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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
The general applicability of an isonomogram developed by Czys and coauthors to diagnose the position of the geographic boundary between freezing precipitation (freezing rain or freezing drizzle) and ice pellets (sleet or snow grains) was tested using a 25-yr sounding database consisting of 1051 soundings, 581 where stations were reporting freezing drizzle, 391 reporting freezing rain, and 79 reporting ice pellets. Of the 1051 soundings, only 306 clearly had an environmental temperature and moisture profile corresponding to that assumed for the isonomogram. This profile consisted of a three-layer atmosphere with 1) a cold cloud layer aloft that is a source of ice particles, 2) a midlevel layer where the temperature exceeds 0°C and ice particles melt, and 3) a surface layer where T < 0°C. The remaining soundings did not conform to the profile either because 1) the freezing precipitation was associated with the warm rain process or 2) the ice pellets formed due to riming rather than melting and refreezing. For soundings conforming to the profile, the isonomogram showed little diagnostic skill. Freezing rain or freezing drizzle occurred about 50% of the time that ice pellets were expected. Ice pellets occurred in nearly a third of the cases where freezing precipitation was diagnosed. Possible reasons for the poor diagnostic skill of the method are suggested.
Abstract
The general applicability of an isonomogram developed by Czys and coauthors to diagnose the position of the geographic boundary between freezing precipitation (freezing rain or freezing drizzle) and ice pellets (sleet or snow grains) was tested using a 25-yr sounding database consisting of 1051 soundings, 581 where stations were reporting freezing drizzle, 391 reporting freezing rain, and 79 reporting ice pellets. Of the 1051 soundings, only 306 clearly had an environmental temperature and moisture profile corresponding to that assumed for the isonomogram. This profile consisted of a three-layer atmosphere with 1) a cold cloud layer aloft that is a source of ice particles, 2) a midlevel layer where the temperature exceeds 0°C and ice particles melt, and 3) a surface layer where T < 0°C. The remaining soundings did not conform to the profile either because 1) the freezing precipitation was associated with the warm rain process or 2) the ice pellets formed due to riming rather than melting and refreezing. For soundings conforming to the profile, the isonomogram showed little diagnostic skill. Freezing rain or freezing drizzle occurred about 50% of the time that ice pellets were expected. Ice pellets occurred in nearly a third of the cases where freezing precipitation was diagnosed. Possible reasons for the poor diagnostic skill of the method are suggested.
