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Abstract
Even though five different general circulation models are all currently producing about a 4° ± 1°C warming for doubled CO2, there is still substantial model disagreement about the degree of high latitude amplification of the surface temperature change. The consequences of this disagreement are investigated by comparing doubled CO2 climates with different latitudinal gradients of sea surface temperature. The GISS 4° × 5° general circulation model (GCM) was run with doubled CO2 and two sets of sea surface temperatures: one set derived from the equilibrium doubled CO2 run of the 8° × 10° GISS GCM, with minimal high latitude amplification, and the other set more closely resembling the GFDL results, with greater amplification. While the experiments differ in their latitudinal distribution of warming, they have the same global mean surface air temperature change. The differences in energy balance, atmospheric dynamics and regional climate simulations are discussed.
The results show that the two experiments often produce substantially different climate characteristics. With reduced high latitude amplification, and thus more equatorial warming, there is a greater increase in specific humidity and the greenhouse capacity (the concentration of infrared-absorbing gases) of the atmosphere, resulting in a warmer atmosphere in general. Features such as the low latitude precipitation, Hadley cell intensity, jet stream magnitude and atmospheric energy transports all increase compared to the control run. In contrast, these features all decrease in the experiment with greater high latitude amplification. There are also significant differences in the cloud cover and stationary eddy energy responses between the two experiments, as well as most regional climate changes; for example, there is greater drying of the midlatitude summer continents and greater polar ice melting when the high latitude amplification is greater. Predictions of the coming doubled CO2 climate and its societal consequences must be tempered by the current uncertainty in the degree of high latitude amplification.
Abstract
Even though five different general circulation models are all currently producing about a 4° ± 1°C warming for doubled CO2, there is still substantial model disagreement about the degree of high latitude amplification of the surface temperature change. The consequences of this disagreement are investigated by comparing doubled CO2 climates with different latitudinal gradients of sea surface temperature. The GISS 4° × 5° general circulation model (GCM) was run with doubled CO2 and two sets of sea surface temperatures: one set derived from the equilibrium doubled CO2 run of the 8° × 10° GISS GCM, with minimal high latitude amplification, and the other set more closely resembling the GFDL results, with greater amplification. While the experiments differ in their latitudinal distribution of warming, they have the same global mean surface air temperature change. The differences in energy balance, atmospheric dynamics and regional climate simulations are discussed.
The results show that the two experiments often produce substantially different climate characteristics. With reduced high latitude amplification, and thus more equatorial warming, there is a greater increase in specific humidity and the greenhouse capacity (the concentration of infrared-absorbing gases) of the atmosphere, resulting in a warmer atmosphere in general. Features such as the low latitude precipitation, Hadley cell intensity, jet stream magnitude and atmospheric energy transports all increase compared to the control run. In contrast, these features all decrease in the experiment with greater high latitude amplification. There are also significant differences in the cloud cover and stationary eddy energy responses between the two experiments, as well as most regional climate changes; for example, there is greater drying of the midlatitude summer continents and greater polar ice melting when the high latitude amplification is greater. Predictions of the coming doubled CO2 climate and its societal consequences must be tempered by the current uncertainty in the degree of high latitude amplification.
Abstract
Climate model results are now being used to asses the potential societal impact of climate change, and to compare with paleoclimate indicators. The models used for these purposes currently employ relatively coarse resolution, and a key question is how the results might change as resolution is improved. To examine this issue, doubled-CO2 and ice age simulations with boundary conditions identical for two different resolutions are run with the GISS model. The resolution dependency of climate change sensitivity, atmospheric dynamics, and regional climate depiction are discussed.
The results show that model resolution affects two key processes in the control runs, moist convection and the nonlinear transfer of kinetic energy into the zonal mean flow. The finer resolution model has more penetrative convection but less convection overall, aspects which alter its temperature and wind structure relative to those of the coarser grid model. With finer resolution there are also stronger winds, more evaporation and a more active hydrological cycle. While some of these changes are not particularly, their characteristics are mirrored in the warm and cold climate simulations.
In comparison with the coarser resolution model, the finer grid doubled CO2 run has a greater decrease in high-level cloud cover, eddy energy, and eddy energy transports, and a greater increase in atmospheric temperature surface winds, precipitation, and penetrative convection. The ice age finer grid run shows the opposite effect when compared with the medium grid: greater eddy energy and eddy transport increases, greater reduction in hydrologic cycle and atmospheric temperature. Regional climate changes also differ with resolution, due to both the local expression of the different dynamical responses and the differing spatial possibilities. The development of higher resolution models, and the practical use of climate change results, should incorporate an awareness of the potential impact of resolution on model process and climate change depiction.
Abstract
Climate model results are now being used to asses the potential societal impact of climate change, and to compare with paleoclimate indicators. The models used for these purposes currently employ relatively coarse resolution, and a key question is how the results might change as resolution is improved. To examine this issue, doubled-CO2 and ice age simulations with boundary conditions identical for two different resolutions are run with the GISS model. The resolution dependency of climate change sensitivity, atmospheric dynamics, and regional climate depiction are discussed.
The results show that model resolution affects two key processes in the control runs, moist convection and the nonlinear transfer of kinetic energy into the zonal mean flow. The finer resolution model has more penetrative convection but less convection overall, aspects which alter its temperature and wind structure relative to those of the coarser grid model. With finer resolution there are also stronger winds, more evaporation and a more active hydrological cycle. While some of these changes are not particularly, their characteristics are mirrored in the warm and cold climate simulations.
In comparison with the coarser resolution model, the finer grid doubled CO2 run has a greater decrease in high-level cloud cover, eddy energy, and eddy energy transports, and a greater increase in atmospheric temperature surface winds, precipitation, and penetrative convection. The ice age finer grid run shows the opposite effect when compared with the medium grid: greater eddy energy and eddy transport increases, greater reduction in hydrologic cycle and atmospheric temperature. Regional climate changes also differ with resolution, due to both the local expression of the different dynamical responses and the differing spatial possibilities. The development of higher resolution models, and the practical use of climate change results, should incorporate an awareness of the potential impact of resolution on model process and climate change depiction.
Abstract
Future climate change could have significant repercussions for lightning-caused wildfires. Two empirical fire models are presented relating the frequency of lightning fires and the area burned by these fires to the elective precipitation and the frequency of thunderstorm activity. One model deals with the seasonal variations in lightning fires, while the second model deals with the interannual variations of lightning fires. These fire models are then used with the Goddard Institute for Space Studies General Circulation Model to investigate possible changes in fire frequency and area burned in a 2 × CO2 climate. In the United States, the annual mean number of lightning fires increases by 44%, while the area burned increases by 78%. On a global scale, the largest increase in lightning fires can be expected in untouched tropical ecosystems where few natural fires occur today.
Abstract
Future climate change could have significant repercussions for lightning-caused wildfires. Two empirical fire models are presented relating the frequency of lightning fires and the area burned by these fires to the elective precipitation and the frequency of thunderstorm activity. One model deals with the seasonal variations in lightning fires, while the second model deals with the interannual variations of lightning fires. These fire models are then used with the Goddard Institute for Space Studies General Circulation Model to investigate possible changes in fire frequency and area burned in a 2 × CO2 climate. In the United States, the annual mean number of lightning fires increases by 44%, while the area burned increases by 78%. On a global scale, the largest increase in lightning fires can be expected in untouched tropical ecosystems where few natural fires occur today.
Abstract
A general circulation model (GCM) is used to model global lightning distributions and frequencies. Both total and cloud-to-ground lightning frequencies are modeled using parameterizations that relate the depth of convective clouds to lightning frequencies. The model's simulations of lightning distributions in time and space show good agreement with available observations. The model's annual mean climatology shows a global lightning frequency of 77 flashes per second, with cloud-to-ground lightning making up 25% of the total. The maximum lightning activity in the GCM occurs during the Northern Hemisphere summer, with approximately 91% of all lightning occurring over continental and coastal regions.
Abstract
A general circulation model (GCM) is used to model global lightning distributions and frequencies. Both total and cloud-to-ground lightning frequencies are modeled using parameterizations that relate the depth of convective clouds to lightning frequencies. The model's simulations of lightning distributions in time and space show good agreement with available observations. The model's annual mean climatology shows a global lightning frequency of 77 flashes per second, with cloud-to-ground lightning making up 25% of the total. The maximum lightning activity in the GCM occurs during the Northern Hemisphere summer, with approximately 91% of all lightning occurring over continental and coastal regions.
Abstract
Large-scale snow cover anomalies are thought to cause significant changes in the diabatic heating of the earth's surface in such a way as to produce substantial local cooling in the surface temperatures. This theory was tested using the GISS 3-D GCM (General Circulation Model). The results of the GCM experiment showed that snow cover caused only a short term local decrease in the surface temperature. In the surface energy budget, reduction in absorbed shortwave radiation and the increased latent heat sink of melting snow contributed to lower temperatures. However, all the remaining heating terms contribute to increasing the net heating over a snow covered surface. The results emphasize the negative feedback which limits the impact of snow cover anomalies over longer time scales.
Abstract
Large-scale snow cover anomalies are thought to cause significant changes in the diabatic heating of the earth's surface in such a way as to produce substantial local cooling in the surface temperatures. This theory was tested using the GISS 3-D GCM (General Circulation Model). The results of the GCM experiment showed that snow cover caused only a short term local decrease in the surface temperature. In the surface energy budget, reduction in absorbed shortwave radiation and the increased latent heat sink of melting snow contributed to lower temperatures. However, all the remaining heating terms contribute to increasing the net heating over a snow covered surface. The results emphasize the negative feedback which limits the impact of snow cover anomalies over longer time scales.
Abstract
The relatively low frequency of the sonic boom generated by the Concorde SST permits propagation in the form of infrasound to long range with small attenuation. Signal characteristics at long range are a function of atmospheric propagation parameters. When the relationship of propagation to signal is understood, then propagation conditions can be determined by inversion with good accuracy. We show here how signal recorded at Palisades, New York, from the Dulles-bound SST reveals direction and speed of stratospheric wind variations diurnally and seasonally and also gives details of at least local circulation change at times of stratospheric warmings.
Abstract
The relatively low frequency of the sonic boom generated by the Concorde SST permits propagation in the form of infrasound to long range with small attenuation. Signal characteristics at long range are a function of atmospheric propagation parameters. When the relationship of propagation to signal is understood, then propagation conditions can be determined by inversion with good accuracy. We show here how signal recorded at Palisades, New York, from the Dulles-bound SST reveals direction and speed of stratospheric wind variations diurnally and seasonally and also gives details of at least local circulation change at times of stratospheric warmings.
Abstract
The relationship between midtropospheric synoptic features and midstratospheric temperature in winter is investigated by examining averages of 5–10 yr of observations, monthly mean observations, and daily records. It is found that midstratospheric warm regions lie above midtropospheric troughs and subtropical ridges, while stratospheric cold regions occur above high-latitude tropospheric ridges. Thus, at high latitudes, an inverse correlation exists between 500-mb height and 10-mb temperatures; this correlation seems to be simultaneous in nature. The implications of these results are discussed with relation to the general circulation of the stratosphere, and in particular to the relative importance of hydrostatic adjustment, planetary wave propagation, and tidal energy.
Abstract
The relationship between midtropospheric synoptic features and midstratospheric temperature in winter is investigated by examining averages of 5–10 yr of observations, monthly mean observations, and daily records. It is found that midstratospheric warm regions lie above midtropospheric troughs and subtropical ridges, while stratospheric cold regions occur above high-latitude tropospheric ridges. Thus, at high latitudes, an inverse correlation exists between 500-mb height and 10-mb temperatures; this correlation seems to be simultaneous in nature. The implications of these results are discussed with relation to the general circulation of the stratosphere, and in particular to the relative importance of hydrostatic adjustment, planetary wave propagation, and tidal energy.
Abstract
This is a further study of the use of natural infrasound in the atmosphere to monitor tidal circulation in the lower thermosphere. The height of this circulation is determined with the use of a reference atmospheric model, which is then used to calibrate infrasound/microseism ratios in terms of height. Also, we show from continuous observation over six years at 41°N, 74°W that the winter semidiurnal tide is present at least 62% of the time and the diurnal, at least 42% of the time.
Abstract
This is a further study of the use of natural infrasound in the atmosphere to monitor tidal circulation in the lower thermosphere. The height of this circulation is determined with the use of a reference atmospheric model, which is then used to calibrate infrasound/microseism ratios in terms of height. Also, we show from continuous observation over six years at 41°N, 74°W that the winter semidiurnal tide is present at least 62% of the time and the diurnal, at least 42% of the time.
Abstract
The Hadley cell is involved in the energy, momentum and moisture budgets in the atmosphere; it may be expected to change as sources and sinks of these quantities are altered due to climate perturbations. The nature of the Hadley cell change is complicated since alterations in one budget generally result in alterations in the others. Thus, Hadley cell sensitivity needs to be explored in an interactive system. In the GISS GCM (model I), a number of experiments are performed in which physical processes in each of the three budgets are omitted, the system adjusts, and the resultant circulation is compared to that of the control run. This procedure highlights which effects are most important and reveals the nature of the various interactions.
The results emphasize the wide variety of processes that appear capable of influencing the mean circulation. The intensity of the circulation is related to the coherence of the thermal forcing, and to the thermal opacity of the atmosphere. When all frictional forcing is removed, the circulation is restricted to the equatorial region. The latitudinal extent appears to be controlled primarily by eddy processes (Ferrel cell intensity). The implications for climate modeling and climate projections (e.g., rainfall changes) are discussed.
Abstract
The Hadley cell is involved in the energy, momentum and moisture budgets in the atmosphere; it may be expected to change as sources and sinks of these quantities are altered due to climate perturbations. The nature of the Hadley cell change is complicated since alterations in one budget generally result in alterations in the others. Thus, Hadley cell sensitivity needs to be explored in an interactive system. In the GISS GCM (model I), a number of experiments are performed in which physical processes in each of the three budgets are omitted, the system adjusts, and the resultant circulation is compared to that of the control run. This procedure highlights which effects are most important and reveals the nature of the various interactions.
The results emphasize the wide variety of processes that appear capable of influencing the mean circulation. The intensity of the circulation is related to the coherence of the thermal forcing, and to the thermal opacity of the atmosphere. When all frictional forcing is removed, the circulation is restricted to the equatorial region. The latitudinal extent appears to be controlled primarily by eddy processes (Ferrel cell intensity). The implications for climate modeling and climate projections (e.g., rainfall changes) are discussed.
Abstract
Results of experiments with a GCM involving changes in UV input (±25%, ±10%, ±5% at wavelengths below 0.3 µm) and simulated equatorial QBO are presented, with emphasis on the middle atmosphere response. The UV forcing employed is larger than observed during the last solar cycle and does not vary with wavelength, hence the relationship of these results to those from actual solar UV forcing should be treated with caution. The QBO alters the location of the zero wind line and the horizontal shear of the zonal wind in the low to middle stratosphere, while the UV change alters the magnitude of the polar jet and the vertical shear of the zonal wind. Both mechanisms thus affect planetary wave propagation. The east phase of the QBO leads to tropical cooling and high-latitude warming in the lower stratosphere, with opposite effects in the upper stratosphere. This quadrupole pattern is also wen in the observations. The high-latitude responses are due to altered planetary wave effect, while the model's tropical response in the upper stratosphere is due to gravity wave drag.
Increased UV forcing warms tropical latitudes in the middle atmosphere, resulting in stronger extratropical wen winds, an effect which peaks in the upper stratosphere/lower mesosphere with the more extreme UV forcing but at lower altitudes and smaller wind variations with the more realistic forcing. The increased vertical gradient of the zonal wind leads to increased vertical propagation of planetary warm altering energy convergences and temperatures. The exact altitudes affected depend upon the UV forcing applied.
Results with combined QBO and UV forcing show that in the Northern Hemisphere, polar warming for the east QBO is stronger when the UV input is reduced by 25% and 5% as increased wave propagation to high latitudes(east QB0 effect) is prevented from then propagating vertically (reduced UV effect). The model results are thus in general agreement with observations associated with solar UV/QBO variations, although the west phase is not absolutely warmer with increased UV. Questions remain concerning the actual variation of stratospheric winds with the solar cycle as the magnitude of the variations reported in some observations cannot be associated with UV variations in this model (but do arise in the model without any external forcing). The model results actually come closer to reproducing observations with the reduced magnitude of UV forcing due to the lower altitude of west wind response, despite the smaller wind variations involved. An evaluation of the reality of the reported effects of combined QBO and solar UV variations on the middle atmosphere requires the use of proper UV solar cycle forcing and should include possible ozone variations.
Abstract
Results of experiments with a GCM involving changes in UV input (±25%, ±10%, ±5% at wavelengths below 0.3 µm) and simulated equatorial QBO are presented, with emphasis on the middle atmosphere response. The UV forcing employed is larger than observed during the last solar cycle and does not vary with wavelength, hence the relationship of these results to those from actual solar UV forcing should be treated with caution. The QBO alters the location of the zero wind line and the horizontal shear of the zonal wind in the low to middle stratosphere, while the UV change alters the magnitude of the polar jet and the vertical shear of the zonal wind. Both mechanisms thus affect planetary wave propagation. The east phase of the QBO leads to tropical cooling and high-latitude warming in the lower stratosphere, with opposite effects in the upper stratosphere. This quadrupole pattern is also wen in the observations. The high-latitude responses are due to altered planetary wave effect, while the model's tropical response in the upper stratosphere is due to gravity wave drag.
Increased UV forcing warms tropical latitudes in the middle atmosphere, resulting in stronger extratropical wen winds, an effect which peaks in the upper stratosphere/lower mesosphere with the more extreme UV forcing but at lower altitudes and smaller wind variations with the more realistic forcing. The increased vertical gradient of the zonal wind leads to increased vertical propagation of planetary warm altering energy convergences and temperatures. The exact altitudes affected depend upon the UV forcing applied.
Results with combined QBO and UV forcing show that in the Northern Hemisphere, polar warming for the east QBO is stronger when the UV input is reduced by 25% and 5% as increased wave propagation to high latitudes(east QB0 effect) is prevented from then propagating vertically (reduced UV effect). The model results are thus in general agreement with observations associated with solar UV/QBO variations, although the west phase is not absolutely warmer with increased UV. Questions remain concerning the actual variation of stratospheric winds with the solar cycle as the magnitude of the variations reported in some observations cannot be associated with UV variations in this model (but do arise in the model without any external forcing). The model results actually come closer to reproducing observations with the reduced magnitude of UV forcing due to the lower altitude of west wind response, despite the smaller wind variations involved. An evaluation of the reality of the reported effects of combined QBO and solar UV variations on the middle atmosphere requires the use of proper UV solar cycle forcing and should include possible ozone variations.
