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- Author or Editor: Richard Bintanja x
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Abstract
The shortwave and longwave fluxes at the top of the atmosphere (TOA) and at the surface are parameterized in term of solar constant, solar zenith angle, cloud parameters (amount, optical depth, height, and emissivity), surface albedo, surface air temperature, surface temperature, and atmospheric CO2 concentration. Detailed radiative models are used to calculate up- and downward radiative fluxes at TOA and at the surface with input from standard vertical atmospheric profiles of temperature, water vapor, and ozone. Expressions for clear-sky and completely overcast conditions are presented. It is shown that there is reasonably good agreement between the radiative fluxes calculated with this parameterization, with the detailed radiation models and with standard radiation codes (ICRCCM). Furthermore, it is shown that the parameterization is able to produce with reasonable accuracy several aspects of the latitudinally and seasonally varying, zonally averaged shortwave and longwave radiative fluxes at TOA and at the surface. The effect of clouds on the radiative fluxes as calculated with the parameterization compares reasonably well with observations, which is an important aspect for climate modeling studies. The radiation parameterization presented here is particularly useful in zonal average climate models (such as energy balance climate models) that treat the atmosphere as one bulk layer, since it is computationally efficient.
Abstract
The shortwave and longwave fluxes at the top of the atmosphere (TOA) and at the surface are parameterized in term of solar constant, solar zenith angle, cloud parameters (amount, optical depth, height, and emissivity), surface albedo, surface air temperature, surface temperature, and atmospheric CO2 concentration. Detailed radiative models are used to calculate up- and downward radiative fluxes at TOA and at the surface with input from standard vertical atmospheric profiles of temperature, water vapor, and ozone. Expressions for clear-sky and completely overcast conditions are presented. It is shown that there is reasonably good agreement between the radiative fluxes calculated with this parameterization, with the detailed radiation models and with standard radiation codes (ICRCCM). Furthermore, it is shown that the parameterization is able to produce with reasonable accuracy several aspects of the latitudinally and seasonally varying, zonally averaged shortwave and longwave radiative fluxes at TOA and at the surface. The effect of clouds on the radiative fluxes as calculated with the parameterization compares reasonably well with observations, which is an important aspect for climate modeling studies. The radiation parameterization presented here is particularly useful in zonal average climate models (such as energy balance climate models) that treat the atmosphere as one bulk layer, since it is computationally efficient.
Abstract
Detailed concurrent summer observations of surface meteorological variables, high-resolution boundary layer profiles, and upper-air profiles were carried in the intermediate region between the high plateau and the coast in Dronning Maud Land, Antarctica, a region that includes several blue ice areas. The main goal is to find out to what extent the flow is influenced by 1) slope-inversion pressure gradients (katabatic force), 2) synoptic pressure gradients (geostrophic wind), and 3) thermal gradients (thermal wind), and how this compares with the situation near the coast. Both synoptic and katabatic forcings generally induce easterly winds in the boundary layer, with a clear diurnal cycle in their relative importance. Westerly surface winds occur less often than at nearby coastal stations Halley and Neumayer, indicating that the influence of synoptic and thermal effects decreases toward the interior of Antarctica. Hence, katabatic forcing dominates, in particular during clear-sky conditions. Under weak synoptic forcing, the boundary layer over the main blue ice area is susceptible to local-scale circulations, which produces noneasterly winds at the surface. This result is caused mainly by reduced cooling over blue ice, which renders the katabatic force weak or absent, and by the typical topographic setting.
Abstract
Detailed concurrent summer observations of surface meteorological variables, high-resolution boundary layer profiles, and upper-air profiles were carried in the intermediate region between the high plateau and the coast in Dronning Maud Land, Antarctica, a region that includes several blue ice areas. The main goal is to find out to what extent the flow is influenced by 1) slope-inversion pressure gradients (katabatic force), 2) synoptic pressure gradients (geostrophic wind), and 3) thermal gradients (thermal wind), and how this compares with the situation near the coast. Both synoptic and katabatic forcings generally induce easterly winds in the boundary layer, with a clear diurnal cycle in their relative importance. Westerly surface winds occur less often than at nearby coastal stations Halley and Neumayer, indicating that the influence of synoptic and thermal effects decreases toward the interior of Antarctica. Hence, katabatic forcing dominates, in particular during clear-sky conditions. Under weak synoptic forcing, the boundary layer over the main blue ice area is susceptible to local-scale circulations, which produces noneasterly winds at the surface. This result is caused mainly by reduced cooling over blue ice, which renders the katabatic force weak or absent, and by the typical topographic setting.
Abstract
This paper presents snowdrift sublimation rates evaluated from meteorological and snowdrift data observed over Antarctic snow surfaces during austral summer. Snowdrift sublimation is found to be the major contributor to the total surface–atmosphere moisture flux in strong winds (equivalent latent heat fluxes up to 250 W m−2), during which surface sublimation becomes negligible because of formation of a near-surface saturated layer. Both surface and snowdrift sublimation interact strongly with the surface moisture budget of the near-surface atmospheric layer. The sum of surface and snowdrift sublimation rates compares reasonably well with the directly measured latent heat fluxes. On average, surface and snowdrift sublimation contributed about equally to the total latent heat flux of 13.1 W m−2 at one site, whereas snowdrift sublimation was estimated to contribute two-thirds of the total sublimation at three other sites. Spatial variations in snowdrift sublimation depend on differences in wind speed, temperature, and humidity in a complex manner. For instance, the highest and windiest location, with the largest snowdrift transport rates, experienced the lowest sublimation rates because of low ambient temperatures.
Abstract
This paper presents snowdrift sublimation rates evaluated from meteorological and snowdrift data observed over Antarctic snow surfaces during austral summer. Snowdrift sublimation is found to be the major contributor to the total surface–atmosphere moisture flux in strong winds (equivalent latent heat fluxes up to 250 W m−2), during which surface sublimation becomes negligible because of formation of a near-surface saturated layer. Both surface and snowdrift sublimation interact strongly with the surface moisture budget of the near-surface atmospheric layer. The sum of surface and snowdrift sublimation rates compares reasonably well with the directly measured latent heat fluxes. On average, surface and snowdrift sublimation contributed about equally to the total latent heat flux of 13.1 W m−2 at one site, whereas snowdrift sublimation was estimated to contribute two-thirds of the total sublimation at three other sites. Spatial variations in snowdrift sublimation depend on differences in wind speed, temperature, and humidity in a complex manner. For instance, the highest and windiest location, with the largest snowdrift transport rates, experienced the lowest sublimation rates because of low ambient temperatures.
Abstract
The Arctic is warming 2 to 3 times faster than the global average. Arctic sea ice cover is very sensitive to this warming and has reached historic minima in late summer in recent years (e.g., 2007 and 2012). Considering that the Arctic Ocean is mainly ice covered and that the albedo of sea ice is very high compared to that of open water, any change in sea ice cover will have a strong impact on the climate response through the radiative surface albedo feedback. Since sea ice area is projected to shrink considerably, this feedback will likely vary considerably in time. Feedbacks are usually evaluated as being constant in time, even though feedbacks and climate sensitivity depend on the climate state. Here the authors assess and quantify these temporal changes in the strength of the surface albedo feedback in response to global warming. Analyses unequivocally demonstrate that the strength of the surface albedo feedback exhibits considerable temporal variations. Specifically, the strength of the surface albedo feedback in the Arctic, evaluated for simulations of the future climate (CMIP5 RCP8.5) using a kernel method, shows a distinct peak around the year 2100. This maximum is found to be linked to increased seasonality in sea ice cover when sea ice recedes, in which sea ice retreat during spring turns out to be the dominant factor affecting the strength of the annual surface albedo feedback in the Arctic. Hence, changes in sea ice seasonality and the associated fluctuations in surface albedo feedback strength will exert a time-varying effect on Arctic amplification during the projected warming over the next century.
Abstract
The Arctic is warming 2 to 3 times faster than the global average. Arctic sea ice cover is very sensitive to this warming and has reached historic minima in late summer in recent years (e.g., 2007 and 2012). Considering that the Arctic Ocean is mainly ice covered and that the albedo of sea ice is very high compared to that of open water, any change in sea ice cover will have a strong impact on the climate response through the radiative surface albedo feedback. Since sea ice area is projected to shrink considerably, this feedback will likely vary considerably in time. Feedbacks are usually evaluated as being constant in time, even though feedbacks and climate sensitivity depend on the climate state. Here the authors assess and quantify these temporal changes in the strength of the surface albedo feedback in response to global warming. Analyses unequivocally demonstrate that the strength of the surface albedo feedback exhibits considerable temporal variations. Specifically, the strength of the surface albedo feedback in the Arctic, evaluated for simulations of the future climate (CMIP5 RCP8.5) using a kernel method, shows a distinct peak around the year 2100. This maximum is found to be linked to increased seasonality in sea ice cover when sea ice recedes, in which sea ice retreat during spring turns out to be the dominant factor affecting the strength of the annual surface albedo feedback in the Arctic. Hence, changes in sea ice seasonality and the associated fluctuations in surface albedo feedback strength will exert a time-varying effect on Arctic amplification during the projected warming over the next century.
Abstract
Little is known about the surface energy balance of Antartic blue-ice areas although there have been some studies of the surface energy balance of snow surfaces. Therefore, a detailed meteorological experiment was carded out in the vicinity of a blue-ice area in the Heimefrontfjella, Dronning Maud Land, Antarctica, during the austral summer of 1992/93. Since not all the surface fluxes could be measured directly, the use of a model was necessary. The main purpose of the model is to calculate the surface and subsurface temperatures from which the emitted longwave radiation and the turbulent fluxes can be calculated. The surface energy balance was evaluated at four locations: one on blue ice, and three on snow. Differences are due mainly to the fact that ice has a lower albedo (0.56) than snow (0.80). To compensate for the larger solar absorption of ice, upward fluxes of longwave radiation and turbulent fluxes are larger over ice. Moreover, the energy flux into the ice is larger than into snow due to the differences in the radiative and conductive properties. Surface temperatures, snow subsurface temperatures, and ice sublimation rates evaluated with the model compare well with the measurements, which yields confidence in the surface energy balance results. The latent heat flux is particularly important since the spatial variability of the sublimation rate largely influences the extent of a blue-ice area. This study helps to explain the heat exchange processes over Antarctic surfaces.
Abstract
Little is known about the surface energy balance of Antartic blue-ice areas although there have been some studies of the surface energy balance of snow surfaces. Therefore, a detailed meteorological experiment was carded out in the vicinity of a blue-ice area in the Heimefrontfjella, Dronning Maud Land, Antarctica, during the austral summer of 1992/93. Since not all the surface fluxes could be measured directly, the use of a model was necessary. The main purpose of the model is to calculate the surface and subsurface temperatures from which the emitted longwave radiation and the turbulent fluxes can be calculated. The surface energy balance was evaluated at four locations: one on blue ice, and three on snow. Differences are due mainly to the fact that ice has a lower albedo (0.56) than snow (0.80). To compensate for the larger solar absorption of ice, upward fluxes of longwave radiation and turbulent fluxes are larger over ice. Moreover, the energy flux into the ice is larger than into snow due to the differences in the radiative and conductive properties. Surface temperatures, snow subsurface temperatures, and ice sublimation rates evaluated with the model compare well with the measurements, which yields confidence in the surface energy balance results. The latent heat flux is particularly important since the spatial variability of the sublimation rate largely influences the extent of a blue-ice area. This study helps to explain the heat exchange processes over Antarctic surfaces.
Abstract
With Arctic summer sea ice potentially disappearing halfway through this century, the surface albedo and insulating effects of Arctic sea ice will decrease considerably. The ongoing Arctic sea ice retreat also affects the strength of the Planck, lapse rate, cloud, and surface albedo feedbacks together with changes in the heat exchange between the ocean and the atmosphere, but their combined effect on climate sensitivity has not been quantified. This study presents an estimate of all Arctic sea ice related climate feedbacks combined. We use a new method to keep Arctic sea ice at its present-day (PD) distribution under a changing climate in a 50-yr CO2 doubling simulation, using a fully coupled global climate model (EC-Earth, version 2.3). We nudge the Arctic Ocean to the (monthly dependent) year 2000 mean temperature and minimum salinity fields on a mask representing PD sea ice cover. We are able to preserve about 95% of the PD mean March and 77% of the September PD Arctic sea ice extent by applying this method. Using simulations with and without nudging, we estimate the climate response associated with Arctic sea ice changes. The Arctic sea ice feedback globally equals 0.28 ± 0.15 W m−2 K−1. The total sea ice feedback thus amplifies the climate response for a doubling of CO2, in line with earlier findings. Our estimate of the Arctic sea ice feedback agrees reasonably well with earlier CMIP5 global climate feedback estimates and shows that the Arctic sea ice exerts a considerable effect on the Arctic and global climate sensitivity.
Abstract
With Arctic summer sea ice potentially disappearing halfway through this century, the surface albedo and insulating effects of Arctic sea ice will decrease considerably. The ongoing Arctic sea ice retreat also affects the strength of the Planck, lapse rate, cloud, and surface albedo feedbacks together with changes in the heat exchange between the ocean and the atmosphere, but their combined effect on climate sensitivity has not been quantified. This study presents an estimate of all Arctic sea ice related climate feedbacks combined. We use a new method to keep Arctic sea ice at its present-day (PD) distribution under a changing climate in a 50-yr CO2 doubling simulation, using a fully coupled global climate model (EC-Earth, version 2.3). We nudge the Arctic Ocean to the (monthly dependent) year 2000 mean temperature and minimum salinity fields on a mask representing PD sea ice cover. We are able to preserve about 95% of the PD mean March and 77% of the September PD Arctic sea ice extent by applying this method. Using simulations with and without nudging, we estimate the climate response associated with Arctic sea ice changes. The Arctic sea ice feedback globally equals 0.28 ± 0.15 W m−2 K−1. The total sea ice feedback thus amplifies the climate response for a doubling of CO2, in line with earlier findings. Our estimate of the Arctic sea ice feedback agrees reasonably well with earlier CMIP5 global climate feedback estimates and shows that the Arctic sea ice exerts a considerable effect on the Arctic and global climate sensitivity.
Abstract
Recent model studies have indicated that observed stratospheric ozone decline can have a cooling effect on climate. This study intends to investigate the climate response due to changes in the radiative fluxes caused by prescribed changes in stratospheric as well as tropospheric ozone. For this purpose the authors use a simplified climate model, basically consisting of an energy balance atmosphere model coupled to an advection–diffusion ocean model. The coupled climate model simulates the latitudinal and seasonal variations in zonal mean surface air temperature and the average lower (12–22 km) and higher (22–100 km) stratospheric temperatures. First, the quasi-equilibrium response of the model to various uniform ozone perturbations is examined. For instance, a uniform 50% reduction in lower stratospheric ozone results in a global average cooling of 3.5°C in the lower stratosphere with maximum values in the Tropics and of 0.46°C at the surface with maximum cooling in the polar winter. The latter is largely due to the albedo–temperature feedback, mainly through increases in sea ice. The albedo–temperature feedback is consistently stronger in the case of tropospheric and lower stratospheric ozone perturbations than in the case of, for instance, CO2 perturbations. This can be attributed mainly to differences in the meridional gradient in tropopause radiative forcing. This study indicates that one must be cautious when using concepts such as global radiative forcing and global climate sensitivity in quantifying climate change. Finally, the transient model response to various ozone trend scenarios indicates that the net effect of tropospheric ozone increases and stratospheric ozone depletions is a slight global average cooling (−0.001 to −0.003 K yr−1), which offsets by approximately 10% the projected surface warming due to increases in the other greenhouse gases. Results obtained with this climate model provide qualitative insights in the fundamental processes that determine the sensitivity of climate for ozone changes.
Abstract
Recent model studies have indicated that observed stratospheric ozone decline can have a cooling effect on climate. This study intends to investigate the climate response due to changes in the radiative fluxes caused by prescribed changes in stratospheric as well as tropospheric ozone. For this purpose the authors use a simplified climate model, basically consisting of an energy balance atmosphere model coupled to an advection–diffusion ocean model. The coupled climate model simulates the latitudinal and seasonal variations in zonal mean surface air temperature and the average lower (12–22 km) and higher (22–100 km) stratospheric temperatures. First, the quasi-equilibrium response of the model to various uniform ozone perturbations is examined. For instance, a uniform 50% reduction in lower stratospheric ozone results in a global average cooling of 3.5°C in the lower stratosphere with maximum values in the Tropics and of 0.46°C at the surface with maximum cooling in the polar winter. The latter is largely due to the albedo–temperature feedback, mainly through increases in sea ice. The albedo–temperature feedback is consistently stronger in the case of tropospheric and lower stratospheric ozone perturbations than in the case of, for instance, CO2 perturbations. This can be attributed mainly to differences in the meridional gradient in tropopause radiative forcing. This study indicates that one must be cautious when using concepts such as global radiative forcing and global climate sensitivity in quantifying climate change. Finally, the transient model response to various ozone trend scenarios indicates that the net effect of tropospheric ozone increases and stratospheric ozone depletions is a slight global average cooling (−0.001 to −0.003 K yr−1), which offsets by approximately 10% the projected surface warming due to increases in the other greenhouse gases. Results obtained with this climate model provide qualitative insights in the fundamental processes that determine the sensitivity of climate for ozone changes.
Abstract
The Arctic summer sea ice has diminished fast in recent decades. A strong year-to-year variability on top of this trend indicates that sea ice is sensitive to short-term climate fluctuations. Previous studies show that anomalous atmospheric conditions over the Arctic during spring and summer affect ice melt and the September sea ice extent (SIE). These conditions are characterized by clouds, humidity, and heat anomalies that all affect downwelling shortwave (SWD) and longwave (LWD) radiation to the surface. In general, positive LWD anomalies are associated with cloudy and humid conditions, whereas positive anomalies of SWD appear under clear-sky conditions. Here the effect of realistic anomalies of LWD and SWD on summer sea ice is investigated by performing experiments with the Community Earth System Model. The SWD and LWD anomalies are studied separately and in combination for different seasons. It is found that positive LWD anomalies in spring and early summer have significant impact on the September SIE, whereas winter anomalies show only little effect. Positive anomalies in spring and early summer initiate an earlier melt onset, hereby triggering several feedback mechanisms that amplify melt during the succeeding months. Realistic positive SWD anomalies appear only important if they occur after the melt has started and the albedo is significantly reduced relative to winter conditions. Simulations where both positive LWD and negative SWD anomalies are implemented simultaneously, mimicking cloudy conditions, reveal that clouds during spring have a significant impact on summer sea ice while summer clouds have almost no effect.
Abstract
The Arctic summer sea ice has diminished fast in recent decades. A strong year-to-year variability on top of this trend indicates that sea ice is sensitive to short-term climate fluctuations. Previous studies show that anomalous atmospheric conditions over the Arctic during spring and summer affect ice melt and the September sea ice extent (SIE). These conditions are characterized by clouds, humidity, and heat anomalies that all affect downwelling shortwave (SWD) and longwave (LWD) radiation to the surface. In general, positive LWD anomalies are associated with cloudy and humid conditions, whereas positive anomalies of SWD appear under clear-sky conditions. Here the effect of realistic anomalies of LWD and SWD on summer sea ice is investigated by performing experiments with the Community Earth System Model. The SWD and LWD anomalies are studied separately and in combination for different seasons. It is found that positive LWD anomalies in spring and early summer have significant impact on the September SIE, whereas winter anomalies show only little effect. Positive anomalies in spring and early summer initiate an earlier melt onset, hereby triggering several feedback mechanisms that amplify melt during the succeeding months. Realistic positive SWD anomalies appear only important if they occur after the melt has started and the albedo is significantly reduced relative to winter conditions. Simulations where both positive LWD and negative SWD anomalies are implemented simultaneously, mimicking cloudy conditions, reveal that clouds during spring have a significant impact on summer sea ice while summer clouds have almost no effect.
ABSTRACT
Long-term climate variations have the potential to amplify or dampen (human-induced) trends in temperature. Understanding natural climate variability is therefore of vital importance, especially since the variability itself may change with a changing climate. Here, we quantify the magnitude and other characteristics of interannual to decadal variability in Arctic temperature and their dependence on the climate state. Moreover, we identify the processes responsible for the state dependency of the variations, using five quasi-equilibrium climate simulations of a state-of-the-art global climate model with 0.25, 0.5, 1, 2, and 4 times present-day atmospheric CO2 forcing. The natural fluctuations in Arctic temperature, including their dependence on the state of the climate, are linked to anomalous atmospheric and oceanic heat transports toward the Arctic. Model results suggest that atmospheric heat transport leads (and also controls) Arctic temperature variations on interannual time scales, whereas oceanic transport is found to govern the fluctuations on decadal time scales. This time-scale transition of atmospheric to oceanic dominance for Arctic temperature variations is most obvious when there is interannual to decadal variability in Arctic sea ice cover. In warm climates (without Arctic sea ice cover), there is no correlation between oceanic transport and surface air temperature on any time scale. In cold climates (with full Arctic sea ice cover), interaction between ocean and atmosphere is limited, leaving poleward atmospheric heat transport to be the primary driver on all time scales (interannual and decadal).
ABSTRACT
Long-term climate variations have the potential to amplify or dampen (human-induced) trends in temperature. Understanding natural climate variability is therefore of vital importance, especially since the variability itself may change with a changing climate. Here, we quantify the magnitude and other characteristics of interannual to decadal variability in Arctic temperature and their dependence on the climate state. Moreover, we identify the processes responsible for the state dependency of the variations, using five quasi-equilibrium climate simulations of a state-of-the-art global climate model with 0.25, 0.5, 1, 2, and 4 times present-day atmospheric CO2 forcing. The natural fluctuations in Arctic temperature, including their dependence on the state of the climate, are linked to anomalous atmospheric and oceanic heat transports toward the Arctic. Model results suggest that atmospheric heat transport leads (and also controls) Arctic temperature variations on interannual time scales, whereas oceanic transport is found to govern the fluctuations on decadal time scales. This time-scale transition of atmospheric to oceanic dominance for Arctic temperature variations is most obvious when there is interannual to decadal variability in Arctic sea ice cover. In warm climates (without Arctic sea ice cover), there is no correlation between oceanic transport and surface air temperature on any time scale. In cold climates (with full Arctic sea ice cover), interaction between ocean and atmosphere is limited, leaving poleward atmospheric heat transport to be the primary driver on all time scales (interannual and decadal).
