Zhien Wang - Profile on Academia.edu (original) (raw)
Papers by Zhien Wang
A Refined Two-Channel Microwave Radiometer Liquid Water Path Retrieval for Cold Regions by Using Multiple-Sensor Measurements
IEEE Geoscience and Remote Sensing Letters, 2007
AbstractTraditional two-channel microwave radiometers (MWRs) are widely used to measure cloud li... more AbstractTraditional two-channel microwave radiometers (MWRs) are widely used to measure cloud liquid water path (LWP); however, the retrieved LWPs are subject to relatively large uncertainties, particularly for low LWP clouds. By reformulating the statistical retrieval ...
Atmospheric Chemistry and Physics, 2008
Airborne dust is a major environmental hazard in Asia. Using an analysis of the first full year o... more Airborne dust is a major environmental hazard in Asia. Using an analysis of the first full year of CALIPSO lidar measurements, this paper derives unprecedented, altituderesolved seasonal distributions of desert dust transported over the Tibetan Plateau (TP) and the surrounding areas. The CALIPSO lidar observations include numerous 5
Journal of Geophysical Research, 2008
Based on the first year of CALIPSO lidar measurements under cloud-free conditions, a height-resol... more Based on the first year of CALIPSO lidar measurements under cloud-free conditions, a height-resolved global distribution of dust aerosols is presented for the first time. Results indicate that spring is the most active dust season, during which ˜20% and ˜12% of areas between 0 and 60°N are influenced by dust at least 10% and 50% of the time, respectively. In summer within 3-6 km, ˜8.3% of area between 0 and 60°N is impacted by dust at least 50% of the time. Strong seasonal cycles of dust layer vertical extent are observed in major source regions, which are similar to the seasonal variation of the thermally driven boundary layer depth. The arid and semiarid areas in North Africa and the Arabian Peninsula are the most persistent and prolific dust sources. African dust is transported across the Atlantic all yearlong with strong seasonal variation in the transport pathways mainly in the free troposphere in summer and at the low altitudes in winter. However, the trans-Atlantic dust is transported at the low altitudes is important for all seasons, especially transported further cross the ocean. The crossing Atlantic dusty zones are shifted southward from summer to winter, which is accompanied by a similar southward shift of dust-generating areas over North Africa. The Taklimakan and Gobi deserts are two major dust sources in East Asia with long-range transport mainly occurring in spring. The large horizontal and vertical coverage of dust aerosols indicate their importance in the climate system through both direct and indirect aerosol effects.
Classifying clouds around the globe with the CloudSat radar: 1-year of results
Geophysical Research Letters, 2008
CloudSat supports a 94 GHz cloud profiling radar as part of the innovative A-train formation of s... more CloudSat supports a 94 GHz cloud profiling radar as part of the innovative A-train formation of satellites studying the Earths clouds and atmosphere. Using the vertical profiles of clouds and precipitation, an algorithm has been developed to determine the type of clouds present. ...
Testing IWC Retrieval Methods Using Radar and Ancillary Measurements with In Situ Data
Journal of Applied Meteorology and Climatology, 2008
Vertical profiles of ice water content (IWC) can now be derived globally from spaceborne cloud sa... more Vertical profiles of ice water content (IWC) can now be derived globally from spaceborne cloud satellite radar (CloudSat) data. Integrating these data with Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data may further increase accuracy. Evaluations of the ...
Journal of Atmospheric and Oceanic Technology, 2009
The current cloud thermodynamic phase discrimination by Cloud-Aerosol Lidar Pathfinder Satellite ... more The current cloud thermodynamic phase discrimination by Cloud-Aerosol Lidar Pathfinder Satellite Observations (CALIPSO) is based on the depolarization of backscattered light measured by its lidar [Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)]. It assumes that backscattered light from ice crystals is depolarizing, whereas water clouds, being spherical, result in minimal depolarization. However, because of the relationship between the CALIOP field of view (FOV) and the large distance between the satellite and clouds and because of the frequent presence of oriented ice crystals, there is often a weak correlation between measured depolarization and phase, which thereby creates significant uncertainties in the current CALIOP phase retrieval. For water clouds, the CALIOP-measured depolarization can be large because of multiple scattering, whereas horizontally oriented ice particles depolarize only weakly and behave similarly to water clouds. Because of the nonunique depolarization-cloud phase relationship, more constraints are necessary to uniquely determine cloud phase. Based on theoretical and modeling studies, an improved cloud phase determination algorithm has been developed. Instead of depending primarily on layer-integrated depolarization ratios, this algorithm differentiates cloud phases by using the spatial correlation of layer-integrated attenuated backscatter and layer-integrated particulate depolarization ratio. This approach includes a two-step process: 1) use of a simple two-dimensional threshold method to provide a preliminary identification of ice clouds containing randomly oriented particles, ice clouds with horizontally oriented particles, and possible water clouds and 2) application of a spatial coherence analysis technique to separate water clouds from ice clouds containing horizontally oriented ice particles. Other information, such as temperature, color ratio, and vertical variation of depolarization ratio, is also considered. The algorithm works well for both the 0.38 and 38 offnadir lidar pointing geometry. When the lidar is pointed at 0.38 off nadir, half of the opaque ice clouds and about one-third of all ice clouds have a significant lidar backscatter contribution from specular reflections from horizontally oriented particles. At 38 off nadir, the lidar backscatter signals for roughly 30% of opaque ice clouds and 20% of all observed ice clouds are contaminated by horizontally oriented crystals.
Geophysical Research Letters, 2008
The formation of polar stratospheric clouds (PSCs) is critical to the development of polar ozone ... more The formation of polar stratospheric clouds (PSCs) is critical to the development of polar ozone loss. However, the mechanisms of PSC formation remain poorly understood, which affects ozone loss models. Here, based on observations by the NASA A-train satellites, we show that 66% ± 16% and 52% ± 17% of PSCs over west and east Antarctica during the period June -October 2006 were associated with deep tropospheric cloud systems, with maximum depths exceeding 7 km. The development of such deep tropospheric cloud systems should cool the lower stratosphere through adiabatic and radiative processes, favoring PSC development. These deep systems also transport lower tropospheric air into the upper troposphere and lower stratosphere. These new findings suggest that Antarctic PSC formation is closely connected to tropospheric meteorology and thus governed by synoptic scale dynamics, local topography, and large-scale circulation. More dedicated studies are still needed to better understand Antarctic PSC formation.
The Cloudsat Mission and the A-Train
Bulletin of The American Meteorological Society, 2002
CloudSat is a satellite experiment designed to measure the vertical structure of clouds from spac... more CloudSat is a satellite experiment designed to measure the vertical structure of clouds from space. The expected launch of CloudSat is planned for 2004, and once launched, CloudSat will orbit in formation as part of a constellation of satellites (the A-Train) that includes NASA's Aqua and Aura satellites, a NASA-CNES lidar satellite (CALIPSO), and a CNES satellite carrying a polarimeter (PARASOL). A unique feature that CloudSat brings to this constellation is the ability to fly a precise orbit enabling the fields of view of the CloudSat radar to be overlapped with the CALIPSO lidar footprint and the other measurements of the constellation. The precision and near simultaneity of this overlap creates a unique multisatellite observing system for studying the atmospheric processes essential to the hydrological cycle.The vertical profiles of cloud properties provided by CloudSat on the global scale fill a critical gap in the investigation of feedback mechanisms linking clouds to climate. Measuring these profiles requires a combination of active and passive instruments, and this will be achieved by combining the radar data of CloudSat with data from other active and passive sensors of the constellation. This paper describes the underpinning science and general overview of the mission, provides some idea of the expected products and anticipated application of these products, and the potential capability of the A-Train for cloud observations. Notably, the CloudSat mission is expected to stimulate new areas of research on clouds. The mission also provides an important opportunity to demonstrate active sensor technology for future scientific and tactical applications. The CloudSat mission is a partnership between NASA's JPL, the Canadian Space Agency, Colorado State University, the U.S. Air Force, and the U.S. Department of Energy.
Journal of Applied Meteorology, 2000
Apparent depletion of ozone in a cold (ϳ0ЊC), continental stratus cloud system was observed durin... more Apparent depletion of ozone in a cold (ϳ0ЊC), continental stratus cloud system was observed during in situ data collection on 30 April 1994 at the Department of Energy Clouds and Radiation Test Bed site in northern Oklahoma. Analyses of the aircraft data show a significant negative correlation between ozone concentration and liquid water content (LWC) in this cloud. Although droplets of pure water should not significantly affect ozone concentrations, water clouds can potentially perturb ozone through a number of processes, including radiative effects and aqueous-phase reactions in impure cloud droplets. A simple diagnostic model that takes account of cloud effects on the vertical ozone distribution in the boundary layer was constructed to interpret the field data. The results of multifactor regression analysis indicate that aqueous-phase chemistry contributes predominantly to the negative correlation. A depletion of ozone as a function of LWC of about Ϫ6.1 ppbv (g m Ϫ3 ) Ϫ1 was found in this particular stratus. In this case, the average in-cloud reduction of ozone is ϳ6% for an average LWC of ϳ0.3 g m Ϫ3 and ozone mixing ratio of ϳ31 ppbv outside the cloud layer, which is in reasonable agreement with recent model results.
Cloud Type and Macrophysical Property Retrieval Using Multiple Remote Sensors
Journal of Applied Meteorology, 2001
A cloud detection algorithm based on ground-based remote sensors has been developed that can diff... more A cloud detection algorithm based on ground-based remote sensors has been developed that can differentiate among various atmospheric targets such as ice and water clouds, virga, precipitation, and aerosol layers. Standard cloud type and macrophysical properties are identified by combining polarization lidar, millimeter-wave radar, infrared radiometer, and dual-channel microwave radiometer measurements. These algorithms are applied to measurements collected during 1998
Journal of Applied Meteorology, 2002
A retrieval algorithm is described to estimate vertical profiles of cirrus-cloud ice water conten... more A retrieval algorithm is described to estimate vertical profiles of cirrus-cloud ice water content (IWC) and general effective size D ge from combined lidar and radar measurements. In the algorithm, the lidar extinction coefficient is parameterized as ϭ IWC[a 0 ϩ (a 1 /D ge )] and water equivalent radar reflectivity factor Z e is parameterized as Z e ϭ CЈ(IWC/ i ) , where a 0 , a 1 , CЈ, and b are constants based on the assumption of a b D ge modified gamma size distribution and hexagonal ice crystals. A comparison of retrieved results from a cirruscloud case study with aircraft in situ measurements indicates that the algorithm can provide reliable cirrus cloud microphysical properties. A technique to estimate ice water path and layer-mean D ge is also developed using the optical depth and mean radar reflectivity factor of the cloud layer.
Journal of Applied Meteorology, 2002
A series of cirrus cloud simulations performed using a model with explicit cloud microphysics is ... more A series of cirrus cloud simulations performed using a model with explicit cloud microphysics is applied to testing ice water content retrieval algorithms based on millimeter-wave radar reflectivity measurements. The simulated ice particle size spectra over a 12-h growth/dissipation life cycle are converted to equivalent radar reflectivity factors Z e and visible optical extinction coefficients , which are used as a test dataset to intercompare the results of various algorithms. This approach shows that radar Z e -only approaches suffer from significant problems related to basic temperature-dependent cirrus cloud processes, although most algorithms work well under limited conditions (presumably similar to those of the empirical datasets from which each was derived). However, when lidar or radiometric measurements of or cloud optical depth are used to constrain the radar data, excellent agreement with the modeled contents can be achieved under the conditions simulated. Implications for the satellite-based active remote sensing of cirrus clouds are discussed. In addition to showing the utility of sophisticated cloud-resolving models for testing remote sensing algorithms, the results of the simulations for cloud-top temperatures of Ϫ50Њ, Ϫ60Њ, and Ϫ70ЊC illustrate some fundamental properties of cirrus clouds that are regulated by the adiabatic process.
When launched in 2004, CloudSat-part of the new A-train constellation-will provide much needed me... more When launched in 2004, CloudSat-part of the new A-train constellation-will provide much needed measurements of the vertical structure of clouds from space.
Journal of Applied Meteorology, 2004
Mixed-phase clouds are still poorly understood, though studies have indicated that their paramete... more Mixed-phase clouds are still poorly understood, though studies have indicated that their parameterization in general circulation models is critical for climate studies. Most of the knowledge of mixed-phase clouds has been gained from in situ measurements, but reliable remote sensing algorithms to study mixed-phase clouds extensively are lacking. A combined active and passive remote sensing approach for studying supercooled altocumulus with ice virga, using multiple remote sensor observations, is presented. Precipitating altocumulus clouds are a common type of mixed-phase clouds, and their easily identifiable structure provides a simple scenario to study mixedphase clouds. First, ice virga is treated as an independent ice cloud, and an existing lidar-radar algorithm to retrieve ice water content and general effective size profiles is applied. Then, a new iterative approach is used to retrieve supercooled water cloud properties by minimizing the difference between atmospheric emitted radiance interferometer (AERI)-observed radiances and radiances, calculated using the discrete-ordinate radiative transfer model at 12 selected wavelengths. Case studies demonstrate the capabilities of this approach in retrieving radiatively important microphysical properties to characterize this type of mixed-phase cloud. The good agreement between visible optical depths derived from lidar measurement and those estimated from retrieved liquid water path and effective radius provides a closure test for the accuracy of mainly AERI-based supercooled water cloud retrieval.
Cloud and Aerosol Research Capabilities at FARS: The Facility for Atmospheric Remote Sensing
Bulletin of The American Meteorological Society, 2001
Since October 1987, the University of Utah Facility for Atmospheric Remote Sensing (FARS) has bee... more Since October 1987, the University of Utah Facility for Atmospheric Remote Sensing (FARS) has been applied to the probing of the atmosphere, concentrating on the study of high-level clouds. Regular FARS measurements, which currently total ~3000 h of ruby lidar ...
Continental Stratus Clouds: A Case Study Using Coordinated Remote Sensing and Aircraft Measurements
Journal of The Atmospheric Sciences, 1999
A continental stratus cloud layer was studied by advanced ground-based remote sensing instruments... more A continental stratus cloud layer was studied by advanced ground-based remote sensing instruments and aircraft probes on 30 April 1994 from the Cloud and Radiation Testbed site in north-central Oklahoma. The boundary layer structure clearly resembled that of a cloud-topped mixed layer, and the cloud content is shown to be near adiabatic up to the cloud-top entrainment zone. A cloud retrieval algorithm using the radar reflectivity and cloud droplet concentration (either measured in situ or deduced using dual-channel microwave radiometer data) is applied to construct uniquely high-resolution cross sections of liquid water content and mean droplet radius. The combined evidence indicates that the 350-600 m deep, slightly supercooled (2.0° to 2.0°C) cloud, which failed to produce any detectable ice or drizzle particles, contained an average droplet concentration of 347 cm3, and a maximum liquid water content of 0.8 g m3 and mean droplet radius of 9 m near cloud top. Lidar data indicate that the Ka-band radar usually detected the cloud-base height to within 50 m, such that the radar insensitivity to small cloud droplets had a small impact on the findings. Radar-derived liquid water paths ranged from 71 to 259 g m2 as the stratus deck varied, which is in excellent agreement with dual-channel microwave radiometer data, but 20% higher than that measured in situ. This difference appears to be due to the undersampling of the few largest cloud droplets by the aircraft probes. This combination of approaches yields a unique image of the content of a continental stratus cloud, as well as illustrating the utility of modern remote sensing systems for probing nonprecipitating water clouds.
In situ detection of biological particles in cloud ice-crystals
Nature Geoscience, 2009
Each month, Nature Geoscience will bring you top-quality research papers, reviews and opinion pie... more Each month, Nature Geoscience will bring you top-quality research papers, reviews and opinion pieces - in print and online.
Journal of The Atmospheric Sciences, 2002
A retrieval algorithm is described to estimate vertical profiles of cirrus-cloud ice water conten... more A retrieval algorithm is described to estimate vertical profiles of cirrus-cloud ice water content (IWC) and general effective size D ge from combined lidar and radar measurements. In the algorithm, the lidar extinction coefficient is parameterized as ϭ IWC[a 0 ϩ (a 1 /D ge )] and water equivalent radar reflectivity factor Z e is parameterized as Z e ϭ CЈ(IWC/ i ) , where a 0 , a 1 , CЈ, and b are constants based on the assumption of a b D ge modified gamma size distribution and hexagonal ice crystals. A comparison of retrieved results from a cirruscloud case study with aircraft in situ measurements indicates that the algorithm can provide reliable cirrus cloud microphysical properties. A technique to estimate ice water path and layer-mean D ge is also developed using the optical depth and mean radar reflectivity factor of the cloud layer.
Improved Radar Ice Water Content Retrieval Algorithms Using Coincident Microphysical and Radar Measurements
Journal of Applied Meteorology, 2005
Airborne radar reflectivity measurements at frequencies of 9.6 and 94 GHz, with collocated, in si... more Airborne radar reflectivity measurements at frequencies of 9.6 and 94 GHz, with collocated, in situ particle size distribution and ice water content measurements from the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL) Florida Area Cirrus Experiment (FACE) in Florida in July 2002, offer one of the first opportunities to evaluate and improve algorithms for retrieving ice water content from single-wavelength spaceborne radar measurements. Both ice water content and radar reflectivity depend on the distribution of particle mass with size. It is demonstrated that single, power-law, mass dimensional relationships are unable to adequately account for the dominating contribution of small particles at lower reflectivities and large particles at higher reflectivities. To circumvent the need for multiple, or complex, mass dimensional relationships, analytic expressions that use particle ensemble mean ice particle densities that are derived from the coincident microphysical and radar observations are developed. These expressions, together with more than 5000 CRYSTAL FACE size distributions, are used to develop radar reflectivity ice water content relationships for the two radar wavelengths that appear to provide improvements over earlier relationships, at least for convectively generated stratiform ice clouds.
Midlatitude Cirrus Clouds Derived from Hurricane Nora: A Case Study with Implications for Ice Crystal Nucleation and Shape
Journal of The Atmospheric Sciences, 2003
Hurricane Nora traveled up the Baja Peninsula coast in the unusually warm El Niño waters of Septe... more Hurricane Nora traveled up the Baja Peninsula coast in the unusually warm El Niño waters of September 1997 until rapidly decaying as it approached southern California on 24 September. The anvil cirrus blowoff from the final surge of tropical convection became ...
A Refined Two-Channel Microwave Radiometer Liquid Water Path Retrieval for Cold Regions by Using Multiple-Sensor Measurements
IEEE Geoscience and Remote Sensing Letters, 2007
AbstractTraditional two-channel microwave radiometers (MWRs) are widely used to measure cloud li... more AbstractTraditional two-channel microwave radiometers (MWRs) are widely used to measure cloud liquid water path (LWP); however, the retrieved LWPs are subject to relatively large uncertainties, particularly for low LWP clouds. By reformulating the statistical retrieval ...
Atmospheric Chemistry and Physics, 2008
Airborne dust is a major environmental hazard in Asia. Using an analysis of the first full year o... more Airborne dust is a major environmental hazard in Asia. Using an analysis of the first full year of CALIPSO lidar measurements, this paper derives unprecedented, altituderesolved seasonal distributions of desert dust transported over the Tibetan Plateau (TP) and the surrounding areas. The CALIPSO lidar observations include numerous 5
Journal of Geophysical Research, 2008
Based on the first year of CALIPSO lidar measurements under cloud-free conditions, a height-resol... more Based on the first year of CALIPSO lidar measurements under cloud-free conditions, a height-resolved global distribution of dust aerosols is presented for the first time. Results indicate that spring is the most active dust season, during which ˜20% and ˜12% of areas between 0 and 60°N are influenced by dust at least 10% and 50% of the time, respectively. In summer within 3-6 km, ˜8.3% of area between 0 and 60°N is impacted by dust at least 50% of the time. Strong seasonal cycles of dust layer vertical extent are observed in major source regions, which are similar to the seasonal variation of the thermally driven boundary layer depth. The arid and semiarid areas in North Africa and the Arabian Peninsula are the most persistent and prolific dust sources. African dust is transported across the Atlantic all yearlong with strong seasonal variation in the transport pathways mainly in the free troposphere in summer and at the low altitudes in winter. However, the trans-Atlantic dust is transported at the low altitudes is important for all seasons, especially transported further cross the ocean. The crossing Atlantic dusty zones are shifted southward from summer to winter, which is accompanied by a similar southward shift of dust-generating areas over North Africa. The Taklimakan and Gobi deserts are two major dust sources in East Asia with long-range transport mainly occurring in spring. The large horizontal and vertical coverage of dust aerosols indicate their importance in the climate system through both direct and indirect aerosol effects.
Classifying clouds around the globe with the CloudSat radar: 1-year of results
Geophysical Research Letters, 2008
CloudSat supports a 94 GHz cloud profiling radar as part of the innovative A-train formation of s... more CloudSat supports a 94 GHz cloud profiling radar as part of the innovative A-train formation of satellites studying the Earths clouds and atmosphere. Using the vertical profiles of clouds and precipitation, an algorithm has been developed to determine the type of clouds present. ...
Testing IWC Retrieval Methods Using Radar and Ancillary Measurements with In Situ Data
Journal of Applied Meteorology and Climatology, 2008
Vertical profiles of ice water content (IWC) can now be derived globally from spaceborne cloud sa... more Vertical profiles of ice water content (IWC) can now be derived globally from spaceborne cloud satellite radar (CloudSat) data. Integrating these data with Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data may further increase accuracy. Evaluations of the ...
Journal of Atmospheric and Oceanic Technology, 2009
The current cloud thermodynamic phase discrimination by Cloud-Aerosol Lidar Pathfinder Satellite ... more The current cloud thermodynamic phase discrimination by Cloud-Aerosol Lidar Pathfinder Satellite Observations (CALIPSO) is based on the depolarization of backscattered light measured by its lidar [Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)]. It assumes that backscattered light from ice crystals is depolarizing, whereas water clouds, being spherical, result in minimal depolarization. However, because of the relationship between the CALIOP field of view (FOV) and the large distance between the satellite and clouds and because of the frequent presence of oriented ice crystals, there is often a weak correlation between measured depolarization and phase, which thereby creates significant uncertainties in the current CALIOP phase retrieval. For water clouds, the CALIOP-measured depolarization can be large because of multiple scattering, whereas horizontally oriented ice particles depolarize only weakly and behave similarly to water clouds. Because of the nonunique depolarization-cloud phase relationship, more constraints are necessary to uniquely determine cloud phase. Based on theoretical and modeling studies, an improved cloud phase determination algorithm has been developed. Instead of depending primarily on layer-integrated depolarization ratios, this algorithm differentiates cloud phases by using the spatial correlation of layer-integrated attenuated backscatter and layer-integrated particulate depolarization ratio. This approach includes a two-step process: 1) use of a simple two-dimensional threshold method to provide a preliminary identification of ice clouds containing randomly oriented particles, ice clouds with horizontally oriented particles, and possible water clouds and 2) application of a spatial coherence analysis technique to separate water clouds from ice clouds containing horizontally oriented ice particles. Other information, such as temperature, color ratio, and vertical variation of depolarization ratio, is also considered. The algorithm works well for both the 0.38 and 38 offnadir lidar pointing geometry. When the lidar is pointed at 0.38 off nadir, half of the opaque ice clouds and about one-third of all ice clouds have a significant lidar backscatter contribution from specular reflections from horizontally oriented particles. At 38 off nadir, the lidar backscatter signals for roughly 30% of opaque ice clouds and 20% of all observed ice clouds are contaminated by horizontally oriented crystals.
Geophysical Research Letters, 2008
The formation of polar stratospheric clouds (PSCs) is critical to the development of polar ozone ... more The formation of polar stratospheric clouds (PSCs) is critical to the development of polar ozone loss. However, the mechanisms of PSC formation remain poorly understood, which affects ozone loss models. Here, based on observations by the NASA A-train satellites, we show that 66% ± 16% and 52% ± 17% of PSCs over west and east Antarctica during the period June -October 2006 were associated with deep tropospheric cloud systems, with maximum depths exceeding 7 km. The development of such deep tropospheric cloud systems should cool the lower stratosphere through adiabatic and radiative processes, favoring PSC development. These deep systems also transport lower tropospheric air into the upper troposphere and lower stratosphere. These new findings suggest that Antarctic PSC formation is closely connected to tropospheric meteorology and thus governed by synoptic scale dynamics, local topography, and large-scale circulation. More dedicated studies are still needed to better understand Antarctic PSC formation.
The Cloudsat Mission and the A-Train
Bulletin of The American Meteorological Society, 2002
CloudSat is a satellite experiment designed to measure the vertical structure of clouds from spac... more CloudSat is a satellite experiment designed to measure the vertical structure of clouds from space. The expected launch of CloudSat is planned for 2004, and once launched, CloudSat will orbit in formation as part of a constellation of satellites (the A-Train) that includes NASA's Aqua and Aura satellites, a NASA-CNES lidar satellite (CALIPSO), and a CNES satellite carrying a polarimeter (PARASOL). A unique feature that CloudSat brings to this constellation is the ability to fly a precise orbit enabling the fields of view of the CloudSat radar to be overlapped with the CALIPSO lidar footprint and the other measurements of the constellation. The precision and near simultaneity of this overlap creates a unique multisatellite observing system for studying the atmospheric processes essential to the hydrological cycle.The vertical profiles of cloud properties provided by CloudSat on the global scale fill a critical gap in the investigation of feedback mechanisms linking clouds to climate. Measuring these profiles requires a combination of active and passive instruments, and this will be achieved by combining the radar data of CloudSat with data from other active and passive sensors of the constellation. This paper describes the underpinning science and general overview of the mission, provides some idea of the expected products and anticipated application of these products, and the potential capability of the A-Train for cloud observations. Notably, the CloudSat mission is expected to stimulate new areas of research on clouds. The mission also provides an important opportunity to demonstrate active sensor technology for future scientific and tactical applications. The CloudSat mission is a partnership between NASA's JPL, the Canadian Space Agency, Colorado State University, the U.S. Air Force, and the U.S. Department of Energy.
Journal of Applied Meteorology, 2000
Apparent depletion of ozone in a cold (ϳ0ЊC), continental stratus cloud system was observed durin... more Apparent depletion of ozone in a cold (ϳ0ЊC), continental stratus cloud system was observed during in situ data collection on 30 April 1994 at the Department of Energy Clouds and Radiation Test Bed site in northern Oklahoma. Analyses of the aircraft data show a significant negative correlation between ozone concentration and liquid water content (LWC) in this cloud. Although droplets of pure water should not significantly affect ozone concentrations, water clouds can potentially perturb ozone through a number of processes, including radiative effects and aqueous-phase reactions in impure cloud droplets. A simple diagnostic model that takes account of cloud effects on the vertical ozone distribution in the boundary layer was constructed to interpret the field data. The results of multifactor regression analysis indicate that aqueous-phase chemistry contributes predominantly to the negative correlation. A depletion of ozone as a function of LWC of about Ϫ6.1 ppbv (g m Ϫ3 ) Ϫ1 was found in this particular stratus. In this case, the average in-cloud reduction of ozone is ϳ6% for an average LWC of ϳ0.3 g m Ϫ3 and ozone mixing ratio of ϳ31 ppbv outside the cloud layer, which is in reasonable agreement with recent model results.
Cloud Type and Macrophysical Property Retrieval Using Multiple Remote Sensors
Journal of Applied Meteorology, 2001
A cloud detection algorithm based on ground-based remote sensors has been developed that can diff... more A cloud detection algorithm based on ground-based remote sensors has been developed that can differentiate among various atmospheric targets such as ice and water clouds, virga, precipitation, and aerosol layers. Standard cloud type and macrophysical properties are identified by combining polarization lidar, millimeter-wave radar, infrared radiometer, and dual-channel microwave radiometer measurements. These algorithms are applied to measurements collected during 1998
Journal of Applied Meteorology, 2002
A retrieval algorithm is described to estimate vertical profiles of cirrus-cloud ice water conten... more A retrieval algorithm is described to estimate vertical profiles of cirrus-cloud ice water content (IWC) and general effective size D ge from combined lidar and radar measurements. In the algorithm, the lidar extinction coefficient is parameterized as ϭ IWC[a 0 ϩ (a 1 /D ge )] and water equivalent radar reflectivity factor Z e is parameterized as Z e ϭ CЈ(IWC/ i ) , where a 0 , a 1 , CЈ, and b are constants based on the assumption of a b D ge modified gamma size distribution and hexagonal ice crystals. A comparison of retrieved results from a cirruscloud case study with aircraft in situ measurements indicates that the algorithm can provide reliable cirrus cloud microphysical properties. A technique to estimate ice water path and layer-mean D ge is also developed using the optical depth and mean radar reflectivity factor of the cloud layer.
Journal of Applied Meteorology, 2002
A series of cirrus cloud simulations performed using a model with explicit cloud microphysics is ... more A series of cirrus cloud simulations performed using a model with explicit cloud microphysics is applied to testing ice water content retrieval algorithms based on millimeter-wave radar reflectivity measurements. The simulated ice particle size spectra over a 12-h growth/dissipation life cycle are converted to equivalent radar reflectivity factors Z e and visible optical extinction coefficients , which are used as a test dataset to intercompare the results of various algorithms. This approach shows that radar Z e -only approaches suffer from significant problems related to basic temperature-dependent cirrus cloud processes, although most algorithms work well under limited conditions (presumably similar to those of the empirical datasets from which each was derived). However, when lidar or radiometric measurements of or cloud optical depth are used to constrain the radar data, excellent agreement with the modeled contents can be achieved under the conditions simulated. Implications for the satellite-based active remote sensing of cirrus clouds are discussed. In addition to showing the utility of sophisticated cloud-resolving models for testing remote sensing algorithms, the results of the simulations for cloud-top temperatures of Ϫ50Њ, Ϫ60Њ, and Ϫ70ЊC illustrate some fundamental properties of cirrus clouds that are regulated by the adiabatic process.
When launched in 2004, CloudSat-part of the new A-train constellation-will provide much needed me... more When launched in 2004, CloudSat-part of the new A-train constellation-will provide much needed measurements of the vertical structure of clouds from space.
Journal of Applied Meteorology, 2004
Mixed-phase clouds are still poorly understood, though studies have indicated that their paramete... more Mixed-phase clouds are still poorly understood, though studies have indicated that their parameterization in general circulation models is critical for climate studies. Most of the knowledge of mixed-phase clouds has been gained from in situ measurements, but reliable remote sensing algorithms to study mixed-phase clouds extensively are lacking. A combined active and passive remote sensing approach for studying supercooled altocumulus with ice virga, using multiple remote sensor observations, is presented. Precipitating altocumulus clouds are a common type of mixed-phase clouds, and their easily identifiable structure provides a simple scenario to study mixedphase clouds. First, ice virga is treated as an independent ice cloud, and an existing lidar-radar algorithm to retrieve ice water content and general effective size profiles is applied. Then, a new iterative approach is used to retrieve supercooled water cloud properties by minimizing the difference between atmospheric emitted radiance interferometer (AERI)-observed radiances and radiances, calculated using the discrete-ordinate radiative transfer model at 12 selected wavelengths. Case studies demonstrate the capabilities of this approach in retrieving radiatively important microphysical properties to characterize this type of mixed-phase cloud. The good agreement between visible optical depths derived from lidar measurement and those estimated from retrieved liquid water path and effective radius provides a closure test for the accuracy of mainly AERI-based supercooled water cloud retrieval.
Cloud and Aerosol Research Capabilities at FARS: The Facility for Atmospheric Remote Sensing
Bulletin of The American Meteorological Society, 2001
Since October 1987, the University of Utah Facility for Atmospheric Remote Sensing (FARS) has bee... more Since October 1987, the University of Utah Facility for Atmospheric Remote Sensing (FARS) has been applied to the probing of the atmosphere, concentrating on the study of high-level clouds. Regular FARS measurements, which currently total ~3000 h of ruby lidar ...
Continental Stratus Clouds: A Case Study Using Coordinated Remote Sensing and Aircraft Measurements
Journal of The Atmospheric Sciences, 1999
A continental stratus cloud layer was studied by advanced ground-based remote sensing instruments... more A continental stratus cloud layer was studied by advanced ground-based remote sensing instruments and aircraft probes on 30 April 1994 from the Cloud and Radiation Testbed site in north-central Oklahoma. The boundary layer structure clearly resembled that of a cloud-topped mixed layer, and the cloud content is shown to be near adiabatic up to the cloud-top entrainment zone. A cloud retrieval algorithm using the radar reflectivity and cloud droplet concentration (either measured in situ or deduced using dual-channel microwave radiometer data) is applied to construct uniquely high-resolution cross sections of liquid water content and mean droplet radius. The combined evidence indicates that the 350-600 m deep, slightly supercooled (2.0° to 2.0°C) cloud, which failed to produce any detectable ice or drizzle particles, contained an average droplet concentration of 347 cm3, and a maximum liquid water content of 0.8 g m3 and mean droplet radius of 9 m near cloud top. Lidar data indicate that the Ka-band radar usually detected the cloud-base height to within 50 m, such that the radar insensitivity to small cloud droplets had a small impact on the findings. Radar-derived liquid water paths ranged from 71 to 259 g m2 as the stratus deck varied, which is in excellent agreement with dual-channel microwave radiometer data, but 20% higher than that measured in situ. This difference appears to be due to the undersampling of the few largest cloud droplets by the aircraft probes. This combination of approaches yields a unique image of the content of a continental stratus cloud, as well as illustrating the utility of modern remote sensing systems for probing nonprecipitating water clouds.
In situ detection of biological particles in cloud ice-crystals
Nature Geoscience, 2009
Each month, Nature Geoscience will bring you top-quality research papers, reviews and opinion pie... more Each month, Nature Geoscience will bring you top-quality research papers, reviews and opinion pieces - in print and online.
Journal of The Atmospheric Sciences, 2002
A retrieval algorithm is described to estimate vertical profiles of cirrus-cloud ice water conten... more A retrieval algorithm is described to estimate vertical profiles of cirrus-cloud ice water content (IWC) and general effective size D ge from combined lidar and radar measurements. In the algorithm, the lidar extinction coefficient is parameterized as ϭ IWC[a 0 ϩ (a 1 /D ge )] and water equivalent radar reflectivity factor Z e is parameterized as Z e ϭ CЈ(IWC/ i ) , where a 0 , a 1 , CЈ, and b are constants based on the assumption of a b D ge modified gamma size distribution and hexagonal ice crystals. A comparison of retrieved results from a cirruscloud case study with aircraft in situ measurements indicates that the algorithm can provide reliable cirrus cloud microphysical properties. A technique to estimate ice water path and layer-mean D ge is also developed using the optical depth and mean radar reflectivity factor of the cloud layer.
Improved Radar Ice Water Content Retrieval Algorithms Using Coincident Microphysical and Radar Measurements
Journal of Applied Meteorology, 2005
Airborne radar reflectivity measurements at frequencies of 9.6 and 94 GHz, with collocated, in si... more Airborne radar reflectivity measurements at frequencies of 9.6 and 94 GHz, with collocated, in situ particle size distribution and ice water content measurements from the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL) Florida Area Cirrus Experiment (FACE) in Florida in July 2002, offer one of the first opportunities to evaluate and improve algorithms for retrieving ice water content from single-wavelength spaceborne radar measurements. Both ice water content and radar reflectivity depend on the distribution of particle mass with size. It is demonstrated that single, power-law, mass dimensional relationships are unable to adequately account for the dominating contribution of small particles at lower reflectivities and large particles at higher reflectivities. To circumvent the need for multiple, or complex, mass dimensional relationships, analytic expressions that use particle ensemble mean ice particle densities that are derived from the coincident microphysical and radar observations are developed. These expressions, together with more than 5000 CRYSTAL FACE size distributions, are used to develop radar reflectivity ice water content relationships for the two radar wavelengths that appear to provide improvements over earlier relationships, at least for convectively generated stratiform ice clouds.
Midlatitude Cirrus Clouds Derived from Hurricane Nora: A Case Study with Implications for Ice Crystal Nucleation and Shape
Journal of The Atmospheric Sciences, 2003
Hurricane Nora traveled up the Baja Peninsula coast in the unusually warm El Niño waters of Septe... more Hurricane Nora traveled up the Baja Peninsula coast in the unusually warm El Niño waters of September 1997 until rapidly decaying as it approached southern California on 24 September. The anvil cirrus blowoff from the final surge of tropical convection became ...