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S. Bony
,
K-M. Lau
, and
Y. C. Sud

Abstract

Two independent sets of meteorological reanalyses are used to investigate relationships between the tropical sea surface temperature (SST) and the large-scale vertical motion of the atmosphere for spatial and seasonal variations, as well as for El Niño/La Niña episodes of 1987–88. Supergreenhouse effect (SGE) situations are found to be linked to the occurrence of enhanced large-scale rising motion associated with increasing SST. In regions where the large-scale atmospheric motion is largely decoupled from the local SST due to internal or remote forcings, the SGE occurrence is weak. On seasonal and interannual timescales, such regions are found mainly over equatorial regions of the Indian Ocean and western Pacific, especially for SSTs exceeding 29.5°C. In these regions, the activation of feedback processes that regulate the ocean temperature is thus likely to be more related to the large-scale remote processes, such as those that govern the monsoon circulations and the low-frequency variability of the atmosphere, than to the local SST change.

The relationships among SST, clouds, and cloud radiative forcing inferred from satellite observations are also investigated. In large-scale subsidence regimes, regardless of the SST range, the cloudiness, the cloud optical thickness, and the shortwave cloud forcing decrease with increasing SST. In convective regions maintained by the large-scale circulation, the strong dependence of both the longwave (LW) and shortwave (SW) cloud forcing on SST mainly results from changes in the large-scale vertical motion accompanying the SST changes. Indeed, for a given large-scale rising motion, the cloud optical thickness decreases with SST, and the SW cloud forcing remains essentially unaffected by SST changes. However, the LW cloud forcing still increases with SST because the detrainment height of deep convection, and thus the cloud-top altitude, tend to increase with SST. The dependence of the net cloud radiative forcing on SST may thus provide a larger positive climate feedback when the ocean warming is associated with weak large-scale circulation changes than during seasonal or El Niño variations.

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Shian-Jiann Lin
,
Winston C. Chao
,
Y. C. Sud
, and
G. K. Walker

Abstract

A generalized form of the second-order van Leer transport scheme is derived. Several constraints to the implied subgrid linear distribution are discussed. A very simple positive-definite scheme can be derived directly from the generalized form. A monotonic version of the scheme is applied to the Goddard Laboratory for Atmospheres (GLA) general circulation model (GCM) for the moisture transport calculations, replacing the original fourth-order center-differencing scheme. Comparisons with the original scheme are made in idealized tests as well as in a summer climate simulation using the full GLA GCM. A distinct advantage of the monotonic transport scheme is its ability to transport sharp gradients without producing spurious oscillations and unphysical negative mixing ratio. Within the context of low-resolution climate simulations, the aforementioned characteristics are demonstrated to be very beneficial in regions where cumulus convection is active. The model-produced precipitation pattern using the new transport scheme is more coherently organized both in time and in space, and correlates better with observations. The side effect of the filling algorithm used in conjunction with the original scheme is also discussed, in the context of idealized tests.

The major weakness of the proposed transport scheme with a local monotonic constraint is its substantial implicit diffusion at low resolution. Alternative constraints are discussed to counter this problem.

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B. N. Goswami
,
J. Shukla
,
E. K. Schneider
, and
Y. C. Sud

Abstract

The results of some calculations with a zonally symmetric version of the Goddard Laboratory of Atmospheric Sciences (GLAS) climate model are described. The model was first used to study the nature of symmetric circulation in response to various zonally-averaged latent heating fields based on observations. Three experiments with distribution of Intent beating corresponding to the equinox condition, Northern Hemisphere summer condition and south Asian monsoon condition showed reasonable similarity to the observed distribution of surface easterlies and westerlies and the subtropical westerly jets. In the south Asian monsoon experiment, surface westerlies as well as the upper-level easterly jet in the subtropics of the Northern Hemisphere were found. The strength of the subtropical westerly jet increased with decrease in the vertical eddy viscosity.

Additional experiments were carried out in which the model was allowed to determine its own latent beat sources and the results were analyzed to examine the interaction of CISK and the imposed SST in determining the position, structure and transient behavior of the ITCZ. In the small number of cases considered, the model equilibrium was found to be independent of initial conditions, with a narrow ITCZ occurring over the SST maximum. After the equilibrium solution was established, the specfied SST distribution was altered. It was found that the initial ITCZ persisted for a long time (weeks to months); however, finally a new ITCZ became established at the location of the new SST maximum. Initially its development was slow, but was followed by a rapid intensification toward the end. The time needed for the establishment of the ITCZ at its new position depended upon the latitude of maximum SST and the magnitude of the SST anomaly.

The calculations also indicated the properties of some of the parameterizations employed in the climate model, in particular, the moist convection and the effects of clouds on radiative cooling.

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K. M. Lau
,
H. T. Wu
,
Y. C. Sud
, and
G. K. Walker

Abstract

The sensitivity of tropical atmospheric hydrologic processes to cloud microphysics is investigated using the NASA Goddard Earth Observing System (GEOS) general circulation model (GCM). Results show that a faster autoconversion rate leads to (a) enhanced deep convection in the climatological convective zones anchored to tropical land regions; (b) more warm rain, but less cloud over oceanic regions; and (c) an increased convective-to-stratiform rain ratio over the entire Tropics. Fewer clouds enhance longwave cooling and reduce shortwave heating in the upper troposphere, while more warm rain produces more condensation heating in the lower troposphere. This vertical differential heating destabilizes the tropical atmosphere, producing a positive feedback resulting in more rain and an enhanced atmospheric water cycle over the Tropics. The feedback is maintained via secondary circulations between convective tower and anvil regions (cold rain), and adjacent middle-to-low cloud (warm rain) regions. The lower cell is capped by horizontal divergence and maximum cloud detrainment near the freezing–melting (0°C) level, with rising motion (relative to the vertical mean) in the warm rain region connected to sinking motion in the cold rain region. The upper cell is found above the 0°C level, with induced subsidence in the warm rain and dry regions, coupled to forced ascent in the deep convection region.

It is that warm rain plays an important role in regulating the time scales of convective cycles, and in altering the tropical large-scale circulation through radiative–dynamic interactions. Reduced cloud–radiation feedback due to a faster autoconversion rate results in intermittent but more energetic eastward propagating Madden–Julian oscillations (MJOs). Conversely, a slower autoconversion rate, with increased cloud radiation produces MJOs with more realistic westward-propagating transients embedded in eastward-propagating supercloud clusters. The implications of the present results on climate change and water cycle dynamics research are discussed.

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Y. C. Sud
,
G. K. Walker
,
V. M. Mehta
, and
William K-M. Lau

Abstract

A recent version of the Goddard Earth Observing System GCM which contains several upgrades to the model's prognostic cloud physics and microphysics as well as snow and ice hydrology, was used to isolate the influences of the annual cycles of solar irradiation and sea surface temperatures (SSTs) on the annual cycle of circulation and precipitation. Four 50-month-long integrations were produced with the GCM. The first integration, called the control simulation, C, was forced with daily interpolated SSTs from a 30-yr climatology of monthly SST data. In this simulation both SSTs and incoming solar irradiance had their normally prescribed annual cycles. The next two companion simulations were called S1, which used annual mean prescribed incoming solar irradiation, and S2, which used annual mean prescribed SST; everything else was kept similar to C in these two simulations. In the fourth simulation, called S3, both SSTs and incoming solar irradiation at the top of the atmosphere were prescribed to always maintain their annual mean values. This constraint virtually eliminated all the annual cycle forcings in the simulation. Nevertheless, all simulations had the diurnal cycle of solar irradiation. An intercomparison of these simulations revealed the following.

  1. The poleward excursions of the zonal mean ITCZ and precipitation are strongly modulated by the annual cycles of SSTs and solar forcings. For the majority of the regions, particularly in the subtropical monsoonal regions, for example, India, Southeast Asia, and Australia, the influence of the annual cycle of solar heating was found to be stronger than that of the annual cycle of SSTs.

  2. The precipitation and circulation patterns over the Kuroshio Current region off the east coast of Asia were most affected by the SSTs and were strongly linked to the annual cycle(s) of local SSTs.

  3. The annual mode of precipitation over Amazonia had two regimes: an equatorial regime with a maximum in the month of March in S1 and a corresponding maximum in the month of January in S2. Control C, which had both annual cycles, produced both the “month of January” and “month of March” modes of precipitation. This shows how solar and SST annual cycles jointly influence the simulated annual modes of precipitation over South America. Surprisingly, the annual modes of precipitation in C were roughly equal to the sum of the annual modes of precipitation of S1 and S2.

  4. Precipitation over Sahelian Africa is significantly reduced in simulations lacking the annual cycle of solar irradiation. The opposite kind of influence of the annual cycle of solar radiation was noted in almost all other monsoonal regions: India, Southeast Asia, as well as Australia. The only exception is the continental United States, where the solar annual cycle showed only a relatively minor influence on the annual mode of precipitation.

  5. The simulated tropical intraseasonal oscillations (TIOs) were reasonably robust in each of the four simulations. This suggests that TIOs are an outcome of the internal dynamics of the atmosphere that may in turn be forced by the interactions among the physical and dynamical processes of the atmosphere. This conclusion is consistent with the robustness of the observed TIO modes throughout the annual cycle and the significant dependence of TIOs on physical parameterization(s).

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Y. C. Sud
,
D. M. Mocko
,
G. K. Walker
, and
Randal D. Koster

Abstract

Four different Atmospheric Radiation Measurement Program Cloud and Radiation Test Bed (ARM–CART) Single-Column Model (SCM) datasets were used to force an SCM in a number of simulations performed to study the influence of land surface fluxes on precipitation. The SCM employed Goddard Earth Observing System (GEOS-2) GCM physics, which includes a recent version of prognostic cloud scheme (Microphysics of Clouds with Relaxed Arakawa–Schubert), and a land model (Simplified Simple Biosphere Model) coupled to a highly resolved soil hydrological description in the vertical. The four ARM–CART datasets employed in these studies are referred as case 1, case 3, case 4, and case 8. The SCM simulation results broadly confirm the previous findings that an increase in the solar absorption and surface evaporation helps to increase the local rainfall, but they also reveal that the magnitude of the rainfall increase is strongly affected by the ability of the background circulation to promote moist convection. The simulated precipitation increase was as large as 50% of the evapotranspiration increase for case 1 that covered a relatively wet period. It was substantially reduced for cases 3 and 4 covering a normal rainfall period and became negligible for case 8, a dry case. A part of evaporation increase became horizontal divergence of water vapor; this would have the potential of increasing the precipitation downstream of the test region. For a particular background circulation, it was found that the evaporation–precipitation relationship, often defined as recycling ratio, is remarkably robust even for a large range of vegetation covers, soil types, and initial soil moistures. Notwithstanding the limitations of only one-way interaction (i.e., the large scale influencing the regional physics and not vice versa), the current SCM simulations show that recycling ratio is a function of the background circulation and not a regional and/or seasonal feature. Indeed, a vigorous biosphere can help to produce more rainfall under wet conditions but may do little to dislodge a large-scale drought. It is pointed out that even though these inferences are robust, they are prone to weaknesses of the SCM physics as well as the assumption of the large scale remaining unaffected by changes of moist processes.

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Y. C. Sud
,
D. M. Mocko
,
K-M. Lau
, and
R. Atlas

Abstract

Past studies have suggested that the drought of the summer of 1988 over the midwestern United States may have been caused by sea surface temperature (SST) anomalies, an evolving stationary circulation, a soil-moisture feedback on circulation and rainfall, or even by remote forcings. The relative importance of various contributing factors is investigated in this paper through the use of Goddard Earth Observing System (GEOS) GCM simulations. Seven different experiments, each containing an ensemble of four simulations, were conducted with the GCM. For each experiment, the GCM was integrated through the summers of 1987 and 1988 starting from an analyzed atmosphere in early January of each year. In the baseline case, only the SST anomalies and climatological vegetation parameters were prescribed, while everything else (such as soil moisture, snow cover, and clouds) was interactive. The precipitation differences (1988 minus 1987) show that the GCM was successful in simulating reduced precipitation in 1988, but the accompanying low-level circulation anomalies in the Midwest were not well simulated. To isolate the influence of the model’s climate drift, analyzed winds and analyzed soil moisture were prescribed globally as continuous updates (in isolation or jointly). The results show that remotely advected wind biases (emanating from potential errors in the model’s dynamics and physics) are the primary cause of circulation biases over North America. Inclusion of soil moisture helps to improve the simulation as well as to reaffirm the strong feedback between soil moisture and precipitation. In a case with both updated winds and soil moisture, the model produces more realistic evapotranspiration and precipitation differences. An additional case also used soil moisture and winds updates, but only outside North America. Its simulation is very similar to that of the case with globally updated winds and soil moisture, which suggests that North American simulation errors originate largely outside the region. Two additional cases examining the influence of vegetation were built on this case using correct and opposite-year vegetation. The model did not produce a discernible improvement in response to vegetation for the drought year. One may conclude that the soil moisture governs the outcome of the land–atmosphere feedback interaction far more than the vegetation parameters. A primary inference of this study is that even though SSTs have some influence on the drought, model biases strongly influence the prediction errors. It must be emphasized that the results from this study are dependent upon the GEOS model’s identified errors and biases, and that the conclusions do not necessarily apply to results from other models.

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Y. C. Sud
,
W. K-M. Lau
,
G. K. Walker
,
J-H. Kim
,
G. E. Liston
, and
P. J. Sellers

Abstract

Two 3-year (1979–1982) integrations were carried out with a version of the GLA GCM that contains the Simple Biosphere Model (SiB) for simulating land-atmosphere interactions. The control case used the usual SiB vegetation cover (comprising 12 vegetation types), while its twin, the deforestation case, imposed a scenario in which all tropical rainforests were entirely replaced by grassland. Except for this difference, all other initial and prescribed boundary conditions were kept identical in both integrations.

An intercomparison of the integrations shows that tropical deforestation

• decreases evapotranspiration and increases land surface outgoing longwave radiation and sensible heat flux, thereby warming and drying the planetary boundary layer. This happens despite the reduced absorption of solar radiation due to higher surface albedo of the deforested land.

• produces significant and robust local as well as global climate changes. The local effect includes significant changes (mostly reductions) in precipitation and diabatic heating, while the large-scale effect is to weaken the Hadley circulation but invigorate the southern Ferrel cell, drawing larger air mass from the indirect polar cells.

• decreases the surface stress (drag force) owing to reduced surface roughness of deforested land, which in turn intensifies winds in the planetary boundary layer, thereby affecting the dynamic structure of moisture convergence. The simulated surface winds are about 70% stronger and are accompanied by significant changes in the power spectrum of the annual cycle of surface and PBL winds and precipitation.

• Our results broadly confirm several findings of recent tropical deforestation simulation experiments. In addition, some global-scale climatic influences of deforestation not identified in earlier studies are delineated.

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Maeng-Ki Kim
,
William K. M. Lau
,
Mian Chin
,
Kyu-Myong Kim
,
Y. C. Sud
, and
Greg K. Walker

Abstract

The direct effects of aerosols on global and regional climate during boreal spring are investigated based on numerical simulations with the NASA Global Modeling and Assimilation Office finite-volume general circulation model (fvGCM) with Microphyics of Clouds with the Relaxed–Arakawa Schubert Scheme (McRAS), using aerosol forcing functions derived from the Goddard Ozone Chemistry Aerosol Radiation and Transport model (GOCART).

The authors find that anomalous atmospheric heat sources induced by absorbing aerosols (dust and black carbon) excite a planetary-scale teleconnection pattern in sea level pressure, temperature, and geopotential height spanning North Africa through Eurasia to the North Pacific. Surface cooling due to direct effects of aerosols is found in the vicinity and downstream of the aerosol source regions, that is, South Asia, East Asia, and northern and western Africa. Significant atmospheric heating is found in regions with large loading of dust (over northern Africa and the Middle East) and black carbon (over Southeast Asia). Paradoxically, the most pronounced feature in aerosol-induced surface temperature is an east–west dipole anomaly with strong cooling over the Caspian Sea and warming over central and northeastern Asia, where aerosol concentrations are low. Analyses of circulation anomalies show that the dipole anomaly is a part of an atmospheric teleconnection pattern driven by atmospheric heating anomalies induced by absorbing aerosols in the source regions, but the influence was conveyed globally through barotropic energy dispersion and sustained by feedback processes associated with the regional circulations.

The surface temperature signature associated with the aerosol-induced teleconnection bears striking resemblance to the spatial pattern of observed long-term trend in surface temperature over Eurasia. Additionally, the boreal spring wave train pattern is similar to that reported by Fukutomi et al. associated with the boreal summer precipitation seesaw between eastern and western Siberia. The results of this study raise the possibility that global aerosol forcing during boreal spring may play an important role in spawning atmospheric teleconnections that affect regional and global climates.

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Randal D. Koster
,
Y. C. Sud
,
Zhichang Guo
,
Paul A. Dirmeyer
,
Gordon Bonan
,
Keith W. Oleson
,
Edmond Chan
,
Diana Verseghy
,
Peter Cox
,
Harvey Davies
,
Eva Kowalczyk
,
C. T. Gordon
,
Shinjiro Kanae
,
David Lawrence
,
Ping Liu
,
David Mocko
,
Cheng-Hsuan Lu
,
Ken Mitchell
,
Sergey Malyshev
,
Bryant McAvaney
,
Taikan Oki
,
Tomohito Yamada
,
Andrew Pitman
,
Christopher M. Taylor
,
Ratko Vasic
, and
Yongkang Xue

Abstract

The Global Land–Atmosphere Coupling Experiment (GLACE) is a model intercomparison study focusing on a typically neglected yet critical element of numerical weather and climate modeling: land–atmosphere coupling strength, or the degree to which anomalies in land surface state (e.g., soil moisture) can affect rainfall generation and other atmospheric processes. The 12 AGCM groups participating in GLACE performed a series of simple numerical experiments that allow the objective quantification of this element for boreal summer. The derived coupling strengths vary widely. Some similarity, however, is found in the spatial patterns generated by the models, with enough similarity to pinpoint multimodel “hot spots” of land–atmosphere coupling. For boreal summer, such hot spots for precipitation and temperature are found over large regions of Africa, central North America, and India; a hot spot for temperature is also found over eastern China. The design of the GLACE simulations are described in full detail so that any interested modeling group can repeat them easily and thereby place their model’s coupling strength within the broad range of those documented here.

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