scholarly journals Intensification of North American Megadroughts through Surface and Dust Aerosol Forcing*

2013 ◽  
Vol 26 (13) ◽  
pp. 4414-4430 ◽  
Author(s):  
Benjamin I. Cook ◽  
Richard Seager ◽  
Ron L. Miller ◽  
Joseph A. Mason
Keyword(s):  
Boundary Conditions ◽  
Land Surface ◽  
General Circulation ◽  
Atmospheric Stability ◽  
Circulation Model ◽  
Dust Aerosol ◽  
Central Plains ◽  
Model Experiments ◽  
Aerosol Forcing ◽  
The Impact

Abstract Tree-ring-based reconstructions of the Palmer drought severity index (PDSI) indicate that, during the Medieval Climate Anomaly (MCA), the central plains of North America experienced recurrent periods of drought spanning decades or longer. These megadroughts had exceptional persistence compared to more recent events, but the causes remain uncertain. The authors conducted a suite of general circulation model experiments to test the impact of sea surface temperature (SST) and land surface forcing on the MCA megadroughts over the central plains. The land surface forcing is represented as a set of dune mobilization boundary conditions, derived from available geomorphological evidence and modeled as increased bare soil area and a dust aerosol source (32°–44°N, 105°–95°W). In the experiments, cold tropical Pacific SST forcing suppresses precipitation over the central plains but cannot reproduce the overall drying or persistence seen in the PDSI reconstruction. Droughts in the scenario with dust aerosols, however, are amplified and have significantly longer persistence than in other model experiments, more closely matching the reconstructed PDSI. This additional drying occurs because the dust increases the shortwave planetary albedo, reducing energy inputs to the surface and boundary layer. The energy deficit increases atmospheric stability, inhibiting convection and reducing cloud cover and precipitation over the central plains. Results from this study provide the first model-based evidence that dust aerosol forcing and land surface changes could have contributed to the intensity and persistence of the central plains megadroughts, although uncertainties remain in the formulation of the boundary conditions and the future importance of these feedbacks.

The Role of Radiative Interactions in Tropical Cyclone Development under Realistic Boundary Conditions

2020 ◽  
pp. 1-38
Author(s):  
Bosong Zhang ◽  
Brian J. Soden ◽  
Gabriel A. Vecchi ◽  
Wenchang Yang
Keyword(s):  
Boundary Conditions ◽  
General Circulation ◽  
Circulation Model ◽  
Sea Surface ◽  
Latent Heat Release ◽  
Synoptic Scale ◽  
Spatially Varying ◽  
The Impact ◽  
Realistic Boundary Conditions

AbstractThe impact of radiative interactions on tropical cyclones (TC) climatology is investigated using a global, TC-permitting general circulation model (GCM) with realistic boundary conditions. In this model, synoptic-scale radiative interactions are suppressed by overwriting the model-generated atmospheric radiative cooling rates with its monthly-varying climatological values. When radiative interactions are suppressed, the global TC frequency is significantly reduced, indicating that radiative interactions are a critical component of TC development even in the presence of spatially varying boundary conditions. The reduced TC activity is primarily due to a decrease in the frequency of pre-TC synoptic disturbances (“seeds”), whereas the likelihood that the seeds undergo cyclogenesis is less affected. When radiative interactions are suppressed, TC genesis shifts toward coastal regions, whereas TC lysis locations stay almost unchanged; together the distance between genesis and lysis is shortened, reducing TC duration. In a warmer climate, the magnitude of TC reduction from suppressing radiative interactions is diminished due to the larger contribution from latent heat release with increased sea surface temperatures. These results highlight the importance of radiative interactions in modulating the frequency and duration of TCs.

scholarly journals Impacts of soil-aquifer heat and water fluxes on simulated global climate

2013 ◽  
Vol 10 (1) ◽  
pp. 1185-1212 ◽  
Author(s):  
N. Y. Krakauer ◽  
M. J. Puma ◽  
B. I. Cook
Keyword(s):  
Soil Moisture ◽  
Land Surface ◽  
General Circulation ◽  
Soil Column ◽  
Circulation Model ◽  
The Arctic ◽  
Water Fluxes ◽  
Limited Impact ◽  
Mean Climate ◽  
The Impact

Abstract. Climate models have traditionally only represented heat and water fluxes within relatively shallow soil layers, but there is increasing interest in the possible role of heat and water exchanges with the deeper subsurface. Here, we integrate an idealized 50 m deep aquifer into the land surface module of the GISS ModelE general circulation model to test the influence of aquifer-soil moisture and heat exchanges on climate variables. We evaluate the impact on the modeled climate of aquifer-soil heat and water fluxes separately, as well as in combination. The addition of the aquifer to ModelE has limited impact on annual-mean climate, with little change in global mean land temperature, precipitation, or evaporation. The seasonal amplitude of deep soil temperature is strongly damped by the soil-aquifer heat flux. This not only improves the model representation of permafrost area but propagates to the surface, resulting in an increase in the seasonal amplitude of surface air temperature of >1 K in the Arctic. The soil-aquifer water and heat fluxes both slightly decrease interannual variability in soil moisture and land-surface temperature, and decrease the soil moisture memory of the land surface on annual timescales. The results of this experiment suggest that deepening the modeled land surface, compared to modeling only a shallower soil column with a no-flux bottom boundary condition, has limited impact on mean climate but does affect seasonality and interannual persistence.

scholarly journals On the impact of recent developments of the LMDz atmospheric general circulation model on the simulation of CO<sub>2</sub> transport

2018 ◽  
Vol 11 (11) ◽  
pp. 4489-4513 ◽  
Author(s):  
Marine Remaud ◽  
Frédéric Chevallier ◽  
Anne Cozic ◽  
Xin Lin ◽  
Philippe Bousquet
Keyword(s):  
Land Surface ◽  
General Circulation ◽  
Circulation Model ◽  
Surface Model ◽  
Strong Impact ◽  
Vertical Resolution ◽  
Atmospheric General Circulation ◽  
Inverse Systems ◽  
The Impact ◽  
Physical Parameterizations

Abstract. The quality of the representation of greenhouse gas (GHG) transport in atmospheric general circulation models (GCMs) drives the potential of inverse systems to retrieve GHG surface fluxes to a large extent. In this work, the transport of CO2 is evaluated in the latest version of the Laboratoire de Météorologie Dynamique (LMDz) GCM, developed for the Climate Model Intercomparison Project 6 (CMIP6) relative to the LMDz version developed for CMIP5. Several key changes have been implemented between the two versions, which include a more elaborate radiative scheme, new subgrid-scale parameterizations of convective and boundary layer processes and a refined vertical resolution. We performed a set of simulations of LMDz with different physical parameterizations, two different horizontal resolutions and different land surface schemes, in order to test the impact of those different configurations on the overall transport simulation. By modulating the intensity of vertical mixing, the physical parameterizations control the interhemispheric gradient and the amplitude of the seasonal cycle in the Northern Hemisphere, as emphasized by the comparison with observations at surface sites. However, the effect of the new parameterizations depends on the region considered, with a strong impact over South America (Brazil, Amazonian forest) but a smaller impact over Europe, East Asia and North America. A finer horizontal resolution reduces the representation errors at observation sites near emission hotspots or along the coastlines. In comparison, the sensitivities to the land surface model and to the increased vertical resolution are marginal.

scholarly journals On the impact of recent developments of an atmospheric general circulation model on the simulation of CO<sub>2</sub> transport

2018 ◽  
Author(s):  
Marine Remaud ◽  
Frédéric Chevallier ◽  
Anne Cozic ◽  
Xin Lin ◽  
Philippe Bousquet
Keyword(s):  
Land Surface ◽  
General Circulation ◽  
Circulation Model ◽  
Surface Model ◽  
Strong Impact ◽  
Vertical Resolution ◽  
Atmospheric General Circulation ◽  
Inverse Systems ◽  
The Impact ◽  
Physical Parameterizations

Abstract. The quality of the representation of greenhouse gas (GHG) transport in atmospheric General Circulation Models (GCMs) drives the potential of inverse systems to retrieve GHG surface fluxes to a large extent. In this work, the transport of CO2 is evaluated in the latest version of the LMDz GCM, developed for the Climate Model Intercomparison Project 6 (CMIP6) relative to the LMDz version developed for CMIP4. Several key changes have been implemented between the two versions; those include a more elaborate radiative scheme, new sub-grid scale parameterizations of convective and boundary layer processes, and a refined vertical resolution. We performed a set of simulations of LMDz with the different physical parameterizations, two different horizontal resolutions and different land surface schemes, in order to test the impact of those different configurations on the overall transport simulation. By modulating the intensity of vertical mixing, the physical parameterizations control the interhemispheric gradient and the amplitude of the seasonal cycle in the summer northern hemisphere, as emphasized by the comparison with observations at surface sites. However, the effect of the new parameterizations depends on the region considered, with a strong impact over South America (Brazil, Amazonian forest) but a smaller impact over Europe, Eastern Asia and North America. A finer horizontal resolution reduces the representation errors at observation sites near emission-hot spots or along the coastlines. In comparison, the sensitivities to the land surface model and to the increased vertical resolution are marginal.

scholarly journals Impacts of soil–aquifer heat and water fluxes on simulated global climate

2013 ◽  
Vol 17 (5) ◽  
pp. 1963-1974 ◽  
Author(s):  
N. Y. Krakauer ◽  
M. J. Puma ◽  
B. I. Cook
Keyword(s):  
Soil Moisture ◽  
Land Surface ◽  
General Circulation ◽  
Soil Column ◽  
Circulation Model ◽  
The Arctic ◽  
Water Fluxes ◽  
Limited Impact ◽  
Mean Climate ◽  
The Impact

Abstract. Climate models have traditionally only represented heat and water fluxes within relatively shallow soil layers, but there is increasing interest in the possible role of heat and water exchanges with the deeper subsurface. Here, we integrate an idealized 50 m deep aquifer into the land surface module of the GISS ModelE general circulation model to test the influence of aquifer–soil moisture and heat exchanges on climate variables. We evaluate the impact on the modeled climate of aquifer–soil heat and water fluxes separately, as well as in combination. The addition of the aquifer to ModelE has limited impact on annual-mean climate, with little change in global mean land temperature, precipitation, or evaporation. The seasonal amplitude of deep soil temperature is strongly damped by the soil–aquifer heat flux. This not only improves the model representation of permafrost area but propagates to the surface, resulting in an increase in the seasonal amplitude of surface air temperature of > 1 K in the Arctic. The soil–aquifer water and heat fluxes both slightly decrease interannual variability in soil moisture and in land-surface temperature, and decrease the soil moisture memory of the land surface on seasonal to annual timescales. The results of this experiment suggest that deepening the modeled land surface, compared to modeling only a shallower soil column with a no-flux bottom boundary condition, has limited impact on mean climate but does affect seasonality and interannual persistence.

scholarly journals Impact of absorbing aerosols on the simulation of climate over the Indian region in an atmospheric general circulation model

2004 ◽  
Vol 22 (5) ◽  
pp. 1421-1434 ◽  
Author(s):  
A. Chakraborty ◽  
S. K. Satheesh ◽  
R. S. Nanjundiah ◽  
J. Srinivasan
Keyword(s):  
General Circulation Model ◽  
General Circulation ◽  
Monsoon Season ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Indian Region ◽  
Radiative Heating ◽  
Aerosol Forcing ◽  
Absorbing Aerosols ◽  
The Impact

Abstract. The impact of anthropogenic absorbing aerosols (such as soot) on the climate over the Indian region has been studied using the NCMRWF general circulation model. The absorbing aerosols increase shortwave radiative heating of the lower troposphere and reduce the heating at the surface. These effects have been incorporated as heating of the lower troposphere (up to 700hPa) and cooling over the continental surface based on INDOEX measurements. The heating effect is constant in the pre-monsoon season and reduces to zero during the monsoon season. It is shown that even in the monsoon season when the aerosol forcing is zero, there is an overall increase in rainfall and a reduction in surface temperature over the Indian region. The rainfall averaged over the Tropics shows a small reduction in most of the months during the January to September period. The impact of aerosol forcing, the model's sensitivity to this forcing and its interaction with model-physics has been studied by changing the cumulus parameterization from the Simplified Arakawa-Schubert (SAS) scheme to the Kuo scheme. During the pre-monsoon season the major changes in precipitation occur in the oceanic Inter Tropical Convergence Zone (ITCZ), where both the schemes show an increase in precipitation. This result is similar to that reported in Chung2002. On the other hand, during the monsoon season the changes in precipitation in the continental region are different in the SAS and Kuo schemes. It is shown that the heating due to absorbing aerosols changes the vertical moist-static stability of the atmosphere. The difference in the precipitation changes in the two cumulus schemes is on account of the different responses in the two parameterization schemes to changes in vertical stability. Key words. Atmospheric composition and structure (aerosols and particles) – Meteorology and atmospheric dynamics (tropical meteorology; precipitation)

scholarly journals The role of boundary and initial conditions for dynamical seasonal predictability

2003 ◽  
Vol 10 (3) ◽  
pp. 211-232 ◽  
Author(s):  
T. J. Reichler ◽  
J. O. Roads
Keyword(s):  
Boundary Conditions ◽  
Time Scales ◽  
General Circulation ◽  
Initial Conditions ◽  
Spatial Scales ◽  
Circulation Model ◽  
Forecast Skill ◽  
Initial State ◽  
Tropical Forcing ◽  
The Impact

Abstract. The importance of initial state and boundary forcing for atmospheric predictability is explored on global to regional spatial scales and on daily to seasonal time scales. A general circulation model is used to conduct predictability experiments with different combinations of initial and boundary conditions. The experiments are verified under perfect model assumptions as well as against observational data. From initial conditions alone, there is significant instantaneous forecast skill out to 2 months. Different initial conditions show different predictability using the same kind of boundary forcing. Even on seasonal time scales, using observed atmospheric initial conditions leads to a substantial increase in overall skill, especially during periods with weak tropical forcing. The impact of boundary forcing on predictability is detectable after 10 days and leads to measurable instantaneous forecast skill at very long lead times. Over the Northern Hemisphere, it takes roughly 4 weeks for boundary conditions to reach the same effect on predictability as initial conditions. During events with strong tropical forcing, these time scales are somewhat shorter. Over the Southern Hemisphere, there is a strongly enhanced influence of initial conditions during summer. We conclude that the long term memory of initial conditions is important for seasonal forecasting.

scholarly journals Investigating the impact of land surface characteristics on monsoon dynamics with idealized model simulations and theories

2021 ◽  
pp. 1-49
Author(s):  
Jane E. Smyth ◽  
Yi Ming
Keyword(s):  
Land Surface ◽  
General Circulation ◽  
Circulation Model ◽  
South American ◽  
Moist Convection ◽  
Modeling Framework ◽  
Dry Land ◽  
Near Surface ◽  
Deep Moist Convection ◽  
The Impact

AbstractMonsoons emerge over a range of land surface conditions and exhibit varying physical characteristics over the seasonal cycle, from onset to withdrawal. Systematically varying the moisture and albedo parameters over land in an idealized modeling framework allows one to analyze the physics underlying the successive stages of monsoon development. To this end we implement an isolated South American continent with reduced heat capacity but no topography in an idealized moist general circulation model. Irrespective of the local moisture availability, the seasonal cycles of precipitation and circulation over the South American monsoon sector are distinctly monsoonal with the default surface albedo. The dry land case (zero evaporation) is characterized by a shallow overturning circulation with vigorous lower-tropospheric ascent, transporting water vapor from the ocean. By contrast, with bucket hydrology or unlimited land moisture the monsoon features deep moist convection that penetrates the upper troposphere. A series of land albedo perturbation experiments indicates that the monsoon strengthens with the net column energy flux and the near-surface moist static energy with all land moisture conditions. When the land-ocean thermal contrast is strong enough, inertial instability alone is sufficient for producing a shallow but vigorous circulation and converging a large amount of moisture from the ocean even in the absence of land moisture. Once the land is sufficiently moist, convective instability takes hold and the shallow circulation deepens. These results have implications for monsoon onset and intensification, and may elucidate the seasonal variations in how surface warming impacts tropical precipitation over land.

The Impact of Radiative Interactions on Tropical Cyclone Development in a General Circulation Model

2021 ◽  
Author(s):  
Bosong Zhang ◽  
Brian Soden ◽  
Gabriel Vecchi ◽  
Wenchang Yang
Keyword(s):  
Tropical Cyclone ◽  
Boundary Conditions ◽  
General Circulation Model ◽  
General Circulation ◽  
Circulation Model ◽  
Sea Surface ◽  
Latent Heat Release ◽  
Synoptic Scale ◽  
Spatially Varying ◽  
The Impact

&lt;p&gt;The impact of radiative interactions on tropical cyclone (TC) climatology is investigated using a global, TC-permitting general circulation model (GCM) with realistic boundary conditions. In this model, synoptic-scale radiative interactions are suppressed by overwriting the model-generated atmospheric radiative cooling rates with its monthly-varying climatological values. When radiative interactions are suppressed, the global TC frequency is significantly reduced, indicating that radiative interactions are a critical component of TC development even in the presence of spatially varying boundary conditions. The reduced TC activity is primarily due to a decrease in the frequency of pre-TC synoptic disturbances (&amp;#8220;seeds&amp;#8221;), whereas the likelihood that the seeds undergo cyclogenesis is less affected. When radiative interactions are suppressed, TC genesis shifts toward coastal regions, whereas TC lysis locations stay almost unchanged; together the distance between genesis and lysis is shortened, reducing TC duration. In a warmer climate, the magnitude of TC reduction from suppressing radiative interactions is diminished due to the larger contribution from latent heat release with increased sea surface temperatures. These results highlight the importance of radiative interactions in modulating the frequency and duration of TCs.&lt;/p&gt;

The simulated response of the climate system to changes in soil moisture parameterization under paleoclimatic boundary conditions at 6000 years before present

2000 ◽  
Vol 37 (5) ◽  
pp. 635-660 ◽  
Author(s):  
G Vettoretti ◽  
W R Peltier ◽  
N A McFarlane ◽  
P M I P participating groups
Keyword(s):  
Boundary Conditions ◽  
Surface Temperature ◽  
Land Surface ◽  
Radiative Forcing ◽  
General Circulation ◽  
Circulation Model ◽  
Control Model ◽  
Before Present ◽  
Surface Parameterization ◽  
Modern Control

We present the results of a sensitivity study involving modifications to the simple land surface scheme implemented in the second-generation atmospheric general circulation model (GCMII) of the Canadian Climate Centre for Modelling and Analysis (CCCma), under paleoclimatic boundary conditions characteristic of 6000 calendar years before present (6 ka BP). The land surface parameterization is modified in two primary respects. Firstly, we modify the space dependant bucket depth scheme in the original model to one in which this depth is taken to be constant. Secondly, we modify the evapotranspiration parameterization from the space dependant form employed in the control model to a more conventional space independent scheme. In all, 4 experiments have been performed to enable us to resolve both the modern control and the 6 ka BP response to the land surface modifications. A subset of these simulations is also compared with results obtained using other models in the context of the Paleoclimate Model Intercomparison Project (PMIP) to investigate the mid-latitude behaviour of these models to reveal the extent to which model response to a change in radiative forcing may be significantly influenced by changes in the land surface parameterization. These comparisons reveal that the original Canadian GCMII model is an extreme outlier among the members of the set of all models in that its Northern Hemisphere mid-latitude surface continental response to the 6 ka BP insolation anomaly is significantly cold biased in the summer season. We investigate the extent to which this anomalous behaviour may be explained as a consequence of modifications to the land surface parameterizations employed in GCMII. Our results reveal a strong sensitivity in the modern control model to changes in bucket depth, but not to the modification of the evapotranspiration scheme. More interesting, however, is the fact that the model climate sensitivity at 6 ka BP is influenced both by changes in bucket depth and by changes in the evapotranspiration scheme. A detailed investigation of the surface energy balance, cloud cover, surface albedo, and snow cover reveals the role of each of the components that contribute to the 6 ka BP surface temperature response. Comparison of the predictions of the CCCma model with proxy climate indicators of lake levels and surface temperature over Canada, furthermore, demonstrate the extreme sensitivity of climate predictions for this geographical region to changes in the manner in which land surface processes are represented.

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