Abstract

The technique developed by Gal-Chen in 1978 is used to derive vertical velocities, buoyancy, and pressure perturbations from dual-Doppler radar observations of the planetary boundary layer (PBL). Several approaches to verification are pursued. They include: (a) scan-to-scan temporal continuity of the derived fields; (b) an objective test to find out how well the derived pressure perturbations balance the dynamical equations; (c) comparison of dual-Doppler derived, horizontally averaged fluxes of heat versus in situ measurements and other data sets; and (d) a noteworthy improvement in the quality of the retrieved pressure when tendencies are included.

Previous studies indicate that in order for the method to be viable the radars have to resolve the PBL with at least ten vertical levels. One such event occurred on 27 September 1978 during project PHOENIX, conducted at the Boulder Atmospheric Observatory (BAO) 300 m tower. An inversion above a shallow boundary layer of height around 800 m was eroded, and the PBL grew to a height of 2.4 km in less than half an hour. During that period, the vertical profiles of potential temperature and pressure variance derived from the two NOAA/Wave Propagation Laboratory X-band (3 cm wavelength) Doppler radars suggest the existence of two inversions. Two inversions are also indicated by the aircraft data.

Some aspects of the derived heat flux profiles, such as negative heat flux at the top of the mixed layer, are classical and constitute further evidence of the plausibility of the results. Some other aspects such as positive vertical gradient of the heat flux profile near the first inversion (where the heat flux is still positive) are not commonly observed. Based on the available data, it is speculated that this latter feature is transient, indicative of the mixing (during the growth of the PBL) of the potentially warmer upper layer with the potentially colder lower layer.

Several closure approximations for three-dimensional PBL models are tested. Nonlinear eddy viscosities are derived from the observed second moments of the Doppler spectrum and are used to estimate the frictional dissipation in a three-dimensional numerical model of the PBL. Except near the ground, the derived temperature and pressure are only slightly sensitive to factor-of-two variation in the value of the eddy viscosity. Furthermore, it is found that adding frictional dissipation does not reduce the imbalance between the horizontal pressure gradient and the horizontal accelerations. Recalling that in a “perfect” three-dimensional model exact balance must prevail, one concludes that this particular subgrid parameterization could be merely a device to prevent excessive accumulation of energy in the smallest resolvable scale of a numerical model.

Abstract

Details of the structure of a moderate reflectivity microburst were provided by dual-Doppler radar measurements during the Phoenix II convective boundary layer experiment. The dated allowed high resolution of the descending microburst in both time and space. Thermodynamic fields of virtual potential temperature and buoyancy retrieved from the radar measurements indicated that the downdraft was associated with a minimum in virtual potential temperature, rather than coinciding with a maximum in precipitation loading. The physical separation of the downdraft from the reflectivity maximum was especially pronounced during the later stages of the microburst and was partly due to the tilled reflectivity core descending more rapidly than the downdraft. The downdraft corms also descended at a rate slower than the magnitude of the maximum downdraft so that air was continually converging and entraining into the downdraft above the level of its peak value and was detraining and diverging below it. The retrieved pressure fields and simple analytical calculations showed that this slower descent and internal circulation coincided with an upward-directed pressure form. Simple calculations also suggest that this influence of the pressure force on the vertical accelerations depends strongly on the aspect ratio of the negatively buoyant parce1; horizontally narrow and vertically deep negatively buoyant parcels result in stronger downdraft than wider and shallower parcels. Our study suggests the internal circulation and the relatively slow descent of the peak downdraft should be inherent characteristics of microbursts driven by corms of low virtual potential temperature air, while microbursts driven primarily by water loading could be expected to have a different structure. In the case of the microbursts driven by corms of cool air, observation and recognition of the convergence and divergence associated with the internal circulation provides important precursors to microburst activity. In this study, the Doppler measurements showed that the microburst descending into a stable layer may have enhanced the divergence pattern below the peak downdraft.

Abstract

When observing clouds with radars, there are a number of design parameters, such as transmitted power, antenna size, and wavelength, that can affect the detection threshold. In making calculations of radar thresholds, also known as minimum sensitivities, it is usually assumed that the radar pulse volume is completely filled with targets. In this paper, the issue of partial beam filling, which results, for instance, if a cloud is thin with respect to the pulse length, or measurements are being made near cloud edges, is investigated. This study pursues this question by using measurements of radar reflectivities made with a 35-GHz, surface-based radar with 37.5-m pulse lengths, and computing how reflectivity statistics would be affected if the same clouds and/or precipitation had been observed with a radar with a 450-m pulse length. In a dataset measured during winter over a midcontinental site, partial beamfilling degraded the percentage of clouds detected by about 22% if it was assumed that the minimum detection threshold was −30 dBZ. In a second dataset collected during summer over a summertime subtropical site that was dominated by thin, boundary layer stratus, partial beam filling degraded the percentage of clouds detected by 38%, again assuming a minimum detection threshold of −30 dBZ. This study provides a preliminary indication of how radar reflectivity statistics from a spaceborne cloud radar may be impacted by design constraints, which would mandate a pulse length of around 500 m and a minimum detection threshold of around −30 dBZ.

Abstract

A method is presented that estimates particle fall velocities from Doppler velocity and reflectivity measurements taken with a vertically pointing Doppler radar. The method is applicable to uniform, stratified clouds and is applied here to cirrus clouds. A unique aspect of the technique consists of partitioning the Doppler velocities into discrete cloud height and cloud reflectivity bins prior to temporal averaging. The first step of the method is to temporally average the partitioned Doppler velocities over an hour or two to remove the effects of small-scale vertical air motions. This establishes relationships between particle fall velocity and radar reflectivity at various levels within the cloud. Comparisons with aircraft in situ observations from other experiments show consistency with the remote-sensing observations. These results suggest that particle fall speeds can be determined to within 5–10 cm s⁻¹ by means of this technique.

Abstract

A remote-sensing technique called TRACIR (tracking air with circular-polarization radar) was developed recently for studying air-parcel trajectories in clouds. The technique uses a dual-circular-polarization radar to detect microwave chaff fibers that serve as tracers of the air motion. The radar is able to detect the chaff inside clouds and precipitation by measuring the circular-depolarization ratio, which is much higher for chaff than for hydrometeors. Chaff concentrations are also estimated by the technique, thus permitting turbulent diffusion in clouds to be examined. Demonstrations of TRACIR's capabilities are presented for three cases in which chaff was used to simulate the movement of cloud-seeding nuclei in clouds and precipitation. In two cases involving airborne chaff releases, the gradual drift and diffusion of chaff in a stratiform cloud are contrasted with its abrupt transport and dispersion in a convective cloud. In the third case study, the technique successfully detected a plume of chaff released from the ground in a snowstorm. In each case the radar data provided three-dimensional visualizations of the extent of the chaff region and maps of the chaff concentration with excellent spatial and temporal resolution.

Abstract

A vertically pointing 3.2-cm radar is used to observe altostratus and cirrus clouds as they pass overhead. Radar reflectivities are used in combination with an empirical Z _i -IWC (ice water content) relationship developed by Sassen (1987) to parameterize IWC, which is then integrated to obtain estimates of ice water path (IWP). The observed dataset is segregated into all-ice and mixed-phase periods using measurements of integrated liquid water paths (LWP) detected by a collocated, dual-channel microwave radiometer. The IWP values for the all ice periods are compared to measurements of infrared (IR) downward fluxes measured by a collocated narrowband (9.95 − 11.43 um) IR radiometer, which results in scattergrams representing the observed dependence of [R fluxes on IWP. A two-strum model is used to calculate the infrared fluxes expected from ice clouds with boundary conditions specified by the actual clouds, and similar curves relating IWP and infrared fluxes are obtained. The model and observational results suggest that IWP is one of the primary controls on infrared thermal fluxes for ice clouds.

Abstract

Model calculations and measurements of the specific propagation and backscatter differential phase shifts (K _DP and δ _o , respectively) in rain are discussed for X- (λ ∼ 3 cm) and K_a-band (λ ∼ 0.8 cm) radar wavelengths. The details of the drop size distribution have only a small effect on the relationships between K _DP and rainfall rate R. These relationships, however, are subject to significant variations due to the assumed model of the drop aspect ratio as a function of their size. The backscatter differential phase shift at X band for rain rates of less than about 15 mm h⁻¹ is generally small and should not pose a serious problem when estimating K _DP from the total phase difference at range intervals of several kilometers. The main advantage of using X-band wavelengths compared to S-band (λ ∼ 10–11 cm) wavelengths is an increase in K _DP by a factor of about 3 for the same rainfall rate. The relative contribution of the backscatter differential phase to the total phase difference at K_a band is significantly larger than at X band. This makes propagation and backscatter phase shift contributions comparable for most practical cases and poses difficulties in estimating rainfall rate from K_a-band measurements of the differential phase.

Experimental studies of rain using X-band differential phase measurements were conducted near Boulder, Colorado, in a stratiform, intermittent rain with a rate averaging about 4–5 mm h⁻¹. The differential phase shift approach proved to be effective for such modest rains, and finer spatial resolutions were possible in comparison to those achieved with similar measurements at longer wavelengths. A K _DP–R relation derived for the mean drop aspect ratio (R = 20.5 K ^0.80 _DP ) provided a satisfactory agreement between rain accumulations derived from radar measurements of the differential phase and data from several nearby high-resolution surface rain gauges. For two rainfall events, radar estimates based on the assumed mean drop aspect ratio were, on average, quite close to the gauge measurements with about 38% relative standard deviation of radar data from the gauge data.

Abstract

An approach is suggested to relate measurements of radar depolarization ratios and aspect ratios of predominant hydrometeors in nonprecipitating and weakly precipitating layers of winter clouds. The trends of elevation angle dependencies of depolarization ratios are first used to distinguish between columnar-type and plate-type particles. For the established particle type, values of depolarization ratios observed at certain elevation angles, for which the influence of particle orientation is minimal, are then used to estimate aspect ratios when information on particle effective bulk density is assumed or inferred from other measurements. The use of different polarizations, including circular, slant-45° linear, and two elliptical polarizations, is discussed. These two elliptical polarizations are quasi-circular and quasi-linear slant-45° linear, and both are currently achievable with the National Oceanic and Atmospheric Administration Environmental Technology Laboratory’s K_a-band radar. In comparison with the true circular and slant-45° linear polarizations, the discussed elliptical polarizations provide a stronger signal in the “weak” radar receiver channel; however, it is at the expense of diminished dynamic range of depolarization ratio variations. For depolarization measurements at the radar elevation angles that do not show much sensitivity to particle orientations, the available quasi-circular polarization provides a better depolarization contrast between nonspherical and spherical particles than does the available quasi-linear slant-45°polarization. The use of the proposed approach is illustrated with the experimental data collected during a recent field experiment. It is shown that it allows successful differentiation among pristine planar crystals, rimed planar crystals, long columns, blocky columns, and graupel. When a reasonable assumption about particle bulk density is made, quantitative estimates of particle aspect ratios from radar depolarization data are in good agreement with in situ observations. Uncertainties of particle aspect ratios estimated from depolarization measurements due to 0.1 g cm⁻³ variations in the assumed bulk density are about 0.1.

Abstract

An approach to distinguish between various types of ice hydrometeors and to estimate their shapes using radar polarization measurements is discussed. It is shown that elevation angle dependencies of radar depolarization ratios can be used to distinguish between planar crystals, columnar crystals, and aggregates in reasonably homogeneous stratiform clouds. Absolute values of these ratios depend on the reflectivity-weighted mean particle aspect ratio in the polarization plane. Circular depolarization ratios depend on this ratio, and linear depolarization ratios depend on this ratio and particle orientation in the polarization plane. The use of nearly circular elliptical polarization provides a means of measuring depolarization for low reflectivity scatterers when the circular polarization fails due to low signal level in one of the receiving channels. Modeling of radar backscattering was applied to the elliptical depolarization ratios as measured by the K_a-band radar developed at the NOAA Environmental Technology Laboratory. Experimental data taken during the Winter Icing and Storms Instrument Test experiment in 1993 generally confirmed the calculations and demonstrated the applicability of the approach.

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 influence 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 influenced 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-reflectivity clouds, and insensitivity to oscillations in the settling orientations of ice crystals. A 45° 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 verified experimentally in field measurements conducted during the Mount Washington Icing Sensors Project (MWISP). A selectable-dual-polarization K_a-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. Additionally, although less rigorously predicted by theory, the field measurements demonstrated substantial differentiation 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 verified.