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- Author or Editor: Andrea Buzzi x
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Abstract
A recent paper by Mak et al. grants the opportunity to discuss two different definitions of the frontogenetical function proposed in the literature to study the formation and evolution of upper-level fronts. This comment exposes some problems that, in this author’s opinion, are related to the use of the Lagrangian tendency of the 3D (in place of horizontal) gradient of potential temperature, as adopted in the Mak et al. paper.
Abstract
A recent paper by Mak et al. grants the opportunity to discuss two different definitions of the frontogenetical function proposed in the literature to study the formation and evolution of upper-level fronts. This comment exposes some problems that, in this author’s opinion, are related to the use of the Lagrangian tendency of the 3D (in place of horizontal) gradient of potential temperature, as adopted in the Mak et al. paper.
Abstract
Humidity stratification may affect baroclinic wave growth by inducing vertical confinement in the layer where small amplitude updrafts are saturated. By means of a two-dimensional numerical model the authors show that small amplitude modes are trapped in saturated regions, exhibiting horizontal and vertical scales that are smaller and growth rates that are larger than those of Eady waves in a dry atmosphere. Then the small amplitude is contrasted with the finite amplitude growth: whereas in the former a clear-cut modal structure is attained, in the latter the eventual mixing of moisture alters the initial environment to the point that shallow waves are no longer supported. When the alteration of the basic state takes place the amplitude of the shallow wave, although finite, is small enough that the approximations adopted to represent meridional advection of moisture in this two-dimensional model are still valid. The effect of surface drag is also discussed and it is shown that it does not prevent modal and finite amplitude growth in the moist atmosphere.
Abstract
Humidity stratification may affect baroclinic wave growth by inducing vertical confinement in the layer where small amplitude updrafts are saturated. By means of a two-dimensional numerical model the authors show that small amplitude modes are trapped in saturated regions, exhibiting horizontal and vertical scales that are smaller and growth rates that are larger than those of Eady waves in a dry atmosphere. Then the small amplitude is contrasted with the finite amplitude growth: whereas in the former a clear-cut modal structure is attained, in the latter the eventual mixing of moisture alters the initial environment to the point that shallow waves are no longer supported. When the alteration of the basic state takes place the amplitude of the shallow wave, although finite, is small enough that the approximations adopted to represent meridional advection of moisture in this two-dimensional model are still valid. The effect of surface drag is also discussed and it is shown that it does not prevent modal and finite amplitude growth in the moist atmosphere.
Abstract
The intense precipitation event that occurred between 3 and 6 November 1994 and caused extensive flooding over Piedmont in northwestern Italy is simulated and tested with respect to various physical aspects, using a meteorological mesoscale model (BOLAM).
The period when the most intense rain occurred, mainly covering the second half of 4 and all of 5 November, is examined. A control experiment, starting at 1200 UTC 4 November, simulates the two observed precipitation peaks and captures the magnitude and timing of the most intense precipitation well even at relatively low horizontal resolution (about 30 km). The European Centre for Medium-Range Weather Forecasts analyses are used to provide the initial and boundary conditions. Model output diagnostics and comparison with observations indicate that most of the precipitation is associated with a prefrontal low-level jet, ahead of the cold front, impinging upon the orography of the region (Alps and Apennines). The model simulates a multiple rainband and frontal structure whose evolution determines both intensity and location of the prefrontal warm and moist flow. Almost all of the simulated precipitation over the Alps forms in the middle–low troposphere through forced ascent, whereas part of the secondary maximum, observed over the Apennines, is of convective type.
Sensitivity experiments have been conducted to investigate the effects of orography, surface fluxes, and latent heat exchange processes in the atmosphere. The role of the orography is crucial in determining distribution and amount of precipitation, whereas sensible and latent heat fluxes from the Mediterranean Sea (over the period considered) enhanced only the convective precipitation. Distinct dynamical effects, important for the amount and the spatial distribution of precipitation, are found to be associated with warming due to condensation and cooling due to evaporation and melting of precipitation. The latter process seems to be responsible for the simulated formation of rainbands and complex evolution of the cold front over the western Mediterranean. The multiple front life cycle and propagation feeds back on the simulated precipitation distribution, affecting the location of the prefrontal moist flow. Condensation affects the atmospheric effective stratification where the flow impinges on the orography, determining the flow regime (orographic lifting vs blocking and flow around), which, in turn, has an important impact on precipitation.
Abstract
The intense precipitation event that occurred between 3 and 6 November 1994 and caused extensive flooding over Piedmont in northwestern Italy is simulated and tested with respect to various physical aspects, using a meteorological mesoscale model (BOLAM).
The period when the most intense rain occurred, mainly covering the second half of 4 and all of 5 November, is examined. A control experiment, starting at 1200 UTC 4 November, simulates the two observed precipitation peaks and captures the magnitude and timing of the most intense precipitation well even at relatively low horizontal resolution (about 30 km). The European Centre for Medium-Range Weather Forecasts analyses are used to provide the initial and boundary conditions. Model output diagnostics and comparison with observations indicate that most of the precipitation is associated with a prefrontal low-level jet, ahead of the cold front, impinging upon the orography of the region (Alps and Apennines). The model simulates a multiple rainband and frontal structure whose evolution determines both intensity and location of the prefrontal warm and moist flow. Almost all of the simulated precipitation over the Alps forms in the middle–low troposphere through forced ascent, whereas part of the secondary maximum, observed over the Apennines, is of convective type.
Sensitivity experiments have been conducted to investigate the effects of orography, surface fluxes, and latent heat exchange processes in the atmosphere. The role of the orography is crucial in determining distribution and amount of precipitation, whereas sensible and latent heat fluxes from the Mediterranean Sea (over the period considered) enhanced only the convective precipitation. Distinct dynamical effects, important for the amount and the spatial distribution of precipitation, are found to be associated with warming due to condensation and cooling due to evaporation and melting of precipitation. The latter process seems to be responsible for the simulated formation of rainbands and complex evolution of the cold front over the western Mediterranean. The multiple front life cycle and propagation feeds back on the simulated precipitation distribution, affecting the location of the prefrontal moist flow. Condensation affects the atmospheric effective stratification where the flow impinges on the orography, determining the flow regime (orographic lifting vs blocking and flow around), which, in turn, has an important impact on precipitation.
Abstract
A two-dimensional semi-geostrophic model in isentropic coordinates is applied to the study of internal frontogenesis induced by a geostrophic deformation field. A continuous potential vorticity distribution is considered and the upper and lower boundaries are chosen to be isentropic. In the initial conditions a jet structure is prescribed. Internal frontogenesis takes place under such conditions and is thus shown not to be a side effect of intensification of temperature gradients at the ground level. The model reproduces some of the observed features of the process, although the ageostrophic circulation is in a thermodynamically direct sense and a temperature discontinuity only forms in an infinite time.
Abstract
A two-dimensional semi-geostrophic model in isentropic coordinates is applied to the study of internal frontogenesis induced by a geostrophic deformation field. A continuous potential vorticity distribution is considered and the upper and lower boundaries are chosen to be isentropic. In the initial conditions a jet structure is prescribed. Internal frontogenesis takes place under such conditions and is thus shown not to be a side effect of intensification of temperature gradients at the ground level. The model reproduces some of the observed features of the process, although the ageostrophic circulation is in a thermodynamically direct sense and a temperature discontinuity only forms in an infinite time.
Abstract
Since August 2009, the GLOBO atmospheric general circulation model has been running experimentally at the Institute of Atmospheric Sciences and Climate (ISAC) of the National Council of Research of Italy. GLOBO is derived from the Bologna Limited Area Model (BOLAM), a gridpoint limited-area meteorological model that was developed at the same institute and that has been extended to the entire earth atmosphere. The main dynamical features and physical parameterizations of GLOBO are presented. Starting from initial conditions obtained from the analysis of the NCEP Global Forecast System (GFS) model valid at 0000 UTC, 6-day forecasts with average horizontal resolution of 32 km were performed on a daily basis and in real time. The assessment of the forecast skill during the 1.5-yr period included the calculation of the monthly averaged root-mean-square errors (model prediction versus gridded analyses) of geopotential height at 500 hPa and mean sea level pressure for the northern and southern extratropics, performed accordingly to WMO Commission for Basic Systems (CBS) standards. The verification results are compared with models from other global data processing and forecasting system centers, as are available in the literature. The GLOBO skill for medium-range forecasts turns out to be comparable to that of the above models. The lack of analyses based on model forecasts and data assimilation is likely to penalize the scores for shorter-term forecasts.
Abstract
Since August 2009, the GLOBO atmospheric general circulation model has been running experimentally at the Institute of Atmospheric Sciences and Climate (ISAC) of the National Council of Research of Italy. GLOBO is derived from the Bologna Limited Area Model (BOLAM), a gridpoint limited-area meteorological model that was developed at the same institute and that has been extended to the entire earth atmosphere. The main dynamical features and physical parameterizations of GLOBO are presented. Starting from initial conditions obtained from the analysis of the NCEP Global Forecast System (GFS) model valid at 0000 UTC, 6-day forecasts with average horizontal resolution of 32 km were performed on a daily basis and in real time. The assessment of the forecast skill during the 1.5-yr period included the calculation of the monthly averaged root-mean-square errors (model prediction versus gridded analyses) of geopotential height at 500 hPa and mean sea level pressure for the northern and southern extratropics, performed accordingly to WMO Commission for Basic Systems (CBS) standards. The verification results are compared with models from other global data processing and forecasting system centers, as are available in the literature. The GLOBO skill for medium-range forecasts turns out to be comparable to that of the above models. The lack of analyses based on model forecasts and data assimilation is likely to penalize the scores for shorter-term forecasts.
Abstract
We describe a simple and economic method for reducing the errors that can result from the irregular distribution of data points in linear interpolation schemes that use prescribed, isotropic weighting (IW) functions. The method can be applied to single-step analysis as well as to schemes consisting of more than one step.
The starting point of the analysis algorithm is the generation of two datasets (with an IW scheme), one by interpolating the observed field onto the collocated observing sites, the other by interpolating the observed field onto a regular grid. These two datasets are then used independently to estimate two new gridpoint fields as outputs from the same IW analysis scheme. It is assumed that the difference between these two new gridpoint fields is a measure of the error field that results from applying the IW scheme to an inhomogeneous distribution of observing sites, and that this error field is not very different from that associated with the initial gridpoint field analysis. It is therefore used as a basis for correcting the initial gridpoint field analysis. This procedure can be applied iteratively, and is shown to converge when applied to realistic data distributions sampling both real and simulated meteorological fields.
Each step of the iterative scheme is described in terms of a frequency response function in the presence of irregularly spaced data points, in order to illustrate its general convergence properties. The performance of the analysis algorithm has also been investigated in the context of the two-step Barnes analysis scheme and its application to scale separation analysis. Applications of the method to simulated and observed data show that the deviations between analyses and original fields are substantially reduced following a small number of iterations.
Abstract
We describe a simple and economic method for reducing the errors that can result from the irregular distribution of data points in linear interpolation schemes that use prescribed, isotropic weighting (IW) functions. The method can be applied to single-step analysis as well as to schemes consisting of more than one step.
The starting point of the analysis algorithm is the generation of two datasets (with an IW scheme), one by interpolating the observed field onto the collocated observing sites, the other by interpolating the observed field onto a regular grid. These two datasets are then used independently to estimate two new gridpoint fields as outputs from the same IW analysis scheme. It is assumed that the difference between these two new gridpoint fields is a measure of the error field that results from applying the IW scheme to an inhomogeneous distribution of observing sites, and that this error field is not very different from that associated with the initial gridpoint field analysis. It is therefore used as a basis for correcting the initial gridpoint field analysis. This procedure can be applied iteratively, and is shown to converge when applied to realistic data distributions sampling both real and simulated meteorological fields.
Each step of the iterative scheme is described in terms of a frequency response function in the presence of irregularly spaced data points, in order to illustrate its general convergence properties. The performance of the analysis algorithm has also been investigated in the context of the two-step Barnes analysis scheme and its application to scale separation analysis. Applications of the method to simulated and observed data show that the deviations between analyses and original fields are substantially reduced following a small number of iterations.
Demonstration of probabilistic hydrological and atmospheric simulation of flood events in the Alpine region (D-PHASE) is made by the Forecast Demonstration Project in connection with the Mesoscale Alpine Programme (MAP). Its focus lies in the end-to-end flood forecasting in a mountainous region such as the Alps and surrounding lower ranges. Its scope ranges from radar observations and atmospheric and hydrological modeling to the decision making by the civil protection agents. More than 30 atmospheric high-resolution deterministic and probabilistic models coupled to some seven hydrological models in various combinations provided real-time online information. This information was available for many different catchments across the Alps over a demonstration period of 6 months in summer/fall 2007. The Web-based exchange platform additionally contained nowcasting information from various operational services and feedback channels for the forecasters and end users. D-PHASE applications include objective model verification and intercomparison, the assessment of (subjective) end user feedback, and evaluation of the overall gain from the coupling of the various components in the end-to-end forecasting system.
Demonstration of probabilistic hydrological and atmospheric simulation of flood events in the Alpine region (D-PHASE) is made by the Forecast Demonstration Project in connection with the Mesoscale Alpine Programme (MAP). Its focus lies in the end-to-end flood forecasting in a mountainous region such as the Alps and surrounding lower ranges. Its scope ranges from radar observations and atmospheric and hydrological modeling to the decision making by the civil protection agents. More than 30 atmospheric high-resolution deterministic and probabilistic models coupled to some seven hydrological models in various combinations provided real-time online information. This information was available for many different catchments across the Alps over a demonstration period of 6 months in summer/fall 2007. The Web-based exchange platform additionally contained nowcasting information from various operational services and feedback channels for the forecasters and end users. D-PHASE applications include objective model verification and intercomparison, the assessment of (subjective) end user feedback, and evaluation of the overall gain from the coupling of the various components in the end-to-end forecasting system.
Abstract
No Abstract available.
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No Abstract available.
Abstract
The authors evaluate the performance of current regional models in an intercomparison project for a case of explosive secondary marine cyclogenesis occurring during the Canadian Atlantic Storms Project and the Genesis of Atlantic Lows Experiment of 1986. Several systematic errors are found that have been identified in the refereed literature in prior years. There is a high (low) sea level pressure bias and a cold (warm) tropospheric temperature error in the oceanic (continental) regions. Though individual model participants produce central pressures of the secondary cyclone close to the observed during the final stages of its life cycle, systematically weak systems are simulated during the critical early stages of the cyclogenesis. Additionally, the simulations produce an excessively weak (strong) continental anticyclone (cyclone); implications of these errors are discussed in terms of the secondary cyclogenesis. Little relationship between strong performance in predicting the mass field and skill in predicting a measurable amount of precipitation is found. The bias scores in the precipitation study indicate a tendency for all models to overforecast precipitation. Results for the measurable threshold (0.2 mm) indicate the largest gain in precipitation scores results from increasing the horizontal resolution from 100 to 50 km, with a negligible benefit occurring as a consequence of increasing the resolution from 50 to 25 km. The importance of a horizontal resolution increase from 100 to 50 km is also generally shown for the errors in the mass field. However, little improvement in the prediction of the cyclogenesis is found by increasing the horizontal resolution from 50 to 25 km.
Abstract
The authors evaluate the performance of current regional models in an intercomparison project for a case of explosive secondary marine cyclogenesis occurring during the Canadian Atlantic Storms Project and the Genesis of Atlantic Lows Experiment of 1986. Several systematic errors are found that have been identified in the refereed literature in prior years. There is a high (low) sea level pressure bias and a cold (warm) tropospheric temperature error in the oceanic (continental) regions. Though individual model participants produce central pressures of the secondary cyclone close to the observed during the final stages of its life cycle, systematically weak systems are simulated during the critical early stages of the cyclogenesis. Additionally, the simulations produce an excessively weak (strong) continental anticyclone (cyclone); implications of these errors are discussed in terms of the secondary cyclogenesis. Little relationship between strong performance in predicting the mass field and skill in predicting a measurable amount of precipitation is found. The bias scores in the precipitation study indicate a tendency for all models to overforecast precipitation. Results for the measurable threshold (0.2 mm) indicate the largest gain in precipitation scores results from increasing the horizontal resolution from 100 to 50 km, with a negligible benefit occurring as a consequence of increasing the resolution from 50 to 25 km. The importance of a horizontal resolution increase from 100 to 50 km is also generally shown for the errors in the mass field. However, little improvement in the prediction of the cyclogenesis is found by increasing the horizontal resolution from 50 to 25 km.
