Search Results
Abstract
A multiscale analysis reveals diverse atmospheric structure and processes within a mesoscale convective complex (MCC) observed during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central (PRE-STORM) experiment. This midlatitude system was the second in a series of four MCCs that developed and traveled along a quasi-stationary frontal zone over the central United States on 3–4 June 1985. Objectively analyzed mesoscale upper-air soundings encompassing the MCC are interpreted in tandem with more detailed dual-Doppler radar measurements that disclose the storm's internal airflow and precipitation structure. The mature MCC is found to include a variety of local environments and associated weather, ranging from tornadic thunderstorms to more linear convective bands and widespread chilling rains. A corresponding spectrum of mesoscale ver6cW-motion profiles is documented. These findings are related to previous composite-based portrayals of MCCs, as well as detailed case studies of simpler squall-type convective systems.
A hallmark of this storm was its “open-wave” precipitation pattern, in which two convective bands intersected so as to resemble a miniature developing frontal cyclone. This resemblance proves superficial, however, since 1) anticyclonic lower-tropospheric flow was observed in place of the expected cyclonic circulation near the convective apex, and 2) the accompanying wavelike lower-tropospheric temperature pattern was strongly influenced by moist processes intrinsic to the MCC (e.g., evaporative cooling), as opposed to horizontal advection about a developing vortex. The storm's intriguing organization is instead postulated to have resulted from the superposition of two preferred convective modes: one aligned with the mean vertical wind-shear vector, accompanied by marked cross-band thermal contrast and deformation through a deep layer, and another oriented perpendicular to the low-level shear, which exhibited a shallow gust front and mesoscale cold pool as found in squall-line systems. Highly three-dimensional airflow within the mature MCC and a pronounced modulation of convective instability across an embedded frontal-like zone further promoted the storm's asymmetric precipitation pattern.
Abstract
A multiscale analysis reveals diverse atmospheric structure and processes within a mesoscale convective complex (MCC) observed during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central (PRE-STORM) experiment. This midlatitude system was the second in a series of four MCCs that developed and traveled along a quasi-stationary frontal zone over the central United States on 3–4 June 1985. Objectively analyzed mesoscale upper-air soundings encompassing the MCC are interpreted in tandem with more detailed dual-Doppler radar measurements that disclose the storm's internal airflow and precipitation structure. The mature MCC is found to include a variety of local environments and associated weather, ranging from tornadic thunderstorms to more linear convective bands and widespread chilling rains. A corresponding spectrum of mesoscale ver6cW-motion profiles is documented. These findings are related to previous composite-based portrayals of MCCs, as well as detailed case studies of simpler squall-type convective systems.
A hallmark of this storm was its “open-wave” precipitation pattern, in which two convective bands intersected so as to resemble a miniature developing frontal cyclone. This resemblance proves superficial, however, since 1) anticyclonic lower-tropospheric flow was observed in place of the expected cyclonic circulation near the convective apex, and 2) the accompanying wavelike lower-tropospheric temperature pattern was strongly influenced by moist processes intrinsic to the MCC (e.g., evaporative cooling), as opposed to horizontal advection about a developing vortex. The storm's intriguing organization is instead postulated to have resulted from the superposition of two preferred convective modes: one aligned with the mean vertical wind-shear vector, accompanied by marked cross-band thermal contrast and deformation through a deep layer, and another oriented perpendicular to the low-level shear, which exhibited a shallow gust front and mesoscale cold pool as found in squall-line systems. Highly three-dimensional airflow within the mature MCC and a pronounced modulation of convective instability across an embedded frontal-like zone further promoted the storm's asymmetric precipitation pattern.
Abstract
This study uses airborne Doppler radar observations to describe the mesoscale structure and evolution of a cold frontal system as it made landfall on the mountainous coast of Oregon and northern California on 1 December 1995 during the Coastal Observations and Simulations with Topography experiment. This section of coastline constitutes a steep, approximately two-dimensional north–south-oriented orographic barrier. The front exhibited a northeast–southwest orientation and thus intersected the axis of high terrain at an acute angle. The along-barrier pressure gradient and low-level winds increased with time along the coastal zone and reached a maximum as the front made landfall. Stably stratified prefrontal flow was strongly blocked by the orography, resulting in a confluent transition from pervasive southwesterly winds offshore to a narrow zone of accelerated south-southwesterly flow near the coast, where wind speeds approached 30 m s−1 at a height of 750 m above mean sea level. Postfrontal flow was much less affected by the topography, probably because of its weaker static stability. Upstream blocking by the steep coastal terrain also evidently led to modifications of precipitation in the vicinity of the front, including the rapid genesis of a narrow cold-frontal rainband (NCFR) and nearshore enhancement of two prefrontal precipitation bands. This evolution of the NCFR is interpreted in conjunction with changes in prefrontal vertical wind shear, which favored more upright convective ascent as the front neared shore and encountered accelerated along-barrier flow adjacent to the steep terrain. In addition, a statistical examination of observed radar reflectivity patterns shows that the intensity of frontal precipitation systematically decreased with upstream distance away from the orographic barrier.
Abstract
This study uses airborne Doppler radar observations to describe the mesoscale structure and evolution of a cold frontal system as it made landfall on the mountainous coast of Oregon and northern California on 1 December 1995 during the Coastal Observations and Simulations with Topography experiment. This section of coastline constitutes a steep, approximately two-dimensional north–south-oriented orographic barrier. The front exhibited a northeast–southwest orientation and thus intersected the axis of high terrain at an acute angle. The along-barrier pressure gradient and low-level winds increased with time along the coastal zone and reached a maximum as the front made landfall. Stably stratified prefrontal flow was strongly blocked by the orography, resulting in a confluent transition from pervasive southwesterly winds offshore to a narrow zone of accelerated south-southwesterly flow near the coast, where wind speeds approached 30 m s−1 at a height of 750 m above mean sea level. Postfrontal flow was much less affected by the topography, probably because of its weaker static stability. Upstream blocking by the steep coastal terrain also evidently led to modifications of precipitation in the vicinity of the front, including the rapid genesis of a narrow cold-frontal rainband (NCFR) and nearshore enhancement of two prefrontal precipitation bands. This evolution of the NCFR is interpreted in conjunction with changes in prefrontal vertical wind shear, which favored more upright convective ascent as the front neared shore and encountered accelerated along-barrier flow adjacent to the steep terrain. In addition, a statistical examination of observed radar reflectivity patterns shows that the intensity of frontal precipitation systematically decreased with upstream distance away from the orographic barrier.
Abstract
The relative airflow and accompanying precipitation structure of squall lines trailed by mesoscale regions of stratiform rain are examined with emphasis on the occurrence of “rear inflow,” i.e. the intrusion of environmental air into these storms across the trailing precipitation boundary. Three cases from Kansas and Oklahoma provide examples of “Strong Rear Inflow,” which crosses the back edge of the stratiform precipitation area at relative speeds exceeding 10 m s−1. The vertical profiles of relative flow at the trailing precipitation boundary of these three systems were remarkably similar, with the rear inflow confined to a jet-like layer centered at about 550 mb. Doppler radar data for two of these cases showed that the rear inflow jet occupied a continuous channel extending from middle levels at the back edge of the stratiform region to lower levels of the leading convective region, where it merged with outflow from convective downdrafts to bolster the leading gust front. Strong front-to-rear flow occurred both above and below this layer. The front-to-rear flow lying above the rear inflow jet was consistently strengthened in the vicinity of convective cells and was separated from the rear inflow by an interface of strong vertical wind shear that sloped upward toward the rear of the storm. Soundings indicate this interface marked the division between cloudy air (the trailing “anvil”) associated mesoscale updraft (above) and subsaturated air in the mesoscale downdraft (below).
An inclusive review of the literature reveals that these three midlatitude squall lines had stronger rear inflow than any previously described squall lines with trailing stratiform precipitation. Five “Weak Rear Inflow” cases had peak inflows at the back edge of the stratiform rain area between 5 and 10 m s−1. The vertical profiles of relative flow at the trailing edge exhibited a variety of structures, with the rear inflow maximum sometimes at higher altitude and sometimes at lower altitude than in the Strong Rear Inflow cases. The literature further reveals ten studies in which the relative flow at the back edge of the precipitation showed little if any rear inflow (<5 m s−1), suggestive of a stagnation of the midlevel system-relative flow as air at the back edge of the stratiform region moved at or near the speed of the system. These “Stagnation Zone” squall systems, like both the Strong and Weak Rear Inflow cases, exhibited maxima of front-to-rear flow at both upper and lower levels; however, the front-to-rear flow was not as strong as in the Strong Rear Inflow cases. The stagnation layer was located between 650 and 750 mb, considerably below the height of the inflow jet in the Strong Rear Inflow systems. While no appreciable rear inflow occurred at the back edge of Stagnation Zone cases, rear-to-front flow has been observed to develop at midlevels in the interior of their stratiform regions, suggesting that physical processes internal to the mesoscale system are capable of generating rear-to-front flow behind the convective line without the aid of ambient flow entering the storm.
Abstract
The relative airflow and accompanying precipitation structure of squall lines trailed by mesoscale regions of stratiform rain are examined with emphasis on the occurrence of “rear inflow,” i.e. the intrusion of environmental air into these storms across the trailing precipitation boundary. Three cases from Kansas and Oklahoma provide examples of “Strong Rear Inflow,” which crosses the back edge of the stratiform precipitation area at relative speeds exceeding 10 m s−1. The vertical profiles of relative flow at the trailing precipitation boundary of these three systems were remarkably similar, with the rear inflow confined to a jet-like layer centered at about 550 mb. Doppler radar data for two of these cases showed that the rear inflow jet occupied a continuous channel extending from middle levels at the back edge of the stratiform region to lower levels of the leading convective region, where it merged with outflow from convective downdrafts to bolster the leading gust front. Strong front-to-rear flow occurred both above and below this layer. The front-to-rear flow lying above the rear inflow jet was consistently strengthened in the vicinity of convective cells and was separated from the rear inflow by an interface of strong vertical wind shear that sloped upward toward the rear of the storm. Soundings indicate this interface marked the division between cloudy air (the trailing “anvil”) associated mesoscale updraft (above) and subsaturated air in the mesoscale downdraft (below).
An inclusive review of the literature reveals that these three midlatitude squall lines had stronger rear inflow than any previously described squall lines with trailing stratiform precipitation. Five “Weak Rear Inflow” cases had peak inflows at the back edge of the stratiform rain area between 5 and 10 m s−1. The vertical profiles of relative flow at the trailing edge exhibited a variety of structures, with the rear inflow maximum sometimes at higher altitude and sometimes at lower altitude than in the Strong Rear Inflow cases. The literature further reveals ten studies in which the relative flow at the back edge of the precipitation showed little if any rear inflow (<5 m s−1), suggestive of a stagnation of the midlevel system-relative flow as air at the back edge of the stratiform region moved at or near the speed of the system. These “Stagnation Zone” squall systems, like both the Strong and Weak Rear Inflow cases, exhibited maxima of front-to-rear flow at both upper and lower levels; however, the front-to-rear flow was not as strong as in the Strong Rear Inflow cases. The stagnation layer was located between 650 and 750 mb, considerably below the height of the inflow jet in the Strong Rear Inflow systems. While no appreciable rear inflow occurred at the back edge of Stagnation Zone cases, rear-to-front flow has been observed to develop at midlevels in the interior of their stratiform regions, suggesting that physical processes internal to the mesoscale system are capable of generating rear-to-front flow behind the convective line without the aid of ambient flow entering the storm.
Abstract
A squall line exhibiting an extensive trailing region of stratiform precipitation passed over the observational network of the National Severe Storms Laboratory on 22 May 1976. Satellite imagery and conventional radar observations document its evolution from a broken line of thunderstorms to a system of mesoscale proportions, and single-Doppler radar observations describe aspects of its mature structure. Satellite measurements of cloud-top temperature showed the system to be a mesoscale convective complex (MCC). The life cycle of the system exhibited the stages of development seen in tropical cloud clusters.
At maturity, two prominent mesoscale flow regimes were identified at midlevels: one marked by inflow into the system's front and continuing toward its rear, and another associated with inflow entering the extreme rear of the system.
The rear inflow was associated with a cyclonic midlevel vortex in the stratiform precipitation region. It produced a concavity, or “notch”, in the back edge of the precipitation echo. Shortly after the appearance of the notch, a downwind segment of the leading convective line accelerated forward. The notch persisted through the dissipating stage, at which time secondary notches also formed. The last remnant of the stratiform precipitation area took the form of a chain of three comma-shaped vortices, whose origin could be traced in time back to the primary and secondary notches.
The inflow at the front of the system spanned both the leading convective and trailing stratiform regions. Convective-scale velocity maxima were superimposed on this front-to-rear flow in the convective region, while a broad maximum of the rearward current occurred in the stratiform region, just above the melting layer. This rearward system-relative flow apparently promoted the broad structure of the precipitation area. Slowly falling ice particles originating at convective cell tops were evidently advected rearward and dispersed over a 50–100 km wide region, whereupon their melting produced a prominent radar bright band.
Abstract
A squall line exhibiting an extensive trailing region of stratiform precipitation passed over the observational network of the National Severe Storms Laboratory on 22 May 1976. Satellite imagery and conventional radar observations document its evolution from a broken line of thunderstorms to a system of mesoscale proportions, and single-Doppler radar observations describe aspects of its mature structure. Satellite measurements of cloud-top temperature showed the system to be a mesoscale convective complex (MCC). The life cycle of the system exhibited the stages of development seen in tropical cloud clusters.
At maturity, two prominent mesoscale flow regimes were identified at midlevels: one marked by inflow into the system's front and continuing toward its rear, and another associated with inflow entering the extreme rear of the system.
The rear inflow was associated with a cyclonic midlevel vortex in the stratiform precipitation region. It produced a concavity, or “notch”, in the back edge of the precipitation echo. Shortly after the appearance of the notch, a downwind segment of the leading convective line accelerated forward. The notch persisted through the dissipating stage, at which time secondary notches also formed. The last remnant of the stratiform precipitation area took the form of a chain of three comma-shaped vortices, whose origin could be traced in time back to the primary and secondary notches.
The inflow at the front of the system spanned both the leading convective and trailing stratiform regions. Convective-scale velocity maxima were superimposed on this front-to-rear flow in the convective region, while a broad maximum of the rearward current occurred in the stratiform region, just above the melting layer. This rearward system-relative flow apparently promoted the broad structure of the precipitation area. Slowly falling ice particles originating at convective cell tops were evidently advected rearward and dispersed over a 50–100 km wide region, whereupon their melting produced a prominent radar bright band.
Abstract
This paper documents the three-dimensional flow and precipitation structures associated with a weak cold front interacting with the Olympic Mountains and the subsequent development of a Puget Sound convergence zone. This study utilizes data collected during COAST IOP5 (the fifth intensive observing period of the Coastal Observation and Simulation with Topography field experiment) that took place on 11–12 December 1993. One of the most important data sources was a NOAA P-3 aircraft, which provided flight-level data, radar reflectivity, and Doppler winds as it circumnavigated the Olympics. Initially, frontal passage along the western foothills of the Olympics was accompanied by a 2°–3°C temperature drop, a rapid wind shift to northwesterlies, and an intense line of precipitation (35–45 dBZ); however, the wind shift and associated precipitation structures attenuated when the front began to ascend the windward slopes of the Olympics. Surface and P-3 observations document the deformation of the front around the Olympics. The front accelerated and became more intense and shallow to the northeast of the Olympics where it encountered prefrontal downslope warming and strong southerlies. This portion of the front and the associated precipitation band moved southward toward central Puget Sound and eventually became stationary to the east of the Olympics, where a Puget Sound convergence zone developed and subsequently dissipated after a few hours.
This case was simulated down to 3-km horizontal resolution using the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Frontogenesis calculations using the model data suggest that prefrontal flow splitting around the Olympics intensified the front by enhanced stretching deformation as it approached the barrier. A simulation with half-height Olympics showed that reduced flow splitting resulted in a weaker front approaching the windward foothills. As observed by P-3 observations and in accordance with previous theoretical studies, frontolysis occurred as the modeled front ascended the windward slope. Sensitivity experiments showed that diabatic effects were important in maintaining the front as it rounded the north side of the Olympics and pushed southward toward central Puget Sound.
Abstract
This paper documents the three-dimensional flow and precipitation structures associated with a weak cold front interacting with the Olympic Mountains and the subsequent development of a Puget Sound convergence zone. This study utilizes data collected during COAST IOP5 (the fifth intensive observing period of the Coastal Observation and Simulation with Topography field experiment) that took place on 11–12 December 1993. One of the most important data sources was a NOAA P-3 aircraft, which provided flight-level data, radar reflectivity, and Doppler winds as it circumnavigated the Olympics. Initially, frontal passage along the western foothills of the Olympics was accompanied by a 2°–3°C temperature drop, a rapid wind shift to northwesterlies, and an intense line of precipitation (35–45 dBZ); however, the wind shift and associated precipitation structures attenuated when the front began to ascend the windward slopes of the Olympics. Surface and P-3 observations document the deformation of the front around the Olympics. The front accelerated and became more intense and shallow to the northeast of the Olympics where it encountered prefrontal downslope warming and strong southerlies. This portion of the front and the associated precipitation band moved southward toward central Puget Sound and eventually became stationary to the east of the Olympics, where a Puget Sound convergence zone developed and subsequently dissipated after a few hours.
This case was simulated down to 3-km horizontal resolution using the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Frontogenesis calculations using the model data suggest that prefrontal flow splitting around the Olympics intensified the front by enhanced stretching deformation as it approached the barrier. A simulation with half-height Olympics showed that reduced flow splitting resulted in a weaker front approaching the windward foothills. As observed by P-3 observations and in accordance with previous theoretical studies, frontolysis occurred as the modeled front ascended the windward slope. Sensitivity experiments showed that diabatic effects were important in maintaining the front as it rounded the north side of the Olympics and pushed southward toward central Puget Sound.
Abstract
Airborne Doppler radar data, collected off the Pacific Northwest coast by a NOAA WP-3D Orion aircraft over an 8-h period on 8 December 1993 during the Coastal Observations and Simulations with Topography experiment, reveal the mesoscale structure of an intense frontal system while it was well offshore and as it approached within 20 km of the Oregon coastline. During the offshore stage, a portion of the narrow cold-frontal rainband was characterized by deep convective cores. Pseudo-dual-Doppler analyses characterize the kinematic and precipitation structure of the deep convection.
Pseudo-dual-Doppler analyses describe the subsequent evolution of the narrow cold-frontal rainband as it approached to within 20 km of the Oregon coast. Deformation of the frontal zone appeared to cause the dissipation of one of three precipitation cores contained within the dual-Doppler area. The precipitation cores and the strong convergence zone associated with the front conformed to some degree to the shape of the coastline near Cape Blanco, Oregon, as the front neared the coast. Changes in the prefrontal flow that occurred as the front approached the coast were qualitatively consistent with theoretical and numerical studies of upstream orographic influence. Comparison of pseudo-dual-Doppler-derived velocity profiles with idealized numerical model calculations suggests that the nearshore evolution of the frontal rainband was significantly affected by an upstream influence of the coastal orography.
Abstract
Airborne Doppler radar data, collected off the Pacific Northwest coast by a NOAA WP-3D Orion aircraft over an 8-h period on 8 December 1993 during the Coastal Observations and Simulations with Topography experiment, reveal the mesoscale structure of an intense frontal system while it was well offshore and as it approached within 20 km of the Oregon coastline. During the offshore stage, a portion of the narrow cold-frontal rainband was characterized by deep convective cores. Pseudo-dual-Doppler analyses characterize the kinematic and precipitation structure of the deep convection.
Pseudo-dual-Doppler analyses describe the subsequent evolution of the narrow cold-frontal rainband as it approached to within 20 km of the Oregon coast. Deformation of the frontal zone appeared to cause the dissipation of one of three precipitation cores contained within the dual-Doppler area. The precipitation cores and the strong convergence zone associated with the front conformed to some degree to the shape of the coastline near Cape Blanco, Oregon, as the front neared the coast. Changes in the prefrontal flow that occurred as the front approached the coast were qualitatively consistent with theoretical and numerical studies of upstream orographic influence. Comparison of pseudo-dual-Doppler-derived velocity profiles with idealized numerical model calculations suggests that the nearshore evolution of the frontal rainband was significantly affected by an upstream influence of the coastal orography.
Abstract
Radar reflectivity and raingage data obtained during six springtimes indicate the types of mesoscale organization that occur in association with major rain events in Oklahoma (at least 25 mm of rain in 24 h over an area exceeding 12 500 km2). In these storms the primary rain area is found to be a contiguous region of precipitation 10s to 100s of km in scale that consists partly of deep convection and partly of stratiform rain. The patterns of rain formed by the convective and stratiform areas comprise a continuous spectrum of mesoscale structures. About two-thirds of the cases examined exhibited variations on the type of organization in which convective cells arranged in a moving line are followed by a region of stratiform rain. Storm organization was graded according to the degree to which it matched an idealized model of this “leading-line/trailing-stratiform” structure. The precipitation pattern was further graded according to whether its structure was relatively symmetric with respect to an axis normal to and passing through the midpoint of the line, or asymmetric, in which case the storm was biased toward having stronger, more discrete convective structure at the upwind (south or southwestern) end of the line and/or the most extensive stratiform precipitation behind the downwind (north to northeastern) end of the line. About one-third of the cases examined displayed much more chaotic, unclassifiable arrangements of convective and stratiform areas.
Among the cases with leading-line/trailing-stratiform structure, severe weather was most frequent in systems with (i) a strong degree of leading-line/trailing-stratiform structure, in which a solid, relatively uniform, are-shaped line had stratiform rain centered symmetrically behind it, and (ii) a weaker degree of leading-line/trailing-stratiform structure in which a southwest-northeast line was biased toward having narrow, intensely convective, irregularly spaced cell structure at its southwestern (upwind) end and stratiform rain confined to the region behind the broader northeastern (downwind) portion of the line. Although all mesoscale organization types were characterized by all types of severe weather, the type (ii) cases were the most prolific category in terms of tornado and hail production, while type (i) cases were prone to be associated with flooding. The chaotic, unclassifiable cases, which exhibited no line organization, had just as much severe weather as the cases with line organization, but were more likely to produce hail and somewhat less likely to produce tornadoes and flooding than the systems with line structure.
Major rain events occurred whenever a mesoscale convective complex (MCC) was passing over the study area, unless the MCC was dissipating or merely skirting the area. However, 75% of the major rain events occurred under cloud shields that failed to meet the MCC criteria explicitly, although they often resembled MCCs qualitatively. No particular type of mesoscale radar-echo organization was favored when cloud shields meeting the MCC criteria were observed. A slight preference for the more chaotic type of organization was suggested; however, the data sample is not large enough for this finding to be regarded as conclusive.
Mean soundings and hodographs generally show no sign of a low-level jet in environments associated with chaotically arranged rain areas that lacked any line structure. On the other hand, a low-level jet and resulting curved hodograph were typically associated with cases in which line organization was evident. The wind shear in the low-to-mid troposphere, the bulk Richardson number and other familiar parameters characterizing squall fine environments are consistent with results from recent modeling studies. When leading-line/trailing-stratiform structure was present, the cross-line shear in the environment was of a magnitude associated with model simulations in which a rearward sloping updraft circulation favorable to trailing-stratiform anvil formation quickly develops. The along-line component of shear was greater when the squall system structure was of the asymmetric type and the degree of leading-line/trailing-stratiform structure was not as strong, i.e. in those mesoscale systems favoring tornado occurrence.
Abstract
Radar reflectivity and raingage data obtained during six springtimes indicate the types of mesoscale organization that occur in association with major rain events in Oklahoma (at least 25 mm of rain in 24 h over an area exceeding 12 500 km2). In these storms the primary rain area is found to be a contiguous region of precipitation 10s to 100s of km in scale that consists partly of deep convection and partly of stratiform rain. The patterns of rain formed by the convective and stratiform areas comprise a continuous spectrum of mesoscale structures. About two-thirds of the cases examined exhibited variations on the type of organization in which convective cells arranged in a moving line are followed by a region of stratiform rain. Storm organization was graded according to the degree to which it matched an idealized model of this “leading-line/trailing-stratiform” structure. The precipitation pattern was further graded according to whether its structure was relatively symmetric with respect to an axis normal to and passing through the midpoint of the line, or asymmetric, in which case the storm was biased toward having stronger, more discrete convective structure at the upwind (south or southwestern) end of the line and/or the most extensive stratiform precipitation behind the downwind (north to northeastern) end of the line. About one-third of the cases examined displayed much more chaotic, unclassifiable arrangements of convective and stratiform areas.
Among the cases with leading-line/trailing-stratiform structure, severe weather was most frequent in systems with (i) a strong degree of leading-line/trailing-stratiform structure, in which a solid, relatively uniform, are-shaped line had stratiform rain centered symmetrically behind it, and (ii) a weaker degree of leading-line/trailing-stratiform structure in which a southwest-northeast line was biased toward having narrow, intensely convective, irregularly spaced cell structure at its southwestern (upwind) end and stratiform rain confined to the region behind the broader northeastern (downwind) portion of the line. Although all mesoscale organization types were characterized by all types of severe weather, the type (ii) cases were the most prolific category in terms of tornado and hail production, while type (i) cases were prone to be associated with flooding. The chaotic, unclassifiable cases, which exhibited no line organization, had just as much severe weather as the cases with line organization, but were more likely to produce hail and somewhat less likely to produce tornadoes and flooding than the systems with line structure.
Major rain events occurred whenever a mesoscale convective complex (MCC) was passing over the study area, unless the MCC was dissipating or merely skirting the area. However, 75% of the major rain events occurred under cloud shields that failed to meet the MCC criteria explicitly, although they often resembled MCCs qualitatively. No particular type of mesoscale radar-echo organization was favored when cloud shields meeting the MCC criteria were observed. A slight preference for the more chaotic type of organization was suggested; however, the data sample is not large enough for this finding to be regarded as conclusive.
Mean soundings and hodographs generally show no sign of a low-level jet in environments associated with chaotically arranged rain areas that lacked any line structure. On the other hand, a low-level jet and resulting curved hodograph were typically associated with cases in which line organization was evident. The wind shear in the low-to-mid troposphere, the bulk Richardson number and other familiar parameters characterizing squall fine environments are consistent with results from recent modeling studies. When leading-line/trailing-stratiform structure was present, the cross-line shear in the environment was of a magnitude associated with model simulations in which a rearward sloping updraft circulation favorable to trailing-stratiform anvil formation quickly develops. The along-line component of shear was greater when the squall system structure was of the asymmetric type and the degree of leading-line/trailing-stratiform structure was not as strong, i.e. in those mesoscale systems favoring tornado occurrence.
Abstract
An analysis has been carried out of the surface pressure field in a highly complex mesoscale convective system that occurred on 3-4 June 1985 during the Oklahoma-Kansas Preliminary Regional Expeximent for STORM-Central (OK PRE-STORM). During its mature stage the storm consisted of two primary intersecting convective bands approximately 200 km in length, one oriented NIE-SW (to the north) and the other N-S (to the south), with a stratiform precipitation region extending to the northwest of the bands. Stratifonn precipitation was weak to nonexistent in the southernmost portion of the storm.
Although the organization of the storm was complex, the surface pressure field resembled those associated with simpler, quasi-linear squall systems containing trading stratifom regions: a mesohigh existed neat the convective line and a wake low was observed to the rear of the stratiform region. A strong system-relative, descending rear inflow jet was observed in the northern part of the storm near the wake low. Significantly, only the northern portion of the storm had a trailing stratiform region and it was only in that region that a wake low and a descending mu inflow jet occurred.
An analysis of dual-Doppler radar data taken in the northern part of the storm indicates remarkably strong, localized subsidence at low levels within the rear inflow jet, up to 6 m s−1 on a 10-km scale at the back edge of the trailing stratiform region. The maximum sinking occurred (a) to the rear of the highest reflectivity portion of the trailing stratiform region, (b) within the region of the strongest low-level reflectivity gradient, and (c) was coincident with the strongest surface pressure gradient [up to 2 mb (5 km)−1] ahead of the wake low center.
These findings indicate that the trailing stratiform precipitation regions of mesoscale convective systems can be dynamically significant phenomena, generating rapidly descending inflow jets at their back edges and, con-sequently, producing pronounced lower-tropospheric warming, intense surface pressure gradients and strong low-level winds.
Abstract
An analysis has been carried out of the surface pressure field in a highly complex mesoscale convective system that occurred on 3-4 June 1985 during the Oklahoma-Kansas Preliminary Regional Expeximent for STORM-Central (OK PRE-STORM). During its mature stage the storm consisted of two primary intersecting convective bands approximately 200 km in length, one oriented NIE-SW (to the north) and the other N-S (to the south), with a stratiform precipitation region extending to the northwest of the bands. Stratifonn precipitation was weak to nonexistent in the southernmost portion of the storm.
Although the organization of the storm was complex, the surface pressure field resembled those associated with simpler, quasi-linear squall systems containing trading stratifom regions: a mesohigh existed neat the convective line and a wake low was observed to the rear of the stratiform region. A strong system-relative, descending rear inflow jet was observed in the northern part of the storm near the wake low. Significantly, only the northern portion of the storm had a trailing stratiform region and it was only in that region that a wake low and a descending mu inflow jet occurred.
An analysis of dual-Doppler radar data taken in the northern part of the storm indicates remarkably strong, localized subsidence at low levels within the rear inflow jet, up to 6 m s−1 on a 10-km scale at the back edge of the trailing stratiform region. The maximum sinking occurred (a) to the rear of the highest reflectivity portion of the trailing stratiform region, (b) within the region of the strongest low-level reflectivity gradient, and (c) was coincident with the strongest surface pressure gradient [up to 2 mb (5 km)−1] ahead of the wake low center.
These findings indicate that the trailing stratiform precipitation regions of mesoscale convective systems can be dynamically significant phenomena, generating rapidly descending inflow jets at their back edges and, con-sequently, producing pronounced lower-tropospheric warming, intense surface pressure gradients and strong low-level winds.
Abstract
The formative stage of a long-lived mesoscale cyclonic vortex was captured by the NOAA P-3 aircraft as it investigated a developing mesoscale convective system (MCS) near the southeastern coast of Taiwan on 16 June 1987 during the Taiwan Area Mesoscale Experiment. The supporting environment of the mesovortex was characterized by an exceptionally moist atmosphere and moderate ambient vertical shear through a deep layer from the near surface to ∼6 km, with much weaker shear and winds aloft. In addition, a pronounced low-level mesoscale shear/convergence zone, which resulted from the interaction of southeasterly flow with northeasterly flow confined to the near-coast region, existed in the vicinity of the observed mesovortex. Composite three-dimensional wind fields derived via pseudo-dual-Doppler synthesis show the vortex had a horizontal diameter expanding from ∼40 km to ∼70 km in the lower to midtroposphere, respectively, and exhibited considerable tilt through this layer. Contrary to previously documented mesovortices, which have generally been fully developed and observed in the stratiform region of mature-to-decaying MCSs, the present vortex was intimately coupled to convective precipitation within this developing MCS. This study provides unique observational evidence that under appropriate environmental conditions a long-lasting mesovortex may originate in the convective region of an MCS.
Abstract
The formative stage of a long-lived mesoscale cyclonic vortex was captured by the NOAA P-3 aircraft as it investigated a developing mesoscale convective system (MCS) near the southeastern coast of Taiwan on 16 June 1987 during the Taiwan Area Mesoscale Experiment. The supporting environment of the mesovortex was characterized by an exceptionally moist atmosphere and moderate ambient vertical shear through a deep layer from the near surface to ∼6 km, with much weaker shear and winds aloft. In addition, a pronounced low-level mesoscale shear/convergence zone, which resulted from the interaction of southeasterly flow with northeasterly flow confined to the near-coast region, existed in the vicinity of the observed mesovortex. Composite three-dimensional wind fields derived via pseudo-dual-Doppler synthesis show the vortex had a horizontal diameter expanding from ∼40 km to ∼70 km in the lower to midtroposphere, respectively, and exhibited considerable tilt through this layer. Contrary to previously documented mesovortices, which have generally been fully developed and observed in the stratiform region of mature-to-decaying MCSs, the present vortex was intimately coupled to convective precipitation within this developing MCS. This study provides unique observational evidence that under appropriate environmental conditions a long-lasting mesovortex may originate in the convective region of an MCS.
Abstract
This paper describes the kinematic and precipitation evolution accompanying the passage of a cold baroclinic trough over the Central Oregon Coast Range and Cascades during 4–5 December 2001 of the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) field project. In contrast to previously documented IMPROVE-2 cases, the 4–5 December event featured weaker cross-barrier winds (15–20 m s−1), weaker moist static stability (Nm < 0.006 s−1), and convective cells that preferentially intensified over Oregon’s modest coastal mountain range. These cells propagated eastward and became embedded within the larger orographic precipitation shield over the windward slopes of the Cascades. The Weather Research and Forecasting Model (version 2.2) at 1.33-km grid spacing was able to accurately replicate the observed evolution of the precipitation across western Oregon. As a result of the convective cell development, the precipitation enhancement over the Coast Range (500–1000 m MSL) was nearly as large as that over the Cascades (1500–2000 m MSL). Simulations selectively eliminating the elevated coastal range and differential land–sea friction across the Pacific coastline illustrate that both effects were important in triggering convection and in producing the observed coastal precipitation enhancement. A sensitivity run employing a smoothed representation of the Cascades illustrates that narrow ridges located on that barrier’s windward slope had a relatively small (<5%) impact on embedded convection and overall precipitation amounts there. This is attributed to the relatively weak gravity wave motions and low freezing level, which limited precipitation growth by riming.
Abstract
This paper describes the kinematic and precipitation evolution accompanying the passage of a cold baroclinic trough over the Central Oregon Coast Range and Cascades during 4–5 December 2001 of the second Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-2) field project. In contrast to previously documented IMPROVE-2 cases, the 4–5 December event featured weaker cross-barrier winds (15–20 m s−1), weaker moist static stability (Nm < 0.006 s−1), and convective cells that preferentially intensified over Oregon’s modest coastal mountain range. These cells propagated eastward and became embedded within the larger orographic precipitation shield over the windward slopes of the Cascades. The Weather Research and Forecasting Model (version 2.2) at 1.33-km grid spacing was able to accurately replicate the observed evolution of the precipitation across western Oregon. As a result of the convective cell development, the precipitation enhancement over the Coast Range (500–1000 m MSL) was nearly as large as that over the Cascades (1500–2000 m MSL). Simulations selectively eliminating the elevated coastal range and differential land–sea friction across the Pacific coastline illustrate that both effects were important in triggering convection and in producing the observed coastal precipitation enhancement. A sensitivity run employing a smoothed representation of the Cascades illustrates that narrow ridges located on that barrier’s windward slope had a relatively small (<5%) impact on embedded convection and overall precipitation amounts there. This is attributed to the relatively weak gravity wave motions and low freezing level, which limited precipitation growth by riming.
