scholarly journals Modulation of radiative aerosols effects by atmospheric circulation over the Euro-Mediterranean region

2020 ◽  
Author(s):  
Pierre Nabat ◽  
Samuel Somot ◽  
Christophe Cassou ◽  
Marc Mallet ◽  
Martine Michou ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Atmospheric Circulation ◽  
Mediterranean Region ◽  
Cloud Cover ◽  
Regional Climate ◽  
Surface Radiation ◽  
Anthropogenic Aerosols ◽  
Significant Information ◽  
Weather Regimes ◽  
Aerosol Effects

Abstract. The present work aims at better understanding regional climate-aerosol interactions by studying the relationships between aerosols and synoptic atmospheric circulation over the Euro-Mediterranean region. Two 40-year simulations (1979–2018) have been carried out with the CNRM-ALADIN64 regional climate model, one using interactive aerosols and the other one without any aerosol. The simulation with aerosols has been evaluated in terms of different climate and aerosol parameters. This evaluation shows a good agreement between the model and observations, significant improvements compared to the previous model version, and consequently the relevance of using this model for the study of climate-aerosol interactions over this region. A first attempt to explain the climate variability of aerosols is based on the use of the North-Atlantic Oscillation (NAO) index, which explains a significant part of the interannual variability, notably in winter for the export of dust aerosols over the Atlantic Ocean and the Eastern Mediterranean, and in summer for the positive anomalies of anthropogenic aerosols over Western Europe. This index is however not sufficient to fully understand the variations of aerosols in this region, notably at daily scale. The use of weather regimes, namely persisting meteorological patterns, stable at synoptic scale for a few days, provide a relevant description of atmospheric circulation, which drives the emission, transport and deposition of aerosols. The four weather regimes usually defined in this area in winter and in summer bring significant information to answer this question. The blocking and NAO+ regimes are largely favourable to strong aerosol effects on shortwave surface radiation and surface temperature, either because of higher aerosol loads, or because of weaker cloud fraction, which reinforces the direct aerosol effect. Inversely the NAO- and Atlantic Ridge regimes are unfavourable to aerosol radiative effects, because of weaker aerosol concentrations and increased cloud cover. This study thus puts forward the strong dependence of aerosol loads on the synoptic circulation from interannual to daily scales, and as a consequence, the important modulation of the aerosol effects on shortwave surface radiation and surface temperature by atmospheric circulation. The role of cloud cover is essential in this modulation as shown by the use of weather regimes.

scholarly journals Modulation of radiative aerosols effects by atmospheric circulation over the Euro-Mediterranean region

2020 ◽  
Vol 20 (14) ◽  
pp. 8315-8349 ◽  
Author(s):  
Pierre Nabat ◽  
Samuel Somot ◽  
Christophe Cassou ◽  
Marc Mallet ◽  
Martine Michou ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Atmospheric Circulation ◽  
Mediterranean Region ◽  
Cloud Cover ◽  
Regional Climate ◽  
Surface Radiation ◽  
Significant Information ◽  
Near Surface ◽  
Weather Regimes ◽  
Aerosol Effects

Abstract. The present work aims at better understanding regional climate–aerosol interactions by studying the relationships between aerosols and synoptic atmospheric circulation over the Euro-Mediterranean region. Two 40-year simulations (1979–2018) have been carried out with version 6.3 of the Centre National de Recherches Météorologiques (National Centre for Meteorological Research) – Aire Limitée Adaptation dynamique Développement InterNational (CNRM-ALADIN) regional climate model, one using interactive aerosols and the other one without any aerosol. The simulation with aerosols has been evaluated in terms of different climate and aerosol parameters. This evaluation shows a good agreement between the model and observations, significant improvements compared to the previous model version and consequently the relevance of using this model for the study of climate–aerosol interactions over this region. A first attempt to explain the climate variability of aerosols is based on the use of the North Atlantic Oscillation (NAO) index. The latter explains a significant part of the interannual variability, notably in winter for the export of dust aerosols over the Atlantic Ocean and the eastern Mediterranean, and in summer for the positive anomalies of anthropogenic aerosols over western Europe. This index is however not sufficient to fully understand the variations of aerosols in this region, notably at daily scale. The use of “weather regimes”, namely persisting meteorological patterns, stable at synoptic scale for a few days, provides a relevant description of atmospheric circulation, which drives the emission, transport and deposition of aerosols. The four weather regimes usually defined in this area in winter and in summer bring significant information to answer this question. The blocking and NAO+ regimes are largely favourable to strong aerosol effects on shortwave surface radiation and near-surface temperature, either because of higher aerosol loads or because of weaker cloud fraction, which reinforces the direct aerosol effect. Inversely, the NAO− and Atlantic Ridge regimes are unfavourable to aerosol radiative effects, because of weaker aerosol concentrations and increased cloud cover. This study thus puts forward the strong dependence of aerosol loads on the synoptic circulation from interannual to daily scales and, as a consequence, the important modulation of the aerosol effects on shortwave surface radiation and near-surface temperature by atmospheric circulation. The role of cloud cover is essential in this modulation as shown by the use of weather regimes.

scholarly journals Modelling atmospheric structure, cloud and their response to CCN in the central Arctic: ASCOS case studies

2012 ◽  
Vol 12 (7) ◽  
pp. 3419-3435 ◽  
Author(s):  
C. E. Birch ◽  
I. M. Brooks ◽  
M. Tjernström ◽  
M. D. Shupe ◽  
T. Mauritsen ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Cloud Cover ◽  
Cloud Condensation Nuclei ◽  
Late Summer ◽  
Surface Radiation ◽  
The Arctic ◽  
Temperature Structure ◽  
Near Surface ◽  
Low Level ◽  
Arctic Summer

Abstract. Observations made during late summer in the central Arctic Ocean, as part of the Arctic Summer Cloud Ocean Study (ASCOS), are used to evaluate cloud and vertical temperature structure in the Met Office Unified Model (MetUM). The observation period can be split into 5 regimes; the first two regimes had a large number of frontal systems, which were associated with deep cloud. During the remainder of the campaign a layer of low-level cloud occurred, typical of central Arctic summer conditions, along with two periods of greatly reduced cloud cover. The short-range operational NWP forecasts could not accurately reproduce the observed variations in near-surface temperature. A major source of this error was found to be the temperature-dependant surface albedo parameterisation scheme. The model reproduced the low-level cloud layer, though it was too thin, too shallow, and in a boundary-layer that was too frequently well-mixed. The model was also unable to reproduce the observed periods of reduced cloud cover, which were associated with very low cloud condensation nuclei (CCN) concentrations (<1 cm−3). As with most global NWP models, the MetUM does not have a prognostic aerosol/cloud scheme but uses a constant CCN concentration of 100 cm−3 over all marine environments. It is therefore unable to represent the low CCN number concentrations and the rapid variations in concentration frequently observed in the central Arctic during late summer. Experiments with a single-column model configuration of the MetUM show that reducing model CCN number concentrations to observed values reduces the amount of cloud, increases the near-surface stability, and improves the representation of both the surface radiation fluxes and the surface temperature. The model is shown to be sensitive to CCN only when number concentrations are less than 10–20 cm−3.

scholarly journals Impacts of Aerosols on Seasonal Precipitation and Snowpack in California Based on Convection-Permitting WRF-Chem Simulations

2017 ◽  
Author(s):  
Longtao Wu ◽  
Yu Gu ◽  
Jonathan H. Jiang ◽  
Hui Su ◽  
Nanpeng Yu ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Sierra Nevada ◽  
Surface Runoff ◽  
Snow Water Equivalent ◽  
Dominant Role ◽  
Anthropogenic Aerosols ◽  
Surface Absorption ◽  
Dust Aerosols ◽  
Aerosol Sources ◽  
Aerosol Effects

Abstract. A version of the WRF-Chem model with fully coupled aerosol-meteorology-snowpack is employed to investigate the impacts of various aerosol sources on precipitation and snowpack in California. In particular, the impacts of locally emitted anthropogenic and dust aerosols, and aerosols transported from outside of California are studied. We differentiate three pathways of aerosol effects including aerosol-radiation interaction (ARI), aerosol-snow interaction (ASI), and aerosol-cloud interaction (ACI). The convection-permitting model simulations show that precipitation, snow water equivalent (SWE), and surface air temperature averaged over the whole domain (34–42° N, 117–124° W, not including ocean points) are reduced when aerosols are included, therefore reducing the high model biases of these variables when aerosol effects are not considered. Aerosols affect California water resources through the warming of mountain tops and anomalously low precipitation, however, different aerosol sources play different roles in changing surface temperature, precipitation and snowpack in California by means of various weights of the three pathways. ARI by all aerosols mainly cools the surface, leading to slightly increased SWE over the mountains. Locally emitted dust aerosols warm the surface of mountain tops through ASI, in which the reduced snow albedo associated with dirty snow leads to more surface absorption of solar radiation and reduced SWE. Transported and local anthropogenic aerosols play a dominant role in increasing cloud water amount but reducing precipitation through ACI, leading to reduced SWE and runoff over the Sierra Nevada, as well as the warming of mountain tops associated with decreased SWE and hence lower surface albedo. The average changes in surface temperature from October to June are about −0.19 K and 0.22 K for the whole domain and over mountain tops, respectively. Overall, the averaged reduction during October to June is about 7 % for precipitation, 3 % for SWE, and 7 % for surface runoff for the whole domain, while the corresponding numbers are 12 %, 10 %, and 10 % for mountain tops. The reduction in SWE is more significant in a dry year, with 9 % for the whole domain and 16 % for mountain tops. The maximum reduction of ~ 20 % in precipitation occurs in May associated with the maximum of aerosol loadings, leading to the largest decrease in SWE and surface runoff over that time period. It is also found that dust aerosols could cause early snowmelt at the mountain tops and reduced surface runoff after April.

scholarly journals Modelling atmospheric structure, cloud and their response to CCN in the Central Arctic: ASCOS case studies

2012 ◽  
Vol 12 (1) ◽  
pp. 2559-2601 ◽  
Author(s):  
C. E. Birch ◽  
I. M. Brooks ◽  
M. Tjernström ◽  
M. D. Shupe ◽  
T. Mauritsen ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Cloud Cover ◽  
Cloud Condensation Nuclei ◽  
Late Summer ◽  
Surface Radiation ◽  
The Arctic ◽  
Temperature Structure ◽  
Near Surface ◽  
Low Level ◽  
Arctic Summer

Abstract. Observations made during late summer in the Central Arctic Ocean, as part of the Arctic Summer Cloud Ocean Study (ASCOS), are used to evaluate cloud and vertical temperature structure in the Met Office Unified Model (MetUM). The observation period can be split into 5 regimes; the first two regimes had a large number of frontal systems, which were associated with deep cloud. During the remainder of the campaign a layer of low-level cloud occurred, typical of Central Arctic summer conditions, along with two periods of greatly reduced cloud cover. The short-range operational NWP forecasts could not accurately reproduce the observed variations in near-surface temperature. A major source of this error was found to be the temperature-dependant surface albedo parameterisation scheme. The model reproduced the low-level cloud layer, though it was too thin, too shallow, and in a boundary-layer that was too frequently well-mixed. The model was also unable to reproduce the observed periods of reduced cloud cover, which were associated with very low cloud condensation nuclei (CCN) concentrations (<1 cm−3). As with most global NWP models, the MetUM does not have a prognostic aerosol/cloud scheme but uses a constant CCN concentration of 100 cm−3 over all marine environments. It is therefore unable to represent the low CCN number concentrations and the rapid variations in concentration frequently observed in the Central Arctic during late summer. Experiments with a single-column model configuration of the MetUM show that reducing model CCN number concentrations to observed values reduces the amount of cloud, increases the near-surface stability, and improves the representation of both the surface radiation fluxes and the surface temperature. The model is shown to be sensitive to CCN only when number concentrations are less than 10–20 cm−3.

scholarly journals Impacts of aerosols on seasonal precipitation and snowpack in California based on convection-permitting WRF-Chem simulations

2018 ◽  
Vol 18 (8) ◽  
pp. 5529-5547 ◽  
Author(s):  
Longtao Wu ◽  
Yu Gu ◽  
Jonathan H. Jiang ◽  
Hui Su ◽  
Nanpeng Yu ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Sierra Nevada ◽  
Surface Runoff ◽  
Snow Water Equivalent ◽  
Dominant Role ◽  
Anthropogenic Aerosols ◽  
Surface Absorption ◽  
Dust Aerosols ◽  
Aerosol Sources ◽  
Aerosol Effects

Abstract. A version of the WRF-Chem model with fully coupled aerosol–meteorology–snowpack is employed to investigate the impacts of various aerosol sources on precipitation and snowpack in California. In particular, the impacts of locally emitted anthropogenic and dust aerosols, and aerosols transported from outside California are studied. We differentiate three pathways of aerosol effects: aerosol–radiation interaction (ARI), aerosol–snow interaction (ASI), and aerosol–cloud interaction (ACI). The convection-permitting model simulations show that precipitation, snow water equivalent (SWE), and surface air temperature averaged over the whole domain (34–42∘ N, 117–124∘ W, not including ocean points) are reduced when aerosols are included, therefore reducing large biases in these variables due to the absence of aerosol effects in the model. Aerosols affect California water resources through the warming of mountaintops and the reduction of precipitation; however, different aerosol sources play different roles in changing surface temperature, precipitation, and snowpack in California by means of various weights of the three pathways. ARI by all aerosols mainly cools the surface, leading to slightly increased SWE over the mountains. Locally emitted dust aerosols warm the surface of mountaintops through ASI, in which the reduced snow albedo associated with dusty snow leads to more surface absorption of solar radiation and reduced SWE. Transported aerosols and local anthropogenic aerosols play a dominant role in increasing nonprecipitating clouds but reducing precipitation through ACI, leading to reduced SWE and runoff on the Sierra Nevada, as well as the warming of mountaintops associated with decreased SWE and hence lower surface albedo. The average changes in surface temperature from October 2012 to June 2013 are about −0.19 and 0.22 K for the whole domain and over mountaintops, respectively. Overall, the averaged reduction during October to June is about 7 % for precipitation, 3 % for SWE, and 7 % for surface runoff for the whole domain, while the corresponding numbers are 12, 10, and 10 % for the mountaintops. The reduction in SWE is more significant in a dry year, with 9 % for the whole domain and 16 % for the mountaintops. The maximum reduction of ∼ 20 % in precipitation occurs in May and is associated with the maximum aerosol loading, leading to the largest decrease in SWE and surface runoff over that period. It is also found that dust aerosols can cause early snowmelt on the mountaintops and reduced surface runoff after April.

scholarly journals The Sensitivity of Land–Atmosphere Coupling to Modern Agriculture in the Northern Midlatitudes

2018 ◽  
Vol 32 (2) ◽  
pp. 465-484
Author(s):  
Sonali Shukla McDermid ◽  
Carlo Montes ◽  
Benjamin I. Cook ◽  
Michael J. Puma ◽  
Nancy Y. Kiang ◽  
...  
Keyword(s):  
Soil Moisture ◽  
Land Cover ◽  
Surface Temperature ◽  
Regional Climate ◽  
Growing Season ◽  
Climate Impacts ◽  
Surface Radiation ◽  
Climate Conditions ◽  
Simulated Precipitation ◽  
Background Climate

AbstractModern agricultural land cover and management are important as regional climate forcings. Previous work has shown that land cover change can significantly impact key climate variables, including turbulent fluxes, precipitation, and surface temperature. However, fewer studies have investigated how intensive crop management can impact background climate conditions, such as the strength of land–atmosphere coupling and evaporative regime. We conduct sensitivity experiments using a state-of-the-art climate model with modified vegetation characteristics to represent modern crop cover and management, using observed crop-specific leaf area indexes and calendars. We quantify changes in land–atmosphere interactions and climate over intensively cultivated regions situated at transitions between moisture- and energy-limited conditions. Results show that modern intensive agriculture has significant and geographically varying impacts on regional evaporative regimes and background climate conditions. Over the northern Great Plains, modern crop intensity increases the model simulated precipitation and soil moisture, weakening hydrologic coupling by increasing surface water availability and reducing moisture limits on evapotranspiration. In the U.S. Midwest, higher growing season evapotranspiration, coupled with winter and spring rainfall declines, reduces regional soil moisture, while crop albedo changes also reduce net surface radiation. This results overall in reduced dependency of regional surface temperature on latent heat fluxes. In central Asia, a combination of reduced net surface energy and enhanced pre–growing season precipitation amplify the energy-limited evaporative regime. These results highlight the need for improved representations of agriculture in global climate models to better account for regional climate impacts and interactions with other anthropogenic forcings.

scholarly journals A study on the direct effect of anthropogenic aerosols on near surface air temperature over Southeastern Europe during summer 2000 based on regional climate modeling

2009 ◽  
Vol 27 (10) ◽  
pp. 3977-3988 ◽  
Author(s):  
P. Zanis
Keyword(s):  
Atmospheric Circulation ◽  
Air Temperature ◽  
Radiative Forcing ◽  
Regional Climate ◽  
Lower Troposphere ◽  
Balkan Peninsula ◽  
Southeastern Europe ◽  
Anthropogenic Aerosols ◽  
Near Surface ◽  
Induced Changes

Abstract. In the present work it is investigated the direct shortwave effect of anthropogenic aerosols on the near surface temperature over Southeastern Europe and the atmospheric circulation during summer 2000. In summer 2000, a severe heat-wave and droughts affected many countries in the Balkans. The study is based on two yearly simulations with and without the aerosol feedback of the regional climate model RegCM3 coupled with a simplified aerosol model. The surface radiative forcing associated with the anthropogenic aerosols is negative throughout the European domain with the more negative values in Central and Central-eastern Europe. A basic pattern of the aerosol induced changes in air temperature at the lower troposphere is a decrease over Southeastern Europe and the Balkan Peninsula (up to about 1.2°C) thus weakening the pattern of the climatic temperature anomalies of summer 2000. The aerosol induced changes in air temperature from the lower troposphere to upper troposphere are not correlated with the respective pattern of the surface radiative forcing implying the complexity of the mechanisms linking the aerosol radiative forcing with the induced atmospheric changes through dynamical feedbacks of aerosols on atmospheric circulation. Investigation of the aerosol induced changes in the circulation indicates a southward shift of the subtropical jet stream playing a dominant role for the decrease in near surface air temperature over Southeastern Europe and the Balkan Peninsula. The southward shift of the jet exit region over the Balkan Peninsula causes a relative increase of the upward motion at the northern flank of the jet exit region, a relative increase of clouds, less solar radiation absorbed at the surface and hence relative cooler air temperatures in the lower troposphere between 45° N and 50° N. The southward extension of the lower troposphere aerosol induced negative temperature changes in the latitudinal band 35° N–45° N over the Balkan Peninsula is justified from the prevailing northerly flow advecting the relatively cooler air from the latitudinal band 45° N–50° N towards the lower latitudes. The present regional climate modeling study indicates the important role of anthropogenic aerosols for the regional climate and their dynamical feedback on atmospheric circulation.

scholarly journals Aerosol indirect effects on summer precipitation in a regional climate model for the Euro-Mediterranean region

2018 ◽  
Vol 36 (2) ◽  
pp. 321-335 ◽  
Author(s):  
Nicolas Da Silva ◽  
Sylvain Mailler ◽  
Philippe Drobinski
Keyword(s):  
Regional Climate Model ◽  
Indirect Effects ◽  
Mediterranean Region ◽  
Climate Model ◽  
Regional Climate ◽  
Summer Precipitation ◽  
Cloud Microphysics ◽  
Radiative Heating ◽  
Direct Effects ◽  
Anthropogenic Aerosols

Abstract. Aerosols affect atmospheric dynamics through their direct and semi-direct effects as well as through their effects on cloud microphysics (indirect effects). The present study investigates the indirect effects of aerosols on summer precipitation in the Euro-Mediterranean region, which is located at the crossroads of air masses carrying both natural and anthropogenic aerosols. While it is difficult to disentangle the indirect effects of aerosols from the direct and semi-direct effects in reality, a numerical sensitivity experiment is carried out using the Weather Research and Forecasting (WRF) model, which allows us to isolate indirect effects, all other effects being equal. The Mediterranean hydrological cycle has often been studied using regional climate model (RCM) simulations with parameterized convection, which is the approach we adopt in the present study. For this purpose, the Thompson aerosol-aware microphysics scheme is used in a pair of simulations run at 50 km resolution with extremely high and low aerosol concentrations. An additional pair of simulations has been performed at a convection-permitting resolution (3.3 km) to examine these effects without the use of parameterized convection. While the reduced radiative flux due to the direct effects of the aerosols is already known to reduce precipitation amounts, there is still no general agreement on the sign and magnitude of the aerosol indirect forcing effect on precipitation, with various processes competing with each other. Although some processes tend to enhance precipitation amounts, some others tend to reduce them. In these simulations, increased aerosol loads lead to weaker precipitation in the parameterized (low-resolution) configuration. The fact that a similar result is obtained for a selected area in the convection-permitting (high-resolution) configuration allows for physical interpretations. By examining the key variables in the model outputs, we propose a causal chain that links the aerosol effects on microphysics to their simulated effect on precipitation, essentially through reduction of the radiative heating of the surface and corresponding reductions of surface temperature, resulting in increased atmospheric stability in the presence of high aerosol loads. Keywords. Atmospheric composition and structure (aerosols and particles)

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