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- Author or Editor: Mohan K. Ramamurthy x
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Abstract
A severe freezing rainstorm produced as much as 4.5 cm of freezing rain during an 18-h period at Champaign, Illinois, on 14–15 February 1990, resulting in over $12 million in damage, week-long power outages, and a federal disaster declaration. The ice storm occurred during the University of Illinois Winter Precipitation Program based in Champaign. The early mesoscale evolution of this storm was documented for several hours with a 10-cm Doppler radar and Cross-chain Loran Atmospheric Sounding System soundings launched every 3 h. The freezing rain event occurred when convective bands developed over a slow-moving warm front during a period of strong overrunning. The strongest convection developed in a period of about 1 h, with a narrow elongated band northwest of the radar producing very heavy sleet and a band just south of the radar producing heavy freezing rain, along with in-cloud lightning.
An analysis of conditional symmetric instability yielded no evidence that centrifugal accelerations were important to the development of convection in this storm. Frontogenetic forcing was strongest several hours before the development of the bands but apparently was also insufficient to trigger convection until the local atmosphere became marginally unstable to upright convection. The transition from a conditionally stable to an unstable atmosphere in the vicinity of the bands was directly associated with locally strong warm advection above the warm frontal surface.
Forecast guidance, including the nested grid model (NGM) thickness, precipitation, and 850-mb temperature forecasts, and model output statistics of both the limited fine mesh (LFM) model and the NGM all predicted that the warm front would progress northward and that freezing rain would convert to rain before significant glaze accumulations occurred. Forecasts of midtropospheric parameters such as 1000–500-mb thickness and 850-mb temperature indeed verified; however, surface temperature forecasts were significantly in error, with errors ranging from 5° to 10°C during the period of heaviest glaze accumulation. The observed surface temperature never rose above 0°C during the period of ice accumulation or throughout the following day. The isothermal conditions observed during and after the storm appeared to be the result of sublimation and melting of ice that had accumulated on surface objects. The available evidence suggested that ice sublimation and melting, in addition to cooling the boundary layer, maintained a small wedge of cold air at the surface over which warmer air rose as it advected northward. The result of ice sublimation and melting was to retard the movement of the surface warm front, although warm air aloft was free to move over the narrow wedge of cooled surface air. By maintaining the surface temperature near 0°C, diabatic processes extended the duration of time that heavy glaze accumulations remained on trees and wires, allowing more damage to occur.
Abstract
A severe freezing rainstorm produced as much as 4.5 cm of freezing rain during an 18-h period at Champaign, Illinois, on 14–15 February 1990, resulting in over $12 million in damage, week-long power outages, and a federal disaster declaration. The ice storm occurred during the University of Illinois Winter Precipitation Program based in Champaign. The early mesoscale evolution of this storm was documented for several hours with a 10-cm Doppler radar and Cross-chain Loran Atmospheric Sounding System soundings launched every 3 h. The freezing rain event occurred when convective bands developed over a slow-moving warm front during a period of strong overrunning. The strongest convection developed in a period of about 1 h, with a narrow elongated band northwest of the radar producing very heavy sleet and a band just south of the radar producing heavy freezing rain, along with in-cloud lightning.
An analysis of conditional symmetric instability yielded no evidence that centrifugal accelerations were important to the development of convection in this storm. Frontogenetic forcing was strongest several hours before the development of the bands but apparently was also insufficient to trigger convection until the local atmosphere became marginally unstable to upright convection. The transition from a conditionally stable to an unstable atmosphere in the vicinity of the bands was directly associated with locally strong warm advection above the warm frontal surface.
Forecast guidance, including the nested grid model (NGM) thickness, precipitation, and 850-mb temperature forecasts, and model output statistics of both the limited fine mesh (LFM) model and the NGM all predicted that the warm front would progress northward and that freezing rain would convert to rain before significant glaze accumulations occurred. Forecasts of midtropospheric parameters such as 1000–500-mb thickness and 850-mb temperature indeed verified; however, surface temperature forecasts were significantly in error, with errors ranging from 5° to 10°C during the period of heaviest glaze accumulation. The observed surface temperature never rose above 0°C during the period of ice accumulation or throughout the following day. The isothermal conditions observed during and after the storm appeared to be the result of sublimation and melting of ice that had accumulated on surface objects. The available evidence suggested that ice sublimation and melting, in addition to cooling the boundary layer, maintained a small wedge of cold air at the surface over which warmer air rose as it advected northward. The result of ice sublimation and melting was to retard the movement of the surface warm front, although warm air aloft was free to move over the narrow wedge of cooled surface air. By maintaining the surface temperature near 0°C, diabatic processes extended the duration of time that heavy glaze accumulations remained on trees and wires, allowing more damage to occur.
Abstract
Two large-amplitude gravity waves were observed over the midwestern United States on 5 and 14 January 1989 during the University of Illinois Winter Precipitation Program. On both days, an extensive amount of data was recorded, including data from two radars and a radiosonde facility. The waves originated near Missouri, registered pressure fluctuations as large as 10 mb, and produced distinct precipitation bands along their updraft regions.
The waves were long-lived and maintained their identity over 1000 km, a distance several times their wave-lengths. The synoptic features at the surface were dissimilar. A deep cyclone was present on 5 January, while a leeside trough was present on 14 January. However, the middle- and upper-tropospheric flow patterns were similar. In both cases, the axis of a trough was immediately upstream of the gravity-wave genesis area and a jet streak had just propagated through the base of the trough toward a downstream ridge. Soundings taken near the gravity waves were remarkably similar, with both soundings showing a surface inversion capped by a deep layer of near-neutral stability. However, the relationship between the location of the gravity wave and the region of large-scale precipitation differed in the two cases. The 5 January wave occurred at the back edge of the precipitation associated with a comma cloud, while the wave on 14 January was observed at the leading edge of the synoptic-scale precipitation region.
The gravity wave had the structure of a solitary wave of elevation on 5 January, while it appeared as an undular bore with an embedded pressure jump on 14 January. A critical level, with small Richardson numbers, was present in both the cases. A well-defined duct, formed by an inversion below and critical level above, contributed to the maintenance of waves. Shearing instability and geostrophic adjustment were the likely generation mechanisms, though it was difficult to discount the role of convection.
Abstract
Two large-amplitude gravity waves were observed over the midwestern United States on 5 and 14 January 1989 during the University of Illinois Winter Precipitation Program. On both days, an extensive amount of data was recorded, including data from two radars and a radiosonde facility. The waves originated near Missouri, registered pressure fluctuations as large as 10 mb, and produced distinct precipitation bands along their updraft regions.
The waves were long-lived and maintained their identity over 1000 km, a distance several times their wave-lengths. The synoptic features at the surface were dissimilar. A deep cyclone was present on 5 January, while a leeside trough was present on 14 January. However, the middle- and upper-tropospheric flow patterns were similar. In both cases, the axis of a trough was immediately upstream of the gravity-wave genesis area and a jet streak had just propagated through the base of the trough toward a downstream ridge. Soundings taken near the gravity waves were remarkably similar, with both soundings showing a surface inversion capped by a deep layer of near-neutral stability. However, the relationship between the location of the gravity wave and the region of large-scale precipitation differed in the two cases. The 5 January wave occurred at the back edge of the precipitation associated with a comma cloud, while the wave on 14 January was observed at the leading edge of the synoptic-scale precipitation region.
The gravity wave had the structure of a solitary wave of elevation on 5 January, while it appeared as an undular bore with an embedded pressure jump on 14 January. A critical level, with small Richardson numbers, was present in both the cases. A well-defined duct, formed by an inversion below and critical level above, contributed to the maintenance of waves. Shearing instability and geostrophic adjustment were the likely generation mechanisms, though it was difficult to discount the role of convection.
Abstract
The successful deployment of many different observing systems during the summer Monsoon Experiment of 1979 provides a unique opportunity to perform extensive observing system experiments. These numerical studies, accomplished here with a ten-level, limited-area primitive equation model, allow the assessment of the value of individual or combined observing systems to the model's four-dimensional data assimilation system as well as to its subsequent forecasts. The specific objectives of this work include the investigation of (i) the relative merit of ten different data platforms, (ii) the relative role of wind and mass field data, (iii) the effect of different vertical distributions of single-level wind data, and (iv) the dynamical response of the model to different modes of data insertion.
Eight experiments are summarized, all of which involved a 12-h data assimilation period based on the Newtonian relaxation procedure followed by a 36-h forecast. Predictions using all of the data produced very good forecasts of the June 1979 onset vortex over the Arabian Sea. The dropwindsonde data were found to be most responsible for this success, primarily because they resolve the rotational modes of the system and cover a significant depth of the troposphere. While the winds were more important, the dropsonde thermodynamic data were beneficial. All datasets, when tested individually, had a positive impact on the forecasts. When used in combination, however, some datasets became less important or even redundant. The influence of satellite winds was enhanced greatly by spreading the wind increments over a larger vertical depth. It is shown that the dynamical response of the model to the various distributions and amounts of new data is consistent with geostrophic adjustment theory and provides guidance for future observing system.
Abstract
The successful deployment of many different observing systems during the summer Monsoon Experiment of 1979 provides a unique opportunity to perform extensive observing system experiments. These numerical studies, accomplished here with a ten-level, limited-area primitive equation model, allow the assessment of the value of individual or combined observing systems to the model's four-dimensional data assimilation system as well as to its subsequent forecasts. The specific objectives of this work include the investigation of (i) the relative merit of ten different data platforms, (ii) the relative role of wind and mass field data, (iii) the effect of different vertical distributions of single-level wind data, and (iv) the dynamical response of the model to different modes of data insertion.
Eight experiments are summarized, all of which involved a 12-h data assimilation period based on the Newtonian relaxation procedure followed by a 36-h forecast. Predictions using all of the data produced very good forecasts of the June 1979 onset vortex over the Arabian Sea. The dropwindsonde data were found to be most responsible for this success, primarily because they resolve the rotational modes of the system and cover a significant depth of the troposphere. While the winds were more important, the dropsonde thermodynamic data were beneficial. All datasets, when tested individually, had a positive impact on the forecasts. When used in combination, however, some datasets became less important or even redundant. The influence of satellite winds was enhanced greatly by spreading the wind increments over a larger vertical depth. It is shown that the dynamical response of the model to the various distributions and amounts of new data is consistent with geostrophic adjustment theory and provides guidance for future observing system.
Abstract
The dynamic and thermodynamic structure and associated frontal circulations within the trowal and warm-frontal regions of two extratropical winter cyclones are examined using numerical simulations. In each cyclone, the warm, moist airstream originating in the warm sector was found to bifurcate upon reaching the warm front. One branch of the flow turned anticyclonically eastward, corresponding to the warm conveyor belt, while the second branch turned cyclonically westward becoming the trowal airstream. The dynamic forcing of vertical motion within the two airstreams was investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), both from an analysis of the Sawyer–Eliassen (SE) equation in two dimensions and from complete model solutions.
Shearing deformation, associated with the jet stream and the low-level cyclonic flow, dominated confluent deformation near the trowal in both cases. The shearing deformation was accompanied by cold advection associated with an intrusion of cold, dry air aloft. The configuration of isentropes and the wind field led to frontogenesis on the equatorward side of the trowal and frontolysis farther south on the poleward side of the jet stream. The SE solution showed a circulation centered on the frontogenesis–frontolysis couplet, with air rising in the trowal and sinking within the dry air mass on the trowal’s equatorward side. The rising branch of the circulation was responsible for the wide swath of snowfall coincident with the trowal. From the vicinity of the bifurcation axis eastward along the warm-frontal zone, confluent deformation dominated within the troposphere. Frontogenesis in this region produced a direct circulation whose rising branch accounted for the production of precipitation over the warm-frontal zone. Diabatic processes associated with latent heating and cooling produced frontogenesis–frontolysis couplets and significantly modified the transverse frontal circulations. The ascending motion was amplified by a factor of 2 or greater compared with the ascending motion solely due to horizontal deformation. The width of the ascending branch was also narrowed compared with that solely from deformation. Vertical tilting, a result of the secondary circulation generated by horizontal deformation, produced frontogenesis–frontolysis couplets that acted to oppose and reduce the magnitude of the secondary circulation. A conceptual model of the effect of these processes on the production and organization of snowfall in the two cyclones is presented.
Abstract
The dynamic and thermodynamic structure and associated frontal circulations within the trowal and warm-frontal regions of two extratropical winter cyclones are examined using numerical simulations. In each cyclone, the warm, moist airstream originating in the warm sector was found to bifurcate upon reaching the warm front. One branch of the flow turned anticyclonically eastward, corresponding to the warm conveyor belt, while the second branch turned cyclonically westward becoming the trowal airstream. The dynamic forcing of vertical motion within the two airstreams was investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), both from an analysis of the Sawyer–Eliassen (SE) equation in two dimensions and from complete model solutions.
Shearing deformation, associated with the jet stream and the low-level cyclonic flow, dominated confluent deformation near the trowal in both cases. The shearing deformation was accompanied by cold advection associated with an intrusion of cold, dry air aloft. The configuration of isentropes and the wind field led to frontogenesis on the equatorward side of the trowal and frontolysis farther south on the poleward side of the jet stream. The SE solution showed a circulation centered on the frontogenesis–frontolysis couplet, with air rising in the trowal and sinking within the dry air mass on the trowal’s equatorward side. The rising branch of the circulation was responsible for the wide swath of snowfall coincident with the trowal. From the vicinity of the bifurcation axis eastward along the warm-frontal zone, confluent deformation dominated within the troposphere. Frontogenesis in this region produced a direct circulation whose rising branch accounted for the production of precipitation over the warm-frontal zone. Diabatic processes associated with latent heating and cooling produced frontogenesis–frontolysis couplets and significantly modified the transverse frontal circulations. The ascending motion was amplified by a factor of 2 or greater compared with the ascending motion solely due to horizontal deformation. The width of the ascending branch was also narrowed compared with that solely from deformation. Vertical tilting, a result of the secondary circulation generated by horizontal deformation, produced frontogenesis–frontolysis couplets that acted to oppose and reduce the magnitude of the secondary circulation. A conceptual model of the effect of these processes on the production and organization of snowfall in the two cyclones is presented.
Abstract
This paper compares the structure of the trough of warm air aloft (trowal)–warm-frontal region of two continental wintertime cyclones. The cyclones were observed over the central Great Lakes region during the Lake-Induced Convection Experiment/Snowband Dynamics Project field campaign. The cyclones had different origins, with the first forming east of the Colorado Rockies and the second forming over the Gulf of Mexico. They were associated with different upper-level flow regimes, one located just north of a nearly zonal jet and the other located just west of a nearly meridional jet. Both storms produced heavy swaths of snow across the states of Illinois, Wisconsin, and Michigan. High-resolution observations of frontal structure were made during flights of the National Center for Atmospheric Research Electra aircraft using dropsondes and the Electra Doppler Radar tail radar system. The high-resolution observations suggest a different arrangement of air masses in the trowal region compared with the classical occlusion model, where the trowal axis forms at the intersection of a warm front and a cold front that has overtaken and subsequently ascended the warm front. In both cyclones dry air intruded over the warm front, isolating the warm, moist airflow within the trowal airstream. Very sharp moisture gradients were present at the leading edge of the dry air in both cyclones. In each case, relative humidity differences of over 50% were observed over distances of 10–20 km. The thermal gradient near the leading edge of the dry air in one cyclone was diffuse, so that the moist–dry boundary could best be characterized as an upper-level humidity front. In the other cyclone, the thermal gradient was sharper and aligned with the moisture boundary and was best characterized as a cold front aloft. The analyses suggest that the classical conceptual model of the trowal, at least in some cyclones such as the two illustrated here, needs to be revised to include the possibility that the warm moist airstream aloft may sometimes be bounded on its south side by an upper-level front rather than a surface-based cold front. Since the two cyclones discussed here had different origins, tracks, and flow regimes, the similarity of their structure suggests that these features may be common.
Abstract
This paper compares the structure of the trough of warm air aloft (trowal)–warm-frontal region of two continental wintertime cyclones. The cyclones were observed over the central Great Lakes region during the Lake-Induced Convection Experiment/Snowband Dynamics Project field campaign. The cyclones had different origins, with the first forming east of the Colorado Rockies and the second forming over the Gulf of Mexico. They were associated with different upper-level flow regimes, one located just north of a nearly zonal jet and the other located just west of a nearly meridional jet. Both storms produced heavy swaths of snow across the states of Illinois, Wisconsin, and Michigan. High-resolution observations of frontal structure were made during flights of the National Center for Atmospheric Research Electra aircraft using dropsondes and the Electra Doppler Radar tail radar system. The high-resolution observations suggest a different arrangement of air masses in the trowal region compared with the classical occlusion model, where the trowal axis forms at the intersection of a warm front and a cold front that has overtaken and subsequently ascended the warm front. In both cyclones dry air intruded over the warm front, isolating the warm, moist airflow within the trowal airstream. Very sharp moisture gradients were present at the leading edge of the dry air in both cyclones. In each case, relative humidity differences of over 50% were observed over distances of 10–20 km. The thermal gradient near the leading edge of the dry air in one cyclone was diffuse, so that the moist–dry boundary could best be characterized as an upper-level humidity front. In the other cyclone, the thermal gradient was sharper and aligned with the moisture boundary and was best characterized as a cold front aloft. The analyses suggest that the classical conceptual model of the trowal, at least in some cyclones such as the two illustrated here, needs to be revised to include the possibility that the warm moist airstream aloft may sometimes be bounded on its south side by an upper-level front rather than a surface-based cold front. Since the two cyclones discussed here had different origins, tracks, and flow regimes, the similarity of their structure suggests that these features may be common.
