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Takanobu Yamaguchi and David A Randall

Abstract

The relative importance, for cloud-top entrainment, of the cooling rates due to longwave radiation, evaporation, and mixing was assessed through analysis of the results produced by a Lagrangian parcel-tracking model (LPTM) incorporated into a large-eddy simulation model. The LPTM predicts each parcel’s trajectory over time, using the resolved velocity simulated by the host model. An LPTM makes it possible to identify entrained parcels; this is almost impossible to do in an observational study.

A nocturnal stratocumulus cloud was simulated over 4h using a 5-m horizontal grid spacing and a 2.5-m vertical grid spacing. At the start of the last hour of the simulation, over 40 million parcels were placed near the top of the inversion layer and then tracked. Parcel histories were analyzed to identify entrained parcels.

Entrainment occurs in cloud holes, which occur in dry regions of sinking air. Entrainment into the mixed layer is regulated by buoyancy, which requires parcels to be precooled in the inversion layer, prior to entrainment. A mixing fraction analysis was used to separate the cooling due to longwave radiation, evaporation, and mixing. Results show that radiative and evaporative cooling are of comparable importance, but that mixing is by far the dominant cooling mechanism. The radiative cooling rate is strongly inhomogeneous, and only weak radiative cooling is found in regions of entrainment. Therefore, the entrained parcels experience less than the horizontal-mean radiative cooling. Although radiative cooling maintains the boundary layer turbulence, its direct effect on buoyancy of entrained parcels is modest.

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Zachary A. Eitzen and David A. Randall

Abstract

This study uses a numerical model to simulate deep convection both in the Tropics over the ocean and the midlatitudes over land. The vertical grid that was used extends into the stratosphere, allowing for the simultaneous examination of the convection and the vertically propagating gravity waves that it generates. A large number of trajectories are used to evaluate the behavior of tracers in the troposphere, and it is found that the tracers can be segregated into different types based upon their position in a diagram of normalized vertical velocity versus displacement. Conditional sampling is also used to identify updrafts in the troposphere and calculate their contribution to the kinetic energy budget of the troposphere. In addition, Fourier analysis is used to characterize the waves in the stratosphere; it was found that the waves simulated in this study have similarities to those observed and simulated by other researchers. Finally, this study examines the wave energy flux as a means to provide a link between the tropospheric behavior of the convection and the strength of the waves in the stratosphere.

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Harshvardhan, David A. Randall, and Donald A. Dazlich

Abstract

Attempts to map the global longwave surface radiation budget from space have been thwarted by the presence of clouds. Unlike the shortwave, there is no physical relationship between the outgoing longwave and the surface longwave under cloudy skies. Therefore, there is no correlation between spatial and temporal averages of the outgoing longwave radiation and not longwave radiation at the surface. However, in regions where a particular cloud regime exists preferentially, a relationship between the mean longwave cloud radiative forcing (CRF) at the top of the atmosphere and at the surface can he shown to exist. Results from a general circulation model suggest that this relationship for monthly means is coherent over fairly large geographical areas. For example, in tropical convective areas, the longwave CRF at the top is very large, but at the surface it is quite small because of the high opacity of the lowest layers of the atmosphere. On the other hand, in areas of stratus over cool ocean surfaces, the longwave CRF at the top is very small but at the surface, it is quite substantial.

To the extent that the cloudiness simulated in the model mimics the real atmosphere, it may be possible to estimate the monthly mean longwave CRF at the surface from the Earth Radiation Budget Experiment cloud forcing at the top. The net longwave radiation at the surface can then be mapped if monthly means of the clear-sky fluxes are obtained by some independent technique.

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David A. Randall and Bruce A. Wielicki

Measurements in atmospheric science sometimes determine universal functions, but more commonly data are collected in the form of case studies. Models are conceptual constructs that can be used to make predictions about the outcomes of measurements. Hypotheses can be expressed in terms of model results, and the best use of measurements is to falsify such hypotheses. Tuning of models should be avoided because it interferes with falsification. Comparison of models with data would be easier if the minimum data requirements for testing some types of models could be standardized.

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Michael A. Kelly and David A. Randall

Abstract

A simple fixed-SST model of a zonal circulation in the tropical atmosphere has been developed that has separate boxes for the ascending and descending branches of the atmospheric circulation. This circulation resembles the Walker circulation. This is the first box model to determine the fractional widths of the warm and cold pools. The atmospheric model contains an explicit hydrologic cycle, a simplified but physically based radiative transfer parameterization, and interactive clouds.

Results indicate that the intensity of the tropical circulation is crucially dependent on the amount and vertical distribution of water vapor above the cold-pool boundary layer (CPBL). In response to increasing precipitable water over the CPBL, the radiative cooling rate of the free troposphere increases. To a good approximation, subsidence warming balances radiative cooling in the subsiding branches of the circulation. If the fractional width of the cold pool (CP) does not change too much, the circulation must intensify as the subsidence rate increases. To compensate for a stronger circulation and to restore energy balance in the Walker cell, the precipitable water over the warm pool (WP) must decrease. A “moist-outflow” experiment shows that the Walker circulation intensifies if air is advected to the subsiding regions from lower altitudes in the WP. As the advection level decreases, air supplied to the CP becomes warmer and moister, and so the column water vapor in the CP free troposphere increases. The mechanism described above then leads to a strengthening of the circulation. This moist-outflow experiment also shows that when the authors try to moisten the atmosphere by specifying a lower advection level for water vapor, the atmosphere adjusts so as to dry out. This effect is very strong.

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David A. Randall, Harshvardhan, and Donald A. Dazlich

Abstract

This paper presents an analysis of the diurnal and semidiurnal variability of precipitation, evaporation, precipitable water, horizontal moisture flux convergence, cloudiness, and cloud radiative forcing, as simulated by the Colorado State University General Circulation Model (GCM). In broad agreement with observations, the model produces an afternoon precipitation maximum over land in warm rainy regions, such as the tropics and the midlatitude summer continents, and an early morning maximum over the oceans far from land. The statistical significance of these model results is demonstrated using a chi-square test. The observed diurnal variation of temperature in the oceanic tropical middle troposphere is also realistically simulated.

Encouraged by these results, the model was used to investigate the causes of the diurnal cycle of precipitation over the oceans. For this purpose, experiments have been performed with an all-ocean global model. Results show that an oceanic diurnal cycle of precipitation occurs even in the absence of neighboring continents and tends to have a morning maximum. It is generally weaker than observed, however. When the radiative effects of clouds are omitted, the simulated diurnal cycle of precipitation is much weaker but still present, with essentially the same phase.

Several experiments have also been performed with a one-dimensional version of the GCM, in which time-dependent large-scale vertical motion can be prescribed. The results show that even in the absence of any systematic daily variation of the large-scale vertical motion, the model produces a diurnal cycle of precipitation with an amplitude of about 1 mm day−1, and a morning maximum.

Finally, previously published results have been followed up, which show that the diurnal cycle strongly affects the partitioning of precipitation between land and sea. The new analysis is based on comparison of three nondiurnal June-July integrations with three Julys from a multiyear diurnally forced seasonal simulation. The results show major changes in the time-averaged surface energy budget, and much more precipitation in “summer monsoon” regimes when the diurnal cycle is omitted.

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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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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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Cara-Lyn Lappen and David A. Randall

Abstract

In a companion paper, the authors presented a boundary layer parameterization that was based on the mass-flux concept and included an internally consistent representation of the vertical flux of horizontal momentum. In the present paper, the authors show how the framework of that model can be used to determine the perturbation pressure field, by solving the anelastic pressure equation. The pressure covariances needed to close the parameterization can then be diagnosed. Tests show very encouraging agreement of the pressure statistics with results obtained from large-eddy simulations.

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Kuan-Man Xu and David A. Randall

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This study describes some results from several simulations of cumulus ensembles at the Southern Great Plains site of the Atmospheric Radiation Measurement (ARM) program during the July 1995 Intensive Observation Period (IOP). A 2D cloud ensemble model (CEM) is used to simulate the macroscopic properties of midlatitude cumulus ensembles. The observed large-scale, horizontal advective tendencies and large-scale vertical velocity or the total advective tendencies are used to drive the CEM, in addition to nudging of the simulated, domain-averaged horizontal wind components toward the observed winds.

A detailed comparison with available observations and tropical convection is made in this study. In general, the CEM-simulated results agree reasonably well with the available observations from the July 1995 IOP. The differences between simulations and observations are, however, much larger than those obtained in tropical cases, especially those based on the Global Atmospheric Research Program Atlantic Tropical Experiment Phase III data. Significant differences exist between the statistical properties of tropical and midlatitude cumulus convection, especially in the vertical structures of the cumulus mass fluxes, apparent heat source (Q 1), and apparent moisture sink (Q 2). The strong variations of the subcloud-layer thermodynamic structure and the surface fluxes in midlatitude continents have large impacts on the heat and moisture budgets. The radiative budgets and satellite-observed cloud amounts are also compared with observations. Although the agreements are reasonably good, some deficiencies of the simulations and inadequate accuracy of large-scale advective tendencies can be clearly seen from the comparisons. Sensitivity tests are performed to address these issues.

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