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Stephen K. Cox, David S. McDougal, David A. Randall, and Robert A. Schiffer
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Changan Zhang, David A. Randall, Chin-Hoh Moeng, Mark Branson, Kerry A. Moyer, and Qing Wang

Abstract

A new bulk transfer formulation for the surface turbulent fluxes of momentum, heat, and moisture has been developed by using the square root of the vertically averaged turbulent kinetic energy (TKE) in the atmospheric boundary layer as a velocity scale, in place of the mean wind speed. The new parameterization utilizes the surface radiative (skin) temperature instead of the temperature at a “roughness height.” Field observations and large-eddy simulation (LES) results were used to develop the parameterization. It has been tested using an independent dataset from the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE). The predicted surface momentum flux compares very well with the observations, despite the fact that the data used for developing the new parameterization have a very different roughness length from the independent FIFE data. This shows that the parameterization can represent a wide range of surface roughness boundary conditions. The predicted sensible and latent heat fluxes also agree well with the FIFE observations, although the predicted surface sensible heat flux is somewhat less than observed at the FIFE site. The diurnal cycles of the predicted surface sensible heat and latent heat fluxes correspond very well with the observations in both magnitude and phase.

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Bruce A. Wielicki, Robert D. Cess, Michael D. King, David A. Randall, and Edwin F. Harrison

The role of clouds in modifying the earth's radiation balance is well recognized as a key uncertainty in predicting any potential future climate change. This statement is true whether the climate change of interest is caused by changing emissions of greenhouse gases and sulfates, deforestation, ozone depletion, volcanic eruptions, or changes in the solar constant. This paper presents an overview of the role of the National Aeronautics and Space Administration's Earth Observing System (EOS) satellite data in understanding the role of clouds in the global climate system. The paper gives a brief summary of the cloud/radiation problem, and discusses the critical observations needed to support further investigations. The planned EOS data products are summarized, including the critical advances over current satellite cloud and radiation budget data. Key advances include simultaneous observation of radiation budget and cloud properties, additional information on cloud particle size and phase, improved detection of thin clouds and multilayer cloud systems, greatly reduced ambiguity in partially cloud-filled satellite fields of view, improved calibration and stability of satellite-observed radiances, and improved estimates of radiative fluxes at the top of the atmosphere, at the surface, and at levels within the atmosphere. Outstanding sampling and remote sensing issues that affect data quality are also discussed. Finally, the EOS data are placed in the context of other satellite observations as well as the critical surface, field experiment, and laboratory data needed to address the role of clouds in the climate system. It is concluded that the EOS data are a necessary but insufficient condition for solution of the scientific cloud/radiation issues. A balanced approach of satellite, field, and laboratory data will be required. These combined data can span the necessary spatial scales of global, regional, cloud cell, and cloud particle physics (i.e., from 108 to 10−7 m).

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Charlotte A. DeMott, Cristiana Stan, David A. Randall, James L. Kinter III, and Marat Khairoutdinov

Abstract

Three general circulation models (GCMs) are used to analyze the impacts of air–sea coupling and superparameterized (SP) convection on the Asian summer monsoon: Community Climate System Model (CCSM) (coupled, conventional convection), SP Community Atmosphere Model (SP-CAM) (uncoupled, SP convection), and SP-CCSM (coupled, SP). In SP-CCSM, coupling improves the basic-state climate relative to SP-CAM and reduces excessive tropical variability in SP-CAM. Adding SP improves tropical variability, the simulation of easterly zonal shear over the Indian and western Pacific Oceans, and increases negative sea surface temperature (SST) biases in that region.

SP-CCSM is the only model to reasonably simulate the eastward-, westward-, and northward-propagating components of the Asian monsoon. CCSM and SP-CCSM mimic the observed phasing of northward-propagating intraseasonal oscillation (NPISO), SST, precipitation, and surface stress anomalies, while SP-CAM is limited in this regard. SP-CCSM produces a variety of tropical waves with spectral characteristics similar to those in observations. Simulated equatorial Rossby (ER) and mixed Rossby–gravity (MRG) waves may lead to different simulations of the NPISO in each model. Each model exhibits some northward propagation for ER waves but only SP-CCSM produces northward-propagating MRG waves, as in observations. The combination of ER and MRG waves over the Indian Ocean influences the spatiotemporal structure of the NPISO and contributes to the differences seen in each model.

The role of ocean coupling must be considered in terms of the time scale of the SST response compared to the time scale of tropical variability. High-frequency disturbances experience coupling via its changes to the basic state, while lower-frequency disturbances may respond directly to SST fluctuations.

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Charlotte A. DeMott, Cristiana Stan, David A. Randall, and Mark D. Branson

Abstract

The interaction of ocean coupling and model physics in the simulation of the intraseasonal oscillation (ISO) is explored with three general circulation models: the Community Atmospheric Model, versions 3 and 4 (CAM3 and CAM4), and the superparameterized CAM3 (SPCAM3). Each is integrated coupled to an ocean model, and as an atmosphere-only model using sea surface temperatures (SSTs) from the coupled SPCAM3, which simulates a realistic ISO. For each model, the ISO is best simulated with coupling. For each SST boundary condition, the ISO is best simulated in SPCAM3.

Near-surface vertical gradients of specific humidity, (temperature, ), explain ~20% (50%) of tropical Indian Ocean latent (sensible) heat flux variance, and somewhat less of west Pacific variance. In turn, local SST anomalies explain ~5% (25%) of variance in coupled simulations, and less in uncoupled simulations. Ergo, latent and sensible heat fluxes are strongly controlled by wind speed fluctuations, which are largest in the coupled simulations, and represent a remote response to coupling. The moisture budget reveals that wind variability in coupled simulations increases east-of-convection midtropospheric moistening via horizontal moisture advection, which influences the direction and duration of ISO propagation.

These results motivate a new conceptual model for the role of ocean feedbacks on the ISO. Indian Ocean surface fluxes help developing convection attain a magnitude capable of inducing the circulation anomalies necessary for downstream moistening and propagation. The “processing” of surface fluxes by model physics strongly influences the moistening details, leading to model-dependent responses to coupling.

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Bruce A. Albrecht, David A. Randall, and Stephen Nicholls

During June and July 1987, a major collaborative experiment (part of The First ISCCP [International Satellite Cloud Climatology Project] Regional Experiment (FIRE) took place off the coast of California to study the extensive fields of stratocumulus clouds that are a persistent feature of subtropical marine boundary layers. For the first time, measurements were made on both the regional scale and on the detailed local scale to permit the widest possible interpretation of the mean, turbulent, microphysical, radiative, and chemical characteristics of stratocumulus, together with the interactions among these quantities that are believed to be important in controlling the structure and evolution of these clouds. Multiple aircraft were used to make detailed, in situ measurements and to provide a bridge between the microscale and features seen from satellites. Ground-based remote-sensing systems on San Nicolas Island captured the time evolution of the boundary-layer structure during the three-week duration of the experiment, and probes flown from tethered balloons were used to measure turbulence at several levels simultaneously, and to collect cloud-microphysical data and cloud-radiative data.

Excellent cloud conditions were present throughout the experiment, although the data show that even this relatively simple cloud system displays fairly complicated structures on a variety of scales. Overall, the operational goals of the experiment were satisfied and preliminary results look very encouraging. The data collected should provide the observational base needed to increase our understanding of how stratocumulus clouds are generated, maintained, and dissipated, and thus provide for better parameterizations in large-scale numerical models and improved methods for retrieving cloud properties by satellite.

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David A. Randall, Anthony D. Del Genio, Leo J. Donner, William D. Collins, and Stephen A. Klein
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Anna Harper, Ian T. Baker, A. Scott Denning, David A. Randall, Donald Dazlich, and Mark Branson

Abstract

Moisture recycling can be an important source of rainfall over the Amazon forest, but this process relies heavily upon the ability of plants to access soil moisture. Evapotranspiration (ET) in the Amazon is often maintained or even enhanced during the dry season, when net radiation is high. However, ecosystem models often over predict the dry season water stress. The authors removed unrealistic water stress in an ecosystem model [the Simple Biosphere Model, version 3 (SiB3)] and examined the impacts of enhanced ET on the dry season climate when coupled to a GCM. The “stressed” model experiences dry season water stress and limitations on ET, while the “unstressed” model has enhanced root water access and exhibits strong drought tolerance.

During the dry season in the southeastern Amazon, SiB3 unstressed has significantly higher latent heat flux (LH) and lower sensible heat flux (SH) than SiB3 stressed. There are two competing impacts on the climate in SiB3 unstressed: cooling resulting from lower SH and moistening resulting from higher LH. During the average dry season, the cooling plays a larger role and the atmosphere is more statically stable, resulting in less precipitation than in SiB3 stressed. During dry season droughts, significantly higher LH in SiB3 unstressed is a necessary but not sufficient condition for stronger precipitation. The moistening effect of LH dominates when the Bowen ratio (BR = SH/LH) is >1.0 in SiB3 stressed and precipitation is up to 26% higher in SiB3 unstressed. An implication of this analysis is that forest conservation could enable the Amazon to cope with drying conditions in the future.

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V. Krishnamurthy, Cristiana Stan, David A. Randall, Ravi P. Shukla, and James L. Kinter III

Abstract

The simulation of the South Asian monsoon by a coupled ocean–atmosphere model with an embedded cloud-resolving model is analyzed on intraseasonal and interannual time scales. The daily modes of variability in the superparameterized Community Climate System Model, version 3 (SP-CCSM), are compared with those in observation, the superparameterized Community Atmospheric Model, version 3 (SP-CAM3), and the control simulation of CCSM (CT-CCSM) with conventional parameterization of convection. The CT-CCSM fails to simulate the observed intraseasonal oscillations but is able to generate the atmospheric El Niño–Southern Oscillation (ENSO) mode, although with regular biennial variability. The dominant modes of variability extracted from daily anomalies of outgoing longwave radiation, precipitation, and low-level horizontal wind in SP-CCSM consist of two intraseasonal oscillations and two seasonally persisting modes, in good agreement with observation. The most significant observed features of the intraseasonal oscillations correctly simulated by the SP-CCSM are the northward propagation of convection, precipitation, and circulation as well as the eastward and westward propagations. The observed spatial structure and the periods of the oscillations are also well captured by the SP-CCSM, although with lesser magnitude. The SP-CCSM is able to simulate the chaotic variability and spatial structure of the seasonally persisting atmospheric ENSO mode, while the evidence for the Indian Ocean dipole mode is inconclusive. The SP-CAM3 simulates two intraseasonal oscillations and the atmospheric ENSO mode. However, the intraseasonal oscillations in SP-CAM3 do not show northward propagation while their periods and spatial structures are not comparable to observation. The results of this study indicate the necessity of coupled models with sufficiently realistic cloud parameterizations.

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Luke P. Van Roekel, Taka Ito, Patrick T. Haertel, and David A. Randall

Abstract

The Lagrangian ocean model is used as a tool to simulate the response of the basin-scale overturning circulation to spatially variable diapycnal mixing in an idealized ocean basin. The model explicitly calculates the positions, velocities, and tracer properties of water parcels. Owing to its Lagrangian formulation, numerical diffusion is completely eliminated and water parcel pathways and water mass ages can be quantified within the framework of the discrete, advective transit time distribution. To illustrate the ventilation pathways, simulated trajectories were tracked backward in time from the interior ocean to the surface mixed layer where the water parcel was last in contact with the atmosphere. This new diagnostic has been applied to examine the response of the meridional overturning circulation to highly localized diapycnal mixing through sensitivity experiments. In particular, the focus is on three simulations: the first holds vertical diffusivity uniform; in the second, the vertical diffusivity is confined within an equatorial box; and the third simulation has a diffusivity pattern based on idealized hurricane-induced mixing. Domain-integrated deep ventilation rates and heat transport are similar between the first two cases. However, locally enhanced mixing yields about 30% younger water mass age in the tropical thermocline due to intense localized upwelling. In the third simulation, a slower ventilation rate of deep waters is found to be due to the lack of abyssal mixing. These results are interpreted using the classical theories of abyssal circulation, highlighting the strong sensitivity of the ventilation pathways to the spatial distribution of diapycnal mixing.

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