522 research outputs found
The Hearkening Eye
Hildegarde Flanner had lived in what she called the âWestern Earthâ for fifty years when this collection was published in 1979. She began writing in the nineteen twenties and states in her introduction, âIt was not necessary to go to Paris to write poetry.â Her verse is replete with the cadences and landscape of the West. Flannerâs poetry has both urban and contemporary themes, but its main emphasis lies with Earthâcicadas, valley weeds, sassafras and Judas trees. Flannerâs imagery takes the reader beyond the surface of description. Her rhymes take a closer view that nudges the reader into the realm of the metaphysical
Multidecadal Variability in Surface Albedo Feedback Across CMIP5 Models
Previous studies quantify surface albedo feedback (SAF) in climate change, but few assess its variability on decadal time scales. Using the Coupled Model Intercomparison Project Version 5 (CMIP5) multimodel ensemble data set, we calculate time evolving SAF in multiple decades from surface albedo and temperature linear regressions. Results are meaningful when temperature change exceeds 0.5 K. Decadalâscale SAF is strongly correlated with centuryâscale SAF during the 21st century. Throughout the 21st century, multimodel ensemble mean SAF increases from 0.37 to 0.42Â WÂ mâ2Â Kâ1. These results suggest that modelsâ mean decadalâscale SAFs are good estimates of their centuryâscale SAFs if there is at least 0.5Â K temperature change. Persistent SAF into the late 21st century indicates ongoing capacity for Arctic albedo decline despite there being less sea ice. If the CMIP5 multimodel ensemble results are representative of the Earth, we cannot expect decreasing Arctic sea ice extent to suppress SAF in the 21st century.Key PointsPeriods with global warming of at least 0.5Â K provide reasonable estimates of surface albedo feedbackModelsâ 21st century surface albedo feedbacks are strongly correlated with their mean decadalâscale feedbacksSixteen CMIP5 models show significant strengthening in decadalâscale surface albedo feedback throughout the 21st centuryPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/142955/1/grl56989.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/142955/2/grl56989_am.pd
Sensitivity of modeled farâIR radiation budgets in polar continents to treatments of snow surface and ice cloud radiative properties
While most general circulation models assume spectrally independent surface emissivity and nonscattering clouds in their longwave radiation treatment, spectral variation of the index of refraction of ice indicates that in the far IR, snow surface emissivity can vary considerably and ice clouds can cause nonnegligible scattering. These effects are more important for highâelevation polar continents where the dry and cold atmosphere is not opaque in the far IR. We carry out sensitivity studies to show that in a winter month over the Antarctic Plateau including snow surface spectral emissivity and ice cloud scattering in radiative transfer calculation reduces net upward farâIR flux at both top of atmosphere and surface. The magnitudes of such reductions in monthly mean allâsky farâIR flux range from 0.72 to 1.47 Wm â2 , with comparable contributions from the cloud scattering and the surface spectral emissivity. The reduction is also sensitive to sizes of both snow grains and cloud particles. Key Points Ice cloud and snow surface radiative properties vary considerably in the far IR Snow surface emissivity and cloud scattering affect far IR comparably Even for farâIR radiation alone, the impact is nonnegligiblePeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109339/1/Auxiliary_material_Aug27.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/109339/2/grl52118.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/109339/3/TableS01.pd
Diagnosing shortwave cryosphere radiative effect and its 21st century evolution in CESM
We incorporate a new diagnostic called the cryosphere radiative effect (CrRE), the instantaneous influence of surface snow and sea ice on the topâofâmodel solar energy budget, into two released versions of the Community Earth System Model (CESM1 and CCSM4). CrRE offers a more climatically relevant metric of the cryospheric state than snow and sea ice extent and is influenced by factors such as the seasonal cycle of insolation, cloud masking, and vegetation cover. We evaluate CrRE during the late 20th century and over the 21st century, specifically diagnosing the nature of CrRE contributions from terrestrial and marine sources. The radiative influence of ice sheets and glaciers is not considered, but snow on top of them is accounted for. Presentâday global CrRE in both models is â3.8 W m â2 , with a boreal component (â4.2 to â4.6 W m â2 ) that compares well with observationally derived estimates (â3.9 to â4.6 W m â2 ). Similar presentâday CrRE in the two model versions results from compensating differences in cloud masking and sea ice extent. Over the 21st century, radiative forcing in the Representative Concentration Pathway (RCP) 8.5 scenario causes reduced boreal sea ice cover, austral sea ice cover, and boreal snow cover, which all contribute roughly equally to enhancing global absorbed shortwave radiation by 1.4â1.8âWm â2 . Twentyâfirst century RCP8.5 global cryospheric albedo feedback are +0.41 and +0.45âW/m 2 /K, indicating that the two models exhibit similar temperatureânormalized CrRE change. Key Points We implement the first GCM diagnostic calculation of cryosphere radiative effect Global average CrRE from snow and sea ice is â4 W m â2 in presentâday simulations Earth absorbs 1.6 W m â2 more insolation from cryosphere loss by 2099 in RCP8.5Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106707/1/jgrd51156.pd
Climate Response to Negative Greenhouse Gas Radiative Forcing in Polar Winter
Greenhouse gas (GHG) additions to Earthâs atmosphere initially reduce global outgoing longwave radiation, thereby warming the planet. In select environments with temperature inversions, however, increased GHG concentrations can actually increase local outgoing longwave radiation. Negative top of atmosphere and effective radiative forcing (ERF) from this situation give the impression that local surface temperatures could cool in response to GHG increases. Here we consider an extreme scenario in which GHG concentrations are increased only within the warmest layers of winter nearâsurface inversions of the Arctic and Antarctic. We find, using a fully coupled Earth system model, that the underlying surface warms despite the GHG addition exerting negative ERF and cooling the troposphere in the vicinity of the GHG increase. This unique radiative forcing and thermal response is facilitated by the high stability of the polar winter atmosphere, which inhibit thermal mixing and amplify the impact of surface radiative forcing on surface temperature. These findings also suggest that strategies to exploit negative ERF via injections of shortâlived GHGs into inversion layers would likely be unsuccessful in cooling the planetary surface.Key PointsIncreased GHG concentrations in polar inversion layers cause negative top of atmosphere instantaneous and effective radiative forcingPolar and global surface temperatures warm despite this negative radiative forcingSurface warming and tropospheric cooling result from high stability and increased surface downwelling longwave fluxPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/142965/1/grl56994_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/142965/2/grl56994.pd
Sensitivity studies on the impacts of Tibetan Plateau snowpack pollution on the Asian hydrological cycle and monsoon climate
The Tibetan Plateau (TP) has long been identified to be critical in
regulating the Asian monsoon climate and hydrological cycle. In this
modeling study a series of numerical experiments with a global climate model
are designed to simulate radiative effect of black carbon (BC) and dust in
snow, and to assess the relative impacts of anthropogenic CO<sub>2</sub> and
carbonaceous particles in the atmosphere and snow on the snowpack over the
TP and subsequent impacts on the Asian monsoon climate and hydrological
cycle. Simulations results show a large BC content in snow over the TP,
especially the southern slope. Because of the high aerosol content in snow
and large incident solar radiation in the low latitude and high elevation,
the TP exhibits the largest surface radiative flux changes induced by
aerosols (e.g. BC, Dust) in snow compared to any other snow-covered regions
in the world.
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Simulation results show that the aerosol-induced snow albedo perturbations
generate surface radiative flux changes of 5â25 W m<sup>â2</sup> during spring,
with a maximum in April or May. BC-in-snow increases the surface air
temperature by around 1.0 °C averaged over the TP and reduces spring
snowpack over the TP more than pre-industrial to present CO<sub>2</sub> increase
and carbonaceous particles in the atmosphere. As a result, runoff increases
during late winter and early spring but decreases during late spring and
early summer (i.e. a trend toward earlier melt dates). The snowmelt
efficacy, defined as the snowpack reduction per unit degree of warming
induced by the forcing agent, is 1â4 times larger for BC-in-snow than
CO<sub>2</sub> increase during AprilâJuly, indicating that BC-in-snow more
efficiently accelerates snowmelt because the increased net solar radiation
induced by reduced albedo melts the snow more efficiently than snow melt due
to warming in the air.
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The TP also influences the South (SAM) and East (EAM) Asian monsoon through
its dynamical and thermal forcing. Simulation results show that during
boreal spring aerosols are transported by southwesterly, causing some
particles to reach higher altitude and deposit to the snowpack over the TP.
While BC and Organic Matter (OM) in the atmosphere directly absorb sunlight
and warm the air, the darkened snow surface polluted by BC absorbs more
solar radiation and increases the skin temperature, which warms the air
above through sensible heat flux. Both effects enhance the upward motion of
air and spur deep convection along the TP during the pre-monsoon season,
resulting in earlier onset of the SAM and increase of moisture, cloudiness
and convective precipitation over northern India. BC-in-snow has a more
significant impact on the EAM in July than CO<sub>2</sub> increase and
carbonaceous particles in the atmosphere. Contributed by the significant
increase of both sensible heat flux associated with the warm skin
temperature and latent heat flux associated with increased soil moisture
with long memory, the role of the TP as a heat pump is elevated from spring
through summer as the land-sea thermal contrast increases to strengthen the
EAM. As a result, both southern China and northern China become wetter, but
central China (i.e. Yangtze River Basin) becomes drier â a near-zonal
anomaly pattern that is consistent with the dominant mode of precipitation
variability in East Asia.
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The snow impurity effects reported in this study likely represent some upper
limits as snowpack is remarkably overestimated over the TP due to excessive
precipitation. Improving the simulation of precipitation and snowpack will
be important for improved estimates of the effects of snowpack pollution in
future work
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