bluuebunny/crossref_metadata_2025 huggingface dataset:
Droplet accretion during rime growth and the formation of secondary ice crystals
Authors: Dong Ya Yi , Hallett John
Published: February 2007

Laboratory experiments have been performed to study rime growth and its relation to secondary ice crystal production (the Hallett‐Mossop process). The rime was grown in a wind tunnel on a fixed vertical fibre 0.5 mm in diameter. The range of conditions extended from −1.0°C to −13°C in temperature, from 20cms−1 to 200cms−1 in air speed, and liquid water content up to 1.5 gm−3. The droplet size distributions were 5 to 25 μm and 8 to 50 μm, peaking at diameters of 9 and 22 μm respectively. Under conditions favourable for secondary ice production, the accreting droplets spread out on the ice surface; individual frozen droplets formed only at the lower temperatures. A separate experiment showed that supercooled water droplets froze as cones on a flat surface above −10.°C. At temperatures between −1. and −4.°C the spreading ratio of supercooled droplets on the basal surface of ice was less than on other faces, with lower molecular packing density. It is hypothesized that spreading takes place in a quasi‐liquid layer which is thinner on the basal surface. Splinter formation by pressure build‐up inside individual frozen droplets is therefore unlikely to be responsible for shatter; a new model is suggested in which thermal gradients give stresses leading to an ice crack at about − 5°C.
A comparison of incoming solar radiation at marine and continental stations
Authors: Dobson Fred W. , Smith Stuart D.
Published: February 2007

In modelling solar radiation from surface weather observations, different approaches have been successful with marine and with land‐based data. We apply a simple model to a marine and a mid‐latitude continental station to illustrate systematic differences in the dependence of the insolation on solar elevation, cloud, and season. Relative to the continental station, the marine station has less insolation at low solar elevations, particularly in winter, but more insolation at high solar elevations. Two mechanisms are discussed to explain these differences.
Inversion of cloudy satellite sounding radiances by nonlinear optimal estimation. I: Theory and simulation for TOVS
Authors: Eyre J. R.
Published: February 2007

A new approach is proposed for inverting satellite sounding data for use in numerical weather prediction (NWP). It may be considered in two ways: either as a retrieval scheme which uses a forecast profile as a background or ‘first guess’ and the covariance of forecast error as a constraint in order to produce retrieved profiles compatible with the subsequent NWP analysis; or as a module within a NWP analysis scheme to perform the vertical analysis of satellite radiances at the observation points (i.e. to project the information in the radiances on to the NWP analysis levels in the vertical). The new scheme performs an inversion of the radiances to retrieve simultaneously the temperature and humidity profiles, the surface temperature and microwave emissivity, and the cloud‐top pressure and amount. It employs an iterative method which finds the maximum probability solution to a nonlinear retrieval/analysis problem. It operates on ‘raw’, potentially cloud‐affected radiances and thus bypasses the separate pre‐processing and cloud‐clearing stages necessary in many retrieval schemes. The technique is a fully ‘physical’ method, involving calculations for each profile of corresponding radiances (and of derivatives of radiance with respect to profile parameters). It is also a ‘statistical’ method, since it uses the covariance of forecast error as a constraint.

The paper develops the theory of the new approach and examines its convergence properties and inherent quality control characteristics. The application of the scheme to data from the TIROS Operational Vertical Sounder (TOVS) is then discussed. The theoretical error characteristics of the scheme are investigated, and the results of retrievals from simulated TOVS radiances are presented. The theoretical advantages of retrieving cloud parameters simultaneously with other variables are demonstrated, in terms of both the temperature retrieval accuracies in cloudy conditions and the skill with which the cloud parameters themselves may be obtained.
Frontal circulations in the presence of small viscous moist symmetric stability and weak forcing
Authors: Xu Qin
Published: February 2007

The Sawyer‐Eliassen (S‐E) equation for frontal circulation is extended to include the effects of eddy viscosity and small negative moist potential vorticity (MPV) under the condition of weak or, at least, not very strong frontogenetic forcing. When MPV is positive, viscosity can be neglected and this equation becomes the conventional S‐E equation. When forcing is absent and MPV is strongly negative, this equation degenerates into the equation of linear viscous conditional (moist) symmetric instability (CSI). By using an idealized distribution of frontogenetic forcing which attenuates exponentially away from the position of maximum forcing, the extended S‐E equation is solved both analytically and numerically for frontal circulations where frontogenetic forcing and negative MPV coexist (but the negative MPV is not so strong as to initiate viscous CSI). The solutions contain slantwise banded substructures on the warm side of the region of maximum forcing. The intensity, structure and scale of the bands are controlled by the competition between the frontogenetic forcing, negative MPV and eddy viscosity with the following characteristics:

For a horizontally concentrated forcing whose attenuation length Lb (i.e. the length over which the forcing is reduced by a factor of eπ = 23.1 away from the region of maximum forcing) is less than Ls2 (where Ls is the semi‐geostrophic Rossby radius of deformation), the substructure is single‐banded. The band occurs even before MPV becomes negative. As MPV decreases and becomes negative, the band becomes narrow and intense.

For a widespread forcing whose attenuation length Lb is larger than Ls, the substructure is multi‐banded. The multiple bands may occur only if MPV is negative enough, and the first occurring multi‐bands are characterized by wide bands of moist ascent with narrow and weak dry descents embedded between the moist bands. As MPV becomes further negative, the moist bands become narrow, intense and widely spaced.

For a moderately widespread forcing whose attenuation length Lb is between Ls/2 and Ls, the substructure changes from wide and weak multiple bands to a narrow and strong single band as MPV becomes increasingly negative.

As the coefficient of eddy viscosity increases within a moderately wide range (5‐100m2s−1), the band (or bands) become weak and wide.

The theoretical findings are explained physically and compared with observations.
Observations of boundary‐layer structure over complex terrain
Authors: Grant A. L. M. , Mason P. J.
Published: February 2007

Turbulence data collected in an area of complex terrain using a number of instrument packages attached to the tether cable of a captive balloon are presented. Eddy correlation measurements of the turbulent momentum flux are used to determine the magnitude of the roughness length from the drag coefficient and also to show that the roughness length is a useful concept in parametrizing the surface stress in such areas. the relationship between the terrain characteristics and the roughness length is examined using a simple formula, results from a numerical model and previous experimental estimates of the roughness length for complex terrain. It is found that a formula based on numerical model results for separated flows gives reasonable predictions and that the measured roughness length is representative of an area of order 100 km2.

Non‐dimensionalized turbulence data are compared with similar data obtained over flat homogeneous surfaces. This comparison suggests that the structure of the turbulent boundary layer over complex terrain is remarkably similar to that over flat homogeneous surfaces. Because of the limitations of fixed point measurements in complex terrain, results from a numerical study of turbulent flow over two‐dimensional sinusoidal orography are shown to indicate the magnitudes of the perturbations introduced into the boundary‐layer flow by large undulations in the underlying surface. the model results demonstrate the validity of the analysis carried out with the data.
The effect of mixing height on maritime aerosol concentrations over the north atlantic ocean
Authors: Park P. M. , Smith M. H. , Exton H. J.
Published: February 2007

An empirical investigation of the effect of the stability of the atmosphere on near‐surface maritime aerosol concentrations is presented. the data base consists of an extensive series of aerosol experiments conducted on the western coast of the Outer Hebrides.

In the absence of any well‐defined capping inversion over the Atlantic, stability is characterized in terms of convective and mechanical mixing heights, derived from radiosonde ascent data and from friction velocity, respectively. After wind‐speed‐production effects have been removed from the data, a simple relationship of the form \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm concentration}\, = \,A\, + \,B/({\rm mixing}\,{\rm height})\,\,\,(A,B\,{\rm constants}) $\end{document} was found to adequately describe most data subsets. A discussion of the errors involved in such a model is give.
The density of natural ice accretions related to nondimensional icing parameters
Authors: Jones Kathleen F.
Published: February 2007

The meteorological conditions under which atmospheric icing occurs determine the density of the accreted ice. A density relationship developed by Macklin from artificially iced samples is often used to calculate rime ice density as a function of R (effective droplet radius multiplied by droplet impact speed divided by icing surface temperature). In this study icing data collected in natural conditions at the summit of Mt Washington in New Hampshire were used to test Macklin's relationship. the Mt Washington Observatory has been making icing measurements using rotating multicylinders since 1969. Meteorological and icing data from these observations were used along with accretion weight and volume data from each of the six cylinders in the multicylinder set to relate ice accretion density to Macklin's R parameter. the relationship obtained was compared with Macklin's and with those developed by other authors who also related their measured rime densities to R. However, R is difficult to calculate and has been defined differently by various researchers. Therefore, a multiple regression analysis was performed to relate the ice accretion density to easily calculated nondimensional numbers describing the icing process. the inertia coefficient, ϕ parameter, and a heat flux ratio were used to predict rime accretion density. For a given cylinder diameter the cloud median volume droplet diameter and the wind speed were found to be the most important factors in controlling the ice accretion density.
Large‐eddy simulation of the up‐slope boundary layer
Authors: Schumanndlr Ulrich
Published: February 2007

Large‐eddy simulation (LES) is used to determine the turbulent structure of the steady‐state up‐slope boundary layer (UBL) which forms at a uniformly heated inclined or vertical plane surface below a stratified atmosphere at rest. At proper scales, the problem depends solely on the slope angle a and the surface roughness height z0 a relative to a length scale H which is a function of stratification and surface heating. We cover 2° ⩽ α ⩽ 90° and 3 × 10‐5 α ⩽ z0/H α ⩽ 3 × 10‐2 by a series of LESs. Simulations using different grids show that the results are only weakly sensitive to truncation errors. the UBL approaches a steady‐state mean‐profile by slowly decaying oscillations; its frequency equals the Brunt‐Väisälä frequency times sin α. In the LES, convergence towards steady state has been enforced by adjusting the fields according to integral steady‐state conditions. For α ⩽ 10°, a well‐mixed layer is formed which causes a strong temperature inversion and strong down‐slope flow at the outer edge of the UBL. Large‐scale coherence is weak with some indications of cross‐slope rolls for small inclination angles and longitudinal rolls for steep slopes. Several mean quantities have been tabulated as a function of slope angle and surface roughness and approximating power laws are given. the dependence of mean quantities on the slope angle is difficult to explain with simple models, but the influence of surface roughness closely follows earlier results.
Simulations of winter blocking episodes using observed sea surface temperatures
Authors: Kung Ernest C. , Dacamara Carlos C. , Baker Wayman E. , Susskind Joel , Park Chung‐Kyu
Published: February 2007

The formation of major northern hemisphere blockings during January 1979 is studied in numerical simulations of the global atomosphere with a sea surface temperature (s.s.t.) field updated with observations during the model integration. Both the standard 4 × 5 degrees latitude‐longitude and high‐resolution 2 × 2.5 degrees versions of the Goddard Laboratory for Atmospheres general circulation models (GCMs) are employed. the s.s.t. field is provided by a blend analysis of in situ and satellite‐retrieved data.

The simulations by the high‐resolution GCM with a realistic s.s.t. field of the ocean surface are shown to be capable of producing two successive realistic major blockings in the Pacific and Atlantic through a one‐month period. the skill in predicting the blocking formation as obtained in the simulations is due to improved skill in forecasting of ultralong waves. Although the results are encouraging, only one initial state is involved in this series of simulations; therefore, the results should not be generalized yet at this stage of investigation.
Field measurements of wet deposition in an extended region of complex topography
Authors: Dore A. J. , Choularton T. W. , Fowler D. , Storton‐West R.
Published: February 2007

A model designed to predict the enhancement of rainfall and the wet deposition of aerosol in complex terrain has been extended and compared with field data. the results of five separate field studies performed over an extensive region of complex terrain in northern England are presented and compared with the model predictions. It is found that the main features of the data are predicted by the model in widely varying conditions but the full details of a particular case‐study cannot be reproduced quantitatively owing to the complexities of the meteorology over the extended distances and collection periods.

The main physical processes controlling the deposition pattern have been established and it is concluded that a simple model of the seeder‐feeder process is able to give reasonable descriptions of deposition patterns in complex terrain both for individual episodes and for longer‐period averages.
Regimes in the wintertime circulation over northern extratropics. II: Consequences for dynamical predictability
Authors: Molteni Franco , Tibaldi Stefano
Published: February 2007

A classification of hemispheric flow patterns into clusters corresponding to circulation regimes is used to analyse the errors in geopotential fields predicted by the ECMWF models in the medium range during winter. It is found that the probability distributions of both the r.m.s. error over the northern hemisphere and the amplitude of large‐scale planetary waves are bimodal in the region of phase space where the fields have negative or small projection onto an EOF (empirical orthogonal function) that partially resembles the Pacific‐North‐American pattern, and where the separation between clusters is strongest.

In addition to a true systematic error, a regime‐dependent component exists in the error fields, whose pattern is opposite to the anomaly of the observed flow regime. In fact, the ECMWF model (especially its earlier versions) tends to relax the flow towards its most densely populated regime. These results are supported by an analysis of the frequencies of transitions between different clusters, and are interpreted on the basis of simple analytical solutions of the Fokker‐Planck equation for stochastically‐perturbed dynamical systems.

Statistics computed for different ECMWF models show that a considerable improvement in both the systematic and the regime‐dependent component of the medium‐range error occurred with the T106 model, owing principally to its improved physical parametrization. However, even though the bias towards states with low wave amplitude has been strongly reduced in the T106 model, a loss of predictability is still associated with transitions to regimes with large amplitude of planetary waves.
Tests of a scheme for regression retrieval and time‐space interpolation of stratospheric temperature from satellite measurements
Authors: Jackson D. R. , Harwood R. S. , Renshaw E.
Published: February 2007

A scheme to retrieve and analyse stratospheric temperature from satellite measurements is tested. Because of the lack of ‘ground truth’ in the stratosphere, the ‘true’ atmosphere is represented by an atmosphere simulated by a numerical model. Simulated observations are calculated by computing the radiance that would be observed from the ‘true’ atmosphere by a satellite instrument. the radiances are then retrieved and analysed and the resultant analyses compared with the corresponding ‘true’ fields. the tests are made using output from a day when a sudden warming was present. the retrievals are made by using a multiple linear regression model which regresses radiances against Planck function. the corresponding temperatures are then analysed on a grid using a linear time‐space interpolation scheme.

The retrieval scheme is seen not to perform so well within the area of the sudden warming as outside it. However, this may be expected as the vertical structure within the sudden warming is generally too small to be resolved by a satellite instrument. the analysis scheme analyses the stratospheric field well, even in the area of a sudden warming. the effect of varying the distance radius used to select observations for the scheme is also discussed.
Ice particle concentrations and precipitation development in small polar maritime cumuliform clouds
Authors: Rangno Arthur L. , Hobbs Peter V.
Published: February 2007

Increases in the concentrations of ice particles, as well as precipitation development, can proceed very rapidly in even quite small maritime cumuliform clouds. In the upper portions of these clouds, ice particle concentrations can increase from <1 per litre to >100 per litre in ∼10 minutes, even when cloud tops are warmer than −12°C. Just prior to the onset of the high ice particle concentrations in the upper regions of these clouds, a few liquid and frozen drizzle drops appear; these are followed almost immediately by high concentrations of largely vapour‐grown ice crystals. Subsequently, the ice particle concentrations gradually decline as the ice particles grow, aggregate and fall out. Crystal habits and sizes indicate that nearly all of the ice crystals encountered in the upper regions of the clouds form close to those regions.

Prior to the formation of high ice particle concentrations, the clouds generally contained, near cloud top, droplets >30μm diameter in concentrations >1cm−3, and often drizzle drops (100–400μm diameter) in concentrations >1 per litre. For maritime and continental cumuliform clouds with widths D ≥ 3km, the breadth of the droplet spectrum near the cloud top is a better predictor of the maximum ice particle concentrations that will develop in this region than is cloud‐top temperature.

Exceptions to the general picture described above are the very narrow (D<3km), ‘chimney‐type’, maritime cumuliform clouds with tops that quickly subside after reaching their maximum altitude. For cloud‐top temperatures between −7 and −13°C these clouds produce fewer ice particles (∼1–20 per litre), even though droplets with diameters >30μm are present in concentrations >1cm−3 and drizzle drops are often present in concentrations >1 per litre.

Several proposed mechanisms for the formation of high ice particle concentrations in clouds are discussed in the light of these field observations. In a detailed case‐study, ice particle concentrations produced by the ejection of ice splinters during riming were calculated to be about an order of magnitude less than the ice particle concentrations measured in the upper regions of the wider maritime clouds. The freezing of evaporating drops by contact nucleation could be responsible for the frozen drops that precede the high concentrations of vapour‐grown ice crystals. The latter could be produced in localized regions of unexpectedly high supersaturation that may be produced when drops begin to grow by collisions. This explanation is consistent with several of our findings concerning the conditions under which high concentrations of vapour‐grown crystals appear almost spontaneously in maritime clouds. Similarities between these natural crystals, and the conditions under which they appear, and those produced artificially by dry‐ice seeding and by aircraft flying through supercooled clouds are discussed.
On the parametrization of clouds containing water droplets
Authors: Jonas Peter
Published: February 2007

A one‐dimensional model of the growth of droplets by condensation in an entraining cumulus cloud is used to examine the sensitivity of the average spectrum at a given height to the cloud base temperature, liquid water content and condensation nucleus spectrum. It is shown that the equivalent radius of the droplet spectrum is dependent on the nucleus spectrum and cloud water content but is relatively insensitive to other properties of the cloud. The results are used to suggest a method of parametrizing the effects of cumulus clouds in radiative transfer calculations for use in atmospheric circulation models.
Atmospheric soundings from satellites—false expectation or the key to improved weather prediction?
Authors: Smith William L.
Published: February 2007

Temperature and water vapour soundings from satellites have been available for more than 20 years. Because of the global nature of these data, satellite soundings were expected to improve significantly global numerical weather prediction (NWP). Indeed, experiments with the satellite soundings provided during the Global Weather Experiment (GWE) in 1979 revealed the positive impact of these data on extended‐range forecasts, particularly in the southern hemisphere. Unfortunately, since the GWE, the value of satellite soundings has apparently diminished. I believe this to be due largely to improvements in the models which have surpassed the inherently poor vertical resolution of these data relative to that of the contemporary NWP models. The satellite data can even degrade the analysis/forecast operation through the aliasing of small‐scale features which may be properly represented by the vertical scale resolved by the model.

This lecture addresses, from a historical perspective, the dilemma of the decreasing value of satellite soundings in the weather‐forecast application (i.e., the reasons for the unfulfilled expectations of these data). Attention is then turned to the improvements in the satellite technology and the application techniques needed to reverse this trend.
On the effective roughness length for heterogeneous terrain
Authors: Vihma Timo , Savijärvi Hannu
Published: February 2007

Several theories and a mesoscale model were applied in the calculation of the effective roughness length Z over heterogeneous flat terrain having varying proportions of land and water. Rules based on terrain classification tended to give large values of Z, while the simple logarithmic areal average of local roughness lengths gave small values compared with the model output. The most recent theories were quite close to the model results. These theories were then used to produce a map of Z for 150 × 150 km2 grid squares over Finland, based on a detailed knowledge of land use and water coverage.
Measurements of evaporation from fallow Sahelian savannah at the start of the dry season
Authors: Gash J. H. C. , Wallace J. S. , Lloyd C. R. , Dolman A. J. , Sivakumar M. V. K. , Renard C.
Published: February 2007

Micrometeorological measurements of evaporation were made for the first six weeks of the dry season at a fallow savannah site in the Sahel. The evaporation fell from typically 4.5 mm per day at the start of the dry season to 1.5 mm per day six weeks later. The surface conductance was modelled in terms of empirical functions of time of day, and the number of days since the final rain of the season. It was found that there was little variation in surface conductance within each day, with no significant correlation with solar radiation and only a weak correlation with humidity deficit. Daily values of the surface conductance necessary to estimate the actual evaporation, from the data provided by a standard climate station, were also derived.
Lagrangian and K‐theory approaches in modelling evaporation from sparse canopies
Authors: Dolman A. J. , Wallace J. S.
Published: February 2007

An evaporation model based on Lagrangian turbulent diffusion principles is developed and compared with simpler single‐ and dual‐source K‐theory models of evaporation. The performance of the Lagrangian model is assessed against observations of total evaporation by an eddy‐correlation instrument and found to be satisfactory for a millet crop in west Africa. Three versions of existing simpler models containing K‐theory descriptions of within‐canopy turbulence are described and their results also compared. The two K‐theory models that explicitly take into acccount the soil source perform better than the single‐source Penman‐Monteith model. A Lagrangian analysis does not seem to be necessary for this kind of crop because the low source‐density profile of the crop, associated with a low leaf‐area index, caused the near‐field effect to be very small. The overall difference between the evaporation estimates of the dual‐source and Lagrangian models is therefore small. It is concluded that, for practical purposes, K‐theory remains an adequate approximation of turbulent transport in sparse‐crop evaporation models.

Significant anomalies in the surface heat fluxes, particularly those of sensible heat, accompanied the decrease in the sea ice concentration. Substantial atmospheric warming was simulated over and in the vicinity of areas in which leads were considered. In all but one experiment there were anomalous easterlies between about 40 and 60°S with westerly anomalies further to the south. The surface pressure at high latitudes appears to change in a consistent fashion with the fraction of open water, with the largest changes occurring in the Weddell and near the Ross Seas.

Some of the feedbacks which may enhance the responses here, but which are not included in our model, are discussed.
Bubble clouds and the dynamics of the upper ocean
Authors: Thorpe S. A.
Published: February 2007

This is a review of turbulence in the upper ocean close to the sea surface, particularly of the information that has been obtained from sonar observations of bubble clouds produced by breaking wind waves. These clouds provide tracers of the turbulent motions and are important, especially at high wind speeds, in the process of air‐sea gas transfer. the observations of bubble clouds are here related to other measurements of turbulence, particularly to direct measurements of currents and temperatures in lakes or at sea, and to laboratory studies. Some novel observations of bubble clouds and breaking waves, their frequency and relation to Langmuir circulation, are presented.

There is now emerging a pattern of clues that point to the dominance of breaking surface gravity waves as a source of turbulence to a depth below the surface of 0.04 to 0.2 times the wavelength of the dominant breaking waves, although the effect of swell has yet to be clarified. the relative depth appears to increase with increasing values of W,1/c, where W10 is the wind speed and c the phase speed of the dominant waves. Below this region the generation of turbulence may be dominated by shear‐stress or convection. Here, turbulence is generally similar to that in the atmospheric boundary layer. There are, however, coherent motions on the scale of the mixing layer that persist for periods of a few minutes to an hour or so, to which the transport of a large part of the momentum and heat fluxes can be attributed, and which strongly affect the dispersion of buoyant particles or bubbles. These motions deserve special study to establish their contribution to heat and momentum transport, and hence to determine if, or when, they should be specifically represented in models of the upper ocean devised, for example, to describe the dispersion of passive and non‐passive tracers or the air‐sea transfer of gases.
The accuracy of rainfall estimates by radar as a function of range
Authors: Fabry F. , Austin G. L. , Tees D.
Published: February 2007

High‐spatial‐resolution data were collected for nearby convective storms by using a volume scanning weather radar with 200m resolution, and for stratiform events by using a high‐resolution (20m) vertically pointing radar. Errors in the estimation of rainfall by the scanning radar, due to the radar beam intersecting or overshooting features such as the bright band, were simulated by using these high‐resolution images. It was found that these effects define a maximum useful range for the radar that is a strong function of the prevailing meteorological conditions (e.g. bright‐band height, intense cell diameter and height).

Sudden changes in bright‐band height over short times and the large scatter observed in its thickness limit the accuracy with which corrections for the vertical profile of reflectivity may be precalculated in stratiform rain events. A scheme for using a vertically pointing radar to produce these corrections in real time is proposed. the convective events show enough significant storm‐to‐storm variability that caution is required in extending the useful range of the radar too far.
The Boreal Forests and Climate
Authors: Thomas G. , Rowntree P. R.
Published: February 2007

The sensitivity of modelled northern hemisphere climate to modification of the snow‐covered surface albedo is investigated using the United Kingdom Meteorological Office (UKMO) general circulation model (GCM). the UKMO GCM is a global, primitive‐equation model with 11 layers in the atmosphere. Surface processes in the model are highly parametrized, with bulk aerodynamic formulation of the surface fluxes and a ‘bucket’ soil‐moisture accounting method. the experiment represents in a highly simplified fashion the role of the boreal forests in reducing the surface albedo under snow‐covered conditions. A parametrization of snow‐covered land was developed which allows the maximum albedo attainable with a snow cover to be prescribed as a function of vegetation type. In the standard version of the model the maximum snow‐covered surface albedo attainable is 0.60, which exceeds observed values for the forested areas of the northern hemisphere. the model was integrated twice, with different albedos representing forested and deforested conditions. the sensitivity of the heat and hydrologic budgets for the northern hemisphere and deforested areas is discussed. A detailed analysis of the deforested regions reveals systematic reductions in temperature of up to 2.8 K. Precipitation shows a systematic decrease in the affected regions. the largest decreases occur generally in the months with largest evaporation changes. For the case of no masking by forest vegetation (equivalent to boreal deforestation) the model produces a significant change in the pattern of snow‐melt. the removal of forest affects both the magnitude and the timing of spring snow‐melt, and consequently also the runoff. There are delays in snow‐melt‐induced runoff peaks by a month and increases in the magnitudes by on average 32%. the role of large‐scale advection is investigated by comparison with the results from a single‐column‐model experiment. A realistic representation of the snow‐covered surface albedo is evidently a requirement for simulations of the northern hemisphere climate.
Non‐Linear Three‐Dimensional Effects On Gravity‐Wave Drag: Splitting Flow and Breaking Waves
Authors: Miranda P. M. A. , James I. N.
Published: February 2007

A study has been made of some aspects of frictionless stratified flow past three‐dimensional isolated mountains. the study uses a three‐dimensional non‐hydrostatic numerical model to investigate the behaviour of the flow as a function of the Froude number, and produces a picture of the dependence of the gravity‐wave drag on the Froude number for a wide range of that parameter. At the same time, the results of the numerical experiments clarify the behaviour of the flow in the transition from high to low Froude number, showing the relative importance of wave breaking and flow splitting in the transitional regime.
Solar Radiative Transfer Through Clouds Possessing Isotropic Variable Extinction Coefficient
Authors: Barker Howard W.
Published: February 2007

Solar radiative fluxes were computed for heterogeneous clouds using an extension of the Monte Carlo method of photon transport which assumes that clouds possess isotropic variability. Hence, computation of fluxes for three‐dimensional (3‐D) clouds can be achieved with only an extended, characteristic one‐dimensional (1‐D) transect of extinction coefficient, β. These are easily obtained by aircraft measurements. In order to validate the new scheme, fluxes for 3‐D, stochastic multiplicative cascade clouds were computed by the conventional approach to Monte Carlo simulation. 1‐D transects through these clouds were then strung together and used in the new scheme. Not only do both methods of calculation yield statistically identical flux estimates, but they also produce virtually identical distributions of photon optical pathlengths and number of scattering events. Furthermore, the new technique may require an order of magnitude less computation time, depending on the desired level of accuracy.

Cloud microphysical data obtained by aircraft were used to represent characteristic transects of β, and corresponding fluxes were computed with the new scheme. Results suggest that internal inhomogeneity reduces cloud albedo below homogeneous values by about 5‐10% for overcast and isolated cubic clouds. Also, it is predicted that for overcast clouds of optical depth τ < (>) ≈ 40, inhomogeneous clouds absorb less (more) radiation relative to their homogeneous counterparts. Three individual and trivial modifications of a plane‐parallel, homogeneous two‐stream solution of the radiative‐transfer equation appear to be capable of capturing the essential effects of inhomogeneity. This is promising for incorporation of inhomogeneous cloud effects into existing climate‐model solar‐radiation routines. Little evidence, however, was found to suggest that the effects of inhomogeneity alone can explain the spectral aspects of the cloud absorption/albedo anomaly problem.
Impact of Cloud Longwave Radiative Surface Forcing On Interception Loss From A Coniferous Forest Canopy
Authors: Lockwood J. G.
Published: February 2007

Downward longwave radiation at the surface is greater in a cloudy atmosphere than in a clear one. This additional longwave radiative forcing at the surface has an average daily value in Britain of around 40 W m−2, which is small compared to observed fine‐weather shortwave radiative fluxes. However in British rainfall events, the intensity of the incoming shortwave radiation is normally around 100 W m−2 or less during the day and zero at night. A numerical experiment using a multilayer model of a coniferous forest is described that suggests that during typical rainfall under British conditions the additional cloud longwave radiative forcing has a significant impact on interception loss from coniferous forest canopies. Interception loss is reduced from about 31% of rainfall, with normal cloud longwave radiative surface forcing, to about 15% with the cloud longwave forcing removed. This result is confirmed using a single‐layer model of a coniferous forest over an 80‐day period.
Two paradigms of baroclinic‐wave life‐cycle behaviour
Authors: Thorncroft C. D. , Hoskins B. J. , McIntyre M. E.
Published: February 2007

Two idealized baroclinic wave‐6 life cycles examined here suggest a framework of opposite extremes (a) in which to view the behaviour of real synoptic‐scale disturbances in middle latitudes, and (b) in which to interrelate the synoptic and wave‐theoretic viewpoints, using the ‘saturation‐propagation‐saturation’ (SPS) picture of wave‐6 life‐cycle behaviour. The two life cycles. dented by LC1 and LC2, are higher‐resolution versions of the Simmons‐Hoskins ‘basic’ and ‘anomalous’ cases (showing strong and weak late decay of eddy kinetic energy, EKE). They illustrate, in varying degrees. Two extreme types of behaviour here disignated ‘anticyclonic’ and ‘cyclonic’, and epitomized by strongly contrasting upper‐air trough behaviour. ‘Anticyclonic’ behaviour dominates the late stages of LC1 and is characterized by backward‐tilted, thinning troughs being advected anticyclonically and equatorward, as in the commoner cases of planetary‐scale mid‐stratospheric ‘Rossby‐wave breaking’. ‘Cyclonic’ behaviour dominates LC2 and is characterized by forward‐tilted, broadening troughs wrapping themselves up cyclonically and poleward. producing major cut‐off cyclones in high latitudes. These morphologies are visualized by upper‐air maps of potential temperature on the nominal tropopause, defined as a constant‐potential‐vorticity surface. Some atmospheric mid‐latitude disturbances examined here, using the same visualization applied to operational analyses, show the same two extreme types of trough behaviour together with intermediate cases.

The SPS picture is re‐examined, using Eliassen‐palm and refractive‐index cross‐sections. It is shown, in particular, by reference to a wave‐activity theorem of Haynes, that the late stages of LC2 can be looked upon as a remarkably clear, and morphologically novel, large‐amplitude counterpart of the nonlinear reflection scenario of Rossby‐wave critical‐layer theory. The late stages of LC1, by constrast, look more akin to an nonlinear critical‐layer absorption scenario. LC2 exhibits region of largely undular PV contours adjacent to a nonlinear of irreversiby deformen PV contous. In the latter region PV rearrangement, and hence absorption of Rossby‐wave activity, has largely ceased. This accounts for the more persistent EKEE in the LC2 case.
Predictability and finite‐time instability of the northern winter circulation
Authors: Molteni Franco , Palmer T. N.
Published: February 2007

The finite‐time instability and associated predictability of atmospheric of atmospheric circulations are defined in terms of the largest singular values, and associated singular vectors, of the linear evolution operator determined form given equations of motion. These quantities are calculated in both a barotropic and a three‐level quasi‐geostrophic model, using as basic states realistic large‐scale northern wintertime flows that represent the climatological state, regime composites, and specific realizations of these regimes. for time‐invariant basic states, the singular vectors are compared with the corresponding normal‐mode solutions; it is shown that the perturbations defined (at the initial time) by the singular vectors have much larger growth rates than the normal modes, and possess a more localized spatial structure.

The regimes studied have opposite values of the Pacific/North American (PNA) index, and growth rates for the barotropic basis states appear to confirm earlier studies that the barotropic instability of the negative PNA states may be larger than the corresponding positive PNA states. The evolution of the singular‐vector perturbations, with emphasis on the vertical structure, is compared for time‐evolving and time‐invariant baroclinic basis states; the effects of nonlinearity are also discusses. It is shown that, in the baroclinic model, interactions between synoptic‐scale eddies in the time‐evolving basic state and in the perturbation field are fundamental for studying the predictability of transitions in the large‐scale circulation. Consequently results obtained from linear calculations using very smooth basic states cannot properly account for such predictability.

These results form the basis of a technique used to initialize ensembles of forecasts made with a primitive‐equation model, and are described in the companion paper (Mureau et al. 1993).
Atmospheric water‐vapour transport to Antarctica inferred from radiosonde data
Authors: Connolley W. M. , King J. C.
Published: February 2007

Data from 16 radiosonde stations are used to study the transport of water vapour in the antarctic atmosphere. Total column moisture (TCM), winds and moisture flux measurements are presented, together with an analysis of their reliability. Annual TCM values are similar—about 4 kg m−2—at all stations around the coast of East Antarctica but are much smaller on the East Antarctic Plateau. Over a period of 6 years the interannual variation (standard deviation) of the TCM is about 10% of the mean value.

At the coastal stations moisture fluxes reflect the predominantly zonal easterly flow in the lower troposphere, and their meridional components are generally small. As a result of interannual variations in the strength of the atmospheric circulation, and to a lesser extent in the TCM, interannual variability of the fluxes is high, suggesting that there may be large interannual variability in the precipitation over Antarctica.

From the data a water‐vapour budged for East Antarctica is computed. The annual accumulation rate obtained agrees surprisingly well with glaciological estimates. However, the uncertainties are considerable as a result of measurement errors and the representativity of the stations available. The conclusion is that the data are more suitable for evaluating the regional performance of circulation models from which systematic budget estimates may be derived.
Sensitivity of the Extended UGAMP General Circulation Model to the specification of gravity‐wave phase speeds
Authors: Jackson D. R.
Published: February 2007

Two simulations made using the Extended UGAMP General Circulation Model (EUGCM) are compared. The first uses a gravity‐wave parametrization scheme that only includes orographically forced gravity waves with zero phase speeds, whilst the second uses a similar scheme, except that it includes a gravity wave with a phase speed of 20 ms−1. Results from the first simulation show that wintertime westerlies are reasonably estimated, but summertime and equatorial easterlies are excessive owing to an absence of gravity‐wave drag in these region. This absence of gravity‐wave drag also means that the summertime mean meridional circulation is too weak, and summer mesospheric model temperatures are accordingly warmer than observations by up to 35 degC. Inclusion of gravity waves with phase speeds of 20 ms−1 reduces summer and equatorial easterlies and decreases summer mesophere temperatures because, unlike zero phase‐speed waves, these waves can propagate into the summer and equatorial middle atmosphere and thus of the 20 ms−1 gravity wave also improves the simulation of the semiannual oscillation both at the stratopause and at the mesopause.
Interannual variability of the stratospheric circulation in the southern hemisphere
Authors: Shiotani Masato , Shimoda Naoki , Hirota Isamu
Published: February 2007

The variability from year‐to‐year of the seasonal evolution in the southern hemisphere stratosphere over a period of 10 years, from 1980 to 1989, is investigated using the global geopotential height derived from the data from the Stratospheric Sounding Unit on board the TIROS‐NOAA satellites. As a measures of interannual variability, the variance of the zonal mean geostrophic wind over the 10 years was calculated for each day. Although the major warming hardly occurs in the southern hemisphere stratosphere, large variances can be observed during the winter and spring. The maximum variances appear at low latitudes in late autumn, move towards mid latitudes in midwinter and remain near 60°S in spring. The seasonal movement of the zonal mean westerly jet in the southern hemisphere stratosphere can be classified into two categories in terms of the location of the maximum westerlies at the 1 mb level in midwinter, namely

HLJ (high‐latitude‐jet) years with maximum westerlies around 50°S,

LLJ (low‐latitude‐jet) years with maximum westerlies around 40°S.

During the late winter the core of the westerly jet moves polewards earlier in HLJ years in LLJ years. In association with this earlier movement, the growth of the amplitudes of planetary waves 1 and 2 during the period from winter to spring occurs earlier in HLJ years than in LLJ years. In autumn, however, the wave‐1 amplitude, only for LLJ years, develops vigorously in the stratosphere and even the troposphere, before the seasonal movement of the zonal mean westerly jet branches off into the two categories. The appearances of HLJ and LLJ years occur in groups of a few years; this is quite different from the behaviour of the equatorial quasi‐biennial oscillation.
The onset of effective coalescence growth in convective clouds
Authors: Johnson David B.
Published: February 2007

This study examines the microphysical and environmental conditions necessary to support rapid coalescence growth in warm convective clouds. A quantitative threshold for effective coalescence growth can be defined in terms of the mass‐doubling times for growing precipitation embryos.
Cyclone behaviour response to changes in winter southern hemisphere sea‐ice concentration
Authors: Simmonds Ian , Wu Xingren
Published: February 2007

The sensitivity of climatological cyclone behaviour in the southern hemisphere to reductions in winter antarctic sea‐ice concentration is studies in general‐circulation‐model (GMC) experiments. An increase in the number of cyclones over most of the Weddell an Ross Seas is a response common to all cases of reduction of sea‐ice concentration from 100%. There is a tendency, particularly at lower ice concentrations, for more cyclones to be found in the latitude band of about 10 to 15° of latitude width centered on the antarctic coast, and a belt of fewer cyclones to the north. The structure of the changes in cyclogenesis assumes a much lesser zonally oriented form, and all experiments induce enhanced cyclogenesis in the western Weddell Sea. For the most part thee is an increase in cyclogenesis south of about 65°S, although some longitudinal sectors show up as regions of decreased cyclogenesis even through the sea‐ice concentration has been reduced there.

The intensity of the extratropical cyclones has been quantified as the difference between the Mean depth of the systems and the climatological average pressure for each particular sea‐ice configuration. Perhaps contrary to what might have been expected, this ‘relative’ central pressure of lows exhibits an increase over most of the sea‐ice area (i.e. the cyclones become weaker in a relative sense). In an earlier study it was found that the climatological response to imposed anomalies of sea‐ice concentration was that the lowering of the concentration of antarctic sea ice resulted in a lowering of sea‐level pressure, particularly close to the continent. The results presented here strongly suggest that the climatological decreases in pressure result essentially from increases in cyclone numbers as opposed to the possibility that the cyclones are more intense.

We have attempted to relate the regions of cyclogenesis with a measure of baroclinicity in the control simulation. We have also examined the relationship between changes in these parameters in response to anomalous sea‐ice concentration. Despite some similarity between theses, our study emphasizes the important role being played by other factors, including surface heat fluxes and topography, in these experiments. This study demonstrates the utility of GCMs in the study of high‐latitude cyclone sensitivity and suggests it is a valuable tool to be used in tandem with observational‐based analyses.