bluuebunny/crossref_metadata_2025_split huggingface dataset:
The parametrization of entrainment in cloudy boundary layers
Authors: Lock A. P. 
Published: February 2007

AbstractA parametrization for the entrainment rate in shear‐free convective cloud‐capped boundary layers is derived. The only information it requires is the external mixed‐layer turbulence forcings (the surface buoyancy flux and the net radiative‐divergence profile), the inversion jumps of temperature and humidity, the cloud‐top liquid water mixing ratio and the cloud and mixed‐layer depths. Despite this simplicity it is found to compare well against both a wide range of large‐eddy simulations and observations of stratocumulus.The parametrization is an extension of those derived previously for the idealised cases of smoke clouds, where turbulence is driven by combinations of surface heating and radiative cooling, and liquid water clouds driven solely by buoyancy reversal. The radiative forcing is specified as an indirect forcing, through the buoyant production of turbulence within the boundary layer and a direct forcing, which promotes deepening of the boundary layer when undulations in the cloud top cause part of the cooling to occur within the horizontally averaged inversion. Condensation of water in saturated air reduces the strength of both these terms but otherwise, for radiatively driven liquid water clouds (where evaporative cooling of entrained air does not generate buoyancy reversal), the parametrization is unchanged from that for smoke clouds.Where evaporative cooling of entrained air is strong enough to generate buoyancy reversal, and therefore drive convective motions, not only does it provide an additional turbulence source, but it is also found to compensate for the reduction in strength of the radiative‐forcing terms by the presence of saturated air. Allowing for this enhancement, the entrainment rate is predicted with remarkable accuracy by the sum of previously derived parametrizations for the rates that would have been generated by radiative cooling and buoyancy reversal acting in isolation.
A novel approach to atmospheric dispersion modelling: The Puff‐Particle Model
Authors: De Haan Peter , Rotach Mathias W. 
Published: February 2007

AbstractThe Puff–Particle Model (PPM) uses a new approach to problems of three‐dimensional atmospheric dispersion from micro‐ to mesoscale. The pollutant particles are grouped in clusters treated as Gaussian puffs, which are dispersed making use of the concept of relative diffusion. The centre of mass of each puff is moved along a stochastic trajectory. This trajectory is derived from particle trajectories given by a Lagrangian stochastic‐dispersion model. In this way, the PPM retains the advantages of traditional puff models and those of particle models, and is able to take into account the correct probability density function of the stochastic velocity components. The effect of meandering (caused by turbulent eddies larger than the puff but not resolved by the flow field) is simulated by the puff centre trajectories, yielding a complete description of dispersion. The PPM is validated using measurements from three tracer experiments in Copenhagen, Lillestrøm and Kincaid.
Estimation of area‐averaged moisture flux
Authors: Mahrt L. , Kotwica K. , Sun Jielun , Macpherson J. I. , Desjardins R. L. 
Published: February 2007

AbstractToward the goal of predicting area‐averaged evapotranspiration, the evaporative fraction is modelled in terms of surface radiation temperature, air temperature, solar zenith angle, Normalized Difference of the Vegetation Index and albedo. Previous relationships break down when applied simultaneously to a variety of surfaces. The zenith angle is required to account for the influence of shaded ground on the remotely sensed surface radiation temperatures. The model of evaporative fraction is developed from Canadian Twin Otter aircraft data from the BOReal Ecosystem‐Atmosphere Study and the California Ozone Deposition Experiment. The model consists of separate submodels for active agriculture, forested surfaces, bare soil/senescent vegetation and water surfaces. These broad land‐use groups are effectively determined from remotely sensed red and near‐infrared reflectances.The area‐averaged moisture flux predicted by the model compares favourably with that computed from independent data over forested surfaces. However, the model performs poorly over bare soil/senescent surfaces, and data for a wider variety of surface conditions are required before the generality of the model can be evaluated.
The horizontal and vertical structure of east Asian winter monsoon pressure surges
Authors: Compo Gilbert P. , Kiladis George N. , Webster Peter J. 
Published: February 2007

AbstractEast Asian cold‐air outbreaks, accompanied by increasing surface pressure (‘pressure surges’), are shown to be an important aspect of the subseasonal variability of the winter monsoon system. In this study the statistical linear relationship between pressure surges, tropical convection, and tropospheric circulation is assessed using a ten‐year data set (1985/86‐1994/95) of the European Centre for Medium‐Range Weather Forecasts gridded operational analyses. From spectral analysis the pressure, wind, and temperature fluctuations indicative of strong pressure surges are found to have statistically‐significant spectral peaks at submonthly periods (6 to 30 days). Linear regression analysis is used to detail the time evolution of the dominant horizontal and vertical structure of east Asian pressure surges. Surges are shown to relate significantly to circulation anomalies in both the meridional and zonal components of the lower‐tropospheric wind. Relationships between east Asian pressure surges in the submonthly band and tropical circulation anomalies are found over the Bay of Bengal, the eastern Indian Ocean, Indonesia, and the western Pacific regions. Submonthly surges over the South China Sea are related to strong surges and convective activity south of Indonesia, over the South China Sea, the eastern Indian Ocean, and the Philippine regions. Surge‐enhanced convective activity is found to precede an enhancement of the local east Asian Hadley cell. Submonthly surges over the Philippine Sea are related to periods of westerly‐wind anomalies and convective activity in the western Pacific. Upper‐level wave activity over western Asia precedes submonthly surges. the wave activity amplifies in the region of the Pacific jet stream, and can be traced dispersing equatoward through the region of upper‐level westerlies over the eastern tropical Pacific.
An analysis of a relatively rare case of continental blocking
Authors: Lupo Anthony R. , Bosart Lance F. 
Published: February 2007

AbstractPlanetary‐ and synoptic‐scale analyses of a relatively rare continental blocking event that occurred over North America during the spring of 1980 are undertaken to determine whether or not this event was different from its counterparts which occur over oceanic regions. the planetary‐scale analysis demonstrates that during the spring season a ridge was located further inland over the North American continent and amplified with respect to climatology. the position of this ridge may have been linked to a broad region of colder‐than‐normal sea surface temperatures found over the north central Pacific during the spring season and much of the previous winter. Simple ‘Sutcliffe‐type’ and thermodynamic analyses of the accompanying lower‐tropospheric warm anomaly associated with the ridging show that lower‐tropospheric temperature advection and subsidence associated with anticyclonic‐vorticity advection by the time‐mean thermal wind produced much of the anomalous warmth.A simple synoptic‐scale analysis was performed using both the Zwack‐Okossi (ZO) equation and potential vorticity (PV) thinking approaches. These complementary analyses demonstrated that synoptic‐scale cyclones were instrumental in the formation and maintenance (and/or intensification) of this blocking event. the PV analysis demonstrated that low‐PV air was swept polewards and then was advected over the blocking region sustaining the broad region of low potential vorticity associated with the block over North America. the ZO analysis showed that the advection of anticyclonic vorticity was the most important mechanism forcing geopotential‐height rises at 500 hPa over the block centre. the region of low PV and ZO height rises could be associated with the anticyclonic‐shear side of an upstream jet maximum typically found in association with developing and/or intensifying blocking events. Thus, negative PV advection correlated significantly with calculated ZO height rises. Finally, it is suggested that a favourable phase relationship between the upstream cyclones and the large‐scale ridge is necessary for block development or intensification.
Accounting for subgrid‐scale cloud variability in a multi‐layer 1d solar radiative transfer algorithm
Authors: Oreopoulos Lazaros , Barker Howard W. 
Published: February 2007

AbstractA multi‐layer, 1D solar radiative transfer algorithm that accounts for subgrid‐scale cloud variability is presented. This algorithm is efficient and suitable for use in large‐scale models such as global climate and weather prediction models. While it is built on the same principles as standard multi‐layer 1D codes, there are two major differences. First, it is assumed that for all cloudy layers all the time, frequency distributions of optical depth τ are described by gamma probability density functionspr(τ) and characterized by mean optical depth τ and a variance‐related parameterv.Albedos and transmittances for individual layers are estimated by integrals over all τ of the plane‐parallel, homogeneous two‐stream approximation equations weighted bypr(τ). Thus, the model is referred to as the gamma‐weighted two‐stream approximation. Second, in an attempt to counteract the use of horizontally homogeneous fluxes, a method was devised that often reduces layer values of τ.The gamma‐weighted two‐stream approximation was implemented in a well known broadband column model and the parametrizations upon which it is built were tested using 2D and 3D inhomogeneous cloud fields generated by a bounded cascade model and cloud‐resolving models. All fields resolved the lowest 20 km of the atmosphere into at least 30 layers. Reference calculations were obtained by: (i) applying the 1D‐plane‐parallel, homogeneous model to each column and averaging (the independent column approximation); and (ii) a 3D Monte Carlo algorithm. the gamma‐weighted two‐stream approximation, the regular plane‐parallel, homogeneous, and two other 1D models operated on horizontally‐averaged versions of the fields (i.e. 1D vectors of cloud fraction, τ, andv). For several demanding cases, the gamma‐weighted two‐stream approximation reduced plane‐parallel, homogeneous‐biases for TOA albedo and surface irradiance by typically more than 85%. Moreover, its estimates of atmospheric heating rates usually differed from the independent column approximation and Monte Carlo values by less than 10%. This translates into heating rate errors that are four to eight times smaller than those associated with conventional 1D plane‐parallel, homogeneous algorithms. In a large‐scale model, a multi‐layer solar code with the gamma‐weighted two‐stream approximation should require about twice as much CPU time as its plane‐parallel, homogeneous counterpart.
Variational quality control
Authors: Anderson Erik , Järvinen Heikki 
Published: February 2007

AbstractWe describe the implementation of variational quality control in the operational four‐dimensional variational data assimilation scheme at the European Centre for Medium‐Range Weather Forecasts (ECMWF). the quality control takes place during the iterative solution of the variational analysis problem itself, rather than in a separate step prior to the main analysis. This ensures that the quality control is consistent with the analysis in terms of error statistics, background and model constraints, in particular. All observational data are used and quality‐controlled simultaneously. the probability of gross error is computed for each observation, given the preliminary analysis at each iteration. the weight of each observation is smoothly decreased with increased likelihood for gross error. the formulation, the implementation details and results from data assimilation and forecast experiments are presented.
A numerical study of tropical‐cyclone structure: Quasi‐stationary spiral bands
Authors: Weber Harry C. 
Published: February 2007

AbstractThe evolution of quasi‐stationary spiral bands of vorticity, on the periphery of an initially symmetrical tropical‐cyclone‐like vortex moving on a β‐plane, is investigated using a semi‐spectral nondivergent nonlinear barotropic numerical model, formulated in a cylindrical coordinate system moving with the vortex. the results obtained with the numerical model are supported by analytic solutions of the linear barotropic vorticity equation in a moving frame of reference.The analysis of the model data shows the slow evolution and vortex‐relative translation of a spectrum of Rossby waves, forming cyclonically curved spiral bands of positive and negative vorticity anomalies in the rear‐right quadrant of the moving vortex. the analytic solutions of the linear barotropic vorticity equation and examination of the vorticity tendency in the numerical model indicate that the generation of the spiral bands is mainly a result of the vortex drift and the advection of planetary vorticity by the vortex wind field. Investigation of the kinematical structure of the trailing spiral bands of vorticity and their vicinity suggests that the cyclonic spiral bands in the numerical model may be related, either directly or indirectly, to the existence of quasi‐stationary principal spiral bands of convection in real tropical cyclones.
The effect of land‐surface feedbacks on the monsoon circulation
Authors: Ferranti L. , Slingo J. M. , Palmer T. N. , Hoskins B. J. 
Published: February 2007

AbstractSummary the effects of feedbacks from land‐surface forcing on intraseasonal monsoon activity are studied by performing idealized sensitivity experiments with a general circulation model.In agreement with observations, the simulated intraseasonal monsoon activity is mainly described by irregular alternations of active spells and break spells associated with fluctuations of the Tropical Convergence Zone (TCZ) between a continental and an oceanic regime. In the model, the spatial characteristic of the intraseasonal monsoon variability is a robust feature which is primarily related to an internal mode of variability of the system, rather than to a response to land‐surface feedbacks. Experimentation indicates that the simulation of northward propagating events, related to transitions in the regime, does not require the inclusion of interactive surface hydrological processes. This suggests that the transitions are also mainly related to internal atmospheric dynamics.The temporal characteristics of the fluctuations between the two TCZ regimes, however, are influenced by an interactive surface. the low‐frequency intraseasonal monsoon variability is enhanced by hydrological surface feedbacks. When the surface interacts with the atmosphere, the active and break regimes of the monsoon are equally likely. In the absence of surface feedbacks, the probability distribution is modified and the changes depend on the land‐surface conditions imposed. the results show that the probability of a monsoon break exceeds that of an active phase when the imposed land‐surface conditions are based on climatological values for July. This asymmetry in the probability distribution affects intraseasonal monsoon variability. In turn, the time‐mean monsoon circulation, depending as it does on the statistics of the intraseasonal oscillations (such as frequency of occurrence and mean amplitude), is also modified by the surface feedbacks. It follows that the surface conditions play a role in the interannual predictability of the time‐mean monsoon.
Primary ice nucleation in orographic cirrus clouds: A numerical simulation of the microphysics
Authors: Spice A. , Johnson D. W. , Brown P. R. A. , Darlison A. G. , Saunders C. P. R. 
Published: February 2007

AbstractIce particle production by the processes of homogeneous freezing and heterogeneous nucleation in orographic cirrus clouds is studied using a simple adiabatic ascending‐air‐parcel model. Homogeneous freezing rates in the model are based on a formulation by Jeffery and Austin, which matches with observed rates, modified according to the solution effect. Two alternative ice nuclei (IN) activation spectra are applied, one dependent upon temperature and the other upon ice supersaturation. Simulations are performed for air parcels with initial dew‐point temperatures, Td, in the range from −20 to −45°C and constant vertical velocities, w, of 0.1 to 5.0 m s‐1. In modelled clouds with Td ⩾ 30°C, heterogeneous nucleation initiated by IN is found to dominate ice formation, since homogeneous freezing rates are so low. In modelled clouds with Td ⩽ −35°C, pure liquidwater homogeneous freezing rates are large, but inclusion of sufficient quantities of IN may completely suppress aqueous solution droplet growth, dilution and homogeneous freezing, with heterogeneous nucleation then remaining dominant even at very low temperatures. the IN number concentrations required for homogeneous freezing suppression, for a fixed cloud condensation nucleus (CCN) activation spectrum, are found to increase with increasing w‐heterogeneous nucleation at low temperatures is therefore much more viable at the low w values found in non‐orographic compared with orographic cirrus clouds. the common conception of a deficit of IN aloft is challenged, and, because of the apparent sensitivity of cirrus cloud microphysical and radiative properties to these aerosol particles, IN measurements at cirrus levels, in addition to CCN measurements, are suggested as vital. the model results suggest that it may be possible to interpret measurements of peak liquid‐water content, or possibly maximum droplet radius, in orographic cirrus clouds in terms of the dominant ice nucleation mode.
Aircraft observations of sea‐breeze frontal structure
Authors: Wood R. , Stromberg I. M. , Jonas P. R. 
Published: February 2007

AbstractDetailed aircraft observations of sea‐breeze frontal structure and dynamics are presented for two cases of well defined sea‐breeze fronts near the east coast of England. In the first case the sea‐breeze was advancing into a well mixed convective boundary layer with strong turbulence and an offshore breeze of around 3 m s‐1. In the second case the sea‐breeze was penetrating into a convective boundary layer characterized by weaker turbulence and an offshore wind speed of 2‐3 m s‐1. Indeed, during the course of the measurements a stable internal boundary layer was forming in the early evening. Cross‐sections of the frontal structure are derived from aircraft traverses at a range of heights along a fixed line normal to the coast. A typical head‐like structure is observed in both cases, with a region of strong mixing immediately seawards of the head. an approximately exponential fall‐off in dissipation rate with distance seaward of the leading edge of the front is found. Heat and momentum fluxes are used to derive turbulent kinetic energy (TKE) budgets for the mixing region and indicate that turbulence in this mixing region is maintained by TKE generated by strong shear at the top of the cold‐air inflow. In the first case the shear production of TKE is almost twice that of the second case, suggesting that the presence of ambient turbulence in the convective boundary layer has a significant effect upon the frontal dynamics. In the second case, as the turbulence decayed, a curious wave‐like structure appeared behind the leading edge of the front. the waves have wavelengths of 1‐3 km and it is unlikely that they were caused by Kelvin‐Helmholtz instability. It is suggested that the waves might be a solitary wave‐train emerging as the sea‐breeze interacts with a low‐level stable layer forming in the early evening.
Surface‐flux aggregation in heterogeneous terrain
Authors: Hasager C. B. , Jensen N. O. 
Published: February 2007

AbstractAggregation of surface fluxes in non‐homogeneous terrain is a difficult task due to the highly nonlinear processes governing especially the momentum flux. Results of the current work are high‐resolution maps of land‐surface momentum flux in a strongly heterogeneous mid‐latitude area in Europe, and aggregated values of the aerodynamic roughness lengths. This paper describes the development of a flux aggregation model based on a linearized set of the atmospheric flow equations solved by fast Fourier transforms. This is computationally efficient, therefore a large amount of data can be handled quickly. the input data to the model are high‐resolution (30 m x 30 m) roughness maps derived from classified satellite images and assigned Meteorological field values within a region (100 km x 100 km). the model deals with neutral flow only. the momentum‐flux results from this objective (physically based) method compare well with other models and experimental field data.
Retrieval of surface and atmospheric parameters over land from SSM/I: Potential and limitations
Authors: Prigent C. , Rossow W. R. 
Published: February 2007

AbstractThe potential to retrieve surface temperature, water vapour content and cloud liquid water path from the Special Sensor Microwave/Imager (SSM/I) observations over land is examined, using a variational inversion scheme. the method relies upon prior estimates of the monthly mean land surface microwave emissivities and exploits a priori information on the atmospheric situation (International Satellite Cloud Climatology Project (ISCCP) data and National Center for Environmental Prediction analysis). the accuracy of the surface temperature retrieval is of the order of 1 K, does not strongly depend on surface type and is not very sensitive to the presence of thin clouds. the sensitivity to the water vapour is very low except in the most arid areas. With an accuracy of ∼ 0.1 kg m−2, the SSM/I retrieval does not properly characterize thin clouds, but the major cloud structures are well delineated and qualitative agreement is observed between the SSM/I and ISCCP estimations. This inversion process also has some ability to detect rain events.