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- Author or Editor: J. Todd Hawes x
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Abstract
The National Meteorological Center's 72-b spectral model forecasts for the 1983–84 cool season are examined in an appraisal of the model's ability to simulate 500-mb cyclones and anticyclones, defined by the existence of at least one (60-m interval) closed contour. Position and intensity errors we determined from comparison between forecast and observed 500-mb height fields. On the basis of this sample it is concluded that there is a tendency, with some geographical exceptions, for the model to overpredict the heights in these systems. This is particularly true of high latitude anticyclones. One noteworthy error characteristic in the model is a recurring failure to predict closed 500-mb cyclonic circulations which evolve from troughs crossing western North America. This suggests either initial data problems with troughs originating over the Pacific or that the model does not simulate the troughs' interaction with the Rocky Mountains very well on some occasions. Selected examples of model error are presented, accompanied by corresponding 48-h forecasts from the operational, limited-area, fine-mesh, primative equation model in one example. Computations of 850-mb geostrophic temperature advection from this latter model are compared with observed computations in this example and suggest that model errors in surface weather systems may he coupled with model errors in 500-mb systems, in this case through incorrect prediction of lower tropospheric temperature advection.
Abstract
The National Meteorological Center's 72-b spectral model forecasts for the 1983–84 cool season are examined in an appraisal of the model's ability to simulate 500-mb cyclones and anticyclones, defined by the existence of at least one (60-m interval) closed contour. Position and intensity errors we determined from comparison between forecast and observed 500-mb height fields. On the basis of this sample it is concluded that there is a tendency, with some geographical exceptions, for the model to overpredict the heights in these systems. This is particularly true of high latitude anticyclones. One noteworthy error characteristic in the model is a recurring failure to predict closed 500-mb cyclonic circulations which evolve from troughs crossing western North America. This suggests either initial data problems with troughs originating over the Pacific or that the model does not simulate the troughs' interaction with the Rocky Mountains very well on some occasions. Selected examples of model error are presented, accompanied by corresponding 48-h forecasts from the operational, limited-area, fine-mesh, primative equation model in one example. Computations of 850-mb geostrophic temperature advection from this latter model are compared with observed computations in this example and suggest that model errors in surface weather systems may he coupled with model errors in 500-mb systems, in this case through incorrect prediction of lower tropospheric temperature advection.
Abstract
An experiment is reported in which derived diagnostic parameters computed from Limited-area Fine-Mesh (LFM) model gridpoint data were examined to determine subjectively whether their availability in real time would assist the forecaster in interpreting and understanding the model's forecast of the weather. Specifically, model products thought to relate to the development of mesoscale convective weather systems (MCSs) were combined into a composite forecast and compared with the standard ensemble of LFM products for 25 episodes of significant convective activity. An objective verification of the LFM forecasts themselves was not attempted. Both 12 and 24 h forecasts from the 1200 UTC run were considered. In a majority of cases, it was evident that derived diagnostic gridpoint data added information about parameter patterns and values important to MCS development that was not obvious from viewing the conventional model products alone. Two case studies demonstrate how information about 850 mb moisture convergence and lower tropospheric temperature advection can help to understand why the model predicted a maximum in vertical velocity (and precipitation) in a region that did not look favorable for large-scale ascent as diagnosed from the conventional output.
Abstract
An experiment is reported in which derived diagnostic parameters computed from Limited-area Fine-Mesh (LFM) model gridpoint data were examined to determine subjectively whether their availability in real time would assist the forecaster in interpreting and understanding the model's forecast of the weather. Specifically, model products thought to relate to the development of mesoscale convective weather systems (MCSs) were combined into a composite forecast and compared with the standard ensemble of LFM products for 25 episodes of significant convective activity. An objective verification of the LFM forecasts themselves was not attempted. Both 12 and 24 h forecasts from the 1200 UTC run were considered. In a majority of cases, it was evident that derived diagnostic gridpoint data added information about parameter patterns and values important to MCS development that was not obvious from viewing the conventional model products alone. Two case studies demonstrate how information about 850 mb moisture convergence and lower tropospheric temperature advection can help to understand why the model predicted a maximum in vertical velocity (and precipitation) in a region that did not look favorable for large-scale ascent as diagnosed from the conventional output.
Abstract
The geographical and monthly frequencies of 500 mb cyclones and anticyclones in the National Meteorological Center analyses over the western half of the Northern Hemisphere are investigated for the period 1950–85. These cyclones and anticyclones, defined by the appearance of at least one closed (approximately) 6-dekameter contour around relatively low or high heights in the 500 mb height field, are generally observed less than ten percent of the time in any 10° by 10° latitude-longitude quadrangle, with cyclones being more numerous than anticyclones. The 500 mb cyclones are found primarily at middle and high latitudes, while anticyclones are observed most frequently over the subtropics. Cyclone frequency increases over the northern oceanic regions during summer, while anticyclone frequency increases throughout the subtropics during summer, especially over southwestern North America. Exceptions to these rules are observed; relatively high springtime 500 mb anticyclone frequency is found over the northeastern Atlantic Ocean while relatively high 500 mb cyclone frequency is found over the central subtropics Pacific Ocean and near Alaska during summer, southwestern North America during winter, and near southwestern Europe throughout the year. Abnormally strong diffluent flow over southwestern North America is suggested as an antecedent condition for 500 mb cyclogenesis in this same region. The correlation between 500 mb cyclone frequencies and 300 mb westerly momentum transports is also investigated, revealing that 500 mb cyclones may be associated with the convergence of westerly momentum into the 300 mb westerly jet. Finally, temporal trends in the frequencies indicate that 500 mb cyclone frequencies declined from 1950 through 1970 but increased from 1971 through 1985, while 500 mb anticyclone frequencies declined from 1950 through 1985.
Abstract
The geographical and monthly frequencies of 500 mb cyclones and anticyclones in the National Meteorological Center analyses over the western half of the Northern Hemisphere are investigated for the period 1950–85. These cyclones and anticyclones, defined by the appearance of at least one closed (approximately) 6-dekameter contour around relatively low or high heights in the 500 mb height field, are generally observed less than ten percent of the time in any 10° by 10° latitude-longitude quadrangle, with cyclones being more numerous than anticyclones. The 500 mb cyclones are found primarily at middle and high latitudes, while anticyclones are observed most frequently over the subtropics. Cyclone frequency increases over the northern oceanic regions during summer, while anticyclone frequency increases throughout the subtropics during summer, especially over southwestern North America. Exceptions to these rules are observed; relatively high springtime 500 mb anticyclone frequency is found over the northeastern Atlantic Ocean while relatively high 500 mb cyclone frequency is found over the central subtropics Pacific Ocean and near Alaska during summer, southwestern North America during winter, and near southwestern Europe throughout the year. Abnormally strong diffluent flow over southwestern North America is suggested as an antecedent condition for 500 mb cyclogenesis in this same region. The correlation between 500 mb cyclone frequencies and 300 mb westerly momentum transports is also investigated, revealing that 500 mb cyclones may be associated with the convergence of westerly momentum into the 300 mb westerly jet. Finally, temporal trends in the frequencies indicate that 500 mb cyclone frequencies declined from 1950 through 1970 but increased from 1971 through 1985, while 500 mb anticyclone frequencies declined from 1950 through 1985.
