Search Results
You are looking at 1 - 10 of 41 items for
- Author or Editor: Tristan S. L’Ecuyer x
- Refine by Access: All Content x
Abstract
A new method for retrieving rainfall profiles from a spaceborne radar is introduced. As a result of the frequencies necessary in spaceborne radar applications, attenuation by both rainfall and liquid cloud particles is nonnegligible and must be accurately accounted for before quantitative rainfall estimates can be made. The proposed method is based on the minimization of a cost function that allows one to account for attenuation at each level directly in the iteration process. In addition, the algorithm does not invoke the Rayleigh approximation and is, therefore, applicable at wavelengths characteristic of spaceborne radars. The method is flexible with regard to the parameters to be retrieved and is well-suited for the addition of measurements from other sensors, such as a passive microwave radiometer, to constrain the retrieval. Preliminary results, using simplified assumptions of drop size distribution and particle shape, illustrate the utility of the algorithm provided the attenuation is not severe. At the frequency of the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (14 GHz), synthetic retrievals are accurate to within 20% for rain rates up to 40 mm h−1. On the other hand, at 94 GHz, the frequency of the CloudSat cloud profiling radar, attenuation effects are too severe at rain rates greater than 1.5 mm h−1, suggesting the need for additional information to constrain the retrieval. Such information might come in the form of a path-integrated attenuation (PIA) derived from surface echo measurements or, alternatively, a precipitation water path (PWP) estimate from a passive microwave radiometer. Addition of a simple PWP constraint yields improvements in the retrieved rainfall profiles from both instruments when attenuation is severe. At 94 GHz, in particular, it is found that accurate quantitative rainfall estimates can be made provided the near-surface rain rate does not exceed 10 mm h−1.
Abstract
A new method for retrieving rainfall profiles from a spaceborne radar is introduced. As a result of the frequencies necessary in spaceborne radar applications, attenuation by both rainfall and liquid cloud particles is nonnegligible and must be accurately accounted for before quantitative rainfall estimates can be made. The proposed method is based on the minimization of a cost function that allows one to account for attenuation at each level directly in the iteration process. In addition, the algorithm does not invoke the Rayleigh approximation and is, therefore, applicable at wavelengths characteristic of spaceborne radars. The method is flexible with regard to the parameters to be retrieved and is well-suited for the addition of measurements from other sensors, such as a passive microwave radiometer, to constrain the retrieval. Preliminary results, using simplified assumptions of drop size distribution and particle shape, illustrate the utility of the algorithm provided the attenuation is not severe. At the frequency of the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (14 GHz), synthetic retrievals are accurate to within 20% for rain rates up to 40 mm h−1. On the other hand, at 94 GHz, the frequency of the CloudSat cloud profiling radar, attenuation effects are too severe at rain rates greater than 1.5 mm h−1, suggesting the need for additional information to constrain the retrieval. Such information might come in the form of a path-integrated attenuation (PIA) derived from surface echo measurements or, alternatively, a precipitation water path (PWP) estimate from a passive microwave radiometer. Addition of a simple PWP constraint yields improvements in the retrieved rainfall profiles from both instruments when attenuation is severe. At 94 GHz, in particular, it is found that accurate quantitative rainfall estimates can be made provided the near-surface rain rate does not exceed 10 mm h−1.
Abstract
The earth's weather and climate is driven by the meridional transport of energy required to establish a global balance between incoming energy from the sun and outgoing thermal energy emitted by the atmosphere and surface. Clouds and precipitation play an integral role in the exchange of these sources of energy between the surface, atmosphere, and space—enhancing reflection of solar radiation to space, trapping thermal emission from the surface, and providing a mechanism for the direct transfer of energy to the atmosphere through the release of latent heat in precipitation. This paper introduces a new multisensor algorithm for extracting longwave, shortwave, and latent heat fluxes over oceans from the sensors aboard the Tropical Rainfall Measuring Mission (TRMM) satellite. The technique synthesizes complementary information from distinct retrievals of high and low clouds and precipitation from the TRMM Microwave Imager (TMI) and Visible and Infrared Scanner (VIRS) instruments to initialize broadband radiative transfer calculations for deriving the structure of radiative heating in oceanic regions from 40°S to 40°N and its evolution on daily and monthly timescales.
Sensitivity studies using rigorous estimates of the uncertainties in all input parameters and detailed comparisons with flux observations from the Clouds and Earth's Radiant Energy System (CERES) are used to study the dominant influences on the algorithm's performance and to assess the accuracy of its products. The results demonstrate that the technique provides monthly mean estimates of oceanic longwave fluxes at 1° resolution to an accuracy of ∼10 W m−2. Uncertainties in these estimates are found to arise primarily from a lack of explicit vertical cloud boundary information and errors in prescribed temperature and humidity profiles. Corresponding shortwave flux estimates are shown to be accurate to ∼25 W m−2, with uncertainties due to errors in cloud detection, poorly constrained cloud particle sizes, and uncertainties in the prescribed surface albedo. When viewed as a whole, the components of the method provide a tool to diagnose relationships between the climate, hydrologic cycle, and the earth's energy budget.
Abstract
The earth's weather and climate is driven by the meridional transport of energy required to establish a global balance between incoming energy from the sun and outgoing thermal energy emitted by the atmosphere and surface. Clouds and precipitation play an integral role in the exchange of these sources of energy between the surface, atmosphere, and space—enhancing reflection of solar radiation to space, trapping thermal emission from the surface, and providing a mechanism for the direct transfer of energy to the atmosphere through the release of latent heat in precipitation. This paper introduces a new multisensor algorithm for extracting longwave, shortwave, and latent heat fluxes over oceans from the sensors aboard the Tropical Rainfall Measuring Mission (TRMM) satellite. The technique synthesizes complementary information from distinct retrievals of high and low clouds and precipitation from the TRMM Microwave Imager (TMI) and Visible and Infrared Scanner (VIRS) instruments to initialize broadband radiative transfer calculations for deriving the structure of radiative heating in oceanic regions from 40°S to 40°N and its evolution on daily and monthly timescales.
Sensitivity studies using rigorous estimates of the uncertainties in all input parameters and detailed comparisons with flux observations from the Clouds and Earth's Radiant Energy System (CERES) are used to study the dominant influences on the algorithm's performance and to assess the accuracy of its products. The results demonstrate that the technique provides monthly mean estimates of oceanic longwave fluxes at 1° resolution to an accuracy of ∼10 W m−2. Uncertainties in these estimates are found to arise primarily from a lack of explicit vertical cloud boundary information and errors in prescribed temperature and humidity profiles. Corresponding shortwave flux estimates are shown to be accurate to ∼25 W m−2, with uncertainties due to errors in cloud detection, poorly constrained cloud particle sizes, and uncertainties in the prescribed surface albedo. When viewed as a whole, the components of the method provide a tool to diagnose relationships between the climate, hydrologic cycle, and the earth's energy budget.
Abstract
The impact of clouds and precipitation on the climate is a strong function of their spatial distribution and microphysical properties, characteristics that depend, in turn, on the environments in which they form. Simulating feedbacks between clouds, precipitation, and their surroundings therefore places an enormous burden on the parameterized physics used in current climate models. This paper uses multisensor observations from the Tropical Rainfall Measuring Mission (TRMM) to assess the representation of the response of regional energy and water cycles in the tropical Pacific to the strong 1998 El Niño event in (Atmospheric Model Intercomparison Project) AMIP-style simulations from the climate models that participated in the Intergovernmental Panel on Climate Change’s (IPCC’s) most recent assessment report. The relationship between model errors and uncertainties in their representation of the impacts of clouds and precipitation on local energy budgets is also explored.
With the exception of cloud radiative impacts that are often overestimated in both regions, the responses of atmospheric composition and heating to El Niño are generally captured in the east Pacific where the SST forcing is locally direct. Many models fail, however, to correctly predict the magnitude of induced trends in the west Pacific where the response depends more critically on accurate representation of the zonal atmospheric circulation. As a result, a majority of the models examined do not reproduce the apparent westward transport of energy in the equatorial Pacific during the 1998 El Niño event. Furthermore, the intermodel variability in the responses of precipitation, total heating, and vertical motion is often larger than the intrinsic ENSO signal itself, implying an inherent lack of predictive capability in the ensemble with regard to the response of the mean zonal atmospheric circulation in the tropical Pacific to ENSO. While ENSO does not necessarily provide a proxy for anthropogenic climate change, the results suggest that deficiencies remain in the representation of relationships between radiation, clouds, and precipitation in current climate models that cannot be ignored when interpreting their predictions of future climate.
Abstract
The impact of clouds and precipitation on the climate is a strong function of their spatial distribution and microphysical properties, characteristics that depend, in turn, on the environments in which they form. Simulating feedbacks between clouds, precipitation, and their surroundings therefore places an enormous burden on the parameterized physics used in current climate models. This paper uses multisensor observations from the Tropical Rainfall Measuring Mission (TRMM) to assess the representation of the response of regional energy and water cycles in the tropical Pacific to the strong 1998 El Niño event in (Atmospheric Model Intercomparison Project) AMIP-style simulations from the climate models that participated in the Intergovernmental Panel on Climate Change’s (IPCC’s) most recent assessment report. The relationship between model errors and uncertainties in their representation of the impacts of clouds and precipitation on local energy budgets is also explored.
With the exception of cloud radiative impacts that are often overestimated in both regions, the responses of atmospheric composition and heating to El Niño are generally captured in the east Pacific where the SST forcing is locally direct. Many models fail, however, to correctly predict the magnitude of induced trends in the west Pacific where the response depends more critically on accurate representation of the zonal atmospheric circulation. As a result, a majority of the models examined do not reproduce the apparent westward transport of energy in the equatorial Pacific during the 1998 El Niño event. Furthermore, the intermodel variability in the responses of precipitation, total heating, and vertical motion is often larger than the intrinsic ENSO signal itself, implying an inherent lack of predictive capability in the ensemble with regard to the response of the mean zonal atmospheric circulation in the tropical Pacific to ENSO. While ENSO does not necessarily provide a proxy for anthropogenic climate change, the results suggest that deficiencies remain in the representation of relationships between radiation, clouds, and precipitation in current climate models that cannot be ignored when interpreting their predictions of future climate.
Abstract
The east Pacific double intertropical convergence zone (ITCZ) in austral fall is investigated with particular focus on the growing processes of its Southern Hemisphere branch. Satellite measurements from the Tropical Rainfall Measuring Mission (TRMM) and Quick Scatterometer (QuikSCAT) are analyzed to derive 8-yr climatology from 2000 to 2007. The earliest sign of the south ITCZ emerges in sea surface temperature (SST) by January, followed by the gradual development of surface convergence and water vapor. The shallow cumulus population starts growing to form the south ITCZ in February, a month earlier than vigorous deep convection is organized into the south ITCZ. The key factors that give rise to the initial SST enhancement or the southeast Pacific warm band are diagnosed by simple experiments. The experiments are designed to calculate SST, making use of an ocean mixed layer “model” forced by surface heat fluxes, all of which are derived from satellite observations. It is found that the shortwave flux absorbed into the ocean mixed layer is the primary driver of the southeast Pacific warm band. The warm band does not develop in boreal fall because the shortwave flux is seasonally so small that it is overwhelmed by other negative fluxes, including the latent heat and longwave fluxes. Clouds offset the net radiative flux by 10–15 W m−2, which is large enough for the warm band to develop in boreal fall if it were not for clouds reflecting shortwave radiation. Interannual variability of the double ITCZ is also discussed in brief.
Abstract
The east Pacific double intertropical convergence zone (ITCZ) in austral fall is investigated with particular focus on the growing processes of its Southern Hemisphere branch. Satellite measurements from the Tropical Rainfall Measuring Mission (TRMM) and Quick Scatterometer (QuikSCAT) are analyzed to derive 8-yr climatology from 2000 to 2007. The earliest sign of the south ITCZ emerges in sea surface temperature (SST) by January, followed by the gradual development of surface convergence and water vapor. The shallow cumulus population starts growing to form the south ITCZ in February, a month earlier than vigorous deep convection is organized into the south ITCZ. The key factors that give rise to the initial SST enhancement or the southeast Pacific warm band are diagnosed by simple experiments. The experiments are designed to calculate SST, making use of an ocean mixed layer “model” forced by surface heat fluxes, all of which are derived from satellite observations. It is found that the shortwave flux absorbed into the ocean mixed layer is the primary driver of the southeast Pacific warm band. The warm band does not develop in boreal fall because the shortwave flux is seasonally so small that it is overwhelmed by other negative fluxes, including the latent heat and longwave fluxes. Clouds offset the net radiative flux by 10–15 W m−2, which is large enough for the warm band to develop in boreal fall if it were not for clouds reflecting shortwave radiation. Interannual variability of the double ITCZ is also discussed in brief.
Abstract
The equatorial asymmetry of the east Pacific intertropical convergence zone (ITCZ) is explored on the basis of an ocean surface heat budget analysis carried out with a variety of satellite data products. The annual mean climatology of absorbed shortwave flux exhibits a pronounced meridional asymmetry due to a reduction of insolation by high clouds in the north ITCZ. Ocean mixed layer advection has the largest, if not exclusive, effect of counteracting this shortwave-exerted asymmetry. Other heat fluxes, in particular latent heat flux, predominate over the advective heat flux in magnitude but are secondary with respect to equatorial asymmetry. The asymmetry in advective heat flux stems from a warm pool off the Central American coast and, to a lesser extent, the North Equatorial Counter Current, neither of which exist in the Southern Hemisphere. The irregular continental geography presumably comes into play by generating a warm pool north of the equator and bringing cold waters to the south in the far eastern Pacific.
In addition to the annual climatology, the north–south contrast in the seasonal cycle of surface heat flux is instrumental in sustaining the north ITCZ throughout the year. The northeast Pacific is exposed to a seasonal cycle that is considerably weaker than that in the southeast Pacific, arising from multiple causes including the finite eccentricity of the earth’s orbit and meridional gradient in mixed layer absorptivity. Simple experiments generating synthetic sea surface temperature (SST) illustrate that the muted seasonal cycle of heat flux forcing moderates the SST seasonal variability in the northeast Pacific and thus allows the north ITCZ to persist year round. Existing theories on the ITCZ asymmetry are briefly examined in light of the present findings.
Abstract
The equatorial asymmetry of the east Pacific intertropical convergence zone (ITCZ) is explored on the basis of an ocean surface heat budget analysis carried out with a variety of satellite data products. The annual mean climatology of absorbed shortwave flux exhibits a pronounced meridional asymmetry due to a reduction of insolation by high clouds in the north ITCZ. Ocean mixed layer advection has the largest, if not exclusive, effect of counteracting this shortwave-exerted asymmetry. Other heat fluxes, in particular latent heat flux, predominate over the advective heat flux in magnitude but are secondary with respect to equatorial asymmetry. The asymmetry in advective heat flux stems from a warm pool off the Central American coast and, to a lesser extent, the North Equatorial Counter Current, neither of which exist in the Southern Hemisphere. The irregular continental geography presumably comes into play by generating a warm pool north of the equator and bringing cold waters to the south in the far eastern Pacific.
In addition to the annual climatology, the north–south contrast in the seasonal cycle of surface heat flux is instrumental in sustaining the north ITCZ throughout the year. The northeast Pacific is exposed to a seasonal cycle that is considerably weaker than that in the southeast Pacific, arising from multiple causes including the finite eccentricity of the earth’s orbit and meridional gradient in mixed layer absorptivity. Simple experiments generating synthetic sea surface temperature (SST) illustrate that the muted seasonal cycle of heat flux forcing moderates the SST seasonal variability in the northeast Pacific and thus allows the north ITCZ to persist year round. Existing theories on the ITCZ asymmetry are briefly examined in light of the present findings.
Abstract
This paper outlines recent advances in estimating atmospheric radiative heating rate profiles from the sensors aboard the Tropical Rainfall Measuring Mission (TRMM). The approach employs a deterministic framework in which four distinct retrievals of clouds, precipitation, and other atmospheric and surface properties are combined to form input to a broadband radiative transfer model that simulates profiles of upwelling and downwelling longwave and shortwave radiative fluxes in the atmosphere. Monthly, 5° top of the atmosphere outgoing longwave and shortwave flux estimates agree with corresponding observations from the Clouds and the Earth’s Radiant Energy System (CERES) to within 7 W m−2 and 3%, respectively, suggesting that the resulting products can be thought of as extending the eight-month CERES dataset to cover the full lifetime of TRMM.
The analysis of a decade of TRMM data provides a baseline climatology of the vertical structure of atmospheric radiative heating in today’s climate and an estimate of the magnitude of its response to environmental forcings on weekly to interannual time scales. In addition to illustrating the scope and properties of the dataset, the results highlight the strong influence of clouds, water vapor, and large-scale dynamics on regional radiation budgets and the vertical structure of radiative heating in the tropical and subtropical atmospheres. The combination of the radiative heating rate product described here, with profiles of latent heating that are now also being generated from TRMM sensors, provides a unique opportunity to develop large-scale estimates of vertically resolved atmospheric diabatic heating using satellite observations.
Abstract
This paper outlines recent advances in estimating atmospheric radiative heating rate profiles from the sensors aboard the Tropical Rainfall Measuring Mission (TRMM). The approach employs a deterministic framework in which four distinct retrievals of clouds, precipitation, and other atmospheric and surface properties are combined to form input to a broadband radiative transfer model that simulates profiles of upwelling and downwelling longwave and shortwave radiative fluxes in the atmosphere. Monthly, 5° top of the atmosphere outgoing longwave and shortwave flux estimates agree with corresponding observations from the Clouds and the Earth’s Radiant Energy System (CERES) to within 7 W m−2 and 3%, respectively, suggesting that the resulting products can be thought of as extending the eight-month CERES dataset to cover the full lifetime of TRMM.
The analysis of a decade of TRMM data provides a baseline climatology of the vertical structure of atmospheric radiative heating in today’s climate and an estimate of the magnitude of its response to environmental forcings on weekly to interannual time scales. In addition to illustrating the scope and properties of the dataset, the results highlight the strong influence of clouds, water vapor, and large-scale dynamics on regional radiation budgets and the vertical structure of radiative heating in the tropical and subtropical atmospheres. The combination of the radiative heating rate product described here, with profiles of latent heating that are now also being generated from TRMM sensors, provides a unique opportunity to develop large-scale estimates of vertically resolved atmospheric diabatic heating using satellite observations.
Abstract
Temporal variability in the moist static energy (MSE) budget is studied with measurements from a combination of different satellites including the Tropical Rainfall Measuring Mission (TRMM) and A-Train platforms. A composite time series before and after the development of moist convection is obtained from the observations to delineate the evolution of MSE and moisture convergences and, in their combination, gross moist stability (GMS). A new algorithm is then applied to estimate large-scale vertical motion from energy budget constraints through vertical-mode decomposition into first and second baroclinic modes and a background shallow mode. The findings are indicative of a possible mechanism of tropical convection. A gradual destabilization is brought about by the MSE convergence intrinsic to the positive second baroclinic mode (congestus mode) that increasingly counteracts a weak MSE divergence in the background state. GMS is driven to nearly zero as the first baroclinic mode begins to intensify, accelerating the growth of vigorous large-scale updrafts and deep convection. As the convective burst peaks, the positive second mode switches to the negative mode (stratiform mode) and introduces an abrupt rise in MSE divergence that likely discourages further maintenance of deep convection. The first mode quickly dissipates and GMS increases away from zero, eventually returning to the background shallow-mode state. A notable caveat to this scenario is that GMS serves as a more reliable metric when defined with a radiative heating rate included to offset MSE convergence.
Abstract
Temporal variability in the moist static energy (MSE) budget is studied with measurements from a combination of different satellites including the Tropical Rainfall Measuring Mission (TRMM) and A-Train platforms. A composite time series before and after the development of moist convection is obtained from the observations to delineate the evolution of MSE and moisture convergences and, in their combination, gross moist stability (GMS). A new algorithm is then applied to estimate large-scale vertical motion from energy budget constraints through vertical-mode decomposition into first and second baroclinic modes and a background shallow mode. The findings are indicative of a possible mechanism of tropical convection. A gradual destabilization is brought about by the MSE convergence intrinsic to the positive second baroclinic mode (congestus mode) that increasingly counteracts a weak MSE divergence in the background state. GMS is driven to nearly zero as the first baroclinic mode begins to intensify, accelerating the growth of vigorous large-scale updrafts and deep convection. As the convective burst peaks, the positive second mode switches to the negative mode (stratiform mode) and introduces an abrupt rise in MSE divergence that likely discourages further maintenance of deep convection. The first mode quickly dissipates and GMS increases away from zero, eventually returning to the background shallow-mode state. A notable caveat to this scenario is that GMS serves as a more reliable metric when defined with a radiative heating rate included to offset MSE convergence.
Abstract
Satellite observations are used to deduce the relationship between cloud water and precipitation water for low-latitude shallow marine clouds. The specific sensors that facilitate the analysis are the collocated CloudSat profiling radar and the Moderate Resolution Imaging Spectroradiometer (MODIS). The separation of the cloud water and precipitation water signals relies on the relative insensitivity of MODIS to the presence of precipitation water in conjunction with estimates of the path-integrated attenuation of the CloudSat radar beam while explicitly accounting for the effect of precipitation water on the observed MODIS optical depth. Variations in the precipitation water path are shown to be associated with both the cloud water path and the cloud effective radius, suggesting both macrophysical and microphysical controls on the production of precipitation water. The method outlined here is used to place broad bounds on the mean relationship between the precipitation water path and the cloud water path in shallow marine clouds, given certain clearly stated assumptions. The ratio of precipitation water to cloud water is shown to increase from zero at low cloud water path values to roughly 0.5 at 500 g m−2 of cloud water. The retrieval results further show that the median influence of precipitation on the observed optical depth increases monotonically with optical depth varying between 1% and 5% at 500 g m−2 of cloud water with the source of the uncertainty deriving from the assumption of the nature of the precipitation drop size distribution.
Abstract
Satellite observations are used to deduce the relationship between cloud water and precipitation water for low-latitude shallow marine clouds. The specific sensors that facilitate the analysis are the collocated CloudSat profiling radar and the Moderate Resolution Imaging Spectroradiometer (MODIS). The separation of the cloud water and precipitation water signals relies on the relative insensitivity of MODIS to the presence of precipitation water in conjunction with estimates of the path-integrated attenuation of the CloudSat radar beam while explicitly accounting for the effect of precipitation water on the observed MODIS optical depth. Variations in the precipitation water path are shown to be associated with both the cloud water path and the cloud effective radius, suggesting both macrophysical and microphysical controls on the production of precipitation water. The method outlined here is used to place broad bounds on the mean relationship between the precipitation water path and the cloud water path in shallow marine clouds, given certain clearly stated assumptions. The ratio of precipitation water to cloud water is shown to increase from zero at low cloud water path values to roughly 0.5 at 500 g m−2 of cloud water. The retrieval results further show that the median influence of precipitation on the observed optical depth increases monotonically with optical depth varying between 1% and 5% at 500 g m−2 of cloud water with the source of the uncertainty deriving from the assumption of the nature of the precipitation drop size distribution.
Abstract
A satellite data analysis is performed to explore the Madden–Julian oscillation (MJO) focusing on the potential roles of the equatorial Rossby (ER) and Kelvin waves. Measurements from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and Visible/Infrared Scanner (VIRS) are analyzed in the frequency–wavenumber domain to identify and ultimately filter primary low-frequency modes in the Tropics. The space–time spectrum of deep-storm fraction estimated by PR and VIRS exhibits notable Kelvin wave signals at wavenumbers 5–8, a distinct MJO peak at wavenumbers 1–7 and periods of about 40 days, and a signal corresponding to the ER wave. These modes are separately filtered to study the individual modes and possible relationship among them in the time–longitude space. In 10 cases analyzed here, an MJO event is often collocated with a group of consecutive Kelvin waves as well as an intruding ER wave accompanied with the occasional onset of a stationary convective phase. The spatial and temporal relationship between the MJO and Kelvin wave is clearly visible in a lag composite diagram, while the ubiquity of the ER wave leads to a less pronounced relation between the MJO and ER wave. A case study based on the Geostationary Meteorological Satellite (GMS) imagery together with associated dynamic field captures the substructure of the planetary-scale waves. A cross-correlation analysis confirms the MJO-related cycle that involves surface and atmospheric parameters such as sea surface temperature, water vapor, low clouds, shallow convection, and near-surface wind as proposed in past studies. The findings suggest the possibility that a sequence of convective events coupled with the linear waves may play a critical role in MJO propagation. An intraseasonal radiative–hydrological cycle inherent in the local thermodynamic conditions could be also a potential factor responsible for the MJO by loosely modulating the envelope of the entire propagation system.
Abstract
A satellite data analysis is performed to explore the Madden–Julian oscillation (MJO) focusing on the potential roles of the equatorial Rossby (ER) and Kelvin waves. Measurements from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and Visible/Infrared Scanner (VIRS) are analyzed in the frequency–wavenumber domain to identify and ultimately filter primary low-frequency modes in the Tropics. The space–time spectrum of deep-storm fraction estimated by PR and VIRS exhibits notable Kelvin wave signals at wavenumbers 5–8, a distinct MJO peak at wavenumbers 1–7 and periods of about 40 days, and a signal corresponding to the ER wave. These modes are separately filtered to study the individual modes and possible relationship among them in the time–longitude space. In 10 cases analyzed here, an MJO event is often collocated with a group of consecutive Kelvin waves as well as an intruding ER wave accompanied with the occasional onset of a stationary convective phase. The spatial and temporal relationship between the MJO and Kelvin wave is clearly visible in a lag composite diagram, while the ubiquity of the ER wave leads to a less pronounced relation between the MJO and ER wave. A case study based on the Geostationary Meteorological Satellite (GMS) imagery together with associated dynamic field captures the substructure of the planetary-scale waves. A cross-correlation analysis confirms the MJO-related cycle that involves surface and atmospheric parameters such as sea surface temperature, water vapor, low clouds, shallow convection, and near-surface wind as proposed in past studies. The findings suggest the possibility that a sequence of convective events coupled with the linear waves may play a critical role in MJO propagation. An intraseasonal radiative–hydrological cycle inherent in the local thermodynamic conditions could be also a potential factor responsible for the MJO by loosely modulating the envelope of the entire propagation system.
Abstract
This paper explores changes in the principal components of observed energy budgets across the tropical Pacific in response to the strong 1998 El Niño event. Multisensor observations from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), Visible and Infrared Scanner (VIRS), and precipitation radar (PR) instruments aboard TRMM are used to quantify changes in radiative and latent heating in the east and west Pacific in response to the different phases of the El Niño–Southern Oscillation. In periods of normal east–west SST gradients there is substantial heating in the west Pacific and cooling in the east, implying strong eastward atmospheric energy transport. During the active phase of the El Niño, both the east and west Pacific tend toward local radiative–convective equilibrium resulting in their temporary energetic decoupling. It is further demonstrated that the response of these regions to ENSO-induced SST variability is directly related to changes in the characteristics of clouds and precipitation in each region. Through quantitative analysis of the radiative and latent heating properties of shallow, midlevel, and deep precipitation events and an equivalent set of nonprecipitating cloud systems, times of reduced atmospheric heating are found to be associated with a shift toward shallow and midlevel precipitation systems and associated low-level cloudiness. The precipitation from such systems is typically less intense, and they do not trap outgoing longwave radiation as efficiently as their deeper counterparts, resulting in reduced radiative and latent heating of the atmosphere. The results also suggest that the net effect of precipitating systems on top-of-the-atmosphere (TOA) fluxes and the efficiency with which they heat the atmosphere and cool the surface exhibit strong dependence on their surroundings. The sensitivity of cloud radiative impacts to the atmospheric and surface properties they act to modify implies the existence of strong feedbacks whose representation may pose a significant challenge to the climate modeling community.
Abstract
This paper explores changes in the principal components of observed energy budgets across the tropical Pacific in response to the strong 1998 El Niño event. Multisensor observations from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), Visible and Infrared Scanner (VIRS), and precipitation radar (PR) instruments aboard TRMM are used to quantify changes in radiative and latent heating in the east and west Pacific in response to the different phases of the El Niño–Southern Oscillation. In periods of normal east–west SST gradients there is substantial heating in the west Pacific and cooling in the east, implying strong eastward atmospheric energy transport. During the active phase of the El Niño, both the east and west Pacific tend toward local radiative–convective equilibrium resulting in their temporary energetic decoupling. It is further demonstrated that the response of these regions to ENSO-induced SST variability is directly related to changes in the characteristics of clouds and precipitation in each region. Through quantitative analysis of the radiative and latent heating properties of shallow, midlevel, and deep precipitation events and an equivalent set of nonprecipitating cloud systems, times of reduced atmospheric heating are found to be associated with a shift toward shallow and midlevel precipitation systems and associated low-level cloudiness. The precipitation from such systems is typically less intense, and they do not trap outgoing longwave radiation as efficiently as their deeper counterparts, resulting in reduced radiative and latent heating of the atmosphere. The results also suggest that the net effect of precipitating systems on top-of-the-atmosphere (TOA) fluxes and the efficiency with which they heat the atmosphere and cool the surface exhibit strong dependence on their surroundings. The sensitivity of cloud radiative impacts to the atmospheric and surface properties they act to modify implies the existence of strong feedbacks whose representation may pose a significant challenge to the climate modeling community.
