bluuebunny/crossref_metadata_2025_split huggingface dataset:
Development of a revised longwave radiation scheme for an atmospheric general circulation model
Authors: Slingo A. , Wilderspin R. C. 
Published: February 2007

AbstractThis paper describes the development of a new version of the longwave part of the radiation scheme used in the Meteorological Office 11‐layer atmospheric general circulation model. The work was initiated when comparisons with other radiation schemes revealed systematic errors in the longwave fluxes. A detailed investigation of these errors was carried out by examining the sensitivity of the scheme to many of the details of its formulation. Various improvements were made, notably to the treatment of the temperature dependence of the Planck function and the water vapour continuum absorption. It is demonstrated that the fluxes are extremely sensitive to the formulation of the continuum absorption and suggested that more observations are needed to refine the values of the absorption coefficients. Special attention was paid to the treatment of carbon dioxide absorption, as the model is being used to continue research on the effect on climate of increasing CO2 concentrations. A simple scaling of the absorber amounts is applied in order to include the temperature dependence of the CO2 absorption. It is demonstrated that fluxes from the new version of the scheme and the sensitivity of the fluxes to changing CO2 concentrations in realistic atmospheres are in good agreement with those from more detailed models.
A simple model of the tropical atmosphere driven by a circulation‐dependent forcing
Authors: Weare Bryan C. 
Published: February 2007

AbstractA linearized shallow water model is forced by a circulation‐dependent heating term. This heating is derived from a consideration of the moisture budget equation and includes terms representing vertical motion, horizontal advection and evaporation. The model is iterated to achieve steady states. The impact of various terms of the heating expression to initial surface layer specific humidity perturbations near 2.5°N and 22.5°S is explored for a hypothetical mean specific humidity field representing the tropical Pacific Ocean. The vertical motion and advection terms in the heating equation lead to a smaller scale of heating than the initial perturbation. The evaporation term tends, on the other hand, to maintain or increase the spatial scale of the heating. For the initial perturbation at 2.5°N the final heating region is often shifted westward of its original position but remains at the same latitude. The 22.5°S perturbation leads to a heating zone equatorward of the original perturbation. The possible impact of these processes on El Nino theories is briefly discussed.
An observational study of the structure of stratiform cloud sheets: Part I. Structure
Authors: Nicholls S. , Leighton J. 
Published: February 2007

AbstractResults from six flights with an instrumented aircraft in marine, stratiform cloud‐topped boundary layers are presented. The flights took place around the U.K. in a variety of conditions, both during the day and at night. The data comprise simultaneous, high resolution turbulence, cloud microphysics and radiation measurements.On five of the flights, turbulent mixing in the cloud layer was found to be primarily maintained by convection resulting from cloud top radiative cooling. In four cases, this mixed layer was observed to be decoupled from the surface, i.e. the cloud‐induced mixing did not extend down to the surface, and the cloud formed part of a detached turbulent layer. One flight in high surface wind conditions displayed a significantly different structure. Possible reasons for this are suggested.The mean microphysical features of the different cloud layers are compared, as are the measured and theoretically calculated radiation fluxes. The latter show good general agreement, but some systematic differences are apparent. Again, possible causes of these disparities are discussed.Features of the turbulent velocity field are presented and a number of similarities between data from the convective cases and cloud‐free boundary layers driven by heating from below are found. Values of the entrainment velocity are estimated directly from the measured cloud top water fluxes in all but one case, together with an estimate of the errors incurred. The measured vertical turbulent fluxes are found to be reasonably well predicted by diagnostic mixed‐layer models using the measured boundary conditions as long as the heights of the mixed‐layer boundaries are also specified from observations. Such a model is also used to help explain the main observed features of the mixed‐layer structure and the differences between the various cases.
Sea‐ice and the antarctic winter circulation: A numerical experiment
Authors: Mitchell J. F. B. , Hills T. S. 
Published: February 2007

AbstractA numerical experiment has been conducted to test the sensitivity of a global general circulation model to changes in sea‐ice extents in the antarctic during winter. Three 112‐day integrations have been made in which all the antarctic sea‐ice poleward of 66°S was removed, commencing on 10 June in the second, third and fourth years of a control integration. There was a large increase in sensible heat flux over the anomaly, a warming over the antarctic confined to the lowest atmospheric layer, and a 2ms−1 reduction in the westerly flow around the periphery of the (new) sea‐ice margin, in broad agreement with a previous study by I. Simmonds. The increased heating over the anomaly was accompanied by a decrease in surface pressure which was not found by Simmonds, and possible explanations of this discrepancy are considered. The results are related to previous work on the effect of increased CO2 using prescribed changes in sea surface temperature and sea‐ice extents. The implications of the results for the parametrization of the heat flux through sea‐ice cover, and for coupled ocean‐atmosphere models are discussed.
The water vapour budget of the stratosphere studied using LIMS and SAMS satellite data
Authors: Jones R. L. , Pyle J. A. , Harries J. E. , Zavody A. M. , Russell J. M. , Gille J. C. 
Published: February 2007

AbstractMonthly zonal mean observations of H2O and CH4 made by the limb infrared monitor of the stratosphere (LIMS) and the stratospheric and mesospheric sounder (SAMS) instruments on Nimbus 7 have been used to investigate whether the H2O mixing ratios in the stratosphere are consistent with a source via the oxidation of CH4. While both sets of data show considerable seasonally varying structure, total hydrogen (neglecting molecular hydrogen) is relatively featureless with a mean value over the stratosphere of 6.0±0.35 p.p.m.v. (1σ) for the five‐month period studied. The uniformity of the total hydrogen fields points to the validity of the CH4 oxidation hypothesis.The derived fields of total hydrogen are used to deduce a mean H2O mixing ratio for air as it enters the stratosphere of 2.7±0.35 p.p.m.v. (1σ) from which a desiccation temperature may be deduced (for example. −87.2°C at 60 mb).
The skill of dynamical long‐range forecasts, including the effect of sea surface temperature anomalies
Authors: Mansfield D. A. 
Published: February 2007

AbstractThe Meteorological Office 5‐level general circulation model has been used to investigate the skill of wintertime, long‐range (50‐day) forecasts. In spite of its low vertical resolution and relatively simple parametrization (by present day standards), its winter climatology is shown to be realistic not only in terms of the time‐mean flow but also in the behaviour of transients. In order to eliminate the less predictable synoptic scales, 15‐day means have been used to assess the skill at longer ranges. Even with climatological sea surface temperatures, the largest‐scale features of the flow are found on average to have a small but significant correlation with the real atmospheric evolution out to about 20 days. However, the skill is far from uniformly distributed between the 17 forecasts studied. Three forecasts exhibited some skill throughout the 50 days, while in five forecasts the anomaly correlation fell to zero within 10 days. The use of observed tropical sea surface temperatures improved the forecasts in four out of five years. Examination of the model response to tropical sea surface temperature anomalies shows that it is particularly sensitive to anomalies in the west Pacific and to a lesser extent the west Atlantic. In this region anomalies as small as 1 K or less, similar to the uncertainty in their measurement, can have marked mid‐latitude effects.
On the scale of baroclinic instability in deep, compressible atmospheres
Authors: Read P. L. 
Published: February 2007

AbstractA simple model of linearized, inviscid baroclinic instability in an adiabatic, hydrostatic, compressible atmosphere of arbitrary (though finite) depth, based on the well‐known Eady model, is used to investigate the variation of growth rates and favoured horizontal length scales as functions of δ, the ratio of the model depth D to the density scale height Hs. Both geometric height coordinates (with w = 0 horizontal boundary conditions) and log‐pressure (with ω = 0 boundary conditions) are considered. For δ > 3 and a given zonal velocity scale, growth rates are significantly reduced relative to comparable instabilities in an incompressible fluid (δ = 0), and may be suppressed altogether in a laterally‐bounded channel for large enough δ at a given value of static stability. Where instability does occur, the length scales favoured are longer than for an incompressible fluid, and are generally comparable to a deformation radius based on D (rather than Hs). The relationship between these results and those obtained in comparable recent studies (which have found scales comparable to a deformation radius based on Hs to be important) is examined. Some implications for the role of baroclinic instability in the atmospheres of the major planets are also discussed.
Simulations of linear and nonlinear disturbances in the stratosphere
Authors: O'Neill A. , Pope V. D. 
Published: February 2007

AbstractThe aim is to study and contrast the dynamics of linear and nonlinear, large‐scale disturbances generated in the stratosphere by a localized perturbation in the troposphere. The perturbation is applied at the lower boundary of a primitive equation model of the stratosphere and mesosphere. The experiments are diagnosed using a range of dynamical quantities; in particular isentropic maps of Ertel's potential vorticity reveal where and how the stratospheric disturbance becomes nonlinear.The simulations are contrasted with an idealized theoretical model of a nonlinear critical layer, and the relevance for our experiments of ‘wave breaking’ and irreversible mixing of potential vorticity is discussed. Apparent polar focusing of Eliassen‐Palm fluxes is related to localized changes in a widening ‘buckling zone’. Wave amplitudes are far too large to apply simple ideas of wave propagation.The notion of pre‐conditioning as it has been applied to the stratosphere is challenged; there are occasions when it is an oversimplification to regard the troposphere as a wavemaker forcing changes in the stratosphere according to the tenets of wave, mean‐flow theory. Preceding a major warming, the flow is asymmetric about any axis (even in the much‐studied case of February 1979), and strong warmings develop through the nonlinear interaction of large, deep vortices associated with a tropospheric asymmetry. The nonlinearity of the flow is also manifested in an anticorrelation in time between zonal harmonics one and two over a deep layer, and by a phase shift of harmonics as amplitudes change. Associated changes in zonal mean winds give the appearance of distinct minor and major warmings, though a synoptic view allows systematic changes to be seen which caution against dividing the evolution into separate events.
Baroclinic waves and frontogenesis with an embedded zone of small moist symmetric stability
Authors: Xu Qin 
Published: February 2007

AbstractThree‐dimensional semigeostrophic moist baroclinic waves and frontogenesis are studied both analytically and numerically. It is found that a slantwise zone of small (either unconditional or quasi‐conditional) moist symmetric stability embedded in a dry and baroclinic basic state traps the energy of baroclinic waves, forming a strong meridional structure with maximum wave amplitude in the moist zone and exponential decay of the wave amplitude away from the moist zone. The unstable waves absorb energy from both the moist and dry baroclinic basic state, but the strongest energy conversion is in the moist zone, so growth rates decrease with the width of the moist zone. As the moist zone becomes infinitely narrow (wide) the model degenerates into a dry (moist) Eady model. The smaller the moist symmetric stability, the larger the growth rate and the wavenumber of the most unstable wave. The valid range of the model parameters depends on the semigeostrophic approximation, which sets a limit to the smallness of the moist symmetric stability.In semigeostrophic space, the moist zone changes shape due to geostrophic deformation. As the wave develops in real space, the surface pressure low (high) shifts northward (southward) due to geostrophic deformation, tightens (expands) due to ageostrophic convergence (divergence), and goes on to form a coupled warm‐cold frontal system.A numerical model of the semi‐Lagrangian finite element method is developed. The method is shown to be accurate in computing wave growths and efficient in resolving frontal structures, although the semigeostrophic approximation is found to become poor or even invalid locally in the narrow (meso‐β‐scale) cold frontal region.
The seasonal evolution of planetary waves in the southern hemisphere stratosphere and troposphere
Authors: Randel William J. 
Published: February 2007

AbstractSeasonality in planetary wave structure in the southern hemisphere stratosphere and troposphere (1000‐1 mb) is studied based on eight years (1979–1986) of daily geopotential data. Emphasis is placed on the distinctive seasonality in the stratosphere, along with tracing the stratospheric fluctuations into the troposphere. Time filtering is used to separate stationary and transient components. Maximum geopotential height variance is observed in the middle to upper stratosphere in late winter‐spring, with a distinct secondary, smaller maximum in late fall‐early winter; these maxima sandwich the strongest mid‐winter zonal winds. This sequence occurs approximately one month earlier in the upper stratosphere. Two corresponding peaks are observed in stratospheric zonal mean wind and temperature fluctuations. The two maxima in stratospheric wave variance are not in agreement with observed seasonal changes in quasi‐geostrophic refractive index of the zonal mean flow.The stratospheric geopotential variance maxima result from nearly equal amounts of stationary and transient zonal wave‐1 fluctuations, with transient wave 2 also contributing during late winter. Stationary wave 1 shows intriguing seasonal evolution: the early winter maximum is predominantly equivalent barotropic, whereas that of late winter‐spring is highly baroclinic in the stratosphere. Stratospheric poleward fluxes of heat and momentum both exhibit pronounced maxima during September‐October; these result primarily from the stationary wave‐1 contributions, and the transients assume a secondary role. Although not as pronounced as the stationary wave seasonality, seasonal differences in transient wave behaviour are also documented, including three‐dimensional propagation characteristics and wave‐mean flow interaction diagnostics.
Dynamical analyses of marine atmospheric boundary layer structure near the Gulf Stream oceanic front
Authors: Wai Mickey Man‐Kui , Stage Steven A. 
Published: February 2007

AbstractThe effects of the sea surface temperature (s.s.t.) front at the edge of the Gulf Stream on the marine atmospheric boundary layer (MABL) are investigated using a numerical model to study the modification effects of an oceanic front on the MABL structure. The situation simulated is flow from over cold shelf water to over the warm water of the Gulf Stream. The initial temperature and humidity profiles of the air are specified to be near neutral over the cold water and are therefore typical of undisturbed conditions. The differential in s.s.t. across the oceanic front creates a horizontal variation in the surface perturbation pressure and the stability. The surface perturbation pressure and turbulent fluxes modulate the flow and produce horizontal variations in horizontal wind components with associated vertical motions. A thermally direct cell is produced as a result of the s.s.t. difference across the front. The isotherms slope upward towards the warm water. Entrainment of inversion layer air and upward vertical motion over the warm water cause the MABL to be deeper there. A layer of cloud forms over warm water and is associated with mixed layer deepening rather than lowering of the condensation level. Turbulent fluxes in the MABL show considerable spatial variation. Surface stress is much larger over the front and over the warm water than over the cold water. This is mostly caused by wind speed changes associated with the front. Changes in the drag coefficient due to changes in surface roughness and stability are much less important.Mean budgets for temperature and total water indicate that there is a balance between horizontal advection and turbulent flux divergence. The U momentum budget shows that once the geostrophic balance terms are subtracted, the balance is mainly between the pressure gradient force associated with the induced temperature field and turbulent friction, with horizontal advection and the Coriolis force acting on the geostrophic departure playing minor roles. The V momentum budget shows a balance between horizontal advection, Coriolis force and friction.Although there are few data for comparison, the results are in qualitative agreement with observations in the area. This study shows that the s.s.t. front at the Gulf Stream edge produces marked local changes in the nearby atmospheric surface layer.
A study of topographically induced multiple equilibria and low‐frequency variability. I. Idealized topography
Authors: Mak Mankin 
Published: February 2007

AbstractIt is shown that both the steady and the vacillatory states of a two‐layer, quasi‐geostrophic, forced dissipative topographic model with a high spectral resolution can be truly intransitive. There exists only an odd number of equilibria in this geophysical system, as predicted by Benjamin's general theorem for the Navier‐Stokes equation. The overall findings constitute a generalization of previous results for a barotropic model.The analyses specifically establish for the case of a single‐wave topography that, in response to a barotropic forcing, there exist: (1) a low threshold value of the topographic height required for the existence of hysteresis: (2) a higher threshold value for the occurrence of weakly vacillating states with a high frequency; (3) a detuning effect of the nonlinearity due to an increase of the equivalent resonance frequency; (4) a baroclinization effect of an asymmetric friction; and (5) a robustness of the multiple equilibria for an additional weak baroclinic forcing.Under near‐resonance conditions of the topographic waves for the case of a simple multi‐wave topography, as defined according to the linear dynamics, none of the equilibria is stable. The strong wave‐wave interaction in such a situation sustains a dynamically different pronounced vacillation with a dominant long period in the order of tens of days. The existence of multiple vacillations with distinctly different structure and period is established. Outside that range of parametric conditions, there is only one stable equilibrium state.It is demonstrated that the time‐mean wavy flow of a decidedly nonlinear time‐dependent response can be reproduced, in a high degree of resemblance, by tuning the dissipation parameters in a counterpart linearized model that makes use of the ‘known’ mean zonal flow. It would be misleading, however, to conclude, on the basis of such an agreement, that the linear dynamics really accounts for the actual time‐mean wavy flow. This example serves to reveal the potential dangers in interpreting an observed mean flow as consisting of linear planetary waves on a pre‐determined zonally symmetric basic state.
Description and performance of an hourly nowcasting and very short‐range forecasting system
Authors: Heijboer L. C. , Timmerman H. , van der Hoek A. 
Published: February 2007

AbstractSince the disaster in The Netherlands caused by a mesoscale gale on 12 May 1983, an hourly nowcasting and very short‐range forecasting system has been developed using all available hourly surface observations of pressure and wind over the North Sea and the surrounding countries.Hourly analyses and prognoses are produced using hourly interpolated background fields and lateral boundaries from the fine mesh United Kingdom prognoses, which are received from Bracknell twice a day based on 00 and 12 GMT up to 36 hours in advance at 3‐hour intervals.The computational area consists of a grid of 35 × 35 points with a grid distance of 50 km covering the North Sea and surroundings. The update and forecast model is a two‐parameter model based on filtered equations with the geopotential height fields at 500 and 1000 mb as input and output. It is called VIMOLA (vertically integrated model on limited area). A new method is used for the analysis at 1000 mb. The observations of pressure and wind are directly assimilated onto the computational grid, and in this way wind measurements are converted into pressure gradients. All 10‐minute‐mean winds are taken into account. The analysis system is called APW (assimilation of pressure and wind).Every hour analyses and prognoses are produced up to nine hours in advance. The prognoses are merged with the fine mesh U.K. prognoses, i.e. the final prognoses are a linear combination of VIMOLA prognoses and U.K. prognoses. For one hour in advance no merging takes place and after nine hours the final prognoses are identical to the U.K. prognoses.For The Netherlands and surrounding regions winds computed from the forecasts have been verified against the observations. Winds from the U.K. prognoses have been verified in the same way. The U.K. prognoses show a very good quality and a small error growth during the first 24 hours. However, the hourly incorporation of the observations gives an improvement, especially for the first hours of the forecast period.Because no upper air observations are needed, a very short data cut‐off time can be applied (20–45 minutes after the time of observation). In practice, the system is suitable for operational gale warnings and tidal surge forecasts. Due to the hourly character, the system is able to detect temporal variations in the surface wind field such as the diurnal cycle over land and air mass variability. Practical examples are given in this paper, including some results obtained for the storm of 15–16 October 1987, which hit the south‐eastern part of England.