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Search Results: 1 - 10 of 149808 matches for " H. Sobel "
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Delayed Seasonal Cycle and African Monsoon in a Warmer Climate
Michela Biasutti,Adam H. Sobel
Physics , 2009,
Abstract: Increasing greenhouse gases will change many aspects of the Earth's climate, from its annual mean to the frequency of extremes such as heat waves and droughts. Here we report that the current generation of climate models predicts a delay in the seasonal cycle of global rainfall and ocean temperature in response to increasing greenhouse gases, with important implications for the regional monsoons. In particular, the rainy season of the semi-arid African Sahel is projected to start later and become shorter: an undesirable change for local rainfed agriculture and pastoralism. Previous work has highlighted the uncertainty in this region's response to anthropogenic global warming: summer rainfall is predicted either to decrease or increase by up to 30% depending which model is used. The robust agreement across models on the seasonal distribution of rainfall changes signifies that the onset date and length of the rainy season should be more predictable than annual mean anomalies.
Simple Adaptive Delta Operator Aircraft Flight Control for Accommodation of Loss of Control Effectiveness  [PDF]
Alfredo Cano, Kenneth Sobel
Engineering (ENG) , 2016, DOI: 10.4236/eng.2016.84016
Abstract: A new proof for stability of delta operator simple adaptive control is presented in terms of a set of Linear Matrix Inequalities (LMIs). The paper shows how to design a feedforward gain to satisfy the LMIs over a polytope of loss of control effectiveness failures. The MATLAB Robust Control Toolbox is used to find the feedforward gain with the smallest norm that satisfies the LMIs. Examples are presented of the F/A-18 aircraft and the Innovative Control Effectors (ICE) tailless aircraft that show the design of a feedforward gain for a loss of control effectiveness in any one control effector. The designs use a fixed eigenstructure assignment controller for an inner loop augmented with the simple adaptive controller. Simulations of both aircraft include simultaneous loss of control effectiveness failure and lateral wind gust. Simulation results for the F/A-18 aircraft show that the adaptive controller achieves almost perfect tracking whereas the nonadaptive controller cannot achieve a coordinated turn when an aileron failure occurs. The ICE tailless aircraft uses sideslip, washed-out stability axis yaw rate, and stability axis roll rate feedback for both the inner loop eigenstructure assignment controller and the simple adaptive controller. However, the adaptive controller also uses bank angle feedback. Simulation results for the ICE tailless aircraft show that the adaptive controller achieves almost perfect tracking whereas the nonadaptive controller diverges when an all moving tip failure occurs.
Intraseasonal Variability in an Aquaplanet General Circulation Model
Eric D Maloney,Adam H Sobel,Walter M Hannah
Journal of Advances in Modeling Earth Systems , 2010, DOI: 10.3894/james.2010.2.5
Abstract: An aquaplanet atmospheric general circulation model simulation with a robust intraseasonal oscillation is analyzed. The SST boundary condition resembles the observed December-April average with continents omitted, although with the meridional SST gradient reduced to be one-quarter of that observed poleward of 10 latitude. Slow, regular eastward propagation at 5 m s21 in winds and precipitation with amplitude greater than that in the observed MJO is clearly identified in unfiltered fields. Local precipitation rate is a strongly non-linear and increasing function of column precipitable water, as in observations. The model intraseasonal oscillation resembles a moisture mode that is destabilized by wind-evaporation feedback, and that propagates eastward through advection of anomalous humidity by the sum of perturbation winds and mean westerly flow. A series of sensitivity experiments are conducted to test hypothesized mechanisms. A mechanism denial experiment in which intraseasonal latent heat flux variability is removed largely eliminates intraseasonal wind and precipitation variability. Reducing the lower-troposphere westerly flow in the warm pool by reducing the zonal SST gradient slows eastward propagation, supporting the importance of horizontal advection by the low-level wind to eastward propagation. A zonally symmetric SST basic state produces weak and unrealistic intraseasonal variability between 30 and 90 day timescales, indicating the importance of mean low-level westerly winds and hence a realistic phase relationship between precipitation and surface flux anomalies for producing realistic tropical intraseasonal variability.
A Mechanism Denial Study on the Madden-Julian Oscillation
Daehyun Kim,Adam H. Sobel,In-Sik Kang
Journal of Advances in Modeling Earth Systems , 2011,
Abstract: A series of Madden-Julian oscillation (MJO) mechanism-denial experiments is performed using an atmospheric general circulation model (AGCM). Daily climatological seasonal cycles of i) surface latent heat flux, ii) net radiative heating rate, and iii) surface wind stress are obtained from a control simulation and prescribed in place of the normal interactive computations of these fields in order to turn off the i) wind-induced surface heat exchange (WISHE), ii) cloud-radiation interaction (CRI), and iii) frictional wave-CISK (FWC) mechanisms, respectively. Dual and triple mechanism denial experiments are also conducted by switching off multiple mechanisms together. The influence of each mechanism is assessed by comparing experiments with that mechanism turned off to those in which it is not. CRI and WISHE are both found to be important to the simulated MJO amplitude and propagation speed, while FWC has weaker and less systematic effects. The MJO is weakened when CRI is turned off, but strengthened when WISHE is turned off, indicating that CRI amplifies the MJO in the control simulation while WISHE weakens it. The negative influence of WISHE is shown to result from simulated phase relationships between surface winds, surface fluxes and convection which differ significantly from those found in observations, and thus is not interpreted as evidence against a positive role for WISHE in the development and maintenance of the observed MJO. The positive influence of CRI in the model is consistent with a strong simulated relationship between daily grid-point column-integrated radiative and convective heating; the mean ratio of the latter to the former exceeds 0.2 for rain rates less than 14 mm d-1. CRI is also shown to suppress an excessive excitation of the convectively coupled Kelvin wave so that the amplitude and frequency of the MJO is maintained.
Multiple Equilibria in a Single-Column Model of the Tropical Atmosphere
Adam H. Sobel,Gilles Bellon,Julio T. Bacmeister
Physics , 2007, DOI: 10.1029/2007GL031320
Abstract: A single-column model run under the weak temperature gradient approximation, a parameterization of large-scale dynamics appropriate for the tropical atmosphere, is shown to have multiple stable equilibria. Under conditions permitting persistent deep convection, the model has a statistically steady state in which such convection occurs, as well as an extremely dry state in which convection does not occur. Which state is reached depends on the initial moisture profile.
Surface Fluxes and Tropical Intraseasonal Variability: a Reassessment
Adam H Sobel,Eric D Maloney,Gilles Bellon,Dargan M. Frierson
Journal of Advances in Modeling Earth Systems , 2010, DOI: 10.3894/james.2010.2.2
Abstract: The authors argue that interactive feedbacks involving surface moist enthalpy fluxes, both turbulent and radiative, are important to the dynamics of tropical intraseasonal variability. Evidence in favor of this hypothesis includes the observed spatial distribution of intraseasonal variance in precipitation and outgoing longwave radiation, the observed relationship between intraseasonal latent heat flux and precipitation anomalies in regions where intraseasonal variability is strong, and sensitivity experiments performed with a small number of general circulation and idealized models. The authors argue that it would be useful to assess the importance of surface fluxes to intraseasonal variability in a larger number of comprehensive numerical models.
Surface fluxes and tropical intraseasonal variability: a reassessment
Adam H. Sobel,Eric D. Maloney,Gilles Bellon,Dargan M. Frierson
Physics , 2008,
Abstract: The authors argue for the hypothesis that interactive feedbacks involving surface enthalpy fluxes are important to the dynamics of tropical intraseasonal variability. These include cloud-radiative feedbacks as well as surface turbulent flux feedbacks; the former effectively act to transport enthalpy from the ocean to the atmosphere, as do the latter. Evidence in favor of this hypothesis includes the observed spatial distribution of intraseasonal variance in precipitation and outgoing longwave radiation, the observed relationship between intraseasonal latent heat flux and precipitation anomalies in regions where intraseasonal variability is strong, and sensitivity experiments performed with a small number of general circulation and idealized models. The authors argue that it would be useful to assess the importance of surface fluxes to intraseasonal variability in a larger number of comprehensive numerical models. Such an assessment could provide insight into the relevance of interactive surface fluxes to real intraseasonal variability, perhaps making it possible to rule out either theoretical explanations in which surface fluxes are crucial, or those in which they are not.
Planning the Future of U.S. Particle Physics (Snowmass 2013): Chapter 7: Underground Laboratory Capabilities
M. G. Gilchriese,P. Cushman,K. Heeger,J. Klein,K. Scholberg,H. Sobel,M. Witherell
Physics , 2014,
Abstract: These reports present the results of the 2013 Community Summer Study of the APS Division of Particles and Fields ("Snowmass 2013") on the future program of particle physics in the U.S. Chapter 7, on Underground Laboratory Capabilities, discusses the prospects and requirements for large underground experiments such as those for neutrino physics, proton decay, and dark matter.
氧化还原工艺的现状与发展
Sobel,K,E,Foo,K,A,Whellock,J,G,Canterford,J,H,王翠兰译
黄金科学技术 , 2002,
Abstract: 利用硝酸的化学特性所进行的氧化还原技术,已经证明在处理难熔金属和精砂中,具有很大的优势。阐述了该工艺的理论基础、发展状况和未来前景。
Vaginal Mucormycosis: A Case Report
Jack D. Sobel
Infectious Diseases in Obstetrics and Gynecology , 2001, DOI: 10.1155/s1064744901000205
Abstract: Although Zygomycetes cause life-threatening, opportunistic infections in immunocompromised hosts, the first case of vaginitis caused by Mucor species in a healthy woman is reported. Mucor vaginitis, which caused mild symptoms only, was refractory to conventional azole therapy and resistant to flucytosine. Cure was achieved with topical amphotericin B.
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