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Abstract
Data from reanalyses recently carried out by several climate and numerical weather prediction centers will find a variety of applications in different branches of atmospheric science. A careful evaluation of the many aspects of these datasets is a prerequisite for their successful use.
This paper describes the implementation of a fully prognostic cloud scheme into the ECMWF reanalysis system and provides a first assessment of the simulation of cloud cover by comparing it with monthly mean cloud cover derived from satellite observations in the context of the International Satellite Cloud Climatology Project for the years 1983–90. Special emphasis is put on the major cloud regimes and their intra- and interannual variation.
The main deficiencies identified are an underestimation of extratropical cloud cover over the oceans by 10%–15%, an overestimation of trade wind cumulus cover by about 10%–15%, an underestimation of stratocumulus off the west coasts of the subtropical continents by 15%, and an underestimation of the summer maximum in cloud cover over the Eurasian continent. Despite these deficiencies it is shown that the reanalysis system is able to capture the main aspects of the interannual variability, especially those connected to the major El Niño events in the observation period.
Abstract
Data from reanalyses recently carried out by several climate and numerical weather prediction centers will find a variety of applications in different branches of atmospheric science. A careful evaluation of the many aspects of these datasets is a prerequisite for their successful use.
This paper describes the implementation of a fully prognostic cloud scheme into the ECMWF reanalysis system and provides a first assessment of the simulation of cloud cover by comparing it with monthly mean cloud cover derived from satellite observations in the context of the International Satellite Cloud Climatology Project for the years 1983–90. Special emphasis is put on the major cloud regimes and their intra- and interannual variation.
The main deficiencies identified are an underestimation of extratropical cloud cover over the oceans by 10%–15%, an overestimation of trade wind cumulus cover by about 10%–15%, an underestimation of stratocumulus off the west coasts of the subtropical continents by 15%, and an underestimation of the summer maximum in cloud cover over the Eurasian continent. Despite these deficiencies it is shown that the reanalysis system is able to capture the main aspects of the interannual variability, especially those connected to the major El Niño events in the observation period.
Abstract
An objective tropical cloud regime classification based on daytime averaged cloud-top pressure and optical thickness information from the International Satellite Cloud Climatology Project (ISCCP) is combined with precipitation and latent heating characteristics derived using the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). TRMM precipitation information is stratified into the ISCCP regimes in the tropical western Pacific (TWP), revealing the following three major precipitation regimes: a heavy (12 mm day−1) precipitation regime dominated by stratiform precipitation and top-heavy latent heating; a regime with moderate (5 mm day−1) precipitation amounts, mostly convective in nature with more midlevel latent heating; and a low (2 mm day−1) precipitation regime with a relatively large rain contribution from shallow convection, compared to the other regimes. Although three of the ISCCP cloud regimes are linked to the more convective, moderate precipitation regime, only one of the cloud regimes is associated with the more stratiform, top-heavy latent heating regime, making the ISCCP regimes a potentially useful tool for the further study of this dynamically important tropical weather state. Similarly, only one cloud regime is associated with the more shallow convective precipitation regime.
In terms of the TWP, precipitation and latent heating are dominated by the relatively infrequent (15%) occurrence of the strongly precipitating top-heavy latent heating state and by the frequent (>30%) occurrence of one of the more moderately precipitating convective states. The low precipitation/shallow cumulus regime occurs often (i.e., 25% of the time) but does not contribute strongly to the overall precipitation and latent heating. Each of these regimes also shows distinct geographical patterns in the TWP, thus providing insight into the distribution of convective and stratiform rain across the tropics. This study confirms the potential usefulness of the objective regime classification based on ISCCP, and it opens several new avenues for studying the interaction of convection with the large-scale tropical circulation.
Abstract
An objective tropical cloud regime classification based on daytime averaged cloud-top pressure and optical thickness information from the International Satellite Cloud Climatology Project (ISCCP) is combined with precipitation and latent heating characteristics derived using the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). TRMM precipitation information is stratified into the ISCCP regimes in the tropical western Pacific (TWP), revealing the following three major precipitation regimes: a heavy (12 mm day−1) precipitation regime dominated by stratiform precipitation and top-heavy latent heating; a regime with moderate (5 mm day−1) precipitation amounts, mostly convective in nature with more midlevel latent heating; and a low (2 mm day−1) precipitation regime with a relatively large rain contribution from shallow convection, compared to the other regimes. Although three of the ISCCP cloud regimes are linked to the more convective, moderate precipitation regime, only one of the cloud regimes is associated with the more stratiform, top-heavy latent heating regime, making the ISCCP regimes a potentially useful tool for the further study of this dynamically important tropical weather state. Similarly, only one cloud regime is associated with the more shallow convective precipitation regime.
In terms of the TWP, precipitation and latent heating are dominated by the relatively infrequent (15%) occurrence of the strongly precipitating top-heavy latent heating state and by the frequent (>30%) occurrence of one of the more moderately precipitating convective states. The low precipitation/shallow cumulus regime occurs often (i.e., 25% of the time) but does not contribute strongly to the overall precipitation and latent heating. Each of these regimes also shows distinct geographical patterns in the TWP, thus providing insight into the distribution of convective and stratiform rain across the tropics. This study confirms the potential usefulness of the objective regime classification based on ISCCP, and it opens several new avenues for studying the interaction of convection with the large-scale tropical circulation.
Abstract
Convection over the western equatorial Indian Ocean (WEIO) is strongly linked to precipitation over Africa and Australia but is poorly represented in current climate models, and its observed seasonal cycle is poorly understood. This study investigates the seasonal cycle of convection in the WEIO through rainfall and cloud measurements. Rainfall shows a single annual peak in early austral summer, but cloud proxies identify convective activity maxima in both boreal and austral summer. These diverging measures of convection during boreal summer are indicative of a reduction in the intensity of precipitation associated with a given cloud regime or cloud-top height during this time of year but an increase in the overall occurrence of high-top clouds and convectively active cloud regimes. The change in precipitation intensity associated with regimes is found to explain most of the changes in total precipitation during the period from May to November, whereas changes in the occurrence of convective regimes explains most of the changes throughout the rest of the year. The reduction in precipitation intensities associated with cloud regimes over the WEIO during boreal summer appears to be related to large-scale monsoon circulations, which suppress convection through forcing air descent in the midtroposphere and increase the apparent occurrence of convectively active cloud regimes through the advection of high-level cloud from monsoon-active areas toward the WEIO region.
Abstract
Convection over the western equatorial Indian Ocean (WEIO) is strongly linked to precipitation over Africa and Australia but is poorly represented in current climate models, and its observed seasonal cycle is poorly understood. This study investigates the seasonal cycle of convection in the WEIO through rainfall and cloud measurements. Rainfall shows a single annual peak in early austral summer, but cloud proxies identify convective activity maxima in both boreal and austral summer. These diverging measures of convection during boreal summer are indicative of a reduction in the intensity of precipitation associated with a given cloud regime or cloud-top height during this time of year but an increase in the overall occurrence of high-top clouds and convectively active cloud regimes. The change in precipitation intensity associated with regimes is found to explain most of the changes in total precipitation during the period from May to November, whereas changes in the occurrence of convective regimes explains most of the changes throughout the rest of the year. The reduction in precipitation intensities associated with cloud regimes over the WEIO during boreal summer appears to be related to large-scale monsoon circulations, which suppress convection through forcing air descent in the midtroposphere and increase the apparent occurrence of convectively active cloud regimes through the advection of high-level cloud from monsoon-active areas toward the WEIO region.
Abstract
A comparison of marine cold air outbreaks (MCAOs) in the Northern and Southern Hemispheres is presented, with attention to their seasonality, frequency of occurrence, and strength as measured by a cold air outbreak index. When considered on a gridpoint-by-gridpoint basis, MCAOs are more severe and more frequent in the Northern Hemisphere (NH) than the Southern Hemisphere (SH) in winter. However, when MCAOs are viewed as individual events regardless of horizontal extent, they occur more frequently in the SH. This is fundamentally because NH MCAOs are larger and stronger than those in the SH. MCAOs occur throughout the year, but in warm seasons and in the SH they are smaller and weaker than in cold seasons and in the NH. In both hemispheres, strong MCAOs occupy the cold air sector of midlatitude cyclones, which generally appear to be in their growth phase. Weak MCAOs in the SH occur under generally zonal flow with a slight northward component associated with weak zonal pressure gradients, while weak NH MCAOs occur under such a wide range of conditions that no characteristic synoptic pattern emerges from compositing. Strong boundary layer deepening, warming, and moistening occur as a result of the surface heat fluxes within MCAOs.
Abstract
A comparison of marine cold air outbreaks (MCAOs) in the Northern and Southern Hemispheres is presented, with attention to their seasonality, frequency of occurrence, and strength as measured by a cold air outbreak index. When considered on a gridpoint-by-gridpoint basis, MCAOs are more severe and more frequent in the Northern Hemisphere (NH) than the Southern Hemisphere (SH) in winter. However, when MCAOs are viewed as individual events regardless of horizontal extent, they occur more frequently in the SH. This is fundamentally because NH MCAOs are larger and stronger than those in the SH. MCAOs occur throughout the year, but in warm seasons and in the SH they are smaller and weaker than in cold seasons and in the NH. In both hemispheres, strong MCAOs occupy the cold air sector of midlatitude cyclones, which generally appear to be in their growth phase. Weak MCAOs in the SH occur under generally zonal flow with a slight northward component associated with weak zonal pressure gradients, while weak NH MCAOs occur under such a wide range of conditions that no characteristic synoptic pattern emerges from compositing. Strong boundary layer deepening, warming, and moistening occur as a result of the surface heat fluxes within MCAOs.
Abstract
This study investigates the radiative, cloud, and thermodynamic characteristics of the atmosphere separated into objectively defined cloud regimes in the tropical western Pacific (TWP). A cluster analysis is applied to 2 yr of daytime-only data from the International Satellite Cloud Climatology Project (ISCCP) to identify four major cloud regimes in the TWP region. A variety of data collected at the Department of Energy’s Atmospheric Radiation Measurement Program (ARM) site on Manus Island is then used to identify the main characteristics of the regimes. Those include surface and top-of-the-atmosphere radiative fluxes and cloud properties derived from a suite of ground-based active remote sensors, as well as the temperature and water vapor distribution measured from radiosondes.
The major cloud regimes identified in the TWP area are two suppressed regimes—one dominated by the occurrence of mostly shallow clouds, the other by thin cirrus—as well as two convectively active regimes—one exhibiting a large coverage of optically thin cirrus clouds, the other characterized by a large coverage with optically thick clouds. All four of these TWP cloud regimes are shown to exist with varying frequency of occurrence at the ARM site at Manus. It is further shown that the detailed data available at that site can be used to characterize the radiative, cloud, and thermodynamic properties of each of the regimes, demonstrating the potential of the regime separation to facilitate the extrapolation of observations at one location to larger scales. A variety of other potential applications of the regime separation are discussed.
Abstract
This study investigates the radiative, cloud, and thermodynamic characteristics of the atmosphere separated into objectively defined cloud regimes in the tropical western Pacific (TWP). A cluster analysis is applied to 2 yr of daytime-only data from the International Satellite Cloud Climatology Project (ISCCP) to identify four major cloud regimes in the TWP region. A variety of data collected at the Department of Energy’s Atmospheric Radiation Measurement Program (ARM) site on Manus Island is then used to identify the main characteristics of the regimes. Those include surface and top-of-the-atmosphere radiative fluxes and cloud properties derived from a suite of ground-based active remote sensors, as well as the temperature and water vapor distribution measured from radiosondes.
The major cloud regimes identified in the TWP area are two suppressed regimes—one dominated by the occurrence of mostly shallow clouds, the other by thin cirrus—as well as two convectively active regimes—one exhibiting a large coverage of optically thin cirrus clouds, the other characterized by a large coverage with optically thick clouds. All four of these TWP cloud regimes are shown to exist with varying frequency of occurrence at the ARM site at Manus. It is further shown that the detailed data available at that site can be used to characterize the radiative, cloud, and thermodynamic properties of each of the regimes, demonstrating the potential of the regime separation to facilitate the extrapolation of observations at one location to larger scales. A variety of other potential applications of the regime separation are discussed.
Abstract
Summertime (December–February) rainfall over northwestern Australia has increased significantly since the middle of the twentieth century. As a prerequisite to understanding the observed trend, this investigation examines the broad characteristics of rainfall and identifies the physical mechanisms by which rainfall in the region is initiated. This is achieved using a combination of in situ, spaceborne, and numerical reanalysis datasets.
Hourly pluviograph data and the Tropical Rainfall Measuring Mission (TRMM)-3B42 dataset show distinctly different diurnal cycles of rainfall in different geographical subregions; near the coast, rainfall rates peak in the midafternoon, whereas inland (near the maximum rainfall trend) the rainfall rate is largest overnight. These data also indicate that most of the summertime rain falls in events lasting 2–5 days. Analysis of the ECMWF Re-Analysis (ERA-Interim) demonstrates that convergence into the continental heat low controls the diurnal cycle of rainfall but cannot explain the synoptic variability.
Composites of wet and dry conditions from ERA-Interim expose synoptic-scale differences in the environmental flow. Prior to rain falling in the interior of northwestern Australia, there is a distinct shift in the origins of low-level air parcels, such that air with high convective available potential energy is advected from the tropical maritime regions, rather than from over the continent. Preliminary analysis suggests that these flow changes may be linked to transient synoptic disturbances such as midlatitude cyclones and monsoon lows. Rather than reflecting a large-scale change in the ocean state, these results imply that the observed increase in rainfall may be linked more closely to changes in the synoptic weather systems.
Abstract
Summertime (December–February) rainfall over northwestern Australia has increased significantly since the middle of the twentieth century. As a prerequisite to understanding the observed trend, this investigation examines the broad characteristics of rainfall and identifies the physical mechanisms by which rainfall in the region is initiated. This is achieved using a combination of in situ, spaceborne, and numerical reanalysis datasets.
Hourly pluviograph data and the Tropical Rainfall Measuring Mission (TRMM)-3B42 dataset show distinctly different diurnal cycles of rainfall in different geographical subregions; near the coast, rainfall rates peak in the midafternoon, whereas inland (near the maximum rainfall trend) the rainfall rate is largest overnight. These data also indicate that most of the summertime rain falls in events lasting 2–5 days. Analysis of the ECMWF Re-Analysis (ERA-Interim) demonstrates that convergence into the continental heat low controls the diurnal cycle of rainfall but cannot explain the synoptic variability.
Composites of wet and dry conditions from ERA-Interim expose synoptic-scale differences in the environmental flow. Prior to rain falling in the interior of northwestern Australia, there is a distinct shift in the origins of low-level air parcels, such that air with high convective available potential energy is advected from the tropical maritime regions, rather than from over the continent. Preliminary analysis suggests that these flow changes may be linked to transient synoptic disturbances such as midlatitude cyclones and monsoon lows. Rather than reflecting a large-scale change in the ocean state, these results imply that the observed increase in rainfall may be linked more closely to changes in the synoptic weather systems.
Abstract
Interannual variations in the sea surface temperature (SST) to the north of Australia are strongly linked to variations in Australian climate, including winter rainfall and tropical cyclone numbers. The north Australian SSTs are also closely linked to ENSO and tropical Pacific SSTs, with the relationship exhibiting a strong seasonal cycle. Credible predictions of Australian climate change therefore depend on climate models being able to represent ENSO and its connection to north Australian SSTs, the topic of this study.
First, the observational datasets of the Met Office Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) and the NOAA Extended Reconstructed Sea Surface Temperature (ERSST) are used to document the links between the Niño-3.4 index and a north Australian SST index, and the temporal evolution of north Australian SSTs during ENSO events. During austral autumn, the correlation between Niño-3.4 SST and north Australian SST is positive, while in austral spring it is strongly negative. During El Niño events, the north Australian SST anomalies become negative in the austral spring preceding the development of the positive Niño-3.4 SST anomalies.
The coupled models participating in the Coupled Model Intercomparison Project phase 3 (CMIP3) are evaluated in terms of this temporal evolution of Niño-3.4 SST and the relationship to north Australian SST for the twentieth-century simulations. Some of the models perform very well, while some do not capture the seasonal cycle of correlations at all. The way in which these relationships may change in the future is examined using the A2 emissions scenario in those models that do a reasonable job of capturing the present-day observed relationship, and very little change is found.
Abstract
Interannual variations in the sea surface temperature (SST) to the north of Australia are strongly linked to variations in Australian climate, including winter rainfall and tropical cyclone numbers. The north Australian SSTs are also closely linked to ENSO and tropical Pacific SSTs, with the relationship exhibiting a strong seasonal cycle. Credible predictions of Australian climate change therefore depend on climate models being able to represent ENSO and its connection to north Australian SSTs, the topic of this study.
First, the observational datasets of the Met Office Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) and the NOAA Extended Reconstructed Sea Surface Temperature (ERSST) are used to document the links between the Niño-3.4 index and a north Australian SST index, and the temporal evolution of north Australian SSTs during ENSO events. During austral autumn, the correlation between Niño-3.4 SST and north Australian SST is positive, while in austral spring it is strongly negative. During El Niño events, the north Australian SST anomalies become negative in the austral spring preceding the development of the positive Niño-3.4 SST anomalies.
The coupled models participating in the Coupled Model Intercomparison Project phase 3 (CMIP3) are evaluated in terms of this temporal evolution of Niño-3.4 SST and the relationship to north Australian SST for the twentieth-century simulations. Some of the models perform very well, while some do not capture the seasonal cycle of correlations at all. The way in which these relationships may change in the future is examined using the A2 emissions scenario in those models that do a reasonable job of capturing the present-day observed relationship, and very little change is found.
Abstract
Aspects of the climate of Australia are linked to interannual variability of the sea surface temperatures (SSTs) to the north of the country. SST anomalies in this region have been shown to exhibit strong, seasonally varying links to ENSO and tropical Pacific SSTs.
Previously, the models participating in phase 3 of the Coupled Model Intercomparison Project (CMIP3) have been evaluated and found to vary in their abilities to represent both the seasonal cycle of correlations between the Niño-3.4 and north Australian SSTs and the evolution of SSTs during composite El Niño and La Niña events. In this study, the new suite of models participating in the CMIP5 is evaluated using the same method. In the multimodel mean, the representation of the links is slightly improved, but generally the models do not capture the strength of the negative correlations during the second half of the year. The models also still struggle to capture the SST evolution in the north Australian region during El Niño and La Niña events.
Abstract
Aspects of the climate of Australia are linked to interannual variability of the sea surface temperatures (SSTs) to the north of the country. SST anomalies in this region have been shown to exhibit strong, seasonally varying links to ENSO and tropical Pacific SSTs.
Previously, the models participating in phase 3 of the Coupled Model Intercomparison Project (CMIP3) have been evaluated and found to vary in their abilities to represent both the seasonal cycle of correlations between the Niño-3.4 and north Australian SSTs and the evolution of SSTs during composite El Niño and La Niña events. In this study, the new suite of models participating in the CMIP5 is evaluated using the same method. In the multimodel mean, the representation of the links is slightly improved, but generally the models do not capture the strength of the negative correlations during the second half of the year. The models also still struggle to capture the SST evolution in the north Australian region during El Niño and La Niña events.
Abstract
Clouds strongly affect the absorption and reflection of shortwave and longwave radiation in the atmosphere. A key bias in climate models is related to excess absorbed shortwave radiation in the high-latitude Southern Ocean. Model evaluation studies attribute these biases in part to midtopped clouds, and observations confirm significant midtopped clouds in the zone of interest. However, it is not yet clear what cloud properties can be attributed to the deficit in modeled clouds. Present approaches using observed cloud regimes do not sufficiently differentiate between potentially distinct types of midtopped clouds and their meteorological contexts.
This study presents a refined set of midtopped cloud subregimes for the high-latitude Southern Ocean, which are distinct in their dynamical and thermodynamic background states. Active satellite observations from CloudSat and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) are used to study the macrophysical structure and microphysical properties of the new cloud regimes. The subgrid-scale variability of cloud structure and microphysics is quantified within the cloud regimes by identifying representative physical cloud profiles at high resolution from the radar–lidar (DARDAR) cloud classification mask.
The midtopped cloud subregimes distinguish between stratiform clouds under a high inversion and moderate subsidence; an optically thin cold-air advection cloud regime occurring under weak subsidence and including altostratus over low clouds; optically thick clouds with frequent deep structures under weak ascent and warm midlevel anomalies; and a midlevel convective cloud regime associated with strong ascent and warm advection. The new midtopped cloud regimes for the high-latitude Southern Ocean will provide a refined tool for model evaluation and the attribution of shortwave radiation biases to distinct cloud processes and properties.
Abstract
Clouds strongly affect the absorption and reflection of shortwave and longwave radiation in the atmosphere. A key bias in climate models is related to excess absorbed shortwave radiation in the high-latitude Southern Ocean. Model evaluation studies attribute these biases in part to midtopped clouds, and observations confirm significant midtopped clouds in the zone of interest. However, it is not yet clear what cloud properties can be attributed to the deficit in modeled clouds. Present approaches using observed cloud regimes do not sufficiently differentiate between potentially distinct types of midtopped clouds and their meteorological contexts.
This study presents a refined set of midtopped cloud subregimes for the high-latitude Southern Ocean, which are distinct in their dynamical and thermodynamic background states. Active satellite observations from CloudSat and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) are used to study the macrophysical structure and microphysical properties of the new cloud regimes. The subgrid-scale variability of cloud structure and microphysics is quantified within the cloud regimes by identifying representative physical cloud profiles at high resolution from the radar–lidar (DARDAR) cloud classification mask.
The midtopped cloud subregimes distinguish between stratiform clouds under a high inversion and moderate subsidence; an optically thin cold-air advection cloud regime occurring under weak subsidence and including altostratus over low clouds; optically thick clouds with frequent deep structures under weak ascent and warm midlevel anomalies; and a midlevel convective cloud regime associated with strong ascent and warm advection. The new midtopped cloud regimes for the high-latitude Southern Ocean will provide a refined tool for model evaluation and the attribution of shortwave radiation biases to distinct cloud processes and properties.
