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- Author or Editor: H. Zhang x
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Abstract
This study investigates gravity wave spectral characteristics based on high-resolution mesoscale simulations of idealized moist baroclinic jet–front systems with varying degrees of convective instability, with the intent of improving nonorographic gravity wave parameterizations. In all experiments, there is a clear dominance of negative vertical flux of zonal momentum. The westward momentum flux is distributed around the estimated ground-based baroclinic wave phase velocity along the zonal direction, while strong moist runs indicate a dipole structure pattern with stronger westward momentum flux centers at slower phase velocity and weaker eastward momentum flux centers at faster phase velocity. The spectral properties of short-scale wave components (50–200 km) generally differ from those of medium-scale ones (200–600 km). Compared to the medium-scale wave components, the momentum flux phase speed spectra for the short-scale ones appear to be more sensitive to the increasing initial moisture content. The spectral behavior in horizontal wavenumber space or phase velocity space is highly anisotropic, with a noticeable preference along the jet direction, except for the short-scale components in strong moist runs. It is confirmed that the dry gravity wave source (i.e., upper jet and/or surface front) generates a relatively narrow and less symmetrical power spectrum (dominated by negative momentum flux) centered around lower phase velocity and horizontal wavenumber, whereas the moist gravity wave source (i.e., moist convection) generates a broader and more symmetrical power spectrum, with a broader range of phase speeds and horizontal wavenumbers. This study also shows that the properties of gravity wave momentum flux depend on the location relative to the baroclinic jet.
Abstract
This study investigates gravity wave spectral characteristics based on high-resolution mesoscale simulations of idealized moist baroclinic jet–front systems with varying degrees of convective instability, with the intent of improving nonorographic gravity wave parameterizations. In all experiments, there is a clear dominance of negative vertical flux of zonal momentum. The westward momentum flux is distributed around the estimated ground-based baroclinic wave phase velocity along the zonal direction, while strong moist runs indicate a dipole structure pattern with stronger westward momentum flux centers at slower phase velocity and weaker eastward momentum flux centers at faster phase velocity. The spectral properties of short-scale wave components (50–200 km) generally differ from those of medium-scale ones (200–600 km). Compared to the medium-scale wave components, the momentum flux phase speed spectra for the short-scale ones appear to be more sensitive to the increasing initial moisture content. The spectral behavior in horizontal wavenumber space or phase velocity space is highly anisotropic, with a noticeable preference along the jet direction, except for the short-scale components in strong moist runs. It is confirmed that the dry gravity wave source (i.e., upper jet and/or surface front) generates a relatively narrow and less symmetrical power spectrum (dominated by negative momentum flux) centered around lower phase velocity and horizontal wavenumber, whereas the moist gravity wave source (i.e., moist convection) generates a broader and more symmetrical power spectrum, with a broader range of phase speeds and horizontal wavenumbers. This study also shows that the properties of gravity wave momentum flux depend on the location relative to the baroclinic jet.
Abstract
This paper examines changes in isotopic abundances for 18O and deuterium in precipitation over the Amazon basin based on data in the Global Network on Isotopes in Precipitation (GNIP) database from the International Atomic Energy Agency (IAEA)/WMO. The analysis is conducted in the context of recent changes (and anticipated future changes) to the land surface hydrology as a result of tropical deforestation. Statistically significant temporal changes (1965–90) in selected stable isotopic signatures in the Amazon have been compared with global climate model (GCM) predictions revealing notable differences. For example, the wet season deuterium excess differences between Belem and Manaus, Brazil, are consistent with recent GCM simulations only if there has been a relative increase in evaporation from nonfractionating water sources over this period. No significant change in dry season isotopic characteristics is found despite earlier predictions that land-use change signals would be found. Results of GCM simulations of Amazonian deforestation suggest that the recent stable isotope record is more consistent with the predicted effects of greenhouse warming possibly combined with forest removal than with the predicted effects of deforestation alone.
Abstract
This paper examines changes in isotopic abundances for 18O and deuterium in precipitation over the Amazon basin based on data in the Global Network on Isotopes in Precipitation (GNIP) database from the International Atomic Energy Agency (IAEA)/WMO. The analysis is conducted in the context of recent changes (and anticipated future changes) to the land surface hydrology as a result of tropical deforestation. Statistically significant temporal changes (1965–90) in selected stable isotopic signatures in the Amazon have been compared with global climate model (GCM) predictions revealing notable differences. For example, the wet season deuterium excess differences between Belem and Manaus, Brazil, are consistent with recent GCM simulations only if there has been a relative increase in evaporation from nonfractionating water sources over this period. No significant change in dry season isotopic characteristics is found despite earlier predictions that land-use change signals would be found. Results of GCM simulations of Amazonian deforestation suggest that the recent stable isotope record is more consistent with the predicted effects of greenhouse warming possibly combined with forest removal than with the predicted effects of deforestation alone.
Abstract
The influence of the boreal summer intraseasonal oscillation (BSISO) on the diurnal cycle of coastal rainfall over south China during the mei-yu (heavy rainfall) season is investigated using the OLR-based Madden–Julian oscillation index (OMI), satellite rainfall data, and atmospheric reanalysis. Results show that the mei-yu season coastal rainfall is enhanced during the BSISO phase 1 (convectively active phase over the western Indian Ocean), with 25% greater rainfall than the climatological regional mean. Rainfall is suppressed during the BSISO phases 4 and 5 (convectively active phase in the Bay of Bengal and South China Sea), with negative rainfall anomalies of 39% and 46%, respectively. During phase 1, the rainfall enhancement is mostly over the inland region during the afternoon, while there is little diurnal variability of the rainfall anomaly offshore. During phases 4 and 5, the rainfall suppression is considerably stronger over the offshore region in the morning, whereas stronger rainfall suppression occurs inland during the afternoon. In phase 8, positive rainfall anomalies are found over the offshore region with a peak from the morning to the early afternoon, whereas negative rainfall anomalies are found over the inland region with the strongest suppression in the late afternoon. Analysis of phase composites and horizontal moisture advection shows that the diurnal variation of rainfall anomalies over the south China coastal area during different BSISO phases can be interpreted as the interaction between the large-scale anomalous moisture advection and the local land and sea breeze circulations.
Abstract
The influence of the boreal summer intraseasonal oscillation (BSISO) on the diurnal cycle of coastal rainfall over south China during the mei-yu (heavy rainfall) season is investigated using the OLR-based Madden–Julian oscillation index (OMI), satellite rainfall data, and atmospheric reanalysis. Results show that the mei-yu season coastal rainfall is enhanced during the BSISO phase 1 (convectively active phase over the western Indian Ocean), with 25% greater rainfall than the climatological regional mean. Rainfall is suppressed during the BSISO phases 4 and 5 (convectively active phase in the Bay of Bengal and South China Sea), with negative rainfall anomalies of 39% and 46%, respectively. During phase 1, the rainfall enhancement is mostly over the inland region during the afternoon, while there is little diurnal variability of the rainfall anomaly offshore. During phases 4 and 5, the rainfall suppression is considerably stronger over the offshore region in the morning, whereas stronger rainfall suppression occurs inland during the afternoon. In phase 8, positive rainfall anomalies are found over the offshore region with a peak from the morning to the early afternoon, whereas negative rainfall anomalies are found over the inland region with the strongest suppression in the late afternoon. Analysis of phase composites and horizontal moisture advection shows that the diurnal variation of rainfall anomalies over the south China coastal area during different BSISO phases can be interpreted as the interaction between the large-scale anomalous moisture advection and the local land and sea breeze circulations.
Abstract
This is the second in a pair of papers in which the possible impacts of tropical deforestation are examined using a version of the NCAR CCM1. The emphasis in this paper is on the influence of tropical deforestation on the large-scale climate system. This influence is explored through the examination of the regional moisture budget and through an analysis of the Hadley and Walker circulations. Modification of the model surface parameters to simulate tropical deforestation produces significant modifications of both Hadley and Walker circulations, which result in changes distant from the, region of deforestation. A mechanism for propagation to middle and high latitudes of disturbances arising from tropical deforestation is proposed based on Rossby wave propagation mechanisms. These mechanisms, which have also been associated with the extratropical influences of ENSO events, provide a pathway for the dispersion of the tropical disturbances to high latitudes.
Abstract
This is the second in a pair of papers in which the possible impacts of tropical deforestation are examined using a version of the NCAR CCM1. The emphasis in this paper is on the influence of tropical deforestation on the large-scale climate system. This influence is explored through the examination of the regional moisture budget and through an analysis of the Hadley and Walker circulations. Modification of the model surface parameters to simulate tropical deforestation produces significant modifications of both Hadley and Walker circulations, which result in changes distant from the, region of deforestation. A mechanism for propagation to middle and high latitudes of disturbances arising from tropical deforestation is proposed based on Rossby wave propagation mechanisms. These mechanisms, which have also been associated with the extratropical influences of ENSO events, provide a pathway for the dispersion of the tropical disturbances to high latitudes.
Abstract
The potential impacts of deforestation in the humid Tropics are examined using a version of the National Center for Atmospheric Research's CCM1 coupled with the Biosphere-Atmosphere Transfer Scheme package. Tropical deforestation in South America, Africa, and Southeast Asia is studied using the results from an 11-yr deforestation experiment and a 25-yr control integration. It is found that the local-scale impact (here defined as within the area deforested) varies greatly between the three deforested regions due to the differing controls on the local atmospheric circulation: the Southeast Asian monsoon is much less sensitive to deforestation than the low-level flow over South America. The analysis of the changes in cloud radiative forcing suggests that reduction in cloud amount can significantly mitigate the imposed increases in surface albedo. The importance of water recycling by the forest canopy is stressed in the simulation of local precipitation changes. Correlation analysis of the changes resulting from the deforestation has been used to determine the nature of the processes that follow from the removal of the forest canopy and to suggest the important processes. The role of large-scale dynamics is explored in a companion paper.
Abstract
The potential impacts of deforestation in the humid Tropics are examined using a version of the National Center for Atmospheric Research's CCM1 coupled with the Biosphere-Atmosphere Transfer Scheme package. Tropical deforestation in South America, Africa, and Southeast Asia is studied using the results from an 11-yr deforestation experiment and a 25-yr control integration. It is found that the local-scale impact (here defined as within the area deforested) varies greatly between the three deforested regions due to the differing controls on the local atmospheric circulation: the Southeast Asian monsoon is much less sensitive to deforestation than the low-level flow over South America. The analysis of the changes in cloud radiative forcing suggests that reduction in cloud amount can significantly mitigate the imposed increases in surface albedo. The importance of water recycling by the forest canopy is stressed in the simulation of local precipitation changes. Correlation analysis of the changes resulting from the deforestation has been used to determine the nature of the processes that follow from the removal of the forest canopy and to suggest the important processes. The role of large-scale dynamics is explored in a companion paper.
Abstract
The Madden–Julian oscillation (MJO) is often observed to weaken or sometimes completely decay as its convective anomaly moves from the Indian Ocean over to the Maritime Continent (MC), which is known as the MC barrier effect on the MJO. This barrier effect is often exaggerated in numerical models. Using 23 years of the retrospective intraseasonal forecast from two coupled model systems with useful MJO prediction skills, we show that the predictive skill of the real-time multivariate MJO (RMM) index for the continuously propagating MJO events across the MC region is higher than for the blocked MJO events. The greater prediction skill is not related to the higher initial RMM amplitude for the continuous MJO events. Rather the higher skill arises from the more persistent behavior of the propagating MJO events as the convective anomaly moves through the MC region into the western Pacific. The potential predictability is similar for both types of MJO events, suggesting the forecast models hardly differentiate the two types of MJO events in prediction; they only maintain higher RMM magnitudes of the continuously propagating events. The global reanalysis dataset indicates that the blocked events are often associated with persistent higher surface pressures over colder sea surface temperatures in the central Pacific, suggesting the large-scale environment plays a role in promoting or inhibiting the MJO propagation across the MC region. Caveats in the models to reproduce the observed MJO events are also discussed.
Abstract
The Madden–Julian oscillation (MJO) is often observed to weaken or sometimes completely decay as its convective anomaly moves from the Indian Ocean over to the Maritime Continent (MC), which is known as the MC barrier effect on the MJO. This barrier effect is often exaggerated in numerical models. Using 23 years of the retrospective intraseasonal forecast from two coupled model systems with useful MJO prediction skills, we show that the predictive skill of the real-time multivariate MJO (RMM) index for the continuously propagating MJO events across the MC region is higher than for the blocked MJO events. The greater prediction skill is not related to the higher initial RMM amplitude for the continuous MJO events. Rather the higher skill arises from the more persistent behavior of the propagating MJO events as the convective anomaly moves through the MC region into the western Pacific. The potential predictability is similar for both types of MJO events, suggesting the forecast models hardly differentiate the two types of MJO events in prediction; they only maintain higher RMM magnitudes of the continuously propagating events. The global reanalysis dataset indicates that the blocked events are often associated with persistent higher surface pressures over colder sea surface temperatures in the central Pacific, suggesting the large-scale environment plays a role in promoting or inhibiting the MJO propagation across the MC region. Caveats in the models to reproduce the observed MJO events are also discussed.
Abstract
Long-lived, zonally propagating diurnal rainfall disturbances are a highly pronounced and common feature in the Maritime Continent (MC). A recent study argues that these disturbances can be explained as diurnally phase-locked gravity waves. Here we explore the origins of these waves through regional cloud-permitting numerical model experiments. The gravity waves are reproduced and isolated in the model framework through the combined use of realistic geography and diurnally cyclic lateral boundary conditions representative of both characteristic easterly and westerly background zonal flow regimes. These flow regimes are characteristic of the Madden–Julian oscillation (MJO) suppressed and active phase in the MC, respectively. Tests are conducted wherein Borneo, Sumatra, or both islands and/or their orography are removed. These tests imply that the diurnal gravity waves are excited and maintained directly by latent heating from the vigorous mesoscale convective systems (MCSs) that form nocturnally in both Borneo and Sumatra. Removing orography has only a secondary impact on both the MCSs and the gravity waves, implying that it is not critical to these waves. We therefore hypothesize that diurnal gravity waves are fundamentally driven by mesoscale organized deep convection, and are only sensitive to orography to the measure that the convection is affected by the orography and its mesoscale flows. Factor separation further reveals that the nonlinear interaction of synchronized diurnal cycles in Sumatra and Borneo slightly amplifies this gravity wave mode compared to if either island existed in isolation. This nonlinear feedback appears most prominently at longitudes directly between the two islands.
Abstract
Long-lived, zonally propagating diurnal rainfall disturbances are a highly pronounced and common feature in the Maritime Continent (MC). A recent study argues that these disturbances can be explained as diurnally phase-locked gravity waves. Here we explore the origins of these waves through regional cloud-permitting numerical model experiments. The gravity waves are reproduced and isolated in the model framework through the combined use of realistic geography and diurnally cyclic lateral boundary conditions representative of both characteristic easterly and westerly background zonal flow regimes. These flow regimes are characteristic of the Madden–Julian oscillation (MJO) suppressed and active phase in the MC, respectively. Tests are conducted wherein Borneo, Sumatra, or both islands and/or their orography are removed. These tests imply that the diurnal gravity waves are excited and maintained directly by latent heating from the vigorous mesoscale convective systems (MCSs) that form nocturnally in both Borneo and Sumatra. Removing orography has only a secondary impact on both the MCSs and the gravity waves, implying that it is not critical to these waves. We therefore hypothesize that diurnal gravity waves are fundamentally driven by mesoscale organized deep convection, and are only sensitive to orography to the measure that the convection is affected by the orography and its mesoscale flows. Factor separation further reveals that the nonlinear interaction of synchronized diurnal cycles in Sumatra and Borneo slightly amplifies this gravity wave mode compared to if either island existed in isolation. This nonlinear feedback appears most prominently at longitudes directly between the two islands.
Abstract
Future changes in floods and water availability across China under representative concentration pathway 2.6 (RCP2.6) and RCP8.5 are studied by analyzing discharge simulations from the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP) with the consideration of uncertainties among global climate models (GCMs) and hydrologic models. Floods and water availability derived from ISI-MIP simulations are compared against observations. The uncertainties among models are quantified by model agreement. Only model agreement >50% is considered to generate reliable projections of floods and water availability and their relationships with climate change. The results show five major points. First, ISI-MIP simulations have acceptable ability in modeling floods and water availability. The spatial patterns of changes in floods and water availability highly depend on the outputs of GCMs. Uncertainties from GCMs/hydrologic models predominate the uncertainties in the wet/dry areas in eastern/northwestern China. Second, the magnitudes of floods throughout China increase during 2070–99 under RCP8.5 relative to those with the same return periods during 1971–2000. The increase rates of larger floods are higher than those of the smaller ones. Third, water availability decreases/increases in southern/northern China under RCP8.5, but changes negligibly under RCP2.6. Fourth, more severe floods in the future are driven by more intense precipitation extremes over China. The negligible change in mean precipitation and the increase in actual evapotranspiration reduce the water availability in southern China. Fifth, model agreements are higher in simulated floods than water availability because increasing precipitation extremes are more consistent among different GCM outputs compared to mean precipitation.
Abstract
Future changes in floods and water availability across China under representative concentration pathway 2.6 (RCP2.6) and RCP8.5 are studied by analyzing discharge simulations from the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP) with the consideration of uncertainties among global climate models (GCMs) and hydrologic models. Floods and water availability derived from ISI-MIP simulations are compared against observations. The uncertainties among models are quantified by model agreement. Only model agreement >50% is considered to generate reliable projections of floods and water availability and their relationships with climate change. The results show five major points. First, ISI-MIP simulations have acceptable ability in modeling floods and water availability. The spatial patterns of changes in floods and water availability highly depend on the outputs of GCMs. Uncertainties from GCMs/hydrologic models predominate the uncertainties in the wet/dry areas in eastern/northwestern China. Second, the magnitudes of floods throughout China increase during 2070–99 under RCP8.5 relative to those with the same return periods during 1971–2000. The increase rates of larger floods are higher than those of the smaller ones. Third, water availability decreases/increases in southern/northern China under RCP8.5, but changes negligibly under RCP2.6. Fourth, more severe floods in the future are driven by more intense precipitation extremes over China. The negligible change in mean precipitation and the increase in actual evapotranspiration reduce the water availability in southern China. Fifth, model agreements are higher in simulated floods than water availability because increasing precipitation extremes are more consistent among different GCM outputs compared to mean precipitation.
Abstract
Diabatic heating (or Q 1) profiles associated with specific cloud types are produced by matching synoptic cloud observations with a sounding budget analysis during the Tropical Rainfall Measuring Mission (TRMM) Kwajalein Experiment (KWAJEX), which took place in the Marshall Islands from late July through mid-September 1999. Fair-weather cumulus clouds produce up to 1 K day−1 of heating below 850 hPa and are associated with cooling throughout much of the rest of the troposphere. Cumulus congestus clouds produce heating on the order of 1 K day−1 up to 575 hPa and cooling in the mid- to upper troposphere. Cumulonimbus clouds produce heating through the depth of the troposphere, with a maximum of 3.7 K day−1 near 550 hPa. Cloud types indicating widespread rain (stratus or cumulus fractus of bad weather at low levels and nimbostratus at midlevels) have the largest and most elevated heating, with values >10 K day−1 above 600 hPa. Other mid- and high-level cloud types are shown to be consistent with area-averaged rain rates and Q 1 profiles. Profiles of the divergence and apparent moisture sink (or Q 2) for convective clouds are also analyzed and are shown to be consistent with the physics of the heating profiles just described.
Abstract
Diabatic heating (or Q 1) profiles associated with specific cloud types are produced by matching synoptic cloud observations with a sounding budget analysis during the Tropical Rainfall Measuring Mission (TRMM) Kwajalein Experiment (KWAJEX), which took place in the Marshall Islands from late July through mid-September 1999. Fair-weather cumulus clouds produce up to 1 K day−1 of heating below 850 hPa and are associated with cooling throughout much of the rest of the troposphere. Cumulus congestus clouds produce heating on the order of 1 K day−1 up to 575 hPa and cooling in the mid- to upper troposphere. Cumulonimbus clouds produce heating through the depth of the troposphere, with a maximum of 3.7 K day−1 near 550 hPa. Cloud types indicating widespread rain (stratus or cumulus fractus of bad weather at low levels and nimbostratus at midlevels) have the largest and most elevated heating, with values >10 K day−1 above 600 hPa. Other mid- and high-level cloud types are shown to be consistent with area-averaged rain rates and Q 1 profiles. Profiles of the divergence and apparent moisture sink (or Q 2) for convective clouds are also analyzed and are shown to be consistent with the physics of the heating profiles just described.
Abstract
To understand the factors causing the interannual variations in the summer retreat of the Beaufort Sea ice edge, Seasonal Ice Zone Reconnaissance Surveys (SIZRS) aboard U.S. Coast Guard Arctic Domain Awareness flights were made monthly from June to October in 2012, 2013, and 2014. The seasonal ice zone (SIZ) is where sea ice melts and reforms annually and encompasses the nominally narrower marginal ice zone (MIZ) where a mix of open-ocean and ice pack processes prevail. Thus, SIZRS provides a regional context for the smaller-scale MIZ processes. Observations with aircraft expendable conductivity–temperature–depth probes reveal a salinity pattern associated with large-scale gyre circulation and the seasonal formation of a shallow (~20 m) fresh layer moving with the ice edge position. Repeat occupations of the SIZRS lines from 72° to 76°N on 140° and 150°W allow a comparison of observed hydrography to atmospheric indices. Using this relationship, the basinwide salinity signals are separated from the fresh layer associated with the ice edge. While this layer extends northward under the ice edge as the melt season progresses, low salinities and warm temperatures appear south of the edge. Within this fresh layer, average salinity is correlated with distance from the ice edge. The salinity observations suggest that the upper-ocean freshening over the summer is dominated by local sea ice melt and vertical mixing. A Price–Weller–Pinkel model analysis reveals that observed changes in heat content and density structure are also consistent with a 1D mixing process.
Abstract
To understand the factors causing the interannual variations in the summer retreat of the Beaufort Sea ice edge, Seasonal Ice Zone Reconnaissance Surveys (SIZRS) aboard U.S. Coast Guard Arctic Domain Awareness flights were made monthly from June to October in 2012, 2013, and 2014. The seasonal ice zone (SIZ) is where sea ice melts and reforms annually and encompasses the nominally narrower marginal ice zone (MIZ) where a mix of open-ocean and ice pack processes prevail. Thus, SIZRS provides a regional context for the smaller-scale MIZ processes. Observations with aircraft expendable conductivity–temperature–depth probes reveal a salinity pattern associated with large-scale gyre circulation and the seasonal formation of a shallow (~20 m) fresh layer moving with the ice edge position. Repeat occupations of the SIZRS lines from 72° to 76°N on 140° and 150°W allow a comparison of observed hydrography to atmospheric indices. Using this relationship, the basinwide salinity signals are separated from the fresh layer associated with the ice edge. While this layer extends northward under the ice edge as the melt season progresses, low salinities and warm temperatures appear south of the edge. Within this fresh layer, average salinity is correlated with distance from the ice edge. The salinity observations suggest that the upper-ocean freshening over the summer is dominated by local sea ice melt and vertical mixing. A Price–Weller–Pinkel model analysis reveals that observed changes in heat content and density structure are also consistent with a 1D mixing process.
