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- Author or Editor: Wayne C. Bresky x
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Abstract
A cyclone that developed over the eastern United States during December 1992 is investigated using a potential vorticity (PV) framework. Upon partitioning the perturbation PV field (which includes the near-surface potential temperature distribution) into upper-level (UL), low-level (LL), and lower boundary (LB) components, the extent to which particular PV anomalies contribute to the cyclone development is quantified by inverting the PV distribution associated with each component. In addition, the accuracy of a 48–84-h forecast, produced by the CCM2 version of NCAR's Community Climate Model, is assessed. The assessment primarily concerns the 36-h geopotential height tendencies forced at 850 mb by the individual components of the analyzed and forecast PV tendency fields.
It is found that the lower boundary (923 mb) thermal wave accompanying the storm is amplified mostly by the winds associated with an upper-level disturbance and that latent heating plays an important, but secondary, role in near-surface development. The upper-level disturbance, which is accompanied by the formation of a trough of low-θ air at the tropopause, existed in an amplified form prior to the onset of rapid surface deepening. The winds associated with the upper-level perturbation PV initiate the growth of this trough, whereas the winds associated with the LL and LB components further amplify it. In general, the development is characterized at first by the mutual amplification of PV anomalies (i.e., baroclinic instability more important) and later by the superposition of anomalies.
Although the total height tendency fields forced by the analyzed and forecast data were quite similar, the fields forced by the individual components (which sum to the total) showed substantial differences. This suggests that the model may have been right for the wrong reason. In particular, the model overforecast the upper-level disturbance. This was manifested in the development of a tropopause-θ anomaly in the model that was 4 K too cold. The strong circulation induced by this anomaly undoubtedly contributed to the overamplification of the lower boundary wave in the model. On the other hand, the model underestimated a low-level height fall center, apparently of diabatic origin, and displaced it to the northwest of the analyzed feature. These findings point to the importance, in numerical weather prediction, of accurately resolving the near-discontinuity in PV values at the tropopause and of properly handling the release of latent heat throughout the troposphere.
Abstract
A cyclone that developed over the eastern United States during December 1992 is investigated using a potential vorticity (PV) framework. Upon partitioning the perturbation PV field (which includes the near-surface potential temperature distribution) into upper-level (UL), low-level (LL), and lower boundary (LB) components, the extent to which particular PV anomalies contribute to the cyclone development is quantified by inverting the PV distribution associated with each component. In addition, the accuracy of a 48–84-h forecast, produced by the CCM2 version of NCAR's Community Climate Model, is assessed. The assessment primarily concerns the 36-h geopotential height tendencies forced at 850 mb by the individual components of the analyzed and forecast PV tendency fields.
It is found that the lower boundary (923 mb) thermal wave accompanying the storm is amplified mostly by the winds associated with an upper-level disturbance and that latent heating plays an important, but secondary, role in near-surface development. The upper-level disturbance, which is accompanied by the formation of a trough of low-θ air at the tropopause, existed in an amplified form prior to the onset of rapid surface deepening. The winds associated with the upper-level perturbation PV initiate the growth of this trough, whereas the winds associated with the LL and LB components further amplify it. In general, the development is characterized at first by the mutual amplification of PV anomalies (i.e., baroclinic instability more important) and later by the superposition of anomalies.
Although the total height tendency fields forced by the analyzed and forecast data were quite similar, the fields forced by the individual components (which sum to the total) showed substantial differences. This suggests that the model may have been right for the wrong reason. In particular, the model overforecast the upper-level disturbance. This was manifested in the development of a tropopause-θ anomaly in the model that was 4 K too cold. The strong circulation induced by this anomaly undoubtedly contributed to the overamplification of the lower boundary wave in the model. On the other hand, the model underestimated a low-level height fall center, apparently of diabatic origin, and displaced it to the northwest of the analyzed feature. These findings point to the importance, in numerical weather prediction, of accurately resolving the near-discontinuity in PV values at the tropopause and of properly handling the release of latent heat throughout the troposphere.
Abstract
Comparisons between satellite-derived winds and collocated rawinsonde observations often show a pronounced slow speed bias at mid- and upper levels of the atmosphere. A leading cause of the slow speed bias is the improper assignment of the tracer to a height that is too high in the atmosphere. Height errors alone cannot fully explain the slow bias, however. Another factor influencing the speed bias is the size of the target window used in the tracking step. Tracking with a large target window can cause excessive averaging to occur and a smoothing of the instantaneous wind field. Conversely, if too small a window is specified, there is an increased risk of finding a false match. The authors have developed a new “nested tracking” approach that isolates the dominant local motion within a cloud scene and minimizes the smoothing of the motion estimate. A major advantage of the new approach is the ability to identify which pixels within the cloud scene are contributing to the tracking solution. Knowing which pixels contribute to the dominant motion allows for a more representative height to be derived, thereby directly linking the height assignment to the tracking process, which is an important goal for producers of global atmospheric motion vector (AMV) data. When compared with equivalent rawinsondes, the AMVs derived with the new approach show a considerable improvement in the speed bias and root-mean-square error over a control set of AMVs derived with more-conventional methods.
Abstract
Comparisons between satellite-derived winds and collocated rawinsonde observations often show a pronounced slow speed bias at mid- and upper levels of the atmosphere. A leading cause of the slow speed bias is the improper assignment of the tracer to a height that is too high in the atmosphere. Height errors alone cannot fully explain the slow bias, however. Another factor influencing the speed bias is the size of the target window used in the tracking step. Tracking with a large target window can cause excessive averaging to occur and a smoothing of the instantaneous wind field. Conversely, if too small a window is specified, there is an increased risk of finding a false match. The authors have developed a new “nested tracking” approach that isolates the dominant local motion within a cloud scene and minimizes the smoothing of the motion estimate. A major advantage of the new approach is the ability to identify which pixels within the cloud scene are contributing to the tracking solution. Knowing which pixels contribute to the dominant motion allows for a more representative height to be derived, thereby directly linking the height assignment to the tracking process, which is an important goal for producers of global atmospheric motion vector (AMV) data. When compared with equivalent rawinsondes, the AMVs derived with the new approach show a considerable improvement in the speed bias and root-mean-square error over a control set of AMVs derived with more-conventional methods.
