Search Results
You are looking at 1 - 10 of 23 items for
- Author or Editor: J. W. Strapp x
- Refine by Access: All Content x
Abstract
The Gerber Scientific, Inc. Particle Volume Monitor (PVM) is widely used to measure the liquid water content (LWC) and droplet effective radius (r eff) of water clouds. The LWC response of the airborne version of this instrument, the PVM-100A, was evaluated in two independent wet wind tunnel experiments under well-controlled conditions. Earlier studies predict that the PVM-100 (the ground-based version of the PVM) theoretical response to monodisperse droplets diminishes for droplet diameters larger than about 40 μm. However, results from the wind tunnel experiments presented in this paper show that the response of the PVM-100A to monodisperse droplets begins to decrease when the droplet diameter is between 20- and 30-μm diameter. For polydisperse droplet populations (such as those found in natural clouds) the efficiency of the airborne PVM-100A for sensing LWC begins to decrease when the median volume diameter (MVD) of the droplet size distribution is above 20 μm, falling to 50% efficiency for an MVD of 50 μm. Therefore, measurements of LWC obtained from the PVM-100A in natural clouds with broad droplet size distributions (and large values of MVD) should be treated with caution.
Abstract
The Gerber Scientific, Inc. Particle Volume Monitor (PVM) is widely used to measure the liquid water content (LWC) and droplet effective radius (r eff) of water clouds. The LWC response of the airborne version of this instrument, the PVM-100A, was evaluated in two independent wet wind tunnel experiments under well-controlled conditions. Earlier studies predict that the PVM-100 (the ground-based version of the PVM) theoretical response to monodisperse droplets diminishes for droplet diameters larger than about 40 μm. However, results from the wind tunnel experiments presented in this paper show that the response of the PVM-100A to monodisperse droplets begins to decrease when the droplet diameter is between 20- and 30-μm diameter. For polydisperse droplet populations (such as those found in natural clouds) the efficiency of the airborne PVM-100A for sensing LWC begins to decrease when the median volume diameter (MVD) of the droplet size distribution is above 20 μm, falling to 50% efficiency for an MVD of 50 μm. Therefore, measurements of LWC obtained from the PVM-100A in natural clouds with broad droplet size distributions (and large values of MVD) should be treated with caution.
Abstract
Analysis of the aircraft icing environments of East Coast winter storms have been made from 3 1 flights duringthe second Canadian Atlantic Storms Program. Microphysical parameters have been summarized and are compared to common icing intensity envelopes and to other icing datasets. Cloud regions with supercooled liquid water had an average horizontal extent of 4.3 km, with average droplet concentrations of 130 μ, liquid water contents of 0.13 g m-3, and droplet median volume diameters of 18 pm. In general, the icing intensity observed was classified as light, although moderate to severe icing was observed in several common synoptic situationsand several cases are discussed. Freezing drizzle was observed on four flights, and represented the most severeicing environment encountered.
Abstract
Analysis of the aircraft icing environments of East Coast winter storms have been made from 3 1 flights duringthe second Canadian Atlantic Storms Program. Microphysical parameters have been summarized and are compared to common icing intensity envelopes and to other icing datasets. Cloud regions with supercooled liquid water had an average horizontal extent of 4.3 km, with average droplet concentrations of 130 μ, liquid water contents of 0.13 g m-3, and droplet median volume diameters of 18 pm. In general, the icing intensity observed was classified as light, although moderate to severe icing was observed in several common synoptic situationsand several cases are discussed. Freezing drizzle was observed on four flights, and represented the most severeicing environment encountered.
Abstract
Comparisons of particle-size distributions measured by Particle Measuring Systems FSSP-300 and PCASP-100X probes through a range of relative humidities reveal that the deiced PCASP-100X probe dries hydrated submicron aerosols before measurement. The FSSP-300 appears to measure the particles in their hydrated state and detects the expected growth in the particle spectrum with increasing relative humidity. Calibration changes fox refractive-index changes with hydration are not applicable to the deiced PCASP-100X probe but are for the FSSP-300. The combined use of the two probes with their differing responses to hydrated aerosols may provide information related to the chemical composition of the aerosol.
Abstract
Comparisons of particle-size distributions measured by Particle Measuring Systems FSSP-300 and PCASP-100X probes through a range of relative humidities reveal that the deiced PCASP-100X probe dries hydrated submicron aerosols before measurement. The FSSP-300 appears to measure the particles in their hydrated state and detects the expected growth in the particle spectrum with increasing relative humidity. Calibration changes fox refractive-index changes with hydration are not applicable to the deiced PCASP-100X probe but are for the FSSP-300. The combined use of the two probes with their differing responses to hydrated aerosols may provide information related to the chemical composition of the aerosol.
Ice Crystal Sizes in High Ice Water Content Clouds. Part I: On the Computation of Median Mass Diameter from In Situ MeasurementsAbstract
Engine and air data probe manufacturers, as well as aviation agencies, are interested in better characterization of high ice water content (HIWC) areas close to thunderstorms, since HIWC conditions are suspected to cause in-service engine power loss and air data events on commercial aircraft. In this context, a collaborative field campaign has been conducted by high-altitude ice crystals (HAIC) and HIWC projects in order to provide ice water content and median mass diameter (MMD) of ice crystals in the HIWC environment.
The computation of MMD from in situ measurements relies mainly on the definition of the crystal dimension D and on the 
Since MMDs are quite sensitive to β, this study suggests a generic method for deducing β solely from optical array probes (OAPs) image data for various size definitions. The method is based on simulations of 3D crystal objects projected onto a 2D plane, thereby relating crystal mass to 2D area (projection) and perimeter. The MMD values calculated for different size definitions are quite similar, at least much closer than MMDs derived from different m(D) relationships in the literature.
Abstract
Engine and air data probe manufacturers, as well as aviation agencies, are interested in better characterization of high ice water content (HIWC) areas close to thunderstorms, since HIWC conditions are suspected to cause in-service engine power loss and air data events on commercial aircraft. In this context, a collaborative field campaign has been conducted by high-altitude ice crystals (HAIC) and HIWC projects in order to provide ice water content and median mass diameter (MMD) of ice crystals in the HIWC environment.
The computation of MMD from in situ measurements relies mainly on the definition of the crystal dimension D and on the
Since MMDs are quite sensitive to β, this study suggests a generic method for deducing β solely from optical array probes (OAPs) image data for various size definitions. The method is based on simulations of 3D crystal objects projected onto a 2D plane, thereby relating crystal mass to 2D area (projection) and perimeter. The MMD values calculated for different size definitions are quite similar, at least much closer than MMDs derived from different m(D) relationships in the literature.
Abstract
In this study, observations from aircraft, Doppler radar, and LANDSAT are used to better understand dynamical and microphysical characteristics of low-level Arctic clouds for climate change studies. Observations during the Beaufort and Arctic Storms Experiment were collected over the southern Beaufort Sea and the northern Mackenzie River Basin during 1 September–14 October 1994. Measurements from the cases of 8 September and 24–25 September are analyzed. In situ observations were made by instruments mounted on a Convair-580 research aircraft. Reflectivity and radial winds were obtained from an X-band Doppler radar located near Inuvik. The reflectivity field from LANDSAT observations concurrent with the aircraft and radar observations was also obtained. Dynamical activity, representing vertical air velocity (w a ) and turbulent fluxes, is found to be larger in cloud regions. The sizes of coherent structures (e.g., cells) are from 0.1 to 15 km as determined by wavelet analysis and time series of aircraft data. This size is comparable with LANDSAT and Doppler radar–derived cell sizes. Reflectivity in embedded cells for the 8 September case was larger than that of single convective cells for the 24–25 September case. The effective radius for ice crystals (droplets) ranged from 37(7.5) μm to 70(9.5) μm for both cases. Using observations, parameterization of the ice crystal number concentration (N i ) is obtained from a heat budget equation. Results showed that N i is a function of w a , radiative cooling, particle size, and supersaturation. The large-scale models may have large uncertainties related to microphysical and dynamical processes (e.g., particle size and vertical air velocity, respectively), which can directly or indirectly influence radiative processes. Overall, the results suggest that the microphysical and dynamical properties of Arctic clouds need to be further explored for climate change studies.
Abstract
In this study, observations from aircraft, Doppler radar, and LANDSAT are used to better understand dynamical and microphysical characteristics of low-level Arctic clouds for climate change studies. Observations during the Beaufort and Arctic Storms Experiment were collected over the southern Beaufort Sea and the northern Mackenzie River Basin during 1 September–14 October 1994. Measurements from the cases of 8 September and 24–25 September are analyzed. In situ observations were made by instruments mounted on a Convair-580 research aircraft. Reflectivity and radial winds were obtained from an X-band Doppler radar located near Inuvik. The reflectivity field from LANDSAT observations concurrent with the aircraft and radar observations was also obtained. Dynamical activity, representing vertical air velocity (w a ) and turbulent fluxes, is found to be larger in cloud regions. The sizes of coherent structures (e.g., cells) are from 0.1 to 15 km as determined by wavelet analysis and time series of aircraft data. This size is comparable with LANDSAT and Doppler radar–derived cell sizes. Reflectivity in embedded cells for the 8 September case was larger than that of single convective cells for the 24–25 September case. The effective radius for ice crystals (droplets) ranged from 37(7.5) μm to 70(9.5) μm for both cases. Using observations, parameterization of the ice crystal number concentration (N i ) is obtained from a heat budget equation. Results showed that N i is a function of w a , radiative cooling, particle size, and supersaturation. The large-scale models may have large uncertainties related to microphysical and dynamical processes (e.g., particle size and vertical air velocity, respectively), which can directly or indirectly influence radiative processes. Overall, the results suggest that the microphysical and dynamical properties of Arctic clouds need to be further explored for climate change studies.
Abstract
In this paper, statistical properties of rainfall are derived from 14 years of Tropical Rainfall Measuring Mission data to optimize the use of flight hours for the upcoming High Altitude Ice Crystals (HAIC)/High Ice Water Content (HIWC) program. This program aims to investigate the convective processes responsible for the generation of the high ice water content that has been recognized as a threat to civil aviation. The probability that convective cells are conducive to HIWC is also further investigated using three years of C-band polarimetric radar data. Further insights into the variability of convective rainfall and favorable conditions for HIWC are also gained using two different methods to characterize the large-scale atmospheric conditions around Darwin, Australia (the Madden–Julian oscillation and the Darwin atmospheric regimes), and the underlying surface type (oceanic vs continental). The main results from the climatology relevant to flight-plan decision making are (i) convective cells conducive to HIWC should be found close to Darwin, (ii) at least 90% of convective cells are conducive to HIWC at 10- and 12-km flight levels, (iii) multiple flights per day in favorable large-scale conditions will be needed so as to utilize the 150 project flight hours, (iv) the largest numbers of HIWC radar pixels are found around 0300 and 1500 local time, and (v) to fulfill the requirement to fly 90 h in oceanic convection and 60 h in or around continental convection, a minimum “acceptable” size of the convective area has been derived and should serve as a guideline for flight-decision purposes.
Abstract
In this paper, statistical properties of rainfall are derived from 14 years of Tropical Rainfall Measuring Mission data to optimize the use of flight hours for the upcoming High Altitude Ice Crystals (HAIC)/High Ice Water Content (HIWC) program. This program aims to investigate the convective processes responsible for the generation of the high ice water content that has been recognized as a threat to civil aviation. The probability that convective cells are conducive to HIWC is also further investigated using three years of C-band polarimetric radar data. Further insights into the variability of convective rainfall and favorable conditions for HIWC are also gained using two different methods to characterize the large-scale atmospheric conditions around Darwin, Australia (the Madden–Julian oscillation and the Darwin atmospheric regimes), and the underlying surface type (oceanic vs continental). The main results from the climatology relevant to flight-plan decision making are (i) convective cells conducive to HIWC should be found close to Darwin, (ii) at least 90% of convective cells are conducive to HIWC at 10- and 12-km flight levels, (iii) multiple flights per day in favorable large-scale conditions will be needed so as to utilize the 150 project flight hours, (iv) the largest numbers of HIWC radar pixels are found around 0300 and 1500 local time, and (v) to fulfill the requirement to fly 90 h in oceanic convection and 60 h in or around continental convection, a minimum “acceptable” size of the convective area has been derived and should serve as a guideline for flight-decision purposes.
Abstract
This paper considers the theory of diffraction image formation of spherical particles and peculiarities of particle sizing by discrete imaging probes. The diffraction images of spherical water droplets are approximated by Fresnel diffraction by an opaque disc. The approach developed in the paper is applicable to all types of array and matrix imaging probes. The analysis measurement accuracy is performed for the PMS Optical Array Prove (OAP)-2D-C and OAP-2Dgray probes. It is shown that a 25-μm resolution PMS OAP-2D-C probe can both oversize and undersize droplets smaller than approximately 100 μm in diameter, and oversize droplets larger than approximately 100 μm. The errors in droplet sizing increase with decreasing size. The discrete manner of particle image registration also leads to losses of particles with sizes smaller than 100 μm. For the ideal case with zero photodiode response time, these losses reach 70% for 25-μm droplets. A nonzero response time will increase these losses. These findings help explain discrepancies observed in the overlap region of the PMS FSSP and OAP droplet spectra. A variety of calculated digital images for PMS OAP-2D-C and OAP-2Dgray probes is presented. Different methods of particle image sizing are discussed. Several methods of size correction of individual droplets and droplet ensembles are suggested. Correction algorithms for these effects are derived, and distortion and correction retrieval matrices are calculated. Several examples of actual and measured size distributions are presented.
Abstract
This paper considers the theory of diffraction image formation of spherical particles and peculiarities of particle sizing by discrete imaging probes. The diffraction images of spherical water droplets are approximated by Fresnel diffraction by an opaque disc. The approach developed in the paper is applicable to all types of array and matrix imaging probes. The analysis measurement accuracy is performed for the PMS Optical Array Prove (OAP)-2D-C and OAP-2Dgray probes. It is shown that a 25-μm resolution PMS OAP-2D-C probe can both oversize and undersize droplets smaller than approximately 100 μm in diameter, and oversize droplets larger than approximately 100 μm. The errors in droplet sizing increase with decreasing size. The discrete manner of particle image registration also leads to losses of particles with sizes smaller than 100 μm. For the ideal case with zero photodiode response time, these losses reach 70% for 25-μm droplets. A nonzero response time will increase these losses. These findings help explain discrepancies observed in the overlap region of the PMS FSSP and OAP droplet spectra. A variety of calculated digital images for PMS OAP-2D-C and OAP-2Dgray probes is presented. Different methods of particle image sizing are discussed. Several methods of size correction of individual droplets and droplet ensembles are suggested. Correction algorithms for these effects are derived, and distortion and correction retrieval matrices are calculated. Several examples of actual and measured size distributions are presented.
Abstract
Airborne measurements of ice water content (IWC) in both ice and mixed-phase clouds remain one of the long-standing problems in experimental cloud physics. For nearly three decades, IWC has been measured with the help of the Nevzorov hot-wire total water content (TWC) sensor, which had an inverted cone shape. It was assumed that ice particles would be captured inside the cone and then completely melt and evaporate. However, wind tunnel experiments conducted with the help of high-speed video recordings showed that ice particles may bounce out of the TWC cone, resulting in the underestimation of the measured IWC. The TWC sensor was modified to improve the capture efficiency of ice particles. The modified sensor was mounted on the National Research Council (NRC) Convair-580 and its measurements in ice clouds were compared with the measurements of the original Nevzorov TWC sensor, a Droplet Measurement Technologies (DMT) counterflow virtual impactor (CVI), and IWC calculated from the particle size distribution measured by optical array probes (OAPs). Results indicated that the IWC measured by the modified TWC hot-wire sensor as well as the CVI and that deduced from the OAP size distributions agreed reasonably well when the maximum size of ice particles did not exceed 4 mm. However, IWC measured by the original TWC sensor was approximately 3 times lower than that measured by the other three techniques. This result can be used for the retrieval of the past IWC measurements obtained with this TWC sensor. For clouds with ice particles larger than 4 mm, the IWC measured by the modified TWC sensor and CVI exhibited diverging measurements.
Abstract
Airborne measurements of ice water content (IWC) in both ice and mixed-phase clouds remain one of the long-standing problems in experimental cloud physics. For nearly three decades, IWC has been measured with the help of the Nevzorov hot-wire total water content (TWC) sensor, which had an inverted cone shape. It was assumed that ice particles would be captured inside the cone and then completely melt and evaporate. However, wind tunnel experiments conducted with the help of high-speed video recordings showed that ice particles may bounce out of the TWC cone, resulting in the underestimation of the measured IWC. The TWC sensor was modified to improve the capture efficiency of ice particles. The modified sensor was mounted on the National Research Council (NRC) Convair-580 and its measurements in ice clouds were compared with the measurements of the original Nevzorov TWC sensor, a Droplet Measurement Technologies (DMT) counterflow virtual impactor (CVI), and IWC calculated from the particle size distribution measured by optical array probes (OAPs). Results indicated that the IWC measured by the modified TWC hot-wire sensor as well as the CVI and that deduced from the OAP size distributions agreed reasonably well when the maximum size of ice particles did not exceed 4 mm. However, IWC measured by the original TWC sensor was approximately 3 times lower than that measured by the other three techniques. This result can be used for the retrieval of the past IWC measurements obtained with this TWC sensor. For clouds with ice particles larger than 4 mm, the IWC measured by the modified TWC sensor and CVI exhibited diverging measurements.
Abstract
A counterflow virtual impactor (CVI) designed for aircraft use was evaluated at the NASA Icing Research Tunnel in Cleveland, Ohio. Tests were conducted for tunnel speeds of 67 and 100 m s−1, for liquid water contents of 0.23–1.4 g m−3, and for a wide range of droplet median volume diameters (MVDs). For droplet distributions with MVDs between about 30 and 240 μm, liquid water content (LWC) measured by the CVI agreed with reference values within the uncertainty of the measurements. For a range of LWCs at 30-μm MVD, the relationship was near 1:1, and no systematic dependence of CVI results on LWC or airspeed was observed. For smaller MVDs, the CVI underestimated LWC. Decreased collection efficiency for small droplets can partially explain this effect, but the difference from reference values was larger than expected based on previous calibrations and comparisons with in situ data. Tunnel runs conducted with a flow-straightening shroud around the CVI inlet produced approximately 20% enhancements in LWC at small MVDs, which are expected for these speeds based on previous modeling studies. The effect of large drop breakup on CVI droplet number concentration was evaluated both theoretically and experimentally; drop breakup was predicted to occur for drops larger than 169 μm at 67 m s−1 and larger than 76 μm at 100 m s−1. Enhancement in number concentration measured by the CVI was found to be strongly related to observed large drop concentrations, particularly to those in the 312–700-μm-diameter range.
Abstract
A counterflow virtual impactor (CVI) designed for aircraft use was evaluated at the NASA Icing Research Tunnel in Cleveland, Ohio. Tests were conducted for tunnel speeds of 67 and 100 m s−1, for liquid water contents of 0.23–1.4 g m−3, and for a wide range of droplet median volume diameters (MVDs). For droplet distributions with MVDs between about 30 and 240 μm, liquid water content (LWC) measured by the CVI agreed with reference values within the uncertainty of the measurements. For a range of LWCs at 30-μm MVD, the relationship was near 1:1, and no systematic dependence of CVI results on LWC or airspeed was observed. For smaller MVDs, the CVI underestimated LWC. Decreased collection efficiency for small droplets can partially explain this effect, but the difference from reference values was larger than expected based on previous calibrations and comparisons with in situ data. Tunnel runs conducted with a flow-straightening shroud around the CVI inlet produced approximately 20% enhancements in LWC at small MVDs, which are expected for these speeds based on previous modeling studies. The effect of large drop breakup on CVI droplet number concentration was evaluated both theoretically and experimentally; drop breakup was predicted to occur for drops larger than 169 μm at 67 m s−1 and larger than 76 μm at 100 m s−1. Enhancement in number concentration measured by the CVI was found to be strongly related to observed large drop concentrations, particularly to those in the 312–700-μm-diameter range.
Abstract
The present study discusses the meso- and microscale structures of Precipitation regions within a midlatitude winter storm over the North Atlantic, observed during the Experiment on Rapidly Intensifying Cyclones over the Atlantic. Two wide regions of precipitation separated by a narrow band were observed at low levels by airborne radar. These regions were aligned parallel to the cold front and were sampled by aircraft at three different levels. The calculated mesoscale frontogenetical forcing is dominated at low levels by confluence and at mid-levels by the tilting term. The absolute magnitudes are smaller than those reported by Shapiro, and Bond and Fleagle, and are consistent with the broader and less intense front in this study. The frontogenetical forcing due to melting of ice crystals was estimated from observations of precipitation particles. The analysis indicates that the cooling due to melting of ice particles is not a dominant frontogenetical forcing at the observed stage in storm evolution. Precipitation rates larger than those observed (by a factor of 3) behind the cold front are needed before the thermal impact of melting could contribute to frontogenesis as much as confluence at the same level. The region of precipitation ahead of the cold front appears to be linked to convective instability observed in the warm sector. The observed precipitation region to the west of the cold front is consistent with the trajectories of failing particles carried by the relative wind flowing toward the back of the system. The decrease in precipitation rate observed right behind the front can be interpreted as ice particles failing through a deep region in which temperatures are close to 0°C. The presence of such a region leads to a nonuniform precipitation distribution, with areas that would appear as precipitation bands in radar images, and others in which precipitation is reduced.
Abstract
The present study discusses the meso- and microscale structures of Precipitation regions within a midlatitude winter storm over the North Atlantic, observed during the Experiment on Rapidly Intensifying Cyclones over the Atlantic. Two wide regions of precipitation separated by a narrow band were observed at low levels by airborne radar. These regions were aligned parallel to the cold front and were sampled by aircraft at three different levels. The calculated mesoscale frontogenetical forcing is dominated at low levels by confluence and at mid-levels by the tilting term. The absolute magnitudes are smaller than those reported by Shapiro, and Bond and Fleagle, and are consistent with the broader and less intense front in this study. The frontogenetical forcing due to melting of ice crystals was estimated from observations of precipitation particles. The analysis indicates that the cooling due to melting of ice particles is not a dominant frontogenetical forcing at the observed stage in storm evolution. Precipitation rates larger than those observed (by a factor of 3) behind the cold front are needed before the thermal impact of melting could contribute to frontogenesis as much as confluence at the same level. The region of precipitation ahead of the cold front appears to be linked to convective instability observed in the warm sector. The observed precipitation region to the west of the cold front is consistent with the trajectories of failing particles carried by the relative wind flowing toward the back of the system. The decrease in precipitation rate observed right behind the front can be interpreted as ice particles failing through a deep region in which temperatures are close to 0°C. The presence of such a region leads to a nonuniform precipitation distribution, with areas that would appear as precipitation bands in radar images, and others in which precipitation is reduced.
