The Experts below are selected from a list of 291 Experts worldwide ranked by ideXlab platform

D. L. Hartmann - One of the best experts on this subject based on the ideXlab platform.

  • Tropical cirrus and water vapor: an Effective Earth infrared iris feedback?
    Atmospheric Chemistry and Physics, 2002
    Co-Authors: Mark D. Baker, D. L. Hartmann
    Abstract:

    We revisit a model of feedback processes proposed by Lindzen et al. (2001), in which an assumed 22% reduction in the area of tropical high clouds per degree increase in sea surface temperature produces negative feedbacks associated with upper tropospheric water vapor and cloud radiative effects. We argue that the water vapor feedback is overestimated in Lindzen et al. (2001) by at least 60%, and that the high cloud feedback is small. Although not mentioned by Lindzen et al. (2001), tropical low clouds make a significant contribution to their negative feedback, which is also overestimated. Using more realistic parameters in the model of Lindzen et al. (2001), we obtain a feedback factor in the range of -0.15 to -0.51, compared to their larger negative feedback factor of -0.45 to -1.03. It is noted that our feedback factor could still be overestimated due to the assumption of constant low cloud cover in the simple radiative-convective model.

  • Tropical cirrus and water vapor: an Effective Earth infrared iris feedback?
    Atmospheric Chemistry and Physics Discussions, 2001
    Co-Authors: Mark D. Baker, D. L. Hartmann
    Abstract:

    We revisit a model of feedback processes proposed by Lindzen et al. (2001), in which an assumed 22% reduction in the area of tropical high clouds per degree of sea surface temperature increase produces negative feedbacks associated with upper tropospheric water vapor and cloud radiative effects. We argue that the water vapor feedback is overestimated in Lindzen et al. (2001) by at least 60%, and that the high cloud feedback should be small. Although not mentioned by Lindzen et al, tropical low clouds make a significant contribution to their negative feedback, which is also overestimated. Using more realistic parameters in the model of Lindzen et al., we obtain a feedback factor in the range of ?0.15 to ?0.51, compared to their larger negative feedback factor of ?0.45 to ?1.03.

Mark D. Baker - One of the best experts on this subject based on the ideXlab platform.

  • Tropical cirrus and water vapor: an Effective Earth infrared iris feedback?
    Atmospheric Chemistry and Physics, 2002
    Co-Authors: Mark D. Baker, D. L. Hartmann
    Abstract:

    We revisit a model of feedback processes proposed by Lindzen et al. (2001), in which an assumed 22% reduction in the area of tropical high clouds per degree increase in sea surface temperature produces negative feedbacks associated with upper tropospheric water vapor and cloud radiative effects. We argue that the water vapor feedback is overestimated in Lindzen et al. (2001) by at least 60%, and that the high cloud feedback is small. Although not mentioned by Lindzen et al. (2001), tropical low clouds make a significant contribution to their negative feedback, which is also overestimated. Using more realistic parameters in the model of Lindzen et al. (2001), we obtain a feedback factor in the range of -0.15 to -0.51, compared to their larger negative feedback factor of -0.45 to -1.03. It is noted that our feedback factor could still be overestimated due to the assumption of constant low cloud cover in the simple radiative-convective model.

  • Tropical cirrus and water vapor: an Effective Earth infrared iris feedback?
    Atmospheric Chemistry and Physics Discussions, 2001
    Co-Authors: Mark D. Baker, D. L. Hartmann
    Abstract:

    We revisit a model of feedback processes proposed by Lindzen et al. (2001), in which an assumed 22% reduction in the area of tropical high clouds per degree of sea surface temperature increase produces negative feedbacks associated with upper tropospheric water vapor and cloud radiative effects. We argue that the water vapor feedback is overestimated in Lindzen et al. (2001) by at least 60%, and that the high cloud feedback should be small. Although not mentioned by Lindzen et al, tropical low clouds make a significant contribution to their negative feedback, which is also overestimated. Using more realistic parameters in the model of Lindzen et al., we obtain a feedback factor in the range of ?0.15 to ?0.51, compared to their larger negative feedback factor of ?0.45 to ?1.03.

A. Y. Hou - One of the best experts on this subject based on the ideXlab platform.

  • Reply to: "Tropical cirrus and water vapor: an Effective Earth infrared iris feedback?
    Atmospheric Chemistry and Physics, 2002
    Co-Authors: M.-d. Chou, R. S. Lindzen, A. Y. Hou
    Abstract:

    In assessing the iris effect suggested by Lindzen et al. (2001), Fu et al. (2002) found that the response of high-level clouds to the sea surface temperature had an effect of reducing the climate sensitivity to external radiative forcing, but the effect was not as strong as LCH found. The approach of FBH to specifying longwave emission and cloud albedos appears to be inappropriate, and the derived cloud optical properties may not have real physical meaning. The cloud albedo calculated by FBH is too large for cirrus clouds and too small for boundary layer clouds, which underestimates the iris effect.

  • Reply to: "Tropical cirrus and water vapor: an Effective Earth infrared iris feedback?
    Atmospheric Chemistry and Physics Discussions, 2002
    Co-Authors: M.-d. Chou, R. S. Lindzen, A. Y. Hou
    Abstract:

    In assessing the iris effect suggested by Lindzen et al. (2001), Fu et al. (2001, 2002) found that the response of high-level clouds to the sea surface temperature had an effect of reducing the climate sensitivity to external radiative forcing, but the effect was not as strong as Lindzen et al. (2001) found. The approach of Fu et al. (2001, 2002) to specifying longwave emission and cloud albedos appears to be inappropriate, and the derived cloud optical properties may not have real physical meaning. The cloud albedo calculated by Fu et al. (2001, 2002) is too large for cirrus clouds and too small for boundary layer clouds, which underestimates the iris effect.

A.j. Palmer - One of the best experts on this subject based on the ideXlab platform.

  • A Novel Simple Semi-Empirical Model for the Effective Earth Radius Factor
    IEEE Transactions on Broadcasting, 2006
    Co-Authors: A.j. Palmer, D.c. Baker
    Abstract:

    This paper presents a dual cumulative probability distribution model for predicting the cumulative distribution of the probability that a particular value of the propagation k-factor will be exceeded. The distributions relate to so-called "dry" and "wet" (or "climatic") components. The model is based on meteorological radiosonde data observed over a period of some 11 years and published by other workers. The model is extended by a simple height regression analysis in order to predict the cumulative behavior of the k-factor at different ground level heights in the summer rainfall area of South Africa during different months. Comparisons are made between predicted and observed data. The model may have potential applications in other parts of the world where data on k-factor behavior are scarce

  • Predicting the monthly cumulative distribution of the Effective Earth radius factor for South Africa
    2004 IEEE Africon. 7th Africon Conference in Africa (IEEE Cat. No.04CH37590), 2004
    Co-Authors: A.j. Palmer, D.c. Baker
    Abstract:

    In previous work a model was developed to predict the annual average cumulative distribution of the Effective Earth radius factor, the k-factor, for telecommunications applications in South Africa. Long term meteorological radiosonde data from eight stations were used for this purpose. In this paper, the model is used to predict the cumulative Effective Earth radius, or k-factor, distribution for different months as a function of height. Comparisons between observed and predicted results are presented.

  • Predicting the long-term average of the Effective Earth radius factor for South Africa using ground-based observations
    2004 IEEE Africon. 7th Africon Conference in Africa (IEEE Cat. No.04CH37590), 2004
    Co-Authors: A.j. Palmer, D.c. Baker
    Abstract:

    The work reported in This work further extends work presented in a companion paper and uses ground based meteorological observations to predict the long-term annual average value of the k-factor. These results are compared with those obtained from the radiosonde data used in the companion paper. The results are in broad agreement with those obtained from radiosonde observations. However, it is found that there are discrepancies in the comparison for Alexander Bay data. This may be due to the influence of the cold Benguela current.

  • A model for the fraction of time availability of the Effective Earth radius factor for communications planning in South Africa: Part II: Inclusion ofthe climatic term
    2002
    Co-Authors: D.c. Baker, A.j. Palmer
    Abstract:

    This paper proposes a novel model for the cumulative distribution of the k-factor, which describes the Effective Earth radius in terrestrial communications applications. The model includes a climatic term to account for the "wet" component of the radio refractive index. The model is tested against available data, and used to predict the annual average cumulative distribution as a function of height for the inland summer rainfall areas of South Africa. Agreement between the observed and predicted average annual data is generally to within a standard deviation for the inland observing stations.

  • A proposed empirical model of the Effective Earth radius factor for telecommunications use in South Africa
    IEEE AFRICON. 6th Africon Conference in Africa, 2002
    Co-Authors: D.c. Baker, A.j. Palmer
    Abstract:

    Available data for the cumulative probability of occurrence of various values of the radio refractive k-factor for South Africa have been subjected to regression analysis in terms of a proposed equation to describe such a distribution. The results presented have considerable promise in predicting the average cumulative trend of the k-factor in the inland summer rainfall areas. The model is expected to be of value in predicting large values of the k-factor, which may only be exceeded relatively rarely, but which could give rise to long-range interference in radio communication systems.

Frank Y Chen - One of the best experts on this subject based on the ideXlab platform.

  • experimental study of soil arching effect under seepage condition
    Acta Geotechnica, 2019
    Co-Authors: Luju Liang, Qizhi Chen, Wenjun Luo, Frank Y Chen
    Abstract:

    Soil arching effect, which relates to the load transfer and stress redistribution in a soil mass, exists commonly in various geotechnical situations. Many researchers have conducted trapdoor tests and theoretical analyses to study the soil arching and its development in recent years. However, little attention has been paid to the interaction between soil arching and seepage flow, both occurring during the tunnelling of a seabed tunnel. To study the influence of the seepage flow on soil arching, a series of two-dimensional trapdoor tests were carried out considering different fill heights and water level heights. Two subvertical slip surfaces were observed during the tests using the PIV technique. It was found that seepage flow increased the displacement of the particles and the Effective vertical stress acting at the top of the trapdoor. However, there was little difference in the development of slip surfaces between the seepage condition and the saturated/no-seepage condition. In addition, a nonuniform distribution of vertical stresses at the top of the trapdoor was observed. The Effective Earth pressure measured along the centreline of the trapdoor was larger than that on the two edges of the trapdoor. But this nonuniformity decreased with an increasing water level height in the test chamber.