296 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 19
Evaluation of a 45؇ Slant Quasi-Linear Radar Polarization State for Distinguishing Drizzle Droplets, Pristine Ice Crystals, and Less Regular Ice Particles
ROGER F. R EINKING NOAA/OAR/Environmental Technology Laboratory, Boulder, Colorado
SERGEY Y. M ATROSOV Cooperative Institute for Research in the Environmental Sciences, University of Colorado, NOAA/Environmental Technology Laboratory, Boulder, Colorado
ROBERT A. KROPFLI AND BRUCE W. B ARTRAM NOAA/OAR/Environmental Technology Laboratory, Boulder, Colorado
(Manuscript received 21 February 2001, in ®nal form 20 June 2001)
ABSTRACT A remote sensing capability is needed to detect clouds of supercooled, drizzle-sized droplets, which are a major aircraft icing hazard. Discrimination among clouds of differing ice particle types is also important because both the presence and type of ice in¯uence the survival of liquid in a cloud and the chances for occurrence of these large, most hazardous droplets. This work shows how millimeter-wavelength dual-polarization radar can be used to identify these differing hydrometeors. It also shows that by measuring the depolarization ratio (DR), the estimation of the hydrometeor type can be accomplished deterministically for drizzle droplets; ice particles of regular shapes; and to a considerable extent, the more irregular ice particles, and that discrimination is strongly in¯uenced by the polarization state of the transmitted microwave radiation. Thus, appropriate selection of the polarization state is emphasized. The selection of an optimal polarization state involves trade-offs in competing factors such as the functional dynamic range of DR, sensitivity to low-re¯ectivity clouds, and insensitivity to oscillations in the settling orientations of ice crystals. A 45Њ slant, quasi-linear polarization state, one in which only slight ellipticity is introduced, was found to offer a very good compromise, providing considerable advantages over standard horizontal and substantially elliptical polarizations. This was determined by theoretical scattering calculations that were veri®ed experimentally in ®eld measurements conducted during the Mount Washington Icing Sensors
Project (MWISP). A selectable-dual-polarization Ka-band (8.66-mm wavelength) radar was used. A wide variety of hydrometeor types was sampled. Clear differentiation among planar crystals, columnar crystals, and drizzle droplets was achieved. Also, differentiation among crystals of fundamentally different shapes (aspect ratios) within each of the planar and columnar families was found possible. These distinctions matched calculations of DR, usually to within 1 or 2 dB. The results from MWISP and from previous experiments with other polarizations have demonstrated that the agreement between theory and measurements by this method is repeatable. Addi- tionally, although less rigorously predicted by theory, the ®eld measurements demonstrated substantial differ- entiation among the more irregular and more spherical ice particles, including aggregates, elongated aggregates, heavily rimed dendrites, and graupel. Measurable separation between these various irregular ice particle types and drizzle droplets was also veri®ed.
1. Introduction from clouds of the various ice particles. The wavelength Through nearly a decade of studies beginning with at Ka band (8.66 mm) is suitable for detection of these the Winter Icing and Storms Projects (Rasmussen et al. droplets, which are small compared to raindrops, snow- 1992), a radar remote sensing capability has been sought ¯akes, and hail. Explicit identi®cation of these droplets to identify ice particles in glaciated and mixed-phase is the focal interest because they can present a partic- clouds and, speci®cally, to distinguish clouds of super- ularly severe aircraft icing hazard when supercooled cooled, 50±500-m-diameter, drizzle-sized droplets (Politovich 1996; Ashendon et al. 1996; Ashendon and Marwitz 1997, as summarized by Reinking et al. 1997b). Explicit identi®cation of the differing ice par- Corresponding author address: Dr. Roger F. Reinking, NOAA/ ETL/ET6, 325 Broadway, Boulder, CO 80305. ticles is important as well because some types will them- E-mail: [email protected] selves create hazards to aircraft, and because ice par-
Unauthenticated | Downloaded 10/01/21 04:03 AM UTC MARCH 2002 REINKING ET AL. 297 ticles of differing growth characteristics, through vapor ization applied to particle identi®cation has been con- consumption and riming, can in¯uence liquid water con- sidered theoretically by Matrosov (1991) and Matrosov tent, droplet size, and droplet lifetime in mixed phase et al. (2001). clouds. Thus, there are advantages in differentiating The transmitted polarization state determines the among not only the pristine ice crystals of ``regular'' magnitude of the separation or isolation in DR of each growth habits (the basic planar and columnar types) but category of hydrometeor. In selecting from the available also among the more spherical and irregular ice particles continuum, the states nearer to optimal for this purpose such as graupel and aggregates, and between all of these would maximize the isolation, but only so much as al- and drizzle droplets. lowed by practical, compromising limitations in radar The results of the series of studies including this one performance that in¯uence the measurement. Thus, it is show that dual-polarization Ka-band radar can be used appropriate to ask, how much separation in detected DR to identify drizzle and the various ice hydrometeors in is suf®cient for an explicit and deterministic measure- clouds and precipitation. However, selection of the po- ment, and what polarizations can be transmitted to ac- larization state is important. This is demonstrated complish this? We de®ne a deterministic measurement through scattering calculations and measurements. En- in the normal sense, as one that will estimate particle sembles of ice crystals settle with the mean orientation type directly by measuring one parameter, the values of of their major dimensions near horizontal, but the stan- which are uniquely de®ned by theory for the particular dard deviations of angles from this preferred orientation particle types. This estimate is made without relying on are variable and unpredictable (where, for example, 3Њ the use of a probabilistic approach, such as techniques is minimal and 15Њ is substantial). Thus, the basic pre- involving neural networks, statistical decision theory, mise of this paper is that the best polarization states will or fuzzy logic, which combine several parameters that be those that maximize the measured signals and dif- only in combination will tend to uniquely identify a ferentiate the hydrometeors but are not very sensitive particle type (e.g., Liu and Chandrasekar 2000). The to the standard deviations of the orientation angle, and next question is this: Given that the more regular crys- a linear or even better, an approximately linear polari- tals can be most readily differentiated by this method, zation transmitted at a 45Њ slant is therefore a natural can the more spherical and irregular shapes of ice par- choice. ticles, with wide-ranging bulk densities, such as blocky Any of several polarimetric measurands singularly or columns, graupel, and aggregates also be differentiated in combination demonstrate some sensitivity to particle among themselves and from drizzle droplets by mea- type (Table 8.1 in Doviak and Zrnic 1993; Zrnic and suring the same single parameter? Ryzhkov 1999). In this study, the radar polarization pa- Several polarization states have now been investi- rameter measured for this purpose is the depolarization gated using NOAA/K, the Environmental Technology ratio (DR) a parameter in¯uenced predominantly by hy- Laboratory's (ETL) cloud-sensing, selectable dual-po- drometeor type if the appropriate polarization state is larization, scanning Doppler Ka-band (8.6-mm) radar used. DR is de®ned as the logarithm of the ratio of (Krop¯i et al. 1995; Krop¯i and Kelly 1996). In the next power returned in the ``weak'' channel to power re- section, the method for measuring the depolarization turned in the ``strong'' channel when a polarized signal ratio is outlined for the NOAA/K technology, and our is transmitted, where these receiving channels are or- previous investigations are reviewed as some of the op- thogonal. Hydrometeors of the various types will de- tions for the appropriate polarization state are examined polarize and backscatter the transmitted microwave ra- and required compromises are considered. The reasons diation according to their aspect ratio (prolate or oblate are established for exploring a 45Њ slant, quasi-linear shape), settling orientation, and bulk density, and the polarization state. The justi®cation for the slight ellip- polarization state of transmitted radiation. The ®rst three ticity that sets this state apart from a true linear one is factors are determined by cloud properties, but the examined. Then the scattering theory that is the foun- fourth, the polarization state, can be engineered. dation is presented. The transmitted polarization state can be selected To test the performance of this polarization state by from a continuum of possible elliptical polarizations, measuring slant quasi-linear depolarization ratio where linear and circular de®ne the limits. In general, (SLDR) at 45Њ (SLDR*-45) in the ®eld, the radar was linear polarization transmitted in the horizontal plane, deployed for the Mount Washington Icing Sensors Pro- and received in both the same, co-polar plane and the ject (MWISP) to categorize hydrometeors in clouds perpendicular (vertical), cross-polar plane, is most com- formed in the forcing of orographic and cyclonic cir- monly employed for dual-polarization measurements. culations over the slopes of Mt. Washington, New Circular polarization has also been investigated and uti- Hampshire. MWISP was conducted in April 1999 lized for various purposes. These studies include track- (Ryerson et al. 2000). As the acronym indicates, the ing of chaff ®bers that are in some respects an extremely focus was on testing several state-of-the-art remote sen- elongated representation of columnar crystals (Martner sors for their capabilities in detecting and measuring and Krop¯i 1989) and an attempt at hydrometeor iden- cloud characteristics that cause aircraft icing. The radar ti®cation by Hendry and Antar (1984). Circular polar- site was midway up the west slope of the mountain.
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Supporting data on Hydrometeor types and atmospheric TABLE 1. De®nitions. conditions were gathered at the Mount Washington Ob- Word servatory (MWO) at the summit, from the radar site, Depolarization ratio Mathematical de®nition designation and from an aircraft. The results of the polarization Horizontal linear DR (⌿,0Њ), for any ⌿, LDR measurements made at MWISP are interpreted with ver- DR (180Њ,0Њ) in this paper i®cation from the supporting data. Circular DR (90Њ,45Њ) CDR Elliptical DR (79.5Њ,45Њ)* EDR 45Њ slant linear DR (180Њ, 22.5Њ) SLDR-45 2. Background 45Њ slant quasi-linear DR (177.4Њ, 22.5Њ)** SLDR*-45 The design of the NOAA/K radar is such that the * An example that was tested. transmitted polarization state can be selected by rotating ** As applied during MWISP. a phase-retarding plate (PRP). The particular PRP sets the phase shift angle, , in the radio frequency (RF) transmission and reception path for NOAA/K. The and the weak-channel return and Pcr are synonymous transmitted state is then set within a continuous but (for circular polarization the ratio is inverted). restricted range de®ned by the phase shift and the ro- Spheres do not depolarize microwave radiation trans- tation of the PRP to a tilt angle, , from horizontal, the mitted in a linear or circular state, so drizzle-sized drop- position where the radar's base, horizontal polarization lets, which are nearly or truly spherical, show minimal state is transmitted. The principles of the PRP are pre- or no depolarization. This is the limiting case for which sented by Matrosov and Krop¯i (1993) and summarized DR → Ϫϱ for the ideal antenna. However, when the by Reinking et al. (1997b), and the equations describing transmitted signal is not linear or circular, but rather has the polarization transformations by the PRP are pre- some ellipticity, even spheres including drizzle will de- sented in our companion paper (Matrosov et al. 2001). polarize the signal to some degree and return a nonzero The depolarization by hydrometeors of the transmitted, signal in the weak channel. For any polarization state, polarized radiation of power, P, is measured as a ratio the value of DR for spheres will be a constant, inde- of the power returned in the cross-polarized channel pendent of the elevation or azimuth angle at which the radar points at them. (which is orthogonal to the transmission channel), Pcr, and that returned with the same polarization as the trans- In practice, DR has a lower limit that bounds the measurements in either channel and is established by mission in the co-polarized channel, Pco. The depolar- ization ratio for a transmitted polarization state speci®ed the antenna polarization cancellation ratio (cross-talk) by and  may be de®ned as the logarithmic difference and receiver noise. No longer does DR → Ϫϱ for the between power returns in the ``weak'' and ``strong'' nondepolarizing targets; rather their DR is de®ned as channels: the decibel value of the cross-talk. The weak channel signal is usually signi®cantly (e.g., tens of decibels) DR(, ) ϭ 10 log10 [min(P cr, P co)/max(P cr, P co)]. (1) weaker than the strong channel signal. If the re¯ectivity A horizontal polarization (ellipticity ϭ0), for mea- (the strong-channel signal) of a cloud is low, the or- suring the linear depolarization ratio (LDR) is trans- thogonal, weak-channel signal can easily drop below mitted when  ϭ 0Њ and the ®xed has any value (for the receiver noise, irrespective of the phase and other consistency, we use ϭ 180Њ), and a circular polari- properties of the cloud particles. In that case, spherical zation (⑀ϭ1) for measuring a circular depolarization and shaped particles are not separable in DR. The DR ratio (CDR) is achieved when ϭ 90Њ or 270Њ and  of any particular particle type is not measurable as a ϭ 45Њ. A linear polarization slanted at 45Њ, for mea- unique signature at this limit. In contrast, some ellip- suring the slant-linear depolarization ratio (SLDR-45) ticity in the transmission enhances the weak-channel can be transmitted when ϭ 180Њ and  ϭ 22.5Њ. El- return relative to that in the strong channel, so the DR lipticity is introduced when deviates from the noted value for spheres will be a unique, measurable value values. Table 1 summarizes the various depolarization above the cross-talk limit. Since the re¯ectivity of a ratios that we address in this paper. cloud of drizzle-sized droplets can be at least as low as In Eq. (1), the smaller of the returned powers is used Ϫ15 dBZ, the radar must be able to measure a much in the numerator and the larger in the denominator so fainter weak-channel return in low-re¯ectivity clouds. as to maintain the same, conventional negative sign of This is what is needed to uniquely distinguish drizzle DR, because as the polarization state is changed from from all forms of ice. Matrosov and Krop¯i (1993) pre- linear to circular (and ellipticity from zero to unity), the dicted this advantage to the measurement of DR offered main returned power changes from the co- to the cross- by some ellipticity, and it was experimentally demon- polarized channel. Thus, for any linear state, such as strated with an elliptical polarization achieved with our the slant linear one that is the focus of this study, the ®rst PRP, for which the ability to detect and measure equation simpli®es to DR in low-re¯ectivity clouds at relatively long ranges was enhanced by a strengthened return in the weak chan-
DR(, ) ϭ 10 log10 (P cr/P co), (2) nel (Matrosov et al. 1996; Reinking et al. 1997a,b). For
Unauthenticated | Downloaded 10/01/21 04:03 AM UTC MARCH 2002 REINKING ET AL. 299 the linear and circular polarization states, in clouds with imum, DR changes due to variations in canting are min- more substantial re¯ectivities, the weak-channel return imized. Thus, the use of SLDR-45 also diminishes the will place the DR of signi®cantly nonspherical particles sensitivity to the variation of orientation. above the cross-talk, but spherical drizzle and quasi- Since differences in DR serve to separate the hydro- spherical ice particles like graupel can produce weak- meteors of different shapes, a polarization that produces channel returns that still can be lower than receiver a wider range in DR will result in larger separations. noise. Thus, unique identi®cation of spherical droplets For both the horizontal and circular polarization states, by default can fail. The positive effect of ellipticity is the dynamic range in DR is theoretically in®nite. In examined further with our scattering calculations. practice, the widest possible range in DR is that between For particles that are randomly oriented in the hori- 0 dB and the antenna cross-talk limit. This range is zontal plane (and therefore randomly oriented with re- available to differentiate the signatures of the differing spect to the antenna's azimuth angle), depolarizations hydrometeors, although only particles with a rare com- caused by nonspherical particles will be greater than bination of properties would cause complete depolar- those caused by spherical particles for linear, circular, ization, to 0 dB. Horizontal polarization is commonly and slightly elliptical polarizations. (An exception oc- used, but calculations presented later show that the func- curs for the case when all the nonspherical particles are tional dynamic range, that which can actually be utilized aligned along the electric vector of the linear polari- to measure DR to differentiate hydrometeors, is so much zations. In this case, which is conceivable in strong narrower than the available range for this state that the electric ®elds, nonspherical particles will be indistin- advantage of the latter is not realized. Although the guishable from spheres by their depolarization patterns.) dynamic range de®ned by the cross-talk limit is ®xed, Most ice particles will depolarize the signal signi®cantly the magnitude of the effective or available dynamic because they are nonspherical and settle with a preferred range is state dependent. For particles randomly oriented horizontal orientation, although the magnitude of de- in the horizontal plane, some ellipticity in the polari- polarization by ice particles is somewhat diminished by zation reduces the available dynamic range to one the decrease in bulk density relative to that of solid ice, bounded at its lower limit by the DR of spherical targets. and both relative to the density of liquid water. De- This lower limit is above the antenna cross-talk limit, pending on the polarization state, in contrast to spherical so the DR for spheres actually may be measured. droplets, the DR of some of the nonspherical ice particle In practice, it is dif®cult to achieve true horizontal or types will show a dependence on radar elevation angle. circular polarization, and this is true of the PRP tech- Unfortunately, DR for nonspherical hydrometeors nology. This was demonstrated by our ®rst attempt in also depends on the three-dimensional canting of the manufacturing a PRP that produced an elliptical state particles about their preferred horizontal orientation. ( ϭ 79.5Њ instead of 90Њ, ⑀ϭ0.83 instead of 1.0, at Matrosov (1991) theoretically examined the potential  ϭ 45Њ). The measurements of the corresponding el- for hydrometeor identi®cation using the horizontal po- liptical depolarization ratio (EDR; Table 1) for speci®c larization (⑀ϭ0), and a circular polarization (⑀ϭ1). hydrometeor types showed excellent agreement with His calculations demonstrated the high sensitivity of the scattering calculations. However, the available dynamic horizontal depolarization ratio, LDR, to variations in ice range of observable EDR values was, in terms of dB, crystal settling orientations compared to that for CDR. less than half that allowed by the radar's antenna cross- This conclusion derives from considerations of the talk, and the utilized range 25% narrower than the avail- asymmetry of non-spherical ice particles, which imply able range. This resulted in a narrow separation of hy- that if a horizontally polarized incident electric ®eld is drometeors for some types and restricted the differen- aligned with one axis of the particles, there can be no tiation of drizzle from some ice types including graupel orthogonally polarized backscatter, so theoretically LDR and blocky columns (Matrosov et al. 1996; Reinking et → Ϫϱ; however, when falling particles wobble, the re- al. 1997a,b). Reduction of both the available and utilized sulting distribution of canting angles increases LDR dynamic ranges can be minimized, however, by select- (Doviak and Zrnic 1993). CDR is sensitive only to var- ing a polarization state with an ellipticity that deviates iation of canting in the plane perpendicular to the po- only slightly from the true linear or circular state. This larization plane (``front-to-back'' canting, along the compromise can result in a wider utilized dynamic range beam), not to variation of the canting in the polarization than that possible with the linear state, and this allows plane (``side-to-side'' canting, across the beam), so the greater distinction among the hydrometeor types (see sensitivity to the variation of orientation, while not elim- section 4). inated, is diminished (Matrosov et al. 2001). A linear In summation, LDR is subject to the ambiguities im- polarization slanted at 45Њ is sensitive to both side-to- posed by poor weak-channel signal in low-re¯ectivity side and front-to-back canting, but substantially less clouds, indistinguishable droplets versus quasi-spherical sensitive than LDR. This is true because for predomi- ice particles in the noise at the antenna cross-talk even nantly horizontally oriented particles, the weak-channel with higher re¯ectivities, uncertainties due to variations returns maximize when the electric vector of incident in ice particle settling orientation that can be confused radiation is slanted at 45Њ, and in the vicinity of a max- with differences in shape, and a restricted dynamic range
Unauthenticated | Downloaded 10/01/21 04:03 AM UTC 300 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 19 in DR. The horizontal polarization state therefore is a A true half-wave plate (HWP) would induce a 180Њ poor selection for hydrometeor identi®cation. This is phase shift, and rotation would vary the transmitted state demonstrated later by calculations and some measure- through a continuum of slants of linear polarization, ments that build on those of Matrosov (1991) and Ma- including 1) horizontal as noted, 2) a linear polarization trosov et al. (2001). The CDR, by comparison, dem- slanted at 45Њ from horizontal at rotation  ϭ 22.5Њ, onstrates low sensitivity to variations in ¯utter in the and 3) vertical polarization at  ϭ 45Њ, to measure, orientation for columnar crystals, and shows substantial respectively, LDR ϵ DR (180Њ,0Њ), SLDR ϵ DR (180Њ, sensitivity for planar crystals only when the elevation 22.5Њ), and VLDR ϵ DR (180Њ,45Њ) by Eq. (2). In angle exceeds about 50Њ (Fig. 1 in Matrosov et al. 2001). contrast, a true quarter wave plate (QWP) would induce This relative insensitivity of CDR to orientation makes a90Њ phase shift, and rotation would vary the trans- it a better candidate, but the ambiguity of identifying mitted state from horizontal at  ϭ 0Њ, through a con- spherical droplets by default in the cross-talk, including tinuum of states of increasing ellipticity with increasing the effects of poor return in the weak channel in low rotation, to circular at  ϭ 45Њ, to measure, respectively, re¯ectivity clouds, must still be dealt with. LDR ϵ DR (90Њ,0Њ), EDR ϵ DR (90Њ,1Њ±44Њ), and A linear polarization transmitted at a 45Њ slant from CDR ϵ DR (90Њ,45Њ; Table 1). When using any PRP horizontal, and received at 45Њ and 135Њ, is simple to that is not a true HWP or QWP, polarizations will be implement (the PRP technology is useful for research transmitted that will have an ellipticities of 0 Ͻ⑀Ͻ1 but is not required) and is the closest relative to the at all rotations except zero or 180Њ, where the horizontal common horizontal linear polarization. For particles linear base state is transmitted. predominantly oriented in the horizontal, and for For this study, we recognized from experience that spheres, the slant-linear state is in effect similar to cir- precision engineering of the PRP for an exact phase cular. This state is not commonly used, but is presently shift is dif®cult, so a true slant linear ( ϭ 180Њ) was receiving consideration as a possible choice for the next targeted with the expectation that there would be some Weather Surveillance Radar-1988 Doppler Next Gen- ellipticity, which would be advantageous. The PRP eration Weather Radar (Brunkow et al. 1997; Doviak et manufactured for this research induces a phase shift of al. 2000). A slight ellipticity is introduced by imper- approximately 177.4Њ, according to calibrations with fection of the PRP used to transmit slant-linear polar- spherical hydrometeors (section 5). The cross-talk limit ization. The linear state with the added advantages of for the antenna of NOAA/K is approximately Ϫ36 dB, the slant and a slight ellipticity was explored in this so the maximum available dynamic range in DR is ap- study. The quantitative theory and analyses presented proximately 36 dB. This is the dynamic range for linear in following sections show that this slant-45Њ quasi- and circular polarizations for this radar. Ellipticity re- linear state provides depolarizations (measured as duces the dynamic range in which DR may be measured SLDR*-45, Table 1) that substantially overcome the is- to a value above the cross-talk limit, for particles ran- sues present with LDR. domly oriented with respect to the azimuth angle. The previously tested PRP that induced a 79.5Њ phase shift (⑀ϭ0.83) to measure EDR ϭ DR (79.5Њ,45Њ) reduced 3. Radar measurements and supporting data from the dynamic range to 15 dB, which was too narrow MWISP (Matrosov et al. 1996; Reinking et al. 1997a,b). The The measurement methods for NOAA/K were as fol- slightly elliptical, 45Њ slant polarization state produced lows. NOAA/K is equipped to provide a selection of by our new PRP reduced the dynamic range by several transmitted polarization states for testing by installing decibels, but not nearly to the extent realized with the any of several rotatable PRPs. Each engineered PRP more elliptical polarization. This allowed for measure- produces a speci®c, ®xed phase shift in the transmitted ment of the depolarization of drizzle at a value above signal. Measurements of DR (, ) with NOAA/K are the cross-talk limit of the antenna. The ϳ2.6Њ offset from taken by (i) selecting a PRP and setting  for a speci®c ϭ 180Њ induces a slight ellipticity in the transmitted polarization state, setting the radar's antenna at a speci®c radiation, which is approximately 0.02, allowing for an azimuth, and scanning it from horizon through zenith uncertainly of a few tenths of a degree in the 177.4Њ toward the opposite horizon in the vertical plane of an phase shift, which was obtained by matching the cal- elevation (RHI) scan through a sector as wide as 175Њ. culations from the PRP equation (Matrosov et al. 2001, Alternatively, the measurements are taken by (ii) ®xing p. 481) for spheres to the measured Ϫ29 dB. A rotation the radar's beam at a selected elevation and azimuth and of this PRP to 22.5Њ allows a 45Њ slant quasi-linear de- rotating the PRP through 360Њ, to continuously change polarization ratio, SLDR*-45 ϵ DR (177.4Њ, 22.5Њ)to the polarization state over the range and limits de®ned be measured (Table 1). For spheres, and for nonspherical by the particular PRP. Regardless of the phase shift of particles with a preferred horizontal orientation but ran- the particular PRP, at zero rotation the transmitted po- domly oriented with respect to their azimuthal angle (or larization state remains that of the base state, LDR ϵ equivalently, their vertical axis of rotation), the slight DR (, 0Њ), which is independent of the value of and ellipticity of this transmitted polarization state increases is measured in the returned signal according to Eq. (2). the weak-channel return over that possible with either
Unauthenticated | Downloaded 10/01/21 04:03 AM UTC MARCH 2002 REINKING ET AL. 301 true linear or circular polarization. This enhances DR Some rawinsondes from the NCAR CLASS and pro®les measurements in clouds of low re¯ectivity (Matrosov of cloud liquid water content measured with ATEK and Krop¯i 1993) at a sacri®ce of some of the dynamic sondes released from the radar site were also valuable range for particle type separation. Conversely, at zero to this study (Hill 1989, 1994). rotation of this or any other PRP, the transmitted state The polarization measurements will be examined af- is independent of and remains the base state of the ter we present their basis in scattering calculations. radar, where the full dynamic range is available at a sacri®ce of sensitivity. 4. Scattering calculations Propagation effects on the value of DR for our radar have been considered in Matrosov et al. 2001. In brief, NOAA/K can obtain measurements of DR as a func- propagation of the signals through a media of non- tion of antenna elevation or pointing angle, , from the spherical particles to and from the radar resolution vol- RHI scans. The scattering calculations presented here ume increases the apparent depolarization ratio mea- predict the DR signatures of ice crystals of the most sured by the radar over the DR unaffected by propa- common ``regular'' growth habits (Pruppacher and Klett gation. The propagation effects increase as the elevation 1997, 44±46), as a function of , for three polarization angle decreases. The increase is greater for the more states, standard (horizontal) linear, 45Њ slant linear, and non-spherical particles, and it depends on particle con- 45Њ slant quasi-linear obtained with a 177.4Њ phase shift centration. The ice water contents are not very large PRP. Thus, respectively, the relationships LDR(), (usually not exceeding a few tenths of a gram per cubic SLDR-45(), and SLDR*-45() were calculated for a meter). The estimates in Matrosov et al. (2001) show Ka-band radar with a Ϫ36 dB antenna cross-talk limit. that the apparent increase of DR due to propagation The model for these scattering calculations is explained effects does not exceed about 2 dB, for particles with by Matrosov et al. (1996, 2001). Matrosov et al. (2001) a wide range of aspect ratios (0.1±1.0) if the distance present similar calculations for SLDR-45(), and (radial range) is within about 10 km and elevation angles SLDR*-45(), but from the somewhat different per- are above about 30ЊÐwhich is the case for the mea- spective of testing the possibility for measuring particle surements reported in this paper. Note that since prop- aspect ratio, so effects of varying bulk density and as- agation effects tend to increase DR monotonically, if pect ratio are incorporated. The following calculations signi®cant, they could be recognized by analyzing the incorporate median crystal sizes and bulk densities that DR range patterns. Our experimental data collected in are ®eld-measured values recorded in the literature, as the various ®eld campaigns including MWISP indicate explained by Reinking et. al. (1997a). that these effects are very modest for the short ranges and high elevation angles. The images of radar scans a. Horizontal polarization presented later in this paper do not indicate any signif- icant propagation effects. For reference, the calculations for the horizontal po- During MWISP, NOAA/K was operated to measure larization state are presented ®rst. Thus, LDR()is
DR, re¯ectivity (Ze), radial velocity (Ve), and other basic shown in Figs. 1a and 1b for spheres (drizzle) and the radar parameters from a site midway up the west slope regular types of ice crystals. Dimensions affect shape, of Mount Washington, at 0.5-km altitude MSL, 4.1-km so the columnar crystals are separated according as the range west, and 1.1 km below the summit, where MWO length/diameter ratio, L/D, relating to the representative is located. The radar equipped with the 177.4Њ-PRP was size measurements, is greater or smaller than 2. For used to measure the depolarization of signals transmitted LDR, the dynamic range available for separation of hy- at the 45Њ slant, quasi-linear state. Measurement at the drometeor types is maximized. For this polarization horizontal and intermediate states was also possible. state, spheres cause no depolarization, so the backscatter Many kinds of supporting data were gathered during is that of radiation from the co-polar channel at all el- MWISP (Reyerson et al. 2000). Some in situ samples evation angles, and the calculated signature of spheres of hydrometeors from the MWO and at the radar site (drizzle) equals the Ϫ36 dB cross-talk limit at all . are used as in situ truth in this study. At the radar site, Both the planar and columnar families of crystals tend falling ice particles were sampled on black velvet and to settle with their larger dimension near horizontal. photographed and/or noted in the radar's electronic log- Their shapes therefore appear very different when ir- book. Hydrometeor imaging data from the MWO were radiated at zenith ( ϭ 90Њ) and near the horizon ( Ͻ taken with a Cloud Particle Imager CPI; Lawson and 45Њ). Consequently, the selectively shaped ice particles Jensen 1998; Lawson et al. 1998) and a standard Particle cause depolarizations that vary with elevation angle. Measuring Systems (PMS) laser two-dimensional gray- Variations of settling orientation are determined by the scale cloud probe (2DGC) operated by the Cold Regions three-dimensional canting angle variability from axis tilt Research and Engineering Laboratory (CRREL). A few and rotation; we characterize this by the standard de- particle measurements from the National Aeronautics viation of the major particle dimension from the pre- and Space Administration (NASA) Flenn Research Cen- ferred horizontal settling position, . Horizontal po- ter Twin Otter cloud physics aircraft were also used. larization excites only the dipole moment along the hor-
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Unauthenticated | Downloaded 10/01/21 04:03 AM UTC MARCH 2002 REINKING ET AL. 303 izontal principal axis of the scattering hydrometeors small, so relating the results to crystal populations is when the settling position is truly horizontal; however, dif®cult. Nevertheless, collectively, these measurements when particles ¯utter as they fall, LDR becomes more demonstrate that the deviations of the settling orienta- a function of than particle shape. tion from horizontal may frequently be small but may LDR increases with for the columnar family and vary through a wide range, such that might be ex- decreases with for the planar family. This difference pected to vary widely from case to case. Signi®cant is very evident, and the -dependencies differentiate differences between calm and turbulent clouds or due each type from the constant value for spheres (Figs. to variations in riming can be theorized. Some aspects 1a,b). However, of the available 36-dB dynamic range, of the secondary motions of ice particles have been only 19 dB is used, and at both low and near zenith, described (as summarized by Pruppacher and Klett some of the crystals show little differentiation from 1997, 444±446), but is as yet unpredictable. The lack spheres. This differentiation is as little as 1 dB for crys- of consistency and predictability of confuse the hy- tals with settling orientations of small standard deviation drometeor identi®cation using LDR. The large variation
( ϭ 3Њ, Fig. 1a). The separation from drizzle is in- of LDR with is therefore one key aspect of horizontal creased by 5 to 10 dB if the crystals have a much larger polarization that eliminates it from candidacy for the variation in settling orientation ( ϭ 15Њ, Fig. 1b), but optimal transmitted polarization state. the LDR values for the various crystals becomes more overlapped and inseparable. The sensitivity to applies b. 45Њ slant linear polarization to columnar crystals at Ͻ 35Њ, approximately, and to planar crystals at all . In examining the same issue by A45Њ tilt of the transmitted signal's polarization plane another approach, Sturniolo et al. (2000) show that for enhances the cross-polar return of non-spherical parti- ®xed at 15Њ, LDR values for long columns increase cles because it excites dipole moments along both prin- within a range of 18%±39%, depending on aspect ratio, cipal axes of the scattering hydrometeors that are set- when is increased from 6Њ to 30Њ (only slightly more tling with a generally horizontal orientation. These as- change occurs as is increased to 90Њ, where alignment pects of the potential improvement offered by a 45Њ slant is almost random). linear polarization are illustrated in Figs. 2a and 2b. A consistently large separation of crystals from driz- First, the difference between available and utilized dy- zle would suggest an advantage in using LDR to detect namic range must be considered. SLDR-45 ϭ DR (180Њ, these droplets. This separation would be established 22.5Њ) utilizes 29 dB of its available 36 dB dynamic only if the variations in crystal settling orientation were range to differentiate among the noted crystal types and always large, but they are not. Sassen (1980), using a drizzle, 10 dB more than utilized with LDR with the lidar, measured maximum planar crystal ``wobbles'' of same available dynamic range. Second, to within 0.5 only 3Њ, apparently in calm air. Thus, for these crystals, dB, the columnar crystal types become independent of
was evidently of the order of only 1Њ. Mallmann et the difference between 3Њ and 15Њ in and are nearly al. (1998) show both photographic and theoretical ev- independent of as well, although advantageously off- idence of narrow and broad sun pillars that were due, set from drizzle by ϳ11±12 dB. For the same difference respectively, to crystals with ``nearly'' horizontal and in , the SLDR-45 signatures of planar crystals shift less horizontal orientations. The supporting calculations by less than 2 dB at Յ 60Њ; the shift above 60Њ becomes consider canting angles between 2Њ and 10Њ. Canting as large as 8 dB, but the differentiation at low angles angles of 10Њ±25Њ were measured for individual, rimed and the slope of the planer crystal curves is so large columnar crystals by Kajikawa (1976). Zikumunda and that their identi®cation should be unambiguous. Third, Vali (1972) present a frequency distribution for mea- whereas some LDR values for crystals approach that sured canting angles of rimed columnar crystals with for drizzle at both low and high , with SLDR-45, all length-to-diameter axis ratios, L/D primarily in the of the crystal types are differentiated from drizzle by range of 2 to 10. Their distribution shows that ϳ40% 10 dB or more at all Յ 60Њ. This means that a radar of the crystals fell with less than a 5Њ cant, and ϳ90% with the 45Њ-slant linear polarization should be able to fell with less than a 15Њ cant, although canting angles very effectively differentiate drizzle from planar and were as large as 75Њ for a few crystals. The riming columnar crystals with only a ®xed beam set at a low undoubtedly tended to induce imbalances, to increase antenna elevation angle. This cannot be done with LDR. the variations in settling motions over those of pristine Since spherical droplets do not depolarize any truly crystals. The samples for these analyses were very linear signal, their null signature would be SLDR-45 ϭ
←
FIG. 1. Calculations of the horizontal depolarization ratio, LDR ϭ DR (180Њ,0Њ) (dB), as a function of antenna elevation angle (Њ) for spheres (drizzle droplets) and for basic ice crystal types, planar (plates, dendrites, thick plates), and columnar (needles, long hollow columns, blocky solid columns); for standard deviation of canting angle (a) ϭ 3Њ and (b) ϭ 15Њ [also indicated are Magono and Lee crystal
(1966) classi®cations; experimental mean major axes dimensions, Dm and Lm; and assumed ice densities, ].
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LDR → Ϫϱ dB without the antenna cross-talk limit of between of 0Њ and 90Њ, and the signature of each type the radar. With that limit imposed, their signature will of columns is nearly constant, showing an increase of be at Ϫ36 dB for NOAA/K. This is a disadvantage for only ϳ1 dB with increasing , and well separated from both SLDR-45 and LDR that can introduce some am- the Ϫ29 dB signature of drizzle droplets, well above biguity because the null signal (a weak-channel, cross- the antenna's cross-talk. polar return below the noise) might be interpreted either as droplets or simply insuf®cient return to make a dis- d. Summary of comparisons tinction in clouds with low re¯ectivities, thus, for NOAA/K when the strong-channel re¯ectivity is above Some of the comparisons of the three polarization the noise level by less than 36 dB. This situation can states, as derived from Figs. 1±3, are summarized in be remedied in part by using a slightly elliptical polar- Table 2. The tabulated parameters include 1) the avail- ization because it raises the weak-channel return. able but rarely achievable effective dynamic range for each depolarization ratio, 2) the portions of that range that are actually functional and utilized for differenti- c. 45Њ slant, quasi-linear polarization ating among the regular crystals and droplets and for For the SLDR*-45 ϭ DR (177.4Њ, 22.5Њ), the cal- differentiating among only the various crystal types, 3) culations as a function of are presented in Figs. 3a an example of the shift in each DR due to the 12Њ change and 3b. The key features of the truly linear SLDR-45 in (from 3Њ to 15Њ), and ®nally, 4) the minimum are retained but compressed into a dynamic range that difference between the DRs of droplets and crystals at is 7 dB narrower than allowed by the Ϫ36 dB cross- Յ 60Њ, where the best differentiation is achieved. The talk limit of the radar. In this case, all of the regular values in Table 2 and Figs. 1±3 show that SLDR*-45 crystal types are differentiated from drizzle by 5 dB or should be quite superior to LDR due to its (i) relative more at all Յ 60Њ. This difference is substantial and insensitivity to , (ii) wider utilized dynamic range for easily measured. The spheres, now depolarizing but at separation of hydrometeors, and (iii) invariant and siz- a constant value independent of due to the signal's able minimum difference between the DR values for ellipticity, de®ne the lower limit of this range for this regular ice particles and for droplets at Յ 60Њ. From polarization state as ϳϪ29 dB. However, according to the same table and ®gures we see that the slant linear the calculations, SLDR*-45 utilizes more of its available depolarization ratio, SLDR-45, would offer distinctions dynamic range than LDR for both large and small var- superior to both LDR and SLDR*-45, were it not for iations in settling orientation (Figs. 3a,b vs Figs. 1a,b). two factors that do not appear in the table. Spherical Some 20±22 dB of the available 29 dB is utilized to scatterers are still identi®ed only by default at the DR differentiate among the noted crystals and drizzle; this value of the antenna cross-talk; and the weak-channel is still 1±3 dB more than the range utilized in LDR with return of nonspherical scatters comprising clouds with the broader available range. The range utilized for dif- a (strong-channel) re¯ectivity less than the absolute val- ferentiating among crystals only is 17±22 dB in SLDR*- ue of the cross-talk above the noise will by default have 45, 4±5 dB more than that in LDR (both ranges de- the same DR as spheres. Thus, for any true linear state, pending on ). including SLDR-45 (or the circular state), unambiguous, For further perspective on the effect of crystal ¯utter, deterministic identi®cation of spherical droplets in low- examine the curves for the crystals that exhibit the great- re¯ectivity clouds will be dif®cult, and range sensitivity est dependency on , the hexagonal plates (P1a). When is reduced, relative to the results using SLDR*-45. Also, is changed from 3Њ to 15Њ, SLDR*-45 shifts by only scatters with higher strong-channel re¯ectivity but are Ϫ2dBat ϭ 0Њ and ϩ4dBat ϭ 90Њ, but LDR shifts nearly spherical may also have a SLDR-45 that ap- by ϩ8dBat ϭ 0Њ and ϩ9dBat ϭ 90Њ. For ϭ proximates that of spheres, due to a weak-channel return 15Њ, LDR provides a separation from drizzle of at least that is not measureable and by default is also in the 6 dB in the worst case, which is better than the 3 dB noise. for SLDR*-45. Thus, if were always relatively large Some ellipticity introduces a weak-channel return (e.g., 15Њ), LDR would provide the greatest differenti- from drizzle and from ice particles with randomness in ation from drizzle, but that condition cannot be reliably orientation in the horizontal plane (Matrosov and Krop- assumed. Also, LDR does not provide the greatest sep- ¯i 1993). This covers most atmospheric situations, so aration among crystals only. Therefore, measurement of ellipticity will normally provide a strengthened weak- SLDR*-45 is expected to be much more dependent on channel echo to enhance detection and differentiation crystal shape and less on orientation than LDR and, of low-re¯ectivity clouds of drizzle and the other par- therefore, much more consistent. As with SLDR-45, ticles. Slight ellipticity, as in SLDR*-45, raises the SLDR*-45 of the planar crystals exhibits a steep slope weak-channel signal for spheres several decibels above
←
FIG. 2. Calculations, as in Fig. 1, of the SLDR-45 ϭ DR (180Њ, 22.5Њ) (dB) for (a) ϭ 3Њ and (b) ϭ 15Њ.
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Unauthenticated | Downloaded 10/01/21 04:03 AM UTC MARCH 2002 REINKING ET AL. 307 the antenna cross talk limit and allows for a de®nitive, TABLE 2. Comparisons of available vs functional or utilized dy- directly measurable cross-polar power. This also in- namic ranges in three depolarization ratios (DR), corresponding shifts in DR due to two differing ice crystal canting angles, and minimum creases the radial range of minimum detectability of differentiation in DR of regular ice crystals from spherical droplets low-re¯ectivity cloud. Of course, the radial range is also at Յ 60Њ. a function of radar hardware (power, antenna size, etc.) SLDR-45 SLDR*-45 and mode of operation (dwell time, pulse length, etc.). LDR (dB) (dB) (dB) Once these basic radar parameters are set, and the dwell- time-dependent minimum detectable polarization dif- Available dyn. range 36 36 29 ference needed to distinguish all ice from low-DR water Utilized dyn. range, droplets is determined, the ellipticity needed to generate crystals ϩ droplets ϭ 3Њ 19 29 22 this minimum difference can then be determined exactly ϭ 15Њ 19 27 20 through scattering calculations. However, with too much ellipticity, the dynamic range in DR can be decreased Utilized dyn. range, crystals only too much to allow effective distinction among the var- ϭ 3Њ 18 28 22 ious particles, so a reasonable compromise is necessary. ϭ 15Њ 13 20 17 From the scattering calculations, in summary, com- Shift in DR at ϭ 0Њ, pared to horizontal polarization, the primary functions ϭ 3Њ vs ϭ 15Њ and advantages of the transmission at a slant are 1) to Plates 8 2 2 relatively immunize the measured depolarization ratio Solid columns 5 1 1 against unpredictable variations due to particle oscil- Min DR difference, lations in around their preferred horizontal orientation, droplets vs crystals and 2) to substantially increase the effective dynamic ϭ 3Њ 1±6 11 5 range that is utilized to differentiate the DR signatures ϭ 15Њ 6±11 11 5 of drizzle and the various ice hydrometeors. The pri- mary function and advantage of the added ellipticity is to increase weak-channel, cross-polar return such that with the PRP rotated to a ®xed position to establish the the ratio for least depolarization (that for spherical drop- state. lets) is at an explicitly measurable level above the an- Beginning with an exemplary RHI scan, the MWISP tenna cross-talk and need not be determined by default. PRP was ®xed at  ϭ 22.5Њ to measure SLDR*-45 (Fig. Equivalently, this gains radial range sensitivity. The el- 4a, top panel; 1232 UTC 07 April 99). This RHI shows lipticity in SLDR*-45 compromises the utilized dynam- a cloud with ice aloft, where SLDR*-45 up to about ic range somewhat relative to SLDR-45, but does not Ϫ16 dB was measured. Ice precipitating from the cloud reduce it to the range for LDR. Given these factors, melted to produce a highly depolarizing bright band, results in particle identi®cation from the measurement where SLDR*-45 ഠ Ϫ10 dB. The hydrometeors below of SLDR*-45 should be superior to those using SLDR- the bright band showed very uniform and minimal de- 45, and both should be far superior to those from mea- polarization at any , indicating drizzle, which was ob- surements of LDR. served at the ground. The differentiation of ice from liquid is immediately evident, as it was in measurements 5. The measured signature of drizzle droplets using EDR (Reinking 1997a,b), but it is the quantitative increase in separation that is of interest here. The cor-
Measurement of the signature of drizzle and experi- responding cloud (strong-channel) re¯ectivity, Z e , mental calibration of the designed polarization state are shown in the bottom panel of Fig. 4a, reveals the melting intertwined because the actual phase shift of each PRP level but otherwise only hints at differentiation of these is determined by ®eld observation of spherical particles features. A plot of the function, SLDR*-45(), at con- (drizzle droplets). The calibration can be accomplished stant altitude through the drizzle in Fig. 4a, is invariant by measuring return signals in drizzle from ®xed beam at Ϫ29 Ϯ 0.5 dB; this is curve in Fig. 5 labeled measurements and RHI scans. With the radar pointed at ``drizzle.'' Thus, the effective dynamic range and the a ®xed elevation angle, the PRP is continuously rotated signature of drizzle for this PRP con®guration is indi- through at least 180Њ, to determine DR at all rotations, cated to be Ϫ29 dB. but particularly at  ϭ 0Њ and 22.5Њ corresponding, re- A calibration of the other type, with a PRP rotation, spectively, to the horizontal and slant-linear states. The in another MWISP drizzle situation is shown in Fig. 6a RHI measurements calibrate the ®xed value of DR in (1559 UTC 26 April 99). At  ϭ 0Њ (as well as  ϭ drizzle for a speci®c polarization; this is accomplished Ϯ90Њ), the measurement is that of LDR ϭ DR (, 0Њ)
←
FIG. 3. Calculations as in Fig. 1, of the SLDR*-45 ϭ DR (177.4Њ, 22.5Њ) (dB) for (a) ϭ 3Њ and (b) ϭ 15Њ.
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FIG. 4. Over-the-top RHI scans, from east (right, azimuth 86.4Њ) through zenith to west (left), observing clouds of drizzle and ice particles of three pristine, regular growth habits. Each part of this ®gure shows a pair of images where the top panel is the depolarization ratio,
SLDR*-45 (dB, upper color scale), and the bottom panel is the corresponding radar equivalent re¯ectivity, Ze (dBZ, lower color scale). The color scale for SLDR*-45 is the same in each case: range Ϫ6toϪ32 dB, center at Ϫ20 dB. The scale for Ze was varied according to cloud intensity. Radial lines indicate antenna elevation angle in increments of 30Њ. Range ring intervals are 1 km from the radar located at bottom center. The upper, steep west slope of Mount Washington is evident as the line of ground clutter in view at the right in each ®gure, between the 2.7 and 3.7 km range. The MWO is located at the 4.1-km range, just under the horizon beyond the upper end of the clutter. These ®gures de®ne the signatures of the following hydrometeors: (a) a cloud with ice particles aloft, the bright band (melting layer) near 600 m AGL, and drizzle below (1232 UTC 7 Apr 1999); (b) pristine planar crystals: unrimed dendrites were predominant in surface samples at the radar
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FIG. 5. Measured curves of SLDR*-45 a function of radar antenna elevation angle, SLDR* -45() (dB, Њ), for drizzle-sized droplets and several types ice particles of regular growth habits, as labeled. The curves SLDR*-45()at Ͼ 90Њ approximate mirror images of those for Ͻ 90Њ (i.e., the measurement at ϭ 150Њ is equivalent to that at ϭ 30Њ, etc., such that either half of the curve can be compared to the calculations in Fig. 2). Valid signal strength was ensured by
restricting cross-polar intensity to Icr Ͼ 0.04 volts, somewhat above the 0.015 V threshold. Each measurement is at a constant altitude h, as follows: (a) drizzle: h ϭ 0.2 km AGL in Fig. 4a, 1232 UTC 7 Apr 1999; (b) hexagonal plates (classi®cation P1a): h ϭ 2.2 km AGL, 1442 UTC 15 Apr; (c) dendrites (P1e): h ϭ 1.4 km AGL in Fig. 4b, 1257 UTC 14 Apr; (d) long columns (C1f ϩ N1e): h ϭ 0.3 km AGL in Fig. 4c, 1404 UTC 17 Apr; and (e) blocky columns (C1e): h ϭ 0.4 km AGL in Fig. 4d, 1043 UTC 27 Apr. and is independent of phase shift, . At any intermediate measured as DR (177.4Њ, 22.5Њ) ഠ Ϫ28 Ϯ 0.5 dB. A rotation, DR depends on , so rotation through  provides measured depolarization of ϳϪ26.5 dB at a rotation of the measurements to determine the actual phase shift of Ϫ67.5Њ, or 22.5Њ from Ϫ90Њ, is indicated at the intersec- this PRP. The smooth curve in Fig. 6a is the calculated tion with the vertical line at the left in Fig. 6a. This would DR ϭ f() for ϭ 177.4Њ and the antenna cross-talk ®t a curve for ഠ 153Њ; the calculated calibration curve of 36 dB. Curves from measurements at three altitudes for this would not account for the Ϫ29 dB drizzle in the drizzle with the radar pointing vertically are also signature veri®ed by the analysis in Fig. 5 of the RHI in shown. Unlike the calibrations for our PRP with ϭ Fig. 4a. Thus, the curve for 177.4Њ approximates the cor- 79.5Њ that we used to measure EDR (Matrosov et al. 1996; rect choice to de®ne the calibration. The slight ellipticity Reinking et al. 1997b), a nearly exact ®t between the induced by the 177.4Њ phase shift in the PRP at 22.5Њ calculations and measurements was not obtained. The rotation (for the 45Њ slant) is illustrated in Fig. 6b. departures of the experimental curves from the theoretical The calibrations from the separate RHI and rotating curves in Fig. 6a are explained subjectively as the result PRP measurements differ only by approximately 1 dB. of internal microwave re¯ections in the PRP. The cali- They experimentally establish the effective or functional bration is somewhat disappointing in this respect and dynamic range for this PRP as 28.5 Ϯ 1 dB (corre- signi®cantly deviant from theory at some rotations. How- sponding to the signature of drizzle) and the phase shift ever, it does suf®ce. As the three samples indicate, the as approximately 177.4Њ. Therefore, SLDR*-45 ϭ DR patterns are reproducible and symmetric around  ϭ 0Њ. (177.4Њ, 22.5Њ) as calculated in Figs. 3a and 3b where The minima in the measured curves indicate that LDR the dynamic range is 29 dB should provide reasonable ഠ Ϫ36 or Ϫ37 dB. This provides a foundation for the predictions of the depolarizations by the basic types of calibration because it approximates the radar cross-talk hydrometeors. This dynamic range in depolarization ra- limit, as it should. The curve for ϭ 177.4Њ approximates tio is about 14 dB wider than it was for EDR with a the best ®t at the intersection of the experimental curves 79.5Њ phase shift and ensures much greater separations and the vertical line at  ϭ 22.5Њ, where SLDR*-45 is among drizzle and the other hydrometeors.
←
(1257 UTC 14 Apr 1999); (c) a mix of regular long columns and sheaths (1404 UTC 17 Apr 1999); and (d) blocky columns comprising a weak-re¯ectivity cloud (1043 UTC 27 Apr 1999).
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FIG. 6. (a) DR (dB), as a function of the rotation angle, , of the PRP used in MWISP (1559 UTC 26 Apr 1999). The antenna was pointed to zenith ( ϭ 90Њ), and the PRP was rotated through  at 1 rpm to produce the experimental measurements in drizzle at 3 altitudes above the radar. The smooth curve is that calculated for a PRP with a 177.4Њ phase shift. The triple intersection of the vertical line at rotation  ϭ 22.5Њ, the calculated curve, and the experimental curves indicates that SLDR*-45 ഠ Ϫ28.5 dB in drizzle (spheres). The minimum values of the curves show LDR ഠ Ϫ36 or Ϫ37 dB in drizzle, at the cross-talk limit of the radar; (b) the polarization ellipse representing the PRP with the 177.4Њ phase shift, rotated to 22.5Њ to establish the 45Њ slant in the transmitted signal.
The absence of a cross-polar power returned when of (a) DR (), and (b) the corresponding Icr and standard measuring LDR, which is not readily discerned in the (copolar) re¯ectivity, Ze. This 6-min period includes the DR measurements because the effect occurs as a signal PRP spin in Fig. 6a that was used for calibration. These at the Ϫ36-dB cross-talk level, is evident in the mea- data are from a slightly longer range. From the beginning surements of cross-polar intensity, Icr, which is used to of the series, the pattern in DR that de®nes drizzle re- derive Pcr (in this case, the weak-channel echo) and DR. peated itself as the rotating PRP cycled during approx- An inspection of considerable data shows that the noise imately the ®rst 3.5 min. Here, the minima in DR at Ϫ35 threshold occurs approximately where Icr ഠ 0.015 V, and to Ϫ37 dB occurred where Icr is below the noise thresh- the DR measurement is clearly discernable when Icr Ͼ old, indicating no cross-polarized signal; these are the 0.03±0.04 V; this value offers a threshold to identify values for LDR. SLDR*-45 is represented by the sec- reliable measurements. In contrast, re¯ectivity, for ex- ondary peaks, where DR was Ϫ27 to Ϫ29 dB, and Icr ample, is range dependent and less rigid in separating rose to easily measured values above 0.1 volt. good and unreliable signals. Figure 7 shows a time series After about 3.5 min, DR became erratic (Fig. 7a).
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A constant-altitude sample of SLDR*-45() at 1.4 km AGL (``dendrites,'' Fig. 5) from the data in the RHI scan shows depolarization ratios of approximately Ϫ12 or Ϫ13 dB at ϭ 20Њ (and equivalently in the ``re¯ection'' at 160Њ) and Ϫ27 dB at 90Њ (zenith). This measured curve closely approximates the calculated curves for dendrites, especially that in Fig. 3b. Due to their simple structure, hexagonal plates (class P1a) are predicted to produce a ``V'' signature that is even more pronounced than that of dendrites. This is demonstrated by the measured curve labeled as such in Fig. 5, where the depolarization at low was some 3 dB greater than that caused by the dendrites. The pres- ence of the plates early in the evolution of this layer cloud was con®rmed by the very ®rst of a set of PMS 2DC probe measurements obtained over the radar with the NASA aircraft (Fig. 8b). This cloud rapidly changed to dendrites and aggregates, and some sector crystals were already present at this time, just 6 min after the radar sample, but the sector type would have depolarized the signal only slightly less than plates. Compare the curves for the plates and dendrites in FIG.6.(Continued) Fig. 5 for near zenith. Whereas the plates show a depolarization nearly equivalent to spheres and drizzle at this elevation and closely approximate the calculated This occurred when Z ®rst dropped below about Ϫ25 e curve for ϭ 3Њ (Fig. 3a), those for dendrites show dBZ, and I decreased to values mostly under ϳ0.04 V. cr about 3 dB less depolarization at the same angles and At this time, the cross-polar re¯ectivity (not shown) more closely approximate that for ϭ 15Њ. This sug- dropped to nearly 50 dBZ. In the time series, the de- Ϫ gests that the dendrites exhibited considerable random- cline in I for SLDR*-45 shows that there is a transition cr ness in their settling orientation, whereas the plates did to a signal dominated by noise, whereas the I for LDR cr not. This effect is of no consequence to particle iden- remains below the noise level and the signal is not re- ti®cation for Ͻ 60Њ, as a comparison of the measured liably measurable. This supports the arguments in favor and calculated curves demonstrates. In view of the pre- of having some ellipticity in the selected polarization vious discussion about the effects of , the selection state to gain a cross-polar signal even in low-re¯ectivity of SLDR*-45 over LDR is further supported. droplets. b. Columnar crystals 6. Measured depolarization by regular planar and Long regular columnar crystals with length-to-di- columnar crystals ameter ratios, L/D Ͼ 2, were observed simultaneously a. Planar crystals with the radar (Fig. 4c, 1404 UTC 17 April 1999) and the CPI at the MWO (Fig. 8c). The CPI images indicate Planar crystals settling with a preferred horizontal ori- a mix of hollow columns and sheaths (respectively, type entation present a quasi-circular cross-section and there- classi®cations C1f and N1e) with L/D ϳ 4±5. A ra- fore depolarize the signal minimally when observed near winsonde released at 1300 UTC from the radar site zenith, but appear as more linear objects and depolarize shows saturation from 0Њ to Ϫ8ЊC, which matches the the signal substantially when observed near the horizons. growth regime for those crystal types. A constant-alti- The consequent -dependent ``V'' pattern in the depo- tude plot from the RHI scan (``long columns'' in Fig. larization signature is illustrated in the RHI scan in the 5) shows that SLDR*-45 increased by only ϳ2 dB, from top panel in Fig. 4b (1257 UTC 14 April 1999). Sub- about Ϫ18 dB to about Ϫ16 dB, as was increased stantially branched, unrimed dendrites, the most common from 30Њ to 90Њ. The experimental curve has approxi- of planar crystals [crystal type P1e, Magono and Lee mately the same slope and lies between those calculated (1966) classi®cation, in Pruppacher and Klett (1997, 44± for needles (or sheaths) and hollow columns (Figs. 3a 46)], were observed on black velvet at the surface and or 3b), which are themselves separated by only 1.3 dB. noted in the radar's electronic log book; they were very Blocky columns (L/D Ͻ 2, classi®cation C1e) are similar but less rimed than crystals photographed 15±30 more spherical than other types of pristine ice crystals, min later (Fig. 8a). These targets are broadest at zenith, with the possible exception of bullets (C1c), so their where even their re¯ectivities reached a maximum, a sig- depolarizations will be closer to that of spheres. Ob- nature unique to planar crystals (bottom panel, Fig. 4b). served blocky columns depolarized the signal 6±8 dB
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FIG. 7. Time series of radar parameters measured during PRP rotations in drizzle: (a) DR () (dB), where the pattern of LDR and SLDR*
-45 are noted, and LDR is repeated once in each 90Њ rotation; (b) the corresponding cross-polar intensity, Icr (V), and re¯ectivity, Ze (dBZ), 1556±1602 UTC 26 Apr 1999. less than the long columns, but 4±5 dB more than the dicted slight slope toward increasing depolarization nondepolarizing drizzle, as predicted in Figs. 3a,b and (less negative DR) between low and zenith seems to measured in RHI scans. The RHI scans showed a uni- be reversed in the measurement but is within the mea- formity of SLDR*-45 with similar to that in Fig. 4c surement error. Nevertheless, the measured depolariza- for the long columns, but in this case at Ϫ24 dB, as tion ratio equals that predicted within less than 1 dB. illustrated in Fig. 4d and the corresponding constant- Photographs and CPI images from the same period ver- altitude sample of SLDR*-45() labeled ``blocky col- i®ed the crystal type (Fig. 8d) and showed that L/D ϳ umns'' in Fig. 5 (1043 UTC 27 April 1999). The pre- 2 in this case.
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FIG. 8. Samples of regular ice crystals corresponding to radar measurements summarized in Fig. 5: (a) dendrites (class P1e; photographs at radar site, ϳ1315±1327 UTC 14 Apr 1999); (b) hexagonal plates and a few sectors (P1a and P1c, NASA aircraft PMS 2DGC samples, 1448 UTC 15 Apr); (c) long columns and sheaths, with small interstitial droplets (N1e, N1d, L/D Ͼ 2, CPI images at MWO, 1359±1402 UTC 17 Apr); and (d) blocky columns (C1e, L/D Յ 2 predominantly; photograph ϳ1044 UTC, CPI image ϳ1117 UTC 27 Apr).
Larger crystal densities would shift the curves for the regular. The more spherical aggregates predictably de- columnar and the planar crystals upward to greater de- polarize less than the individual crystals that comprise polarizations (Matrosov et al. 2001), so the good ®ts of them, according to observations and calculations of the measurements of SLDR*-45() to the calculations EDR based on Rayleigh scattering (Matrosov et al. with either suggest that the densities used for the 1996). Such calculations can only approximate the ef- calculations are reasonable selections (Fig. 3). fect because aggregates are normally too large to be The depolarization caused by regular crystals will precisely represented by the Rayleigh calculations for dominate any signature of small interstitial droplets, the Ka band (maximum dimension D Յ 2 mm, approx- which are common to mixed phase clouds. PMS FSSP imately). Upon aggregation, general aspects of the probe measurements con®rmed that the droplets among DR() of the individual crystals are nevertheless re- the columns in Fig. 8d had diameters under 10 m, so tained, although subdued. For example, the ``V'' sig- those of approximately the same magni®cations in Fig. nature of dendrites is suppressed but still evident be- 8c reached the order of 15 m. Aircraft are required to cause such clouds still contain some relatively pristine be designed to ¯y through droplets of such small sizes, crystals. Likewise, spatial dendrites will present rela- which are generally not regarded as an icing hazard tively spherical, non-Rayleigh targets to the radar, so unless present in large concentrations producing very the depolarization is expected to be diminished from large liquid water contents. that of pristine dendrites and less well described by calculations. To quantitatively describe the depolariza- 7. Measured depolarization by aggregated and tions by such particles, we go directly to the measure- irregular quasi-spherical ice particles ments. Aggregates of dendrites and spatial dendrites with a. Common aggregates individual crystals as large as 3 to 5 mm were observed When crystals aggregate, the composite particles are during MWISP (Fig. 9a). A characteristic distinct ``V'' usually more spherical than individual crystals. Some- signature in SLDR*-45 and a strong Ze near zenith to times they are elongated, but they are always more ir- ϳ8dBZ were both evident in corresponding RHI scans
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FIG. 9. Samples of irregular Ice particle corresponding to depolarization measurements in Figs. 10 and 11: (a) Unrimed and moderately rimed spatial dendrites from one aggregate, photographed at the radar site (ϳ1508 UTC 15 Apr 1999); (b) elongated aggregate of crystals mainly of dendritic structure, L ϳ 2.2 cm (ϳ1906 UTC 27 Apr); (c) a CPI image near the same time of an aggregate of thick plates with sector-like extensions (P1f) and an attached column (1902 UTC 27 Apr 1999); (d) photograph of conical graupel (ϳ2105 UTC 20 Apr); (e) CPI images of lump graupel and very small interstitial cloud droplets (ϳ1445±1446 UTC 15 Apr); (f) and photograph of heavily rimed branched planar crystals, or hexagonal graupel (ϳ1217 UTC 13 Apr).
(Fig. 10a), in spite of the irregularity and relative sphe- ogous to those of drifting feathers; the long, albeit dis- ricity of the aggregates and the particles composing torted axes were oriented horizontally on average but them. SLDR*-45() (Fig. 11, ``aggregates of den- exhibited the glide-pitch oscillations observed in fall drites,'' 1508 UTC 15 April 1999) was nevertheless motion studies (Pruppacher and Klett 1997, p. 445). CPI subdued at low elevation angles, by ϳ6dBat30Њ com- images indicate that plates and very large dendrites pared to the signature of unaggregated dendrites, but dominated, but some columnar crystals also formed in tended to retain the signature of individual dendrites at the cloud (1902 UTC 27 April 1999, Fig. 9c). Earlier zenith (Ϫ26 to Ϫ27 dB), although with more statistical RHI scans revealed columns below about 0.9 km AGL. variance in the signal. The effect is qualitatively the Thus, the aggregates at higher altitudes formed from the same as that for EDR shown for progressive aggregation planar crystals, but probably collected some columnar in the Figs. 7a±c in Matrosov et al. (1996). That study crystals before falling to the surface at the radar or and this one show that aggregates do depolarize the advecting to the summit as suggested by the depicted transmitted signal more than spheres, and this depolar- CPI sample. The effect on DR of the elongation of ag- ization is somewhat predictable from Rayleigh scatter- gregates of dendrites, relative to that of more spherical ing theory, despite their relatively large sizes. aggregates of dendrites, was isolated from the effect of added columns by determining DR() at an altitude b. Elongated aggregates above the columnar growth regime. The sample at 1.6 km AGL (``elongated aggregates'' in Fig. 11) shows Extremely large (2±5 cm), non-Rayleigh, elongated that elongation tends toward ¯attening the SLDR*- aggregates formed on 27 April 1999 (1906 UTC; Fig. 45() curve by increasing the depolarization by several 9b). The settling motions of these crystals were anal- decibels at all antenna elevation angles, but more so
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FIG. 10. Over-the-top RHI scans, from east (right, azimuth 86.4Њ) through zenith to west (left), through clouds with aggregated and irregular ice particles and a cloud of supercooled drizzle-sized droplets. The format is the same as in Fig. 4, with the top panel of each image pair showing the depolarization ratio, SLDR*-45 (dB, upper color scale) and the bottom panel showing corresponding radar equivalent re¯ectivity,
Ze (dBZ, lower color scale). These ®gures show the signatures of the following hydrometeors: (a) common aggregates of dendrites (1508 UTC 15 Apr 1999); (b) conical graupel, with melting layer just above the surface (2110 UTC 20 Apr 1999); (c) hexagonal graupel (1211 UTC 13 Apr 2000); (d) large supercooled droplets (2010 UTC 14 Apr 1999).
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FIG. 11. Measured curves of SLDR*-45() (dB, Њ) from RHI scans from constant-altitude samples through the supercooled drizzle-sized droplets and several types of the more irregular and spherical ice particles (Figs. 9 and 10). Valid signal strength was ensured by restricting cross-polar intensity to Icr Ͼ 0.04 volts. Each measurement is at a constant altitude h, as follows: (a) drizzle: h ϭ 0.5 km AGL 2010 UTC 14 Apr 1999; (b) aggregates, quasi-spherical, of dendrites and spatial dendrites (1.0 km AGL, 1508 UTC 15 Apr 1999); (c) elongated aggregates, extremely large, of planar crystals (1.6 km AGL, 1902 UTC 27 Apr 1999); (d) conical graupel (1.5 km AGL, 2110 UTC 20 Apr 1999); (e) lump graupel (dotted line, 0.3 km AGL, 1442 UTC 15 Apr 1999); and (f) hexagonal graupel (0.4 km AGL, 1211 UTC 13 Apr 1999).
near zenith. Consequently, the curve is distinctly dif- through a convective shower of particles positively iden- ferent from that of more spherical aggregates, although ti®ed as large conical graupel (Fig. 9d) provided signals
Ze was similar (6±8 dBZ). The curve for elongated ag- in SLDR*-45 that were quite uniform with . From the gregates approached the values for long columns (Fig. scan in Fig. 10b, a constant-altitude plot shows that 3, L/D Ͼ 2; and Fig. 5) but still maintained a weakened SLDR*-45() was approximately Ϫ23 to Ϫ25 dB, but de®ned ``V'' signature of about 5-dB depth as a clearly distinguishing it by 4 to 6 dB from the Ϫ29 dB distinguishing factor. signature of drizzle (``conical graupel'' in Fig. 11). The addition of the columns in the lower part of the The relatively strong re¯ectivity common for graupel cloud did ¯atten the SLDR*45() curve to a value of (Ze ഠ 8 Ϫ 20 dBZ; Fig. 10b) also distinguished it from ϳ18 dB, approximately matching the signature of pris- the weak re¯ectivity of drizzle (predominantly, Ze Ͻ 4 tine long columns. The aggregates were distinguished, dBZ in Fig. 4a, more commonly less than 0 dB). The however, by their much greater size and re¯ectivity (8± signature of conical graupel in SLDR*-45() might be 10 dBZ), compared to Ϫ3toϪ14 dBZ in the cloud of mistaken for blocky columns, except that the columns long columns (Fig. 4c). Long columns also readily ag- present a signature more uniform with throughout the gregate, but such a case with columns alone has not cloud and, in general, have a low Ze that is comparable been identi®ed in the measurements. to drizzle (e.g., Ze ϽϪ4dBZ predominantly; Fig. 10b). Rotation of the PRP with the antenna pointed to zenith provided a comparison of LDR and SLDR*-45 for the c. Conical, lump, and hexagonal graupel conical graupel (Fig. 12). Here, SLDR*-45 was consis- Conical graupel (classi®cation R4c) is quite spherical tently Ϫ24 to Ϫ26 dB and separated by a readily mea- compared to most other types of ice particles because surable 3±5 dB from the Ϫ29 dB of spheres, whereas L/D ഠ 1, despite the de®ned shape. However, the conical LDR varied between Ϫ33 and Ϫ35 dB, allowing a dis- form develops as a result of a preferred settling orien- tinction of only 1±3 dB from the Ϫ36 dB representing tation, with the apex of the cone upward. RHI scans spheres using the horizontal polarization state (Fig.
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FIG. 12. As in Fig. 7a: (a) DR() from ®xed-elevation rotation of the PRP in conical graupel, 2 km AGL, 211145±211444 UTC 20 Apr
1999; (b) and corresponding cross-polar intensity (Icr) and main-channel re¯ectivity (Ze). (Compare to data from spin in drizzle, Fig. 7.)
12a). This was true despite cross-polar intensities far were beginning to settle into the lower layer that was above the measurable threshold of 0.03±0.04 V and generating the graupel, and the depolarization values at strong re¯ectivities (ϳ14±19 dBZ; Fig. 12b). Thus, the the low angles in this RHI sample approximate those 45Њ slant, quasi-linear polarization is also superior to measured for a third, hexagonal variety of graupel (see LDR for distinction of such relatively spherical ice par- next paragraph). Overall, the irregularity of SLDR*-45 ticles. of the lump and conical graupel with was in sharp An RHI measurement of SLDR*-45() through small, contrast with the uniform signature caused by drizzle. irregular, lump graupel or snow pellets (classi®cation The CPI captured images of this graupel that reached R4b) that was patchy within the cloud is included in maximum sizes of about 0.8±1.0 mm (Fig. 9e). The Fig. 11. Where the lump graupel was clearly dominant, smaller sizes, and very small concentrations discerned the depolarizations are approximately equal to those from the minimal fallout rate, contributed to re¯ectiv- caused by the conical graupel. Some hexagonal plates ities of approximately ϩ2toϪ4dBZ, as low as those
Unauthenticated | Downloaded 10/01/21 04:03 AM UTC 318 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 19 of drizzle. Therefore, a high re¯ectivity can reinforce measured as Ϫ29.5 dB (Fig. 10d, and ``drizzle,'' Fig. differentiation by depolarization but cannot be expected 11). for graupel occurring in very low concentrations. The surface temperature was near Ϫ1ЊC during this Branched planar crystals without rime depolarize the period, and small, extremely sparse, quasi-spherical ice incident signal minimally at zenith but very substan- pellets were collected at the radar site, indicating that tially at lower elevation angles (e.g., ``dendrites''; Fig. some of the drizzle drops were freezing. A few such 5). However, branched planar crystals that collected pellets, indicated by their irregular outline in Fig. 13, heavy rime, such that they reached the graupel stage were imaged among the drizzle droplets by the 2DGC (Fig. 9f; hexagonal graupel, classi®cation R4a), caused probe. However, a substantial icing rate of ϳ5±6 g hϪ1 some of the lowest levels of depolarization observed measured with a Rosemount icing probe at the summit for ice particles. This hexagonal graupel maintained the and the distinct images of the large droplets con®rmed distinguishing ``V'' signature of planar crystals in the that a hazard existed and was detected by the radar's RHI scans of SLDR*-45 (Fig. 10c), making it distin- polarization measurement. guishable from drizzle, although the depolarizations at low antenna elevations were greatly suppressed. For ex- 9. Conclusions ample, at an antenna elevation as low as 30Њ, SLDR* -45 was still a very distinguishable 2±4 dB above the Supercooled droplets of drizzle size are known to Ϫ29 dB drizzle signature (``hexagonal graupel;'' Figs. present a potentially severe aircraft icing hazard. An 10c and 11). The re¯ectivity of this graupel was ϩ2to approximation to an optimal radar polarization state has ϩ8dBZ. been sought for differentiating among drizzle and the various types of ice particles. The functional dynamic range in depolarization ratio, DR, varies according to 8. A test case for supercooled drizzle detection polarization state and determines the potential separa- tion of signatures for ice crystals and drizzle. The op- On 14 April 1999, an aircraft icing alert would have timization requires compromise due to trade-offs be- been issued if based on algorithms derived from the tween signal power of the weak-channel return and the radar's depolarization signature and cloud temperature functional dynamic range, and it requires consideration measurements. The supporting data show that this alert of the effect of variations of particle settling orientation, would have been justi®ed. which differs among the polarization states. A10msϪ1 upslope ¯ow below ϳ1.7 km AGL (rel- A45Њ slant quasi-linear polarization state, for mea- ative to the radar site) generated a cloud with vertically surement of SLDR*-45, was considered a reasonable integrated liquid water reaching 0.5±0.6 mm by 2000 option because of its practical simplicity, direct rela- UTC. A CLASS rawinsonde from the site at that time tionship to the single-horizontal polarization of opera- revealed an adiabatic saturated layer supercooled to tional radars, the SLDR*-45 value for drizzle-sized Ϫ6ЊC at its base at ϳ1.3 km AGL and to Ϫ11ЊC at the droplets at a decibel level above the radar antenna's base of a capping inversion near 2.1 km AGL. The cross-talk, good ``weak'' or cross-channel sensitivity to inversion, itself, was saturated to ϳ2.5 km AGL at Ϫ11Њ low re¯ectivity clouds, relative insensitivity to varia- to Ϫ10ЊC. This temperature regime is optimal for air- tions in ice crystal canting angle, and a retained wide craft icing. The cloud above approximately 1.7 km, in effective dynamic range that can be utilized for superior cross-slope ¯ow and comprised predominantly of non- separation of hydrometeor types compared to that in precipitating planar crystals, dissipated quickly after LDR, for particles with a preferred horizontal orienta- 2000 UTC, leaving only the upslope cloud, which per- tion. These features are substantiated by our scattering sistently engulfed the summit as MWO weather ob- calculations and measurements. Deterministic, measur- servers recorded ``fog'' and ``riming.'' able differentiation was achieved with this polarization, Large supercooled droplets, the drizzle-sized droplets in that the MWISP measurements of SLDR*-45 differ- that are most likely to be a very signi®cant icing hazard, entiate among the crystals of the various regular planar were consistently imaged with the 2DGC PMS probe and columnar growth habits and among the more irreg- at the summit (Fig. 13). The diameters of these drizzle ular ice particles, and segregate drizzle from all of these drops were predominantly in the 50±250-m range; the ice particles. For droplets and the ice particles of regular modal and median diameters were both ϳ150 m, and growth habits, which can be quite accurately modeled, the median volume diameter was ϳ185 m. The droplet these measurements of SLDR*-45 show extremely good images indicate slight distortion due to some mismatch agreement with the theoretical scattering calculations between the sampling rate and wind speed, but the drop- that model the depolarization ratios, to within about Ϯ1 lets were very nearly spherical. These were engulfed to 2 dB. Thus, this and supporting previous studies in with tiny cloud droplets, which accounted for the ob- the series con®rm that the depolarization of transmitted servation of fog. Together, these caused a re¯ectivity of polarized radiation by the regular ice crystals and drizzle only 0 to Ϫ10 dBZ (Fig. 10d), and the SLDR*-45 sig- can be well predicted by theory and uniquely separated nature was clearly that of drizzle, independent of and from one another in corresponding measurements in DR.
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FIG. 13. PMS 2DGC images of drizzle-sized droplets, predominantly of 50±250-m diameters (particles with smooth circumference and, commonly, a bright center or ``doughnut'' appearance) and rare ice pellets of similar size (less regular particles with ragged circumference), sampled at MWO, 1945±2015 UTC 14 Apr 1999; width of image volume is 800 m. Appearance of non- sphericity of the droplets is due to a slight mismatch between the wind speed and data recording rate of the PMS probe.
The many measurements show that these results are zle-sized droplets (Krop¯i et al. 1995), some of the crys- repeatable. tals of regular growth habits and the more spherical and Such pristine ice crystals do form regularly in clouds, irregular particles become too large for the depolariza- but they usually transition to aggregated or rimed stages tion to be strictly described by Rayleigh scattering. Even of snow¯ake or graupel development. Therefore the re- so, Matrosov et al. (1996) and this study show that such sults showing good differentiation among the irregular particles do cause depolarization values that are quan- type of particles are equally important. Also, the ag- titatively related to DR from scattering by the same gregates are consistently well distinguished from parent fundamental shapes of particles in the Rayleigh regime, planar crystals. However, the signatures for some grau- and these depolarizations can be estimated by experi- pel in the irregular category do tend to overlap with mental measurements. some of the signatures of the regular columnar crystal Overall, our experience with the observations has in- types. This introduces some uncertainty in the deter- dicated that reasonable averaging of data will allow us ministic identi®cation of these selective particles with to detect and distinguish ice particles having DR values DR alone, although it is clear that the statistical variance about 2 dB different from the value expected and mea- of DR with elevation angle is large for graupel and small sured for water droplets. Both the 45Њ tilt and the slight of the particle types, so this helps in making the dis- ellipticity of the transmitted signal enhance the cross- tinction. There are some other uncertainties. The irreg- polarized return for ice particles over that obtainable in ular particles collectively are not so accurately modeled, horizontal LDR. The slight rather than large ellipticity so no ®rm prediction of deterministic values for these is also important. The dynamic range for particle sep- particles is possible, although some estimates of the aration in DR reaches its minimum when ϭ0.5, so general effects on DR have been calculated. And for very elliptical states near this one are restricted, al-
Ka-band radar, which is near optimal for detecting driz- though they do result in comparable return signals in
Unauthenticated | Downloaded 10/01/21 04:03 AM UTC 320 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 19 both receiving channels, maximizing the polarization ®xed beam allows for a long dwell time (1 min) that, sensitivity to low re¯ectivity clouds, especially clouds combined with a large antenna (3 m) and long pulse spherical drizzle, that is sacri®ced with the true hori- length (1.5 s), will signi®cantly enhance sensitivity to zontal and circular states. approximately Ϫ55 to Ϫ60 dBZ at 10-km range. Thus, The signatures of crystals in horizontal LDR will re- within this range, the radar will be able to measure the semble spheres (and be indistinguishable from drizzle) weak-channel re¯ectivity and therefore the depolariza- if the scatterers have a zero canting angle. Moreover the tion ratio in clouds with strong-channel re¯ectivities as horizontal LDR signature will change signi®cantly as low as approximately Ϫ25 or Ϫ30 dBZ, to uniquely the standard deviation of the canting angle is increased, identify clouds of drizzle and even smaller droplets. thus creating a family of indistinguishable depolariza- Since horizontal homogeneity of just a few kilometers tions for the several types of particles. This depolariza- in the clouds is all that is needed for clear identi®cations tion ratio is as strongly dependent on the three-dimen- in the RHI scanning mode, a temporal continuity of sional crystal canting as it is on particle shape. LDR several minutes in the clouds should suf®ce to make consequently becomes unpredictable because the vari- identi®cations in the ®xed beam mode. An option to ations in canting angle are unknown and unpredictable. alternate every 5 min between the tilted angle and zenith SLDR*-45, by comparison, is very stable, so it will is being incorporated to enhance ice particle identi®- minimize the potential errors in identi®cation due to cation in DR with a second point from the DR- curves unknown variations in canting angle while establishing and to measure spectra of the vertical velocity. The ve- wide separations in depolarization according to hydro- locity measurement opens the possibility for further par- meteor type. This and other important in¯uences de- ticle differentiation by fallspeed. A selection will be scribed in this paper show that the horizontal depolar- made between the slant-quasi-linear state tested here ization ratio, LDR, is a poor choice for the purpose. with good results, and the quasi-circular option, which Introducing the 45Њ slant and measuring SLDR-45 has the new theory indicates could be slightly better. The very signi®cant advantages, and overall SLDR*-45 is a full system will include a microwave radiometer, point- superior choice. ed at the same elevation angle(s) as the radar, to con- Is SLDR*-45 ( ഠ 0.02) the optimal state? SLDR*- tinuously measure the path-integrated cloud liquid water 45 is a compromise between the very elliptical states content. Hourly temperature pro®les to determine su- and the true linear state, and indeed a very good selec- percooling of the liquid will be ingested from an op- tion. The new calculations by Matrosov et al. (2001) erational numerical model, so both of these comple- indicate that a state with ϭ0.97 would improve iso- ments will enhance identi®cation of an icing hazard. In lation of drizzle over SLDR*-45 by a few decibels. all, the Ka-band radar itself is being designed to provide Thus, states as near to circular (e.g., ϭ0.92±0.97) as a continuous time series of the pro®le of the depolar- SLDR*-45 is near to linear should also be considered. ization ratio, re¯ectivity structure, and optionally the The desired polarization state can be achieved without vertical velocity structure of passing clouds within the the use of a phase-retarding plate, by adjusting the phase depth of the troposphere. difference between the transmitted components. The practical aspects of instrumentation hardware and as- Acknowledgments. This research is in response to re- sociated effects on transmitted power differ for the cir- quirements and funding by the Federal Aviation Ad- cular and linear states. Such factors should be weighed, ministration (FAA). The views expressed are those of and simplicity may drive the selection where other dif- the authors and do not necessarily represent the of®cial ferences are small. However, this study has demonstrat- policy of the FAA. Partial support was provided by ed the capability for hydrometeor differentiation and the NOAA/ETL. The major effort of David Korn in data importance of the selection of the polarization state, and processing made these analyses possible. Marcia Poli- the results signi®cantly narrow the ®eld of possible can- tovich of NCAR provided excellent leadership of didates for the optimal state. MWISP. The CLASS soundings provided by NCAR, The practical application of short-range dual-polari- and the hydrometeor imagery provided by Paul Lawson of Spec, Inc., Charles Ryerson of CRREL, and Dean zation Ka-band radar to identify clouds with potential aircraft icing conditions by pro®ling the depolarization Miller of NASA-Glen were also essential to the anal- ratio is discussed by Reinking et al. (2000). Based on yses. Bruce Bartram and Kurt Clark of NOAA/ETL en- the results in this paper and from the previous decade gineered and operated the radar; Carroll Campbell and of studies by ETL, an operational-grade dual-polariza- Jan Gibson developed an outstanding new radar data tion radar is being designed for this purpose and will acquisition and display system that greatly enhanced the soon be built to operate unattended and continuously data from MWISP. (Reinking et al. 2001). 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