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Ning Zeng and J. David Neelin

Abstract

Using a coupled atmosphere–land–vegetation model of intermediate complexity, the authors explore how vegetation–climate interaction and internal climate variability might influence the vegetation distribution in Africa. When the model is forced by observed climatological sea surface temperature (SST), positive feedbacks from vegetation changes tend to increase the spatial gradient between desert regions and forest regions at the expense of savanna regions. When interannual variation of SST is included, the climate variability tends to reduce rainfall and vegetation in the wetter regions and to increase them in the drier regions along this gradient, resulting in a smoother desert–forest transition. This effect is most dramatically demonstrated in a model parameter regime for which multiple equilibria (either a desertlike or a forestlike Sahel) can exist when strong vegetation–climate feedbacks are allowed. However, the presence of a variable SST drives the desertlike state and the forestlike state toward an intermediate grasslike state, because of nonlinearities in the coupled system. Both vegetation and interannual variability thus play active roles in shaping the subtropical savanna ecosystem.

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J. David Neelin and Ning Zeng

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A class of model for simulation and theory of the tropical atmospheric component of climate variations is introduced. These models are referred to as quasi-equilibrium tropical circulation models, or QTCMs, because they make use of approximations associated with quasi-equilibrium (QE) convective parameterizations. Quasi-equilibrium convective closures tend to constrain the vertical temperature profile in convecting regions. This can be used to generate analytical solutions for the large-scale flow under certain approximations. A tropical atmospheric model of intermediate complexity is constructed by using the analytical solutions as the first basis function in a Galerkin representation of vertical structure. This retains much of the simplicity of the analytical solutions, while retaining full nonlinearity, vertical momentum transport, departures from QE, and a transition between convective and nonconvective zones based on convective available potential energy. The atmospheric model is coupled to a one-layer land surface model with interactive soil moisture and simulates its own tropical climatology. In the QTCM version presented here, the vertical structure of temperature variations is truncated to a single profile associated with deep convection. Though designed to be accurate in and near regions dominated by deep convection, the model simulates the tropical and subtropical climatology reasonably well, and even has a qualitative representation of midlatitude storm tracks.

The model is computationally economical, since part of the solution has been carried out analytically, but the main advantage is relative simplicity of analysis under certain conditions. The formulation suggests a slightly different way of looking at the tropical atmosphere than has been traditional in tropical meteorology. While convective scales are unstable, the large-scale motions evolve with a positive effective stratification that takes into account the partial cancellation of adiabatic cooling by diabatic heating. A consistent treatment of the moist static energy budget aids the analysis of radiative and surface heat flux effects. This is particularly important over land regions where the zero net surface flux links land surface anomalies. The resulting simplification highlights the role of top-of-the-atmosphere fluxes including cloud feedbacks, and it illustrates the usefulness of this approach for analysis of convective regions. Reductions of the model for theoretical work or diagnostics are outlined.

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Ning Zeng and J. David Neelin

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A theoretical framework is developed in understanding the mechanisms and processes determining the response of the land–atmosphere system to tropical deforestation. The analytical approach is made possible by simplifications in the vertical from the quasi-equilibrium moist convective closure, and in the horizontal from the dynamical temperature homogenization process. The theory emphasizes the energy and water balance. It highlights the interaction among processes of moist convection, cloud, radiation, and surface hydrology while each individual process is simplified. The zero surface energy flux condition, due to the small heat capacity of land, makes land–atmosphere interaction distinctly different from ocean–atmosphere interaction. This imposes a constraint on the sensitivity to the details of surface energy partitioning. Consequently, land surface temperature is largely a response to the energy and water balance, rather than a forcing as in the case of sea surface temperature.

Results from a wet-season surface albedo change case compare well with a recent RCCM2/BATS simulation, with the theory depicting the mechanisms and the roles of the intertwining processes. The precipitation has a significant decrease, initiated by ground radiative forcing as increased surface albedo reflects more solar radiation into space. A positive feedback by moisture convergence is essential for this tendency, with another positive feedback from reduced evaporation providing further enhancement. These are opposed by a negative feedback due to the reduced magnitude of negative cloud radiative forcing as cloud cover decreases. This sheds light on the higher sensitivity in some GCM studies with prescribed clouds. The cloud radiative forcing also has a negative feedback on the initial cooling tendency in ground temperature. Together with reduced evaporation, this leads to little change in the ground temperature. Sensitivities of precipitation and ground temperature changes to individual processes are found to depend on the reference state parameter values, implying a sensitivity of anomaly response to simulated climatology for GCMs. The analysis here also serves as an example of the tight coupling between convection, large-scale atmospheric dynamics, and land processes in the tropical land–atmosphere system.

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Ning Zeng, Katrina Hales, and J. David Neelin

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Although the global vegetation distribution is largely controlled by the large-scale climate pattern, the observed vegetation–rainfall relationship is also influenced by vegetation feedback and climate variability. Using a simplified coupled atmosphere–vegetation model, this work focuses on the effects of these on the gradient of desert–forest transition. A positive feedback from interactive vegetation leads to a wetter and greener state everywhere compared to a state without vegetation. As a result, the gradient in vegetation and rainfall is enhanced at places with moderate rainfall. Climate variability is found to reduce vegetation and rainfall in higher rainfall regions, while enhancing them in lower rainfall regions, thus smoothing out the desert–forest gradient. This latter effect is due to the nonlinear vegetation response to precipitation and it is particularly effective in the savanna regions. The analyses explain results from a three-dimensional climate model. The results suggest that in a varying environment, vegetation plays an active role in determining the observed vegetation–rainfall distributions.

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Katrina Hales, J. David Neelin, and Ning Zeng

Abstract

Paleoevidence indicates that generally wetter conditions existed in the Sahara during the mid-Holocene. Climate modeling studies addressing this issue generally agree that mid-Holocene values of the earth’s orbital parameters favored an enhanced North African summer monsoon but also suggest that land surface and vegetation feedbacks must have been important factors. Attempts to reproduce the “green” mid-Holocene Sahara in model studies with interactive vegetation may be interpreted to indicate that the problem is highly sensitive to the atmospheric dynamics of each model employed. In other work, dynamical mechanisms have been hypothesized to affect monsoon poleward extent, particularly ventilation, by import of low-moist static energy air to the continent. Here, interactive vegetation and the ventilation mechanism are studied in an intermediate complexity atmospheric model coupled to simple land and vegetation components. Interactive vegetation is found to be effective at enhancing the precipitation and vegetation amount in regions where the monsoon has advanced because of changes in orbital parameters or ventilation yet not very effective in moving the monsoon boundary if ventilation is strong. The poleward extent of the mid-Holocene monsoon and the steppe boundary are primarily controlled by the strength of ventilation in the atmospheric model. Within this boundary, the largest changes in monsoon precipitation and vegetation occur when interactive vegetation and reduced ventilation act simultaneously, as these greatly reinforce each other.

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Katrina Hales, J. David Neelin, and Ning Zeng

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Tropical land climate sensitivities to surface properties are studied using an intermediate complexity atmosphere model. The focus here is on land surface vegetation feedbacks to the atmosphere through surface conductance and albedo. Both properties are linked by a parameterization on leaf area index so that their relative impacts can be compared. For a given percent change in leaf area index, it is found that low and moderate vegetation regions such as the Sahel have a higher sensitivity than rain forest regions such as the Amazon in local total precipitation anomaly, as well as fractional change in precipitation. Comparison of sensitivities to changes in surface conductance and albedo shows that neither is negligible and their relative influence differs among local climatic regions, typified by different vegetation types. High precipitation rain forest regions are more influenced by surface conductance due to the large water recycling ratio there, while albedo has a larger influence in arid, low vegetation regions by modifying the energy balance and large-scale atmospheric circulation. In regions of moderate precipitation and vegetation, altered surface conductance and albedo have comparable effects on precipitation. Surface conductance and albedo have opposing effects on surface temperature but surface conductance has the dominant impact on both surface temperature and evapotranspiration.

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Ning Zeng, J. David Neelin, and Chia Chou

Abstract

The quasi-equilibrium tropical circulation model (QTCM1) is implemented and tested. The formulation, described by Neelin and Zeng, uses a Galerkin framework in the vertical, but with basis functions tailored to quasi-equilibrium deep convective physics via analytical solutions. QTCM1 retains a single vertical structure of temperature and humidity. For a balanced treatment of dynamics and subgrid-scale physics, a physics parameterization package of intermediate complexity is developed. This includes a linearized longwave radiation scheme, a simple cloud prediction method, simple shortwave radiation schemes, and an intermediate land surface model.

The simulated climatology has a reasonable spatial pattern and seasonal evolution of the tropical convergence zones, including over land regions. Outgoing longwave radiation and net surface heat flux both appear satisfactory. The Asian monsoon is slightly weak but depicts the northward progression of the monsoon onset, and a monsoon wind shear index exhibits interannual variability associated with observed SST that is similar to general circulation model (GCM) results. The extent and position of the main El Niño–Southern Oscillation rainfall anomalies are simulated, as well as a number of the observed tropical and subtropical teleconnections. The seasonal cycle and interannual variability of the Amazon water budget, including evapotranspiration, interception loss, and surface and subsurface runoff, illustrate reasonable simulation of the hydrologic cycle. Sensitivity studies on effects of topography, evaporation formulation, and land surface processes are also conducted. While the results are imperfect with respect to observations, many aspects are comparable to or better than GCMs of the previous generation. Considering the complexity of these simulated phenomena, the model is computationally light and easy to diagnose. It thus provides a useful tool filling the niche between GCMs and simpler models.

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Johnny Wei-Bing Lin, J. David Neelin, and Ning Zeng

Abstract

An intraseasonal tropical oscillation with a period of 20–80 days is simulated in the Neelin–Zeng Quasi-Equilibrium Tropical Circulation Model. This model is an intermediate-level atmospheric model that includes primitive equation nonlinearity, radiative– convective feedbacks, a simple land model with soil moisture, and a Betts–Miller-type moist convective adjustment parameterization. Vertical temperature and moisture structures in the model are based on quasi-equilibrium profiles taken from deep convective regions. The tropical intraseasonal variability is reasonably broadband. The eastward propagating 20–80-day variability is dominated by zonal wavenumber 1, shows features similar to an irregular Madden–Julian oscillation (MJO), and exhibits amplitude and phase speeds that vary both seasonally and between events. At higher wavenumbers, the model has a distinction between the low-frequency MJO-like band and the moist Kelvin wave band, similar to that found in observations. In the model, it is conjectured that this arises by interaction of the wavenumber-1 moist Kelvin wave with the zonally asymmetric basic state.

Experiments using climatological sea surface temperature forcing are conducted using this model to examine the effects of evaporation–wind feedback and extratropical excitation on the maintenance of intraseasonal variability, with particular attention paid to the low wavenumber mode in the 20–80-day band. These experiments indicate that evaporation–wind feedback partially organizes this intraseasonal variability by reducing damping, but it is not by itself sufficient to sustain this oscillation for the most realistic parameters. Excitation by extratropical variability is a major source of energy for the intraseasonal variability in this model. When midlatitude storms are suppressed, tropical intraseasonal variability is nearly eliminated. However, the eastward propagating intraseasonal signal appears most clearly when midlatitude excitation is aided by the evaporation–wind feedback.

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L. Huang, J. Zhai, C. Y. Sun, J. Y. Liu, J. Ning, and G.S. Zhao

Abstract

Land-use changes (LUCs) strongly influence regional climates through both the biogeochemical and biogeophysical processes. However, many studies have ignored the biogeophysical processes, which in some cases can offset the biogeochemical impacts. We integrated the field observations, satellite-retrieved data, and a conceptual land surface energy balance model to provide new evidence to fill our knowledge gap concerning how regional warming or cooling is affected by the three main types of LUCs (afforestation, cropland expansion, and urbanization) in different climate zones of China. According to our analyses, similar LUCs presented varied, even reverse, biogeophysical forcing on local temperatures across different climate regimes. Afforestation in arid and semiarid regions has caused increased net radiation that has typically outweighed increased latent evapotranspiration, thus warming has been the net biogeophysical effect. However, it has resulted in cooling in subtropical zones because the increase in net radiation has been exceeded by the increase in latent evapotranspiration. Cropland expansion has decreased the net radiation more than latent evapotranspiration, which has resulted in biogeophysical cooling in arid and semiarid regions. Conversely, it has caused warming in subtropical zones as a result of increases in net radiation and decreases in latent evapotranspiration. In all climatic regions, the net biogeophysical effects of urbanization have generally resulted in more or less warming because urbanization has led to smaller net radiation decreases than latent evapotranspiration. This study reinforces the need to adjust land-use policies to consider biogeophysical effects across different climate regimes and to adapt to and mitigate climate change.

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T. Ning, J. Wickert, Z. Deng, S. Heise, G. Dick, S. Vey, and T. Schöne

Abstract

The potential temporal shifts in the integrated water vapor (IWV) time series obtained from reprocessed data acquired from global navigation satellite systems (GNSS) were comprehensively investigated. A statistical test, the penalized maximal t test modified to account for first-order autoregressive noise in time series (PMTred), was used to identify the possible mean shifts (changepoints) in the time series of the difference between the GPS IWV and the IWV obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) data. The approach allows for identification of the changepoints not only in the GPS IWV time series but also in ERA-Interim. The IWV difference time series formed for 101 GPS sites were tested, where 47 of them were found to contain in total 62 changepoints. The results indicate that 45 detected changepoints were due to the inconsistencies in the GPS IWV time series, and 16 were related to ERA-Interim, while one point was left unverified. After the correction of the mean shifts for the GPS data, an improved consistency in the IWV trends is evident between nearby sites, while a better agreement is seen between the trends from the GPS and ERA-Interim data on a global scale. In addition, the IWV trends estimated for 47 GPS sites were compared to the corresponding IWV trends obtained from nearby homogenized radiosonde data. The correlation coefficient of the trends increases significantly by 38% after using the homogenized GPS data.

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