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Laura D. Fowler and David A. Randall

Abstract

A prognostic equation for the mass of condensate associated with large-scale cloudiness introduces a direct coupling between the atmospheric moisture budget and the radiation budget through interactive cloud amounts and cloud optical properties. We have compared the cloudiness, the top-of-the-atmosphere and surface radiation budgets, the radiative forcing of clouds, and the atmospheric general circulation simulated with the Colorado State University general circulation model with and without such a prognostic cloud parameterization. In the EAULIQ run, the radiative effects of cloud water, cloud ice, and snow are considered; those of rain are omitted. The cloud optical depth and cloud infrared emissivity depend on the cloud water, cloud ice, and snow paths predicted by a bulk cloud microphysics parameterization. In the CONTROL run, a conventional large-scale condensation scheme is used. Cloud optical properties depend on the mean cloud temperatures. Results are presented in terms of January and July means.

Comparisons with data from the International Satellite Cloud Climatology Project and the Earth Radiation Budget Experiment show that EAULIQ yields improved simulations of the geographical distributions of the simulated cloudiness, the top-of-the-atmosphere radiation budget, and the longwave and shortwave cloud radiative forcings. Differences between EAULIQ and CONTROL are largest in the Tropics and are mostly due to a decrease, in the EAULIQ run, in the amount and optical thickness of upper-tropospheric clouds. In particular, the cold bias in the outgoing longwave radiation and the overestimation of the planetary albedo obtained in the CONTROL run over the tropical convective regions are substantially reduced. Differences in the radiative and latent heating rates between EAULIQ and CONTROL lead to some improvements in the atmospheric general circulation simulated by EAULIQ when compared against statistics on the observed circulation assembled by the European Centre for Medium-Range Weather Forecasts. When compared to CONTROL, EAULIQ yields a warmer troposphere except below 8 km between 3°N and 30°S. The mean meridional circulation is significantly weakened in the EAULIQ run.

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D. L. Randall and A. B. J. Clark

A system is described with which air flow in the lower one or two thousand feet of the atmosphere can be studied by the use of a radio-command-control smoke puffer. Several applications of this system are proposed in connection with balloon sounding techniques.

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C-H. Moeng and D. A. Randall

Abstract

No abstract available.

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Laura D. Fowler and David A. Randall

Abstract

In the Colorado State University general circulation model, cumulus detrainment of cloud water and cloud ice has been, up to now, the only direct coupling between convective and large-scale condensation processes. This one-way interaction from the convective to the large-scale environment parameterizes, in a highly simplified manner, the growth of anvils spreading horizontally at the tops of narrow cumulus updrafts. The reverse interaction from the large-scale to the convective updrafts, through which large-scale cloud water and cloud ice can affect microphysical processes occurring in individual convective updrafts, is missing. In addition, the effects of compensating subsidence on cloud water and cloud ice are not taken into account.

A new parameterization of convection, called “EAUCUP,” has been developed, in which large-scale water vapor, cloud water, and cloud ice are allowed to enter the sides of the convective updrafts and can be lifted to the tops of the clouds. As the various water species are lifted, cloud microphysical processes take place, removing excess cloud water and cloud ice in the form of rain and snow. The partitioning of condensed vapor between cloud water and cloud ice, and between rain and snow, is based on temperature. The effects of compensating subsidence on the large-scale water vapor, cloud water, and cloud ice are computed separately. Convective rain is assumed to fall instantaneously to the surface. Three treatments of the convective snow are tested: 1) assuming that all snow is detrained at the tops of convective updrafts, 2) assuming that all snow falls outside of the updrafts and may evaporate, and 3) assuming that snow falls entirely inside the updrafts and melts to form rain.

Including entrainment of large-scale cloud water and cloud ice inside the updrafts, large-scale compensating subsidence unifies the parameterizations of large-scale cloud microphysics and convection, but have a lesser impact than the treatment of convective snow on the simulated climate. Differences between the three alternate treatments of convective snow are discussed. Emphasis is on the change in the convective, large-scale, and radiative tendencies of temperature, and change in the convective and large-scale tendencies of water vapor, cloud water, cloud ice, and snow. Below the stratiform anvils, the change in latent heating due to the change in both convective and large-scale heatings contributes a major part to the differences in diabatic heating among the three simulations. Above the stratiform anvils, differences in the diabatic heating between the three simulations result primarily because of differences in the longwave radiative cooling. In particular, detraining convective snow at the tops of convective updrafts yields a strong increase in the longwave radiative cooling associated with increased upper-tropospheric cloudiness. The simulated climate is wetter and colder when convective snow is detrained at the tops of the updrafts than when it is detrained on the sides of the updrafts or when it falls entirely inside the updrafts. This result highlights the importance of the treatment of the ice phase and associated precipitation in the convective cloud models used in cumulus parameterizations.

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Todd D. Ringler and David A. Randall

Abstract

Shallow-water equations discretized on a perfect hexagonal grid are analyzed using both a momentum formulation and a vorticity-divergence formulation. The vorticity-divergence formulation uses the unstaggered Z grid that places mass, vorticity, and divergence at the centers of the hexagons. The momentum formulation uses the staggered ZM grid that places mass at the centers of the hexagons and velocity at the corners of the hexagons. It is found that the Z grid and the ZM grid are identical in their simulation of the physical modes relevant to geostrophic adjustment. Consistent with the continuous system, the simulated inertia–gravity wave phase speeds increase monotonically with increasing total wavenumber and, thus, all waves have nonzero group velocities.

Since a grid of hexagons has twice as many corners as it has centers, the ZM grid has twice as many velocity points as it has mass points. As a result, the ZM-grid velocity field is discretized at a higher resolution than the mass field and, therefore, resolves a larger region of wavenumber space than the mass field. We solve the ∇2 f = λf eigenvalue problem with periodic boundary conditions on both the Z grid and ZM grid to determine the modes that can exist on each grid. The mismatch between mass and momentum leads to computational modes in the velocity field. Two techniques that can be used to control these computational modes are discussed. One technique is to use a dissipation operator that captures or “sees” the smallest-scale variations in the velocity field. The other technique is to invert elliptic equations in order to filter the high wavenumber part of the momentum field.

Results presented here lead to the conclusion that the ZM grid is an attractive alternative to the Z grid, and might be particularly useful for ocean modeling.

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Todd D. Ringler and David A. Randall

Abstract

Using the shallow water equations, a numerical framework on a spherical geodesic grid that conserves domain-integrated mass, potential vorticity, potential enstrophy, and total energy is developed. The numerical scheme is equally applicable to hexagonal grids on a plane and to spherical geodesic grids. This new numerical scheme is compared to its predecessor and it is shown that the new scheme does considerably better in conserving potential enstrophy and energy. Furthermore, in a simulation of geostrophic turbulence, the new numerical scheme produces energy and enstrophy spectra with slopes of approximately K −3 and K −1, respectively, where K is the total wavenumber. These slopes are in agreement with theoretical predictions. This work also exhibits a discrete momentum equation that is compatible with the Z-grid vorticity-divergence equation.

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Michael D. Toy and David A. Randall

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The isentropic system of equations has particular advantages in the numerical modeling of weather and climate. These include the elimination of the vertical velocity in adiabatic flow, which simplifies the motion to a two-dimensional problem and greatly reduces the numerical errors associated with vertical advection. The mechanism for the vertical transfer of horizontal momentum is simply the pressure drag acting on isentropic coordinate surfaces under frictionless, adiabatic conditions. In addition, vertical resolution is enhanced in regions of high static stability, which leads to better resolution of features such as the tropopause. Negative static stability and isentropic overturning frequently occur in finescale atmospheric motion. This presents a challenge to nonhydrostatic modeling with the isentropic vertical coordinate. This paper presents a new nonhydrostatic atmospheric model based on a generalized vertical coordinate. The coordinate is specified in a manner similar to that of Konor and Arakawa, but “arbitrary Eulerian–Lagrangian” (ALE) methods are used to maintain coordinate monotonicity in regions of negative static stability and to return the coordinate surfaces to their isentropic “targets” in statically stable regions. The model is mass conserving and implements a vertical differencing scheme that satisfies two additional integral constraints for the limiting case of z coordinates. The hybrid vertical coordinate model is tested with mountain-wave experiments including a downslope windstorm with breaking gravity waves. The results show that the advantages of the isentropic coordinate are realized in the model with regard to vertical tracer and momentum transport. Also, the isentropic overturning associated with the wave breaking is successfully handled by the coordinate formulation.

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Laura D. Fowler and David A. Randall

Abstract

The inclusion of cloud microphysical processes in general circulation models makes it possible to study the multiple interactions among clouds, the hydrological cycle, and radiation. The gaps between the temporal and spatial scales at which such cloud microphysical processes work and those at which general circulation models presently function force climate modelers to crudely parameterize and simplify the various interactions among the different water species (namely, water vapor, cloud water, cloud ice, rain, and snow) and to use adjustable parameters to which large-scale models can be highly sensitive. Accordingly, the authors have investigated the sensitivity of the climate, simulated with the Colorado State University general circulation model, to various aspects of the parameterization of cloud microphysical processes and its interactions with the cumulus convection and radiative transfer parameterizations.

The results of 120-day sensitivity experiments corresponding to perpetual January conditions have been compared with those of a control simulation in order to 1 ) determine the importance of advecting cloud water, cloud ice, rain, and snow at the temporal and spatial scale resolutions presently used in the model; 2) study the importance of the formation of extended stratiform anvils at the tops of cumulus towers, 3) analyze the role of mixed-phase clouds in determining the partitioning among cloud water, cloud ice, rain, and snow and, hence, their impacts on the simulated cloud optical properties; 4) evaluate the sensitivity of the atmospheric moisture budget and precipitation rates to a change in the fall velocities of rain and snow; 5) determine the model's sensitivity to the prescribed thresholds of autoconversion of cloud water to rain and cloud ice to snow; and 6) study the impact of the collection of supercooled cloud water by snow, as well as accounting for the cloud optical properties of snow.

Results are presented in terms of 30-day mean differences between the sensitivity experiments and control run. The authors find that three-dimensional advection of the water species has little influence on their geographical distributions and globally averaged amounts. The simulated climate remains unchanged when detrained condensed water at the tops of cumulus towers is used as a source of rain and snow rather than as a source of cloud water and cloud ice. In contrast, instantaneously removing cloud water and cloud ice detrained at the tops of cumulus towers in the form of precipitation yields a strong drying of the atmosphere and a significant reduction in the size of the anvils. Altering the partitioning between cloud ice and supercooled cloud water produces significant changes in the vertical distributions of the cloud optical depth and effective cloud fraction, hence producing significant variations in the top-of-the-atmosphere longwave and shortwave cloud radiative forcings. Increasing the fall speeds of rain and snow leads to a decrease in cloudiness and an increase in stratiform rainfall. Increasing the thresholds for autoconversion of cloud water to rain and cloud ice to snow yields a significant increase in middle- and high-level clouds and a reduction of the cumulus precipitation rate. The collection of supercooled cloud water by snow appeared to be an important microphysical process for mixed-phase clouds. Finally, the optical effects of snow have little impact upon the top-of-the-atmosphere radiation budget.

This study illustrates the need for in-depth analysis of the spatial and temporal scale dependence of the different microphysical parameters of the cloud parameterizations used in general circulation models.

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Laura D. Fowler, David A. Randall, and Steven A. Rutledge

Abstract

Microphysical processes responsible for the formation and dissipation of water and ice clouds have been incorporated into the Colorado State University General Circulation Model in order to 1) yield a more physically based representation of the components of the atmospheric moisture budget, 2) link the distribution and optical properties of the model-generated clouds to the predicted cloud water and ice amounts, and 3) produce more realistic simulations of cloudiness and the earth's radiation budget.

The bulk cloud microphysics scheme encompasses five prognostic variables for the mass of water vapor, cloud water, cloud ice, rain, and snow. Graupel and hail are neglected. Cloud water and cloud ice are predicted to form through large-scale condensation and deposition processes and also through detrainment at the tops of cumulus towers. The production of rain and snow occur through autoconversion of cloud water and cloud ice. Rain drops falling through clouds can grow by collecting cloud water, and falling snow can collect both cloud water and cloud ice. These collection processes are formulated using the continuous collection equation. Evaporation of cloud water, cloud ice, rain, and snow are allowed in subsaturated layers. Melting and freezing are included. We also provide a coupling between convective clouds and stratiform anvils through the detrainment of cloud water and cloud ice at the tops of cumulus towers. Interactive cloud optical properties provide the link between the cloud n-microphysics and radiation parameterizations; the optical depths and infrared emissivities of large-scale stratiform clouds are parameterized in terms of the cloud water and cloud ice paths.

Two annual-cycle numerical simulations are performed to assess the impact of cloud microphysics on the hydrological cycle. In the “EAULIQ” run, large-scale moist processes and cloud optical properties are driven by the bulk cloud microphysics parameterization. In the “CONTROL” run, condensed water is immediately removed from the atmosphere in the form of rain, which may evaporate as it falls through subsaturated layers. Stratiform ice clouds are not considered in CONTROL. When clouds are present, cloud optical depths and cloud infrared emissivities are dependent on the mean cloud temperatures.

Results are presented in terms of January and July monthly averages. Emphasis is placed on the spatial distributions of cloud water, cloud ice, rain, and snow produced by the cloud microphysics scheme. In EAULIQ, cloud water and cloud ice are more abundant in the middle latitudes than in the Tropics, suggesting that large-scale condensation contributes a major part to the production of condensed water. Comparisons between the simulated vertically integrated cloud water and die columnar cloud water retrievals from satellite microwave measurements over the global oceans indicate a reasonable agreement. Interactions between the cloud micro- physics and cumulus convection parameterizations lead to smaller, more realistic precipitation rates. In particular, the cumulus precipitation rate is strongly reduced when compared to CONTROL.

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Melissa A. Burt, David A. Randall, and Mark D. Branson

Abstract

As the Arctic sea ice thins and ultimately disappears in a warming climate, its insulating power decreases. This causes the surface air temperature to approach the temperature of the relatively warm ocean water below the ice. The resulting increases in air temperature, water vapor, and cloudiness lead to an increase in the surface downwelling longwave radiation (DLR), which enables a further thinning of the ice. This positive ice–insulation feedback operates mainly in the autumn and winter. A climate change simulation with the Community Earth System Model shows that, averaged over the year, the increase in Arctic DLR is 3 times stronger than the increase in Arctic absorbed solar radiation at the surface. The warming of the surface air over the Arctic Ocean during fall and winter creates a strong thermal contrast with the colder surrounding continents. Sea level pressure falls over the Arctic Ocean, and the high-latitude circulation reorganizes into a shallow “winter monsoon.” The resulting increase in surface wind speed promotes stronger surface evaporation and higher humidity over portions of the Arctic Ocean, thus reinforcing the ice–insulation feedback.

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