bluuebunny/crossref_metadata_2025_split huggingface dataset:
Solar radiation anomalies caused by the El Chichon volcanic cloud: Measurements and model comparisons
Authors: Rao C. R. Nagaraja , Takashima T. 
Published: February 2007

AbstractThe eruption of the volcano El Chichon (17.33°N 93.20°W) in southern Mexico over the period 28 March to 4 April 1982 introduced massive amounts of sulphur dioxide into the stratosphere. The resulting sulphuric acid particulate cloud has affected appreciably the received solar radiation at the surface; measurements made at several locations in the continental United States and Hawaii have indicated that the directly transmitted solar radiation was depleted by up to 25%; diffuse sky radiation was enhanced by factors of up to 3; and global radiation was lowered by up to 5%. These reported anomalies are compared with the results of radiative transfer computations in a realistic model of a cloud‐free, vertically inhomogeneous, plane‐parallel turbid atmosphere. The model yields the observed trends in the direct, diffusely transmitted and global solar radiation under normal and anomalous conditions.
Toward atmospheres without tops: Absorbing upper boundary conditions for numerical models
Authors: Rasch Philip J. 
Published: February 2007

AbstractThe appropriate upper boundary condition (UBC) formulation for the dynamical equations used in atmospheric physics is discussed in terms of both theoretical and computational aspects. The previous work on the UBC formulation is reviewed in the context of a linear mid‐latitude primitive equation (PE) model. A new technique for constructing the UBC is introduced. The technique depends upon the existence of analytic solutions to simplifications to the equations of motion. These analytic solutions are used to construct the exact radiation UBCs (often used in tidal theory and studies of the upper atmosphere) which are non‐local in time and space. Approximate UBCs, which are local in time, are formed through rational approximations to the exact radiation UBCs. The technique is demonstrated to be effective for both Rossby and gravity modes. The UBC is tested for computational problems initially in the linear PE model, and subsequently in a forced, damped nonlinear quasi‐geostrophic model.
Non‐stationary Markov chains for modelling daily radiation data
Authors: Spirkl Wolfgang , Ries Harald 
Published: February 2007

AbstractContinuous seasonal variations are described by expressing each transition probability of the Markov chain as the normalized exponential of a truncated Fourier polynomial. The Fourier coefficients are found by maximum likelihood estimation. The method does not make use of the orthogonality of the Fourier functions and therefore is applicable to data sets with gaps. The appropriate number of Fourier coefficients as well as the appropriate order of the Markov chain is determined by means of Akaike's information criterion. The method was applied to 19 years of data concerning the duration of sunshine at Weihenstephan (48.4°N 11.7°E). Five categories were identified according to the daily relative duration of sunshine. It was found that an order‐one chain with five Fourier coefficients provides an adequate model. Significant seasonal variations were detected especially for the extreme categories of overcast and clear days.
The effect of a sea surface temperature anomaly on a prediction of the onset of the south‐west monsoon over India
Authors: Kershaw R. 
Published: February 2007

AbstractAt the time of the onset of the monsoon in June 1979 the sea surface temperatures in the eastern Arabian Sea were higher than normal. A prediction experiment designed to reveal the effects of this anomaly on the onset is described. This involves two 8‐day forecasts made using the same model and the same initial conditions. They differ only in using different sea surface temperatures in the eastern Arabian Sea. A better prediction of the onset is obtained by using the anomalously high temperatures rather than the normal climatological values. In particular, the development of a tropical storm over the Arabian Sea, the strengthening of the Somali jet, and the northward movement of the rainfall over India are all better predicted. The mechanism of the onset in the model is discussed and two important feedback loops are identified, one involving moisture‐flux convergence and the other involving the surface fluxes of sensible and latent heat. The most important factor in both loops is the release of latent heat over the Arabian Sea. A linear model is used to show that the onset may be thought of as a response to this heating. The roles of barotropic and baroclinic processes in the onset are also discussed.
A two‐dimensional model of mesoscale frontogenesis in the ocean
Authors: Bleck R. , Onken R. , Woods J. D. 
Published: February 2007

AbstractA two‐dimensional primitive equation model conserving potential vorticity is used to simulate the effect of a horizontal barotropic deformation field on a pre‐existing isopycnic potential vorticity gradient. The novel feature of the prognostic model is that it accurately describes the development of the density and velocity distributions up to local Rossby number 0.5 and beyond. The model coordinates are based in the vertical on quasi‐isopycnic vertical coordinates and in the horizontal on geostrophic coordinates. The model is integrated forward in time for three days creating a density front and a jet of 0.4 m s−1in the surface layers. Cross‐jet velocities are one order of magnitude smaller. Conservation of potential vorticity leads to a modulation of spacing between isopycnals as both the cyclonic and anticyclonic vorticity associated with the horizontal shear of the accelerating jet increase. Vortex stretching and compression sustain a strong vertical circulation. The paper shows how the jet kinetic energy is limited by the amount of available potential energy in a catchment area defined by the extent of the region of confluence. The jet becomes shallower as frontogenesis approaches this limit.The model is also applicable to the development of permanent streams in the large‐scale circulation of the upper ocean, by slow (order one month) frontogenesis due to gyre‐scale deformation acting on an isopycnic potential vorticity gradient in the seasonal thermocline.The vigorous upwelling on the anticyclonic side of the jet raises nutrients through the euphotic zone, increasing primary productivity. This explains observed meoscale modulations of biomass.
A four‐layer model for the heat budget of homogeneous land surfaces
Authors: Choudhury B. J. , Monteith J. L. 
Published: February 2007

AbstractThe model is an explicit solution of the equations which define the conservation of heat and water vapour in the system consisting of uniform vegetation and soil. The vegetation has two layers, the first extending from a reference height in the atmosphere to the virtual sink for momentum and the second from the virtual sink to the soil surface. Soil between the surface and the damping depth is divided into an upper, completely dry layer and a lower wet layer. Throughout the system, differences of potential per unit flux are specified by resistances, notably the conventional surface resistance governing the loss of vapour from foliage and a new soil resistance, assumed proportional to the accumulated loss of water by evaporation from the soil surface, which is proportional to the square root of elapsed time. Since the total energy available to the system is limited by absorption of radiation, the increase of transpiration from foliage as it expands decreases evaporation from the soil. Conversely, as soil dries, transpiration rate per unit of foliage area increases. This interaction of vapour fluxes is governed by the behaviour of the saturation vapour pressure deficit within the vegetation, identified as a major variable.General implications of the model are consistent with observations and estimates of evaporation rate from a stand of wheat grown in Arizona agree well with measurements from lysimeters. The model is applicable to a wide range of problems in agricultural meteorology and hydrology. With simplification, it could be used to obtain surface exchanges of sensible and latent heat in atmospheric general circulation models.
Modelling tropical deforestation: A study of GCM land‐surface parametrizations
Authors: Dickinson Robert E. , Henderson‐Sellers Ann 
Published: February 2007

AbstractTropical deforestation, by changing land surfaces, may have important consequences for the climate system. Predicting even the local, immediate effects of replacing tropical broadleaf forest with impoverished grassland has been difficult, because the land‐surface parametrization schemes used previously in climate models have been inadequate. The forest canopy is particularly important for the surface‐energy budget in tropical regions, and models neglecting the occurrence of such a canopy may give an unrealistic partitioning between various surface‐energy fluxes. Inclusion of a land‐surface scheme with a vegetation canopy into a version of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM) with a diurnal as well as a seasonal cycle permits an exploratory study of the possible effects of tropical deforestation. In a 13‐month integration that assumes that all of the Amazon tropical forest in South America is replaced by impoverished grassland, surface hydrological and temperature effects dominate the response. Reduced mixing and less interception and evaporation from the canopy cause runoff to increase and surface temperatures to rise by 3–5 K. The period of driest soil is increased in the model from one month to several, but the possibility that this change is random cannot be excluded. Increased temperatures and drier soil could have a detrimental impact on survival of the remaining forest and on attempts at cultivation in deforested areas. The land‐surface model, driven in a stand‐alone mode by prescribed atmospheric conditions and with an imposed seasonal cycle of rainfall, mimics the seasonal cycle of soil moisture and runoff found in the CCM. Hence, it is used to estimate the relative contribution of the various changes imposed to simulate deforestation in the CCM with respect to the model's response at the surface. The change in surface roughness interacting with the canopy hydrology is evidently a major factor in determining the surface response to deforestation. However, the response to change in roughness is less pronounced for simpler models.
Stably stratified shear flow over low hills
Authors: Hunt J. C. R. , Richards K. J. , Brighton P. W. M. 
Published: February 2007

AbstractAn analysis is presented for the mean flow over hills with low slopes, for various kinds of stable stratification in the approach flow. (The Froude numberFbased on the height is always greater than unity.) The analysis is based on dividing up the flow into different regions with different dynamics.We concentrate on the flow in the inviscid outer region of the flow. It is shown that in the lower part of the outer region, the middle layer, where the shear is large, the upwind velocity profile and the shape of the hill, rather than the upwind density profile, largely determine the vertical perturbation velocity. In the ‘upper layer’ the structure of the flow depends largely on theformof the upwind density profile. For example, this largely determines the location of maximum surface wind. New solutions forstableconditions are obtained when there is a constant density gradient, when (N/UB)2∞ z−2and when there is an elevated inversion layer. In the first and third of these cases, we compare the results with the hydrostatic approximation, and also obtain new general formulae for the near‐surface winds. An approximate formula is developed for surface winds at arbitrary values ofF(>1) for uniform stable stratification.
Surface airflow around Windless Bight, Ross Island, Antarctica
Authors: O'Connor William P. , Bromwich David H. 
Published: February 2007

AbstractRoss Island, Antarctica is located along the Transantarctic Mountains and is subjected to a strong southerly mountain‐parallel wind regime. Despite this, Windless Bight on the island's southern coast is a region of anomalous calm. The atmospheric boundary layer dynamics that gives rise to this phenomenon is analysed both theoretically and observationally. It is the strong static stability of the boundary layer air encountering the high steep topography of Ross Island, that causes a stagnation zone resulting in the calm conditions of Windless Bight.Direct and proxy observations of boundary layer winds provide a detailed description of airflow around the island. The anomalous (with respect to the synoptic pressure field) but persistent north‐easterly winds at Scott Base are due to the deflection of highly stable, low‐level air around Hut Point Peninsula. By contrast, the less frequent, strong southerly winds which override the peninsula are associated with the influx of warm maritime air from cyclonic systems to the east. It is inferred that flow of air around the terrain of Ross Island gives rise to locally strong winds; these are responsible for the ice breakout and polynya occurrences in McMurdo Sound.The surface airflow past Ross Island can be modelled by a two‐dimensional, steady, frictionless, irrotational, incompressible flow past an obstacle, with a shape based on an island height contour. The flow is assumed to separate from the eastern and western sides of the island and form a wake downstream. A solution for this flow is presented, based on potential theory for streaming motion past an obstacle. The streamline and isobar patterns clearly depict the stagnation region in Windless Bight. For approaching winds of 20 ms−1 the local pressure field is perturbed by several millibars.
On the modelling of warm orographic rain by the seeder‐feeder mechanism
Authors: Robichaud Alain J. , Austin Geoffrey L. 
Published: February 2007

AbstractA two‐dimensional model of the seeder–feeder mechanism for the orographic enhancement of rain is presented. It satisfactorily reproduces observed rainfall rate distributions and is helpful in studying some aspects of the physics of orographic rain. The modelling approach is based on the principle of continuity of the water substance and on the two‐dimensional theory of small perturbations of airflow over topographical ridges of modest dimensions. The model is significantly different from those found in the literature and is used to clarify two major points. Firstly, it emphasizes that there is a range of hill dimensions for which the seederfeeder mechanism plays a major role. That is, the model predicts that a change of regime occurs for larger hills (half length > 20 km) where the seeder‐feeder mechanism is no longer dominant because of the possibility of rain development within the feeder cloud and also because the horizontal scale of rain drift becomes small compared with that of the topography. Secondly, it is also suggested that the difference of wind curvature below and above a low‐level jet may be important in determining the intensity of orographic rain. The physical process behind the latter finding is closely linked to partial reflection of upward propagating gravity waves which may lead to ‘resonance’ of the orographically induced vertical velocity.
Selection of baroclinic waves
Authors: Appleby J. C. 
Published: February 2007

AbstractA two‐layer baroclinic flow simultaneously unstable to two discrete wavelengths is altered by the self‐interaction of the waves so that either may attain an equilibrium at finite amplitude. The stability of the two possible states, and hence the ‘selection’ by the flow of one predominant wavelength, is determined, in the absence of hysteresis, by a principle of minimum available potential energy. In the limit of weak nonlinearity this is equivalent to maximum barotropic amplitude, but not in general to maximum wave amplitude within each layer as previously suggested.
The effects of entrainment on the growth of droplets in continental cumulus clouds
Authors: Bower K. N. , Choularton T. W. 
Published: February 2007

AbstractIn this paper data are presented from aircraft passes through continental cumulus clouds obtained during the Cooperative Convective Precipitation Experiment. The effect of evaporation of liquid water from the cloud following the entrainment of free tropospheric air is to totally remove a fraction of the droplets of all sizes rather than partially evaporate most of the droplets. Supersaturations in the cloud are found on average to be considerably higher than those predicted for an adiabatic cloud and the supersaturation is not closely tied to the vertical wind.These observations are found to have a number of important consequences for cloud droplet growth: 1. The average lifetime of individual droplets is considerably less than the age of the cloud and is not expected to exceed about five minutes. This has important consequences for the oxidation of SO2 to sulphate. 2. Statistically favoured droplets that avoid evaporation are observed to grow much larger than average droplets and to sizes larger than would occur at the same altitude in an adiabatic cloud.
Spin vectors and rates of change of wind direction
Authors: White A. A. 
Published: February 2007

AbstractEstablished formulae for various rates of change of horizontal wind direction may be unified by a treatment in terms of the spin vector, S, of a vector field M. The spin vector measures the rate of change of direction of M, in the appropriate osculating plane, and may be defined with respect to any infinitesimal displacement in space and time. When M is a horizontal vector field, the osculating plane is horizontal, and S immediately gives the rate of change of direction relative to a fixed horizontal axis. The formulae involved are those of Hide and of Bryden for the rate of change of geostrophic wind direction with height, and of Burk and Staley and of Lecluyse and Neumann for the Lagrangian rate of change of wind direction. When M is a 3‐dimensional vector field, the orientation of the osculating plane is not in general constant and the Cartesian components of S do not represent the rate of change of direction in the most concise terms. The Lagrangian spin vector of the 3‐dimensional velocity field may nevertheless be considered as a basic kinematic quantity, on a par with the rates of change of kinetic energy, vorticity and divergence.
The slow equations
Authors: Lynch Peter 
Published: February 2007

AbstractA filtered system of equations is derived, using the ideas of normal mode initialization. This slow equation system models the low frequency rotational atmospheric motions; there are no solutions corresponding to gravity waves. The prognostic element of the system is an equation expressing the conservation of potential vorticity. The slow equations differ from the general balance system at second order in the Rossby number, and are free from the spurious solutions found in that system. Integration of a barotropic model with the slow equations shows them to be highly accurate when compared with the primitive equations. Since the slow equation model has no high frequency solutions and is free from data shock, it may be useful for continuous assimilation of observational data.
Sources and sinks of zonal momentum in the middle atmosphere diagnosed using the diabatic circulation
Authors: Shine Keith 
Published: February 2007

AbstractRadiative heating rates calculated using monthly mean climatological fields of temperature and ozone are used to diagnose the diabatic circulation in the middle atmosphere. This circulation is then used to infer the zonal force per unit mass.Previous diagnostic studies of the zonal force per unit mass have considered only the northern hemisphere outside the summer period and for heights up to about 60 km. The present study allows calculations for both hemispheres in all seasons at heights up to about 75 km. Strong inter‐hemispheric differences in the diagnosed fields are found in the winter and spring hemispheres, whilst the summer and autumn hemispheres are quite similar. The drag on the mean wind in the southern winter mesosphere is found to be up to double that found in the northern winter mesosphere. It is argued that this feature may be due to an enhanced transmission of gravity waves through the southern winter stratosphere; this may, in turn, explain observed inter‐hemispheric differences in temperature for the winter months. Differences between the diagnosed fields and those inferred in modelling studies are also discussed.Some attention is also paid to a problem that pervades this and similar studies. The calculated radiative heating rates do not allow a unique diabatic circulation to be diagnosed because of unexplained departures from the global mean mass balance. The diagnosed circulation is thus subject to some uncertainty which puts definite limits on the confidence that can be placed on diagnosed quantities, especially in the stratosphere. It must be recognized that derived quantities are admixtures of the true quantity and imperfections in the data and radiative transfer scheme.
Atmospheric loadings of marine aerosol during a Hebridean cyclone
Authors: Smith M. H. , Consterdine I. E. , Park P. M. 
Published: February 2007

AbstractDuring a marine aerosol field trial at a coastal site in the Outer Hebrides, a depression formed in the North Atlantic, and moved directly over the site. Wind speeds on site gusted past 100m.p.h., and some data were gathered at very high wind speeds.We found that the volume of sea salt aerosol at 30 ms−1 was more than an order of magnitude higher than that at 15 ms−1. This is contrary to previous, rather speculative, findings that the aerosol loadings of the atmosphere became less sensitive to wind speed as the wind speed increases beyond 15 ms−1.
Inversion of cloudy satellite sounding radiances by nonlinear optimal estimation. II: Application to TOVS data
Authors: Eyre J. R. 
Published: February 2007

AbstractA new approach to inverting satellite sounding data for use in numerical weather prediction (NWP) was proposed in a previous paper, where it was applied to simulated data from the TIROS Operational Vertical Sounder (TOVS). In this paper the method has been applied successfully to real TOVS data from the NOAA‐9 satellite, in a case study taken from the period of the Generation of Atlantic Lows Experiment (GALE). Temperature and humidity profiles, surface temperature and microwave emissivity, and cloud‐top pressure and fractional coverage have been retrieved simultaneously. The retrieved profiles have been compared statistically with co‐located radiosonde measurements. Also, fields of retrieved values have been compared qualitatively with corresponding NWP background fields (which provided the ‘first guess’ for the inversion) and with image data from TOVS itself and from the Advanced Very High Resolution Radiometer on the same satellite.Impact on the temperature field was found to be small, as expected over an area for which the conventional observing network is relatively dense. However, changes to the background field were generally found to be beneficial. At mid to upper tropospheric levels, the retrievals modified the background humidity field considerably. Retrieved values were both horizontally coherent and consistent with corresponding images and the expected dynamics. Retrieved cloud parameters have not been studied in great detail but have been found to be reasonable when compared with image data.