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- Author or Editor: A. J. Riordan x
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Abstract
Thermal structure of the marine boundary layer (MBL) was studied during a five-day cruise over the coastal Atlantic Ocean off North Carolina. Three different synoptic conditions were present: ahead of a low moving along the coast, in the area of a frontal zone and during a cold air outbreak. The marine boundary layer height was deeper (approximately 1500 m) and more sharply defined during the cold air outbreak than when the flow was southwesterly with a long fetch over water; the height was only about 1000 m for the latter case. Latent heat fluxes were significantly larger than sensible heat, but during the cold air outbreak, sensible heat fluxes increased appreciably.
Abstract
Thermal structure of the marine boundary layer (MBL) was studied during a five-day cruise over the coastal Atlantic Ocean off North Carolina. Three different synoptic conditions were present: ahead of a low moving along the coast, in the area of a frontal zone and during a cold air outbreak. The marine boundary layer height was deeper (approximately 1500 m) and more sharply defined during the cold air outbreak than when the flow was southwesterly with a long fetch over water; the height was only about 1000 m for the latter case. Latent heat fluxes were significantly larger than sensible heat, but during the cold air outbreak, sensible heat fluxes increased appreciably.
Abstract
A substantial decline in North American cyclone and anticyclone activity has been documented by several recent studies based on counts of disturbance tracks. An independent method of assessing long-term trends in synoptic-scale activity based on sequential spectral analysis of station pressure is suggested. The efficacy of this approach is supported by previous studies relating the spatial distribution of variance of band-pass filtered pressures to preferred cyclone tracks. However, examples of a preliminary application of the spectral method to three widely separated stations using approximately 30 years of winter data fail to reveal any significant long-term trends in the variance of pressure for synoptic-scale time periods.
Abstract
A substantial decline in North American cyclone and anticyclone activity has been documented by several recent studies based on counts of disturbance tracks. An independent method of assessing long-term trends in synoptic-scale activity based on sequential spectral analysis of station pressure is suggested. The efficacy of this approach is supported by previous studies relating the spatial distribution of variance of band-pass filtered pressures to preferred cyclone tracks. However, examples of a preliminary application of the spectral method to three widely separated stations using approximately 30 years of winter data fail to reveal any significant long-term trends in the variance of pressure for synoptic-scale time periods.
Abstract
This study describes a simple objective method to identify cases of coastal frontogenesis offshore of the Carolinas and to characterize the sensible weather associated with frontal passage at measurement sites near the coast. The identification method, based on surface hourly data from offshore and adjacent land stations, was applied to an 11-yr dataset (1984–94). A total of 379 coastal fronts was found, 70 of which eventually made landfall along the North Carolina coast; 112 that remained offshore, and 197 were termed diurnal since they remained offshore but disappeared during daylight hours.
Results show that most coastal and offshore sites experience a wind shift of about 40°–70° and a warming of about 2°–3°C during the hour of frontal passage. Exceptions include sites near colder waters where the rates are markedly reduced and frontal passage is often less discernible. Excluding diurnal fronts, just over half the cases were associated with cold-air damming (CAD) during the cold season of 16 October–15 April. Most of these winter cases linked with CAD were onshore fronts. During the warm season, most fronts were diurnal, but the association with CAD was still significant.
To explore the synoptic-scale environment, composite maps for the cold season were generated for all three frontal subtypes from NCEP–NCAR reanalysis data. Results show a strong surface anticyclone centered north of the region of frontogenesis for all three composites. However, several features in the synoptic-scale regimes appear to differentiate the three frontal types. For example, cyclogenesis in the Gulf of Mexico and onshore southeasterly low-level flow along the southeast Atlantic coast accompanied by warm advection distinguish onshore fronts from the other two types. The offshore fronts are accompanied by more nearly zonal flow aloft and a surface anticyclone that stalls near the New England coastline. Finally, the diurnal type is associated with much weaker pressure and height fields and an east–west elongated surface anticyclone centered much farther south than in the other cases.
Abstract
This study describes a simple objective method to identify cases of coastal frontogenesis offshore of the Carolinas and to characterize the sensible weather associated with frontal passage at measurement sites near the coast. The identification method, based on surface hourly data from offshore and adjacent land stations, was applied to an 11-yr dataset (1984–94). A total of 379 coastal fronts was found, 70 of which eventually made landfall along the North Carolina coast; 112 that remained offshore, and 197 were termed diurnal since they remained offshore but disappeared during daylight hours.
Results show that most coastal and offshore sites experience a wind shift of about 40°–70° and a warming of about 2°–3°C during the hour of frontal passage. Exceptions include sites near colder waters where the rates are markedly reduced and frontal passage is often less discernible. Excluding diurnal fronts, just over half the cases were associated with cold-air damming (CAD) during the cold season of 16 October–15 April. Most of these winter cases linked with CAD were onshore fronts. During the warm season, most fronts were diurnal, but the association with CAD was still significant.
To explore the synoptic-scale environment, composite maps for the cold season were generated for all three frontal subtypes from NCEP–NCAR reanalysis data. Results show a strong surface anticyclone centered north of the region of frontogenesis for all three composites. However, several features in the synoptic-scale regimes appear to differentiate the three frontal types. For example, cyclogenesis in the Gulf of Mexico and onshore southeasterly low-level flow along the southeast Atlantic coast accompanied by warm advection distinguish onshore fronts from the other two types. The offshore fronts are accompanied by more nearly zonal flow aloft and a surface anticyclone that stalls near the New England coastline. Finally, the diurnal type is associated with much weaker pressure and height fields and an east–west elongated surface anticyclone centered much farther south than in the other cases.