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- Author or Editor: R. Subramanian x
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Abstract
Broadband flux emissivity schemes are often used to model infrared radiative exchanges in the atmosphere. In particular, such schemes help highlight the interaction of radiation with other transport processes, an aspect that is crucial to an understanding of phenomena relevant to the nocturnal boundary layer (NBL). Although the original schemes were restricted to radiatively black bounding surfaces, an extension of the same to nonblack surfaces has since been frequently used in NBL modeling. Herein, it is shown that the nonblack extension is erroneous and leads to a spurious yet intense near-surface cooling in the opaque bands. This paper presents the correct formulation that eliminates this cooling and discusses in some detail earlier NBL calculations affected by this error.
Abstract
Broadband flux emissivity schemes are often used to model infrared radiative exchanges in the atmosphere. In particular, such schemes help highlight the interaction of radiation with other transport processes, an aspect that is crucial to an understanding of phenomena relevant to the nocturnal boundary layer (NBL). Although the original schemes were restricted to radiatively black bounding surfaces, an extension of the same to nonblack surfaces has since been frequently used in NBL modeling. Herein, it is shown that the nonblack extension is erroneous and leads to a spurious yet intense near-surface cooling in the opaque bands. This paper presents the correct formulation that eliminates this cooling and discusses in some detail earlier NBL calculations affected by this error.
Abstract
In an accompanying paper by Ponnulakshmi et al., the prevailing flux emissivity scheme for nonblack surfaces was shown to include an erroneous reflected flux term. The error leads to a spurious cooling within the opaque bands; an expression for the correct broadband-reflected flux was given that eliminated this spurious cooling contribution. Herein, it is shown that the error is generic in nature, and is relevant to any frequency-parameterized radiation scheme applied to nonblack surfaces; such schemes are typically used in longwave radiation budget calculations. The correct reflected flux, previously developed within the framework of a broadband emissivity scheme in Ponnulakshmi et al., is generalized here so as to be applicable to any frequency-parameterized radiation model. The error is illustrated by comparing the bandwise fluxes, obtained using the prevailing and correct narrowband formulations, for a model (tropical) atmosphere. The flux discrepancy is the smallest for opaque bands within which the participating medium emits like a blackbody, and it is largest in frequency intervals where the medium is nearly transparent.
Abstract
In an accompanying paper by Ponnulakshmi et al., the prevailing flux emissivity scheme for nonblack surfaces was shown to include an erroneous reflected flux term. The error leads to a spurious cooling within the opaque bands; an expression for the correct broadband-reflected flux was given that eliminated this spurious cooling contribution. Herein, it is shown that the error is generic in nature, and is relevant to any frequency-parameterized radiation scheme applied to nonblack surfaces; such schemes are typically used in longwave radiation budget calculations. The correct reflected flux, previously developed within the framework of a broadband emissivity scheme in Ponnulakshmi et al., is generalized here so as to be applicable to any frequency-parameterized radiation model. The error is illustrated by comparing the bandwise fluxes, obtained using the prevailing and correct narrowband formulations, for a model (tropical) atmosphere. The flux discrepancy is the smallest for opaque bands within which the participating medium emits like a blackbody, and it is largest in frequency intervals where the medium is nearly transparent.
Abstract
This study quantifies, from a systematic set of regional ocean–atmosphere coupled model simulations employing various coupling intervals, the effect of subdaily sea surface temperature (SST) variability on the onset and intensity of Madden–Julian oscillation (MJO) convection in the Indian Ocean. The primary effect of diurnal SST variation (dSST) is to raise time-mean SST and latent heat flux (LH) prior to deep convection. Diurnal SST variation also strengthens the diurnal moistening of the troposphere by collocating the diurnal peak in LH with those of SST. Both effects enhance the convection such that the total precipitation amount scales quasi-linearly with preconvection dSST and time-mean SST. A column-integrated moist static energy (MSE) budget analysis confirms the critical role of diurnal SST variability in the buildup of column MSE and the strength of MJO convection via stronger time-mean LH and diurnal moistening. Two complementary atmosphere-only simulations further elucidate the role of SST conditions in the predictive skill of MJO. The atmospheric model forced with the persistent initial SST, lacking enhanced preconvection warming and moistening, produces a weaker and delayed convection than the diurnally coupled run. The atmospheric model with prescribed daily-mean SST from the coupled run, while eliminating the delayed peak, continues to exhibit weaker convection due to the lack of strong moistening on a diurnal basis. The fact that time-evolving SST with a diurnal cycle strongly influences the onset and intensity of MJO convection is consistent with previous studies that identified an improved representation of diurnal SST as a potential source of MJO predictability.
Abstract
This study quantifies, from a systematic set of regional ocean–atmosphere coupled model simulations employing various coupling intervals, the effect of subdaily sea surface temperature (SST) variability on the onset and intensity of Madden–Julian oscillation (MJO) convection in the Indian Ocean. The primary effect of diurnal SST variation (dSST) is to raise time-mean SST and latent heat flux (LH) prior to deep convection. Diurnal SST variation also strengthens the diurnal moistening of the troposphere by collocating the diurnal peak in LH with those of SST. Both effects enhance the convection such that the total precipitation amount scales quasi-linearly with preconvection dSST and time-mean SST. A column-integrated moist static energy (MSE) budget analysis confirms the critical role of diurnal SST variability in the buildup of column MSE and the strength of MJO convection via stronger time-mean LH and diurnal moistening. Two complementary atmosphere-only simulations further elucidate the role of SST conditions in the predictive skill of MJO. The atmospheric model forced with the persistent initial SST, lacking enhanced preconvection warming and moistening, produces a weaker and delayed convection than the diurnally coupled run. The atmospheric model with prescribed daily-mean SST from the coupled run, while eliminating the delayed peak, continues to exhibit weaker convection due to the lack of strong moistening on a diurnal basis. The fact that time-evolving SST with a diurnal cycle strongly influences the onset and intensity of MJO convection is consistent with previous studies that identified an improved representation of diurnal SST as a potential source of MJO predictability.
Abstract
In earlier papers it was shown that tropospheric vertical wind shear in the layer 850–200 mb decreases appreciably prior to formation of depressions in the Indian summer monsoon area. Further analysis reveals that this decrease in shear stems almost entirely from the upper troposphere between 400 mb (7.4 km) and 250 mb (10.5 km).
During the southwest monsoon period (June–September) the tropospheric wind shear over the Indian subcontinent is found to wax and wane in periods of 5–10 days in the latitudinal belt 9–27°N with an amplitude of the order of 35 m s−1 (650 mb)−1. These oscillations in wind shear thus appear to be a characteristic of the monsoon atmosphere. The phase of the shear oscillation south of about 16°N is opposite to that in the north.
Abstract
In earlier papers it was shown that tropospheric vertical wind shear in the layer 850–200 mb decreases appreciably prior to formation of depressions in the Indian summer monsoon area. Further analysis reveals that this decrease in shear stems almost entirely from the upper troposphere between 400 mb (7.4 km) and 250 mb (10.5 km).
During the southwest monsoon period (June–September) the tropospheric wind shear over the Indian subcontinent is found to wax and wane in periods of 5–10 days in the latitudinal belt 9–27°N with an amplitude of the order of 35 m s−1 (650 mb)−1. These oscillations in wind shear thus appear to be a characteristic of the monsoon atmosphere. The phase of the shear oscillation south of about 16°N is opposite to that in the north.
Abstract
A framework for an empirical parameterization (EP) of heterogeneous nucleation of ice crystals by multiple species of aerosol material in clouds was proposed in a 2008 paper by the authors. The present paper reports improvements to specification of a few of its empirical parameters. These include temperatures for onset of freezing, baseline surface areas of aerosol observed in field campaigns over Colorado, and new parameters for properties of black carbon, such as surface hydrophilicity and organic coatings. The EP’s third group of ice nucleus (IN) aerosols is redefined as that of primary biological aerosol particles (PBAPs), replacing insoluble organic aerosols. A fourth group of IN is introduced—namely, soluble organic aerosols.
The new EP predicts IN concentrations that agree well with aircraft data from selected traverses of shallow wave clouds observed in five flights (1, 3, 4, 6, and 12) of the 2007 Ice in Clouds Experiment–Layer Clouds (ICE-L). Selected traverses were confined to temperatures between about −25° and −29°C in layer cloud without homogeneously nucleated ice from aloft. Some of the wave clouds were affected by carbonaceous aerosols from biomass burning and by dust from dry lakebeds and elsewhere. The EP predicts a trend between number concentrations of heterogeneously nucleated ice crystals and apparent black carbon among the five wave clouds, observed by aircraft in ICE-L. It is predicted in terms of IN activity of black carbon.
The EP’s predictions are consistent with laboratory and field observations not used in its construction, for black carbon, dust, primary biological aerosols, and soluble organics. The EP’s prediction of biological ice nucleation is validated using coincident field observations of PBAP IN and PBAPs in Colorado.
Abstract
A framework for an empirical parameterization (EP) of heterogeneous nucleation of ice crystals by multiple species of aerosol material in clouds was proposed in a 2008 paper by the authors. The present paper reports improvements to specification of a few of its empirical parameters. These include temperatures for onset of freezing, baseline surface areas of aerosol observed in field campaigns over Colorado, and new parameters for properties of black carbon, such as surface hydrophilicity and organic coatings. The EP’s third group of ice nucleus (IN) aerosols is redefined as that of primary biological aerosol particles (PBAPs), replacing insoluble organic aerosols. A fourth group of IN is introduced—namely, soluble organic aerosols.
The new EP predicts IN concentrations that agree well with aircraft data from selected traverses of shallow wave clouds observed in five flights (1, 3, 4, 6, and 12) of the 2007 Ice in Clouds Experiment–Layer Clouds (ICE-L). Selected traverses were confined to temperatures between about −25° and −29°C in layer cloud without homogeneously nucleated ice from aloft. Some of the wave clouds were affected by carbonaceous aerosols from biomass burning and by dust from dry lakebeds and elsewhere. The EP predicts a trend between number concentrations of heterogeneously nucleated ice crystals and apparent black carbon among the five wave clouds, observed by aircraft in ICE-L. It is predicted in terms of IN activity of black carbon.
The EP’s predictions are consistent with laboratory and field observations not used in its construction, for black carbon, dust, primary biological aerosols, and soluble organics. The EP’s prediction of biological ice nucleation is validated using coincident field observations of PBAP IN and PBAPs in Colorado.
Abstract
Ice concentrations in orographic wave clouds at temperatures between −24° and −29°C were shown to be related to aerosol characteristics in nearby clear air during five research flights over the Rocky Mountains. When clouds with influence from colder temperatures were excluded from the dataset, mean ice nuclei and cloud ice number concentrations were very low, on the order of 1–5 L−1. In this environment, ice number concentrations were found to be significantly correlated with the number concentration of larger particles, those larger than both 0.1- and 0.5-μm diameter. A variety of complementary techniques was used to measure aerosol size distributions and chemical composition. Strong correlations were also observed between ice concentrations and the number concentrations of soot and biomass-burning aerosols. Ice nuclei concentrations directly measured in biomass-burning plumes were the highest detected during the project. Taken together, this evidence indicates a potential role for biomass-burning aerosols in ice formation, particularly in regions with relatively low concentrations of other ice nucleating aerosols.
Abstract
Ice concentrations in orographic wave clouds at temperatures between −24° and −29°C were shown to be related to aerosol characteristics in nearby clear air during five research flights over the Rocky Mountains. When clouds with influence from colder temperatures were excluded from the dataset, mean ice nuclei and cloud ice number concentrations were very low, on the order of 1–5 L−1. In this environment, ice number concentrations were found to be significantly correlated with the number concentration of larger particles, those larger than both 0.1- and 0.5-μm diameter. A variety of complementary techniques was used to measure aerosol size distributions and chemical composition. Strong correlations were also observed between ice concentrations and the number concentrations of soot and biomass-burning aerosols. Ice nuclei concentrations directly measured in biomass-burning plumes were the highest detected during the project. Taken together, this evidence indicates a potential role for biomass-burning aerosols in ice formation, particularly in regions with relatively low concentrations of other ice nucleating aerosols.
Abstract
During the Ice in Clouds Experiment–Layer Clouds (ICE-L), aged biomass-burning particles were identified within two orographic wave cloud regions over Wyoming using single-particle mass spectrometry and electron microscopy. Using a suite of instrumentation, particle chemistry was characterized in tandem with cloud microphysics. The aged biomass-burning particles comprised ∼30%–40% by number of the 0.1–1.0-μm clear-air particles and were composed of potassium, organic carbon, elemental carbon, and sulfate. Aerosol mass spectrometry measurements suggested these cloud-processed particles were predominantly sulfate by mass. The first cloud region sampled was characterized by primarily homogeneously nucleated ice particles formed at temperatures near −40°C. The second cloud period was characterized by high cloud droplet concentrations (∼150–300 cm−3) and lower heterogeneously nucleated ice concentrations (7–18 L−1) at cloud temperatures of −24° to −25°C. As expected for the observed particle chemistry and dynamics of the observed wave clouds, few significant differences were observed between the clear-air particles and cloud residues. However, suggestive of a possible heterogeneous nucleation mechanism within the first cloud region, ice residues showed enrichments in the number fractions of soot and mass fractions of black carbon, measured by a single-particle mass spectrometer and a single-particle soot photometer, respectively. In addition, enrichment of biomass-burning particles internally mixed with oxalic acid in both the homogeneously nucleated ice and cloud droplets compared to clear air suggests either preferential activation as cloud condensation nuclei or aqueous phase cloud processing.
Abstract
During the Ice in Clouds Experiment–Layer Clouds (ICE-L), aged biomass-burning particles were identified within two orographic wave cloud regions over Wyoming using single-particle mass spectrometry and electron microscopy. Using a suite of instrumentation, particle chemistry was characterized in tandem with cloud microphysics. The aged biomass-burning particles comprised ∼30%–40% by number of the 0.1–1.0-μm clear-air particles and were composed of potassium, organic carbon, elemental carbon, and sulfate. Aerosol mass spectrometry measurements suggested these cloud-processed particles were predominantly sulfate by mass. The first cloud region sampled was characterized by primarily homogeneously nucleated ice particles formed at temperatures near −40°C. The second cloud period was characterized by high cloud droplet concentrations (∼150–300 cm−3) and lower heterogeneously nucleated ice concentrations (7–18 L−1) at cloud temperatures of −24° to −25°C. As expected for the observed particle chemistry and dynamics of the observed wave clouds, few significant differences were observed between the clear-air particles and cloud residues. However, suggestive of a possible heterogeneous nucleation mechanism within the first cloud region, ice residues showed enrichments in the number fractions of soot and mass fractions of black carbon, measured by a single-particle mass spectrometer and a single-particle soot photometer, respectively. In addition, enrichment of biomass-burning particles internally mixed with oxalic acid in both the homogeneously nucleated ice and cloud droplets compared to clear air suggests either preferential activation as cloud condensation nuclei or aqueous phase cloud processing.
Abstract
The initiation of ice in an isolated orographic wave cloud was compared with expectations based on ice nucleating aerosol concentrations and with predictions from new ice nucleation parameterizations applied in a cloud parcel model. Measurements of ice crystal number concentrations were found to be in good agreement both with measured number concentrations of ice nuclei feeding the clouds and with ice nuclei number concentrations determined from the residual nuclei of cloud particles collected by a counterflow virtual impactor. Using lognormal distributions fitted to measured aerosol size distributions and measured aerosol chemical compositions, ice nuclei and ice crystal concentrations in the wave cloud were reasonably well predicted in a 1D parcel model framework. Two different empirical parameterizations were used in the parcel model: a parameterization based on aerosol chemical type and surface area and a parameterization that links ice nuclei number concentrations to the number concentrations of particles with diameters larger than 0.5 μm. This study shows that aerosol size distribution and composition measurements can be used to constrain ice initiation by primary nucleation in models. The data and model results also suggest the likelihood that the dust particle mode of the aerosol size distribution controls the number concentrations of the heterogeneous ice nuclei, at least for the lower temperatures examined in this case.
Abstract
The initiation of ice in an isolated orographic wave cloud was compared with expectations based on ice nucleating aerosol concentrations and with predictions from new ice nucleation parameterizations applied in a cloud parcel model. Measurements of ice crystal number concentrations were found to be in good agreement both with measured number concentrations of ice nuclei feeding the clouds and with ice nuclei number concentrations determined from the residual nuclei of cloud particles collected by a counterflow virtual impactor. Using lognormal distributions fitted to measured aerosol size distributions and measured aerosol chemical compositions, ice nuclei and ice crystal concentrations in the wave cloud were reasonably well predicted in a 1D parcel model framework. Two different empirical parameterizations were used in the parcel model: a parameterization based on aerosol chemical type and surface area and a parameterization that links ice nuclei number concentrations to the number concentrations of particles with diameters larger than 0.5 μm. This study shows that aerosol size distribution and composition measurements can be used to constrain ice initiation by primary nucleation in models. The data and model results also suggest the likelihood that the dust particle mode of the aerosol size distribution controls the number concentrations of the heterogeneous ice nuclei, at least for the lower temperatures examined in this case.
Abstract
The Indian Ocean Observing System (IndOOS), established in 2006, is a multinational network of sustained oceanic measurements that underpin understanding and forecasting of weather and climate for the Indian Ocean region and beyond. Almost one-third of humanity lives around the Indian Ocean, many in countries dependent on fisheries and rain-fed agriculture that are vulnerable to climate variability and extremes. The Indian Ocean alone has absorbed a quarter of the global oceanic heat uptake over the last two decades and the fate of this heat and its impact on future change is unknown. Climate models project accelerating sea level rise, more frequent extremes in monsoon rainfall, and decreasing oceanic productivity. In view of these new scientific challenges, a 3-yr international review of the IndOOS by more than 60 scientific experts now highlights the need for an enhanced observing network that can better meet societal challenges, and provide more reliable forecasts. Here we present core findings from this review, including the need for 1) chemical, biological, and ecosystem measurements alongside physical parameters; 2) expansion into the western tropics to improve understanding of the monsoon circulation; 3) better-resolved upper ocean processes to improve understanding of air–sea coupling and yield better subseasonal to seasonal predictions; and 4) expansion into key coastal regions and the deep ocean to better constrain the basinwide energy budget. These goals will require new agreements and partnerships with and among Indian Ocean rim countries, creating opportunities for them to enhance their monitoring and forecasting capacity as part of IndOOS-2.
Abstract
The Indian Ocean Observing System (IndOOS), established in 2006, is a multinational network of sustained oceanic measurements that underpin understanding and forecasting of weather and climate for the Indian Ocean region and beyond. Almost one-third of humanity lives around the Indian Ocean, many in countries dependent on fisheries and rain-fed agriculture that are vulnerable to climate variability and extremes. The Indian Ocean alone has absorbed a quarter of the global oceanic heat uptake over the last two decades and the fate of this heat and its impact on future change is unknown. Climate models project accelerating sea level rise, more frequent extremes in monsoon rainfall, and decreasing oceanic productivity. In view of these new scientific challenges, a 3-yr international review of the IndOOS by more than 60 scientific experts now highlights the need for an enhanced observing network that can better meet societal challenges, and provide more reliable forecasts. Here we present core findings from this review, including the need for 1) chemical, biological, and ecosystem measurements alongside physical parameters; 2) expansion into the western tropics to improve understanding of the monsoon circulation; 3) better-resolved upper ocean processes to improve understanding of air–sea coupling and yield better subseasonal to seasonal predictions; and 4) expansion into key coastal regions and the deep ocean to better constrain the basinwide energy budget. These goals will require new agreements and partnerships with and among Indian Ocean rim countries, creating opportunities for them to enhance their monitoring and forecasting capacity as part of IndOOS-2.
