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Martin I. Hoffert
and
Y. C. Sud

Abstract

A similarity model is developed for the vertical profiles of turbulent flow variables in an entraining turbulent boundary layer of arbitrary buoyant stability. In the general formulation the vertical profiles, internal rotation of the velocity vector, discontinuities or jumps at a capping inversion and bulk aerodynamic coefficients of the boundary layer are given by solutions to a system of ordinary differential equations in the similarity variable η = z/h, where h is the physical height or thickness, where the system includes six parameters associated with surface roughness, buoyant stability of the turbulence near the surface, Coriolis effects, baroclinicity and stability of the air mass above the boundary layer. To close the system a new formulation for buoyantly interactive eddy diffusivity in the boundary layer is introduced which recovers Monin-Obukhov similarity near the surface and incorporates a hypothesis accounting for the observed variation of mixing length throughout the boundary layer.

The model is tested in simplified versions which depend only on roughness, surface buoyancy and Coriolis effects by comparison with Clarke's planetary boundary layer wind and temperature profile observations, Arya's measurements of flat-plate boundary layers in a thermally stratified wind tunnel, and Lenschow's observations of profiles of terms in the turbulent kinetic energy budget of convective planetary boundary layers. On balance, the simplified model reproduced the trend of these various observations and experiments reasonably well, suggesting that the full similarity formulation be pursued further.

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Y. C. Sud
and
G. K. Walker

Abstract

A prognostic cloud scheme named McRAS (Microphysics of Clouds with Relaxed Arakawa–Schubert Scheme) has been designed and developed with the aim of improving moist processes, microphysics of clouds, and cloud–radiation interactions in GCMs. McRAS distinguishes three types of clouds: convective, stratiform, and boundary layer. The convective clouds transform and merge into stratiform clouds on an hourly timescale, while the boundary layer clouds merge into the stratiform clouds instantly. The cloud condensate converts into precipitation following the autoconversion equations of Sundqvist that contain a parametric adaptation for the Bergeron–Findeisen process of ice crystal growth and collection of cloud condensate by precipitation. All clouds convect, advect, as well as diffuse both horizontally and vertically with a fully interactive cloud microphysics throughout the life cycle of the cloud, while the optical properties of clouds are derived from the statistical distribution of hydrometeors and idealized cloud geometry.

An evaluation of McRAS in a single-column model (SCM) with the Global Atmospheric Research Program Atlantic Tropical Experiment (GATE) Phase III data has shown that, together with the rest of the model physics, McRAS can simulate the observed temperature, humidity, and precipitation without discernible systematic errors. The time history and time mean in-cloud water and ice distribution, fractional cloudiness, cloud optical thickness, origin of precipitation in the convective anvils and towers, and the convective updraft and downdraft velocities and mass fluxes all simulate a realistic behavior. Some of these diagnostics are not verifiable with data on hand. These SCM sensitivity tests show that (i) without clouds the simulated GATE-SCM atmosphere is cooler than observed; (ii) the model’s convective scheme, RAS, is an important subparameterization of McRAS; and (iii) advection of cloud water substance is helpful in simulating better cloud distribution and cloud–radiation interaction. An evaluation of the performance of McRAS in the Goddard Earth Observing System II GCM is given in a companion paper (Part II).

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Y. C. Sud
and
G. K. Walker

Abstract

A prognostic cloud scheme named the Microphysics of Clouds with the Relaxed Arakawa–Schubert Scheme (McRAS) and the Simple Biosphere Model have been implemented in a version of the Goddard Earth Observing System (GEOS) II GCM at a 4° latitude × 5° longitude × 20 sigma-layer resolution. The McRAS GCM was integrated for 50 months. The integration was initialized with the European Centre for Medium-Range Weather Forecasts analysis of observations for 1 January 1987 and was forced with the observed sea surface temperatures and sea-ice distribution; on land, the permanent ice and vegetation properties (biomes and soils) were climatological, while the soil moisture and snow cover were prognostic. The simulation shows that the McRAS GCM yields realistic structures of in-cloud water and ice, and cloud-radiative forcing (CRF) even though the cloudiness has some discernible systematic errors. The simulated intertropical convergence zone (ITCZ) has a realistic time mean structure and seasonal cycle. The simulated CRF is sensitive to vertical distribution of cloud water, which can be affected hugely with the choice of minimum in-cloud water for the onset of autoconversion or critical cloud water amount that regulates the autoconversion itself. The generation of prognostic cloud water is accompanied by reduced global precipitation and interactive CRF. These feedbacks have a profound effect on the ITCZ. Even though somewhat weaker than observed, the McRAS GCM simulation produces robust 30–60-day oscillations in the 200-hPa velocity potential. Comparisons of CRFs and precipitation produced in a parallel simulation with the GEOS II GCM are included.

Several seasonal simulations were performed with the McRAS–GEOS II GCM for the summer (June–July–August) and winter (December–January–February) periods to determine how the simulated clouds and CRFs would be affected by (i) advection of clouds, (ii) cloud-top entrainment instability, (iii) cloud water inhomogeneity correction, and (iv) cloud production and dissipation in different cloud processes. The results show that each of these processes contributes to the simulated cloud fraction and CRF. Because inclusion of these processes helps to improve the simulated CRF, it is inferred that they would be useful to include in other cloud microphysics schemes as well.

Two ensembles of four summer (July–August–September) simulations, one each for 1987 and 1988, were produced with the earlier 17-layer GEOS I GCM with McRAS. The differences show that the model simulates realistic and statistically significant precipitation differences over India, Central America, and tropical Africa. These findings were also confirmed in the new 20-layer GEOS II GCM with McRAS in the 1987 minus 1988 differences.

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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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Y. C. Sud
,
Winston C. Chao
, and
G. K. Walker

Abstract

Several integrations were made with a coarse (4° × 5° nine-sigma level) version of the GLA GCM, which has the Arakawa–Schubert cumulus parameterization, predicted fractional cloud cover, and a parameterization of evaporation of falling rainfall. All model simulation experiments started from the ECMWF analysis for 15 December 1982 and were integrated until 31 January 1983 using climatological boundary conditions. The first ten days of model integrations show that the model-simulated tropics dries and warms as a result of excessive precipitation.

Three types of model development-cum-analysis studies were made with the cumulus scheme. First, the Critical Cloud Work Function (CCWF) dataset for different sigma layers were reworked using the Cloud Work Function (CWF) database of Lord et al. as representative of time-average CWF and not the actual CCWF values as in the Arakawa–Schubert implementation of cumulus convection. The experiments with the new CCWF dataset helped to delineate the influence of changing CCWF on model simulations. Larger values of CCWF partially alleviated the problem of excessive heating and drying during spinup and sharpened the tropical ITCZ (Intertropical Convergence Zone). Second, by comparing two simulations, one with and one without cumulus convection, the role of cumulus convection in maintaining the observed tropical rainfall and 850 mb easterly winds is clarified. Third, by using Simpson's relations between cloud radii and cumulus entrainment parameter, λ, in the Arakawa–Schubert cumulus scheme, realistic upper and lower bounds on λ were obtained. This improvement had a significant impact on the time evolution of tropical temperature and humidity simulation. It also significantly suppressed the excessive rainfall during spinup. Finally, by invoking λ min = 0.0002 m−1 (R max = 1.00 km) another simulation was made. In this simulation, not only the excessive initial rainfall was virtually eliminated, but a more realistic vertical distribution of specific humidity in the tropics was produced. Despite the conceptual simplicity of the latter, it has made some very significant improvement to the monthly simulation in the tropics.

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B. N. Goswami
,
J. Shukla
,
E. K. Schneider
, and
Y. C. Sud

Abstract

The results of some calculations with a zonally symmetric version of the Goddard Laboratory of Atmospheric Sciences (GLAS) climate model are described. The model was first used to study the nature of symmetric circulation in response to various zonally-averaged latent heating fields based on observations. Three experiments with distribution of Intent beating corresponding to the equinox condition, Northern Hemisphere summer condition and south Asian monsoon condition showed reasonable similarity to the observed distribution of surface easterlies and westerlies and the subtropical westerly jets. In the south Asian monsoon experiment, surface westerlies as well as the upper-level easterly jet in the subtropics of the Northern Hemisphere were found. The strength of the subtropical westerly jet increased with decrease in the vertical eddy viscosity.

Additional experiments were carried out in which the model was allowed to determine its own latent beat sources and the results were analyzed to examine the interaction of CISK and the imposed SST in determining the position, structure and transient behavior of the ITCZ. In the small number of cases considered, the model equilibrium was found to be independent of initial conditions, with a narrow ITCZ occurring over the SST maximum. After the equilibrium solution was established, the specfied SST distribution was altered. It was found that the initial ITCZ persisted for a long time (weeks to months); however, finally a new ITCZ became established at the location of the new SST maximum. Initially its development was slow, but was followed by a rapid intensification toward the end. The time needed for the establishment of the ITCZ at its new position depended upon the latitude of maximum SST and the magnitude of the SST anomaly.

The calculations also indicated the properties of some of the parameterizations employed in the climate model, in particular, the moist convection and the effects of clouds on radiative cooling.

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