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Zengquan Fan
and
Robert J. Oglesby

Abstract

The year to year variability in North China's summertime hydrologic cycle is analyzed in a 100-yr CCM1 simulation. Eastern North America is included for comparative purposes with earlier work. On the basis of the simulated inherent variability of the regionally averaged soil moisture, each year's climate pattern over these two regions is classified into one of three regimes: normal, dry, and wet. Features of the hydrologic cycle, the related large-scale atmospheric circulation, and the water budget are examined for each of the three defined climate regimes for each region.

The relative importance of mechanisms leading to soil moisture anomalies over North China is found to he different from that over eastern North America. For North China, precipitation anomalies, which are related to large-scale circulation, appear to be relatively more important in determining soil moisture, and the preceding springtime soil moisture is of less importance. For eastern North America, the preceding springtime soil moisture anomalies, which help to induce subsequent changes in precipitation and the large-scale circulation, appear to be relatively more important. Overall, the processes yielding summertime hydrologic anomalies over North China are more complicated and less straightforward than for eastern North America.

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Augustus F. Fanning
and
Andrew J. Weaver

Abstract

An idealized coupled ocean–atmosphere model is utilized to study the influence of horizontal resolution and parameterized eddy processes on the poleward heat transport in the climate system. A series of experiments ranging from 4° to 0.25° resolution, with appropriate horizontal viscosities and diffusivities in each case, are performed. The coupled atmosphere–ocean model results contradict earlier studies, which showed that the heat transport associated with time-varying circulations counteracted increases in the time mean so that the total remained unchanged as resolution was increased. Even though the total oceanic heat transport has not converged, the net planetary heat transport has essentially converged owing to the strong constraint of energy balance at the top of the atmosphere. Consequently, the atmospheric heat transport is reduced to offset the increasing oceanic heat transport.

To interpret these results, the oceanic heat transport is decomposed into its baroclinic overturning (related to the meridional overturning and Ekman transports), barotropic gyre (that in the horizontal plane), and baroclinic gyre (associated with the jet core within the western boundary current) components. The increase in heat transport occurs in the steady currents. In particular, the baroclinic gyre transport increases by a factor of 5 from the coarsest- to the finest-resolution case, equaling the baroclinic overturning transport at mid- to high latitudes.

To further assess the results, a parallel series of experiments under restoring conditions are performed to elucidate the differences between heat transport in coupled versus uncoupled models and models driven by temperature and salinity or equivalent buoyancy. Although heat transport is more strongly constrained in the restoring experiments, results are similar to those in the coupled model. Again, the total heat transport is increased due to an increasing baroclinic gyre component.

These results point to the importance of higher resolution in the oceanic component of current coupled climate models. These results also stress the need to adequately represent the heat transport associated with the “warm core” region of the Gulf Stream (the baroclinic gyre transport) in order to adequately represent oceanic poleward heat transport.

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Augustus F. Fanning
and
Andrew J. Weaver

Abstract

An idealized coupled ocean–atmosphere model is utilized to study the influence of horizontal resolution and parameterized eddy processes on the thermohaline circulation. A series of experiments ranging from 4° to 0.25° resolution, with appropriate horizontal viscosities and diffusivities in each case, is performed for both coupled and ocean-only models. Spontaneous internal variability (primarily on the decadal timescale) is found to exist in the higher-resolution cases (with the exception of one of the restoring experiments). The decadal oscillation (whose period varies slightly between cases) is described as an advective–convective mechanism that is thermally driven and linked to the value of the horizontal diffusivity utilized in the model. Increasing the diffusivity in the high-resolution cases presented in this paper is enough to destroy the variability, whereas decreasing the diffusivity in the moderately coarse-resolution cases is capable of inducing decadal-scale variability. As the resolution is increased still further, baroclinic instability within the western boundary current adds a more stochastic component to the solution such that the variability is less regular and more chaotic (giving rise to intradecadal timescales). These results point to the importance of higher resolution in the ocean component of coupled models, revealing the existence of richer variability in models that require less parameterized diffusion.

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Guang J. Zhang
,
Jiwen Fan
, and
Kuan-Man Xu
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Paul G. Myers
,
Augustus F. Fanning
, and
Andrew J. Weaver

Abstract

A diagnostic, finite element, barotropic ocean model has been used to simulate the mean circulation in the North Atlantic. With the inclusion of the joint effect of baroclinicity and relief (JEBAR), the Gulf Stream is found to separate at the correct latitude, ∼35°N, off Cape Hatteras. Results suggest that the JEBAR term in three key regions (offshore of the separation point in the path of the main jet, along the slope region of the North Atlantic Bight, and in the central Irminger Sea) is crucial in determining the separation point. The transport driven by the bottom pressure torque component of JEBAR dominates the solution, except in the subpolar gyre, and is also responsible for the separation of the Gulf Stream. Excluding high latitudes (in the deep-water formation regions) density variations in the upper 1000 m of the water column govern the generation of the necessary bottom pressure torque in our model.

Examination of results from the World Ocean Circulation Experiment-Community Modelling Effort indicates that the bottom pressure torque component of JEBAR is underestimated by almost an order of magnitude, when compared to our diagnostic results. The reason for this is unclear but may be associated with the diffuse nature of the modeled thermocline in the CME as suggested by our model’s sensitivity to the density field above 1000 m.

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Songmiao Fan
,
Daniel A. Knopf
,
Andrew J. Heymsfield
, and
Leo J. Donner

Abstract

In this study, two parameterizations of ice nucleation rate on dust particles are used in a parcel model to simulate aircraft measurements of ice crystal number concentration N i in the Arctic. The parcel model has detailed microphysics for droplet and ice nucleation, growth, and evaporation with prescribed vertical air velocities. Three dynamic regimes are considered, including large-scale ascent, cloud-top generating cells, and their combination. With observed meteorological conditions and aerosol concentrations, the parcel model predicts the number concentrations of size-resolved ice crystals, which may be compared to aircraft measurements. Model results show rapid changes with height/time in relative humidity, N i , and thermodynamic phase partitioning, which is not resolved in current climate and weather forecasting models. Parameterizations for ice number and nucleation rate in mixed-phase stratus clouds are thus developed based on the parcel model results to represent the time-integrated effect of some microphysical processes in large-scale models.

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Leo J. Donner
,
Charles J. Seman
,
Richard S. Hemler
, and
Songmiao Fan

Abstract

A cumulus parameterization based on mass fluxes, convective-scale vertical velocities, and mesoscale effects has been incorporated in an atmospheric general circulation model (GCM). Most contemporary cumulus parameterizations are based on convective mass fluxes. This parameterization augments mass fluxes with convective-scale vertical velocities as a means of providing a method for incorporating cumulus microphysics using vertical velocities at physically appropriate (subgrid) scales. Convective-scale microphysics provides a key source of material for mesoscale circulations associated with deep convection, along with mesoscale in situ microphysical processes. The latter depend on simple, parameterized mesoscale dynamics. Consistent treatment of convection, microphysics, and radiation is crucial for modeling global-scale interactions involving clouds and radiation.

Thermodynamic and hydrological aspects of this parameterization in integrations of the Geophysical Fluid Dynamics Laboratory SKYHI GCM are analyzed. Mass fluxes, phase changes, and heat and moisture transport by the mesoscale components of convective systems are found to be large relative to those of convective (deep tower) components, in agreement with field studies. Partitioning between the convective and mesoscale components varies regionally with large-scale flow characteristics and agrees well with observations from the Tropical Rainfall Measuring Mission (TRMM) satellite.

The effects of the mesoscale components of convective systems include stronger Hadley and Walker circulations, warmer upper-tropospheric Tropics, and moister Tropics. The mass fluxes for convective systems including mesoscale components differ appreciably in both magnitude and structure from those for convective systems consisting of cells only. When mesoscale components exist, detrainment is concentrated in the midtroposphere instead of the upper troposphere, and the magnitudes of mass fluxes are smaller. The parameterization including mesoscale components is consistent with satellite observations of the size distribution of convective systems, while the parameterization with convective cells only is not.

The parameterization of convective vertical velocities is an important control on the intensity of the mesoscale stratiform circulations associated with deep convection. The mesoscale components are less intense than in TRMM observations if spatially and temporally invariant convective vertical velocities are used instead of parameterized, variable velocities.

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Paul A. Hwang
,
Yalin Fan
,
Francisco J. Ocampo-Torres
, and
Héctor García-Nava

Abstract

Directional wave spectra acquired in hurricane reconnaissance missions are compared with wind-wave spectral models. The comparison result is quantified with two indices of model–measurement spectral agreement. In the main region of hurricane coverage, the indices vary sinusoidally with the azimuth angle referenced to the hurricane heading while showing a weak dependence on the radial distance from the hurricane center. The measured spectra agree well with three models evaluated in the back and right quarters, and they are underdeveloped in the front and left quarters. The local wind and wave directions also show a weak radial dependence and sinusoidal variation along the azimuth. The wind and wave vectors are almost collinear in the back and right quarters; they diverge azimuthally and become almost perpendicular in the left quarter. The azimuthally cyclical correlation between the indices of spectral agreement and the wind-wave directional difference is well described by the sinusoidal variations. Also discussed is the wide range of the spectral slopes observed in both hurricane and nonhurricane field data. It is unlikely that the observed spectral slope variation is caused by Doppler frequency shift from background currents. No clear correlation is found between spectral slope and various wind and wave parameters. The result suggests that the spectral slope needs to be treated as a stochastic random variable. Complementing the existing wind-wave spectral models that prescribe a fixed spectral slope of either −4 or −5, a general spectral model with its spectral parameters accommodating a variable spectral slope is introduced.

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Songmiao Fan
,
Paul Ginoux
,
Charles J. Seman
,
Levi G. Silvers
, and
Ming Zhao

Abstract

Mixed-phase clouds are frequently observed in the atmosphere. Here we present a parameterization for ice crystal concentration and ice nucleation rate based on parcel model simulations for mixed-phase stratocumulus clouds, as a complement to a previous parameterization for stratus clouds. The parcel model uses a singular (time independent) description for deposition nucleation and a time-dependent description for condensation nucleation and immersion freezing on mineral dust particles. The mineral dust and temperature-dependent parameterizations have been implemented in the Geophysical Fluid Dynamics Laboratory atmosphere model, version 4.0 (AM4.0) (new), while the standard AM4.0 (original) uses a temperature-dependent parameterization. Model simulations with the new and original AM4.0 show significant changes in cloud properties and radiative effects. In comparison to measurements, cloud-phase (i.e., liquid and ice partitioning) simulation appears to be improved in the new AM4.0. More supercooled liquid cloud is predicted in the new model, it is sustained even at temperatures lower than −25°C unlike in the original model. A more accurate accounting of ice nucleating particles and ice crystals is essential for improved cloud-phase simulation in the global atmosphere.

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Z. Q. Fan
,
Z. Sheng
,
H. Q. Shi
,
X. H. Zhang
, and
C. J. Zhou

Abstract

Global stratospheric temperature measurement is an important field in the study of climate and weather. Dynamic and radiative coupling between the stratosphere and troposphere has been demonstrated in a number of studies over the past decade or so. However, studies of the stratosphere were hampered by a shortage of observation data before satellite technology was used in atmospheric sounding. Now, the data from the Thermosphere, Ionosphere, Mesosphere Energetics, and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) observations make it easier to study the stratosphere. The precision and accuracy of TIMED/SABER satellite soundings in the stratosphere are analyzed in this paper using refraction error data and temperature data obtained from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) radio occultation sounding system and TIMED/SABER temperature data between April 2006 and December 2009. The results show high detection accuracy of TIMED/SABER satellite soundings in the stratosphere. The temperature standard deviation (STDV) errors of SABER are mostly in the range from of 0–3.5 K. At 40 km the STDV error is usually less than 1 K, which means that TIMED/SABER temperature is close to the real atmospheric temperature at this height. The distributions of SABER STDV errors follow a seasonal variation: they are approximately similar in the months that belong to the same season. As the weather situation is complicated and fickle, the distribution of SABER STDV errors is most complex at the equator. The results in this paper are consistent with previous research and can provide further support for application of the SABER’s temperature data.

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