bluuebunny/crossref_metadata_2025_split huggingface dataset:
Frontal circulations in the presence of small viscous moist symmetric stability and weak forcing
Authors: Xu Qin 
Published: February 2007

AbstractThe 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

AbstractTurbulence 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

AbstractAn 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

AbstractThe 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

AbstractLarge‐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

AbstractThe 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

AbstractA 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

AbstractA 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

AbstractA 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

AbstractIncreases 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

AbstractA 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

AbstractTemperature 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

AbstractSeveral 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

AbstractMicrometeorological 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

AbstractAn 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.