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Abstract
A photograph of vertically aligned Kelvin–Helmholtz billows in the eastern eyewall of Hurricane Erin on 10 September 2001 is presented. The vertical shear instability in the horizontal winds necessary to produce the billows is confirmed with a high-altitude dropwindsonde observation. This shear instability is not known to be common in tropical cyclone eyewalls and is likely only in cases with a very large eyewall tilt. However, research and reconnaissance aircraft pilots need to be aware of the possibility of their existence, along with other types of hazardous conditions, in such rare circumstances.
Abstract
A photograph of vertically aligned Kelvin–Helmholtz billows in the eastern eyewall of Hurricane Erin on 10 September 2001 is presented. The vertical shear instability in the horizontal winds necessary to produce the billows is confirmed with a high-altitude dropwindsonde observation. This shear instability is not known to be common in tropical cyclone eyewalls and is likely only in cases with a very large eyewall tilt. However, research and reconnaissance aircraft pilots need to be aware of the possibility of their existence, along with other types of hazardous conditions, in such rare circumstances.
Abstract
A cold-air outbreak east of the Rocky Mountains on 23 January 2003 produced banded clouds and snow across the central and southeastern United States. The bands occurred through two processes: 1) thermal instability in the planetary boundary layer produced horizontal convective rolls (HCRs) over widespread areas, and 2) lake-effect processes downstream of small lakes (fetch < 100 km) produced localized bands. Characteristics of the observed bands associated with the HCRs, such as horizontal scale, depth of circulation, orientation, duration, and dynamics, are explored through observations, previous literature, and theoretical models. Snow from clouds produced by HCRs over land during the cold season has not been extensively studied previously. In this event, cold-air advection over the warm ground led to an upward sensible heat flux, promoting the occurrence of the HCR circulations. As the surface temperature decreased, the height of the lifting condensation level decreased, eventually forming cloud bands within the ascending portion of the HCR circulations. Ice crystals are inferred to have fallen from a large-scale precipitation system aloft into the cloud bands in the planetary boundary layer, which was within the favored temperature regime for dendritic growth of ice crystals. The ice crystals grew and reached the surface as light snow. This seeder–feeder process suggests one way to anticipate development of such snowbands in the future, as demonstrated by other similar events on other days in Oklahoma and Illinois. As the cloud bands were advected equatorward, they ingested drier air and dissipated. Among the several lake-effect bands observed on 23 January 2003, one notable band occurred downwind of Lake Kentucky. Midlake convergence of the land breeze may have initially produced a narrow cloud band that broadened as the land breeze ended. That the snowbands due to the HCRs and lake effect were both associated with heat and/or moisture fluxes from the earth's surface highlights the potential importance of ground- and water-surface temperature measurements for accurate numerical weather prediction.
Abstract
A cold-air outbreak east of the Rocky Mountains on 23 January 2003 produced banded clouds and snow across the central and southeastern United States. The bands occurred through two processes: 1) thermal instability in the planetary boundary layer produced horizontal convective rolls (HCRs) over widespread areas, and 2) lake-effect processes downstream of small lakes (fetch < 100 km) produced localized bands. Characteristics of the observed bands associated with the HCRs, such as horizontal scale, depth of circulation, orientation, duration, and dynamics, are explored through observations, previous literature, and theoretical models. Snow from clouds produced by HCRs over land during the cold season has not been extensively studied previously. In this event, cold-air advection over the warm ground led to an upward sensible heat flux, promoting the occurrence of the HCR circulations. As the surface temperature decreased, the height of the lifting condensation level decreased, eventually forming cloud bands within the ascending portion of the HCR circulations. Ice crystals are inferred to have fallen from a large-scale precipitation system aloft into the cloud bands in the planetary boundary layer, which was within the favored temperature regime for dendritic growth of ice crystals. The ice crystals grew and reached the surface as light snow. This seeder–feeder process suggests one way to anticipate development of such snowbands in the future, as demonstrated by other similar events on other days in Oklahoma and Illinois. As the cloud bands were advected equatorward, they ingested drier air and dissipated. Among the several lake-effect bands observed on 23 January 2003, one notable band occurred downwind of Lake Kentucky. Midlake convergence of the land breeze may have initially produced a narrow cloud band that broadened as the land breeze ended. That the snowbands due to the HCRs and lake effect were both associated with heat and/or moisture fluxes from the earth's surface highlights the potential importance of ground- and water-surface temperature measurements for accurate numerical weather prediction.
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Abstract
Geostationary Operational Environmental Satellite (GOES) enhanced infrared (IR) imagery depicted very cold temperatures over Colorado on the morning of 8 December 1978. The situation was unusual because skies were clear and the cold temperatures were not associated with high cloud tops. Instead, satellite data mapped large areas that were experiencing extremely cold surface temperatures. The GOES data were also examined using the Colorado State University interactive data processing system and it was found that the cold IR readings corresponded well with early morning low temperatures over the state. GOES data can be of use in monitoring surface temperatures and can, in certain situations, provide detailed spatial and temporal information over regions experiencing extreme temperatures.
Abstract
Geostationary Operational Environmental Satellite (GOES) enhanced infrared (IR) imagery depicted very cold temperatures over Colorado on the morning of 8 December 1978. The situation was unusual because skies were clear and the cold temperatures were not associated with high cloud tops. Instead, satellite data mapped large areas that were experiencing extremely cold surface temperatures. The GOES data were also examined using the Colorado State University interactive data processing system and it was found that the cold IR readings corresponded well with early morning low temperatures over the state. GOES data can be of use in monitoring surface temperatures and can, in certain situations, provide detailed spatial and temporal information over regions experiencing extreme temperatures.
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PICTURE OF THE MONTH
Observations of Isolated Mesoscale Cellular Convection Cells
Abstract
What appeared to he isolated mesoscale convection cells embedded in an otherwise solid deck of stratocumulus clouds were observed from a commercial air liner as it approached the coast of the state of Washington on a flight from Tokyo. The presence of cells of the correct dimensions was confirmed by a satellite photograph of the area.
Abstract
What appeared to he isolated mesoscale convection cells embedded in an otherwise solid deck of stratocumulus clouds were observed from a commercial air liner as it approached the coast of the state of Washington on a flight from Tokyo. The presence of cells of the correct dimensions was confirmed by a satellite photograph of the area.
Abstract
A brief case history of a mid-Pacific upper tropospheric low which developed into a surface system that rapidly attained tropical character is presented. Satellite and conventional data suggest that hurricane-force winds were probably attained, marking, perhaps, the first such documented case north of 30° latitude in the Central Pacific.
Abstract
A brief case history of a mid-Pacific upper tropospheric low which developed into a surface system that rapidly attained tropical character is presented. Satellite and conventional data suggest that hurricane-force winds were probably attained, marking, perhaps, the first such documented case north of 30° latitude in the Central Pacific.
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