Search Results
You are looking at 1 - 10 of 74 items for
- Author or Editor: Frederick Sanders x
- Refine by Access: All Content x
Abstract
Increasing evidence indicates that surface cyclogenesis is predominantly a response to the approach of a preexisting trough at upper levels. A question then arises about the origin of the upper-level predecessor. As an initial approach to this question, mobile troughs in the major band of westerlies were crudely tracked in daily Northern Hemispheric 500-mb analysis during nine recent cold seasons. These troughs were identified only in the 552-dam height contour. Between 8 and 15 of them were present on a given day. Study of a particular cold season showed a median duration of 12 days and a mode of 5 days. Average zonal phase speed was 13 m s−1.
Locations or origin and termination of individual troughs were distributed over all longitudes, but births greatly exceeded deaths over and east of the Rocky Mountains in North America and the highlands of central Asia. Trough terminations dominated over the eastern portions of the oceans. Within the quasi-steady planetary waves, origins and terminations of the smaller mobile troughs occurred preferentially in northwesterly and southwesterly flow, respectively.
More detailed studies of the structure during episodes of origin over North America showed prominent vertical and lateral shear in the time-averaged 500-mb flow, rapid growth of the perturbations through the depth of the troposphere, with a vertical tilt upshear only in the lower half, pronounced maximum amplitude near the tropopause, and a variety of circumstances in which troughs became organized in the belt of major westerlies.
Abstract
Increasing evidence indicates that surface cyclogenesis is predominantly a response to the approach of a preexisting trough at upper levels. A question then arises about the origin of the upper-level predecessor. As an initial approach to this question, mobile troughs in the major band of westerlies were crudely tracked in daily Northern Hemispheric 500-mb analysis during nine recent cold seasons. These troughs were identified only in the 552-dam height contour. Between 8 and 15 of them were present on a given day. Study of a particular cold season showed a median duration of 12 days and a mode of 5 days. Average zonal phase speed was 13 m s−1.
Locations or origin and termination of individual troughs were distributed over all longitudes, but births greatly exceeded deaths over and east of the Rocky Mountains in North America and the highlands of central Asia. Trough terminations dominated over the eastern portions of the oceans. Within the quasi-steady planetary waves, origins and terminations of the smaller mobile troughs occurred preferentially in northwesterly and southwesterly flow, respectively.
More detailed studies of the structure during episodes of origin over North America showed prominent vertical and lateral shear in the time-averaged 500-mb flow, rapid growth of the perturbations through the depth of the troposphere, with a vertical tilt upshear only in the lower half, pronounced maximum amplitude near the tropopause, and a variety of circumstances in which troughs became organized in the belt of major westerlies.
Abstract
Tracks and central values of surface low-pressure centre and 500-mb absolute vorticity maxima were gathered from operational analysis for 48 casts of expressive cyclogenesis in the west-central North Atlantic. The cases were stratified into relatively strong moderate and weak categories. Tracking was done from 36 h before to 24 h after the time of most rapid surface deepening.
The surface center often first appeared only 12 to 24 h before maximum deepening, but the upper vorticity maximum was present 36 h or more in advance, often many days. Mean motion of the surface low was rapidly northeastward from the southeastern states, or just offshore, to Newfoundland. Intense deepening occurred in a period of no more than 24–36 h. Strong cyclones moved most rapidly and most meridionally, traveled farther over warm water, and deepened dramatically in a single 12-h period while crossing the closely spaced isotherms of sea surface temperature north of the Gulf Stream. The upper vorticity center moved rapidly eastward from an initial position far northwest of the surface low to a final position close by to its south. Modest intensification of this center occurred during overtaking as the surface cyclone deepened explosively. Detailed study of two cases illustrates the range of behaviors as well as problems of oceanic analysis.
A high correlation was found, for the sample means, between upper-level cyclonic vorticity advection over the surface cyclone and simultaneous surface-deepening rate. Thus the explosive maritime cyclone appears to be a fundamentally baroclinic disturbance in which the low-level response to a given upper-level forcing is dramatically large.
Abstract
Tracks and central values of surface low-pressure centre and 500-mb absolute vorticity maxima were gathered from operational analysis for 48 casts of expressive cyclogenesis in the west-central North Atlantic. The cases were stratified into relatively strong moderate and weak categories. Tracking was done from 36 h before to 24 h after the time of most rapid surface deepening.
The surface center often first appeared only 12 to 24 h before maximum deepening, but the upper vorticity maximum was present 36 h or more in advance, often many days. Mean motion of the surface low was rapidly northeastward from the southeastern states, or just offshore, to Newfoundland. Intense deepening occurred in a period of no more than 24–36 h. Strong cyclones moved most rapidly and most meridionally, traveled farther over warm water, and deepened dramatically in a single 12-h period while crossing the closely spaced isotherms of sea surface temperature north of the Gulf Stream. The upper vorticity center moved rapidly eastward from an initial position far northwest of the surface low to a final position close by to its south. Modest intensification of this center occurred during overtaking as the surface cyclone deepened explosively. Detailed study of two cases illustrates the range of behaviors as well as problems of oceanic analysis.
A high correlation was found, for the sample means, between upper-level cyclonic vorticity advection over the surface cyclone and simultaneous surface-deepening rate. Thus the explosive maritime cyclone appears to be a fundamentally baroclinic disturbance in which the low-level response to a given upper-level forcing is dramatically large.
Abstract
Synoptic and Doppler radar data are used to study the roles of large-scale frontogenetical forcing and of moist symmetric instability in the New England snowstorm of 5–6 December 1981, associated with an explosively intensifying cyclone offshore. Radar reflectivity patterns showed a tendency toward banded structure, particularly near the leading (northwestern) edge of the storm. Only a minor portion of the snowfall, however, was associated with this pronounced bandedness.
From a set of constant-pressure analyses, the frontogenetical forcing was measured from the variation along the temperature gradient of the geostrophic wind component in the direction of this gradient. Over southeastern New England maximum forcing, found near 500 mb at the outset of the storm, descended to the layer between 850 and 700 mb 24 h later. Magnitudes were (3–7) × 10−10 deg m−1 s−1. Observed rates of strengthening of temperature gradient were less than half this value, implying relative adiabatic cooling in the rising warmer air. Doppler radar observations showed strong convergence just above the zone of maximum frontogenesis and at the base of a region of vigorous ascent, with magnitude of a few tens of cm s−1.
Symmetric stability was evaluated, for a geostrophic base-state flow, from a series of vertical cross sections as claw as possible to the radar site. Only small areas of instability appeared in the saturated middle and upper troposphere near the outset of the storm. An evaluation based on gradient-wind balance, on the assumption that the base-state flow 1ocally represented a portion of a steady circular vortex, enlarged these regions of small or negative stability in the northwestern portions of the major cloud mass. Strong (moist or dry) symmetric stability was indicated, however, in the inner portions of the developing cyclonic circulation.
The small stability initially accompanying the frontogenetical forcing was consistent with recent analytic and numerical models showing a vigorous and concentrated frontal updraft. Details of the structure shown by the Doppler data, and in particular the prominence of the bandedness at the northwestern edge of the storm, could be attributed to symmetric instability. The ascent was driven, however, by the frontogenetical forcing, but with an intensity and sharpness due to the small stability of the warmer air.
Abstract
Synoptic and Doppler radar data are used to study the roles of large-scale frontogenetical forcing and of moist symmetric instability in the New England snowstorm of 5–6 December 1981, associated with an explosively intensifying cyclone offshore. Radar reflectivity patterns showed a tendency toward banded structure, particularly near the leading (northwestern) edge of the storm. Only a minor portion of the snowfall, however, was associated with this pronounced bandedness.
From a set of constant-pressure analyses, the frontogenetical forcing was measured from the variation along the temperature gradient of the geostrophic wind component in the direction of this gradient. Over southeastern New England maximum forcing, found near 500 mb at the outset of the storm, descended to the layer between 850 and 700 mb 24 h later. Magnitudes were (3–7) × 10−10 deg m−1 s−1. Observed rates of strengthening of temperature gradient were less than half this value, implying relative adiabatic cooling in the rising warmer air. Doppler radar observations showed strong convergence just above the zone of maximum frontogenesis and at the base of a region of vigorous ascent, with magnitude of a few tens of cm s−1.
Symmetric stability was evaluated, for a geostrophic base-state flow, from a series of vertical cross sections as claw as possible to the radar site. Only small areas of instability appeared in the saturated middle and upper troposphere near the outset of the storm. An evaluation based on gradient-wind balance, on the assumption that the base-state flow 1ocally represented a portion of a steady circular vortex, enlarged these regions of small or negative stability in the northwestern portions of the major cloud mass. Strong (moist or dry) symmetric stability was indicated, however, in the inner portions of the developing cyclonic circulation.
The small stability initially accompanying the frontogenetical forcing was consistent with recent analytic and numerical models showing a vigorous and concentrated frontal updraft. Details of the structure shown by the Doppler data, and in particular the prominence of the bandedness at the northwestern edge of the storm, could be attributed to symmetric instability. The ascent was driven, however, by the frontogenetical forcing, but with an intensity and sharpness due to the small stability of the warmer air.
Abstract
A study was made of the performance of the Limited-Area Fine-Mesh (LFM) operational forecasts for cases of explosive cyclogenesis in the west-central North Atlantic Ocean during 1981–84. For 51 instances in which the observed 12-h deepening was at least 10 mb, the LFM forecasts for 12–24 h range indicated 58% of this deepening on the average, but accounted for only 30% of the variance in individual cases. In a comparison of central pressures from manual analyses with those in LFM initializations and predictions, the latter were insufficiently deep once rapid intensification began, the discrepancy increasing from about 4 mb in the initializations to about 10 mb in the forecasts from 36 to 48 h. In a number of instances the LFM did not detect the initial appearance of the cyclone. Mean position errors increased from about 75 n mi (140 km) initially to about 185 n mi (340 km) at 48-h range. Mean vector errors were shortly southeast of the analyzed center initially and about 65 n mi (120 km) east-northeast finally, indicating a forecast track slightly too fast and slightly too far to the right. Two individual case studies showed that even when there are large quantitative discrepancies between events in the LFM and real atmospheres, the model is qualitatively correct. These results indicate the essentially baroclinic nature of the cyclogenesis, but the intensity of response to the baroclinic forcing remains intractable.
Abstract
A study was made of the performance of the Limited-Area Fine-Mesh (LFM) operational forecasts for cases of explosive cyclogenesis in the west-central North Atlantic Ocean during 1981–84. For 51 instances in which the observed 12-h deepening was at least 10 mb, the LFM forecasts for 12–24 h range indicated 58% of this deepening on the average, but accounted for only 30% of the variance in individual cases. In a comparison of central pressures from manual analyses with those in LFM initializations and predictions, the latter were insufficiently deep once rapid intensification began, the discrepancy increasing from about 4 mb in the initializations to about 10 mb in the forecasts from 36 to 48 h. In a number of instances the LFM did not detect the initial appearance of the cyclone. Mean position errors increased from about 75 n mi (140 km) initially to about 185 n mi (340 km) at 48-h range. Mean vector errors were shortly southeast of the analyzed center initially and about 65 n mi (120 km) east-northeast finally, indicating a forecast track slightly too fast and slightly too far to the right. Two individual case studies showed that even when there are large quantitative discrepancies between events in the LFM and real atmospheres, the model is qualitatively correct. These results indicate the essentially baroclinic nature of the cyclogenesis, but the intensity of response to the baroclinic forcing remains intractable.
Abstract
Analysis of conventional data and of information provided by a number of the competing skippers yields an unusually detailed picture of environmental conditions during the Newport, R.I.–Bermuda Yacht Race in June 1970. Sea-surface temperature data indicate the presence of a warm meander of the Gulf Stream just west of the rhumb-line course, a position intermediate between that of a warm meander to the west in May disclosed by bathythermograph observations from the RMS Franconia and that of a warm eddy to the east in August found by a Naval Oceanographic Office survey.
The fleet was harassed by two groups of severe thunder-squalls during the night of June 21–22, in the vicinity of the warm meander. Even the anomalously high sea-surface temperatures, however, were cool relative to the air in which the thunderstorms were rooted. The storms originated in the Chesapeake Bay area during the day on June 21, and they appeared, surprisingly, to gain intensity over the ocean after being cut off from their surface source of warmth and moisture. Offshore forecasts for June 21–22 took no specific account of the presence of the severe thunderstorm systems.
On June 25, part of the fleet experienced an unexpected southerly gale just northwest of Bermuda. From the yacht data, it is found that the gale was attributable to a small cyclone that formed in an old frontal cloud band and moved northeastward, remaining undetected by the conventional data network throughout its life history. Analysis of the surface wind field suggests that baroclinic effects played only a minor role in the behavior of this cyclone, which at least in some respects resembled a tropical cyclone. Study of the forecasts available at the time indicate that in neither case did small-scale convective activity have a significant direct effect upon the larger scales of motion.
Abstract
Analysis of conventional data and of information provided by a number of the competing skippers yields an unusually detailed picture of environmental conditions during the Newport, R.I.–Bermuda Yacht Race in June 1970. Sea-surface temperature data indicate the presence of a warm meander of the Gulf Stream just west of the rhumb-line course, a position intermediate between that of a warm meander to the west in May disclosed by bathythermograph observations from the RMS Franconia and that of a warm eddy to the east in August found by a Naval Oceanographic Office survey.
The fleet was harassed by two groups of severe thunder-squalls during the night of June 21–22, in the vicinity of the warm meander. Even the anomalously high sea-surface temperatures, however, were cool relative to the air in which the thunderstorms were rooted. The storms originated in the Chesapeake Bay area during the day on June 21, and they appeared, surprisingly, to gain intensity over the ocean after being cut off from their surface source of warmth and moisture. Offshore forecasts for June 21–22 took no specific account of the presence of the severe thunderstorm systems.
On June 25, part of the fleet experienced an unexpected southerly gale just northwest of Bermuda. From the yacht data, it is found that the gale was attributable to a small cyclone that formed in an old frontal cloud band and moved northeastward, remaining undetected by the conventional data network throughout its life history. Analysis of the surface wind field suggests that baroclinic effects played only a minor role in the behavior of this cyclone, which at least in some respects resembled a tropical cyclone. Study of the forecasts available at the time indicate that in neither case did small-scale convective activity have a significant direct effect upon the larger scales of motion.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
No abstract available.
