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- Author or Editor: Jordan C. Alpert x
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Abstract
An analysis of the kinetic energy budget is made for two intensely developing cyclones over North America. The principal kinetic energy source for the first cyclone is the net horizontal transport of kinetic energy across the boundaries of the region enclosing the cyclone. For the second cyclone, it is the local kinetic energy generation. By investigating the kinetic energy budget of the vertically averaged flow (barotropic part) and the vertical shear flow (baroclinic part) it is found that the horizontal transport contribution to the kinetic energy budget of the first cyclone is evenly divided between the barotropic and baroclinic components. However, the kinetic energy generation is the dominant energy source of the second cyclone, and the horizontal transport is an energy sink. The vertical shear kinetic energy reservoir did not act as a “catalyst” as in hemispheric studies but varied during cyclone development.
Abstract
An analysis of the kinetic energy budget is made for two intensely developing cyclones over North America. The principal kinetic energy source for the first cyclone is the net horizontal transport of kinetic energy across the boundaries of the region enclosing the cyclone. For the second cyclone, it is the local kinetic energy generation. By investigating the kinetic energy budget of the vertically averaged flow (barotropic part) and the vertical shear flow (baroclinic part) it is found that the horizontal transport contribution to the kinetic energy budget of the first cyclone is evenly divided between the barotropic and baroclinic components. However, the kinetic energy generation is the dominant energy source of the second cyclone, and the horizontal transport is an energy sink. The vertical shear kinetic energy reservoir did not act as a “catalyst” as in hemispheric studies but varied during cyclone development.
Abstract
The systematic error in the National Meterological Center's (NMC) medium-range operational forecasts of the global divergence during 1987 and 1988 is examined in this study. As in other operational NWP models the NMC model has too weak an annual mean, annual cycle, and 30–60 day oscillation. This weakness shows up after only a few days, especially over the monsoon region and tropical areas. When the intraseasonal oscillation amplitude is large the model predictive skill is improved. The eastward propagation of the low-frequency mode is well predicted throughout the 10-day forecast period. The north-south migration of the tropical Hadley circulation, depicted primarily by the annual-cycle mode, is also weaker although the phase pattern predicted by the MRF is consistent with analyses.
Abstract
The systematic error in the National Meterological Center's (NMC) medium-range operational forecasts of the global divergence during 1987 and 1988 is examined in this study. As in other operational NWP models the NMC model has too weak an annual mean, annual cycle, and 30–60 day oscillation. This weakness shows up after only a few days, especially over the monsoon region and tropical areas. When the intraseasonal oscillation amplitude is large the model predictive skill is improved. The eastward propagation of the low-frequency mode is well predicted throughout the 10-day forecast period. The north-south migration of the tropical Hadley circulation, depicted primarily by the annual-cycle mode, is also weaker although the phase pattern predicted by the MRF is consistent with analyses.
Abstract
The possibility of planetary wave coupling between the troposphere and solar-induced alterations in the upper atmosphere providing a viable mechanism for giving rise to sun-weather relationships is investigated. Some of the observational evidence for solar-activity-induced effects on levels of the upper atmosphere ranging from the thermosphere down to the lower stratosphere are reviewed. It is concluded that there is evidence for such effects extending down to the middle stratosphere and below. Evidence is also reviewed that these effects are due to changes in solar ultraviolet emission during disturbed solar conditions. A theoretical planetary wave model is then used to see at what levels in the upper atmosphere moderate changes in the mean zonal wind state would result in tropospheric changes. It is concluded that changes in the mean zonal flow of ∼20% at levels in the vicinity of 35 km or below would give rise to changes in the tropospheric planetary wave pattern that are less than but on the same order as the observed interannual variability in the tropospheric wave pattern at middle and high latitudes. Thus, planetary wave coupling between the troposphere and the upper atmosphere appears to be a plausible mechanism to give a tropospheric response to solar activity. This mechanism is not viable, however, to provide for short-period changes such as the suggested solar sector boundary vorticity index relation, but rather is applicable to changes of longer period such as the 11- or 22-year solar cycles.
Abstract
The possibility of planetary wave coupling between the troposphere and solar-induced alterations in the upper atmosphere providing a viable mechanism for giving rise to sun-weather relationships is investigated. Some of the observational evidence for solar-activity-induced effects on levels of the upper atmosphere ranging from the thermosphere down to the lower stratosphere are reviewed. It is concluded that there is evidence for such effects extending down to the middle stratosphere and below. Evidence is also reviewed that these effects are due to changes in solar ultraviolet emission during disturbed solar conditions. A theoretical planetary wave model is then used to see at what levels in the upper atmosphere moderate changes in the mean zonal wind state would result in tropospheric changes. It is concluded that changes in the mean zonal flow of ∼20% at levels in the vicinity of 35 km or below would give rise to changes in the tropospheric planetary wave pattern that are less than but on the same order as the observed interannual variability in the tropospheric wave pattern at middle and high latitudes. Thus, planetary wave coupling between the troposphere and the upper atmosphere appears to be a plausible mechanism to give a tropospheric response to solar activity. This mechanism is not viable, however, to provide for short-period changes such as the suggested solar sector boundary vorticity index relation, but rather is applicable to changes of longer period such as the 11- or 22-year solar cycles.
Abstract
The spatial and temporal densities of Weather Surveillance Radar-1988 Doppler (WSR-88D) raw radar radial wind represent a rich source of high-resolution observations for initializing numerical weather prediction models. A characteristic of these observations is the presence of a significant degree of redundant information imposing a burden on an operational assimilation system. Potential improvement in data assimilation efficiency can be achieved by constructing averages, called super-obs. In the past, transmission of the radar radial wind from each radar site to a central site was confined to data feeds that filter the resolution and degrade the precision. At the central site, super-obs were constructed from this data feed and called level-3 super-obs. However, the precision and information content of the radial wind can be improved if data at each radar site are directly utilized at the highest resolution and precision found at the WSR-88D radar and then transmitted to a central site for processing in assimilation systems. In addition, with data compression from using super-obs, the volume of data is reduced, allowing quality control information to be included in the data transmission. The super-ob product from each WSR-88D radar site is called level-2.5 super-obs. Parallel, operational runs and case studies of the impact of the level-2.5 radar radial wind super-ob on the NCEP operational 12-km Eta Data Assimilation System (EDAS) and forecast system are compared with Next-Generation Weather Radar level-3 radial wind super-obs, which are spatially filtered and delivered at reduced precision. From the cases studied, it is shown that the level-3 super-obs make little or no impact on the Eta data analysis and subsequent forecasts. The assimilation of the level-2.5 super-ob product in the EDAS and forecast system shows improved precipitation threat scores as well as reduction in RMS and bias height errors, particularly in the upper troposphere. In the few cases studied, the predicted mesoscale precipitation patterns benefit from the level-2.5 super-obs, and more so when greater weight is given to these high-resolution/precision observations. Direct transmission of raw (designated as level 2) radar data to a central site and its use are now imminent, but this study shows that the level-2.5 super-ob product can be used as an operational benchmark to compare with new quality control and assimilation schemes.
Abstract
The spatial and temporal densities of Weather Surveillance Radar-1988 Doppler (WSR-88D) raw radar radial wind represent a rich source of high-resolution observations for initializing numerical weather prediction models. A characteristic of these observations is the presence of a significant degree of redundant information imposing a burden on an operational assimilation system. Potential improvement in data assimilation efficiency can be achieved by constructing averages, called super-obs. In the past, transmission of the radar radial wind from each radar site to a central site was confined to data feeds that filter the resolution and degrade the precision. At the central site, super-obs were constructed from this data feed and called level-3 super-obs. However, the precision and information content of the radial wind can be improved if data at each radar site are directly utilized at the highest resolution and precision found at the WSR-88D radar and then transmitted to a central site for processing in assimilation systems. In addition, with data compression from using super-obs, the volume of data is reduced, allowing quality control information to be included in the data transmission. The super-ob product from each WSR-88D radar site is called level-2.5 super-obs. Parallel, operational runs and case studies of the impact of the level-2.5 radar radial wind super-ob on the NCEP operational 12-km Eta Data Assimilation System (EDAS) and forecast system are compared with Next-Generation Weather Radar level-3 radial wind super-obs, which are spatially filtered and delivered at reduced precision. From the cases studied, it is shown that the level-3 super-obs make little or no impact on the Eta data analysis and subsequent forecasts. The assimilation of the level-2.5 super-ob product in the EDAS and forecast system shows improved precipitation threat scores as well as reduction in RMS and bias height errors, particularly in the upper troposphere. In the few cases studied, the predicted mesoscale precipitation patterns benefit from the level-2.5 super-obs, and more so when greater weight is given to these high-resolution/precision observations. Direct transmission of raw (designated as level 2) radar data to a central site and its use are now imminent, but this study shows that the level-2.5 super-ob product can be used as an operational benchmark to compare with new quality control and assimilation schemes.
Abstract
The magnitude of the divergent component of the wind is relatively small compared to that of the nondivergent component in large-scale atmospheric flows; nevertheless, it plays an important role in the case of explosive cyclogenesis examined here. We have calculated the kinetic energy budget for the life cycle of an intense, developing cyclone over North America. The principal kinetic energy source is the net horizontal transport across the boundaries of the region enclosing the cyclone. By investigating the relative importance of the divergent and nondivergent wind components in the kinetic energy budget, we found, as expected, that neglecting the divergent wind component in calculating the magnitude of the kinetic energy is of little consequence, but that the horizontal flux convergence and generation of kinetic energy depend crucially upon the divergent component. Modification of the divergent wind component can result in significant changes in the kinetic energy budget of the synoptic system.
Abstract
The magnitude of the divergent component of the wind is relatively small compared to that of the nondivergent component in large-scale atmospheric flows; nevertheless, it plays an important role in the case of explosive cyclogenesis examined here. We have calculated the kinetic energy budget for the life cycle of an intense, developing cyclone over North America. The principal kinetic energy source is the net horizontal transport across the boundaries of the region enclosing the cyclone. By investigating the relative importance of the divergent and nondivergent wind components in the kinetic energy budget, we found, as expected, that neglecting the divergent wind component in calculating the magnitude of the kinetic energy is of little consequence, but that the horizontal flux convergence and generation of kinetic energy depend crucially upon the divergent component. Modification of the divergent wind component can result in significant changes in the kinetic energy budget of the synoptic system.
Abstract
Climatological and predictability features of a simplified moist general circulation model are described herein. The simplified model is driven by empirical forcings designed to eliminate systematic errors and maintain computational efficiency. Resulting perpetual January climatological features of forced variables such as the tropospheric heights and rotational winds, as well as unforced variables such as the velocity potential, compare well with the observations. Unforced temporal variations in the midlatitude 500-mb geopotential, as well as the tropical circulation's intraseasonal oscillation, are also simulated reasonably well.
Short-range persistence and predictability features of this model replicate geographical persistence and predictability features from simpler models and from numerical weather prediction. The streamfunction is highly persistent in the extratropics, less so in the tropical regions; similarly, the streamfunction is predicted better in midlatitude regions than in the tropics. By contrast, the velocity potential is more persistent in tropical regions but, like the streamfunction, is still predicted best in extratropical regions for short-range forecasts. At longer forecast ranges, the velocity potential is better predicted in the tropics than in midlatitudes. Interestingly, during prominent tropical intraseasonal oscillations, the model consistently demonstrates lower tropical forecasting skill. Predictions are more skillful during stagnant tropical periods.
Abstract
Climatological and predictability features of a simplified moist general circulation model are described herein. The simplified model is driven by empirical forcings designed to eliminate systematic errors and maintain computational efficiency. Resulting perpetual January climatological features of forced variables such as the tropospheric heights and rotational winds, as well as unforced variables such as the velocity potential, compare well with the observations. Unforced temporal variations in the midlatitude 500-mb geopotential, as well as the tropical circulation's intraseasonal oscillation, are also simulated reasonably well.
Short-range persistence and predictability features of this model replicate geographical persistence and predictability features from simpler models and from numerical weather prediction. The streamfunction is highly persistent in the extratropics, less so in the tropical regions; similarly, the streamfunction is predicted better in midlatitude regions than in the tropics. By contrast, the velocity potential is more persistent in tropical regions but, like the streamfunction, is still predicted best in extratropical regions for short-range forecasts. At longer forecast ranges, the velocity potential is better predicted in the tropics than in midlatitudes. Interestingly, during prominent tropical intraseasonal oscillations, the model consistently demonstrates lower tropical forecasting skill. Predictions are more skillful during stagnant tropical periods.
