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S. A. Lebedeff
and
S. Hameed

Abstract

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S. A. Lebedeff
and
S. Hameed

Abstract

The two-dimensional diffusion equation has been solved by an integral method to obtain the distribution of ground-level concentration of an inert effluent emitted from a semi-infinite area source in a steady-state and horizontally homogeneous atmospheric surface layer. Mean wind velocity and eddy diffusivity profiles derived from empirically determined flux-profile relations of Businger et al. (1971) for stable and unstable surface layers were used. It is found that concentration as a function of downwind distance can be described by a simple formula over distances of practical interest in surface layer dispersion. Corresponding results for a cross-wind infinite line source are obtained by simple differentiation. The concentration distribution is completely determined by the friction velocity u *, the Monin-Obukhov length L, the roughness length z0, and the effluent source strength Q. The generalization of the integral method needed to obtain accurate solutions of the diffusion equation with the given wind velocity and diffusivity profiles is discussed in an appendix.

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S. A. Lebedeff
and
S. Hameed

Abstract

Turbulent transport of material emitted from a surface may be described by the steady-state, two-dimensional, semi-empirical diffusion equation. It is shown that, with wind velocity and eddy diffusivity expressed as power functions of the vertical coordinate, this equation can be solved exactly by introducing a similarity variable. The solution gives the vertical distribution of concentration for area sources in terms of the incomplete gamma function. Implications of the solution are discussed.

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Bradford S. Barrett
and
Sultan Hameed

Abstract

Monthly precipitation in Chile (30°–55°S) was found to vary by intensity, latitude, and longitude of the South Pacific high (SPH). In austral winter, precipitation was higher when the SPH was weaker and when it was centered farther west. In austral spring, precipitation was higher when the SPH was weaker, similar to winter. However, spring precipitation was not found to be related to SPH longitude, and higher precipitation was found when the SPH was centered farther north. In austral summer, no relationship was found between precipitation and either SPH intensity or longitude, but positive correlations were found between precipitation and latitude of the SPH. In austral autumn, correlation patterns between precipitation and all three SPH metrics more closely resembled those seen in winter. The results of a multiple linear regression confirmed the importance of two SPH metrics (intensity and longitude) and the unimportance of a third SPH metric (latitude) in understanding variability in winter, summer, and autumn precipitation in central and southern Chile. In spring, regression results confirmed a relationship between precipitation and SPH intensity and latitude. Furthermore, the SPH intensity and longitude in winter combined to hindcast monthly precipitation with a better goodness of fit than five El Niño–Southern Oscillation metrics traditionally related to Chilean precipitation. Anomalies of lower-tropospheric circulation and vertical velocities were found to support the observed relationships between SPH and precipitation. Based on these results, a physical mechanism is proposed that employs the SPH as a metric to aid in understanding variability in precipitation in central and south-central Chile in all seasons.

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P. H. Wyant
,
A. Mongroo
, and
S. Hameed

Abstract

Entropy production has been calculated as a function of the coefficient of meridional heat transfer for two seasonal energy-balance climate models. Both models display extrema in entropy production at values of the coefficient appropriate to the present climate. Inclusion of time dependence in the models is found to be the essential feature for successful application of entropy extremization.

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Hameed Rasheed
,
A. S. Aldabagh
, and
Murur V. Ramamoorthy

Abstract

Power transformation was used to normalize the peak daily and peak monthly rainfall at various raingage stations in Iraq. Excellent correlations were found between the coefficient of skewness (Cs ) and a parameter for power transformation (λ), coefficient of kurtosis (Ck ) and λ, and between Cs and Ck . The relationship between Cs and λ is used to develop an estimation procedure for calculation of the transformation parameter. The method eliminates the use of trial and error for estimating λ. The method has been used for estimating T-year peak daily and peak monthly rainfall by power transformation. The results are compared with results from other methods.

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Meredith S. Croke
,
Robert D. Cess
, and
Sultan Hameed

Abstract

Land-based observations of cloud cover, for the period 1900–87 and averaged over three geographical regions of the United States (coastal southwest, coastal northeast, and southern plains), show strong positive correlations with one estimate of global mean surface temperature, a finding consistent with prior investigations that suggest cloud cover over land has increased during global warm periods relative to cold periods. It is also found that the strengths of three permanent high/low pressure systems (North Pacific high, Icelandic low, and Azores high) are negatively correlated with global mean surface temperature, suggesting a possible connection between regional cloud cover, for certain locations, and the strengths of adjacent high/low pressure systems. Specifically, for the regions considered it is suggested that the coastal southwest cloud cover is related to changes in the strength of the subtropical North Pacific high, that for the southern plains also to the strength of the North Pacific high, and that for the coastal northeast to the strength of the Icelandic low. Thus the climate-induced change in cloud cover for certain regions appears related, at least in part, to climate-induced change in the strengths of adjacent high/low pressure systems, and plausible physical explanations for this relation are provided for the three regions that have been studied. This does not, of course, provide a direct physical cause-and-effect explanation for the changes in regional cloud cover, because the mechanisms that cause the intensities of the high/low pressure systems to change are not understood.

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Kennetu R. Sperber
,
Sultan Hameed
,
Gerald L. Potter
, and
James S. Boyle

Abstract

The ability of the ECMWF model (cycle 33) to simulate the Indian and East Asian summer monsoons is evaluated at four different horizontal resolutions: T21, T42, T63, and T1O6. Generally, with respect to the large-scale features of the circulation, the largest differences among the simulations occur at T42 relative to T21. However, on regional scales, important differences among the high-frequency temporal variability serve as a further critical rest of the model's ability to simulate the monsoon.

T106 best captures both the spatial and temporal characteristics of the Indian and East Asian monsoons, whereas T42 fails to correctly simulate the sequence and development of synoptic-scale milestones that characterize the monsoon flow. In particular, T106 is superior at simulating the development and migration of the monsoon trough over the Bay of Bengal. In the T42 simulation, the development of the monsoon occurs one month earlier than typically observed. At this time the trough is incorrectly located adjacent to the east coast of India, which results in an underestimate of precipitation over the Burma-Thailand region. This early establishment of the monsoon trough affects the evolution of the East Asian monsoon and yields excessive preseason rainfall over the Mei-yu region. EOF analysis of precipitation over China indicates that T106 best simulates the Mei-yu mode of variability, which is associated with an oscillation of the rainband that gives rise to periods of enhanced rainfall over the Yangtze River valley. The coarse resolution of T21 precludes simulation of the aforementioned regional-scale monsoon flows.

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