摘要:

A comprehensive comparison of top of atmosphere radiative quantities from the Earth Radiation Budget Experiment (ERBE) with the same quantities from the latest version of the National Center for Atmospheric Research Community Climate Model (CCM) is presented. The ERBE data set offers a unique collection of top of atmosphere radiation fields for it includes clear‐sky radiation fields that have in the past not been available for such comparisons. Comparison between ERBE clear‐sky longwave fluxes and CCMl indicates larger outgoing flux in the model than measured by ERBE. This overestimation is ascribed to the prevalent dryness of the model. Comparison of clear sky albedo indicates good agreement over oceans and land. Total outgoing longwave flux also reflects the moisture deficiency of the model, but differences due to an underprediction of “effective” high cloud are also apparent. The clear‐sky and total fluxes are combined to form the cloud radiative forcing from the ERBE data and the CCM. Comparison of shortwave cloud radiative forcing indicates deficiencies in the model where marine stratus clouds are absent. Large longwave cloud forcing over the tropical deep convective regions in Indonesia and South America are present in the model but are underestimated compared with the ERBE results. Three regions, located over Indonesia, the equatorial Pacific, and the North Atlantic, are considered in detail. For the Indonesian region where deep convection is present, we consider the statistical correlation between the longwave cloud forcing and the shortwave cloud forcing from the ERBE data and CCM. Results indicate a near cancellation between the SWCF and LWCF for these regions, whereas the model predicts a net cooling. Another major area of discrepancy is over the North Atlantic and Pacific oceans where ERBE shows that clouds significantly reduce the solar heating of the oceans. While the model simulates this cooling, the magnitude is underpredicted by more than a fa

ISSN:0148-0227    

DOI:10.1029/JD095iD08p11679

年代:1990

数据来源: WILEY

摘要:

In order to facilitate study of very inhomogeneous optical media such as clouds, the difficult angular part of radiative transfer calculations is simplified by considering systems in which scattering occurs only in certain directions. These directions are selected in such a way that the intensity field decouples into an infinite number of independent (e.g., orthogonal) families in direction space, each coupled only within its family. Further discretization, this time in space, lends itself readily to both analytical renormalization approaches (part 2) and to numerical calculations (part 2). We are particularly interested in scaling systems in which the optical density field has no characteristic size over a wide range of scales; these include internally homogeneous media of any shape but are more generally internally inhomogeneous and better described as fractals or multifractals. In this case, the albedo and transmission obey power laws in the thick cloud limit if scattering is conservative. By deriving powerful discrete angle (DA) similarity relations, we show that the scaling exponents that characterize these laws are “universal” in the sense that they are independent of the DA phase functions. We argue that these universality classes may be generally expected to extend beyond DA to include standard (continuous angle) phase functions and transfer equations. By comparing the DA equations with the diffusion equation, we show that in general the thick cloud limits of the two will be different: the thick cloud regime will only be “diffusive” in very homogeneous clouds, hence the term “universality class” is more appropriate. The DA similarity relations indicate that in scaling systems spatial variability is of primary importance, this suggests that far more research should be made to realistically model the spatial variability and to investigate its effect on radiative response, even if the angular aspect of the transfer process is made much less sophisticated than is possible in the classical plane‐paralle

ISSN:0148-0227    

DOI:10.1029/JD095iD08p11699

年代:1990

数据来源: WILEY

摘要:

The discrete angle radiative transfer systems discussed in part 1 readily lend themselves to approximation schemes in which simple scaling systems with known radiative transfer properties can be doubled in size yielding analytic expressions relating the transfer coefficients corresponding to the initial and doubled scale. This “real space renormalization” method can be viewed as a generalization of conventional invariant imbedding techniques to scaling systems. Analytic nonlinear doubling mappings are obtained for homogeneous square, cubic and triangular systems, as well as for a simple fractal system with both open and cyclic horizontal boundary conditions. The doubling mappings have both thick and thin cloud fixed points; to which the transmission and albedoes are respectively algebraically attracted and repelled, with universal (phase function independent) exponents we estimate analytically. The method is approximate since it systematically neglects small‐scale intensity gradients; however, the results are qualitatively correct, arid it therefore establishes the connection between the scaling of the cloud optical density field and the scaling of the corresponding transfer coefficients. We also discuss the limitations of the method; in part 3 we compare it with a numerical app

ISSN:0148-0227    

DOI:10.1029/JD095iD08p11717

年代:1990

数据来源: WILEY

摘要:

In the first two installments of this series, various cloud models were studied with angularly discretized versions of radiative transfer. This simplification allows the effects of cloud inhomogeneity to be studied in some detail. The families of scattering media investigated were those whose members are related to each other by scale changing operations that involve only ratios of their sizes (“scaling” geometries). In part 1 it was argued that, in the case of conservative scattering, the reflection and transmission coefficients of these families should vary algebraically with cloud size in the asymptotically thick regime, thus allowing us to define scaling exponents and corresponding “universality” classes. In part 2 this was further justified (by using analytical renormalization methods) for homogeneous clouds in one, two, and three spatial dimensions (i.e., slabs, squares, or triangles and cubes, respectively) as well as for a simple deterministic fractal cloud. Here the same systems are studied numerically. The results confirm (1) that renormalization is qualitatively correct (while quantitatively poor), and (2) more importantly, they support the conjecture that the universality classes of discrete and continuous angle radiative transfer are generally identical. Additional numerical results are obtained for a simple class of scale invariant (fractal) clouds that arises when modeling the concentration of cloud liquid water into ever smaller regions by advection in turbulent cascades. These so‐called random “β models” are (also) characterized by a single fractal dimension. Both open and cyclical horizontal boundary conditions are considered. These and previous results are contrasted with plane‐parallel predictions, and measures of systematic error are defined as “packing factors” which are found to diverge algebraically with average optical thickness and are significant even when the scaling behavior is very limited in range. Several meteorological consequences, especially concerning the “albedo paradox” and global climate models, are discussed, and future directions of investigation are outlined. Throughout this series it is shown that spatial variability of the optical density field (i.e., cloud geometry) determines the exponent of optical thickness (hence universality class), whereas changes in phase function can only affect the multiplicative prefactors. It is therefore argued that much more emphasis should be placed on modeling spatial inhomogeneity and investigating its radiative signature, even if this implies crude treatment of the angular aspect of the ra

ISSN:0148-0227    

DOI:10.1029/JD095iD08p11729

年代:1990

数据来源: WILEY

摘要:

The absolute absorption cross sections of the (2, 1)–(12, 1) Schumann‐Runge bands of O2at room temperature have been obtained from measurements of the total absolute absorption cross sections arising from transitions from the ground‐state levels with υ″ = 0 and 1 by subtraction of theυ″ = 0 contributions, which have been synthesized from our previous determinations of the band oscillator strengths, line center positions and predissociation line widths of the (υ′, 0) Schumann‐Runge bands of O2. The resultant absolute absorption cross sections of the (2, 1)–(12, 1) bands have been integrated numerically to obtain the band oscillator strengths. Our experimental band oscillator strengths are somewhat lower than the experimental values of Lewis et al.(1986) but are in good agreement with the recently calculated values

ISSN:0148-0227    

DOI:10.1029/JD095iD08p11743

年代:1990

数据来源: WILEY

摘要:

In this paper we deal with the problem of retrieving temperature and moisture profiles in the Antarctic regions using radiance observations from the TIROS operational vertical sounder (TOVS). The high‐resolution infrared sounder (HIRS) data are calibrated with the internal cold and warm blackbodies in order to alleviate apparent calibration discrepancies resulting from using the space as a cold reference. Significant improvements of the simultaneous retrieval results have been achieved in using a climatological first guess and selected channels of HIRS as a result of this change in calibration procedure. The retrieval results of the orbit at December 21, 1987 (2345 UT) are compared with radiosonde data and analyses of the European Center of Medium Range Weather Forecasts (ECMWF) of December 22, 1987 (0000 UT

ISSN:0148-0227    

DOI:10.1029/JD095iD08p11747

年代:1990

数据来源: WILEY

摘要:

An analytical expression for the bidirectional reflectance field of a vegetation canopy is derived from physical and geometrical considerations of the transfer of radiation through a porous medium. The reflectance pattern is shown to depend explicitly on the optical properties of the scatterers (for example, leaves), and on the structural parameters of the canopy, such as the statistical distribution of the orientation of these scatterers, the leaf area density, the size of the scatterers and their interspacing. This theory provides a simple and accurate way to understand the anisotropy of the radiation field over a vegetated surface. It can be useful for modeling applications (for example, the albedo is a by‐product which can be numerically estimated), as well as for extracting some of the structural and physical properties of the surface. These applications are discussed in the accompanying paper (Pinty et al., this issue

ISSN:0148-0227    

DOI:10.1029/JD095iD08p11755

年代:1990

数据来源: WILEY

摘要:

A physically based, analytical model of the bidirectional reflectance of porous media was derived in a companion paper (Verstraete et al., this issue). This model is validated against laboratory and ground‐based measurements taken over two vegetation covers, both in the visible and near‐infrared spectral regions. An inversion procedure, based on a nonlinear optimization technique, is used to infer the intrinsic optical properties of the leaves, as well as information on the morphology of the canopies, that is, on the geometrical arrangements of these scatterers in space. The model is then used to generate theoretical bidirectional reflectances, using the values of the relevant parameters retrieved from the inversion procedure, and these values compare favorably with the actual observations over the entire range of illumination and observation angles. The values of the parameters retrieved from the inversion procedure are discussed, validated against actual independent measurements, and interpreted in terms of the physical and morphological properties of the vegetation cov

ISSN:0148-0227    

DOI:10.1029/JD095iD08p11767

年代:1990

数据来源: WILEY

摘要:

Near‐inertial oscillations of the surface wind were observed at Marcus Island (24°N, 154°E) in the subtropical Pacific area. Kinetic energy spectra of the surface wind show a peak at a period of 1.1 days in the clockwise component spectrum. This period is slightly (6%) shorter than the local inertial period (1.2 days). The peak period varies slightly during the record analyzed, but tends to remain below the local inertial period. Complex demodulation with a filter shows that the large inertial amplitudes occur intermittently and decay rapidly. Most of the occurrences of large inertial events seem to be related to passing cold frontal systems or migrating low‐pressure sy

ISSN:0148-0227    

DOI:10.1029/JD095iD08p11777

年代:1990

数据来源: WILEY

摘要:

The Earth Radiation Budget Experiment nonscanner measurements are simulated with the scanner measurements. The error in simulating a single measurement is 1% for longwave and 3% for shortwave. Errors in simulating the average daily measurements are half these amounts. Four months of Earth Radiation Budget Satellite measurements were analyzed. The results show that changing sun geometry affects the accuracy of the nonscanner measurements. The medium field of view (MFOV) total channel and scanner agree to within 2% on average. The wide field of view (WFOV) total channel and scanner agree to within 1%. For the shortwave channels, the agreement with the scanner is 2% for the MFOV and 2.5% for the WFOV.

ISSN:0148-0227    

DOI:10.1029/JD095iD08p11785

年代:1990

数据来源: WILEY