2,231 research outputs found
On the relationships among cloud cover, mixed-phase partitioning, and planetary albedo in GCMs
In this study, it is shown that CMIP5 global climate models (GCMs) that convert supercooled water to ice at relatively warm temperatures tend to have a greater mean-state cloud fraction and more negative cloud feedback in the middle and high latitude Southern Hemisphere. We investigate possible reasons for these relationships by analyzing the mixed-phase parameterizations in 26 GCMs. The atmospheric temperature where ice and liquid are equally prevalent (T5050) is used to characterize the mixed-phase parameterization in each GCM. Liquid clouds have a higher albedo than ice clouds, so, all else being equal, models with more supercooled liquid water would also have a higher planetary albedo. The lower cloud fraction in these models compensates the higher cloud reflectivity and results in clouds that reflect shortwave radiation (SW) in reasonable agreement with observations, but gives clouds that are too bright and too few. The temperature at which supercooled liquid can remain unfrozen is strongly anti-correlated with cloud fraction in the climate mean state across the model ensemble, but we know of no robust physical mechanism to explain this behavior, especially because this anti-correlation extends through the subtropics. A set of perturbed physics simulations with the Community Atmospheric Model Version 4 (CAM4) shows that, if its temperature-dependent phase partitioning is varied and the critical relative humidity for cloud formation in each model run is also tuned to bring reflected SW into agreement with observations, then cloud fraction increases and liquid water path (LWP) decreases with T5050, as in the CMIP5 ensemble
The global aerosol-cloud first indirect effect estimated using MODIS, MERRA, and AeroCom
Aerosol-cloud interactions (ACI) represent a significant source of forcing uncertainty in global climate models (GCMs). Estimates of radiative forcing due to ACI in Fifth Assessment Report range from â0.5 to â2.5âWâmâ2. A portion of this uncertainty is related to the first indirect, or Twomey, effect whereby aerosols act as nuclei for cloud droplets to condense upon. At constant liquid water content this increases the number of cloud droplets (Nd) and thus increases the cloud albedo. In this study we use remote-sensing estimates of Nd within stratocumulus regions in combination with state-of-the-art aerosol reanalysis from Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA2) to diagnose how aerosols affect Nd. As in previous studies, Nd is related to sulfate mass through a power law relationship. The slope of the log-log relationship between Nd and SO4 in maritime stratocumulus is found to be 0.31, which is similar to the range of 0.2â0.8 from previous in situ studies and remote-sensing studies in the pristine Southern Ocean. Using preindustrial emissions models, the change in Nd between preindustrial and present day is estimated. Nd is inferred to have more than tripled in some regions. Cloud properties from Moderate Resolution Imaging Spectroradiometer (MODIS) are used to estimate the radiative forcing due to this change in Nd. The Twomey effect operating in isolation is estimated to create a radiative forcing of â0.97â±â0.23âWâmâ2 relative to the preindustrial era
The Change in Low Cloud Cover in a Warmed Climate Inferred from AIRS, MODIS, and ERA-Interim
Decreases in subtropical low cloud cover (LCC) occur in climate model simulations of global warming. In this study 8-day-averaged observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Atmospheric Infrared Sounder (AIRS) spanning 2002â14 are combined with European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis to compute the dependence of the observed variability of LCC on various predictor variables. Large-scale thermodynamic and dynamic predictors of LCC are selected based on insight from large-eddy simulations (LESs) and observational analysis. It is found that increased estimated inversion strength (EIS) is associated with increased LCC. Drying of the free troposphere is associated with decreased LCC. Decreased LCC accompanies subsidence in regions of relatively low EIS; the opposite is found in regions of high EIS. Finally, it is found that increasing sea surface temperature (SST) leads to a decrease in LCC. These results are in keeping with previous studies of monthly and annual data. Based upon the observed response of LCC to natural variability of the control parameters, the change in LCC is estimated for an idealized warming scenario where SST increases by 1 K and EIS increases by 0.2 K. For this change in EIS and SST the LCC is inferred to decrease by 0.5%â2.7% when the regression models are trained on data observed between 40°S and 40°N and by 1.1%â1.4% when trained on data from trade cumulusâdominated regions. When the data used to train the regression model are restricted to stratocumulus-dominated regions the change in LCC is highly uncertain and varies between â1.6% and +1.4%, depending on the stratocumulus-dominated region used to train the regression model
Cloud feedback mechanisms and their representation in global climate models
Cloud feedbackâthe change in topâofâatmosphere radiative flux resulting from the cloud response to warmingâconstitutes by far the largest source of uncertainty in the climate response to CO2 forcing simulated by global climate models (GCMs). We review the main mechanisms for cloud feedbacks, and discuss their representation in climate models and the sources of intermodel spread. Globalâmean cloud feedback in GCMs results from three main effects: (1) rising freeâtropospheric clouds (a positive longwave effect); (2) decreasing tropical low cloud amount (a positive shortwave [SW] effect); (3) increasing highâlatitude low cloud optical depth (a negative SW effect). These cloud responses simulated by GCMs are qualitatively supported by theory, highâresolution modeling, and observations. Rising high clouds are consistent with the fixed anvil temperature (FAT) hypothesis, whereby enhanced upperâtropospheric radiative cooling causes anvil cloud tops to remain at a nearly fixed temperature as the atmosphere warms. Tropical low cloud amount decreases are driven by a delicate balance between the effects of vertical turbulent fluxes, radiative cooling, largeâscale subsidence, and lowerâtropospheric stability on the boundaryâlayer moisture budget. Highâlatitude low cloud optical depth increases are dominated by phase changes in mixedâphase clouds. The causes of intermodel spread in cloud feedback are discussed, focusing particularly on the role of unresolved parameterized processes such as cloud microphysics, turbulence, and convection
Mixed-phase cloud physics and Southern Ocean cloud feedback in climate models
Increasing optical depth poleward of 45° is a robust response to warming in global climate models. Much of this cloud optical depth increase has been hypothesized to be due to transitions from iceâdominated to liquidâdominated mixedâphase cloud. In this study, the importance of liquidâice partitioning for the optical depth feedback is quantified for 19 Coupled Model Intercomparison Project Phase 5 models. All models show a monotonic partitioning of ice and liquid as a function of temperature, but the temperature at which ice and liquid are equally mixed (the glaciation temperature) varies by as much as 40âK across models. Models that have a higher glaciation temperature are found to have a smaller climatological liquid water path (LWP) and condensed water path and experience a larger increase in LWP as the climate warms. The iceâliquid partitioning curve of each model may be used to calculate the response of LWP to warming. It is found that the repartitioning between ice and liquid in a warming climate contributes at least 20% to 80% of the increase in LWP as the climate warms, depending on model. Intermodel differences in the climatological partitioning between ice and liquid are estimated to contribute at least 20% to the intermodel spread in the highâlatitude LWP response in the mixedâphase region poleward of 45°S. It is hypothesized that a more thorough evaluation and constraint of global climate model mixedâphase cloud parameterizations and validation of the total condensate and iceâliquid apportionment against observations will yield a substantial reduction in model uncertainty in the highâlatitude cloud response to warming
Silicon-Organic Hybrid (SOH) and Plasmonic-Organic Hybrid (POH) integration
Silicon photonics offers tremendous potential for inexpensive high-yield photonic-electronic integration. Besides conventional dielectric waveguides, plasmonic structures can also be efficiently realized on the silicon photonic platform, reducing device footprint by more than an order of magnitude. However, nei-ther silicon nor metals exhibit appreciable second-order optical nonlinearities, thereby making efficient electro-optic modulators challenging to realize. These deficiencies can be overcome by the concepts of silicon-organic hybrid (SOH) and plasmonic-organic hybrid integration, which combine SOI waveguides and plasmonic nanostructures with organic electro-optic cladding materials
The relationship between the ITCZ and the Southern Hemispheric eddy-driven jet
[1] We study the effect of a thermal forcing confined to the midlatitudes of one hemisphere on the eddyâdriven jet in the opposite hemisphere. We demonstrate the existence of an âinterhemispheric teleconnection,â whereby warming (cooling) the Northern Hemisphere causes both the intertropical convergence zone (ITCZ) and the Southern Hemispheric midlatitude jet to shift northward (southward). The interhemispheric teleconnection is effected by a change in the asymmetry of the Hadley cells: as the ITCZ shifts away from the Equator, the crossâequatorial Hadley cell intensifies, fluxing more momentum toward the subtropics and sustaining a stronger subtropical jet. Changes in subtropical jet strength, in turn, alter the propagation of extratropical waves into the tropics, affecting eddy momentum fluxes and the eddyâdriven westerlies. The relevance of this mechanism is demonstrated in the context of future climate change simulations, where shifts of the ITCZ are significantly related to shifts of the Southern Hemispheric eddyâdriven jet in austral winter. The possible relevance of the proposed mechanism to paleoclimates is discussed, particularly with regard to theories of ice age terminations
Return of the Great Spaghetti Monster : Learnings from a Twelve-Year Adventure in Web Software Development
The widespread adoption of the World Wide Web has fundamentally changed the landscape of software development. Only ten years ago, very few developers would write software for the Web, let alone consider using JavaScript or other web technologies for writing any serious software applications. In this paper, we reflect upon a twelve-year adventure in web development that began with the development of the Lively Kernel system at Sun Microsystems Labs in 2006. Back then, we also published some papers that identified important challenges in web-based software development based on established software engineering principles. We will revisit our earlier findings and compare the state of the art in web development today to our earlier learnings, followed by some reflections and suggestions for the road forward.Peer reviewe
Client-Side Cornucopia : Comparing the Built-In Application Architecture Models in the Web Browser
Peer reviewe
An extragalactic supernebula confined by gravity
Little is known about the origins of the giant star clusters known as
globular clusters. How can hundreds of thousands of stars form simultaneously
in a volume only a few light years across the distance of the sun to its
nearest neighbor? Radiation pressure and winds from luminous young stars should
disperse the star-forming gas and disrupt the formation of the cluster.
Globular clusters in our Galaxy cannot provide answers; they are billions of
years old. Here we report the measurement of infrared hydrogen recombination
lines from a young, forming super star cluster in the dwarf galaxy, NGC 5253.
The lines arise in gas heated by a cluster of an estimated million stars, so
young that it is still enshrouded in gas and dust, hidden from optical view. We
verify that the cluster contains 4000-6000 massive, hot "O" stars. Our
discovery that the gases within the cluster are bound by gravity may explain
why these windy and luminous O stars have not yet blown away the gases to allow
the cluster to emerge from its birth cocoon. Young clusters in "starbursting"
galaxies in the local and distant universe may be similarly gravitationally
confined and cloaked from view.Comment: Letter to Natur
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