bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the zonal‐scale selection and vertical structure of equatorial intraseasonal waves
Authors: Wang Bin , Chen Jiukang 
Published: February 2007

AbstractObserved tropical intraseasonal oscillations exhibit a preferential zonal scale of wavenumber one or two and a baroclinic structure with largest flow field amplitude in the upper troposphere. Unstable interaction of boundary layer friction‐induced moisture convergence with condensational heating could cause the scale selection and result in the asymmetric baroclinic structure by means of coupling of vertical normal modes. A vertically continuous linear model describing two‐dimensional equatorial motion demonstrates this feedback mechanism and permits detailed study of vertical structure of unstable moist Kelvin waves.
Analytical model for the growth of the coastal internal boundary layer during onshore flow
Authors: Gryning Sven‐Erik , Batchvarova Ekaterina 
Published: February 2007

AbstractA model of near‐neutral and convective steady‐state internal boundary layer evolution is presented. the model deals with the internal boundary layer that forms over land in coastal and lake areas during onshore winds. Near the ground, the growth of the internal boundary layer is controlled by friction velocity in accordance with surface layer theory. Farther downwind, the growth is determined by the atmospheric stability and friction velocity within the internal boundary layer, and the temperature gradient in the air above. the wind profile inside the internal boundary layer is assumed to follow Obukhov similarity theory. an expression for the strength of the inversion that caps the layer is derived and used in the model. A comparison is carried out with independent experimental observations of internal boundary layer growth in the sea‐land transition.Kinematic heat flux through the top of the internal boundary layer is described by the formulation − \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {(w'\theta')} $\end{document}h = 0.2\documentclass{article}\pagestyle{empty}\begin{document}$ \overline {(w'\theta')} $\end{document}s + 2.5 uT/gh + the Zilitinkevich correction. the terms 0.2\documentclass{article}\pagestyle{empty}\begin{document}$ \overline {(w'\theta')} $\end{document}s and 2.5u T/gh account for convective and mechanical turbulence, respectively, and the Zilitinkevich correction is a turbulent kinetic‐energy storage term that ensures finite growth rate of the internal boundary layer near the ground. the relative importance of mechanical and convective turbulence as well as the Zilitinkevich correction is discussed. the Zilitinkevich correction dominates the growth process of the internal boundary layer when it is lower than roughly 50m. As the layer grows, the importance of the Zilitinkevich correction diminishes. Then mechanical turbulence dominates the growth process until the internal boundary layer has reached a height of approximately −1.4L. Further growth is controlled mainly by convective turbulence. Conditions of high wind speed and large values of the potential temperature gradient over water result in a deep zone where the growth of the internal boundary layer is controlled by mechanical turbulence. With near‐zero potential temperature gradient over water, the zone becomes shallow and may vanish.
Onset of sahelian drought viewed as a fluctuation‐induced transition
Authors: Demaree G. R. , Nicolis C. 
Published: February 2007

AbstractA statistical analysis of representative sets of rainfall data in the west Sahel region is performed and a jump transition in the late sixties is detected. It is suggested that the main features associated with the record can be accounted for by a fluctuation‐induced transition between two stable precipitation regimes: a ‘humid’ and a ‘dry’ state. A minimal model involving the dominant nonlinearities is constructed and its parameters determined from the data. an attempt is made to arrive at a statistical prediction of the duration of the alternate regimes through the relative stability of the two stable states as deduced from mean values and higher moments of residence times. the behaviour predicted by the model is in good qualitative agreement with the record.
On the cloud absorption anomaly
Authors: Stephens Graeme L. , Tsay Si‐Chee 
Published: February 2007

AbstractThis paper provides an overview of the subject of absorption of solar radiation by water clouds in the earth's atmosphere. the paper summarizes the available evidence which points to disagreements between theoretical and observed values of both cloud absorption and reflection. the importance of these discrepancies, particularly to remote sensing of clouds as well as to studies of cloud physics and earth radiation budgets, is emphasized. Existing cloud absorption and reflection measurements are reviewed and the persistent differences that exist between calculated and measured near‐infrared cloud albedos are highlighted. Various explanations for these reflection and absorption discrepancies are discussed and a simple outline of the theory of cloud absorption is provided. This outline is used to examine the large‐droplet hypothesis as well as the effects of absorbing aerosol and enhanced water vapour continuum absorption. A further hypothesis regarding the effects of cloud inhomogeneities is also examined. While the theory of cloud absorption is not completely understood, especially with regard to inhomogeneous clouds, the underlying conclusion of this paper points to the need for better measurements of solar radiation in clouds, water vapour absorption and microphysics properties of clouds.
The modelling problems associated with spatial averaging
Authors: Pyle J. A. , Zavody A. M. 
Published: February 2007

AbstractThe problem of spatial averaging in photochemical models of the stratosphere is considered. It is shown that correlation between reacting species, or between species and temperature, can lead to a bias in the model treatment of the reaction rate. Using satellite data it is demonstrated that for reactions controlling the ozone budget the resulting error is generally not large, although it may be appreciable during times of strong dynamical activity. Finally, we speculate that the errors could be more significant for species with strong gradients in the vertical, or for variability on a smaller scale than is resolved by satellite measurements. This could afford an explanation of some current modelling problems.
Some experiments to investigate the assimilation of SEASAT altimeter wave height data into a global wave model
Authors: Francis P. E. , Stratton R. A. 
Published: February 2007

AbstractA scheme for assimilating SEASAT altimeter wave heights into a global wave model, with the additional use of the collocated altimeter wind speeds, is reported. Control experiments are described in which the wave model was run with two sets of winds, one with SEASAT scatterometer wind data included in the NWP analysis and the other without, both for a period from 0000 GMT 15 September 1978 to 0000 GMT 20 September 1978. the results of forcing the wave model with these wind fields are compared with the altimeter wave heights. the differences are discussed in order to quantify errors in both the wave model and the altimetric data. Three experiments in which altimeter wave heights were assimilated are then described. the results of these are compared with the wave fields from the model runs without wave data assimilation. the assimilation experiments show that a significant improvement in model wave height is possible and that the impact of one day of assimilation is retained for at least five days.
The influence of soil wetness distribution on short‐range rainfall forecasting in the West African Sahel
Authors: Rowell D. P. , Blondin C. 
Published: February 2007

AbstractSurface moisture supply is known to be a factor which can be important in triggering convection. This paper aims to investigate its impact on short‐range numerical forecasts of tropical rainfall.The ECMWF (European Centre for Medium Range Weather Forecasts) operational forecasting model was integrated for 5 days with realistic positive and negative anomalies of the initial surface moisture distribution in the West African Sahel. the large‐scale flow is found to be relatively unaffected, but significant differences occur in the rainfall forecasts for about the first 4 days. Rainfall is greatest with a moister surface, except on days 3 and 4 in the south Sahel/Savanna region, for which a decrease of rainfall occurs. the opposite result is seen when an initially drier surface is introduced. the mechanisms involved are investigated by carrying out a detailed hydrological budget, and by studying the evolution of the atmospheric profiles of moisture and temperature.We conclude that an improved surface moisture analysis is likely to result in some improvement of shortrange rainfall forecasts in the Sahel, and presumably also in other tropical regions. In the coming years it should be possible to achieve this using satellite‐derived soil wetness maps.
The calculation of geopotential and the pressure gradient in the ECMWF atmospheric model: Influence on the simulation of the polar atmosphere and on temperature analyses
Authors: Simmons A. J. , Jiabin Chen 
Published: February 2007

AbstractThe spectral atmospheric model used for prediction at the European Centre for Medium Range Weather Forecasts (ECMWF) is modified to change the spectrally‐represented thermodynamic variable from temperature to the deviation of temperature from a reference profile which depends analytically on pressure. There is only a minor change to the form of the thermodynamic equation, and the calculation of the pressure gradient is modified to eliminate some of the cancellation between the Δ and RTΔInp terms which occurs in the standard formulation. At T42 resolution the revised scheme significantly improves southern hemispheric forecasts, with differences originating over Antarctica. There is also a small improvement over the Arctic. Sensitivity is much less at T106 resolution, but a minor advantage can still be seen in high southern latitudes, and there is a reduction in noise in the vertical velocity field near the Andes at tropical latitudes. Similar results are found for a version in which surface pressure, rather than its logarithm, is chosen as a spectrally represented prognostic variable. This gives mass conservation but the stability of the semi‐implicit time integration is somewhat reduced.The improvement seen at high latitudes is also captured by a simpler revision which retains temperature as the spectrally‐represented variable, and uses a reference temperature only in the computation of the pressure‐gradient terms in gridpoint space. Results indicate that much of the systematic difference in behaviour between T42 and T106 simulations at high latitudes can be removed by changing the pressure‐gradient calculation. In particular, the new schemes correct a systematic tendency for erroneously high pressures east of the Ross Ice Shelf over Antarctica in lower resolution simulations at medium and longer time ranges. Improvement occurs because the schemes reduce an inconsistent treatment of the Δ and RTΔInp terms in the model's semi‐implicit treatment of the vorticity equation.The reference temperature profile has also been used in data assimilation to reduce a systematic error in the calculation of first‐guess geopotential heights at standard pressure levels. The resulting height analyses agree slightly better with radiosonde measurements, and initialization causes less of a degradation of the fit to observed data. Temperatures are systematically warmer in the upper troposphere (by almost 0.5 K in the global mean at 200 hPa) and cooler in the lower troposphere, and are closer to observed values. Results from forecasts carried out after one and two days of assimilation show small improvements in the short and early medium range, and are inconclusive at longer time ranges.
Boundary‐layer moisture regimes
Authors: Mahrt L. 
Published: February 2007

AbstractData from fifty‐two aircraft flight legs 100−150m above ground from HAPEX and FIFE are analysed to estimate characteristics of boundary‐layer moisture fluctuations in boundary layers with different bulk stability and surface energy regimes. In HAPEX, considerable effort was devoted to the quality of measurement of moisture fluctuations. The data include repeated 120km flight legs flown at 150m over the same relatively homogeneous terrain, allowing statistical examination of motion characteristics on horizontal scales up to 10km.This study finds two prototype boundary‐layer regimes. With significant boundary‐layer instability, relatively weak surface evaporation and drier air aloft, the entrainment‐drying boundary layer may develop. This boundary layer is characterized by vertical divergence of the moisture flux and significant diffusion of dry air from the top downwards (top‐down). In this type of boundary layer, dry air occasionally reaches the lower boundary layer, leading to negative moisture skewness in spite of positive temperature and vertical velocity skewness associated with warm moist updraughts. In contrast, the moistening boundary layer associated with greater surface evaporation is characterized by positive moisture skewness near the surface.In addition to the above turbulent moisture fluctuations, some of the above data are characterized by 10km moisture variations with horizontal gradients often concentrated in narrow zones of horizontal convergence. Since corresponding signatures of vertical velocity and temperature are weaker, these zones are referred to as ‘mesoscale’ moisture fronts. As a more general feature, moisture variations on scales of 5km and greater are negatively correlated with temperature variations associated with cool moist regions and warm dry regions. On scales of tens of kilometres and greater, such negative correlation may be due to inhomogeneity of the surface energy budget. The negative moisture‐temperature correlation leads to large mesoscale variations of relative humidity and the lifted condensation level.
Pressure drag and momentum fluxes due to the Alps. I: Comparison between numerical simulations and observations
Authors: Hoinka Klaus P. , Clark Terry L. 
Published: February 2007

AbstractA three‐dimensional, non‐hydrostatic, anelastic model using interactive grid nesting is employed to simulate the airflow over and around the Alps during a strong foehn event on 8 November 1982. In the model, a single upstream sounding is used to initialize the mean flow. This single sounding was obtained from the output of a three‐dimensional objective analysis of rawinsonde data. The model results show that in low levels the airflow is forced around the Alps, in particular at its western edges, whereas above 700 hPa the flow is over the Alps. Above northern Italy a convergence line between the easterly flow and the foehn forced by the Apennine Mountains is simulated in accordance with observations. To the west of Milan vertical vortices are simulated in agreement with the complex circulation observed in this region.The simulations show that there is significant horizontal variability in the local vectors of the pressure drag and the momentum flux. This variability complicates the comparison between model and observational data as well as restricting one's ability to extrapolate accurately local cross‐sectional observations of pressure drag and momentum flux to values representative for the entire Alpine complex. Unfortunately, there were no measurements of surface pressure drag for the entire Alps on 8 November 1982. However, measurements during ALPEX during four foehn events obtained meridional and zonal values of −4.3 and 1.8 × 1011 N. The current values of −4.7 and 1.7 × 1011 N obtained for the simulations of the 8 November 1982 case using 5 km horizontal resolution are very close to the previous observations. There were observations on a cross‐section between Vicenza and Munich on this day. The values obtained were −0.67 × 106 N m−1 for the surface pressure drag and −0.75 × 105 N m−1 for the momentum flux averaged between 5 and 10 km above mean sea level. The simulated values for the same cross‐section were −0.62 and −0.50 × 106Nm−1 for the pressure drag and momentum flux, respectively. Overall, the averaged simulated meridional momentum flux between 5 and 10 km was −2.2 × 1011 N which is about 47% of the surface pressure drag. These results and comparisons with observations suggest that while the model appears to predict reasonable values for the surface pressure drag the amplitude of the momentum flux values is too large. The simulated values are, for example, six times those observed for the Vicenza to Munich cross‐section. Simulations using even finer resolution over a reduced region suggest that poorly simulated dissipative forces by the model may be responsible for some of this discrepancy. Some other factors such as surface friction are also discussed in the text.
Size‐resolved measurements of cloud droplet deposition velocity to a forest canopy using an eddy correlation technique
Authors: Beswick K. M. , Hargreaves K. J. , Gallagher M. W. , Choularton T. W. , Fowler D. 
Published: February 2007

AbstractTwo case studies are presented of the direct measurement of cloud droplet fluxes to a forest canopy by the eddy correlation technique. Two instruments have been used for the liquid water measurements, and close agreement for values of liquid water content and liquid water flux have been obtained. The case studies were conducted in different weather conditions: case 1 with high values of liquid water content (typically 0.3 g m−3) and low wind speeds (2–3 m s−1), and case 2 with low liquid water content (0.1 g m−3) and higher wind speeds (typically 6 m s−1). In both cases the deposition velocity for liquid water was markedly less than for momentum. For case study 2 the cause of the small deposition velocity for liquid water was the small droplet size; the typical modal radius was 4.4 μm. The deposition velocity increased as a function of droplet size, exceeding that for momentum for droplets larger than about 6 μm. When normalized, the results agreed reasonably well with those obtained over grassland using the gradient technique. The results are compared with the predictions of the models of both Lovett and Slinn.
Validation of a regional atmospheric model over Europe: Sensitivity of wintertime and summertime simulations to selected physics parametrizations and lower boundary conditions
Authors: Giorgi Filippo , Marinucci Maria Rosaria 
Published: February 2007

AbstractThis paper analyses wintertime and summertime simulations over Europe using the National Center for Atmospheric Research (NCAR) regional atmospheric model (MM4). The sensitivity of the model to selected physics parametrizations (explicit moisture scheme; slower release of condensation heat; horizontal diffusion on s̀‐surfaces), and lower boundary conditions (sea surface temperature; initial soil moisture; snow cover) is examined. The simulation periods are January 1979 and June 1979; initial and lateral meteorological boundary conditions are provided by analyses of observations. The main focus of the analysis is on prediction by the model of surface air temperature and precipitation, but also sea‐level pressure and upper‐air variables are considered. In general, the model reproduces the main features of the synoptic events which prevailed during the two simulation periods. Mid‐tropospheric and upper‐tropospheric biases in the model are generally small. Surface maximum and minimum air temperature biases do not exceed a few degrees K over various regions of Europe. The largest model bias (2‐3 K) is found for summertime minimum temperature. Wintertime surface temperatures are sensitive to the precipitation parametrizations tested, as these affect cloud formation and, in turn, the surface radiative fluxes. Also, temperature biases in the driving large‐scale fields are partially transmitted to the surface air temperature calculations. Summertime temperatures are sensitive to the soil moisture content. Sea surface temperature variations influence land temperatures more strongly for wintertime than for summertime. Precipitation is sensitive to the parametrizations used. The explicit moisture scheme generally induces underprediction of precipitation. When this is not used, the biases are of the order of −10 to −20% of the observations. Summertime precipitation over the highest Alpine regions is overpredicted, except when diffusion of moisture and temperature along terrain‐following s̀‐surfaces is strongly reduced. Sea surface temperature variations substantially affect precipitation, especially over coastal areas, both in January and June. Summertime precipitation is significantly modified by the initialization of soil water content.
Baroclinic Waves Propagating From A High‐Latitude Source
Authors: Watterson I. G. , James I. N. 
Published: February 2007

AbstractA time‐dependent primitive‐equation model of the southern hemisphere winter troposphere with an idealized dome‐shaped continent centred at 80°S has been used to assess the potential of Antarctica to force stationary waves and to determine the location of the mid‐latitude baroclinic storm track. Several integrations of the model for two hundred days were made with the zonal‐mean fields fixed at the climatological values. In the time mean of each run an anticyclonic wave response formed over the mountain and a planetary wave propagated into the mid latitudes with substantial amplitude. This suggests that much of the observed stationary asymmetry in the southern, mid and high latitudes may be due to Antarctica.The response to the Antarctic forcing was different in a steady, linear version of the model. However, interaction between the steady waves appears to be of minor importance to the time‐mean pattern. Rather, E‐vector fields point to a substantial effect of the transients on the pattern. In particular, the low‐frequency transients appear to be important in the balance of the time‐mean equations near the mountain.
Enhanced Absorption of Solar Radiation By Cloud Droplets Containing Soot Particles In Their Surface
Authors: Chylek Petr , Hallett John 
Published: February 2007

AbstractLaboratory studies show that when soot particles from acetylene combustion are captured by drops of size 0.1 to 1 mm they remain partially submerged in the surface for several hours. the intensity of solar radiation passing through a drop is calculated to be much greater near the surface, and therefore with such particles on the surface will give enhanced absorption compared with the situation in which the particles are uniformly distributed throughout the drop volume. This gives a possible explanation for enhanced cloud absorption when soot particles are present in amounts consistent with field measurements.