scholarly journals Radar Scattering from Ice Aggregates Using the Horizontally Aligned Oblate Spheroid Approximation

2012 ◽  
Vol 51 (3) ◽  
pp. 655-671 ◽  
Author(s):  
Robin J. Hogan ◽  
Lin Tian ◽  
Philip R. A. Brown ◽  
Christopher D. Westbrook ◽  
Andrew J. Heymsfield ◽  
...  
Keyword(s):  
Water Content ◽  
Millimeter Wave ◽  
Mie Scattering ◽  
Ice Particles ◽  
Size Relationship ◽  
Wave Radar ◽  
Mass Size ◽  
Ice Water Content ◽  
Ice Water ◽  
Reflectivity Factor

AbstractThe assumed relationship between ice particle mass and size is profoundly important in radar retrievals of ice clouds, but, for millimeter-wave radars, shape and preferred orientation are important as well. In this paper the authors first examine the consequences of the fact that the widely used “Brown and Francis” mass–size relationship has often been applied to maximum particle dimension observed by aircraft Dmax rather than to the mean of the particle dimensions in two orthogonal directions Dmean, which was originally used by Brown and Francis. Analysis of particle images reveals that Dmax ≃ 1.25Dmean, and therefore, for clouds for which this mass–size relationship holds, the consequences are overestimates of ice water content by around 53% and of Rayleigh-scattering radar reflectivity factor by 3.7 dB. Simultaneous radar and aircraft measurements demonstrate that much better agreement in reflectivity factor is provided by using this mass–size relationship with Dmean. The authors then examine the importance of particle shape and fall orientation for millimeter-wave radars. Simultaneous radar measurements and aircraft calculations of differential reflectivity and dual-wavelength ratio are presented to demonstrate that ice particles may usually be treated as horizontally aligned oblate spheroids with an axial ratio of 0.6, consistent with them being aggregates. An accurate formula is presented for the backscatter cross section apparent to a vertically pointing millimeter-wave radar on the basis of a modified version of Rayleigh–Gans theory. It is then shown that the consequence of treating ice particles as Mie-scattering spheres is to substantially underestimate millimeter-wave reflectivity factor when millimeter-sized particles are present, which can lead to retrieved ice water content being overestimated by a factor of 4.

scholarly journals Effective Radius of Ice Particles in Cirrus and Contrails

2011 ◽  
Vol 68 (2) ◽  
pp. 300-321 ◽  
Author(s):  
U. Schumann ◽  
B. Mayer ◽  
K. Gierens ◽  
S. Unterstrasser ◽  
P. Jessberger ◽  
...  
Keyword(s):  
Water Content ◽  
Effective Radius ◽  
Cross Sectional Area ◽  
Sectional Area ◽  
Small Particles ◽  
Particle Volume ◽  
Cross Sectional ◽  
Ice Particles ◽  
Ice Water Content ◽  
Ice Water

Abstract This paper discusses the ratio C between the volume mean radius and the effective radius of ice particles in cirrus and contrails. The volume mean radius is proportional to the third root of the ratio between ice water content and number of ice particles, and the effective radius measures the ratio between ice particle volume and projected cross-sectional area. For given ice water content and number concentration of ice particles, the optical depth scales linearly with C. Hence, C is an important input parameter for radiative forcing estimates. The ratio C in general depends strongly on the particle size distribution (PSD) and on the particle habits. For constant habits, C can be factored into a PSD and a habit factor. The PSD factor is generally less than one, while the habit factor is larger than one for convex or concave ice particles with random orientation. The value of C may get very small for power-law PSDs with exponent n between −4 and 0, which is often observed. For such PSDs, most of the particle volume is controlled by a few large particles, while most of the cross-sectional area is controlled by the many small particles. A new particle habit mix for contrail cirrus including small droxtal-shape particles is suggested. For measured cirrus and contrails, the dependence of C on volume mean particle radius, ambient humidity, and contrail age is determined. For cirrus, C varies typically between 0.4 and 1.1. In contrails, C = 0.7 ± 0.3, with uncertainty ranges increasing with the volume radius and contrail age. For the small particles in young contrails, the extinction efficiency in the solar range deviates considerably from the geometric optics limit.

scholarly journals High ice water content at low radar reflectivity near deep convection – Part 2: Evaluation of microphysical pathways in updraft parcel simulations

2015 ◽  
Vol 15 (20) ◽  
pp. 11729-11751 ◽  
Author(s):  
A. S. Ackerman ◽  
A. M. Fridlind ◽  
A. Grandin ◽  
F. Dezitter ◽  
M. Weber ◽  
...  
Keyword(s):  
Water Vapor ◽  
Water Content ◽  
Deep Convection ◽  
Radar Reflectivity ◽  
Doppler Velocity ◽  
Diffusional Growth ◽  
Ice Particles ◽  
Ice Water Content ◽  
Water Drops ◽  
Ice Water

Abstract. The aeronautics industry has established that a threat to aircraft is posed by atmospheric conditions of substantial ice water content (IWC) where equivalent radar reflectivity (Ze) does not exceed 20–30 dBZ and supercooled water is not present; these conditions are encountered almost exclusively in the vicinity of deep convection. Part 1 (Fridlind et al., 2015) of this two-part study presents in situ measurements of such conditions sampled by Airbus in three tropical regions, commonly near 11 km and −43 °C, and concludes that the measured ice particle size distributions are broadly consistent with past literature with profiling radar measurements of Ze and mean Doppler velocity obtained within monsoonal deep convection in one of the regions sampled. In all three regions, the Airbus measurements generally indicate variable IWC that often exceeds 2 g m-3 with relatively uniform mass median area-equivalent diameter (MMDeq) of 200–300 μm. Here we use a parcel model with size-resolved microphysics to investigate microphysical pathways that could lead to such conditions. Our simulations indicate that homogeneous freezing of water drops produces a much smaller ice MMDeq than observed, and occurs only in the absence of hydrometeor gravitational collection for the conditions considered. Development of a mass mode of ice aloft that overlaps with the measurements requires a substantial source of small ice particles at temperatures of about −10 °C or warmer, which subsequently grow from water vapor. One conceivable source in our simulation framework is Hallett–Mossop ice production; another is abundant concentrations of heterogeneous ice freezing nuclei acting together with copious shattering of water drops upon freezing. Regardless of the production mechanism, the dominant mass modal diameter of vapor-grown ice is reduced as the ice-multiplication source strength increases and as competition for water vapor increases. Both mass and modal diameter are reduced by entrainment and by increasing aerosol concentrations. Weaker updrafts lead to greater mass and larger modal diameters of vapor-grown ice, the opposite of expectations regarding lofting of larger ice particles in stronger updrafts. While stronger updrafts do loft more dense ice particles produced primarily by raindrop freezing, we find that weaker updrafts allow the warm rain process to reduce competition for diffusional growth of the less dense ice expected to persist in convective outflow.

scholarly journals A Comparison of Airborne In Situ Cloud Microphysical Measurement with Ground-Based C-Band Radar Observations in Deep Stratiform Regions of African Squall Lines

2015 ◽  
Vol 54 (12) ◽  
pp. 2461-2477 ◽  
Author(s):  
E. Drigeard ◽  
E. Fontaine ◽  
W. Wobrock ◽  
A. Schwarzenböck ◽  
C. Duroure ◽  
...  
Keyword(s):  
Water Content ◽  
Ice Crystals ◽  
Southwestern Part ◽  
Radar Observations ◽  
Squall Lines ◽  
Ice Water Content ◽  
Ice Water ◽  
Reflectivity Factor

AbstractThis study addresses clouds with significant ice water content (IWC) in the stratiform regions downwind of the convective cores of African squall lines in the framework of the French–Indian satellite Megha-Tropiques project, observed in August 2010 next to Niamey (13.5°N, 2°E) in the southwestern part of Niger. The objectives included comparing the IWC–Z reflectivity relationship for precipitation radars in deep stratiform anvils, collocating reflectivity observed from ground radar with the calculated reflectivity from in situ microphysics for all aircraft locations inside the radar range, and interpreting the role of large ice crystals in the reflectivity of centimeter radars through analysis of their microphysical characteristics as ice crystals larger than 5 mm frequently occurred. It was found that, in the range of 20–30 dBZ, IWC and C-band reflectivity are not really correlated. Cloud regions with high IWC caused by important crystal number concentrations can lead to the same reflectivity factor as cloud regions with low IWC formed by a few millimeter-sized ice crystals.

scholarly journals A comparison of ice water content measurement techniques on the FAAM BAe-146 aircraft

2014 ◽  
Vol 7 (9) ◽  
pp. 3007-3022 ◽  
Author(s):  
S. J. Abel ◽  
R. J. Cotton ◽  
P. A. Barrett ◽  
A. K. Vance
Keyword(s):  
Water Content ◽  
Water Vapour ◽  
Measurement Techniques ◽  
Total Water Content ◽  
Total Water ◽  
Convective Clouds ◽  
Ice Particles ◽  
Mass Dimension ◽  
Ice Water Content ◽  
Ice Water

Abstract. This paper presents a comparison of ice water content (qi) data from a variety of measurement techniques on the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 research aircraft. Data are presented from a range of cloud types measured during the PIKNMIX field experiment that include mixed-phase stratocumulus, cumulus congestus and cirrus clouds. These measurements cover a broad range of conditions in which atmospheric ice particles are found in nature, such as the low-ice-water-content environments typically found in midlatitude cirrus and the environments with much higher ice water content often observed in cold convective clouds. The techniques include bulk measurements from (i) a Nevzorov hot-wire probe, (ii) the difference between the measured total water content (condensed plus vapour) and the water vapour content of the atmosphere and (iii) a counterflow virtual impactor (CVI) (only for cirrus measurements). We also estimate the qi from integration of the measured particle size distribution (PSD) with assumptions on how the density of ice particles varies as a function of size. The results show that the only bulk ice water content technique capable of measuring high qi values (several g m−3) was the method of total water content minus water vapour. For low ice water contents we develop a new parametrisation of the Nevzorov baseline drift that enables the probe to be sensitive to qi ± 0.002 g m−3. In cirrus clouds the agreement between the Nevzorov and other bulk measurements was typically better than a factor of 2 for the CVI (qi > 0.008 g m−3) and the method of total water content minus water vapour (qi > 0.02 g m−3). Good agreement with the bulk measurements for all cases could be obtained with the estimate from the PSD provided that appropriate a priori assumptions on the mass–dimension relationship were made. This is problematic in the convective clouds sampled because pristine ice particles, heavily rimed particles and supercooled liquid drops were all present. In a cirrus case, we show that using a temperature-dependent mass–dimension relation was required to match the bulk measurement of qi.

scholarly journals Airborne measurements of the nitric acid partitioning in persistent contrails

2009 ◽  
Vol 9 (3) ◽  
pp. 14165-14187
Author(s):  
D. Schäuble ◽  
C. Voigt ◽  
B. Kärcher ◽  
P. Stock ◽  
H. Schlager ◽  
...  
Keyword(s):  
Nitric Acid ◽  
Water Content ◽  
Particle Diameter ◽  
Cirrus Clouds ◽  
Airborne Measurements ◽  
Molar Ratios ◽  
Ice Particles ◽  
Ice Water Content ◽  
Ice Water ◽  
Ice Phase

Abstract. This study reports the first systematic measurements of nitric acid (HNO3) uptake in contrail ice particles at typical aircraft cruise altitudes. During the CIRRUS-III campaign cirrus clouds and almost 40 persistent contrails were probed with in situ instruments over Germany and Northern Europe in November 2006. Besides reactive nitrogen, water vapor, cloud ice water content, ice particle size distributions, and condensation nuclei were measured during 6 flights. Contrails with ages up to 8 hours were detected at altitudes 10–11.5 km and temperatures 211–220 K. These contrails had a larger ice phase fraction of total nitric acid (HNO3ice/HNO3tot = 6%) than the ambient cirrus layers (3%). On average, the contrails contained twice as much HNO3ice as the cirrus clouds, 14 pmol/mol and 6 pmol/mol, respectively. Young contrails with ages below 1 h had a mean HNO3ice of 21 pmol/mol. The contrails had higher nitric acid to water molar ratios in ice and slightly higher ice water contents than the cirrus clouds under similar meteorological conditions. The differences in ice phase fractions and molar ratios between developing contrails and cirrus are likely caused by high plume concentrations of HNO3 prior to contrail formation. The location of the measurements in the top region of frontal cirrus layers might account for slight differences in the ice water content between contrails and adjacent cirrus clouds. The observed dependence of molar ratios as a function of the mean ice particle diameter suggests that ice-bound HNO3 concentrations are controlled by uptake of exhaust HNO3 in the freezing plume aerosols in young contrails and subsequent trapping of ambient HNO3 in growing ice particles in older (age > 1 h) contrails.

scholarly journals In-situ measurements of tropical cloud properties in the West African monsoon: upper tropospheric ice clouds, mesoscale convective system outflow, and subvisual cirrus

2011 ◽  
Vol 11 (1) ◽  
pp. 745-812 ◽  
Author(s):  
W. Frey ◽  
S. Borrmann ◽  
D. Kunkel ◽  
R. Weigel ◽  
M. de Reus ◽  
...  
Keyword(s):  
Water Content ◽  
Potential Temperature ◽  
In Situ Measurements ◽  
Size Distributions ◽  
Ice Crystal ◽  
Cloud Particle ◽  
Ice Particles ◽  
Ice Water Content ◽  
Ice Water

Abstract. In-situ measurements of ice crystal size distributions in tropical upper troposphere/lower stratosphere (UT/LS) clouds were performed during the SCOUT-AMMA campaign over West Africa in August 2006. The cloud properties were measured with a Forward Scattering Spectrometer Probe (FSSP-100) and a Cloud Imaging Probe (CIP) operated aboard the Russian high altitude research aircraft M-55 ''Geophysica'' with the mission base in Ouagadougou, Burkina Faso. A total of 117 ice particle size distributions were obtained from the measurements in the vicinity of Mesoscale Convective Systems (MCS). Two or three modal lognormal size distributions were fitted to the average size distributions for different potential temperature bins. The measurements showed proportionate more large ice particles compared to former measurements above maritime regions. With the help of trace gas measurements of NO, NOy, CO2, CO, and O3, and satellite images clouds in young and aged MCS outflow were identified. These events were observed at altitudes of 11.0 km to 14.2 km corresponding to potential temperature levels of 346 K to 356 K. In a young outflow (developing MCS) ice crystal number concentrations of up to 8.3 cm−3 and rimed ice particles with maximum dimensions exceeding 1.5 mm were found. A maximum ice water content of 0.05 g m−3 was observed and an effective radius of about 90 μm. In contrast the aged outflow events were more diluted and showed a maximum number concentration of 0.03 cm−3, an ice water content of 2.3 × 10−4 g m−3, an effective radius of about 18 μm, while the largest particles had a maximum dimension of 61 μm. Close to the tropopause subvisual cirrus were encountered four times at altitudes of 15 km to 16.4 km. The mean ice particle number concentration of these encounters was 0.01 cm−3 with maximum particle sizes of 130 μm, and the mean ice water content was about 1.4 × 10−4 g m−3. All known in-situ measurements of subvisual tropopause cirrus are compared and an exponential fit on the size distributions is established in order to give a parameterisation for modelling. A comparison of aerosol to ice crystal number concentrations, in order to obtain an estimate on how many ice particles result from activation of the present aerosol, yielded low activation ratios for the subvisual cirrus cases of roughly one cloud particle per 30 000 aerosol particles, while for the MCS outflow cases this resulted in a high ratio of one cloud particle per 300 aerosol particles.

scholarly journals On the observation of unusual high concentration of small chain-like aggregate ice crystals and large ice water contents near the top of a deep convective cloud during the CIRCLE-2 experiment

2011 ◽  
Vol 11 (8) ◽  
pp. 23911-23958
Author(s):  
J.-F. Gayet ◽  
G. Mioche ◽  
L. Bugliaro ◽  
A. Protat ◽  
A. Minikin ◽  
...  
Keyword(s):  
Optical Properties ◽  
Water Content ◽  
Convective Cloud ◽  
Ice Crystals ◽  
Convective Clouds ◽  
Microphysical Properties ◽  
Ice Particles ◽  
Ice Water Content ◽  
Ice Water

Abstract. During the CIRCLE-2 experiment carried out over Western Europe in May 2007, combined in situ and remote sensing observations allowed to describe microphysical and optical properties near-top of an overshooting convective cloud (11 080 m/−58 °C). The airborne measurements were performed with the DLR Falcon aircraft specially equipped with a unique set of instruments for the extensive in situ cloud measurements of microphysical and optical properties (Polar Nephelometer, FSSP-300, Cloud Particle Imager and PMS 2D-C) and nadir looking remote sensing observations (DLR WALES Lidar). Quasi-simultaneous space observations from MSG/SEVIRI, CALIPSO/CALIOP-WFC-IIR and CloudSat/CPR combined with airborne RASTA radar reflectivity from the French Falcon aircraft flying above the DLR Falcon depict very well convective cells which overshoot by up to 600 m the tropopause level. Unusual high values of the concentration of small ice particles, extinction, ice water content (up to 70 cm−3, 30 km−1 and 0.5 g m−3, respectively) are experienced. This very dense cloud causes a strong attenuation of the WALES and CALIOP lidar returns. The mean effective diameter is of 43 μm and the maximum particle size is about 300 μm. The SEVIRI retrieved parameters confirm the occurrence of small ice crystals at the top of the convective cell. Smooth and featureless phase functions with asymmetry factors of 0.776 indicate fairly uniform optical properties. Due to small ice crystals the power-law relationship between ice water content (IWC) and radar reflectivity appears to be very different from those usually found in cirrus and anvil clouds. For a given equivalent reflectivity factor, IWCs are significantly larger for the overshooting cell than for the cirrus. Assuming the same prevalent microphysical properties over the depth of the overshooting cell, RASTA reflectivity profiles scaled into ice water content show that retrieved IWC up to 1 g m−3 may be observed near the cloud top. Extrapolating the relationship for stronger convective clouds with similar ice particles, IWC up to 5 g m−3 could be experienced with reflectivity factors no larger than about 20 dBZ. This means that for similar situations, indication of rather weak radar echo does not necessarily warn the occurrence of high ice water content carried by small ice crystals. All along the cloud penetration the shape of the ice crystals is dominated by chain-like aggregates of frozen droplets. Our results confirm previous observations that the chains of ice crystals are found in a continental deep convective systems which are known generally to generate intense electric fields causing efficient ice particle aggregation processes. Vigorous updrafts could lift supercooled droplets which are frozen extremely rapidly by homogeneous nucleation near the −37 °C level, producing therefore high concentrations of very small ice particles at upper altitudes. They are sufficient to deplete the water vapour and suppress further nucleation as confirmed by humidity measurements. These observations address scientific issues related to the microphysical properties and structure of deep convective clouds and confirm that particles smaller than 50 μm may control the radiative properties in convective-related clouds. These unusual observations may also provide some possible insights regarding engineering issues related to the failure of jet engines commonly used on commercial aircraft during flights through areas of high ice water content.

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