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- Author or Editor: R. Paul Lawson x
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Abstract
A new airborne thermometer has been designed using results from numerical simulators of airflow and particle (drop) trajectories. Initial flight tests with the NCAR King Air show that the new thermometer, which uses a fine-wire thermocouple for the sensor and lacks a probe housing, has a response time that is significantly faster than thermometers currently in use. An example of heat-flux calculations in a convective boundary layer shows that, compared to measurements using the Rosemount thermometer and NCAR K probes, the turbulent heat flux is greater by about 20% when using measurements from the new thermometer. Theoretical calculations of time response support the claim that the improved response is due to the absence of a probe housing.
The new thermometer was designed to inertially separate cloud drops from the airflow, and flights in warm clouds suggest that the thermocouple sensor stays dry except in clouds that contain high concentrations of drizzle-size drops. In small cumulus clouds with approximately 1 g m−3 of liquid water that contained low concentrations (∼10 l−1) of drizzle drops, the new thermocouple probe consistently measured warmer temperatures than the reverse-flow and Rosemount thermometers, suggesting that in these clouds the thermocouple probe may not have been affected by errors from sensor wetting. Thus, static temperature measured by the new thermometer in clouds with continental drop spectra should be reliable. An example of data collected in a mixed region of a small cumulus cloud shows that there may be more temperature structure at scales of 2–50 m than previously observed.
Abstract
A new airborne thermometer has been designed using results from numerical simulators of airflow and particle (drop) trajectories. Initial flight tests with the NCAR King Air show that the new thermometer, which uses a fine-wire thermocouple for the sensor and lacks a probe housing, has a response time that is significantly faster than thermometers currently in use. An example of heat-flux calculations in a convective boundary layer shows that, compared to measurements using the Rosemount thermometer and NCAR K probes, the turbulent heat flux is greater by about 20% when using measurements from the new thermometer. Theoretical calculations of time response support the claim that the improved response is due to the absence of a probe housing.
The new thermometer was designed to inertially separate cloud drops from the airflow, and flights in warm clouds suggest that the thermocouple sensor stays dry except in clouds that contain high concentrations of drizzle-size drops. In small cumulus clouds with approximately 1 g m−3 of liquid water that contained low concentrations (∼10 l−1) of drizzle drops, the new thermocouple probe consistently measured warmer temperatures than the reverse-flow and Rosemount thermometers, suggesting that in these clouds the thermocouple probe may not have been affected by errors from sensor wetting. Thus, static temperature measured by the new thermometer in clouds with continental drop spectra should be reliable. An example of data collected in a mixed region of a small cumulus cloud shows that there may be more temperature structure at scales of 2–50 m than previously observed.
Abstract
The ability of airborne instruments to measure temperature in cloud is studied using theoretical analyses and experimental data. Theoretical predictions of the effects of sensor wetting are reviewed and modified, and are then compared to measurements. Two airborne immersion thermometers, the NCAR “reverse-flow” thermometer and the Rosemount 102 thermometer, are compared to each other and to a new radiometric thermometer. The comparisons show that out of cloud all three thermometers agree well with each other. However, there is clear evidence that the immersion thermometers become wet in some clouds and measure erroneously low temperatures as a result. The evidence, particularly from measurements in unmixed parcels, supports the validity of the measurements from the radiometric thermometer both inside and outside clouds. Supporting evidence that the immersion sensors are susceptible to wetting is provided from tests in a wind tunnel and from measurements using a conductivity sensor placed at the location of the immersion sensors. The scientific consequences of these measurement errors, particularly in studies of entrainment and of cloud buoyancy, are discussed.
Abstract
The ability of airborne instruments to measure temperature in cloud is studied using theoretical analyses and experimental data. Theoretical predictions of the effects of sensor wetting are reviewed and modified, and are then compared to measurements. Two airborne immersion thermometers, the NCAR “reverse-flow” thermometer and the Rosemount 102 thermometer, are compared to each other and to a new radiometric thermometer. The comparisons show that out of cloud all three thermometers agree well with each other. However, there is clear evidence that the immersion thermometers become wet in some clouds and measure erroneously low temperatures as a result. The evidence, particularly from measurements in unmixed parcels, supports the validity of the measurements from the radiometric thermometer both inside and outside clouds. Supporting evidence that the immersion sensors are susceptible to wetting is provided from tests in a wind tunnel and from measurements using a conductivity sensor placed at the location of the immersion sensors. The scientific consequences of these measurement errors, particularly in studies of entrainment and of cloud buoyancy, are discussed.
Abstract
The in situ cloud lidar is designed to measure cloud volumes of millions of cubic meters to overcome the sampling limitations of traditional cloud probes in inhomogeneous clouds. This technique sends laser pulses horizontally from an aircraft inside an optically thick cloud and measures the time series of the multiply scattered light with wide field-of-view detectors viewing upward and downward. The extinction in liquid clouds averaged over tens to hundreds of meters and the distance to cloud boundaries can be retrieved from the signal measured by a single-wavelength in situ lidar. This paper describes the design and operation of an in situ cloud lidar. A laser in the aircraft cabin outputs 532-nm wavelength pulses at 10 Hz, which are sent through beam-expanding optics for eye safety. The upward- and downward-viewing detectors use photomultiplier tubes and operate with either daytime (3° half angle; 0.37-nm solar-blocking filter) or nighttime (30°) optics. Example daytime lidar signals in dense cloud have a dynamic range of 1000 after solar background subtraction. Results from a nighttime flight in marine stratus are analyzed in detail. The variations in the lidar signals with aircraft travel are much smoother for the longer photon travel times, indicating that the later times sample volumes hundreds of meters in size. Extinction retrievals for 25-m-radius volumes have high correlation (R 2 = 0.84) with Forward Scattering Spectrometer Probe (FSSP)-derived extinction, while the correlation is relatively low (R 2 = 0.40) for 200-m volumes due to cloud inhomogeneity. Lidar retrievals of cloud-base and -top height from inside the cloud are consistent with cloud boundaries obtained from aircraft penetrations on ascents and descents.
Abstract
The in situ cloud lidar is designed to measure cloud volumes of millions of cubic meters to overcome the sampling limitations of traditional cloud probes in inhomogeneous clouds. This technique sends laser pulses horizontally from an aircraft inside an optically thick cloud and measures the time series of the multiply scattered light with wide field-of-view detectors viewing upward and downward. The extinction in liquid clouds averaged over tens to hundreds of meters and the distance to cloud boundaries can be retrieved from the signal measured by a single-wavelength in situ lidar. This paper describes the design and operation of an in situ cloud lidar. A laser in the aircraft cabin outputs 532-nm wavelength pulses at 10 Hz, which are sent through beam-expanding optics for eye safety. The upward- and downward-viewing detectors use photomultiplier tubes and operate with either daytime (3° half angle; 0.37-nm solar-blocking filter) or nighttime (30°) optics. Example daytime lidar signals in dense cloud have a dynamic range of 1000 after solar background subtraction. Results from a nighttime flight in marine stratus are analyzed in detail. The variations in the lidar signals with aircraft travel are much smoother for the longer photon travel times, indicating that the later times sample volumes hundreds of meters in size. Extinction retrievals for 25-m-radius volumes have high correlation (R 2 = 0.84) with Forward Scattering Spectrometer Probe (FSSP)-derived extinction, while the correlation is relatively low (R 2 = 0.40) for 200-m volumes due to cloud inhomogeneity. Lidar retrievals of cloud-base and -top height from inside the cloud are consistent with cloud boundaries obtained from aircraft penetrations on ascents and descents.
Abstract
Aircraft in situ observations of precipitation during the Rain in Cumulus over the Ocean (RICO) field project are used to study and parameterize the effects of precipitation on cloud probes. Specifically, the effects of precipitation on the Forward Scattering Spectrometer Probe, the King cloud liquid water hot-wire probe, and the particle volume monitor are parameterized as linear functions of the precipitation water content.
Abstract
Aircraft in situ observations of precipitation during the Rain in Cumulus over the Ocean (RICO) field project are used to study and parameterize the effects of precipitation on cloud probes. Specifically, the effects of precipitation on the Forward Scattering Spectrometer Probe, the King cloud liquid water hot-wire probe, and the particle volume monitor are parameterized as linear functions of the precipitation water content.
Abstract
Due to the spatially inhomogeneous nature of clouds there are large uncertainties in validating remote sensing retrievals of cloud properties with traditional in situ cloud probes, which have sampling volumes measured in liters. This paper introduces a new technique called in situ cloud lidar, which can measure extinction in liquid clouds with sampling volumes of millions of cubic meters. In this technique a laser sends out pulses of light horizontally from an aircraft inside an optically thick cloud, and wide-field-of-view detectors viewing upward and downward measure the time series of the number of photons returned. Diffusion theory calculations indicate that the expected in situ lidar time series depends on the extinction and has a functional form of a power law times an exponential, with the exponential scale depending on the distance to the cloud boundary. Simulations of 532-nm wavelength in situ lidar time series are made with a Monte Carlo radiative transfer model in stochastically generated inhomogeneous stratocumulus clouds. Retrieval simulations are performed using a neural network trained on three parameters fit to the time series of each detector to predict 1) the extinction at four volume-averaging scales, 2) the cloud geometric thickness, and 3) the optical depth at four averaging scales. Even with an assumed 20% lidar calibration error the rms extinction and optical depth retrieval accuracy is only 12%. Simulations with a dual wavelength lidar (532 and 1550 nm) give accurate retrievals of liquid water content and effective radius. The results of a mountain-top demonstration of the in situ lidar technique show the expected power-law time series behavior.
Abstract
Due to the spatially inhomogeneous nature of clouds there are large uncertainties in validating remote sensing retrievals of cloud properties with traditional in situ cloud probes, which have sampling volumes measured in liters. This paper introduces a new technique called in situ cloud lidar, which can measure extinction in liquid clouds with sampling volumes of millions of cubic meters. In this technique a laser sends out pulses of light horizontally from an aircraft inside an optically thick cloud, and wide-field-of-view detectors viewing upward and downward measure the time series of the number of photons returned. Diffusion theory calculations indicate that the expected in situ lidar time series depends on the extinction and has a functional form of a power law times an exponential, with the exponential scale depending on the distance to the cloud boundary. Simulations of 532-nm wavelength in situ lidar time series are made with a Monte Carlo radiative transfer model in stochastically generated inhomogeneous stratocumulus clouds. Retrieval simulations are performed using a neural network trained on three parameters fit to the time series of each detector to predict 1) the extinction at four volume-averaging scales, 2) the cloud geometric thickness, and 3) the optical depth at four averaging scales. Even with an assumed 20% lidar calibration error the rms extinction and optical depth retrieval accuracy is only 12%. Simulations with a dual wavelength lidar (532 and 1550 nm) give accurate retrievals of liquid water content and effective radius. The results of a mountain-top demonstration of the in situ lidar technique show the expected power-law time series behavior.
Abstract
The design, laboratory calibrations, and flight tests of a new optical imaging instrument, the two-dimensional stereo (2D-S) probe, are presented. Two orthogonal laser beams cross in the middle of the sample volume. Custom, high-speed, 128-photodiode linear arrays and electronics produce shadowgraph images with true 10-μm pixel resolution at aircraft speeds up to 250 m s−1. An overlap region is defined by the two laser beams, improving the sample volume boundaries and sizing of small (<∼100 μm) particles, compared to conventional optical array probes. The stereo views of particles in the overlap region can also improve determination of three-dimensional properties of some particles.
Data collected by three research aircraft are examined and discussed. The 2D-S sees fine details of ice crystals and small water drops coexisting in mixed-phase cloud. Measurements in warm cumuli collected by the NCAR C-130 during the Rain in Cumulus over the Ocean (RICO) project provide a test bed to compare the 2D-S with 2D cloud (2D-C) and 260X probes. The 2D-S sees thousands of cloud drops <∼150 μm when the 2D-C and 260X probes see few or none. The data suggest that particle images and size distributions ranging from 25 to ∼150 μm and collected at airspeeds >100 m s−1 by the 2D-C and 260X probes are probably (erroneously) generated from out-of-focus particles. Development of the 2D-S is in its infancy, and much work needs to be done to quantify its performance and generate software to analyze data.
Abstract
The design, laboratory calibrations, and flight tests of a new optical imaging instrument, the two-dimensional stereo (2D-S) probe, are presented. Two orthogonal laser beams cross in the middle of the sample volume. Custom, high-speed, 128-photodiode linear arrays and electronics produce shadowgraph images with true 10-μm pixel resolution at aircraft speeds up to 250 m s−1. An overlap region is defined by the two laser beams, improving the sample volume boundaries and sizing of small (<∼100 μm) particles, compared to conventional optical array probes. The stereo views of particles in the overlap region can also improve determination of three-dimensional properties of some particles.
Data collected by three research aircraft are examined and discussed. The 2D-S sees fine details of ice crystals and small water drops coexisting in mixed-phase cloud. Measurements in warm cumuli collected by the NCAR C-130 during the Rain in Cumulus over the Ocean (RICO) project provide a test bed to compare the 2D-S with 2D cloud (2D-C) and 260X probes. The 2D-S sees thousands of cloud drops <∼150 μm when the 2D-C and 260X probes see few or none. The data suggest that particle images and size distributions ranging from 25 to ∼150 μm and collected at airspeeds >100 m s−1 by the 2D-C and 260X probes are probably (erroneously) generated from out-of-focus particles. Development of the 2D-S is in its infancy, and much work needs to be done to quantify its performance and generate software to analyze data.
Abstract
A tethered-balloon system capable of making microphysical and radiative measurements in clouds is described and examples of measurements in boundary layer stratus clouds in the Arctic and at the South Pole are presented. A 43-m3 helium-filled balloon lofts an instrument package that is powered by two copper conductors in the tether. The instrument package can support several instruments, including, but not limited to, a cloud particle imager; a forward-scattering spectrometer probe; temperature, pressure, humidity, and wind sensors; ice nuclei filters; and a 4-π radiometer that measures actinic flux at 500 and 800 nm. The balloon can stay aloft for an extended period of time (in excess of 24 h) and conduct vertical profiles up to about 1–2 km, contingent upon payload weight, wind speed, and surface elevation. Examples of measurements in mixed-phase clouds at Ny-Ålesund, Svalbard (79°N), and at the South Pole are discussed. The stratus clouds at Ny-Ålesund ranged in temperature from 0° to −10°C and were mostly mixed phase with heavily rimed ice particles, even when cloud-top temperatures were warmer than −5°C. Conversely, mixed-phase clouds at the South Pole contained regions with only water drops at temperatures as cold as −32°C and were often composed of pristine ice crystals. The radiative properties of mixed-phase clouds are a critical component of radiative transfer in polar regions, which, in turn, is a lynch pin for climate change on a global scale.
Abstract
A tethered-balloon system capable of making microphysical and radiative measurements in clouds is described and examples of measurements in boundary layer stratus clouds in the Arctic and at the South Pole are presented. A 43-m3 helium-filled balloon lofts an instrument package that is powered by two copper conductors in the tether. The instrument package can support several instruments, including, but not limited to, a cloud particle imager; a forward-scattering spectrometer probe; temperature, pressure, humidity, and wind sensors; ice nuclei filters; and a 4-π radiometer that measures actinic flux at 500 and 800 nm. The balloon can stay aloft for an extended period of time (in excess of 24 h) and conduct vertical profiles up to about 1–2 km, contingent upon payload weight, wind speed, and surface elevation. Examples of measurements in mixed-phase clouds at Ny-Ålesund, Svalbard (79°N), and at the South Pole are discussed. The stratus clouds at Ny-Ålesund ranged in temperature from 0° to −10°C and were mostly mixed phase with heavily rimed ice particles, even when cloud-top temperatures were warmer than −5°C. Conversely, mixed-phase clouds at the South Pole contained regions with only water drops at temperatures as cold as −32°C and were often composed of pristine ice crystals. The radiative properties of mixed-phase clouds are a critical component of radiative transfer in polar regions, which, in turn, is a lynch pin for climate change on a global scale.
