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Abstract
The effects of a variable-resolution mesh on simulated midlatitude baroclinic eddies in idealized settings are examined. Both aquaplanet and Held–Suarez experiments are performed using the Model for Prediction Across Scales-Atmosphere (MPAS-A) hydrostatic dynamical core implemented within the National Science Foundation–Department of Energy (NSF–DOE) Community Atmosphere Model (CAM-MPAS-A). In the real world, midlatitude eddy activity is organized by orography, land–sea contrasts, and sea surface temperature anomalies. In these zonally symmetric idealized settings, transients should have an equal probability of occurring at any longitude. However, the use of a variable-resolution mesh with a circular high-resolution region centered at 30°N results in a maximum in eddy kinetic energy on the eastern side and downstream of this high-resolution region in both aquaplanet and Held–Suarez CAM-MPAS-A simulations. The presence of a geographically confined maximum in both simulations suggests this response is mainly attributable to CAM-MPAS-A’s ability to resolve eddies via the model dynamics as resolution increases. However, in the aquaplanet simulation, a secondary maximum in eddy kinetic energy is present, which is probably linked to the resolution dependencies of the CAM physics. These mesh responses must be considered when interpreting real-world variable-resolution CAM-MPAS-A simulations, particularly in climate change experiments.
Abstract
The effects of a variable-resolution mesh on simulated midlatitude baroclinic eddies in idealized settings are examined. Both aquaplanet and Held–Suarez experiments are performed using the Model for Prediction Across Scales-Atmosphere (MPAS-A) hydrostatic dynamical core implemented within the National Science Foundation–Department of Energy (NSF–DOE) Community Atmosphere Model (CAM-MPAS-A). In the real world, midlatitude eddy activity is organized by orography, land–sea contrasts, and sea surface temperature anomalies. In these zonally symmetric idealized settings, transients should have an equal probability of occurring at any longitude. However, the use of a variable-resolution mesh with a circular high-resolution region centered at 30°N results in a maximum in eddy kinetic energy on the eastern side and downstream of this high-resolution region in both aquaplanet and Held–Suarez CAM-MPAS-A simulations. The presence of a geographically confined maximum in both simulations suggests this response is mainly attributable to CAM-MPAS-A’s ability to resolve eddies via the model dynamics as resolution increases. However, in the aquaplanet simulation, a secondary maximum in eddy kinetic energy is present, which is probably linked to the resolution dependencies of the CAM physics. These mesh responses must be considered when interpreting real-world variable-resolution CAM-MPAS-A simulations, particularly in climate change experiments.
Abstract
The goal of this study is to evaluate the effects of anthropogenic climate change on air quality, in particular on ozone, during the summer in the U.S. mid-Atlantic region. First, we establish a connection between high-ozone (HO) days, defined as those with observed 8-h average ozone concentration greater than 70 parts per billion (ppb), and certain weather patterns, called synoptic types. We identify four summer synoptic types that most often are associated with HO days based on a 30-yr historical period (1986–2015) using NCEP–NCAR reanalysis. Second, we define thresholds for mean near-surface temperature and precipitation that characterize HO days during the four HO synoptic types. Next, we look at climate projections from five models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) for the early and late midcentury (2025–34 and 2045–54) and analyze the frequency of HO days. We find a general increasing trend, weaker in the early midcentury and stronger in the late midcentury, with 2 and 5 extra HO days per year, respectively, from 16 in 2015. These 5 extra days are the result of two processes. On one hand, the four HO synoptic types will increase in frequency, which explains about 1.5–2 extra HO days. The remaining 3–3.5 extra days are explained by the increase in near-surface temperatures during the HO synoptic types. Future air quality regulations, which have been successful in the historical period at reducing ozone concentrations in the mid-Atlantic, may need to become stricter to compensate for the underlying increasing trends from global warming.
Abstract
The goal of this study is to evaluate the effects of anthropogenic climate change on air quality, in particular on ozone, during the summer in the U.S. mid-Atlantic region. First, we establish a connection between high-ozone (HO) days, defined as those with observed 8-h average ozone concentration greater than 70 parts per billion (ppb), and certain weather patterns, called synoptic types. We identify four summer synoptic types that most often are associated with HO days based on a 30-yr historical period (1986–2015) using NCEP–NCAR reanalysis. Second, we define thresholds for mean near-surface temperature and precipitation that characterize HO days during the four HO synoptic types. Next, we look at climate projections from five models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) for the early and late midcentury (2025–34 and 2045–54) and analyze the frequency of HO days. We find a general increasing trend, weaker in the early midcentury and stronger in the late midcentury, with 2 and 5 extra HO days per year, respectively, from 16 in 2015. These 5 extra days are the result of two processes. On one hand, the four HO synoptic types will increase in frequency, which explains about 1.5–2 extra HO days. The remaining 3–3.5 extra days are explained by the increase in near-surface temperatures during the HO synoptic types. Future air quality regulations, which have been successful in the historical period at reducing ozone concentrations in the mid-Atlantic, may need to become stricter to compensate for the underlying increasing trends from global warming.
Abstract
This study compares the error characteristics associated with two grid refinement approaches including global variable resolution and nesting for high-resolution regional climate modeling. The global variable-resolution model, Model for Prediction Across Scales-Atmosphere (MPAS-A), and the limited-area model, Weather Research and Forecasting Model (WRF), are compared in an idealized aquaplanet context. For MPAS-A, simulations have been performed with a quasi-uniform-resolution global domain at coarse (1°) and high (0.25°) resolution, and a variable-resolution domain with a high-resolution region at 0.25° configured inside a coarse-resolution global domain at 1° resolution. Similarly, WRF has been configured to run on a coarse (1°) and high (0.25°) tropical channel domain as well as a nested domain with a high-resolution region at 0.25° nested two-way inside the coarse-resolution (1°) tropical channel. The variable-resolution or nested simulations are compared against the high-resolution simulations. Both models respond to increased resolution with enhanced precipitation and significant reduction in the ratio of convective to nonconvective precipitation. The limited-area grid refinement induces zonal asymmetry in precipitation (heating), accompanied by zonal anomalous Walker-like circulations and standing Rossby wave signals. Within the high-resolution limited area, the zonal distribution of precipitation is affected by advection in MPAS-A and by the nesting strategy in WRF. In both models, the propagation characteristics of equatorial waves are not significantly affected by the variations in resolution.
Abstract
This study compares the error characteristics associated with two grid refinement approaches including global variable resolution and nesting for high-resolution regional climate modeling. The global variable-resolution model, Model for Prediction Across Scales-Atmosphere (MPAS-A), and the limited-area model, Weather Research and Forecasting Model (WRF), are compared in an idealized aquaplanet context. For MPAS-A, simulations have been performed with a quasi-uniform-resolution global domain at coarse (1°) and high (0.25°) resolution, and a variable-resolution domain with a high-resolution region at 0.25° configured inside a coarse-resolution global domain at 1° resolution. Similarly, WRF has been configured to run on a coarse (1°) and high (0.25°) tropical channel domain as well as a nested domain with a high-resolution region at 0.25° nested two-way inside the coarse-resolution (1°) tropical channel. The variable-resolution or nested simulations are compared against the high-resolution simulations. Both models respond to increased resolution with enhanced precipitation and significant reduction in the ratio of convective to nonconvective precipitation. The limited-area grid refinement induces zonal asymmetry in precipitation (heating), accompanied by zonal anomalous Walker-like circulations and standing Rossby wave signals. Within the high-resolution limited area, the zonal distribution of precipitation is affected by advection in MPAS-A and by the nesting strategy in WRF. In both models, the propagation characteristics of equatorial waves are not significantly affected by the variations in resolution.
Abstract
This paper addresses several hypotheses designed to explain why AOGCM simulations of future climate in the third phase of the Coupled Model Intercomparison Project (CMIP3) feature an intensified reduction of precipitation over the Meso-America (MA) region. While the drying is consistent with an amplification of the subtropical high pressure cells and an equatorward contraction of convective regions due to the “upped ante” for convection in a warmer atmosphere, the physical mechanisms behind the intensity and robustness of the MA drying signal have not been fully explored. Regional variations in sea surface temperature (SST) warming may play a role. First, SSTs over the tropical North Atlantic (TNA) do not warm as much as the surrounding ocean. The troposphere senses a TNA that is cooler than the tropical Pacific, potentially exciting a Gill-type response, increasing the strength of the North Atlantic subtropical high. Second, the warm ENSO-like state simulated in the eastern tropical Pacific could decrease precipitation over MA, as warm ENSO events are associated with drying over MA.
The authors use the International Centre for Theoretical Physics (ICTP) AGCM to investigate the effects of these regional SST warming variations on the projected drying over MA. First, the change of SSTs [Special Report on Emissions Scenarios (SRES) A1B’s Twentieth-Century Climate in Coupled Model (A1B-20C)] in the ensemble average of the CMIP3 models is applied to determine if the ICTP AGCM can replicate the future drying. Then the effects of 1) removing the reduced warming over the TNA, 2) removing the warm ENSO-event-like pattern in the eastern tropical Pacific, and 3) applying uniform SST warming throughout the tropics are tested. The ICTP AGCM can reproduce the general pattern and amount of precipitation over MA. Simulations in which the CMIP3 A1B-20C ensemble-average SSTs are added to climatological SSTs show drying of more than 20% over the MA region, similar to the CMIP3 ensemble average. Replacing the relatively cooler SSTs over the TNA excites a Gill response consistent with an off-equatorial heating anomaly, showing that the TNA relative cooling is responsible for about 16% (31%) of the drying in late spring (early summer). The warm ENSO-like SST pattern over the eastern Pacific also affects precipitation over the MA region, with changes of 19% and 31% in March–June (MMJ) and June–August (JJA), respectively. This work highlights the importance of understanding even robust signals in the CMIP3 future scenario simulations, and should aid in the design and analysis of future climate change studies over the region.
Abstract
This paper addresses several hypotheses designed to explain why AOGCM simulations of future climate in the third phase of the Coupled Model Intercomparison Project (CMIP3) feature an intensified reduction of precipitation over the Meso-America (MA) region. While the drying is consistent with an amplification of the subtropical high pressure cells and an equatorward contraction of convective regions due to the “upped ante” for convection in a warmer atmosphere, the physical mechanisms behind the intensity and robustness of the MA drying signal have not been fully explored. Regional variations in sea surface temperature (SST) warming may play a role. First, SSTs over the tropical North Atlantic (TNA) do not warm as much as the surrounding ocean. The troposphere senses a TNA that is cooler than the tropical Pacific, potentially exciting a Gill-type response, increasing the strength of the North Atlantic subtropical high. Second, the warm ENSO-like state simulated in the eastern tropical Pacific could decrease precipitation over MA, as warm ENSO events are associated with drying over MA.
The authors use the International Centre for Theoretical Physics (ICTP) AGCM to investigate the effects of these regional SST warming variations on the projected drying over MA. First, the change of SSTs [Special Report on Emissions Scenarios (SRES) A1B’s Twentieth-Century Climate in Coupled Model (A1B-20C)] in the ensemble average of the CMIP3 models is applied to determine if the ICTP AGCM can replicate the future drying. Then the effects of 1) removing the reduced warming over the TNA, 2) removing the warm ENSO-event-like pattern in the eastern tropical Pacific, and 3) applying uniform SST warming throughout the tropics are tested. The ICTP AGCM can reproduce the general pattern and amount of precipitation over MA. Simulations in which the CMIP3 A1B-20C ensemble-average SSTs are added to climatological SSTs show drying of more than 20% over the MA region, similar to the CMIP3 ensemble average. Replacing the relatively cooler SSTs over the TNA excites a Gill response consistent with an off-equatorial heating anomaly, showing that the TNA relative cooling is responsible for about 16% (31%) of the drying in late spring (early summer). The warm ENSO-like SST pattern over the eastern Pacific also affects precipitation over the MA region, with changes of 19% and 31% in March–June (MMJ) and June–August (JJA), respectively. This work highlights the importance of understanding even robust signals in the CMIP3 future scenario simulations, and should aid in the design and analysis of future climate change studies over the region.
Abstract
No Abstract available.
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No Abstract available.
Abstract
This study investigates the moisture budgets and resolution dependency of precipitation extremes in an aquaplanet framework based on the Community Atmosphere Model, version 4 (CAM4). Moisture budgets from simulations using two different dynamical cores, the Model for Prediction Across Scales-Atmosphere (MPAS-A) and High Order Method Modeling Environment (HOMME), but the same physics parameterizations suggest that during precipitation extremes the intensity of precipitation is approximately balanced by the vertical advective moisture transport. The resolution dependency in extremes from simulations at their native grid resolution originates from that of vertical moisture transport, which is mainly explained by changes in dynamics (related to vertical velocity ω) with resolution. When assessed at the same grid scale by area-weighted averaging the fine-resolution simulations to the coarse grids, simulations with either dynamical core still demonstrate resolution dependency in extreme precipitation with no convergence over the tropics, but convergence occurs at a wide range of latitudes over the extratropics. The use of lower temporal frequency data (i.e., daily vs 6 hourly) reduces the resolution dependency. Although thermodynamic (moisture) changes become significant in offsetting the effect of dynamics when assessed at the same grid scale, especially over the extratropics, changes in dynamics with resolution are still large and explain most of the resolution dependency during extremes. This suggests that the effects of subgrid-scale variability of ω and vertical moisture transport during extremes are not adequately parameterized by the model at coarse resolution. The aquaplanet framework and analysis described in this study provide an important metric for assessing sensitivities of cloud parameterizations to spatial resolution and dynamical cores under extreme conditions.
Abstract
This study investigates the moisture budgets and resolution dependency of precipitation extremes in an aquaplanet framework based on the Community Atmosphere Model, version 4 (CAM4). Moisture budgets from simulations using two different dynamical cores, the Model for Prediction Across Scales-Atmosphere (MPAS-A) and High Order Method Modeling Environment (HOMME), but the same physics parameterizations suggest that during precipitation extremes the intensity of precipitation is approximately balanced by the vertical advective moisture transport. The resolution dependency in extremes from simulations at their native grid resolution originates from that of vertical moisture transport, which is mainly explained by changes in dynamics (related to vertical velocity ω) with resolution. When assessed at the same grid scale by area-weighted averaging the fine-resolution simulations to the coarse grids, simulations with either dynamical core still demonstrate resolution dependency in extreme precipitation with no convergence over the tropics, but convergence occurs at a wide range of latitudes over the extratropics. The use of lower temporal frequency data (i.e., daily vs 6 hourly) reduces the resolution dependency. Although thermodynamic (moisture) changes become significant in offsetting the effect of dynamics when assessed at the same grid scale, especially over the extratropics, changes in dynamics with resolution are still large and explain most of the resolution dependency during extremes. This suggests that the effects of subgrid-scale variability of ω and vertical moisture transport during extremes are not adequately parameterized by the model at coarse resolution. The aquaplanet framework and analysis described in this study provide an important metric for assessing sensitivities of cloud parameterizations to spatial resolution and dynamical cores under extreme conditions.
Abstract
Results from aquaplanet experiments performed using the Model for Prediction across Scales (MPAS) hydrostatic dynamical core implemented within the Department of Energy (DOE)–NCAR Community Atmosphere Model (CAM) are presented. MPAS is an unstructured-grid approach to climate system modeling that supports both quasi-uniform and variable-resolution meshing of the sphere based on conforming grids. Using quasi-uniform simulations at resolutions of 30, 60, 120, and 240 km, the authors evaluate the performance of CAM-MPAS via its kinetic energy spectra, general circulation, and precipitation characteristics. By analyzing an additional variable-resolution simulation with grid spacing that varies from 30 km in a spherical, continental-sized equatorial region to 240 km elsewhere, the CAM-MPAS’s potential for use as a regional climate simulation tool is explored.
Similar to other quasi-uniform aquaplanet simulations, tropical precipitation increases with resolution, indicating the resolution sensitivity of the physical parameterizations. Comparison with the finite volume (FV) dynamical core suggests a weaker tropical circulation in the CAM-MPAS simulations, which is evident in reduced tropical precipitation and a weaker Hadley circulation. In the variable-resolution simulation, the kinetic energy spectrum within the high-resolution region closely resembles the quasi-uniform 30-km simulation, indicating a robust simulation of the fluid dynamics. As suggested by the quasi-uniform simulations, the CAM4 physics behave differently in the high and low resolution regions. A positive precipitation anomaly occurs on the western edge of the high-resolution region, exciting a Gill-type response; this zonal asymmetry represents the errors incurred in a variable resolution setting. When paired with a multiresolution mesh, the aquaplanet test case offers an exceptional opportunity to examine the response of physical parameterizations to grid resolution.
Abstract
Results from aquaplanet experiments performed using the Model for Prediction across Scales (MPAS) hydrostatic dynamical core implemented within the Department of Energy (DOE)–NCAR Community Atmosphere Model (CAM) are presented. MPAS is an unstructured-grid approach to climate system modeling that supports both quasi-uniform and variable-resolution meshing of the sphere based on conforming grids. Using quasi-uniform simulations at resolutions of 30, 60, 120, and 240 km, the authors evaluate the performance of CAM-MPAS via its kinetic energy spectra, general circulation, and precipitation characteristics. By analyzing an additional variable-resolution simulation with grid spacing that varies from 30 km in a spherical, continental-sized equatorial region to 240 km elsewhere, the CAM-MPAS’s potential for use as a regional climate simulation tool is explored.
Similar to other quasi-uniform aquaplanet simulations, tropical precipitation increases with resolution, indicating the resolution sensitivity of the physical parameterizations. Comparison with the finite volume (FV) dynamical core suggests a weaker tropical circulation in the CAM-MPAS simulations, which is evident in reduced tropical precipitation and a weaker Hadley circulation. In the variable-resolution simulation, the kinetic energy spectrum within the high-resolution region closely resembles the quasi-uniform 30-km simulation, indicating a robust simulation of the fluid dynamics. As suggested by the quasi-uniform simulations, the CAM4 physics behave differently in the high and low resolution regions. A positive precipitation anomaly occurs on the western edge of the high-resolution region, exciting a Gill-type response; this zonal asymmetry represents the errors incurred in a variable resolution setting. When paired with a multiresolution mesh, the aquaplanet test case offers an exceptional opportunity to examine the response of physical parameterizations to grid resolution.
Abstract
The potential of an experimental nested prediction system to improve the simulation of subseasonal rainfall statistics including daily precipitation intensity, rainy season onset and withdrawal, and the frequency and duration of dry spells is evaluated by examining a four-member ensemble of regional climate model simulations performed for the period 1982–2002 over South America. The study employs the International Centre for Theoretical Physics (ICTP) regional climate model, version 3 (RegCM3), driven with the NCEP–NCAR reanalysis and the European Centre–Hamburg GCM, version 4.5. Statistics were examined for five regions: the northern Amazon, southern Amazon, the monsoon region, Northeast Brazil, and southeastern South America. RegCM3 and the GCM are able to replicate the distribution of daily rainfall intensity in most regions. The analysis of the rainy season timing shows the observed onset occurring first over the monsoon region and then spreading northward into the southern Amazon, in contrast to some previous studies. Correlations between the onset and withdrawal date and SSTs reveal a strong relationship between the withdrawal date in the monsoon region and SSTs in the equatorial Pacific, with above-average SSTs associated with late withdrawal. Over Northeast Brazil, the regional model errors are smaller than those shown by the GCM, and the strong interannual variability in the timing of the rainy season is better simulated by RegCM3. However, the regional model displays an early bias in onset and withdrawal over the southern Amazon and the monsoon regions. Both RegCM3 and the GCM tend to underestimate (overestimate) the frequency of shorter (longer) dry spells, although the differences in dry spell frequency during warm and cold ENSO events are well simulated. The results presented here show that there is potential for added value from the regional model in simulating subseasonal statistics; however, improvements in the physical parameterizations are needed for this tropical region.
Abstract
The potential of an experimental nested prediction system to improve the simulation of subseasonal rainfall statistics including daily precipitation intensity, rainy season onset and withdrawal, and the frequency and duration of dry spells is evaluated by examining a four-member ensemble of regional climate model simulations performed for the period 1982–2002 over South America. The study employs the International Centre for Theoretical Physics (ICTP) regional climate model, version 3 (RegCM3), driven with the NCEP–NCAR reanalysis and the European Centre–Hamburg GCM, version 4.5. Statistics were examined for five regions: the northern Amazon, southern Amazon, the monsoon region, Northeast Brazil, and southeastern South America. RegCM3 and the GCM are able to replicate the distribution of daily rainfall intensity in most regions. The analysis of the rainy season timing shows the observed onset occurring first over the monsoon region and then spreading northward into the southern Amazon, in contrast to some previous studies. Correlations between the onset and withdrawal date and SSTs reveal a strong relationship between the withdrawal date in the monsoon region and SSTs in the equatorial Pacific, with above-average SSTs associated with late withdrawal. Over Northeast Brazil, the regional model errors are smaller than those shown by the GCM, and the strong interannual variability in the timing of the rainy season is better simulated by RegCM3. However, the regional model displays an early bias in onset and withdrawal over the southern Amazon and the monsoon regions. Both RegCM3 and the GCM tend to underestimate (overestimate) the frequency of shorter (longer) dry spells, although the differences in dry spell frequency during warm and cold ENSO events are well simulated. The results presented here show that there is potential for added value from the regional model in simulating subseasonal statistics; however, improvements in the physical parameterizations are needed for this tropical region.
Abstract
Analyses of phase 5 of the Coupled Model Intercomparison Project (CMIP5) experiments show that the global monsoon is expected to increase in area, precipitation, and intensity as the climate system responds to anthropogenic forcing. Concurrently, detailed analyses for several individual monsoons indicate a redistribution of rainfall from early to late in the rainy season. This analysis examines CMIP5 projected changes in the annual cycle of precipitation in monsoon regions, using a moist static energy framework to evaluate competing mechanisms identified to be important in precipitation changes over land. In the presence of sufficient surface moisture, the local response to the increase in downwelling energy is characterized by increased evaporation, increased low-level moist static energy, and decreased stability with consequent increases in precipitation. A remote mechanism begins with warmer oceans and operates on land regions via a warmer tropical troposphere, increased stability, and decreased precipitation. The remote mechanism controls the projected changes during winter, and the local mechanism controls the switch to increased precipitation during summer in most monsoon regions. During the early summer transition, regions where boundary layer moisture availability is reduced owing to decreases in evaporation and moisture convergence experience an enhanced convective barrier. Regions characterized by adequate evaporation and moisture convergence do not experience reductions in early summer precipitation.
This enhanced convective barrier leads to a redistribution of rainfall from early to late summer, and is robust in the American and African monsoons but muddled in Asia. As described here, viewing monsoons from their inherent ties to the annual cycle could help to fingerprint changes as they evolve.
Abstract
Analyses of phase 5 of the Coupled Model Intercomparison Project (CMIP5) experiments show that the global monsoon is expected to increase in area, precipitation, and intensity as the climate system responds to anthropogenic forcing. Concurrently, detailed analyses for several individual monsoons indicate a redistribution of rainfall from early to late in the rainy season. This analysis examines CMIP5 projected changes in the annual cycle of precipitation in monsoon regions, using a moist static energy framework to evaluate competing mechanisms identified to be important in precipitation changes over land. In the presence of sufficient surface moisture, the local response to the increase in downwelling energy is characterized by increased evaporation, increased low-level moist static energy, and decreased stability with consequent increases in precipitation. A remote mechanism begins with warmer oceans and operates on land regions via a warmer tropical troposphere, increased stability, and decreased precipitation. The remote mechanism controls the projected changes during winter, and the local mechanism controls the switch to increased precipitation during summer in most monsoon regions. During the early summer transition, regions where boundary layer moisture availability is reduced owing to decreases in evaporation and moisture convergence experience an enhanced convective barrier. Regions characterized by adequate evaporation and moisture convergence do not experience reductions in early summer precipitation.
This enhanced convective barrier leads to a redistribution of rainfall from early to late summer, and is robust in the American and African monsoons but muddled in Asia. As described here, viewing monsoons from their inherent ties to the annual cycle could help to fingerprint changes as they evolve.
Abstract
Observations of robust scaling behavior in clouds and precipitation are used to derive constraints on how partitioning of precipitation should change with model resolution. Analysis indicates that 90%–99% of stratiform precipitation should occur in clouds that are resolvable by contemporary climate models (e.g., with 200-km or finer grid spacing). Furthermore, this resolved fraction of stratiform precipitation should increase sharply with resolution, such that effectively all stratiform precipitation should be resolvable above scales of ~50 km. It is shown that the Community Atmosphere Model (CAM) and the Weather Research and Forecasting model (WRF) also exhibit the robust cloud and precipitation scaling behavior that is present in observations, yet the resolved fraction of stratiform precipitation actually decreases with increasing model resolution. A suite of experiments with multiple dynamical cores provides strong evidence that this “scale-incognizant” behavior originates in one of the CAM4 parameterizations. An additional set of sensitivity experiments rules out both convection parameterizations, and by a process of elimination these results implicate the stratiform cloud and precipitation parameterization. Tests with the CAM5 physics package show improvements in the resolution dependence of resolved cloud fraction and resolved stratiform precipitation fraction.
Abstract
Observations of robust scaling behavior in clouds and precipitation are used to derive constraints on how partitioning of precipitation should change with model resolution. Analysis indicates that 90%–99% of stratiform precipitation should occur in clouds that are resolvable by contemporary climate models (e.g., with 200-km or finer grid spacing). Furthermore, this resolved fraction of stratiform precipitation should increase sharply with resolution, such that effectively all stratiform precipitation should be resolvable above scales of ~50 km. It is shown that the Community Atmosphere Model (CAM) and the Weather Research and Forecasting model (WRF) also exhibit the robust cloud and precipitation scaling behavior that is present in observations, yet the resolved fraction of stratiform precipitation actually decreases with increasing model resolution. A suite of experiments with multiple dynamical cores provides strong evidence that this “scale-incognizant” behavior originates in one of the CAM4 parameterizations. An additional set of sensitivity experiments rules out both convection parameterizations, and by a process of elimination these results implicate the stratiform cloud and precipitation parameterization. Tests with the CAM5 physics package show improvements in the resolution dependence of resolved cloud fraction and resolved stratiform precipitation fraction.
