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Y. Mintz and G. K. Walker

Abstract

The global fields of normal monthly soil moisture and land surface evapotranspiration are derived with a simple water budget model that has precipitation and potential evapotranspiration as inputs. The precipitation is observed and the potential evapotranspiration is derived from the observed surface air temperature with the empirical regression equation of Thornthwaite. It is shown that at locations where the net surface radiation flux has been measured, the potential evapotranspiration given by the Thornthwaite equation is in good agreement with those obtained with the radiation-based formulations of Priestley and Taylor, Penman, and Budyko, and this provides the justification for the use of the Thornthwaite equation.

After deriving the global fields of soil moisture and evapotranspiration, the assumption is made that the potential evapotranspiration given by the Thornthwaite equation and by the Priestley–Taylor equation will everywhere be about the same; and the inverse of the Priestley–Taylor equation is used to obtain the normal monthly global fields of net surface radiation flux minus ground heat storage. This and the derived evapotranspiration are then used in the equation for energy conservation at the surface of the earth to obtain the global fields of normal monthly sensible heat flux from the land surface to the atmosphere.

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Y. C. Sud, J. Shukla, and Y. Mintz

Abstract

The influence of land surface roughness on the large scale atmospheric circulation and rainfall was examined by comparing three sets of simulations made with a general circulation model in which the land surface roughness length, z 0, was reduced from 45 cm to 0.02 cm. The reduced surface roughness produced about a two-fold increase in the boundary layer wind speed and, at the same time, a two-fold decrease in the magnitude of the surface stress. There was almost no change in the surface evaporation and surface sensible heat flux. There was, however, a larger change in the horizontal convergence of the water vapor transport in the boundary layer and a corresponding large change in the rainfall distribution mainly as a consequence of the change in the cut of the surface stress. The result suggests that the height of the earth' vegetation cover, which is the main determinant of the land surface roughness, has a large influence on the boundary layer water vapor transport convergence and the rainfall distribution.

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P. J. Sellers, Y. Mintz, Y. C. Sud, and A. Dalcher

Abstract

A simple but realistic biosphere model has been developed for calculating the transfer of energy, mass and momentum between the atmosphere and the vegetated surface of the earth. The model is designed for use in atmospheric general circulation models.

The vegetation in each terrestrial model grid area is represented by two distinct layers, either or both of which may be present or absent at any given location and time. The upper vegetation layer represents the perennial canopy of trees or shrubs, while the lower layer represents the annual ground cover of grasses and other herbaceous species. The local coverage of each vegetation layer may be fractional or complete but as the individual vegetation elements are considered to be evenly spaced, their root systems are assumed to extend uniformly throughout the entire grid area. Besides the vegetation morphology, the physical and physiological properties of the vegetation layers are also prescribed. These properties determine (i) the reflection, transmission, absorption and emission of direct and diffuse radiation in the visible, near infrared and thermal wavelength intervals; (ii) the interception of rainfall and its evaporation from the leaf surfaces; (iii) the infiltration, drainage and storage of the residual rainfall in the soil; (iv) the control by the photosynthetically active radiation and the soil moisture potential, inter alia, over the stomatal functioning and thereby over the return transfer of the soil moisture to the atmosphere through the root-stem-leaf system of the vegetation; and (v) the aerodynamic transfer of water vapor, sensible heat and momentum from the vegetation and soil to a reference level within the atmospheric boundary layer.

The Simple Biosphere (SiB) has seven prognostic physical-state variables: two temperatures (one for the canopy and one for the ground cover and soil surface); two interception water stores (one for the canopy and one for the ground cover); and three soil moisture stores (two of which can be reached by the vegetation root systems and one underlying recharge layer into and out of which moisture is transferred only by hydraulic diffusion and gravitational drainage).

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A. R. Robinson, D. E. Harrison, Y. Mintz, and A. J. Semtner

Abstract

We present the results of a multi-level, constant depth, primitive equation general ocean numerical circulation simulation with mesoscale resolution. A single mid-latitude model gyre is driven by wind and heating. After 30 years of spin-up with a relatively coarse grid and large diffusion coefficients, the grid size and diffusion coefficients are reduced. The circulation then adjusts into a nonlinear and time-dependent flow with periods of tens of days and space scales of hundreds of kilometers. After a quasi-equilibrium state is achieved, two years of data are obtained which are separated into time-mean and time-dependent fluctuations, and analyzed. Dynamically distinct regions are intensified, momentum, heat and vorticity balances examined, and energy integrals calculated. Statistical measures of significance and of uncertainty are computed where possible. Eddy energy is produced primarily by Reynolds stress work (barotropic instability) on the mean circulation shear in the recirculation and near-field region of the northern current system. Mean fluctuation correlation terms are presented in some regions at order 1 in the mean heat and vorticity balance and can be the leading ageostrophic effect in the mean momentum balance. The flow is non-quasigeostrophic in some parts of the intense boundary currents.

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