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Mao-Sung Yao

Abstract

The maintenance of the quasi-stationary waves obtained through numerically integrating a two-level quasi-geostrophic spectral model on a β-plane is studied. An idealized topography which has only wave-number n in the zonal direction and the first mode in the meridional direction is used to force the quasi-stationary waves. However, the model's motion contains wavenumbers 0, n and 2n in the zonal direction, while the first three modes in the meridional direction are allowed for each wave. The cases n = 2 and n = 3 are considered.

The mechanism for maintaining the quasi-stationary waves is investigated by varying the imposed thermal equilibrium temperature gradient, ΔTe, and the reciprocal of the internal frictional coefficient, 0.5 kI −1. If the flow is not highly irregular, the available potential energy of quasi-stationary waves (As) is maintained by the energy conversion AzAS, where Az is the available potential energy of the time-averaged zonal mean flow. For n = 3 and moderately large ΔTe and kI −1, the kinetic energy of these waves (Ks) is maintained by the energy conversion AsKs. If ΔTe, or kI −1 is smaller while n=3, kinetic energy is supplied to the quasi-stationary waves by the energy conversion KzKs through the topographic forcing, where Kz is the kinetic energy of the time-averaged zonal mean flow. The latter mechanism also maintains the kinetic energy of the quasi-stationary waves for n=2 with relatively small ΔTe and kI −1 is sufficiently large, the flow is highly irregular and a unique regime cannot be defined for either n = 2 or n = 3.

In the case of n = 3 and moderately large ΔTe and kI −1, the energy cycle, spectra and form of the quasi-stationary waves suggest that the quasi-stationary waves are largely baroclinic waves which draw their energy from the forced waves.

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Mao-Sung Yao and Ye Cheng

Abstract

The response of cloud simulations to turbulence parameterizations is studied systematically using the GISS general circulation model (GCM) E2 employed in the Intergovernmental Panel on Climate Change’s (IPCC) Fifth Assessment Report (AR5). Without the turbulence parameterization, the relative humidity (RH) and the low cloud cover peak unrealistically close to the surface; with the dry convection or with only the local turbulence parameterization, these two quantities improve their vertical structures, but the vertical transport of water vapor is still weak in the planetary boundary layers (PBLs); with both local and nonlocal turbulence parameterizations, the RH and low cloud cover have better vertical structures in all latitudes due to more significant vertical transport of water vapor in the PBL. The study also compares the cloud and radiation climatologies obtained from an experiment using a newer version of turbulence parameterization being developed at GISS with those obtained from the AR5 version. This newer scheme differs from the AR5 version in computing nonlocal transports, turbulent length scale, and PBL height and shows significant improvements in cloud and radiation simulations, especially over the subtropical eastern oceans and the southern oceans. The diagnosed PBL heights appear to correlate well with the low cloud distribution over oceans. This suggests that a cloud-producing scheme needs to be constructed in a framework that also takes the turbulence into consideration.

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Peter H. Stone and Mao-Sung Yao

Abstract

Vertical eddy fluxes of heat are calculated from simulations with a variety of climate models, ranging from three-dimensional GCMs to a one-dimensional radiative-convective model. The models’ total eddy flux in the lower troposphere is found to agree well with Hantel's analysis from observations, but in the mid- and upper troposphere the models’ values are systematically 30% to 50% smaller than Hantel's. The models nevertheless give very good results for the global temperature profile, and the reason for the discrepancy is unclear. The model results show that the manner in which the vertical eddy flux is carried is very sensitive to the parameterization of moist convection. When a moist adiabatic adjustment scheme with a critical value for the relative humidity of 100% is used, the vertical transports by large-scale eddies and small-scale convection on a global basis are equal; but when a penetrative convection scheme is used, the large-scale flux on a global basis is only about one-fifth to one-fourth the small-scale flux. Comparison of the model results with observations indicates that the results with the latter scheme are more realistic. However, even in this case, in mid- and high latitudes the large and small-scale vertical eddy fluxes of heat are comparable in magnitude above the planetary boundary layer.

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Mao-Sung Yao and Anthony D. Del Genio

Abstract

Climate changes obtained from five doubled CO2 experiments with different parameterizations of large-scale clouds and moist convection are studied by use of the Goddard Institute for Space Studies (GISS) GCM at 4° lat × 5° long resolution. The baseline for the experiments is GISS Model II, which uses a diagnostic cloud scheme with fixed optical properties and a convection scheme with fixed cumulus mass fluxes and no downdrafts. The global and annual mean surface air temperature change (ΔT s) of 4.2°C obtained by using the Model II physics at 8° lat × 10° long resolution is reduced to 3.55°C at the finer resolution. This is due to a significant reduction of tropical cirrus clouds in the warmer climate when a finer resolution is used, despite the fact that the relative humidity increases there with a doubling of CO2. When the new moist convection parameterization of and prognostic large-scale cloud parameterization of are used, ΔT s is reduced to 3.09°C from 3.55°C. This is the net result of the inclusion of the feedback of cloud optical thickness and phase change of cloud water, and the presence of areally extensive cumulus anvil clouds. Without the optical thickness feedback, ΔT s is further reduced to 2.74°C, suggesting that this feedback is positive overall. Without anvil clouds, ΔT s is increased from 3.09° to 3.7°C, suggesting that anvil clouds of large optical thickness reduce the climate sensitivity. The net effect of using the new large-scale cloud parameterization without including the detrainment of convective cloud water is a slight increase of ΔT s from 3.56° to 3.7°C. The net effect of using the new moist convection parameterization without anvil clouds is insignificant (from 3.55° to 3.56°C). However, this is a result of a combination of many competing differences in other climate parameters. Despite the global cloud cover decrease simulated in most of the experiments, middle- and high-latitude continental cloudiness generally increases with warming, consistent with the sense of observed twentieth-century cloudiness trends; an indirect aerosol effect may therefore not be the sole explanation of these observations.

An analysis of climate sensitivity and changes in cloud radiative forcing (CRF) indicates that the cloud feedback is positive overall in all experiments except the one using the new moist convection and large-scale cloud parameterization with prescribed cloud optical thickness, for which the cloud feedback is nearly neutral. Differences in ΔCRF among the different experiments cannot reliably be anticipated by the analogous differences in current climate CRF. The meridional distribution of ΔCRF suggests that the cloud feedback is positive mostly in the low and midlatitudes, but in the high latitudes, the cloud feedback is mostly negative and the amplification of ΔT s is due to other processes, such as snow/ice–albedo feedback and changes in the lapse rate. The authors’ results suggest that when a sufficiently large variety of cloud feedback mechanisms are allowed for, significant cancellations between positive and negative feedbacks result, causing overall climate sensitivity to be less sensitive to uncertainties in poorly understood cloud physics. In particular, the positive low cloud optical thickness correlations with temperature observed in satellite data argue for a minimum climate sensitivity higher than the 1.5°C that is usually assumed.

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Peter H. Stone and Mao-Sung Yao

Abstract

The effect of eddy momentum fluxes on the general circulation is investigated with the aid of perpetual January simulations with a two-dimensional, zonally averaged model. Sensitivity experiments with this model show that the vertical eddy flux has a negligible effect on the general circulation, while the meridional eddy flux has a substantial effect. The experiments on the effect of the mefidional eddy flux essentially confirm the resultsfound by Schneider in a similar (but not identical) set of sensitivity experiments, and, in addition, show that the vertical structure of the mefidional eddy flux has a relatively small effect on the general circulation.

In order to parameterize the vertically integrated mefidional eddy momentum flux, we take Green's parameterization of this quantity and generalize it to allow for the effects of condensation. In order to do this, it is necessary to use Leovy's approximation for the eddy fluctuations in specific humidity. With this approximation the equivalent potential vorticity defined by Saltzman is conserved even when condensation occurs. Leovy's approximation also allows one to generalize the relation between quasi-geostrophic potential vorticity and theEliassen-Palm flux by replacing the potential vorticity and potential temperature by the corresponding equivalent quantities. Thus, the eddy momentum flux can be related to the eddy fluxes of two conserved quantities even when condensation is present. The eddy fluxes of the two conserved quantities are parametefized by mixing-length expressions, with the mixing coefficient taken to be the sum of Branscome's mixing coefficient, plus a correction which allows for nonlinear effects onthe eddy structure and ensures global momentum conservation.

The parametefization of the mefidional eddy transport is tested in another perpetual January simulation with the two-dimensional averaged model. The results are compared with a parallel three-dimensional simulation which calculates the eddy transport explicitly. The parameterization reproduces the latitudinal and seasonal (interhemisphefic) variations and the magnitude of the eddy transport calculated in the three-dimensional simulation reasonably well.

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Mao-Sung Yao and Anthony D. Del Genio

Abstract

An improved version of the GISS Model II cumulus parameterization designed for long-term climate integrations is used to study the effects of entrainment and multiple cloud types on the January climate simulation. Instead of prescribing convective mass as a fixed fraction of the cloud base grid-box mass, it is calculated based on the closure assumption that the cumulus convection restores 0the atmosphere to a neutral most convective state at cloud base. This change alone significantly improves the distribution of precipitation, convective mass exchanges and frequencies in the January climate. The vertical structure of the tropical atmosphere exhibits quasi-equilibrium behavior when this closure is used, even though there is no explicit constraint applied above cloud base. Global aspects of the simulation using the neutral buoyancy closure are almost identical to those obtained in a previous study with a closure relating cumulus mass flux explicitly to large-scale forcing.

A prescription of 0.2 km−1 for the fractional rate of entrainment lower the peak of the convective heating profile, reduces equatorial specific humidifies in the upper atmosphere to more realistic values, and greatly increases eddy kinetic energy at the equator due to reduced momentum mixing. With two cloud types per convective event, each cloud type having a prescribed size and entrainment rate, a clear bimodal distribution of convective mass flux is obtained in strong convective events. At the same time, many of the desirable climate features produced by the neutral buoyancy and entrainment experiments are preserved.

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Mao-Sung Yao and Anthony D. Del Genio

Abstract

The influence of the sea surface temperature distribution on cloud feedbacks is studied by making two sets of doubled CO2 experiments with the Goddard Institute for Space Studies (GISS) GCM at 4° latitude × 5° longitude resolution. One set uses Q fluxes obtained by prescribing observed sea surface temperatures (MODELII′), and the other set uses Q fluxes obtained by prescribing the simulated sea surface temperature of a coupled ocean–atmosphere model (MODELIIO). The global and annual mean surface air temperature change (ΔT s) obtained in MODELII′ is reduced from 4.11° to 3.02°C in MODELIIO. This reduced sensitivity, aside from reduced sea ice/snow–albedo feedback, is mainly due to cloud feedback that becomes nearly neutral in MODELIIO. Furthermore, the negative effect on climate sensitivity of anvil clouds of large optical thickness identified by Yao and Del Genio changes its sign in MODELIIO primarily due to sharply reduced increases of cloud water in the tropical upper troposphere. Colder tropical sea surface temperature in MODELIIO results in weaker deep convective activity and a more humid lower atmosphere in the warmer climate relative to MODELII′, which then removes the negative feedback of anvil clouds and sharply reduces the positive feedback of low clouds. However, an overall positive cloud optical thickness feedback is still maintained in MODELIIO.

It is suggested that the atmospheric climate sensitivity, partially due to changes in cloud feedbacks, may be significantly different for climate changes associated with different patterns of sea surface temperature change, as for example in warm versus cold paleoclimate epochs. Likewise, the climate sensitivity in coupled atmosphere–ocean models is also likely to be significantly different from the results obtained in Q-flux models due to the different simulations of sea surface temperature patterns in each type of model.

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Anthony D. Del Genio and Mao-Sung Yao

Abstract

We examine the response of the GISS global climate model to different parameterizations of moist convective man flux. A control run with arbitrarily specified updraft mass flux is compared to experiments that predict cumulus mass flux on the basis of low-level convergence, convergence plus surface evaporation, or convergence and evaporation modified by varying boundary layer height. An experiment that includes a simple parameterization of saturated convective-scale downdrafts is also described. Convergence effects on cumulus mass flux significantly improve the model's January climatology by increasing the frequency of occurrence of deep convection in the tropics and decreasing it at high latitudes, shifting the ITCZ from 12°N to 4°5, strengthening convective heating in the western Pacific, and increasing tropical long-wave eddy kinetic energy. Surface evaporation effects generally oppose the effects of convergence but are necessary to produce realistic continental convective heating and well-defined marine shallow cumulus regions. Varying boundary layer height (as prescribed by variations in lifting condensation level) has little effect on the model climatology. Downdrafts, however, reinforce many of the positive effects of convergence while also improving the model's vertical humidity profile and radiation balance. The diurnal cycle of precipitation over the West Pacific is best simulated when convergence determines cumulus mass flux, while surface flux effects are needed to reproduce diurnal variations in the continental ITCZ. In each experiment the model correctly simulates the observed correlation between deep convection strength and tropical sea surface temperature; the parameterization of cumulus mass flux has little effect on this relationship. The experiments have several implications for cloud effects on climate sensitivity. The dependence of cumulus mass flux on vertical motions, and the insensitivity of mean vertical motions to changes in forcing, suggests that the convective response to climate forcing may be weaker than that estimated in previous global climate model simulations that link convection only to moist static instability. This implies that changes in cloud cover and hence positive cloud feedback have been overestimated in these climate change experiments. Downdrafts may affect the feedback in the same sense by replenishing boundary layer moisture relative to cumulus parameterization schemes with only dry compensating subsidence.

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Peter H. Stone and Mao-Sung Yao

Abstract

A number of perpetual January simulations are carried out with a two-dimensional (2-D) zonally averaged model employing various parameterizations of the eddy fluxes of heat (potential temperature) and moisture. The parameterizations are evaluated by comparing these results with the eddy fluxes calculated in a parallel simulation using a three-dimensional (3-D) general circulation model with zonally symmetric forcing. The 3-D model's performance in turn is evaluated by comparing its results using realistic (nonsymmetric) boundary conditions with observations.

Branscome's parameterization of the meridional eddy flux of heat and Leovy's parameterization of the meridional eddy flux of moisture simulate the seasonal and latitudinal variations of these fluxes reasonably well, while somewhat underestimating their magnitudes. In particular, Branscome's parameterization underestimates the vertically integrated flux of heat by about 30%, mainly because it misses out the secondary peak in this flux near the tropopause; and Leovy's parameterization of the meridional eddy flux of moisture underestimates the magnitude of this flux by about 20%. The analogous parameterizations of the vertical eddy fluxes of heat and moisture are found to perform much more poorly, i.e., they give fluxes only one quarter to one half as strong as those calculated in the 3-D model. New parameterizations of the vertical eddy fluxes are developed that take into account the enhancement of the eddy mixing slope in a growing baroclinic wave due to condensation, and also the effect of eddy fluctuations in relative humidity. The new parameterizations, when tested in the 2-D model, simulate the seasonal, latitudinal, and vertical variations of the vertical eddy fluxes quite well, when compared with the 3-D model, and only underestimate the magnitude of the fluxes by 10% to 20%.

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Mao-Sung Yao and Peter H. Stone

Abstract

The moist convection parameterization used in the GISS 3-D GCM is adapted for use in a two-dimensional (2-D) zonally averaged statisticai-dynamical model. Experiments with different versions of the parameterization show that its impact on the general circulation in the 2-D model does not parallel its impact in the 3-D model unless the effect of zonal variations is parameterized in the moist convection calculations. A parameterization of the variations in moist static energy is introduced in which the temperature variations are calculated from baroclinic stability theory, and the relative humidity is assumed to be constant. Inclusion of the zonal variations of moist static energy in the 2-D moist convection parameterization allows just a fraction of a latitude circle to be unstable and enhances the amount of deep convection. This leads to a 2-D simulation of the general circulation very similar to that in the 3-D model.

The experiments show that the general circulation is sensitive to the parameterized amount of deep convection in the subsident branch of the Hadley cell. The more there is, the weaker are the Hadley cell circulations and the westerly jets. The experiments also confirm the effects of momentum mixing associated with moist convection found by earlier investigator and, in addition, show that the momentum mixing weakens the Ferrel cell. An experiment in which the moist convection was removed while the hydrological cycle was retained and the eddy forcing was held fixed shows that moist convection by itself stabilizes the tropics, reduces the Hadley circulation, and reduces the maximum speeds in the westerly jets.

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