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WILLIAM D. SELLERS

Abstract

The eigenvector or “empirical orthogonal function” approach is used to determine the dominant precipitation anomaly patterns for the western United States for each month during the last 36 yr. In all months there is enough intercorrelation among monthly precipitation amounts in different parts of the region that at least 45 percent of the total variance can be explained by only three eigenvectors. Usually the most important pattern is one with a single large region of anomalous precipitation, centered in southern California, Arizona, or Nevada in winter and in Washington, Idaho, or Montana in summer. Also important in all months is a pattern with anomalies of opposite sign in the Pacific Northwest and the Arizona-New Mexico-Texas area.

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William D. Sellers

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William D. Sellers

Abstract

The concept of potential evapotranspiration, as applied to arid regions, is examined using an energy balance approach suggested by Budyko. Using data for Yuma, Ariz., it is shown that a 50 per cent increase in the relative humidity of the air above an irrigated field is accompanied by a 10 to 15 per cent decrease in potential evapotranspiration, a negligible decrease in the net radiation, and a temperature increase of the surface relative to the air of 2 to 3C.

The estimated annual potential evapotranspiration at Yuma is about 2000 mm, which is 30 to 50 per cent higher than estimates based on Thornthwaite's method.

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William D. Sellers

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A simple global climatic model described earlier is modified slightly and reapplied to the CO2 problem. The modifications include using a more efficient numerical method to solve the system of equations and changing the expression used to estimate the infrared emissivity of an atmospheric column. After runs of 300 model years, considered the period necessary to reach a true steady state because of the large heat capacity of the oceans, the average global surface temperature drops 1.64C if the amount of CO2 in the atmosphere is halved and rises 1.32C if the amount is doubled.

Some comments on the future of this type of modeling experiment are appended.

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William D. Sellers

Abstract

A quasi-three-dimensional global climate model designed to trace the transient evolution of climate is described. The model, which has an interactive ocean, differs from earlier models in including a 5-layer atmospheric structure and in utilizing a 5-day time step. It is essentially a coarse-grid general circulation model with simplified dynamics. The basic nonlinear system of equations used includes a prognostic equation for surface temperature and diagnostic equations for all the other variables, including sea-level pressure and wind velocity.

Several 20-year runs were made with the model, primarily to test its stability, its simulation of the real climate, and its sensitivity to variations in the solar constant. The model is quite stable and shows no obvious tendency to drift with time toward either a very cold or a very warm climate. It simulates most of the major features of the atmospheric circulation and the hydrologic cycle. It fails, however, to give strong Northern Hemisphere subtropical highs over the oceans in summer. The model response to variations in the solar constant is weaker than in other models. These variations are buttered by an atmosphere whose lapse rate varies in order to preserve a global and annual radiation balance.

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William D. Sellers

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A quasi-three-dimensional global climate model is used to study the effect of a transient change in solar radiation on the model's climate. The solar constant is decreased abruptly by 5 percent in the 10th year of a 50-year run. It is then returned to its original value at increments of 1 percent per year. Although significant changes occur during the perturbation including a drop in the average global surface temperature of about 2.6 K, the long-term effects on the model's climate are small.

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William D. Sellers

Abstract

A relatively simple numerical model of the energy balance of the earth-atmosphere is set up and applied. The dependent variable is the average annual sea level temperature in 10° latitude belts. This is expressed basically as a function of the solar constant, the planetary albedo, the transparency of the atmosphere to infrared radiation, and the turbulent exchange coefficients for the atmosphere and the oceans.

The major conclusions of the analysis are that removing the arctic ice cap would increase annual average polar temperatures by no more than 7C, that a decrease of the solar constant by 2–5% might be sufficient to initiate another ice age, and that man's increasing industrial activities may eventually lead to a global climate much warmer than today.

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William D. Sellers

Abstract

A two–dimensional global climatic model has been developed, using a 10° longitude by 10° latitude box grid and a one-month time step. The lapse rates of temperature and the meridional component of the flow in the oceans and atmosphere and of specific humidity in the atmosphere are parameterized in terms of their respective sea-level values. The model then utilizes the vertically-averaged thermodynamic energy equation, the equation of motion for the boundary layer, the thermal wind equation, the hydrostatic equation, the surface water balance equation, and empirical relations between the sea-level temperature and pressure gradient fields and between cloud cover and precipitation to determine the global distribution by months of sea-level pressure, temperature, wind speed and direction, and relative humidity, precipitation, evaporation, runoff, soil mositure content, ice and snow cover and thickness, cloud cover, and poleward energy transport. A simple ocean model is included.

Two 100 model-year runs were made on the NCAR CDC 7600 computer. Each required about 27 min of computer time. They differed only in the value of the solar constant used. Both runs indicate that the model is extremely stable. In almost all grid boxes only minor changes in any variable in any month occurred after the first 40 to 50 years. There are strong similarities between computed and observed fields of many of the variables. However, there are also several cases where the model predictions are completely unrealistic.

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William D. Sellers

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William D. Sellers

Abstract

An expression for the diabatic wind profile is derived using dimensional analysis and the assumption that the turbulent exchange coefficient for momentum can be partitioned into two parts, one associated with forced convection and the other with natural convection. The limitation of the profile to neutral or unstable conditions is stressed.

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