Quantifying the Cloud Particle‐Size Feedback in an Earth System Model

Physical process‐based two‐moment cloud microphysical parameterizations, in which effective cloud particle size evolves prognostically with climate change, have recently been incorporated into global climate models. The impacts of cloud particle‐size change on the cloud feedback, however, have never been explicitly quantified. Here we develop a partial radiative perturbation‐based method to estimate the cloud feedback associated with particle‐size changes in the Community Earth System Model. We find an increase of cloud particle size in the upper troposphere in response to an instantaneous doubling of atmospheric CO2. The associated net, shortwave, and longwave cloud feedbacks are estimated to be 0.18, 0.33, and −0.15 Wm−2 K−1, respectively. The cloud particle‐size feedback is dominated by its shortwave component with a maximum greater than 1.0 Wm−2 K−1 in the tropics and the Southern Ocean. We suggest that the cloud particle‐size feedback is an underappreciated contributor to the spread of cloud feedback and climate sensitivity among current models.Plain Language SummaryEffects of clouds on Earth’s radiation budget vary with their spatial and temporal distribution and their physical properties, including water content and its partitioning between liquid and ice, and cloud particle size. Changes in cloud distribution and physical properties can amplify or damp anthropogenic global warming and is the largest source of uncertainty in predictions of future climate. The simulation of cloud physical properties in climate models is limited due to a lack of understanding from theory and observations about what controls these properties. Recent progress has been made in some models to predict cloud particle sizes based on physical processes. In this study, we find an increase of cloud particle size in response to anthropogenic warming and estimate the resulting cloud radiative effects. The larger particles increase scattering of solar radiation in the downward direction leading to an amplification of surface warming. We suggest cloud particle‐size changes play a role in the large spread of warming in model predictions of future climate.Key PointsCloud particle size increases with warming in an Earth system modelThe associated cloud particle‐size feedback is estimated to be 0.18, 0.33, and −0.15 Wm−2 K−1 for net, shortwave, and longwave componentsCloud particle‐size feedback is an underappreciated contributor to the spread of climate sensitivity in current modelsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151978/1/grl59600.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151978/2/grl59600_am.pd

Biogeochemical effects of volcanic degassing on the oxygen-state of the oceans during the Cenomanian/Turonian Anoxic Event 2

ABSTRACT FINAL ID: PP11A-1769 Cretaceous anoxic events may have been triggered by massive volcanic CO2 degassing as large igneous provinces (LIPs) were emplaced on the seafloor. Here, we present a comprehensive modeling study to decipher the marine biogeochemical consequences of enhanced volcanic CO2 emissions. A biogeochemical box model has been developed for transient model runs with time-dependent volcanic CO2 forcing. The box model considers continental weathering processes, marine export production, degradation processes in the water column, the rain of particles to the seafloor, benthic fluxes of dissolved species across the seabed, and burial of particulates in marine sediments. The ocean is represented by twenty-seven boxes. To estimate horizontal and vertical fluxes between boxes, a coupled ocean–atmosphere general circulation model (AOGCM) is run to derive the circulation patterns of the global ocean under Late Cretaceous boundary conditions. The AOGCM modeling predicts a strong thermohaline circulation and intense ventilation in the Late Cretaceous oceans under high pCO2 values. With an appropriate choice of parameter values such as the continental input of phosphorus, the model produces ocean anoxia at low to mid latitudes and changes in marine δ13C that are consistent with geological data such as the well established δ13C curve. The spread of anoxia is supported by an increase in riverine phosphorus fluxes under high pCO2 and a decrease in phosphorus burial efficiency in marine sediments under low oxygen conditions in ambient bottom waters. Here, we suggest that an additional mechanism might contribute to anoxia, an increase in the C:P ratio of marine plankton which is induced by high pCO2 values. According to our AOGCM model results, an intensively ventilated Cretaceous ocean turns anoxic only if the C:P ratio of marine organic particles exported into the deep ocean is allowed to increase under high pCO2 conditions. Being aware of the uncertainties such as diagenesis, this modeling study implies that potential changes in Redfield ratios might be a strong feedback mechanism to attain ocean anoxia via enhanced CO2 emissions. The formation of C-enriched marine organic matter may also explain the frequent occurrence of global anoxia during other geological periods characterized by high pCO2 values

Palaeoclimate - A balmy Arctic

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62910/1/432814a.pd

Vegetation‐precipitation controls on Central Andean topography

Climatic controls on fluvial landscapes are commonly characterized in terms of mean annual precipitation. However, physical erosion processes are driven by extreme events and are therefore more directly related to the intensity, duration, and frequency of individual rainfall events. Climate also influences erosional processes indirectly by controlling vegetation. In this study, we explore how interdependent climate and vegetation properties affect landscape morphology at the scale of the Andean orogen. The mean intensity, duration, and frequency of precipitation events are derived from the TRMM 3B42v7 product. Relationships between mean hillslope gradients and precipitation event metrics, mean annual precipitation, vegetation, and bedrock lithology in the central Andes are examined by correlation analyses and multiple linear regression. Our results indicate that mean hillslope gradient correlates most strongly with percent vegetation cover ( r  = 0.56). Where vegetation cover is less than 95%, mean hillslope gradients increase with mean annual precipitation ( r  = 0.60) and vegetation cover ( r  = 0.69). Where vegetation cover is dense (>95%), mean hillslope gradients increase with increasing elevation ( r  = 0.74), decreasing inter‐storm duration ( r  = −0.69), and decreasing precipitation intensity by ~0.5°/(mm d −1 ) ( r  = −0.56). Thus, we conclude that at the orogen scale, climate influences on topography are mediated by vegetation, which itself is dependent on mean annual precipitation ( r  = 0.77). Observations from the central Andes are consistent with landscape evolution models in which hillslope gradients are a balance between rock uplift, climatic erosional efficiency and erosional resistance of the landscape determined by bedrock lithology and vegetation. Key Points Hillslope gradients in central Andes increase with increasing vegetation cover Precipitation intensity affects topography most in densely vegetated areas Mean annual precipitation affects erosional efficiency through vegetation coverPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108109/1/jgrf20258.pd

Quantifying the role of paleoclimate and Andean Plateau uplift on river incision

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/99035/1/jgrf20055-sup-0002-2012JF002533fs02.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99035/2/jgrf20055.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/99035/3/jgrf20055-sup-0001-2012JF002533fs01.pd

Stable Water Isotopes Reveal Effects of Intermediate Disturbance and Canopy Structure on Forest Water Cycling

Forests play an integral role in the terrestrial water cycle and link exchanges of water between the land surface and the atmosphere. To examine the effects of an intermediate disturbance on forest water cycling, we compared vertical profiles of stable water vapor isotopes in two closely located forest sites in northern lower Michigan. At one site, all canopy‐dominant early successional species were stem girdled to induce mortality and accelerate senescence. At both sites, we measured the isotopic composition of atmospheric water vapor at six heights during three seasons (spring, summer, and fall) and paired vertical isotope profiles with local meteorology and sap flux. Disturbance had a substantial impact on local water cycling. The undisturbed canopy was moister, retained more transpired vapor, and at times was poorly mixed with the free atmosphere above the canopy. Differences between the disturbed and undisturbed sites were most pronounced in the summer when transpiration was high. Differences in forest structure at the two sites also led to more isotopically stratified vapor within the undisturbed canopy. Our findings suggest that intermediate disturbance may increase mixing between the surface layer and above‐canopy atmosphere and alter ecosystem‐atmosphere gas exchange.Plain Language SummaryForests play an important role in the climate system and link water fluxes between the land surface and the atmosphere. Here we compare water vapor isotopes in two adjacent forest sites in the northern lower peninsula of Michigan to understand the effects of intermediate disturbance and canopy structure on forest water cycling. One site is dominated by aspen and birch and has a thick, closed canopy. All of the aspen and birch were killed at the second site. As a result, the disturbed site has a more open‐canopy structure. From our comparison, we found that both the species of tree and the spacing around trees are important controls on forest water cycling. With more space between trees, air mixes more freely into the canopy, which dries the forest air. Alternatively, air may be poorly mixed within and above thick, closed canopies.Key PointsIntermediate disturbance can change the contribution of entrained, evaporated, and transpired water vapor to forest canopiesCanopy gaps increase hydrologic mixing between the surface layer and the free atmosphereThe assumption of a well‐mixed canopy atmosphere may be violated in the case of thick, homogeneous forest canopiesPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/152563/1/jgrg21482_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152563/2/jgrg21482.pd

An isotopic approach to partition evapotranspiration in a mixed deciduous forest

Transpiration (T) is perhaps the largest fluxes of water from the land surface to the atmosphere and is susceptible to changes in climate, land use and vegetation structure. However, predictions of future transpiration fluxes vary widely and are poorly constrained. Stable water isotopes can help expand our understanding of land–atmosphere water fluxes but are limited by a lack of observations and a poor understanding of how the isotopic composition of transpired vapour (δT) varies. Here, we present isotopic data of water vapour, terrestrial water and plant water from a deciduous forest to understand how vegetation affects water budgets and land–atmosphere water fluxes. We measured subdiurnal variations of δ18OT from three tree species and used water isotopes to partition T from evapotranspiration (ET) to quantify the role of vegetation in the local water cycle. We find that δ18OT deviated from isotopic steady‐state during the day but find no species‐specific patterns. The ratio of T to ET varied from 53% to 61% and was generally invariant during the day, indicating that diurnal evaporation and transpiration fluxes respond to similar atmospheric and micrometeorological conditions at this site. Finally, we compared the isotope‐inferred ratio of T to ET with results from another ET partitioning approach that uses eddy covariance and sap flux data. We find broad midday agreement between these two partitioning techniques, in particular, the absence of a diurnal cycle, which should encourage future ecohydrological isotope studies. Isotope‐inferred estimates of transpiration can inform land surface models and improve our understanding of land–atmosphere water fluxes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/162787/2/eco2229.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162787/1/eco2229_am.pd

Quantifying uncertainty in satellite-retrieved land surface temperature from cloud detection errors

Clouds remain one of the largest sources of uncertainty in remote sensing of surface temperature in the infrared, but this uncertainty has not generally been quantified. We present a new approach to do so, applied here to the Advanced Along-Track Scanning Radiometer (AATSR). We use an ensemble of cloud masks based on independent methodologies to investigate the magnitude of cloud detection uncertainties in area-average Land Surface Temperature (LST) retrieval. We find that at a grid resolution of 625 km^2 (commensurate with 0.25 degrees grid size at the tropics), cloud detection uncertainties are positively correlated with cloud-cover fraction in the cell, and are larger during the day than at night. Daytime cloud detection uncertainties range between 2.5 K for clear-sky fractions of 10-20 % and 1.03 K for clear-sky fractions of 90-100 %. Corresponding nighttime uncertainties are 1.6 K and 0.38 K respectively. Cloud detection uncertainty shows a weaker positive correlation with the number of biomes present within a grid cell, used as a measure of heterogeneity in the background against which the cloud detection must operate (eg. surface temperature, emissivity and reflectance). Uncertainty due to cloud detection errors is strongly dependent on the dominant land cover classification. We find cloud detection uncertainties of magnitude 1.95 K over permanent snow and ice, 1.2 K over open forest, 0.9-1 K over bare soils and 0.09 K over mosaic cropland, for a standardised clear-sky fraction of 74.2 %. As the uncertainties arising from cloud detection errors are of a significant magnitude for many surface types, and spatially heterogeneous where land classification varies rapidly, LST data producers are encouraged to quantify cloud-related uncertainties in gridded products

Recent contrasting winter temperature changes over North America linked to enhanced positive Pacific‐North American pattern

Recently enhanced contrasts in winter (December‐January‐February) mean temperatures and extremes (cold southeast and warm northwest) across North America have triggered intensive discussion both within and outside of the scientific community, but the mechanisms responsible for these contrasts remain unresolved. Here we use a combination of observations and reanalysis data sets to show that the strengthened contrasts in winter mean temperatures and extremes across North America are closely related to an enhancement of the positive Pacific‐North American (PNA) pattern during the second half of the 20th century. Recent intensification of positive PNA events is associated with amplified planetary waves over North America, driving cold‐air outbreaks into the southeast and warm tropical/subtropical air into the northwest. This not only results in a strengthened winter mean temperature contrast but increases the occurrence of the opposite‐signed extremes in these two regions.Key PointsThe enhanced contrasts in winter mean temperatures and extremes in North America are observedRecent enhancement of positive PNA is a main cause of the contrasting winter temperature changesThe study provides a framework for detection and attribution of climate change in North AmericaPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/115952/1/grl53404_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/115952/2/grl53404.pd

Orbital and CO 2 forcing of late Paleozoic continental ice sheets

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94883/1/grl23653.pd