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Satellite and CALJET Aircraft Observations of Atmospheric Rivers over the Eastern North Pacific Ocean during the Winter of 1997/98

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  • 1 NOAA/Environmental Technology Laboratory, Boulder, Colorado
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Abstract

This study uses a unique combination of airborne and satellite observations to characterize narrow regions of strong horizontal water vapor flux associated with polar cold fronts that occurred over the eastern North Pacific Ocean during the winter of 1997/98. Observations of these “atmospheric rivers” are compared with past numerical modeling studies to confirm that such narrow features account for most of the instantaneous meridional water vapor transport at midlatitudes.

Wind and water vapor profiles observed by dropsondes deployed on 25–26 January 1998 during the California Land-falling Jets Experiment (CALJET) were used to document the structure of a modest frontal system. The horizontal water vapor flux was focused at low altitudes in a narrow region ahead of the cold front where the combination of strong winds and large water vapor content were found as part of a low-level jet. A close correlation was found between these fluxes and the integrated water vapor (IWV) content. In this case, 75% of the observed flux through a 1000-km cross-front baseline was within a 565-km-wide zone roughly 4 km deep. This zone contained 1.5 × 108 kg s−1 of meridional water vapor flux, the equivalent of ∼20% of the global average at 35°N.

By compositing polar-orbiting satellite Special Sensor Microwave Imager (SSM/I) data from 46 dates containing long, narrow zones of large IWV, it was determined that the single detailed case was representative of the composite in terms of both the IWV amplitude (3.09 cm vs 2.81 cm) and the width of the area where IWV ≥ 2 cm (424 km vs 388 km). The SSM/I composites also showed that the width scales (defined by the 75% cumulative fraction along a 1500-km cross-plume baseline) for cloud liquid water and rain rate were 176 and 141 km, respectively, which are narrower than the 417 km for IWV. Examination of coincident Geostationary Operational Environmental Satellite (GOES) and SSM/I satellite data revealed that GOES cloud-top temperatures were coldest and cloud-top pressures were lowest in the core of the IWV plumes, and that the core cloud tops became substantially colder and deeper for larger IWV. A strong latitudinal dependence of the satellite-derived cross-river characteristics was also found.

Atmospheric rivers form a critical link between weather and climate scales. They strongly influence both short-term weather and flood prediction, as well as seasonal climate anomalies and the global water cycle, through their cumulative effects. However, the rivers remain poorly observed by the existing global atmospheric observing system in terms of their horizontal water vapor fluxes.

Corresponding author address: Dr. F. Martin Ralph, NOAA/Environmental Technology Laboratory, Mail Code R/ET7, 325 Broadway, Boulder, CO 80305. Email: MartyRalph@noaa.gov

Abstract

This study uses a unique combination of airborne and satellite observations to characterize narrow regions of strong horizontal water vapor flux associated with polar cold fronts that occurred over the eastern North Pacific Ocean during the winter of 1997/98. Observations of these “atmospheric rivers” are compared with past numerical modeling studies to confirm that such narrow features account for most of the instantaneous meridional water vapor transport at midlatitudes.

Wind and water vapor profiles observed by dropsondes deployed on 25–26 January 1998 during the California Land-falling Jets Experiment (CALJET) were used to document the structure of a modest frontal system. The horizontal water vapor flux was focused at low altitudes in a narrow region ahead of the cold front where the combination of strong winds and large water vapor content were found as part of a low-level jet. A close correlation was found between these fluxes and the integrated water vapor (IWV) content. In this case, 75% of the observed flux through a 1000-km cross-front baseline was within a 565-km-wide zone roughly 4 km deep. This zone contained 1.5 × 108 kg s−1 of meridional water vapor flux, the equivalent of ∼20% of the global average at 35°N.

By compositing polar-orbiting satellite Special Sensor Microwave Imager (SSM/I) data from 46 dates containing long, narrow zones of large IWV, it was determined that the single detailed case was representative of the composite in terms of both the IWV amplitude (3.09 cm vs 2.81 cm) and the width of the area where IWV ≥ 2 cm (424 km vs 388 km). The SSM/I composites also showed that the width scales (defined by the 75% cumulative fraction along a 1500-km cross-plume baseline) for cloud liquid water and rain rate were 176 and 141 km, respectively, which are narrower than the 417 km for IWV. Examination of coincident Geostationary Operational Environmental Satellite (GOES) and SSM/I satellite data revealed that GOES cloud-top temperatures were coldest and cloud-top pressures were lowest in the core of the IWV plumes, and that the core cloud tops became substantially colder and deeper for larger IWV. A strong latitudinal dependence of the satellite-derived cross-river characteristics was also found.

Atmospheric rivers form a critical link between weather and climate scales. They strongly influence both short-term weather and flood prediction, as well as seasonal climate anomalies and the global water cycle, through their cumulative effects. However, the rivers remain poorly observed by the existing global atmospheric observing system in terms of their horizontal water vapor fluxes.

Corresponding author address: Dr. F. Martin Ralph, NOAA/Environmental Technology Laboratory, Mail Code R/ET7, 325 Broadway, Boulder, CO 80305. Email: MartyRalph@noaa.gov

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