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Noah P Molotch - One of the best experts on this subject based on the ideXlab platform.
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spatial variation of the rain snow temperature threshold across the Northern Hemisphere
Nature Communications, 2018Co-Authors: Keith Jennings, Taylor Winchell, Ben Livneh, Noah P MolotchAbstract:Despite the importance of precipitation phase to global hydroclimate simulations, many land surface models use spatially uniform air temperature thresholds to partition rain and snow. Here we show, through the analysis of a 29-year observational dataset (n = 17.8 million), that the air temperature at which rain and snow fall in equal frequency varies significantly across the Northern Hemisphere, averaging 1.0 °C and ranging from –0.4 to 2.4 °C for 95% of the stations. Continental climates generally exhibit the warmest rain–snow thresholds and maritime the coolest. Simulations show precipitation phase methods incorporating humidity perform better than air temperature-only methods, particularly at relative humidity values below saturation and air temperatures between 0.6 and 3.4 °C. We also present the first continuous Northern Hemisphere map of rain–snow thresholds, underlining the spatial variability of precipitation phase partitioning. These results suggest precipitation phase could be better predicted using humidity and air temperature in large-scale land surface model runs. Land surface models often use a spatially uniform air temperature threshold when partitioning rain and snow. Here Jennings et al. show that the threshold varies significantly across the Northern Hemisphere and that threshold selection is a large source of uncertainty in snowfall simulations.
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spatial variation of the rain snow temperature threshold across the Northern Hemisphere
Nature Communications, 2018Co-Authors: Keith Jennings, Taylor Winchell, Ben Livneh, Noah P MolotchAbstract:Despite the importance of precipitation phase to global hydroclimate simulations, many land surface models use spatially uniform air temperature thresholds to partition rain and snow. Here we show, through the analysis of a 29-year observational dataset (n = 17.8 million), that the air temperature at which rain and snow fall in equal frequency varies significantly across the Northern Hemisphere, averaging 1.0 °C and ranging from -0.4 to 2.4 °C for 95% of the stations. Continental climates generally exhibit the warmest rain-snow thresholds and maritime the coolest. Simulations show precipitation phase methods incorporating humidity perform better than air temperature-only methods, particularly at relative humidity values below saturation and air temperatures between 0.6 and 3.4 °C. We also present the first continuous Northern Hemisphere map of rain-snow thresholds, underlining the spatial variability of precipitation phase partitioning. These results suggest precipitation phase could be better predicted using humidity and air temperature in large-scale land surface model runs.
Dennis L. Hartmann - One of the best experts on this subject based on the ideXlab platform.
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The Life Cycle of the Northern Hemisphere Sudden Stratospheric Warmings
Journal of Climate, 2004Co-Authors: Varavut Limpasuvan, David W. J. T Hompson, Dennis L. HartmannAbstract:Motivated by recent evidence of strong stratospheric‐tropospheric coupling during the Northern Hemisphere winter, this study examines the evolution of the atmospheric flow and wave fluxes at levels throughout the stratosphere and troposphere during the composite life cycle of a sudden stratospheric warming. The composite comprises 39 major and minor warming events using 44 years of NCEP‐NCAR reanalysis data. The incipient stage of the life cycle is characterized by preconditioning of the stratospheric zonal flow and anomalous, quasistationary wavenumber-1 forcing in both the stratosphere and troposphere. As the life cycle intensifies, planetary wave driving gives rise to weakening of the stratospheric polar vortex and downward propagation of the attendant easterly wind and positive temperature anomalies. When these anomalies reach the tropopause, the life cycle is marked by momentum flux and mean meridional circulation anomalies at tropospheric levels that are consistent with the negative phase of the Northern Hemisphere annular mode. The anomalous momentum fluxes are largest over the Atlantic half of the Hemisphere and are associated primarily with waves of wavenumber 3 and higher.
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eddy zonal flow feedback in the Northern Hemisphere winter
Journal of Climate, 2003Co-Authors: David J Lorenz, Dennis L. HartmannAbstract:Abstract The variability of the zonal-mean zonal wind in the Northern Hemisphere winter (December–March) is studied using EOF analysis and momentum budget diagnostics of NCEP–NCAR reanalysis data (1976–2001). The leading EOF of the zonal-mean zonal wind is well separated from the remaining EOFs and represents the north–south movement of the midlatitude westerlies. Analysis of the momentum budget shows that a positive feedback between the zonal-mean wind anomalies and the eddy momentum fluxes selects the leading EOF of midlatitude variability. Like the Southern Hemisphere, the baroclinic eddies reinforce the zonal wind anomalies while external Rossby waves damp the wind anomalies. In the Northern Hemisphere, the quasi-stationary eddies also reinforce the zonal wind anomalies, but the baroclinic eddies are most important for the positive eddy–zonal flow feedback. The observations support the following feedback mechanisms. 1) Above-normal baroclinic wave activity is generated in the region of enhanced wester...
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eddy zonal flow feedback in the Northern Hemisphere winter
Journal of Climate, 2003Co-Authors: David J Lorenz, Dennis L. HartmannAbstract:The variability of the zonal-mean zonal wind in the Northern Hemisphere winter (December‐March) is studied using EOF analysis and momentum budget diagnostics of NCEP‐NCAR reanalysis data (1976‐2001). The leading EOF of the zonal-mean zonal wind is well separated from the remaining EOFs and represents the north‐south movement of the midlatitude westerlies. Analysis of the momentum budget shows that a positive feedback between the zonal-mean wind anomalies and the eddy momentum fluxes selects the leading EOF of midlatitude variability. Like the Southern Hemisphere, the baroclinic eddies reinforce the zonal wind anomalies while external Rossby waves damp the wind anomalies. In the Northern Hemisphere, the quasi-stationary eddies also reinforce the zonal wind anomalies, but the baroclinic eddies are most important for the positive eddy‐zonal flow feedback. The observations support the following feedback mechanisms. 1) Above-normal baroclinic wave activity is generated in the region of enhanced westerlies. This leads to wave propagation out of the westerlies that is associated with reinforcing eddy momentum fluxes. 2) The westerly jet is a waveguide for external Rossby waves that tend to propagate into the jet and remove momentum from it. 3) The quasi-stationary waves respond to a refractive index anomaly in the high latitudes below the tropopause. During the high (low) index this anomaly is negative (positive) leading to an acceleration (deceleration) of the zonal wind in the high latitudes.
Keith Jennings - One of the best experts on this subject based on the ideXlab platform.
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spatial variation of the rain snow temperature threshold across the Northern Hemisphere
Nature Communications, 2018Co-Authors: Keith Jennings, Taylor Winchell, Ben Livneh, Noah P MolotchAbstract:Despite the importance of precipitation phase to global hydroclimate simulations, many land surface models use spatially uniform air temperature thresholds to partition rain and snow. Here we show, through the analysis of a 29-year observational dataset (n = 17.8 million), that the air temperature at which rain and snow fall in equal frequency varies significantly across the Northern Hemisphere, averaging 1.0 °C and ranging from –0.4 to 2.4 °C for 95% of the stations. Continental climates generally exhibit the warmest rain–snow thresholds and maritime the coolest. Simulations show precipitation phase methods incorporating humidity perform better than air temperature-only methods, particularly at relative humidity values below saturation and air temperatures between 0.6 and 3.4 °C. We also present the first continuous Northern Hemisphere map of rain–snow thresholds, underlining the spatial variability of precipitation phase partitioning. These results suggest precipitation phase could be better predicted using humidity and air temperature in large-scale land surface model runs. Land surface models often use a spatially uniform air temperature threshold when partitioning rain and snow. Here Jennings et al. show that the threshold varies significantly across the Northern Hemisphere and that threshold selection is a large source of uncertainty in snowfall simulations.
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spatial variation of the rain snow temperature threshold across the Northern Hemisphere
Nature Communications, 2018Co-Authors: Keith Jennings, Taylor Winchell, Ben Livneh, Noah P MolotchAbstract:Despite the importance of precipitation phase to global hydroclimate simulations, many land surface models use spatially uniform air temperature thresholds to partition rain and snow. Here we show, through the analysis of a 29-year observational dataset (n = 17.8 million), that the air temperature at which rain and snow fall in equal frequency varies significantly across the Northern Hemisphere, averaging 1.0 °C and ranging from -0.4 to 2.4 °C for 95% of the stations. Continental climates generally exhibit the warmest rain-snow thresholds and maritime the coolest. Simulations show precipitation phase methods incorporating humidity perform better than air temperature-only methods, particularly at relative humidity values below saturation and air temperatures between 0.6 and 3.4 °C. We also present the first continuous Northern Hemisphere map of rain-snow thresholds, underlining the spatial variability of precipitation phase partitioning. These results suggest precipitation phase could be better predicted using humidity and air temperature in large-scale land surface model runs.
M A Tschudi - One of the best experts on this subject based on the ideXlab platform.
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radiative forcing and albedo feedback from the Northern Hemisphere cryosphere between 1979 and 2008
Nature Geoscience, 2011Co-Authors: M G Flanner, Karen M Shell, Michael Barlage, Donald K Perovich, M A TschudiAbstract:The extent of snow cover and sea ice in the Northern Hemisphere has declined since 1979, suggesting a positive feedback of surface reflectivity on climate. A synthesis of a variety of remote sensing and field measurements suggests that this albedo feedback from the Northern Hemisphere cryosphere falls between 0.3 and 1.1 W m−2 K−1.
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radiative forcing and albedo feedback from the Northern Hemisphere cryosphere between 1979 and 2008
Nature Geoscience, 2011Co-Authors: M G Flanner, Karen M Shell, Michael Barlage, Donald K Perovich, M A TschudiAbstract:The extent of snow cover and sea ice in the Northern Hemisphere has declined since 1979, suggesting a positive feedback of surface reflectivity on climate. A synthesis of a variety of remote sensing and field measurements suggests that this albedo feedback from the Northern Hemisphere cryosphere falls between 0.3 and 1.1 W m−2 K−1. The extent of snow cover1 and sea ice2 in the Northern Hemispherehas declined since 1979, coincident with hemispheric warming and indicative of a positive feedback of surface reflectivity on climate. This albedo feedback of snow on land has been quantified from observations at seasonal timescales3,4,5,6, and century-scale feedback has been assessed using climate models7,8,9,10. However, the total impact of the cryosphere on radiative forcing and albedo feedback has yet to be determined from measurements. Here we assess the influence of the Northern Hemisphere cryosphere on Earth’s radiation budget at the top of the atmosphere—termed cryosphere radiative forcing—by synthesizing a variety of remote sensing and field measurements. We estimate mean Northern Hemisphere forcing at −4.6 to −2.2 W m−2, with a peak in May of −9.0±2.7 W m−2. We find that cyrospheric cooling declined by 0.45 W m−2 from 1979 to 2008, with nearly equal contributions from changes in land snow cover and sea ice. On the basis of these observations, we conclude that the albedo feedback from the Northern Hemisphere cryosphere falls between 0.3 and 1.1 W m−2 K−1, substantially larger than comparable estimates obtained from 18 climate models.
Ben Livneh - One of the best experts on this subject based on the ideXlab platform.
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spatial variation of the rain snow temperature threshold across the Northern Hemisphere
Nature Communications, 2018Co-Authors: Keith Jennings, Taylor Winchell, Ben Livneh, Noah P MolotchAbstract:Despite the importance of precipitation phase to global hydroclimate simulations, many land surface models use spatially uniform air temperature thresholds to partition rain and snow. Here we show, through the analysis of a 29-year observational dataset (n = 17.8 million), that the air temperature at which rain and snow fall in equal frequency varies significantly across the Northern Hemisphere, averaging 1.0 °C and ranging from –0.4 to 2.4 °C for 95% of the stations. Continental climates generally exhibit the warmest rain–snow thresholds and maritime the coolest. Simulations show precipitation phase methods incorporating humidity perform better than air temperature-only methods, particularly at relative humidity values below saturation and air temperatures between 0.6 and 3.4 °C. We also present the first continuous Northern Hemisphere map of rain–snow thresholds, underlining the spatial variability of precipitation phase partitioning. These results suggest precipitation phase could be better predicted using humidity and air temperature in large-scale land surface model runs. Land surface models often use a spatially uniform air temperature threshold when partitioning rain and snow. Here Jennings et al. show that the threshold varies significantly across the Northern Hemisphere and that threshold selection is a large source of uncertainty in snowfall simulations.
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spatial variation of the rain snow temperature threshold across the Northern Hemisphere
Nature Communications, 2018Co-Authors: Keith Jennings, Taylor Winchell, Ben Livneh, Noah P MolotchAbstract:Despite the importance of precipitation phase to global hydroclimate simulations, many land surface models use spatially uniform air temperature thresholds to partition rain and snow. Here we show, through the analysis of a 29-year observational dataset (n = 17.8 million), that the air temperature at which rain and snow fall in equal frequency varies significantly across the Northern Hemisphere, averaging 1.0 °C and ranging from -0.4 to 2.4 °C for 95% of the stations. Continental climates generally exhibit the warmest rain-snow thresholds and maritime the coolest. Simulations show precipitation phase methods incorporating humidity perform better than air temperature-only methods, particularly at relative humidity values below saturation and air temperatures between 0.6 and 3.4 °C. We also present the first continuous Northern Hemisphere map of rain-snow thresholds, underlining the spatial variability of precipitation phase partitioning. These results suggest precipitation phase could be better predicted using humidity and air temperature in large-scale land surface model runs.
