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Abstract
The finescale, spatial inhomogeneity of cloud-droplet concentration in stratiform clouds is studied using FSSP-100 data with 10-m or better resolution from 1700 km of cloud penetrations between +5° and −20°C. The data are analyzed in terms of the sizes and spacings of zones where the droplet concentration is significantly below average (droplet-free being one category) or significantly above average. Such zones typically occur throughout stratiform cloud layers, most frequently near their boundaries. The average length of flight path through zones of significant departure from average droplet concentration is up to several tens of meters. These zones probably arise from turbulent entrainment of clear air at cloud top; inhomogeneities of temperature, humidity, and vertical velocity at cloud base; and turbulent velocity fluctuations within cloud that either activate interstitial condensation nuclei or evaporate significant numbers of droplets. Calculations confirm that vertical velocity fluctuations can produce inhomogeneities like those observed.
Abstract
The finescale, spatial inhomogeneity of cloud-droplet concentration in stratiform clouds is studied using FSSP-100 data with 10-m or better resolution from 1700 km of cloud penetrations between +5° and −20°C. The data are analyzed in terms of the sizes and spacings of zones where the droplet concentration is significantly below average (droplet-free being one category) or significantly above average. Such zones typically occur throughout stratiform cloud layers, most frequently near their boundaries. The average length of flight path through zones of significant departure from average droplet concentration is up to several tens of meters. These zones probably arise from turbulent entrainment of clear air at cloud top; inhomogeneities of temperature, humidity, and vertical velocity at cloud base; and turbulent velocity fluctuations within cloud that either activate interstitial condensation nuclei or evaporate significant numbers of droplets. Calculations confirm that vertical velocity fluctuations can produce inhomogeneities like those observed.
Abstract
A new conceptual model is proposed for enhanced cloud droplet growth during condensation. Rapid droplet growth may occur in zones of high supersaturation resulting from isobaric mixing of saturated volumes with different temperatures. Cloud volumes having a temperature different from the general cloud environment may form due to turbulent vertical motions in a temperature lapse rate that is not pseudoadiabatic. This mechanism is most effective in the vicinity of cloud-top inversions. It is also shown that the isobaric mixing of saturated and dry volumes with different temperatures may also lead to high supersaturations. The high supersaturations are associated with zones of molecular mixing, and they have a characteristic size of the order of millimeters with a characteristic lifetime near tenths of a second. Some small proportion of cloud droplets, over many supersaturation events, may grow large enough to grow effectively through collision–coalescence. This hypothesis of isobaric mixing may help explain freezing and warm drizzle formation.
Abstract
A new conceptual model is proposed for enhanced cloud droplet growth during condensation. Rapid droplet growth may occur in zones of high supersaturation resulting from isobaric mixing of saturated volumes with different temperatures. Cloud volumes having a temperature different from the general cloud environment may form due to turbulent vertical motions in a temperature lapse rate that is not pseudoadiabatic. This mechanism is most effective in the vicinity of cloud-top inversions. It is also shown that the isobaric mixing of saturated and dry volumes with different temperatures may also lead to high supersaturations. The high supersaturations are associated with zones of molecular mixing, and they have a characteristic size of the order of millimeters with a characteristic lifetime near tenths of a second. Some small proportion of cloud droplets, over many supersaturation events, may grow large enough to grow effectively through collision–coalescence. This hypothesis of isobaric mixing may help explain freezing and warm drizzle formation.
Abstract
A droplet generator was used to calibrate and study some features of the forward-scattering spectrometer probe (FSSP). The generator produces a stable jet of monodisperse droplets with the rate of 50–500 droplets per second and a velocity between 0.4 and 1.3 m s−1. The estimated standard deviation of the diameter of the droplets is not larger than 1%. Because of the low rate of generated droplets, there were no coincidence problems during the measurements, that is, no multiple droplets in the laser beam. The results of FSSP calibration in its operating ranges 0 and 1 are presented in this paper. The experiments revealed minor differences (<15%) to the size calibration given by the manufacturer. A comparison of the measurements with theoretical Mie curves yielded good agreement.
Furthermore, the effects of the laser beam inhomogeneity on the droplet sizing were studied. Scanning the droplet jet perpendicular and parallel to the axis of the laser beam of the FSSP showed that underestimates of the droplet size up to 9 µm measured by the FSSP may occur. The undersizing depends on the position, in which the droplets cross the sample volume, and the size of the droplets. This contributes to artificial broadening of the size spectra by the FSSP. An algorithm for droplet spectra retrieval due, to light inhomogeneity and particle velocity is discussed.
The results obtained during this study were obtained using one specific FSSP owned by the Institute for Tropospheric Research. However, the range of effects will certainly be similar for other FSSPs.
Abstract
A droplet generator was used to calibrate and study some features of the forward-scattering spectrometer probe (FSSP). The generator produces a stable jet of monodisperse droplets with the rate of 50–500 droplets per second and a velocity between 0.4 and 1.3 m s−1. The estimated standard deviation of the diameter of the droplets is not larger than 1%. Because of the low rate of generated droplets, there were no coincidence problems during the measurements, that is, no multiple droplets in the laser beam. The results of FSSP calibration in its operating ranges 0 and 1 are presented in this paper. The experiments revealed minor differences (<15%) to the size calibration given by the manufacturer. A comparison of the measurements with theoretical Mie curves yielded good agreement.
Furthermore, the effects of the laser beam inhomogeneity on the droplet sizing were studied. Scanning the droplet jet perpendicular and parallel to the axis of the laser beam of the FSSP showed that underestimates of the droplet size up to 9 µm measured by the FSSP may occur. The undersizing depends on the position, in which the droplets cross the sample volume, and the size of the droplets. This contributes to artificial broadening of the size spectra by the FSSP. An algorithm for droplet spectra retrieval due, to light inhomogeneity and particle velocity is discussed.
The results obtained during this study were obtained using one specific FSSP owned by the Institute for Tropospheric Research. However, the range of effects will certainly be similar for other FSSPs.
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
The polarization difference ΔT b between the vertical and horizontal components of thermal radiation emitted by clouds was studied using 37- and 85-GHz radiometers. The measurements were conducted during the Alliance Icing Research Project in Ottawa, Canada, during the winter of 1999/2000. Polarization differences (ΔT b ) greater than 0.1 K were observed in approximately 30% of the cloudy periods. Characteristic values of the polarization difference at 85 GHz were about 2 K with a maximum value of about 4.5 K. Polarization difference at 37 GHz usually did not exceed 2.5 K and was typically 2–6 times less than that at 85 GHz. Both positive and negative polarization differences were observed. It is suggested that the microwave polarization results from scattering of atmospheric thermal radiation by cloud ice particles. The observations were interpreted with a model of radiative transfer in mixed-phase clouds. The characteristic polarization difference observed during ground-based measurements was found to agree with predictions of the radiative transfer model for typical values of cloud liquid and ice water content.
Abstract
The polarization difference ΔT b between the vertical and horizontal components of thermal radiation emitted by clouds was studied using 37- and 85-GHz radiometers. The measurements were conducted during the Alliance Icing Research Project in Ottawa, Canada, during the winter of 1999/2000. Polarization differences (ΔT b ) greater than 0.1 K were observed in approximately 30% of the cloudy periods. Characteristic values of the polarization difference at 85 GHz were about 2 K with a maximum value of about 4.5 K. Polarization difference at 37 GHz usually did not exceed 2.5 K and was typically 2–6 times less than that at 85 GHz. Both positive and negative polarization differences were observed. It is suggested that the microwave polarization results from scattering of atmospheric thermal radiation by cloud ice particles. The observations were interpreted with a model of radiative transfer in mixed-phase clouds. The characteristic polarization difference observed during ground-based measurements was found to agree with predictions of the radiative transfer model for typical values of cloud liquid and ice water content.
Abstract
The technique of using shadow images of particles, obtained in coherent illumination to measure particle size, is analyzed. The theory of Fresnel diffraction for an opaque disc was used to analyze shadow images of transparent spherical particles. A comparison of theoretical calculations with laboratory results supports the application of this approach to optical array probes. It is shown that the shadow image size of spherical particles is essentially dependent on the distance from the object plane. In particular, for drops with diameters of less than 100 µm, the errors in size measurement from the PMS OAP-200X may reach 65%. These results agree well with laboratory calibrations that use monodisperse water droplets.
On the basis of calculated particle diffraction images, the shape of the sample area and its dependence on drop size were calculated. It was found that the sample area has a complicated sawtooth shape. Gaps oriented perpendicular to the axis of a laser beam occur in the sample area in the case of large droplets. Comparison of the sample area quoted by the manufacturer to the calculated one is considered.
Abstract
The technique of using shadow images of particles, obtained in coherent illumination to measure particle size, is analyzed. The theory of Fresnel diffraction for an opaque disc was used to analyze shadow images of transparent spherical particles. A comparison of theoretical calculations with laboratory results supports the application of this approach to optical array probes. It is shown that the shadow image size of spherical particles is essentially dependent on the distance from the object plane. In particular, for drops with diameters of less than 100 µm, the errors in size measurement from the PMS OAP-200X may reach 65%. These results agree well with laboratory calibrations that use monodisperse water droplets.
On the basis of calculated particle diffraction images, the shape of the sample area and its dependence on drop size were calculated. It was found that the sample area has a complicated sawtooth shape. Gaps oriented perpendicular to the axis of a laser beam occur in the sample area in the case of large droplets. Comparison of the sample area quoted by the manufacturer to the calculated one is considered.
Abstract
In situ measurements of microphysics conditions, obtained during 38 research flights into winter storms, have been used to characterize the performance of a Rosemount Icing Detector (RID). Characteristics of the RID were determined under a wide range of cloud environments, which included icing conditions within mixed phase, freezing rain, and freezing drizzle environments. Cloud conditions observed included temperatures between 0° and −29°C and liquid water contents (LWCs) up to 0.7 g m−3. The detection threshold for LWC was found to be 0.007 ± 0.010 g m−3 for the RID operated at an air speed of 97 ± 10 m s−1, which agrees well with theoretical predictions. A signal level of 0 ± 2 mV s−1 accounted for 99.6% of the measurements in clear air and 98.5% of the measurements in glaciated clouds, when the data were averaged over 30-s intervals. No significant response to glaciated clouds was found during any of the research flights, implying that the instrument can be used to segregate glaciated and mixed phase clouds. There was no change in the RID response between liquid and mixed phase conditions, suggesting that ice crystals neither eroded ice accumulation nor accreted to the RID surface under the range of conditions experienced. During sustained icing conditions, a linear relationship between the RID signal and LWC was observed after the RID signal exceeded 400 mV above the clear-air signal level. The LWC derived from the RID was found to agree with LWC measurements from Nevzorov probes within ±50% for 92% of the data. The relationship between the RID signal and LWC was unchanged for freezing precipitation environments with drop median volume diameters >100 μm. The Ludlam limit was estimated for low LWC values and was found to agree well with theoretical calculations. The analysis provides considerable insight into the strengths and weaknesses of the instrument for operations in natural icing conditions.
Abstract
In situ measurements of microphysics conditions, obtained during 38 research flights into winter storms, have been used to characterize the performance of a Rosemount Icing Detector (RID). Characteristics of the RID were determined under a wide range of cloud environments, which included icing conditions within mixed phase, freezing rain, and freezing drizzle environments. Cloud conditions observed included temperatures between 0° and −29°C and liquid water contents (LWCs) up to 0.7 g m−3. The detection threshold for LWC was found to be 0.007 ± 0.010 g m−3 for the RID operated at an air speed of 97 ± 10 m s−1, which agrees well with theoretical predictions. A signal level of 0 ± 2 mV s−1 accounted for 99.6% of the measurements in clear air and 98.5% of the measurements in glaciated clouds, when the data were averaged over 30-s intervals. No significant response to glaciated clouds was found during any of the research flights, implying that the instrument can be used to segregate glaciated and mixed phase clouds. There was no change in the RID response between liquid and mixed phase conditions, suggesting that ice crystals neither eroded ice accumulation nor accreted to the RID surface under the range of conditions experienced. During sustained icing conditions, a linear relationship between the RID signal and LWC was observed after the RID signal exceeded 400 mV above the clear-air signal level. The LWC derived from the RID was found to agree with LWC measurements from Nevzorov probes within ±50% for 92% of the data. The relationship between the RID signal and LWC was unchanged for freezing precipitation environments with drop median volume diameters >100 μm. The Ludlam limit was estimated for low LWC values and was found to agree well with theoretical calculations. The analysis provides considerable insight into the strengths and weaknesses of the instrument for operations in natural icing conditions.
Abstract
The Rosemount Icing Detector (RICE) has been used extensively over the last three decades for aircraft measurements of the rate of ice riming in supercooled liquid and mixed clouds. Because of difficulties related to calibration and postprocessing, the RICE probe was mainly used as an indicator of the presence of supercooled liquid water. The accuracy of the RICE probe for measurements of supercooled liquid water content is studied here. The theory of ice accretion on an unheated cylinder is applied to the RICE probe. A steady-state heat balance on the surface of a riming cylinder is considered in detail. It is shown that the threshold sensitivity of the RICE probe is limited by the rate of sublimation of ice and it may exceed 0.01 g m−3 at airspeed 200 m s−1. The rate of ice sublimation limits the use of the RICE probe for measurements of low liquid water contents in clouds. The maximum possible measured liquid water content is restricted by the Ludlam limit. A new calibration technique of the RICE probe, based on the measurements of the rate of ice sublimation in cloud-free air, is developed here. The calibration coefficient derived using the “sublimation” technique is compared to that obtained using the conventional technique, that is, when ice is accreting on the cylinder. The sublimation technique was found to be more accurate compared to the conventional one. The accuracies of both methods are discussed.
Abstract
The Rosemount Icing Detector (RICE) has been used extensively over the last three decades for aircraft measurements of the rate of ice riming in supercooled liquid and mixed clouds. Because of difficulties related to calibration and postprocessing, the RICE probe was mainly used as an indicator of the presence of supercooled liquid water. The accuracy of the RICE probe for measurements of supercooled liquid water content is studied here. The theory of ice accretion on an unheated cylinder is applied to the RICE probe. A steady-state heat balance on the surface of a riming cylinder is considered in detail. It is shown that the threshold sensitivity of the RICE probe is limited by the rate of sublimation of ice and it may exceed 0.01 g m−3 at airspeed 200 m s−1. The rate of ice sublimation limits the use of the RICE probe for measurements of low liquid water contents in clouds. The maximum possible measured liquid water content is restricted by the Ludlam limit. A new calibration technique of the RICE probe, based on the measurements of the rate of ice sublimation in cloud-free air, is developed here. The calibration coefficient derived using the “sublimation” technique is compared to that obtained using the conventional technique, that is, when ice is accreting on the cylinder. The sublimation technique was found to be more accurate compared to the conventional one. The accuracies of both methods are discussed.
Abstract
The Nevzorov liquid water content (LWC) and total water content (TWC) probe is a constant-temperature, hot-wire probe designed for aircraft measurements of the ice and liquid water content of clouds. The probe consists of two separate sensors for measurements of cloud liquid and total (ice plus liquid) water content. Each sensor consists of a collector and a reference winding. The reference sensors are shielded from impact with cloud particles, specifically to provide an automatic compensation for convective heat losses. This results in a potentially improved sensitivity over uncompensated probes such as the King LWC probe. The Nevzorov probe has been used in four Canadian field experiments on the National Research Council (NRC) Convair580 since 1994. Intercomparison of Nevzorov LWC, TWC, King, and two PMS Forward Scattering Spectrometer Probes show good agreement in liquid clouds, although the Nevzorov probe displays distinct advantages in low-LWC situations due to a more stable baseline. The sensitivity of the probe is estimated to be approximately 0.003–0.005 g m−3. The accuracy of LWC measurements in nonprecipitating liquid clouds is estimated as 10%–15%. Tests at the NRC high-speed icing tunnel have provided verification of the TWC measurement for small frozen droplets to an accuracy of approximately 10%–20%, but verification in snow and natural ice crystals has not yet been possible due to the absence of any accurate standards. The TWC measurement offers not only the possibility of direct measurements of ice content but also improved liquid water contents in drizzle situations. Airborne measurements have provided data on the baseline drift and sensitivity of the probe and have provided comparisons to other conventional instruments. Several cases have been documented that exhibit the unique capabilities of the instrument to separate the ice and liquid components of supercooled clouds.
Abstract
The Nevzorov liquid water content (LWC) and total water content (TWC) probe is a constant-temperature, hot-wire probe designed for aircraft measurements of the ice and liquid water content of clouds. The probe consists of two separate sensors for measurements of cloud liquid and total (ice plus liquid) water content. Each sensor consists of a collector and a reference winding. The reference sensors are shielded from impact with cloud particles, specifically to provide an automatic compensation for convective heat losses. This results in a potentially improved sensitivity over uncompensated probes such as the King LWC probe. The Nevzorov probe has been used in four Canadian field experiments on the National Research Council (NRC) Convair580 since 1994. Intercomparison of Nevzorov LWC, TWC, King, and two PMS Forward Scattering Spectrometer Probes show good agreement in liquid clouds, although the Nevzorov probe displays distinct advantages in low-LWC situations due to a more stable baseline. The sensitivity of the probe is estimated to be approximately 0.003–0.005 g m−3. The accuracy of LWC measurements in nonprecipitating liquid clouds is estimated as 10%–15%. Tests at the NRC high-speed icing tunnel have provided verification of the TWC measurement for small frozen droplets to an accuracy of approximately 10%–20%, but verification in snow and natural ice crystals has not yet been possible due to the absence of any accurate standards. The TWC measurement offers not only the possibility of direct measurements of ice content but also improved liquid water contents in drizzle situations. Airborne measurements have provided data on the baseline drift and sensitivity of the probe and have provided comparisons to other conventional instruments. Several cases have been documented that exhibit the unique capabilities of the instrument to separate the ice and liquid components of supercooled clouds.
Abstract
Ice particle shattering poses a serious problem to the airborne characterization of ice cloud microstructure. Shattered ice fragments may contaminate particle measurements, resulting in artificially high concentrations of small ice. The ubiquitous observation of small ice particles has been debated over the last three decades. The present work is focused on the study of the effect of shattering based on the results of the Airborne Icing Instrumentation Evaluation (AIIE) experiment flight campaign. Quantitative characterization of the shattering effect was studied by comparing measurements from pairs of identical probes, one modified to mitigate shattering using tips designed for this study (K-tips) and the other in the standard manufacturer’s configuration. The study focused on three probes: the forward scattering spectrometer probe (FSSP), the optical array probe (OAP-2DC), and the cloud imaging probe (CIP). It has been shown that the overestimation errors of the number concentration in size distributions measured by 2D probes increase with decreasing size, mainly affecting particles smaller than approximately 500 μm. It was found that shattering artifacts may increase measured particle number concentration by 1 to 2 orders of magnitude. However, the associated increase of the extinction coefficient and ice water content derived from 2D data is estimated at only 20%–30%. Existing antishattering algorithms alone are incapable of filtering out all shattering artifacts from OAP-2DC and CIP measurements. FSSP measurements can be completely dominated by shattering artifacts, and it is not recommended to use this instrument for measurements in ice clouds, except in special circumstances. Because of the large impact of shattering on ice measurements, the historical data collected by FSSP and OAP-2DC should be reexamined by the cloud physics community.
Abstract
Ice particle shattering poses a serious problem to the airborne characterization of ice cloud microstructure. Shattered ice fragments may contaminate particle measurements, resulting in artificially high concentrations of small ice. The ubiquitous observation of small ice particles has been debated over the last three decades. The present work is focused on the study of the effect of shattering based on the results of the Airborne Icing Instrumentation Evaluation (AIIE) experiment flight campaign. Quantitative characterization of the shattering effect was studied by comparing measurements from pairs of identical probes, one modified to mitigate shattering using tips designed for this study (K-tips) and the other in the standard manufacturer’s configuration. The study focused on three probes: the forward scattering spectrometer probe (FSSP), the optical array probe (OAP-2DC), and the cloud imaging probe (CIP). It has been shown that the overestimation errors of the number concentration in size distributions measured by 2D probes increase with decreasing size, mainly affecting particles smaller than approximately 500 μm. It was found that shattering artifacts may increase measured particle number concentration by 1 to 2 orders of magnitude. However, the associated increase of the extinction coefficient and ice water content derived from 2D data is estimated at only 20%–30%. Existing antishattering algorithms alone are incapable of filtering out all shattering artifacts from OAP-2DC and CIP measurements. FSSP measurements can be completely dominated by shattering artifacts, and it is not recommended to use this instrument for measurements in ice clouds, except in special circumstances. Because of the large impact of shattering on ice measurements, the historical data collected by FSSP and OAP-2DC should be reexamined by the cloud physics community.
