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Abstract
Three cases of colliding outflow boundaries are examined using data collected from the NOAA Doppler lidar and a meteorological tower during the summer of 1986 near Boulder, Colorado. The data are unique because the lidar and the 300 m tower were colocated, providing measurements of both kinematic and thermodynamic properties. Lidar data reveal small-scale vortex roll instabilities within the leading edge of the outflow. Observations of the post-collision interactions showed that the warmer of the two outflows was deflected upward by the colder outflow to heights of 2 km. In all cases, this forced mechanical lifting was sufficient to produce convection. A simple model of two colliding density currents also suggests that deeper outflows are more efficient in initiating convection.
Abstract
Three cases of colliding outflow boundaries are examined using data collected from the NOAA Doppler lidar and a meteorological tower during the summer of 1986 near Boulder, Colorado. The data are unique because the lidar and the 300 m tower were colocated, providing measurements of both kinematic and thermodynamic properties. Lidar data reveal small-scale vortex roll instabilities within the leading edge of the outflow. Observations of the post-collision interactions showed that the warmer of the two outflows was deflected upward by the colder outflow to heights of 2 km. In all cases, this forced mechanical lifting was sufficient to produce convection. A simple model of two colliding density currents also suggests that deeper outflows are more efficient in initiating convection.
Abstract
The paper presents the results of retrieving characteristic particle sizes for the November 26 1991 FIRE II case using two methods that utilize ground-based remotes sensors. The size information for the complete vertical depth of the cloud was obtained for a 3-hour period from 1830 to 2130 UTC using combined Doppler radar and IR radiometer measurements and for two shorter periods using radar reflectivity and CO2 lidar backscatter measurements. The results obtained with both remote sensing techniques are compared for these two periods. Possible retrieval uncertainties are discussed. Comparisons yielded an agreement with a relative standard deviation of 15%-20% between the two methods. Particle sizes retrieved by both methods were compared with 2D particle probe data sampled during 10 time intervals when a research aircraft was crossing the hub area. The relative standard deviation of particle sizes retrieved with the radar-radiometer method from those obtained from 2D probes is about 30% for nine compared times. The corresponding deviation for the lidar-radar method is about 35% for three compared times. The relative standard deviation between particle concentrations retrieved with the radar-radiometer method and those obtained from 2D probes is about 60% for nine compared times.
Abstract
The paper presents the results of retrieving characteristic particle sizes for the November 26 1991 FIRE II case using two methods that utilize ground-based remotes sensors. The size information for the complete vertical depth of the cloud was obtained for a 3-hour period from 1830 to 2130 UTC using combined Doppler radar and IR radiometer measurements and for two shorter periods using radar reflectivity and CO2 lidar backscatter measurements. The results obtained with both remote sensing techniques are compared for these two periods. Possible retrieval uncertainties are discussed. Comparisons yielded an agreement with a relative standard deviation of 15%-20% between the two methods. Particle sizes retrieved by both methods were compared with 2D particle probe data sampled during 10 time intervals when a research aircraft was crossing the hub area. The relative standard deviation of particle sizes retrieved with the radar-radiometer method from those obtained from 2D probes is about 30% for nine compared times. The corresponding deviation for the lidar-radar method is about 35% for three compared times. The relative standard deviation between particle concentrations retrieved with the radar-radiometer method and those obtained from 2D probes is about 60% for nine compared times.
The Land/Sea Breeze Experiment (LASBEX) was conducted at Moss Landing, California, 15–30 September 1987. The experiment was designed to study the vertical structure and mesoscale variation of the land/sea breeze. A Doppler lidar, a triangular array of three sodars, two sounding systems (one deployed from land and one from a ship), and six surface weather stations (one shipborne) were sited around the Moss Landing area. Measurements obtained included ten sea-breeze and four land-breeze events. This paper describes the objectives and design of the experiment, as well as the observing systems that were used. Some preliminary results and selected observations are presented, called from the data collected, as well as the ensuing analysis plans.
The Land/Sea Breeze Experiment (LASBEX) was conducted at Moss Landing, California, 15–30 September 1987. The experiment was designed to study the vertical structure and mesoscale variation of the land/sea breeze. A Doppler lidar, a triangular array of three sodars, two sounding systems (one deployed from land and one from a ship), and six surface weather stations (one shipborne) were sited around the Moss Landing area. Measurements obtained included ten sea-breeze and four land-breeze events. This paper describes the objectives and design of the experiment, as well as the observing systems that were used. Some preliminary results and selected observations are presented, called from the data collected, as well as the ensuing analysis plans.
Abstract
During spring and early summer, a surface confluence zone, often referred to as the dryline, forms in the midwestern United States between continental and maritime air masses. The dewpoint temperature across the dryline can vary in excess of 18°C in a distance of less than 10 km. The movement of the dryline varies diurnally with boundary layer growth over sloping terrain leading to an eastward apparent propagation of the dryline during the day and a westward advection or retrogression during the evening. In this study, we examine the finescale structure of a retrogressing, dryline using data taken by a Doppler lidar, a dual-channel radiometer, and serial rawinsonde ascents. While many previous studies were unable to accurately measure the vertical motions in the vicinity of the dryline, our lidar measurements suggest that the convergence at the dryline is intense with maximum vertical motions of ∼5 m s−1. The winds obtained from the Doppler lidar Measurements were combined with the equations of motion to derive perturbation fields of pressure and virtual potential temperature θ v . Our observations indicate that the circulations associated with this retrogressing dryline were dominated by hot, dry air riding over a westward moving denser, moist flow in a manner similar to a density current. Gravity waves were observed above the dryline interface. Previous observational and numerical studies have shown that differential heating across the dryline may sometimes enhance regional pressure gradients and thus impact dryline movement. We propose that this regional gradient in surface heating in the presence of a confluent flow results in observed intense wind shifts and large horizontal gradients in θ v across the dryline. The local gradient in θ v influences the movement and flow characteristics of the dryline interface. This study is one of the most complete and novel uses of Doppler lidar to date.
Abstract
During spring and early summer, a surface confluence zone, often referred to as the dryline, forms in the midwestern United States between continental and maritime air masses. The dewpoint temperature across the dryline can vary in excess of 18°C in a distance of less than 10 km. The movement of the dryline varies diurnally with boundary layer growth over sloping terrain leading to an eastward apparent propagation of the dryline during the day and a westward advection or retrogression during the evening. In this study, we examine the finescale structure of a retrogressing, dryline using data taken by a Doppler lidar, a dual-channel radiometer, and serial rawinsonde ascents. While many previous studies were unable to accurately measure the vertical motions in the vicinity of the dryline, our lidar measurements suggest that the convergence at the dryline is intense with maximum vertical motions of ∼5 m s−1. The winds obtained from the Doppler lidar Measurements were combined with the equations of motion to derive perturbation fields of pressure and virtual potential temperature θ v . Our observations indicate that the circulations associated with this retrogressing dryline were dominated by hot, dry air riding over a westward moving denser, moist flow in a manner similar to a density current. Gravity waves were observed above the dryline interface. Previous observational and numerical studies have shown that differential heating across the dryline may sometimes enhance regional pressure gradients and thus impact dryline movement. We propose that this regional gradient in surface heating in the presence of a confluent flow results in observed intense wind shifts and large horizontal gradients in θ v across the dryline. The local gradient in θ v influences the movement and flow characteristics of the dryline interface. This study is one of the most complete and novel uses of Doppler lidar to date.
Abstract
The microphysical characteristics, radiative impact, and life cycle of a long-lived, surface-based mixed-layer, mixed-phase cloud with an average temperature of approximately −20°C are presented and discussed. The cloud was observed during the Surface Heat Budget of the Arctic experiment (SHEBA) from 1 to 10 May 1998. Vertically resolved properties of the liquid and ice phases are retrieved using surface-based remote sensors, utilize the adiabatic assumption for the liquid component, and are aided by and validated with aircraft measurements from 4 and 7 May. The cloud radar ice microphysical retrievals, originally developed for all-ice clouds, compare well with aircraft measurements despite the presence of much greater liquid water contents than ice water contents. The retrieved time-mean liquid cloud optical depth of 10.1 ± 7.8 far surpasses the mean ice cloud optical depth of 0.2, so that the liquid phase is primarily responsible for the cloud’s radiative (flux) impact. The ice phase, in turn, regulates the overall cloud optical depth through two mechanisms: sedimentation from a thin upper ice cloud, and a local ice production mechanism with a time scale of a few hours, thought to reflect a preferred freezing of the larger liquid drops. The liquid water paths replenish within half a day or less after their uptake by ice, attesting to strong water vapor fluxes. Deeper boundary layer depths and higher cloud optical depths coincide with large-scale rising motion at 850 hPa, but the synoptic activity is also associated with upper-level ice clouds. Interestingly, the local ice formation mechanism appears to be more active when the large-scale subsidence rate implies increased cloud-top entrainment. Strong cloud-top radiative cooling rates promote cloud longevity when the cloud is optically thick. The radiative impact of the cloud upon the surface is significant: a time-mean positive net cloud forcing of 41 W m−2 with a diurnal amplitude of ∼20 W m−2. This is primarily because a high surface reflectance (0.86) reduces the solar cooling influence. The net cloud forcing is primarily sensitive to cloud optical depth for the low-optical-depth cloudy columns and to the surface reflectance for the high-optical-depth cloudy columns. Any projected increase in the springtime cloud optical depth at this location (76°N, 165°W) is not expected to significantly alter the surface radiation budget, because clouds were almost always present, and almost 60% of the cloudy columns had optical depths >6.
Abstract
The microphysical characteristics, radiative impact, and life cycle of a long-lived, surface-based mixed-layer, mixed-phase cloud with an average temperature of approximately −20°C are presented and discussed. The cloud was observed during the Surface Heat Budget of the Arctic experiment (SHEBA) from 1 to 10 May 1998. Vertically resolved properties of the liquid and ice phases are retrieved using surface-based remote sensors, utilize the adiabatic assumption for the liquid component, and are aided by and validated with aircraft measurements from 4 and 7 May. The cloud radar ice microphysical retrievals, originally developed for all-ice clouds, compare well with aircraft measurements despite the presence of much greater liquid water contents than ice water contents. The retrieved time-mean liquid cloud optical depth of 10.1 ± 7.8 far surpasses the mean ice cloud optical depth of 0.2, so that the liquid phase is primarily responsible for the cloud’s radiative (flux) impact. The ice phase, in turn, regulates the overall cloud optical depth through two mechanisms: sedimentation from a thin upper ice cloud, and a local ice production mechanism with a time scale of a few hours, thought to reflect a preferred freezing of the larger liquid drops. The liquid water paths replenish within half a day or less after their uptake by ice, attesting to strong water vapor fluxes. Deeper boundary layer depths and higher cloud optical depths coincide with large-scale rising motion at 850 hPa, but the synoptic activity is also associated with upper-level ice clouds. Interestingly, the local ice formation mechanism appears to be more active when the large-scale subsidence rate implies increased cloud-top entrainment. Strong cloud-top radiative cooling rates promote cloud longevity when the cloud is optically thick. The radiative impact of the cloud upon the surface is significant: a time-mean positive net cloud forcing of 41 W m−2 with a diurnal amplitude of ∼20 W m−2. This is primarily because a high surface reflectance (0.86) reduces the solar cooling influence. The net cloud forcing is primarily sensitive to cloud optical depth for the low-optical-depth cloudy columns and to the surface reflectance for the high-optical-depth cloudy columns. Any projected increase in the springtime cloud optical depth at this location (76°N, 165°W) is not expected to significantly alter the surface radiation budget, because clouds were almost always present, and almost 60% of the cloudy columns had optical depths >6.
Abstract
The goals of the current study are threefold: 1) to present a multispectral, multiresolution (MSMR) methodology for analysis of scenes containing multiple cloud layers; 2) to apply the MSMR method to two multilevel cloud scenes recorded by the NOAA Advanced Very High Resolution Radiometer (AVHRR) and the High Resolution Infrared Radiometer Sounder (HIRS/2) instruments during the First International Satellite Cloud Climatology Program (ISCCP) Regional Experiment (FIRE) on 28 November 1991; and 3) to validate the cloud-top height results from the case study analyses through comparison with lidar, radar, aircraft and rawin-sonde data. The measurements available from FIRE Cirrus II enable detailed examination of two complex cloud scenes in which cirrus and stratus appear simultaneously.
A “fuzzy logic” classification system is developed to determine whether a 32×32 array of AVHRR data contains clear sky, low-level cloud, midlevel cloud, high-level cloud, or multiple cloud layers. With the addition of the fray logic cloud classification system, it is possible for the first time to find evidence of more than one cloud layer within each HMS field of view. Low cloud heights are determined through application of the spatial coherence method to the AVHRR data, while mid- to high-level cloud heights are calculated from the HIRS/2 15-µm CO2 band radiometric data that are collocated with the AVHRR data. Cirrus cloud heights retrieved from HIRS 15-µm CO2 band data are improved for optically thin cirrus through the use of the upper-tropospheric humidity profile. The MSMR-derived cloud heights are consistent with coincident lidar, radar, and aircraft data. Cirrus and stratus cloud-top heights and cirrus effective emittances are retrieved for data within an ISCCP 2.5° grid cell that encompasses the FIRE experimental region.
Abstract
The goals of the current study are threefold: 1) to present a multispectral, multiresolution (MSMR) methodology for analysis of scenes containing multiple cloud layers; 2) to apply the MSMR method to two multilevel cloud scenes recorded by the NOAA Advanced Very High Resolution Radiometer (AVHRR) and the High Resolution Infrared Radiometer Sounder (HIRS/2) instruments during the First International Satellite Cloud Climatology Program (ISCCP) Regional Experiment (FIRE) on 28 November 1991; and 3) to validate the cloud-top height results from the case study analyses through comparison with lidar, radar, aircraft and rawin-sonde data. The measurements available from FIRE Cirrus II enable detailed examination of two complex cloud scenes in which cirrus and stratus appear simultaneously.
A “fuzzy logic” classification system is developed to determine whether a 32×32 array of AVHRR data contains clear sky, low-level cloud, midlevel cloud, high-level cloud, or multiple cloud layers. With the addition of the fray logic cloud classification system, it is possible for the first time to find evidence of more than one cloud layer within each HMS field of view. Low cloud heights are determined through application of the spatial coherence method to the AVHRR data, while mid- to high-level cloud heights are calculated from the HIRS/2 15-µm CO2 band radiometric data that are collocated with the AVHRR data. Cirrus cloud heights retrieved from HIRS 15-µm CO2 band data are improved for optically thin cirrus through the use of the upper-tropospheric humidity profile. The MSMR-derived cloud heights are consistent with coincident lidar, radar, and aircraft data. Cirrus and stratus cloud-top heights and cirrus effective emittances are retrieved for data within an ISCCP 2.5° grid cell that encompasses the FIRE experimental region.
Abstract
Simultaneous multiwavelength measurements of a developing cloud system were obtained by NOAA Doppler lidar, Doppler radar, Fourier transform infrared interferometer, and microwave and infrared radiometers on 26 November 1991. The evolution of the cloud system is described in terms of lidar backscatter, radar reflectivity and velocity, interferometer atmospheric spectra, and radiometer brightness temperature, integrated liquid water, and water vapor paths. Utilizing the difference in wavelength between the radar and lidar, and therefore their independent sensitivity to different regions of the same cloud, the cloud top, base, depth, and multiple layer heights can he determined with better accuracy than with either instrument alone. Combining the radar, lidar, and radiometer measurements using two different techniques allows an estimation of the vertical profile of cloud microphysical properties such as particle sizes. Enhancement of lidar backscatter near zenith revealed when highly oriented ice crystals were present. The authors demonstrate that no single instrument is sufficient to accurately describe cirrus clouds and that measurements in combination can provide important details on their geometric, radiative, and microphysical properties.
Abstract
Simultaneous multiwavelength measurements of a developing cloud system were obtained by NOAA Doppler lidar, Doppler radar, Fourier transform infrared interferometer, and microwave and infrared radiometers on 26 November 1991. The evolution of the cloud system is described in terms of lidar backscatter, radar reflectivity and velocity, interferometer atmospheric spectra, and radiometer brightness temperature, integrated liquid water, and water vapor paths. Utilizing the difference in wavelength between the radar and lidar, and therefore their independent sensitivity to different regions of the same cloud, the cloud top, base, depth, and multiple layer heights can he determined with better accuracy than with either instrument alone. Combining the radar, lidar, and radiometer measurements using two different techniques allows an estimation of the vertical profile of cloud microphysical properties such as particle sizes. Enhancement of lidar backscatter near zenith revealed when highly oriented ice crystals were present. The authors demonstrate that no single instrument is sufficient to accurately describe cirrus clouds and that measurements in combination can provide important details on their geometric, radiative, and microphysical properties.
Abstract
The variability of precipitation and water supply along the U.S. West Coast creates major challenges to the region’s economy and environment, as evidenced by the recent California drought. This variability is strongly influenced by atmospheric rivers (ARs), which deliver much of the precipitation along the U.S. West Coast and can cause flooding, and by aerosols (from local sources and transported from remote continents and oceans) that modulate clouds and precipitation. A better understanding of these processes is needed to reduce uncertainties in weather predictions and climate projections of droughts and floods, both now and under changing climate conditions.
To address these gaps, a group of meteorologists, hydrologists, climate scientists, atmospheric chemists, and oceanographers have created an interdisciplinary research effort, with support from multiple agencies. From 2009 to 2011 a series of field campaigns [California Water Service (CalWater) 1] collected atmospheric chemistry, cloud microphysics, and meteorological measurements in California and associated modeling and diagnostic studies were carried out. Based on the remaining gaps, a vision was developed to extend these studies offshore over the eastern North Pacific and to enhance land-based measurements from 2014 to 2018 (CalWater-2). The dataset and selected results from CalWater-1 are summarized here. The goals of CalWater-2, and measurements to date, are then described.
CalWater is producing new findings and exploring new technologies to evaluate and improve global climate models and their regional performance and to develop tools supporting water and hydropower management. These advances also have potential to enhance hazard mitigation by improving near-term weather prediction and subseasonal and seasonal outlooks.
Abstract
The variability of precipitation and water supply along the U.S. West Coast creates major challenges to the region’s economy and environment, as evidenced by the recent California drought. This variability is strongly influenced by atmospheric rivers (ARs), which deliver much of the precipitation along the U.S. West Coast and can cause flooding, and by aerosols (from local sources and transported from remote continents and oceans) that modulate clouds and precipitation. A better understanding of these processes is needed to reduce uncertainties in weather predictions and climate projections of droughts and floods, both now and under changing climate conditions.
To address these gaps, a group of meteorologists, hydrologists, climate scientists, atmospheric chemists, and oceanographers have created an interdisciplinary research effort, with support from multiple agencies. From 2009 to 2011 a series of field campaigns [California Water Service (CalWater) 1] collected atmospheric chemistry, cloud microphysics, and meteorological measurements in California and associated modeling and diagnostic studies were carried out. Based on the remaining gaps, a vision was developed to extend these studies offshore over the eastern North Pacific and to enhance land-based measurements from 2014 to 2018 (CalWater-2). The dataset and selected results from CalWater-1 are summarized here. The goals of CalWater-2, and measurements to date, are then described.
CalWater is producing new findings and exploring new technologies to evaluate and improve global climate models and their regional performance and to develop tools supporting water and hydropower management. These advances also have potential to enhance hazard mitigation by improving near-term weather prediction and subseasonal and seasonal outlooks.
An overview is given of the First ISCCP Regional Experiment Arctic Clouds Experiment that was conducted during April–July 1998. The principal goal of the field experiment was to gather the data needed to examine the impact of arctic clouds on the radiation exchange between the surface, atmosphere, and space, and to study how the surface influences the evolution of boundary layer clouds. The observations will be used to evaluate and improve climate model parameterizations of cloud and radiation processes, satellite remote sensing of cloud and surface characteristics, and understanding of cloud–radiation feedbacks in the Arctic. The experiment utilized four research aircraft that flew over surface-based observational sites in the Arctic Ocean and at Barrow, Alaska. This paper describes the programmatic and scientific objectives of the project, the experimental design (including research platforms and instrumentation), the conditions that were encountered during the field experiment, and some highlights of preliminary observations, modeling, and satellite remote sensing studies.
An overview is given of the First ISCCP Regional Experiment Arctic Clouds Experiment that was conducted during April–July 1998. The principal goal of the field experiment was to gather the data needed to examine the impact of arctic clouds on the radiation exchange between the surface, atmosphere, and space, and to study how the surface influences the evolution of boundary layer clouds. The observations will be used to evaluate and improve climate model parameterizations of cloud and radiation processes, satellite remote sensing of cloud and surface characteristics, and understanding of cloud–radiation feedbacks in the Arctic. The experiment utilized four research aircraft that flew over surface-based observational sites in the Arctic Ocean and at Barrow, Alaska. This paper describes the programmatic and scientific objectives of the project, the experimental design (including research platforms and instrumentation), the conditions that were encountered during the field experiment, and some highlights of preliminary observations, modeling, and satellite remote sensing studies.
