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- Author or Editor: Harald Sodemann x
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Abstract
The time water vapor spends in the atmosphere from evaporation to precipitation, termed here the water vapor lifetime, is of fundamental relevance for characterizing the water cycle, for the turnover of mass and energy, causes of precipitation extremes, and the recycling of precipitation over land. While the global average lifetime of water vapor is commonly considered as about 8–10 days, recent work indicates that the distribution of water vapor lifetimes is highly skewed, and that a large part of the water vapor could have average lifetimes of about 4–5 days. Besides calling for scrutiny of these new estimates, these findings also prompt an investigation of the factors shaping the distribution of the lifetime of water vapor. Using idealized setups and reanalysis data, I explore the influence of heterogeneity and nonstationarity on water vapor age and lifetime. The combination of nonstationarity and heterogeneity allows for short and long local lifetimes and water vapor ages, while maintaining the global average mass balance and corresponding mean water vapor lifetime. A plausibility argument based on humidity-weighted winds suggests that median lifetimes of 4–5 days are more consistent with weather system patterns in the extratropics. I propose that the median of the lifetime is more representative, since its mean value is affected by uncertainty originating from a long, thin tail. To more comprehensively understand the water vapor lifetime, methods will need to report the full lifetime distribution. Simulations with artificial water tracers could thereby provide the framework to compare different methods consistently in the future, while stable water isotopes could serve as an observational constraint.
Abstract
The time water vapor spends in the atmosphere from evaporation to precipitation, termed here the water vapor lifetime, is of fundamental relevance for characterizing the water cycle, for the turnover of mass and energy, causes of precipitation extremes, and the recycling of precipitation over land. While the global average lifetime of water vapor is commonly considered as about 8–10 days, recent work indicates that the distribution of water vapor lifetimes is highly skewed, and that a large part of the water vapor could have average lifetimes of about 4–5 days. Besides calling for scrutiny of these new estimates, these findings also prompt an investigation of the factors shaping the distribution of the lifetime of water vapor. Using idealized setups and reanalysis data, I explore the influence of heterogeneity and nonstationarity on water vapor age and lifetime. The combination of nonstationarity and heterogeneity allows for short and long local lifetimes and water vapor ages, while maintaining the global average mass balance and corresponding mean water vapor lifetime. A plausibility argument based on humidity-weighted winds suggests that median lifetimes of 4–5 days are more consistent with weather system patterns in the extratropics. I propose that the median of the lifetime is more representative, since its mean value is affected by uncertainty originating from a long, thin tail. To more comprehensively understand the water vapor lifetime, methods will need to report the full lifetime distribution. Simulations with artificial water tracers could thereby provide the framework to compare different methods consistently in the future, while stable water isotopes could serve as an observational constraint.
Abstract
During December 2006 many cyclones traveled across the North Atlantic, causing temperature and precipitation in Norway to be well above average. Large excursions of high vertically integrated water vapor, often referred to as atmospheric rivers, reached from the subtropics to high latitudes, inducing precipitation over western Scandinavia. The sources and transport of atmospheric water vapor in the North Atlantic storm track during that month are examined by means of a mesoscale model fitted with water vapor tracers. Decomposition of the modeled total water vapor field into numerical water vapor tracers tagged by evaporation latitude shows that when an atmospheric river was present, a higher fraction of water vapor from remote, southerly source regions caused more intense precipitation. The tracer transport analysis revealed that the atmospheric rivers were composed of a sequence of meridional excursions of water vapor, in close correspondence with the upper-level flow configuration. In cyclone cores, fast turnover of water vapor by evaporation and condensation were identified, leading to a rapid assimilation of water from the underlying ocean surface. In the regions of long-range transport, water vapor tracers from the southern midlatitudes and subtropics dominated over local contributions. By advection of water vapor along their trailing cold fronts cyclones were reinforcing the atmospheric rivers. At the same time the warm conveyor belt circulation was feeding off the atmospheric rivers by large-scale ascent and precipitation. Pronounced atmospheric rivers could persist in the domain throughout more than one cyclone's life cycle. These findings emphasize the interrelation between midlatitude cyclones and atmospheric rivers but also their distinction from the warm conveyor belt airstream.
Abstract
During December 2006 many cyclones traveled across the North Atlantic, causing temperature and precipitation in Norway to be well above average. Large excursions of high vertically integrated water vapor, often referred to as atmospheric rivers, reached from the subtropics to high latitudes, inducing precipitation over western Scandinavia. The sources and transport of atmospheric water vapor in the North Atlantic storm track during that month are examined by means of a mesoscale model fitted with water vapor tracers. Decomposition of the modeled total water vapor field into numerical water vapor tracers tagged by evaporation latitude shows that when an atmospheric river was present, a higher fraction of water vapor from remote, southerly source regions caused more intense precipitation. The tracer transport analysis revealed that the atmospheric rivers were composed of a sequence of meridional excursions of water vapor, in close correspondence with the upper-level flow configuration. In cyclone cores, fast turnover of water vapor by evaporation and condensation were identified, leading to a rapid assimilation of water from the underlying ocean surface. In the regions of long-range transport, water vapor tracers from the southern midlatitudes and subtropics dominated over local contributions. By advection of water vapor along their trailing cold fronts cyclones were reinforcing the atmospheric rivers. At the same time the warm conveyor belt circulation was feeding off the atmospheric rivers by large-scale ascent and precipitation. Pronounced atmospheric rivers could persist in the domain throughout more than one cyclone's life cycle. These findings emphasize the interrelation between midlatitude cyclones and atmospheric rivers but also their distinction from the warm conveyor belt airstream.
Abstract
Air masses in marine cold air outbreaks (CAOs) at high latitudes undergo a remarkable diabatic transformation because of the uptake of heat and moisture from the ocean surface, and the formation of precipitation. In this study, the fundamental characteristics of the water cycle during an intense and persistent, yet archetypal basinwide CAO from Fram Strait into the Nordic seas are analyzed with the aid of the tracer-enabled mesoscale limited-area numerical weather prediction model COSMO. A water budget of the CAO water cycle is performed based on tagged water tracers that follow moisture picked up by the CAO at various stages of its evolution. The atmospheric dynamical factors and boundary conditions that shape this budget are thereby analyzed. The water tracer analysis reveals a highly local water cycle associated with the CAO. Rapid turnover of water vapor results in an average residence time of precipitating waters of about one day. Approximately one-third of the total moisture taken up by the CAO falls as precipitation by convective overturning in the marine CAO boundary layer. Furthermore, precipitation efficiency increases as the CAO air mass matures and is exposed to warmer waters in the Norwegian Sea. These properties of the CAO water cycle are in strong contrast to situations dominated by long-range moisture transport that occur in the dynamically active regions of extratropical cyclones. It is proposed that CAOs in the confined Nordic seas provide a natural laboratory for studying local characteristics of the water cycle and evaluating its representation in models.
Abstract
Air masses in marine cold air outbreaks (CAOs) at high latitudes undergo a remarkable diabatic transformation because of the uptake of heat and moisture from the ocean surface, and the formation of precipitation. In this study, the fundamental characteristics of the water cycle during an intense and persistent, yet archetypal basinwide CAO from Fram Strait into the Nordic seas are analyzed with the aid of the tracer-enabled mesoscale limited-area numerical weather prediction model COSMO. A water budget of the CAO water cycle is performed based on tagged water tracers that follow moisture picked up by the CAO at various stages of its evolution. The atmospheric dynamical factors and boundary conditions that shape this budget are thereby analyzed. The water tracer analysis reveals a highly local water cycle associated with the CAO. Rapid turnover of water vapor results in an average residence time of precipitating waters of about one day. Approximately one-third of the total moisture taken up by the CAO falls as precipitation by convective overturning in the marine CAO boundary layer. Furthermore, precipitation efficiency increases as the CAO air mass matures and is exposed to warmer waters in the Norwegian Sea. These properties of the CAO water cycle are in strong contrast to situations dominated by long-range moisture transport that occur in the dynamically active regions of extratropical cyclones. It is proposed that CAOs in the confined Nordic seas provide a natural laboratory for studying local characteristics of the water cycle and evaluating its representation in models.
Abstract
A climatology of cold air outbreaks (CAOs) in the high latitudes of the South Pacific and an analysis of the dynamical mechanisms leading to their formation are presented. Two major and distinct regions with frequent CAOs from autumn to spring are identified: one in the Ross Sea and another in the Amundsen and Bellingshausen Seas. Using an objective method to attribute CAOs to extratropical cyclones, it is shown that about 80% of the CAOs occur in association with the cyclonic flow induced by the passage of extratropical cyclones. Based on kinematic backward trajectories it is quantified that more than 40% of the air masses leading to CAOs originate from Antarctica and descend substantially, with the Ross Ice Shelf corridor as the major pathway. CAO trajectories descending from Antarctica differ from those originating over sea ice by a much lower specific humidity, stronger diabatic cooling, and much more intense adiabatic warming, while potential vorticity evolves similarly in both categories. In winter, CAOs are the major contributor to the net turbulent heat flux off the sea ice edge and CAO frequency strongly determines its interannual variation. Wintertime variations of the frequency of extratropical cyclones are strongly imprinted on the frequency of CAOs and the net turbulent heat and freshwater fluxes. In particular, much of the precipitation associated with the passage of extratropical cyclones is compensated by intense evaporation in cyclone-induced CAOs. This highlights the dominant role of the extratropical storm track in determining the variability of the buoyancy flux forcing of the Southern Ocean.
Abstract
A climatology of cold air outbreaks (CAOs) in the high latitudes of the South Pacific and an analysis of the dynamical mechanisms leading to their formation are presented. Two major and distinct regions with frequent CAOs from autumn to spring are identified: one in the Ross Sea and another in the Amundsen and Bellingshausen Seas. Using an objective method to attribute CAOs to extratropical cyclones, it is shown that about 80% of the CAOs occur in association with the cyclonic flow induced by the passage of extratropical cyclones. Based on kinematic backward trajectories it is quantified that more than 40% of the air masses leading to CAOs originate from Antarctica and descend substantially, with the Ross Ice Shelf corridor as the major pathway. CAO trajectories descending from Antarctica differ from those originating over sea ice by a much lower specific humidity, stronger diabatic cooling, and much more intense adiabatic warming, while potential vorticity evolves similarly in both categories. In winter, CAOs are the major contributor to the net turbulent heat flux off the sea ice edge and CAO frequency strongly determines its interannual variation. Wintertime variations of the frequency of extratropical cyclones are strongly imprinted on the frequency of CAOs and the net turbulent heat and freshwater fluxes. In particular, much of the precipitation associated with the passage of extratropical cyclones is compensated by intense evaporation in cyclone-induced CAOs. This highlights the dominant role of the extratropical storm track in determining the variability of the buoyancy flux forcing of the Southern Ocean.
Abstract
Forecasts of marine cold air outbreaks critically rely on the interplay of multiple parameterization schemes to represent subgrid-scale processes, including shallow convection, turbulence, and microphysics. Even though such an interplay has been recognized to contribute to forecast uncertainty, a quantification of this interplay is still missing. Here, we investigate the tendencies of temperature and specific humidity contributed by individual parameterization schemes in the operational weather prediction model AROME-Arctic. From a case study of an extensive marine cold air outbreak over the Nordic seas, we find that the type of planetary boundary layer assigned by the model algorithm modulates the contribution of individual schemes and affects the interactions between different schemes. In addition, we demonstrate the sensitivity of these interactions to an increase or decrease in the strength of the parameterized shallow convection. The individual tendencies from several parameterizations can thereby compensate each other, sometimes resulting in a small residual. In some instances this residual remains nearly unchanged between the sensitivity experiments, even though some individual tendencies differ by up to an order of magnitude. Using the individual tendency output, we can characterize the subgrid-scale as well as grid-scale responses of the model and trace them back to their underlying causes. We thereby highlight the utility of individual tendency output for understanding process-related differences between model runs with varying physical configurations and for the continued development of numerical weather prediction models.
Abstract
Forecasts of marine cold air outbreaks critically rely on the interplay of multiple parameterization schemes to represent subgrid-scale processes, including shallow convection, turbulence, and microphysics. Even though such an interplay has been recognized to contribute to forecast uncertainty, a quantification of this interplay is still missing. Here, we investigate the tendencies of temperature and specific humidity contributed by individual parameterization schemes in the operational weather prediction model AROME-Arctic. From a case study of an extensive marine cold air outbreak over the Nordic seas, we find that the type of planetary boundary layer assigned by the model algorithm modulates the contribution of individual schemes and affects the interactions between different schemes. In addition, we demonstrate the sensitivity of these interactions to an increase or decrease in the strength of the parameterized shallow convection. The individual tendencies from several parameterizations can thereby compensate each other, sometimes resulting in a small residual. In some instances this residual remains nearly unchanged between the sensitivity experiments, even though some individual tendencies differ by up to an order of magnitude. Using the individual tendency output, we can characterize the subgrid-scale as well as grid-scale responses of the model and trace them back to their underlying causes. We thereby highlight the utility of individual tendency output for understanding process-related differences between model runs with varying physical configurations and for the continued development of numerical weather prediction models.
Abstract
In this study, the important role of extratropical cyclones and fronts for the atmospheric freshwater flux over the Southern Ocean is analyzed. Based on the Interim ECMWF Re-Analysis (ERA-Interim), the freshwater flux associated with cyclones is quantified and it is revealed that the structure of the Southern Hemispheric storm track is strongly imprinted on the climatological freshwater flux. In particular, during austral winter the spiraliform shape of the storm track leads to a band of negative freshwater flux bending toward and around Antarctica, complemented by a strong freshwater input into the midlatitude Pacific, associated with the split storm track. The interannual variability of the wintertime high-latitude freshwater flux is shown to be largely determined by the variability of strong precipitation (>75th percentile). Using a novel and comprehensive method to attribute strong precipitation uniquely to cyclones and fronts, it is demonstrated that over the Southern Ocean between 60% and 90% of the strong precipitation events are due to these synoptic systems. Cyclones are the dominant cause of strong precipitation around Antarctica and in the midlatitudes of the Atlantic and the Pacific, while in the south Indian Ocean and the eastern Atlantic fronts bring most of the strong precipitation. A detailed analysis of the spatial variations of intense front and cyclone precipitation associated with the interannual variability of the wintertime frequency of cyclones in the midlatitude and high-latitude branches of the Pacific storm track underpins the importance of considering both fronts and cyclones in the analysis of the interannual variability of freshwater fluxes.
Abstract
In this study, the important role of extratropical cyclones and fronts for the atmospheric freshwater flux over the Southern Ocean is analyzed. Based on the Interim ECMWF Re-Analysis (ERA-Interim), the freshwater flux associated with cyclones is quantified and it is revealed that the structure of the Southern Hemispheric storm track is strongly imprinted on the climatological freshwater flux. In particular, during austral winter the spiraliform shape of the storm track leads to a band of negative freshwater flux bending toward and around Antarctica, complemented by a strong freshwater input into the midlatitude Pacific, associated with the split storm track. The interannual variability of the wintertime high-latitude freshwater flux is shown to be largely determined by the variability of strong precipitation (>75th percentile). Using a novel and comprehensive method to attribute strong precipitation uniquely to cyclones and fronts, it is demonstrated that over the Southern Ocean between 60% and 90% of the strong precipitation events are due to these synoptic systems. Cyclones are the dominant cause of strong precipitation around Antarctica and in the midlatitudes of the Atlantic and the Pacific, while in the south Indian Ocean and the eastern Atlantic fronts bring most of the strong precipitation. A detailed analysis of the spatial variations of intense front and cyclone precipitation associated with the interannual variability of the wintertime frequency of cyclones in the midlatitude and high-latitude branches of the Pacific storm track underpins the importance of considering both fronts and cyclones in the analysis of the interannual variability of freshwater fluxes.
