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Search Results: 1 - 10 of 448512 matches for " L. J. Donner "
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Thunderstorm and stratocumulus: how does their contrasting morphology affect their interactions with aerosols?
S. S. Lee, L. J. Donner,J. E. Penner
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2010,
Abstract: It is well-known that aerosols affect clouds and that the effect of aerosols on clouds is critical for understanding human-induced climate change. Most climate model studies have focused on the effect of aerosols on warm stratiform clouds (e.g., stratocumulus clouds) for the prediction of climate change. However, systems like the Asian and Indian Monsoon, storm tracks, and the intertropical convergence zone, play important roles in the global hydrological cycle and in the circulation of energy and are driven by thunderstorm-type convective clouds. Here, we show that the different morphologies of these two cloud types lead to different aerosol-cloud interactions. Increasing aerosols are known to suppress the conversion of droplets to rain (i.e., so-called autoconversion). This increases droplets as a source of evaporative cooling, leading to an increased intensity of downdrafts. The acceleration of the intensity of downdrafts is larger in convective clouds due to their larger cloud depths (providing longer paths for downdrafts to follow to the surface) than in stratiform clouds. More accelerated downdrafts intensify the gust front, leading to significantly increased updrafts, condensation and thus the collection of cloud liquid by precipitation, which offsets the suppressed autoconversion. This leads to an enhancement of precipitation with increased aerosols in convective clouds. However, the downdrafts are less accelerated in stratiform clouds due to their smaller cloud depths, and they are not able to induce changes in updrafts as large as those in convective clouds. Thus, the offset is not as effective, and this allows the suppression of precipitation with increased aerosols. Thus aerosols affect these cloud systems differently. The dependence of the effect of aerosols on clouds on the morphology of clouds should be taken into account for a more complete assessment of climate change.
Sensitivity of aerosol and cloud effects on radiation to cloud types: comparison between deep convective clouds and warm stratiform clouds over one-day period
S. S. Lee, L. J. Donner,V. T. J. Phillips
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2009,
Abstract: Cloud and aerosol effects on radiation in two contrasting cloud types, a deep mesoscale convective system (MCS) and warm stratocumulus clouds, are simulated and compared. At the top of the atmosphere, 45–81% of shortwave cloud forcing (SCF) is offset by longwave cloud forcing (LCF) in the MCS, whereas warm stratiform clouds show the offset of less than ~20%. 28% of increased negative SCF is offset by increased LCF with increasing aerosols in the MCS at the top of the atmosphere. However, the stratiform clouds show the offset of just around 2–5%. Ice clouds as well as liquid clouds play an important role in the larger offset in the MCS. Lower cloud-top height and cloud depth, characterizing cloud types, lead to the smaller offset of SCF by LCF and the offset of increased negative SCF by increased LCF at high aerosol in stratocumulus clouds than in the MCS. Supplementary simulations show that this dependence of modulation of LCF on cloud depth and cloud-top height is also simulated among different types of convective clouds.
Cloud and aerosol effects on radiation in deep convective clouds: comparison with warm stratiform clouds
S. S. Lee,L. J. Donner,V. T. J. Phillips
Atmospheric Chemistry and Physics Discussions , 2008,
Abstract: Cloud and aerosol effects on radiation in two contrasting cloud types, a deep convective mesoscale cloud ensemble (MCE) and warm stratocumulus clouds, are simulated and compared. At the top of the atmosphere, 45–81% of shortwave cloud forcing (SCF) is offset by longwave cloud forcing (LCF) in the MCE, whereas warm stratiform clouds show the offset of less than ~20%. 28% of increased negative SCF is offset by increased LCF with increasing aerosols in the MCE at the top of the atmosphere. However, the stratiform clouds show the offset of just around 2–5%. Ice clouds as well as liquid clouds play an important role in the larger offset in the MCE. Hence, this study indicates effects of deep convective clouds on radiation and responses of deep convective clouds to aerosols are quite different from those of shallow clouds through the different modulation of longwave radiation; the presence of ice clouds in deep convective clouds contributes to the different modulation of longwave radiation significantly. Different cloud types, characterized by cloud depth and cloud-top height, play critical roles in those different modulations of LCF between the MCE and stratocumulus clouds. Lower cloud-top height and cloud depth lead to smaller offset of SCF by LCF and offset of increased negative SCF by increased LCF at high aerosol in stratocumulus clouds than in the MCE. Supplementary simulations show this dependence of modulation of LCF on cloud depth and cloud-top height is not limited to those two contrasting cloud types. The dependence is also simulated among different types of convective clouds, indicating the assessment of effects of varying cloud types on radiation due to climate changes can be critical to better prediction of climate.
Thunderstorm and stratocumulus: how does their contrasting morphology affect their interactions with aerosols?
S. S. Lee,L. J. Donner,J. E. Penner
Atmospheric Chemistry and Physics Discussions , 2010,
Abstract: It is well-known that aerosols affect clouds and that the effect of aerosols on clouds is critical for understanding human-induced climate change. Most climate model studies have focused on the effect of aerosols on warm stratiform clouds (e.g., stratocumulus clouds) for the prediction of climate change. However, systems like the Asian and Indian Monsoon, storm tracks, and the intertropical convergence zone, play important roles in the global hydrological cycle and in the circulation of energy and are driven by thunderstorm-type convective clouds. Here, we show that the different morphologies of these two cloud types lead to different aerosol-cloud interactions. Increasing aerosols are known to suppress the conversion of droplets to rain (i.e., so-called autoconversion). This increases droplets as a source of evaporative cooling, leading to an increased intensity of downdrafts. The acceleration of the intensity of downdrafts is larger in convective clouds due to their larger cloud depths (providing longer paths for downdrafts to follow to the surface) than in stratiform clouds. More accelerated downdrafts intensify the gust front, leading to significantly increased updrafts, condensation and thus the collection of cloud liquid by precipitation, which offsets the suppressed autoconversion. This leads to an enhancement of precipitation with increased aerosols in convective clouds. However, the downdrafts are less accelerated in stratiform clouds due to their smaller cloud depths, and they are not able to induce changes in updrafts as large as those in convective clouds. Thus, the offset is not as effective, and this allows the suppression of precipitation with increased aerosols. Thus aerosols affect these cloud systems differently. The dependence of the effect of aerosols on clouds on the morphology of clouds should be taken into account for a more complete assessment of climate change.
A quantitative analysis of grid-related systematic errors in oxidising capacity and ozone production rates in chemistry transport models
M. Salzmann, M. G. Lawrence, V. T. J. Phillips,L. J. Donner
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2004,
Abstract: The vertical transport of tracers by a cumulus ensemble at the TOGA-COARE site is modelled during a 7 day episode using 2-D and 3-D cloud-resolving setups of the Weather Research and Forecast (WRF) model. Lateral boundary conditions (LBC) for tracers, water vapour, and wind are specified and the horizontal advection of trace gases across the lateral domain boundaries is considered. Furthermore, the vertical advection of trace gases by the large-scale motion (short: vertical large-scale advection of tracers, VLSAT) is considered. It is shown that including VLSAT partially compensates the calculated net downward transport from the middle and upper troposphere (UT) due to the mass balancing mesoscale subsidence induced by deep convection. Depending on whether the VLSAT term is added or not, modelled domain averaged vertical tracer profiles can differ significantly. Differences between a 2-D and a 3-D model run were mainly attributed to an increase in horizontal advection across the lateral domain boundaries due to the meridional wind component not considered in the 2-D setup.
Model sensitivity studies regarding the role of the retention coefficient for the scavenging and redistribution of highly soluble trace gases by deep convective cloud systems
M. Salzmann, M. G. Lawrence, V. T. J. Phillips,L. J. Donner
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2007,
Abstract: The role of the retention coefficient (i.e. the fraction of a dissolved trace gas which is retained in hydrometeors during freezing) for the scavenging and redistribution of highly soluble trace gases by deep convective cloud systems is investigated using a modified version of the Weather Research and Forecasting (WRF) model. Results from cloud system resolving model runs (in which deep convection is initiated by small random perturbations in association with so-called "large scale forcings (LSF)") for a tropical oceanic (TOGA COARE) and a mid-latitude continental case (ARM) are compared to two runs in which bubbles are used to initiate deep convection (STERAO, ARM). In the LSF runs, scavenging is found to almost entirely prevent a highly soluble tracer initially located in the lowest 1.5 km of the troposphere from reaching the upper troposphere, independent of the retention coefficient. The release of gases from freezing hydrometeors leads to mixing ratio increases in the upper troposphere comparable to those calculated for insoluble trace gases only in the two runs in which bubbles are used to initiate deep convection. A comparison of the two ARM runs indicates that using bubbles to initiate deep convection may result in an overestimate of the influence of the retention coefficient on the vertical transport of highly soluble tracers. It is, however, found that the retention coefficient plays an important role for the scavenging and redistribution of highly soluble trace gases with a (chemical) source in the free troposphere and also for trace gases for which even relatively inefficient transport may be important. The large difference between LSF and bubble runs is attributed to differences in dynamics and microphysics in the inflow regions of the storms. The dependence of the results on the model setup indicates the need for additional model studies with a more realistic initiation of deep convection, e.g., considering effects of orography in a nested model setup.
A dynamic probability density function treatment of cloud mass and number concentrations for low level clouds in GFDL SCM/GCM
H. Guo,J.-C. Golaz,L. J. Donner,V. E. Larson
Geoscientific Model Development Discussions , 2010, DOI: 10.5194/gmdd-3-541-2010
Abstract: Successful simulation of cloud-aerosol interactions (indirect aerosol effects) in climate models requires relating grid-scale aerosol, dynamic, and thermodynamic fields to small-scale processes like aerosol activation. A turbulence and cloud parameterization, based on multivariate probability density functions (PDFs) of sub-grid vertical velocity, temperature, and moisture, has been extended to treat aerosol activation. This dynamics-PDF approach offers a solution to the problem of the scale gap between the resolution of climate models and the scales relevant for aerosol activation and a means to overcome the limitations of diagnostic estimates of cloud droplet number concentration based only on aerosol concentration. Incorporated into a single-column model for GFDL AM3, the dynamics-PDF parameterization successfully simulates cloud fraction and water content for shallow cumulus, stratocumulus, and cumulus-under-stratocumulus regimes. The extension to treat aerosol activation predicts droplet number concentrations in good agreement with large eddy simulation (LES). The dynamics-PDF droplet number concentrations match LES results more closely than state-of-the-science diagnostic relationships between aerosol concentration and droplet number concentration.
Modelling tracer transport by a cumulus ensemble: lateral boundary conditions and large-scale ascent
M. Salzmann,M. G. Lawrence,V. T. J. Phillips,L. J. Donner
Atmospheric Chemistry and Physics Discussions , 2004,
Abstract: The vertical transport of tracers by a cumulus ensemble at the TOGA-COARE site is modelled during a 7 day episode using 2-D and 3-D cloud-resolving setups of the Weather Research and Forecast (WRF) model. Lateral boundary conditions (LBC) for tracers, water vapour, and wind are specified and the horizontal advection of trace gases across the lateral domain boundaries is considered. Furthermore, the vertical advection of trace gases by the large-scale motion (short: vertical large-scale advection of tracers, VLSAT) is considered. It is shown, that including VLSAT partially compensates the calculated net downward transport from the middle and upper troposphere (UT) due to the mass balancing mesoscale subsidence induced by deep convection. Depending on whether the VLSAT term is added or not, modelled domain averaged vertical tracer profiles can differ significantly. Differences between a 2-D and a 3-D model run were mainly attributed to an increase in horizontal advection across the lateral domain boundaries due to the meridional wind component not considered in the 2-D setup.
Cloud system resolving model study of the roles of deep convection for photo-chemistry in the TOGA COARE/CEPEX region
M. Salzmann,M. G. Lawrence,V. T. J. Phillips,L. J. Donner
Atmospheric Chemistry and Physics Discussions , 2008,
Abstract: A cloud system resolving model including photo-chemistry (CSRMC) has been developed based on a prototype version of the Weather Research and Forecasting (WRF) model and is used to study influences of deep convection on chemistry in the TOGA COARE/CEPEX region. Lateral boundary conditions for trace gases are prescribed from global chemistry-transport simulations, and the vertical advection of trace gases by large scale dynamics, which is not reproduced in a limited area cloud system resolving model, is taken into account. The influences of in situ lightning and other processes on NOx, O3, and HOx(=HO2+OH), in the vicinity of the deep convective systems are investigated in a 7-day 3-D 248×248 km2 horizontal domain simulation and several 2-D sensitivity runs with a 500 km horizontal domain. The fraction of NOx chemically lost within the domain varies between 20 and 24% in the 2-D runs, but is negligible in the 3-D run, in agreement with a lower average NOx concentration in the 3-D run despite a greater number of flashes. In all runs, in situ lightning is found to have only minor impacts on the local O3 budget. Mid-tropospheric entrainment is more important on average for the upward transport of O3 in the 3-D run than in the 2-D runs, but at the same time undiluted O3-poor air from the marine boundary layer reaches the upper troposphere more frequently in the 3-D run than in the 2-D runs, indicating the presence of undiluted convective cores. Near zero O3 volume mixing ratios due to the reaction with lightning-produced NO are only simulated in a 2-D sensitivity run with an extremely high number of NO molecules per flash, which is outside the range of current estimates. Stratosphere to troposphere transport of O3 is simulated to occur episodically in thin filaments, but on average net upward transport of O3 from below ~16 km is simulated in association with mean large scale ascent in the region. Ozone profiles in the TOGA COARE/CEPEX region are suggested to be strongly influenced by the intra-seasonal (Madden-Julian) oscillation.
Cloud system resolving model study of the roles of deep convection for photo-chemistry in the TOGA COARE/CEPEX region
M. Salzmann,M. G. Lawrence,V. T. J. Phillips,L. J. Donner
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2008,
Abstract: A cloud system resolving model including photo-chemistry (CSRMC) has been developed based on a prototype version of the Weather Research and Forecasting (WRF) model and is used to study influences of deep convection on chemistry in the TOGA COARE/CEPEX region. Lateral boundary conditions for trace gases are prescribed from global chemistry-transport simulations, and the vertical advection of trace gases by large scale dynamics, which is not reproduced in a limited area cloud system resolving model, is taken into account. The influences of deep convective transport and of lightning on NOx, O3, and HOx(=HO2+OH), in the vicinity of the deep convective systems are investigated in a 7-day 3-D 248×248 km2 horizontal domain simulation and several 2-D sensitivity runs with a 500 km horizontal domain. Mid-tropospheric entrainment is more important on average for the upward transport of O3 in the 3-D run than in the 2-D runs, but at the same time undiluted O3-poor air from the marine boundary layer reaches the upper troposphere more frequently in the 3-D run than in the 2-D runs, indicating the presence of undiluted convective cores. In all runs, in situ lightning is found to have only minor impacts on the local O3 budget. Near zero O3 volume mixing ratios due to the reaction with lightning-produced NO are only simulated in a 2-D sensitivity run with an extremely high number of NO molecules per flash, which is outside the range of current estimates. The fraction of NOx chemically lost within the domain varies between 20 and 24% in the 2-D runs, but is negligible in the 3-D run, in agreement with a lower average NOx concentration in the 3-D run despite a greater number of flashes. Stratosphere to troposphere transport of O3 is simulated to occur episodically in thin filaments in the 2-D runs, but on average net upward transport of O3 from below ~16 km is simulated in association with mean large scale ascent in the region. Ozone profiles in the TOGA COARE/CEPEX region are suggested to be strongly influenced by the intra-seasonal (Madden-Julian) oscillation.
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