐dimensional vorticity equation. the near non‐divergence of the unforced tropical motion permits a wave to travel largely within a layer of favourable winds.

In a twelve‐level model using observed January and July zonal mean conditions the Hadley circulation changes the response to earth topographic forcing mainly in the subtropics. However, tropical heat forcing can now produce a large mid‐latitude response, in the winter hemisphere especially. the resulting subtropical jets suggest a mechanism for the maintenance of those observed over Australia during the northern monsoon season and adjacent to the anomalous forcing of the 1983 El Niño.
Simulated Sudden Stratospheric Warming; Synoptic Evolution
Authors: Blackshear W. T. , Grose W. L. , Turner R. E.
Published: July 2010

An analysis is presented of a sudden stratospheric warming event which occurred spontaneously during a general circulation model simulation of the global atmospheric circulation. Two separate warming pulses exhibit the same dynamical evolution with a ‘cycle’ of about two weeks. Two distinct phases of the warming cycle are apparent: (1) the generation of an intense localized warm cell in conjunction with significant adiabatic heating associated with cross‐isobar flow which has been induced by vertically propagating long wave disturbances; and (2) the northward transport of that warm cell via advection by the essentially geostrophic windfield corresponding to an intense, offset polar cyclone, in conjunction with a strong Aleutian anticyclone. During the first warming pulse in January, a moderate Aleutian anticyclone was in place prior to the warming cycle and was intensified by interaction with an eastward travelling anticyclone induced by the differential advection of the warm cell. the second warming pulse occurred in early February with a strong Aleutian anticyclone already established. In contrast to the January event, the warming in February culminated with reversal of the zonal westerlies to easterlies over a significant depth of the stratosphere.
On the Relation of 6.7 μm Water Vapour Features to Isentropic Distributions of Potential Vorticity
Authors: Manney Gloria L. , Stanford John L.
Published: July 2010

Currently available isentropic maps of potential vorticity (IPV maps) are of low resolution. In this note, we examine the possibility of using 6.7 μm satellite‐derived water vapour measurements to obtain higher resolution information about ipv distributions. While attempts to relate the two data sets analytically have so far proved unsuccessful, a qualitative comparison shows very similar features in both. Our results suggest that, over limited areas, it may be possible to use 6.7 μm brightness temperatures to obtain approximate contours for water vapour mixing ratio on a particular isentropic surface in the upper troposphere. For areas free of high clouds, these contours are expected to parallel the ipv contours on this surface. In a case study, the resulting map exhibits dramatically increased horizontal resolution compared with present ipv maps, detailing an intriguing upper‐tropospheric feature at sub‐synoptic scales.