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- Author or Editor: Bradley R. Colman x
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Abstract
The first of two papers describing thunderstorms that occur above frontal surfaces, frequently in environments without positive convective available potential energy (CAPE), focuses on the climatology of such storms for the conterminous United States. The dataset used consists of 1093 observations made over a 4-year period. The events were selected using conventional network data and a set of criteria that eliminated thunderstorms rooted in the boundary layer. A composite of the dataset shows that the typical “elevated” thunderstorm occurs northeast of an associated surface low-pressure center, and north of a surface warm front in a region with northeasterly surface winds. The planetary boundary layer is generally very stable as determined by comparisons with both the 50-kPa and 85-kPa air. The thunderstorms are usually found in the left exit region of a low-level wind maximum (an area of horizontal deformation). The large-scale environment is strongly baroclinic with large vertical wind shear and warm advection. Several of the identified characteristics suggest that frequently elevated thunderstorms are the result of physical mechanisms different from those fundamental to surface-based thunderstorms. The most striking of these is that for elevated thunderstorms there is generally very little, if any, positive CAPE in the environment, as the atmosphere is slightly more stable than moist adiabatic above the frontal inversion. The annual frequency distribution of elevated thunderstorms is bimodal, with a primary peak in April and a secondary peak in September. The events are concentrated in an area extending northward from the central Gulf Coast along the Mississippi River valley. The data further show that nearly all winter-season (December through February) thunderstorms east of the Rocky Mountains are of the elevated type. The primary exception involves those over the Florida Peninsula, where surface-based convection persists throughout the year. Most of the winter-season elevated thunderstorms occur near the Gulf Coast downstream from migrating cyclones.
Abstract
The first of two papers describing thunderstorms that occur above frontal surfaces, frequently in environments without positive convective available potential energy (CAPE), focuses on the climatology of such storms for the conterminous United States. The dataset used consists of 1093 observations made over a 4-year period. The events were selected using conventional network data and a set of criteria that eliminated thunderstorms rooted in the boundary layer. A composite of the dataset shows that the typical “elevated” thunderstorm occurs northeast of an associated surface low-pressure center, and north of a surface warm front in a region with northeasterly surface winds. The planetary boundary layer is generally very stable as determined by comparisons with both the 50-kPa and 85-kPa air. The thunderstorms are usually found in the left exit region of a low-level wind maximum (an area of horizontal deformation). The large-scale environment is strongly baroclinic with large vertical wind shear and warm advection. Several of the identified characteristics suggest that frequently elevated thunderstorms are the result of physical mechanisms different from those fundamental to surface-based thunderstorms. The most striking of these is that for elevated thunderstorms there is generally very little, if any, positive CAPE in the environment, as the atmosphere is slightly more stable than moist adiabatic above the frontal inversion. The annual frequency distribution of elevated thunderstorms is bimodal, with a primary peak in April and a secondary peak in September. The events are concentrated in an area extending northward from the central Gulf Coast along the Mississippi River valley. The data further show that nearly all winter-season (December through February) thunderstorms east of the Rocky Mountains are of the elevated type. The primary exception involves those over the Florida Peninsula, where surface-based convection persists throughout the year. Most of the winter-season elevated thunderstorms occur near the Gulf Coast downstream from migrating cyclones.
Abstract
The second of two papers describing thunderstorms that occur above frontal surfaces, frequently in environments without positive convective available potential energy (CAPE), focuses on an impressive outbreak of elevated thunderstorms during AVE-SESAME I. It is shown that the thunderstorms occurred in three convective impulses, each of which developed in the warm sector before propagating onto the frontal surface; subsequent thunderstorms developed over the frontal surface. While in the warm sector, the convection was supported by an extremely unstable boundary layer. However, this convective energy quickly diminished above the frontal surface and thunderstorms continued and developed for many hours in an essentially stable hydrostatic environment. During the lifetime of these impulses, mesoscale updrafts developed and moved with the convective areas, maintaining nearly steady-state systems with strong low-level inflow. The environment was found to be symmetrically neutral in the region of the inflow. Numerous pressure waves were observed in association with the elevated thunderstorms, yet thew features were evidently not important in triggering of the storms. An investigation of a convective band that formed above the frontal surface revealed that the development probably took place in two steps. Initially, high θ e air overlying the frontal inversion was stable to vertical displacements, but inertially unstable. Then, along the instantaneous path of the unstable parcel, the thermodynamic structure changed, the parcel became gravitationally unstable, and upright convection resulted.
Abstract
The second of two papers describing thunderstorms that occur above frontal surfaces, frequently in environments without positive convective available potential energy (CAPE), focuses on an impressive outbreak of elevated thunderstorms during AVE-SESAME I. It is shown that the thunderstorms occurred in three convective impulses, each of which developed in the warm sector before propagating onto the frontal surface; subsequent thunderstorms developed over the frontal surface. While in the warm sector, the convection was supported by an extremely unstable boundary layer. However, this convective energy quickly diminished above the frontal surface and thunderstorms continued and developed for many hours in an essentially stable hydrostatic environment. During the lifetime of these impulses, mesoscale updrafts developed and moved with the convective areas, maintaining nearly steady-state systems with strong low-level inflow. The environment was found to be symmetrically neutral in the region of the inflow. Numerous pressure waves were observed in association with the elevated thunderstorms, yet thew features were evidently not important in triggering of the storms. An investigation of a convective band that formed above the frontal surface revealed that the development probably took place in two steps. Initially, high θ e air overlying the frontal inversion was stable to vertical displacements, but inertially unstable. Then, along the instantaneous path of the unstable parcel, the thermodynamic structure changed, the parcel became gravitationally unstable, and upright convection resulted.
Abstract
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No abstract available.
Abstract
A major snowstorm in Colorado is considered in order to demonstrate the utility of a quasigeostrophic (QG) diagnostic scheme that is capable of separating from the total QG forcing field the cross-isentrope, ageostrophic circulations associated with jet-streak dynamics. The storm did not develop as a consequence of typical baroclinic wave development but instead developed in association with a previously cutoff cyclone. It posed a perplexing forecast problem to Denver area forecasters. It is discovered that at least 12 h before the onset of cyclogenesis there existed QG signatures (computed from rawinsonde data) of the thermally direct-indirect circulations associated with a jet-level wind maximum. These circulations are known to be associated with tropopause folding and the descent of stratospheric potential vorticity into the midtroposphere. It is verified that indeed such a process took place by tracking maxima of potential vorticity on an isentropic surface (295 K) that extended into the midtroposphere. Using analyses of lapse rate and mixing ratio near a “dry slot” in satellite water vapor imagery, our interpretation of the QG signatures are confirmed. The diagnostic scheme can be of value to forecasters who daily must adapt their knowledge of conceptual cyclone models to ascertain the dynamic potential of threatening storms.
Abstract
A major snowstorm in Colorado is considered in order to demonstrate the utility of a quasigeostrophic (QG) diagnostic scheme that is capable of separating from the total QG forcing field the cross-isentrope, ageostrophic circulations associated with jet-streak dynamics. The storm did not develop as a consequence of typical baroclinic wave development but instead developed in association with a previously cutoff cyclone. It posed a perplexing forecast problem to Denver area forecasters. It is discovered that at least 12 h before the onset of cyclogenesis there existed QG signatures (computed from rawinsonde data) of the thermally direct-indirect circulations associated with a jet-level wind maximum. These circulations are known to be associated with tropopause folding and the descent of stratospheric potential vorticity into the midtroposphere. It is verified that indeed such a process took place by tracking maxima of potential vorticity on an isentropic surface (295 K) that extended into the midtroposphere. Using analyses of lapse rate and mixing ratio near a “dry slot” in satellite water vapor imagery, our interpretation of the QG signatures are confirmed. The diagnostic scheme can be of value to forecasters who daily must adapt their knowledge of conceptual cyclone models to ascertain the dynamic potential of threatening storms.
