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While the formulation of most data assimilation schemes assumes an unbiased observation model error, in real applications model error with nontrivial biases is unavoidable. A practical example is errors in the radiative transfer model (which is used to assimilate satellite measurements) in the presence of clouds. Together with the dynamical model error, the result is that many (in fact 99%) of the cloudy observed measurements are not being used although they may contain useful information. This paper presents a novel nonparametric Bayesian scheme that is able to learn the observation model error distribution and correct the bias in incoming observations. This scheme can be used in tandem with any data assimilation forecasting system. The proposed model error estimator uses nonparametric likelihood functions constructed with data-driven basis functions based on the theory of kernel embeddings of conditional distributions developed in the machine learning community. Numerically, positive results are shown with two examples. The first example is designed to produce a bimodality in the observation model error (typical of 'cloudy' observations) by introducing obstructions to the observations that occur randomly in space and time. The second example, which is physically more realistic, is to assimilate cloudy satellite brightness temperature-like quantities, generated from a stochastic multicloud model for tropical convection and a simple radiative transfer model. [ABSTRACT FROM AUTHOR]

The Madden–Julian oscillation (MJO), a prominent feature of the tropical atmospheric circulation at sub seasonal time scales, is known to modulate atmospheric variability in the Euro-Atlantic region. However, current sub seasonal prediction systems fail to accurately reproduce the physical processes involved in these teleconnection mechanisms. This paper explores the observed impact of strong MJO events on surface wind speed over Europe. It is found that some MJO phases are accompanied by strong wind anomalies in Europe. After showing that this teleconnective mechanism is not present in the predictions of the ECMWF monthly forecasting system, a methodology to reconstruct forecasts of daily mean wind speed in the continent weeks ahead is proposed. This method combines MJO forecasts from the S2S project database and the observed teleconnection impacts in the historical records. Although it is found that strong MJO events cannot be skillfully predicted more than 10 days ahead with current prediction systems, a theoretical experiment shows that this method can effectively transform a dynamical MJO forecast into a probabilistic wind speed prediction in Europe. [ABSTRACT FROM AUTHOR]

The Ocean–Land–Atmosphere Model (OLAM) has been developed to extend the capabilities of the Regional Atmospheric Modeling System (RAMS) to a global model domain. OLAM adopts many features of its predecessor, including physical parameterizations, initialization methods, data assimilation, logic and coding structure, and I/O formats. However, its dynamic core is new and is based on a global geodesic grid with triangular mesh cells and a finite-volume discretization of the full compressible Navier–Stokes equations. Local mesh refinement can be activated in OLAM to cover selected geographic areas at very high resolution and hence to simulate atmospheric systems typically studied in regional models. This paper is one in a series whose purpose is to describe the formulation of OLAM and to validate the model’s performance through test results. This paper focuses on global-scale dynamics that can be represented by the shallow-water equations, and provides details of the spatial and temporal discretizations, which contain some unique algorithms that previously have not been applied to atmospheric models. Validation tests are performed using five standard shallow-water simulations that are commonly used benchmarks for global models. OLAM results for all tests are comparable to results from other global models. [ABSTRACT FROM AUTHOR]

In this paper an application of clustering algorithms for statistical downscaling in short-range weather forecasts is presented. The advantages of this technique compared with standard nearest-neighbors analog methods are described both in terms of computational efficiency and forecast skill. Some validation results of daily precipitation and maximum wind speed operative downscaling (lead time 1–5 days) on a network of 100 stations in the Iberian Peninsula are reported for the period 1998–99. These results indicate that the weighting clustering method introduced in this paper clearly outperforms standard analog techniques for infrequent, or extreme, events (precipitation > 20 mm; wind > 80 km h-1). Outputs of an operative circulation model on different local-area or large-scale grids are considered to characterize the atmospheric circulation patterns, and the skill of both alternatives is compared. [ABSTRACT FROM AUTHOR]

In this paper the authors dynamically downscale daily-averaged general circulation model (GCM) datasets over Australia using the Conformal Cubic Atmospheric Model (CCAM). The technique can take advantage of the wider range of Coupled Model Intercomparison Project phase 3 (CMIP3) daily-averaged GCM datasets than is available using 3-hourly datasets. The daily-averaged host GCM atmospheric data are fitted to a time interpolation formula and then differentiated in time to produce a first-order estimate of the atmosphere at 0000 UTC on each simulation day. The processed GCM data are forced into CCAM using a scale-selective filter with an 18°° radius. Since this procedure is unable to account for the diurnal cycle, the forcing data are only applied to winds and air temperatures once per day between 800 and 100 hPa. Lateral boundary conditions are not required since CCAM employs a variable-resolution global grid. The technique is evaluated by downscaling daily-averaged 2.5°° NCEP reanalyses over Australia at 60-km resolution from 1971 to 2000 and comparing the results to downscaling the 6-hourly reanalyses and to simulating with sea surface temperature (SST)-only forcing. The results show that the daily-averaged downscaling technique can simulate average seasonal maximum and minimum screen temperatures and rainfall similar to those obtained downscaling 6-hourly reanalyses. Some implications for regional climate projections are considered by downscaling four daily-averaged GCM datasets from the twentieth-century climate in coupled models (20C3M) experiment over Australia. [ABSTRACT FROM AUTHOR]

Warm and moist air masses, required to generate deep convection east of the Andes Cordillera, are generally the result of humidity transport by the so-called low-level jet (LLJ). In this paper, it is shown from detailed test cases that the eastern part of the continent and the adjacent Atlantic Ocean may constitute another source of moist, warm air, which could be of potential importance even in the presence of a southerly LLJ. The position of the anticyclones crossing South America from the Pacific Ocean to the Atlantic Ocean appears to be a key factor affecting the origin of moisture over the continent. In particular, the LLJ may weaken and even change its direction when the eastern side of the South Pacific anticyclone crosses the mountains; this wind reversal is generally associated with deep convection suppression. Thus, as a South Pacific anticyclone crosses the continent more to the east and its western side reaches the east coast of South America, deep convection can reappear east of the Andes, over the Mendoza region, although the LLJ is frequently suppressed. This is associated with a transport of warm and moist air from Uruguay, southeast Brazil, or even directly from the Atlantic Ocean. [ABSTRACT FROM AUTHOR]

Parameterizations of turbulence often predict several lower-order moments and make closure assumptions for higher-order moments. In principle, the low- and high-order moments share the same probability density function (PDF). One closure assumption, then, is the shape of this family of PDFs. When the higher-order moments involve both velocity and thermodynamic scalars, often the PDF shape has been assumed to be a double or triple delta function. This is equivalent to assuming a mass-flux model with no subplume variability. However, PDF families other than delta functions can be assumed. This is because the assumed PDF methodology is fairly general. This paper proposes closures for several third- and fourth-order moments. To derive the closures, the moments are assumed to be consistent with a particular PDF family, namely, a mixture of two trivariate Gaussians. (This PDF is also called a double Gaussian or binormal PDF by some authors.) Separately from the PDF assumption, the paper also proposes a simplified relationship between scalar and velocity skewnesses. This PDF family and skewness relationship are simple enough to yield simple, analytic closure formulas relating the moments. If certain conditions hold, this set of moments is specifically realizable. By this it is meant that the set of moments corresponds to a real Gaussian-mixture PDF, one that is normalized and nonnegative everywhere. This paper compares the new closure formulas with both large eddy simulations (LESs) and closures based on double and triple delta PDFs. This paper does not implement the closures in a single-column model and test them interactively. Rather, the comparisons are diagnostic; that is, low-order moments are extracted from the LES and treated as givens that are input into the closures. This isolates errors in the closures from errors in a single-column model. The test cases are three atmospheric boundary layers: a trade wind cumulus layer, a stratocumulus layer, and a clear convective case. The new closures have shortcomings, but nevertheless are superior to the double or triple delta closures in most of the cases tested. [ABSTRACT FROM AUTHOR]

Three- and four-dimensional variational assimilation (3DVAR and 4DVAR) systems have been developed for the Océan Parallélisé (OPA) ocean general circulation model of the Laboratoire d'Océanographie Dynamique et de Climatologie. They have been applied to a tropical Pacific version of OPA and cycled over the period 1993-98 using in situ temperature observations from the Global Temperature and Salinity Pilot Programme. The assimilation system is described in detail in Part I of this paper. In this paper, an evaluation of the physical properties of the analyses is undertaken. Experiments performed with a univariate optimal interpolation (OI) scheme give similar results to those obtained with the univariate 3DVAR and are thus not discussed in detail. For the 3DVAR and 4DVAR, it is shown that both the mean state and interannual variability of the thermal field are improved by the assimilation. The fit to the assimilated data in 4DVAR is also very good at timescales comparable to or shorter than the 30-day assimilation window (e.g., at the timescale of tropical instability waves), which demonstrates the effectiveness of the linearized ocean dynamics in carrying information through time. Comparisons with data that are not assimilated are also presented. The intensity of the North Equatorial Counter Current is increased (and improved) in both assimilation experiments. A large eastward bias in the surface currents appears in the eastern Pacific in the 3DVAR analyses, but not in those of 4DVAR. The large current bias is related to a spurious vertical circulation cell that develops along the equatorial strip in 3DVAR. In 4DVAR, the surface current variability is moderately improved. The salinity displays a drift in both experiments but is less accentuated in 4DVAR than in 3DVAR. The better performance of 4DVAR is attributed to multivariate aspects of the 4DVAR analysis coming from the use of the linearized ocean dynamics as a constraint. Even in... [ABSTRACT FROM AUTHOR]

In a recently published paper, spherical nonparametric estimators were applied to feature-track ensembles to determine a range of statistics for the atmospheric features considered. This approach obviates the types of bias normally introduced with traditional estimators. New spherical isotropic kernels with local support were introduced. In this paper the extension to spherical nonisotropic kernels with local support is introduced, together with a means of obtaining the shape and smoothing parameters in an objective way. The usefulness of spherical nonparametric estimators based on nonisotropic kernels is demonstrated with an application to an oceanographic feature-track ensemble. [ABSTRACT FROM AUTHOR]

On 16 May 1995, a supercell storm produced an F1 tornado near Garden City, Kansas, during VORTEX (Verification of the Origins of Rotation in Tornadoes Experiment). This event provided the first opportunity to synthesize data collected by a new airborne radar platform called ELDORA (Electra Doppler radar) developed by the National Center for Atmospheric Research. The evolution of the low- and midlevel mesocyclone was presented in a companion paper. In this paper, the sequence of events that triggered tornadogenesis within the mesocyclone circulation is shown. The occlusion downdraft, documented in Part I, appears to promote "vortex breakdown" resulting in the generation of multiple vorticity centers. It is one of these centers that intensifies into the Garden City tornado. The structural relationship between the observed wall cloud and the radar reflectivity associated with the developing hook echo and the kinematic wind field is also presented. Although the wall cloud appears to be uniform there are pronounced asymmetries in the echo and ground-relative flow fields. [ABSTRACT FROM AUTHOR]

Parameterizing deep convection at model resolutions of a few kilometers or less requires diverging from several simplifying assumptions valid at coarser resolutions. The separation or complementarity between the deep-convection scheme and the model cloud scheme must be addressed properly to prevent a double counting of some phenomena, account for evolution in time, and keep consistent results while approaching resolutions where deep convection can be treated explicitly (without parameterization). In this paper, the author formulates and tests a perturbation approach of the bulk mass-flux representation of deep convective updrafts and downdrafts. The subgrid deep-convection scheme represents only the effect of the unresolved part of the real updrafts, complementing an explicit part associated with the mean gridbox vertical velocity. Special attention is paid to the ordering and interactions of the moist parameterizations, the formulation of the closure and of the triggering, and the accounting of time evolution aspects. The multiresolution behavior of the scheme is assessed in the operational numerical prediction model ALARO [a version of the Action de Recherche Petite Echelle Grande Echelle-Aire Limitée Adaptation Dynamique Développement International (ARPEGE-ALADIN) operational limited area model with a revised and modular structure of the physical parameterizations]. Unlike most mass-flux schemes, the parameterized part gradually fades out when the model resolution is increased, allowing the results to approach those of an explicit treatment of deep convection. [ABSTRACT FROM AUTHOR]

This paper discusses the sparse space-time superparameterization (SSTSP) algorithm and evaluates its ability to represent interactions between moist convection and the large-scale circulation in the context of a Walker cell flow over a planetary scale two-dimensional domain. The SSTSP represents convective motions in each column of the large-scale model by embedding a cloud-resolving model, and relies on a sparse sampling in both space and time to reduce computational cost of explicit simulation of convective processes. Simulations are performed varying the spatial compression and/or temporal acceleration, and results are compared to the cloud-resolving simulation reported previously. The algorithm is able to reproduce a broad range of circulation features for all temporal accelerations and spatial compressions, but significant biases are identified. Precipitation tends to be too intense and too localized over warm waters when compared to the cloud-resolving simulations. It is argued that this is because coherent propagation of organized convective systems from one large-scale model column to another is difficult when superparameterization is used, as noted in previous studies. The Walker cell in all simulations exhibits low-frequency variability on a time scale of about 20 days, characterized by four distinctive stages: suppressed, intensification, active, and weakening. The SSTSP algorithm captures spatial structure and temporal evolution of the variability. This reinforces the confidence that SSTSP preserves fundamental interactions between convection and the large-scale flow, and offers a computationally efficient alternative to traditional convective parameterizations. [ABSTRACT FROM AUTHOR]

This paper documents the interaction processes between mesoscale convective systems (MCS), the tropical easterly jet (TEJ), and the African easterly jet (AEJ) over West Africa during the monsoon peak of 2006 observed during the African Monsoon Multidisciplinary Analyses (AMMA) project. The results highlight the importance of the cloud system localization relative to the jets in order to explain their duration and life cycle. A systematical study reveals that intense and long-lived MCSs correspond to a particular pattern where clouds associated with deep convection are located in entrance regions of TEJ and in exit regions of AEJ. A case study on a particularly well-documented convective event characterizes this link and infers the importance of jet streaks in promoting areas of divergence, favoring the persistence of MCSs. [ABSTRACT FROM AUTHOR]

Using NCEP-NCAR reanalysis and Japanese 25-yr Reanalysis (JRA-25) winter daily (1 December-28 February) data for the period 1979-2012, this paper reveals the leading pattern of winter daily 850-hPa wind variability over northern Eurasia from a dynamic perspective. The results show that the leading pattern accounts for 18% of the total anomalous kinetic energy and consists of two subpatterns: the dipole and the tripole wind patterns. The dipole wind pattern does not exhibit any apparent trend. The tripole wind pattern, however, has displayed significant trends since the late 1980s. The negative phase of the tripole wind pattern corresponds to an anomalous anticyclone over northern Eurasia during winter, as well as two anomalous cyclones occurring over southern Europe and in the mid- to high latitudes of East Asia. These anomalous cyclones in turn lead to enhanced winter precipitation in these two regions, as well as negative surface temperature anomalies over the mid- to high latitudes of Asia. The intensity of the tripole wind pattern and the frequency of its extreme negative phase are significantly correlated with autumn Arctic sea ice anomalies. Simulation experiments further demonstrate that the winter atmospheric response to Arctic sea ice decrease is dynamically consistent with the observed trend in the tripole wind pattern over the past 24 winters, which is one of the causes of the observed declining winter surface air temperature trend over Central and East Asia. The results of this study also imply that East Asia may experience more frequent and/or intense winter extreme weather events in association with the loss of Arctic sea ice. [ABSTRACT FROM AUTHOR]

Time indices of the Madden-Julian oscillation (MJO) are often used to generate empirical forecasts of the global atmospheric circulation. Moist deep convection associated with the MJO initiates eastward-propagating Rossby waves that disperse into the midlatitudes. The background circulation then guides extratropical waves back into the tropics of the eastern Pacific Ocean. Previous works have shown that equatorial Rossby (ER) waves occur following intrusion of extratropical Rossby waves into the tropics. Westward-propagating ER waves and the MJO modulate the total convection. This convection modulates the zonal wind, which influences the location and existence of westerly wind ducts. These wind ducts, in turn, guide extratropical waves into the tropics. This paper demonstrates through a simple composite analysis that a simultaneous assessment of MJO and ER waves yields more information about the extratropical circulation during boreal winter than can be obtained based on either type of disturbance alone, or from a sum of the signals associated with the MJO and ER waves composited separately. This analysis, together with previous results, suggests a feedback loop between the MJO, these waves, and the extratropical circulation. Thus, assessment of the ER wave state during a particular phase of the MJO might yield better empirical prediction of the global atmospheric circulation that follows. [ABSTRACT FROM AUTHOR]

The two-dimensional nonhydrostatic compressible dynamical core for the atmosphere has been developed by using a new nodal-type high-order conservative method, the so-called multimoment constrained finite-volume (MCV) method. Different from the conventional finite-volume method, the predicted variables (unknowns) in an MCV scheme are the values at the solution points distributed within each mesh cell. The time evolution equations to update the unknown point values are derived from a set of constraint conditions based on the multimoment concept, where the constraint on the volume-integrated average (VIA) for each mesh cell is cast into a flux form and thus guarantees rigorously the numerical conservation. Two important features make the MCV method particularly attractive as an accurate and practical numerical framework for atmospheric and oceanic modeling. 1) The predicted variables are the nodal values at the solution points that can be flexibly located within a mesh cell (equidistant solution points are used in the present model). It is computationally efficient and provides great convenience in dealing with complex geometry and source terms. 2) High-order and physically consistent formulations can be built by choosing proper constraints in view of not only numerical accuracy and efficiency but also underlying physics. In this paper the authors present a dynamical core that uses the third- and the fourth-order MCV schemes. They have verified the numerical outputs of both schemes by widely used standard benchmark tests and obtained competitive results. The present numerical core provides a promising and practical framework for further development of nonhydrostatic compressible atmospheric models. [ABSTRACT FROM AUTHOR]

In this paper the authors use the NCEP-Department of Energy (DOE) Reanalysis 2 (NCEP2) data from 1979 to 2004 to expand the daily 500-hPa geopotential height in the Southern Hemisphere (SH, 90°-20°S) into a double Fourier series, and analyze the temporal frequency characteristics of the expansion coefficients over various spatial scales. For the daily series over the whole year, the coefficient series of the extratropical-mean height is characterized by a significant low-frequency (10-30 day) variation. For zonal waves with ( k, l) = (1-5, 1), where k and l are the zonal and meridional wavenumbers, respectively, the low-frequency variability is most pronounced for zonal wavenumbers 3 and 4; while the short wave with zonal wavenumber 5 has significant high-frequency (4-8 day) variability. For meridional waves with ( k, l) = (0, 2-6), the meridional dipole ( l = 2) makes a major contribution to the low-frequency variability, consistent with the intraseasonal space-time features of the southern annular mode (SAM). The meridional tripole ( l = 3) also exhibits low-frequency variability. For two-dimensional waves ( k, l) = (1-5, 2-6), the dipole is a preferred meridional structure for intraseasonal modes with large zonal scales, indicating an out-of-phase relationship between low-frequency planetary-scale waves at mid- and high latitudes. The diagnostic results outlined above can be explained, to a certain extent, by the dispersion relation for Rossby waves. Theoretical analysis indicates that zonal wavenumber 3, zonally symmetric flow such as SAM, and planetary-scale waves with meridional dipole structures may be interpreted as low-frequency eigenmodes of the atmosphere. [ABSTRACT FROM AUTHOR]

This paper deals with the large-scale inertio-gravity (IG) wave energy in the operational ECMWF analyses in July 2007. Energy percentages of the IG waves obtained from the standard-pressure-level data are compared to those derived from various discretizations of the model-level data. The results show a small albeit systematic increase of the IG energy percentage as the vertical level density increases from the standard-pressure levels toward the model-level density; the small relative change is explained by the sufficient vertical resolution to resolve the large-scale IG waves in the tropics that make the majority of the global IG energy on large scales. A relatively larger increase of the IG energy is obtained when the mesospheric model levels are included; however, the analyses at these levels in July 2007 are less reliable. Furthermore, two numerical methods for the normal-mode function (NMF) decomposition are shown to provide similar results. The decomposition of atmospheric analyses into the NMF series is proposed as a tool to analyze the spatial and temporal variations of the large-scale equatorial waves and their role in global energetics. [ABSTRACT FROM AUTHOR]

This paper introduces a relocation scheme for tropical cyclone (TC) initialization in the Advanced Research Weather Research and Forecasting (ARW-WRF) model and demonstrates its application to 70 forecasts of Typhoons Sinlaku (2008), Jangmi (2008), and Linfa (2009) for which Taiwan’s Central Weather Bureau (CWB) issued typhoon warnings. An efficient and dynamically consistent TC vortex relocation scheme for the WRF terrain-following mass coordinate has been developed to improve the first guess of the TC analysis, and hence improves the tropical cyclone initialization. The vortex relocation scheme separates the first-guess atmospheric flow into a TC circulation and environmental flow, relocates the TC circulation to its observed location, and adds the relocated TC circulation back to the environmental flow to obtain the updated first guess with a correct TC position. Analysis of these typhoon cases indicates that the relocation procedure moves the typhoon circulation to the observed typhoon position without generating discontinuities or sharp gradients in the first guess. Numerical experiments with and without the vortex relocation procedure for Typhoons Sinlaku, Jangmi, and Linfa forecasts show that about 67% of the first-guess fields need a vortex relocation to correct typhoon position errors while eliminates the topographical effect. As the vortex relocation effectively removes the typhoon position errors in the analysis, the simulated typhoon tracks are considerably improved for all forecast times, especially in the early periods as large adjustments appeared without the vortex relocation. Comparison of the horizontal and vertical vortex structures shows that large errors in the first-guess fields due to an incorrect typhoon position are eliminated by the vortex relocation scheme and that the analyzed typhoon circulation is stronger and more symmetric without distortions, and better agrees with observations. The result suggests that the main difficulty of objective analysis methods [e.g., three-dimensional variational data assimilation (3DVAR)], in TC analysis comes from poor first-guess fields with incorrect TC positions rather than not enough model resolution or observations. In addition, by computing the eccentricity and correlation of the axes of the initial typhoon circulation, the distorted typhoon circulation caused by the position error without the vortex relocation scheme is demonstrated to be responsible for larger track errors. Therefore, by eliminating the typhoon position error in the first guess that avoids a distorted initial typhoon circulation, the vortex relocation scheme is able to improve the ARW-WRF typhoon initialization and forecasts particularly when using data assimilation update cycling. [ABSTRACT FROM AUTHOR]

The Boundary Layer Evolution (BLE) missions of the International H2O Project (IHOP_2002) were designed to provide comprehensive observations of the distribution of water vapor in the quiescent boundary layer and its evolution during the early morning. The case study discussed in this paper presents detailed observations of the development of the boundary layer from before sunrise through to the period of growth of the mature convective boundary layer (CBL) during the 14 June 2002 BLE mission. The large number of remote sensing platforms, including the multiple instruments collocated at the Homestead Profiling Site, provided a detailed set of measurements of the growth and structure of the CBL. The observations describe the classic evolution of a daytime CBL, beginning with a shallow nocturnal boundary layer (NBL) below the remnants of the previous day’s mixed layer, or residual layer. The vertical distribution of humidity in these layers during the early morning was affected by advection of dry air and by gravity waves. About an hour after sunrise a CBL developed, and gradually deepened with time as it mixed out the NBL and residual layer. The growth of the top of the CBL was particularly well observed because of the strong vertical gradients in temperature, humidity, and aerosol concentration. As the CBL deepened and the average CBL wind speed decreased, the mode of convective organization evolved from horizontal convective rolls to open-celled convection. A unique set of detailed measurements of the structure of the open cells was obtained from multiple instruments including the Doppler-on-Wheels radar, the Mobile Integrated Profiling System wind profiler, and the Scanning Raman lidar. They showed the relationship between open cells, thermals, mantle echoes, and the CBL top. [ABSTRACT FROM AUTHOR]

The ability to model and forecast accretion of ice on structures is very important for many industrial sectors. For example, studies conducted by the power transmission industry indicate that the majority of failures are caused by icing on overhead conductors and other components of power networks. This paper presents an extension to the ice accretion forecasting system (IAFS) that is comprised of a numerical weather prediction model, a precipitation-type algorithm, and an ice accretion model. To optimize the performance of IAFS, the parameters of the precipitation-type algorithm are estimated using a genetic algorithm. The system is developed by hindcasting a well-documented freezing-rain event and calibrated using four additional ice storms. Subsequently, the system is tested using three independent storms. The results show a significant improvement in consistency, accuracy, and skill of IAFS. The methodology described in this contribution is not limited to ice accretion modeling—it provides a general approach for setting operational parameters of data-processing algorithms to achieve interoperability of numerical weather prediction models with add-on applications based on empirical observations. [ABSTRACT FROM AUTHOR]

This paper analyzes synoptic-scale trough-thinning processes using a filamentation time diagnostic. The filamentation time diagnostic is derived from the potential vorticity equation expressed in spherical coordinates in the horizontal plane and the isentropic coordinate in the vertical direction. The diagnostic is an accurate measure of stirring processes under the condition of “slowly varying velocity gradients.” Trough-thinning processes are analyzed for one tropical example and two midlatitude examples. The results indicate that the filamentation time for the tropical trough-thinning event is generally longer than those for the midlatitude trough events. In addition to the effects of stretching and shearing deformation, the filamentation time diagnostic contains the effects of divergence. For the calculation of filamentation time on isentropic surfaces in spherical coordinates, it is acceptable to ignore the curvature effects in the tropics; however, in both the midlatitudes and the tropics, isentropic divergence effects should be retained for improved accuracy. Combining an analysis of cross potential vorticity contour flows on isentropic surfaces with the filamentation time analysis gives a more complete description of the dynamics. The results show that the filamentation time diagnostic can serve as a useful aid in the analysis and prediction of trough thinning and cutoff-low formation. [ABSTRACT FROM AUTHOR]

The isentropic system of equations has particular advantages in the numerical modeling of weather and climate. These include the elimination of the vertical velocity in adiabatic flow, which simplifies the motion to a two-dimensional problem and greatly reduces the numerical errors associated with vertical advection. The mechanism for the vertical transfer of horizontal momentum is simply the pressure drag acting on isentropic coordinate surfaces under frictionless, adiabatic conditions. In addition, vertical resolution is enhanced in regions of high static stability, which leads to better resolution of features such as the tropopause. Negative static stability and isentropic overturning frequently occur in finescale atmospheric motion. This presents a challenge to nonhydrostatic modeling with the isentropic vertical coordinate. This paper presents a new nonhydrostatic atmospheric model based on a generalized vertical coordinate. The coordinate is specified in a manner similar to that of Konor and Arakawa, but “arbitrary Eulerian–Lagrangian” (ALE) methods are used to maintain coordinate monotonicity in regions of negative static stability and to return the coordinate surfaces to their isentropic “targets” in statically stable regions. The model is mass conserving and implements a vertical differencing scheme that satisfies two additional integral constraints for the limiting case of z coordinates. The hybrid vertical coordinate model is tested with mountain-wave experiments including a downslope windstorm with breaking gravity waves. The results show that the advantages of the isentropic coordinate are realized in the model with regard to vertical tracer and momentum transport. Also, the isentropic overturning associated with the wave breaking is successfully handled by the coordinate formulation. [ABSTRACT FROM AUTHOR]

Atmospheric transport of the natural radionuclide 210Pb is simulated by a general circulation model (GCM) and calculated surface concentrations are compared with those recorded at the Tahiti station on a daily scale. Numerical results for 2006 show the underestimation of concentrations for most recorded peaks. The purpose of this paper is to explain the observed discrepancies, to evaluate the GCM physical parameterizations, and to determine by numerical means the concentrations at Tahiti for a pollutant circulating across the South Pacific Ocean. Three meteorological situations in 2006 are further analyzed. Circulation over Tahiti for these periods is simulated by a mesoscale meteorological model using four nested grids with resolutions ranging from 27 to 1 km. The calculated wind fields are validated by those observed at two stations on the northwest coast of Tahiti, which is exposed both to topography-induced vortices and to thermally driven local breezes. Atmospheric dispersion of an offshore plume is then calculated by a particle Lagrangian transport model, driven by the mesoscale model at 1- and 81-km resolutions, representing local and global circulations, respectively. Simulations at 1-km resolution show the complex atmospheric circulation over Tahiti, which results in a large spatial and temporal variability of 210Pb surface concentrations on an hourly scale. The impact of local circulation is, however, limited when daily averaged concentrations at the station are considered. Under the studied regimes, transport simulations at the two resolutions lead to similar daily averaged concentrations. The deficiencies of the GCM in simulating daily averaged 210Pb concentrations could be attributable to the deep convection parameterization. [ABSTRACT FROM AUTHOR]

In this paper a study aimed at comparing the perturbation methodologies based on the singular vector ensemble prediction system (SV-EPS) and the breeding vector ensemble prediction system (BV-EPS) in the same model environment is presented. A simple breeding system (simple BV-EPS) as well as one with regional rescaling dependent on an estimate of the analysis error variance (masked BV-EPS) were used. The ECMWF Integrated Forecast System has been used and the three experiments are compared for 46 forecast cases between 1 December 2005 and 15 January 2006. By studying the distribution of the perturbation energy it was possible to see large differences between the experiments initially, but after 48 h the distributions have converged. Using probabilistic scores, these results show that SV-EPS has a somewhat better performance for the Northern Hemisphere compared to BV-EPS. For the Southern Hemisphere masked BV-EPS and SV-EPS yield almost equal results. For the tropics the masked breeding ensemble shows the best performance during the first 6 days. One reason for this is the current setup of the singular vector ensemble at ECMWF yielding in general very low initial perturbation amplitudes in the tropics. [ABSTRACT FROM AUTHOR]

With many operational centers moving toward order 1-km-gridlength models for routine weather forecasting, this paper presents a systematic investigation of the properties of high-resolution versions of the Met Office Unified Model for short-range forecasting of convective rainfall events. The authors describe a suite of configurations of the Met Office Unified Model running with grid lengths of 12, 4, and 1 km and analyze results from these models for a number of convective cases from the summers of 2003, 2004, and 2005. The analysis includes subjective evaluation of the rainfall fields and comparisons of rainfall amounts, initiation, cell statistics, and a scale-selective verification technique. It is shown that the 4- and 1-km-gridlength models often give more realistic-looking precipitation fields because convection is represented explicitly rather than parameterized. However, the 4-km model representation suffers from large convective cells and delayed initiation because the grid length is too long to correctly reproduce the convection explicitly. These problems are not as evident in the 1-km model, although it does suffer from too numerous small cells in some situations. Both the 4- and 1-km models suffer from poor representation at the start of the forecast in the period when the high-resolution detail is spinning up from the lower-resolution (12 km) starting data used. A scale-selective precipitation verification technique implies that for later times in the forecasts (after the spinup period) the 1-km model performs better than the 12- and 4-km models for lower rainfall thresholds. For higher thresholds the 4-km model scores almost as well as the 1-km model, and both do better than the 12-km model. [ABSTRACT FROM AUTHOR]

This paper deals with the dynamical aspect of variational data assimilation in the tropics and the role of the background-error covariances in the observing system simulation experiments for the tropics. The study uses a model that describes the horizontal structure of the potential temperature and wind fields in regions of deep tropical convection. The assimilation method is three- and four-dimensional variational data assimilation. The background-error covariance model for the assimilation is a multivariate model that includes the mass–wind couplings representative of equatorial inertio-gravity modes and equatorial Kelvin and mixed Rossby–gravity modes in addition to those representative of balanced equatorial Rossby waves. Spectra of the background errors based on these waves are derived from the tropical forecast errors of the European Centre for Medium-Range Weather Forecasts (ECMWF) model. Tropical mass–wind (im)balances are illustrated by studying the potential impact of the spaceborne Doppler wind lidar (DWL) Atmospheric Dynamic Mission (ADM)-Aeolus, which measures horizontal line-of-sight (LOS) wind components. Several scenarios with two DWLs of ADM-Aeolus type are compared under different flow conditions and using different assumptions about the quality of the background-error covariances. Results of three-dimensional variational data assimilation (3DVAR) illustrate the inefficiency of multivariate assimilation in the tropics. The consequence for the assimilation of LOS winds is that the missing part of the wind vector can hardly be reconstructed from the mass-field observations and applied balances as in the case of the midlatitudes. Results of four-dimensional variational data assimilation (4DVAR) show that for large-scale tropical conditions and using reliable background-error statistics, differences among various DWL scenarios are not large. As the background-error covariances becomes less reliable, horizontal scales become smaller and the flow becomes less zonal, the importance of obtaining information about the wind vector increases. The added value of another DWL satellite increases as the quality of the background-error covariances deteriorates and it can be more than twice as large as in the case of reliable covariances. [ABSTRACT FROM AUTHOR]

Tropical Cyclone Gert (1999) experienced an extratropical transition while it merged with an extratropical cyclone upstream. The upstream extratropical cyclone had started to intensify before it merged with the transitioning tropical cyclone, and it continued intensifying afterward (12 hPa in 12 h, according to the Met Office analysis). The question addressed in this paper is the following: what was the impact of the transitioning tropical cyclone on this intensification of the upstream extratropical cyclone? Until now, in the literature, tropical cyclones that experience extratropical transition have been found to have either no impact or a positive impact on the development of extratropical cyclogenesis events. The positive impact involves either a triggering of the development of the extratropical cyclone or simply a contribution to its deepening. However, the case studied here proves to have a negative impact on the developing extratropical cyclone upstream by diminishing its intensification. Forecasts are performed with and without the tropical cyclone in the initial conditions. They show that when Gert is not present in the initial conditions, the peak intensity of the cyclone upstream occurs 9 h earlier and it is 10 hPa deeper than when Gert is present. Thus, Gert acts to weaken the development by contributing to the filling of the extratropical surface low upstream. Quasigeostropic (QG) diagnostics show that the negative impact on the extratropical development is linked to the fact that the transitioning tropical cyclone interacts with a warm front inducing a negative QG vertical velocity over the developing extratropical low upstream. This interpretation is consistent with other contrasting cases in which the transitioning tropical cyclone interacts with a cold front and induces a positive QG vertical velocity over the developing low upstream, thus enhancing its development. The results are also in agreement with idealized experiments in the literature that are aimed at studying the predictability of extratropical storms. These idealized experiments yielded similar results using synoptic-scale and mesoscale vortices as perturbations on warm and cold fronts. [ABSTRACT FROM AUTHOR]

The primary circulation of atmospheric vortices, such as tropical cyclones and tornadoes, can be estimated from single-Doppler radar observations using the ground-based velocity track display (GBVTD) algorithm. The GBVTD algorithm has limitations in the following four areas: 1) distortion in the retrieved asymmetric wind fields, 2) a limited analysis domain, 3) the inability to resolve the cross-beam component of the mean wind, and 4) the inability to separate the asymmetric tangential and radial winds. This paper presents the generalized velocity track display (GVTD) algorithm, which eliminates the first two limitations inherent in the GBVTD technique and demonstrates the possibility of subjectively estimating the mean wind vector when its signature is visible beyond the influence of the vortex circulation. In this new paradigm, the GVTD algorithm fits the atmospheric vortex circulation to a new variable VdD/RT in a linear azimuth angle (θ′), rather than the Doppler velocity Vd in a nonlinear angle (ψ), which is used in GBVTD. Key vortex kinematic structures (e.g., mean wind, axisymmetric tangential wind, etc.) in the VdD/RT space simplify the interpretation of the radar signature and eliminate the geometric distortion inherent in the Vd display. This is a significant improvement in diagnosing vortex structures in both operations and research. The advantages of using VdD/RT are illustrated using analytical atmospheric vortices, and the properties are compared with GBVTD. The characteristics of the VdD/RT display of Typhoon Gladys (1994) can be approximated by a constant mean wind plus an axisymmetric vortex. [ABSTRACT FROM AUTHOR]

Following recent results showing the potential for using surface observations of temperature, water vapor mixing ratio, and winds to determine PBL profiles, this paper reports on experiments with real observations. A 1D column model with soil, surface-layer, and PBL parameterization schemes that are the same as in the Weather Research and Forecasting model is used to estimate PBL profiles with an ensemble filter. Surface observations over the southern Great Plains are assimilated during the spring and early summer period of 2003. To strictly quantify the utility of the observations for determining PBL profiles in the ensemble filter framework, only climatological information is provided for initialization and forcing. The analysis skill, measured against rawinsondes for an independent verification, is compared against climatology to quantify the influence of the observations. Sensitivity to changing parameterization schemes, and to prescribed values of observation error variance, is examined. Temporal propagation of skillful analyses is also assessed, separating the effects of good prior state estimates from the impact of assimilation at night when covariance is weak. Results show that accurate profiles of temperature, mixing ratio, and winds are estimated with the column model and ensemble filter assimilating only surface observations. Results are largely insensitive to choice of parameterization scheme and specified observation error variance. The effects of using different parameterization schemes within the column model depend on whether assimilation is included, showing the importance of evaluating models within assimilation systems. At night, skillful estimates are possible because the influence of the observations from the previous day is temporally propagated, and atmospheric dynamics in the residual layer operate on slow time scales. It is expected that these profiles will have applications for nowcasting and secondary models (e.g., plume dispersion models) that rely on accurate specification of PBL structure. [ABSTRACT FROM AUTHOR]

This paper describes a technique for photogrammetric analysis of stereo pairs of images that is applied to the study of orographic convection. The technique is designed for use with digital images and assumes detailed knowledge of the camera properties (focal length and imaging chip) and that the position and orientation are known as a first guess. An iterative scheme using known landmarks on the frame is used to determine the camera orientation. The scheme is accurate to 10–100 m at a distance of 15 km from the camera pair. The transition from shallow to deep convection over the Santa Catalina Mountains in southern Arizona on 26 July 2005 is presented. The three-dimensional structure of the visible portion of the cloud is determined and compared with the composite reflectivity from the National Weather Service Weather Surveillance Radar-1988 Doppler radar and the tropopause height from the 1200 UTC sounding in Tucson, Arizona, providing additional validation of the scheme. The shallow to deep transition is characterized by tracking individual turrets and determining the maximum height of the cloud top. The cloud tops were limited to beneath 6000 m MSL for the first 1.5 h followed by the development of deep convection. The motion of the turrets and location of the eventual deep convection were consistent with the idea that moistening by shallow convection conditions the atmosphere for further development. [ABSTRACT FROM AUTHOR]

This paper describes the large-scale atmospheric circulation associated with persistent generalized frosts (GFs; at least 75% of the stations report frosts) in the east-central region of Argentina known as the Wet Pampa. The GF events are grouped according to their persistence, and NCEP–NCAR reanalysis data are used to create daily composites of mass and wind field anomalies during the 1961–90 winters. The GFs are caused by an anticyclonic anomaly that enters South America, generating southerly wind anomalies and cold air advection that are strengthened by the meridional layout of a cyclonic anomaly over the South Atlantic Ocean. In the case of the more persistent events the wind anomaly grows during the previous days and becomes quasi-stationary. Also, the study identifies at 250 hPa a double train of eastward-moving Rossby waves along the subtropical and subpolar latitudes, respectively, of the Southern Hemisphere. The layout of both wave trains favors the development of an intense southerly wind anomaly in the entire southern cone of the continent. On the other hand, the propagation pattern during the less persistent GFs shows only one arc-shaped Rossby wave train that reaches South America, and then propagates northeastward. Additionally, there is a subtropical jet entrance/confluence over the western side of the continent that induces a secondary meridional circulation whose subsiding branch facilitates the equatorward displacement of the low-level anticyclone, particularly in the case of the less persistent events. In the case of the more persistent GFs the confluence is located farther east and sustains essentially zonal wave train propagation, so that the surface anticyclone is not able to achieve a major equatorward penetration. [ABSTRACT FROM AUTHOR]

Tropospheric motions can be inferred from geostationary satellites by tracking clouds and water vapor in sequential imagery. These atmospheric motion vectors (AMV) have been operationally assimilated into global models for the past three decades, with positive forecast impacts. This paper presents results from a study to assess the impact of AMV derived from Geostationary Operational Environmental Satellite (GOES) imagery on mesoscale forecasts over the conventional data-poor central North Pacific region. These AMV are derived using the latest automated processing methodologies by the University of Wisconsin—Cooperative Institute for Meteorological Satellite Studies (CIMSS). For a test case, a poorly forecast subtropical cyclone (kona low) that occurred over Hawaii on 23–27 February 1997 was chosen. The Local Analysis and Prediction System (LAPS) was used to assimilate GOES-9 AMV data and to produce fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) initial conditions. The satellite wind assimilation is carried out on the 27-km-resolution domain covering the central Pacific area. The MM5 was run with three two-way nested domains (27, 9, and 3 km), with the innermost domain moving with the kona low. The AMV data are found to influence the cyclone’s development, improving the prediction of the cyclone’s central pressure and the track of the low’s center. Since September 2003, GOES-10 AMV data have been routinely accessed from CIMSS in real time and assimilated into the University of Hawaii (UH) LAPS, providing high-resolution initial conditions for twice-daily runs of MM5 at the Mauna Kea Weather Center collocated at the UH. It is found that the direct assimilation of AMV data into LAPS has a positive impact on the forecast accuracy of the UH LAPS/MM5 operational forecasting system when validated with observations in Hawaii. The implications of the results are discussed. [ABSTRACT FROM AUTHOR]

In the present paper, a 5-yr baseline integration for the period 1987–91 was carried out over a Pan-Canadian domain to validate the performance of the third-generation Canadian Regional Climate Model (CRCM). The CRCM simulated the large-scale circulation over North America well; it also correctly captured the seasonal variability of surface temperature and reproduced the winter precipitation over North America realistically. However, the CRCM systematically overestimated the summer precipitation over the continent when compared with the observed values. Extensive experiments have been conducted to trace down the sources of error of summer precipitation. Particular attention has been given to the water-vapor-related physical parameterization processes such as the mass flux convection scheme in the CRCM. Experiments involving spectral nudging of the specific humidity toward the values of large-scale driving data enabled the authors to link overestimation with abundant water vapor accumulated in the lower boundary layer resulting from an excessive amount of moisture stored in the soil. A strong boundary layer mixing process from the third generation of the Canadian Atmospheric General Circulation Model was then implemented into the CRCM along with an adjustment to the soil water holding capacity. A final analysis of a 14-month experiment showed that these modifications significantly improved the simulation of summer precipitation over North America without adversely affecting the simulation of winter precipitation. [ABSTRACT FROM AUTHOR]

The detailed analysis of the three-dimensional structure of a dryline observed over the Oklahoma panhandle during the International H2O Project (IHOP_2002) on 11 June 2002 is presented. High-resolution observations obtained from the National Center for Atmospheric Research Electra Doppler Radar (ELDORA), S-band dual-polarization Doppler radar (S-Pol), water vapor differential absorption lidar (DIAL) Lidar pour l'Etude des Interactions Aérosols Nuages Dynamique Rayonnement et du Cycle de l'Eau (LEANDRE II; translated as Lidar for the Study of Aerosol–Cloud–Dynamics–Radiation Interactions and of the Water Cycle) as well as Learjet dropsondes are used to reveal the evolution of the dryline structure during late afternoon hours when the dryline was retreating to the northwest. The dryline reflectivity shows significant variability in the along-line direction. Dry air was observed to overrun the moist air in vertical cross sections similar to a density current. The updrafts associated with the dryline were 2–3 m s-1 and were able to initiate boundary-layer-based clouds along the dryline. The formation of this dryline was caused by high equivalent potential temperature air pushing northwestward toward a stationary front in the warm sector. Middle-level clouds with radar reflectivity greater than 18 dBZe near the dryline were detected by ELDORA. A roll boundary, which was associated with larger convergence and moisture content, was evident in the S-Pol data. It is found that the instability parameters most favorable for convection initiation were actually associated with the roll boundary, not the dryline. A storm was initiated near the roll boundary probably as a result of the combination of the favorable instability parameters and stronger upward forcing. It is noted that both the 11 June 2002 dryline and the roll boundary presented in this paper would not be identified if the special datasets from IHOP_2002 were not available. Although all model runs [fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5), Meso Eta, and Rapid Update Cycle (RUC)] suggested deep convection over the Oklahoma panhandle and several cloud lines were observed near the dryline, the dryline itself did not initiate any storms. The reasons why the dryline failed to produce any storm inside the IHOP_2002 intensive observation region are discussed. Both synoptic-scale and mesoscale conditions that were detrimental to convection initiation in this case are investigated in great detail. [ABSTRACT FROM AUTHOR]

Results from a high-resolution numerical simulation of the 24 May 2002 dryline convective initiation (CI) case are presented. The simulation uses a 400 km × 700 km domain with a 1-km horizontal resolution grid nested inside a 3-km domain and starts from an assimilated initial condition at 1800 UTC. Routine as well as special upper-air and surface observations collected during the International H2O Project (IHOP_2002) are assimilated into the initial condition. The initiation of convective storms at around 2015 UTC along a section of the dryline south of the Texas panhandle is correctly predicted, as is the noninitiation of convection at a cold-front–dryline intersection (triple point) located farther north. The timing and location of predicted CI are accurate to within 20 min and 25 km, respectively. The general evolution of the predicted convective line up to 6 h of model time also verifies well. Mesoscale convergence associated with the confluent flow around the dryline is shown to produce an upward moisture bulge, while surface heating and boundary layer mixing are responsible for the general deepening of the boundary layer. These processes produce favorable conditions for convection but the actual triggering of deep moist convection at specific locations along the dryline depends on localized forcing. Interaction of the primary dryline convergence boundary with horizontal convective rolls on its west side provides such localized forcing, while convective eddies on the immediate east side are suppressed by a downward mesoscale dryline circulation. A companion paper analyzes in detail the exact processes of convective initiation along this dryline. [ABSTRACT FROM AUTHOR]

This paper introduces the use of very large ensembles for detailed sensitivity analysis and applies this technique to study the sensitivity of model forecast rainfall to initial boundary layer and soil moisture fields for a particular case from the International H2O Project (IHOP_2002) field program. In total, an aggregate ensemble of over 12 000 mesoscale model forecasts are made, with each forecast having different perturbations of boundary layer moisture, boundary layer wind, or soil moisture. Sensitivity fields are constructed from this ensemble, producing detailed sensitivity fields of defined forecast functions to initial perturbations. This work is based on mesoscale model forecasts of convection of the 24 May 2002 IHOP case, which saw convective initiation along a dryline in western Texas as well as precipitation along a cold front. The large ensemble technique proves to be highly sensitive, and both significant and subtle connections between model state variables are revealed. A number of interesting sensitivity results are obtained. It is found that soil moisture and ABL moisture have opposite effects on the amount of precipitation along the dryline; that moisture on both the dry and moist sides of the dryline was equally important; and that some small perturbations were alone responsible for entire convective storm cells near the cold front, a result implying a high level of nonlinearity. These sensitivity analyses strongly indicate the importance of accurate low-level water vapor characterization for quantitative precipitation forecasting. [ABSTRACT FROM AUTHOR]

A realistic representation of the evolution of the dry convective boundary layer in mesoscale and large-scale atmospheric models has been an elusive goal for many years. In this paper the performance of a new mixing-length formulation for the dry convective boundary layer is evaluated in the context of the Coupled Ocean– Atmosphere Mesoscale Prediction System (COAMPS). In this new formulation, the mixing length is proportional to a time scale and to the square root of the turbulent kinetic energy. The model results are tested against observations from the Climate Impact of Changes in Land Use (CICLUS) field experiment in the south of Portugal. It is shown that COAMPS with the new formulation produces a more realistic simulation of the boundary layer growth. A data assimilation experiment performed with COAMPS shows that the improvements provided by the new formulation are significant, particularly in terms of the humidity vertical distribution. Finally, one-dimensional simulations are used to confirm that the new formulation provides more accurate results because of a more realistic representation of the entrainment and of the vertical mixing in general. [ABSTRACT FROM AUTHOR]

This paper presents an observational analysis of tropical cyclone landfall over the eastern Pacific Ocean. The tropical cyclone that developed during the 2001 season off the Mexican coast was named Juliette and made landfall in the Baja California peninsula. Juliette approached land over the southwestern peninsula, and the evolution of a localized region of cyclonic circulation occurred over the Gulf of California. The storm passage was associated with extensive property damage to the population in the southern peninsula, with most of the damage caused by heavy rainfall and strong winds. The intensity of the circulation associated with Juliette indicates that this was a strong system in the record of landfall during the period 1992–2002. All available sources of observations are collected to investigate characteristics of the circulation motion and structure. In situ observations, including surface and upper-air data, are applied to document the evolution of storm flow while the center was located near and over the southern peninsula. Imagery from a geostationary satellite is used to determine characteristics of convective patterns. The analysis indicates development of distinct bands of deep convection and a period of slow system motion, which are related to large precipitation. Low-level winds are used to identify characteristics of the incident circulation along the Pacific coast. However, the nocturnal nature of the event and the lack of observations at critical times limit the attempts to explain how the incident circulation is associated with that observed over the gulf the next morning. [ABSTRACT FROM AUTHOR]

A new parameterization of shallow cumulus convection is presented. The parameterization consists of a mass flux scheme based on a buoyancy-sorting, entrainment–detrainment plume model. The mass flux scheme is coupled to a 1.5-order turbulence closure model with an entrainment closure for convective boundary layers. Model performance is verified using single-column-model simulations at relatively high vertical resolution of pure trade-cumulus convection and a cumulus to stratocumulus transition. Mixing rates, cloud cover, and vertical flux profiles, as deduced from previously published large-eddy simulation studies, are well reproduced by the parameterization. The model is used to demonstrate that height variations of lateral mixing rates can be successfully captured by a simple implementation of a buoyancy-sorting mechanism in the updraft cloud model. A companion paper describes the implementation of the scheme in a mesoscale model. [ABSTRACT FROM AUTHOR]

The impact of physical parameterizations on simulations of cloud-topped marine boundary layers is investigated using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Three-month MM5 simulations of the northeast and southeast Pacific during June–August 1987 are boundary forced with time- varying ECMWF analyses. Runs with four planetary boundary layer (PBL) parameterizations already implemented in MM5 are compared with runs using new parameterizations of boundary layer turbulence and shallow cumulus convection (ShCu) described in a companion paper. Numerous modifications to the MM5 that allow it to be used as a regional climate model are described. The simulated 3-month mean shortwave cloud radiative forcing (SWCF) and vertical structure of cloud-topped boundary layers in the northeast Pacific are sensitive to the PBL/shallow convection schemes. All four current MM5 PBL schemes [the Blackadar, Medium-Range Forecast (MRF), Burk–Thompson, and Gayno–Seaman schemes] produce overly shallow boundary layers with excessive SWCF throughout this region, especially in the transition from stratocumulus to trade cumulus where their SWCF errors range from 130 (Gayno–Seaman) to 200 W m[sup -2] (MRF). These errors likely reflect inadequate vertical mixing by parameterized turbulence and shallow convection. The only shallow convection scheme available for MM5, the Grell scheme, was almost totally inactive in this region, so no shallow convection scheme was used for the above simulations. The Grenier– Bretherton scheme, which entrains more aggressively above stratocumulus-capped convective layers, has much better regional SWCF and vertical structure. Without shallow cumulus convection, the scheme still produces excessive cloud in the transition regions; the main impact of the ShCu parameterization is to remove this bias. With all schemes, the near-surface air has a cool, dry bias, and surface turbulent fluxes are somewhat larger than observed. Sensitivity studies show that the SWCF is sensitive to a halving of the cloud droplet concentration, to plausible uncertainties in parameterized penetrative mixing at cumulus cloud tops and in stratocumulus entrainment, and (in the coastal zone) to horizontal resolution. Southeast Pacific simulations show that the GB01+ShCu scheme can also accurately represent another subtropical boundary layer cloud regime. [ABSTRACT FROM AUTHOR]

In this paper a Godunov-type methodology is applied to three-dimensional global atmospheric modeling. Numerical issues are addressed regarding the formulation of the tracer advection problem, the application of dimensional splitting, and the implementation of a Godunov-type scheme, based on the WAF approach, on spherical geometries. Particular attention is paid to addressing the problems that arise because of the convergence of the grid lines toward the Poles. A three-dimensional model is then built on the sphere that is based on a uniform longitude-latitude-height grid. This provides the framework within which an adaptive mesh refinement (AMR) algorithm is applied, to enhance the efficiency and accuracy with which results are obtained. These methods are not commonly used in the area of atmospheric modeling, but AMR in particular is commonly used with great success in other areas of computational fluid dynamics. The model is initially validated using a series of idealized case studies that have exact solutions, but is then developed into an offline model of tracer advection, forced by data from meteorological analyses, suitable to study the evolution of trace chemical species in the atmosphere. [ABSTRACT FROM AUTHOR]

An El Niño–Southern Oscillation (ENSO) prediction system with a coupled general circulation model and an ocean data assimilation scheme has been developed at the Australian Bureau of Meteorology Research Centre (BMRC). The coupled model consists of an R21L9 version of the BMRC climate model and a global version of the Geophysical Fluid Dynamics Laboratory modular ocean general circulation model with resolution focused in the tropical region and 25 vertical levels. A univariate statistical interpolation method, with 10-day data ingestion windows, is used to assimilate ocean temperature data and initialize the coupled model. The coupling procedure does not use any flux corrections. Hindcasts have been carried out for the period 1981–95 for each season (60 in all), for up to a lead time of 12 months. This paper will describe these initial experiments and show that the skill of sea surface temperature (SST) hindcasts in the tropical Pacific is comparable to other published coupled models. The skill of the model is strongest in the central Pacific. SST skill tends to be lower during the earlier 1990s than during 1980s in the eastern Pacific but not in the central Pacific. Since the ENSO SST anomaly in the central Pacific is the most important forcing of regional and global climate anomalies, the high SST prediction skill and its insensitivity over the hindcast period in this region in this model give grounds for optimism in the use of coupled general circulation models. [ABSTRACT FROM AUTHOR]

The Operational Multiscale Environment Model with Grid Adaptivity (OMEGA) and its embedded Atmospheric Dispersion Model is a new atmospheric simulation system for real-time hazard prediction, conceived out of a need to advance the state of the art in numerical weather prediction in order to improve the capability to predict the transport and diffusion of hazardous releases. OMEGA is based upon an unstructured grid that makes possible a continuously varying horizontal grid resolution ranging from 100 km down to 1 km and a vertical resolution from a few tens of meters in the boundary layer to 1 km in the free atmosphere. OMEGA is also naturally scale spanning because its unstructured grid permits the addition of grid elements at any point in space and time. In particular, unstructured grid cells in the horizontal dimension can increase local resolution to better capture topography or the important physical features of the atmospheric circulation and cloud dynamics. This means that OMEGA can readily adapt its grid to stationary surface or terrain features, or to dynamic features in the evolving weather pattern. While adaptive numerical techniques have yet to be extensively applied in atmospheric models, the OMEGA model is the first model to exploit the adaptive nature of an unstructured gridding technique for atmospheric simulation and hence real-time hazard prediction. The purpose of this paper is to provide a detailed description of the OMEGA model, the OMEGA system, and a detailed comparison of OMEGA forecast results with data. [ABSTRACT FROM AUTHOR]

This paper documents the development and testing of a new type of atmospheric dynamical core. The model solves the vorticity and divergence equations in place of the momentum equation. The model is discretized in the horizontal using a geodesic grid that is nearly uniform over the entire globe. The geodesic grid is formed by recursively bisecting the triangular faces of a regular icosahedron and projecting those new vertices onto the surface of the sphere. All of the analytic horizontal operators are reduced to line integrals, which are numerically evaluated with second-order accuracy. In the vertical direction the model can use a variety of coordinate systems, including a generalized sigma coordinate that is attached to the top of the boundary layer. Terms related to gravity wave propagation are isolated and an efficient semi-implicit time-stepping scheme is implemented. Since this model combines many of the positive attributes of both spectral models and conventional finite-difference models into a single dynamical core, it represents a distinctively new approach to modeling the atmosphere’s general circulation. The model is tested using the idealized forcing proposed by Held and Suarez. Results are presented for simulations using 2562 polygons (approximately 4.5° × 4.5°) and using 10 242 polygons (approximately 2.25° × 2.25°). The results are compared to those obtained with spectral model simulations truncated at T30 and T63. In terms of first and second moments of state variables such as the zonal wind, meridional wind, and temperature, the geodesic grid model results using 2562 polygons are comparable to those of a spectral model truncated at slightly less than T30, while a simulation with 10 242 polygons is comparable to a spectral model simulation truncated at slightly less than T63. In order to further demonstrate the viability of this modeling approach, preliminary results obtained from a full-physics general... [ABSTRACT FROM AUTHOR]

In this paper extensions to an existing tracking algorithm are described. These extensions implement adaptive tracking constraints in the form of regional upper-bound displacements and an adaptive track smoothness constraint. Together, these constraints make the tracking algorithm more flexible than the original algorithm (which used fixed tracking parameters) and provide greater confidence in the tracking results. The result of applying the new algorithm to high-resolution ECMWF reanalysis data is shown as an example of its effectiveness. [ABSTRACT FROM AUTHOR]

Transient incursions of midlatitude air to the east of the Andes Mountains into subtropical and tropical latitudes are a distinctive feature of the synoptic climatology over South America. The mean synoptic-scale structure of these incursions is documented in this paper on the basis of a compositing analysis of NCEP--NCAR reanalyzed meteorological fields and satellite-based outgoing longwave radiation measurements. Although these incursions are a year-round phenomenon, with relatively modest seasonal changes in their structure, this analysis is focused on the austral summer, when they have their largest impact in the precipitation field. The summertime incursions move equatorward at a mean speed of 10 m s 21 and retain their identities over intervals of about 5 days. The upper-level circulation is characterized by a midlatitude trough--ridge couplet that provides the quasigeostrophic forcing of the system. Before the onset of the incursions of midlatitude air, the approaching upper-level trough tends to deepen the semipermanent area of low pressure over the central plains of the continent (25°--10°S). The strong southward advection of warm, humid air creates favorable conditions for the development of deep convection over central and southern Argentina. During the next two days, a migratory cold anticyclone, steered by the ridge aloft, moves onto the southern plains of South America (40°-- 35°S), establishing a large-scale, southward-directed pressure gradient. Due to the blocking effect of the Andes, the induced low-level circulation assumes the form of an agesotrophic southerly flow that produces the equa-torward advection of the cold air. In this stage, enhanced convection occurs in a well-defined band of the intensified low-level convergence at the leading edge of the cold surge. This synoptic-scale banded structure in the convective cloudiness also emerges as the dominant mode of the day-to-day variability of the deep... [ABSTRACT FROM AUTHOR]

Q-vector partitioning has proven to be a useful tool for the understanding of the frictionless, adiabatic processes responsible for the generation of synoptic-scale vertical motion in the extratropical atmosphere. Partitioning of Q into components parallel and normal to the isotherms on an isobaric surface is standard practice in studies dealing with vertical motion and frontogenesis. This paper is concerned with vertical motion only and examines the consequences of projecting Q onto isohypses, instead of isotherms, on an isobaric surface. Specifically, the Q vector is partitioned in the natural coordinate system that follows the geostrophic wind. The novelty with this partitioning is that it naturally leads to the evaluation of different vertical motion forcing mechanisms, among which are those related to flow curvature and to confluence or diffluence. This evaluation is illustrated by applying the new Q-vector partition to a gridded analysis of a real weather situation. An important conclusion is that the thermal advection by horizontal geostrophic shear is as significant to the forcing of vertical motion as the geostrophic confluence/diffluence. While this result has previously been obtained in the study of frontal dynamics, this is the first application of this finding to the synoptic scale. [ABSTRACT FROM AUTHOR]

In this paper a condensation scheme is implemented in a nonhydrostatic mesoscale model (MEMO). An overview of the model and the modified Sundqvist condensation scheme are given. The model, including condensation (MEMOIC), is tested on two well-known phenomena that are likely to produce clouds in humid atmospheres: sea breeze and flow over a mesoscale mountain. First the sea-breeze simulations are used to show the effect increased horizontal resolution has on the results (wind velocities, cloud water, and precipitation pattern). It is concluded that the circulation generally increases due to increased resolution and that the Sundqvist condensation scheme performs well with horizontal model resolution as high as 2.5 km. Second, a number of simulations of flow over mountain are discussed. Upstream flow parameters and the shape of the mountain are changed to test the usability of MEMOIC. Also, for this flow pattern, MEMOIC performs well. It is shown that it is able to handle released instabilities and that it gives reasonable results for horizontal grid resolution as high as 500 m. For the discussed flow patterns it is hard to give a lower horizontal-resolution limit for MEMOIC. [ABSTRACT FROM AUTHOR]

Examines papers presented by D. J. Parker and C. Liu and M. W. Moncrieff regarding interaction between a cold pool of low-level air and an ambient flow. Information on two-dimensional flow from a fixed-temperature source; Details of three-dimensional flow away from a source of fixed cooling rate; Problems in the application of ideas from the study of bouyant plumes to atmospheric down drafts forced by evaporation and precipitation.