Abstract

It has been shown previously that the use or elliptically polarized radar signals can help to obtain polarization signatures from tenuous cirrus clouds. Estimates of particle dimension ratios and orientations can be made in situations where conventional circular and linear polarizations fail because of weak echoes in one of the received polarization channels. One way of achieving elliptical polarizations is to install a quarter wave plate before the radar transmitter. This paper introduces two new easily measurable elliptical polarization parameters. These parameters are 1) the depolarization ratio for elliptical polarizations that slightly differ from the circular polarization, and 2) the difference in quarter wave plate angular positions that provide equal returns in both received polarization channels. The use of the elliptical depolarization ratio increases echo in the “weak” channel without significant change of echo in the “main” channel. This ratio is used to estimate ice particle deformity (deviation from sphericity). The second suggested parameter can be measured during continuous rotations of the quarter wave plate. It can be used to estimate the combined effect of the degree of common alignment of the ice crystals and their deformity. Operational procedures and possible data interpretations are discussed with emphasis on cloud sensing with Ka-band radar.

Abstract

This paper presents new methods for rainfall estimation from X-band dual-polarization radar observations along with advanced techniques for quality control, hydrometeor classification, and estimation of specific differential phase. Data collected from the Hydrometeorology Testbed (HMT) in orographic terrain of California are used to demonstrate the methodology. The quality control and hydrometeor classification are specifically developed for X-band applications, which use a “fuzzy logic” technique constructed from the magnitude of the copolar correlation coefficient and the texture of differential propagation phase. In addition, an improved specific differential phase retrieval and rainfall estimation method are also applied. The specific differential phase estimation is done for both the melting region and rain region, where it uses a conventional filtering method for the melting region and a self-consistency-based method that distributes the total differential phase consistent with the reflectivity factor for the rain region. Based on the specific differential phase, rainfall estimations were computed using data obtained from the NOAA polarimetric X-band radar for hydrometeorology (HYDROX) and evaluated using HMT rain gauge observations. The results show that the methodology works well at capturing the high-frequency rainfall variations for the events analyzed herein and can be useful for mountainous terrain applications.

Abstract

This paper describes a new method to retrieve vertical profiles of the parameters of cirrus cloud microphysics that are important for the estimation of climatic feedback. These parameters are the particle characteristic size and ice mass content. The method also allows calculations of vertical profiles of particle concentrations and ice mass flux. The method uses measurements of radar reflectivities and Doppler velocities from the ground-based zenith-viewing radar combined with measurements of downwelling brightness temperatures from an infrared radiometer operating in the “window” (10–12 µm) region. The proposed method is illustrated on data obtained on 26 November 1991 during FIRE-II [First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment] in Coffeyville, Kansas. This paper also presents estimates of uncertainties of parameter retrieval due to different a priori assumptions about particle shapes, distributions, fall velocity-size relationships and due to errors in measurements. Comparisons with in situ measurements showed reasonable agreement.

Abstract

Networks of radars that point almost vertically and continuously measure the vertical profile of the horizontal wind will, in the future, be operated at many locations around the world. Although such radars are designed to measure the Doppler-sensed movement of clear-air refractive-index inhomogeneities, they are an exceptional tool for sensing precipitating ice and water particles in clouds. Because of the low detection threshold and long averaging time of these radars water-particle-size distributions can be measured down to 100-µm diameter and mean vertical fall velocities V_f as small as 0.2 m s⁻¹ can be accurately measured. In this paper, data are presented from two events in which clouds form, intensify, and finally produce precipitation. Height profiles are analyzed in terms of ZRV_f plots versus height, where Z is the radar reflectivity factor and R is liquid flux (rainfall rate). The observations provide new insight into drop-growth and breakup processes. Special attention is given to the transition zone through the melting level. It is shown how these radars can provide 1) cloud-layer structure above lower overcast, 2) height profiles of liquid mean drop size, 3) the ice-water transition level compared with the 0° isotherm, 4) height profiles of rain rate, and 5) inferences about the identity of hydrometeors versus height.

Abstract

The paper presents the results of retrieving characteristic particle sizes for the November 26 1991 FIRE II case using two methods that utilize ground-based remotes sensors. The size information for the complete vertical depth of the cloud was obtained for a 3-hour period from 1830 to 2130 UTC using combined Doppler radar and IR radiometer measurements and for two shorter periods using radar reflectivity and CO₂ lidar backscatter measurements. The results obtained with both remote sensing techniques are compared for these two periods. Possible retrieval uncertainties are discussed. Comparisons yielded an agreement with a relative standard deviation of 15%-20% between the two methods. Particle sizes retrieved by both methods were compared with 2D particle probe data sampled during 10 time intervals when a research aircraft was crossing the hub area. The relative standard deviation of particle sizes retrieved with the radar-radiometer method from those obtained from 2D probes is about 30% for nine compared times. The corresponding deviation for the lidar-radar method is about 35% for three compared times. The relative standard deviation between particle concentrations retrieved with the radar-radiometer method and those obtained from 2D probes is about 60% for nine compared times.

Abstract

The microphysical characteristics, radiative impact, and life cycle of a long-lived, surface-based mixed-layer, mixed-phase cloud with an average temperature of approximately −20°C are presented and discussed. The cloud was observed during the Surface Heat Budget of the Arctic experiment (SHEBA) from 1 to 10 May 1998. Vertically resolved properties of the liquid and ice phases are retrieved using surface-based remote sensors, utilize the adiabatic assumption for the liquid component, and are aided by and validated with aircraft measurements from 4 and 7 May. The cloud radar ice microphysical retrievals, originally developed for all-ice clouds, compare well with aircraft measurements despite the presence of much greater liquid water contents than ice water contents. The retrieved time-mean liquid cloud optical depth of 10.1 ± 7.8 far surpasses the mean ice cloud optical depth of 0.2, so that the liquid phase is primarily responsible for the cloud’s radiative (flux) impact. The ice phase, in turn, regulates the overall cloud optical depth through two mechanisms: sedimentation from a thin upper ice cloud, and a local ice production mechanism with a time scale of a few hours, thought to reflect a preferred freezing of the larger liquid drops. The liquid water paths replenish within half a day or less after their uptake by ice, attesting to strong water vapor fluxes. Deeper boundary layer depths and higher cloud optical depths coincide with large-scale rising motion at 850 hPa, but the synoptic activity is also associated with upper-level ice clouds. Interestingly, the local ice formation mechanism appears to be more active when the large-scale subsidence rate implies increased cloud-top entrainment. Strong cloud-top radiative cooling rates promote cloud longevity when the cloud is optically thick. The radiative impact of the cloud upon the surface is significant: a time-mean positive net cloud forcing of 41 W m⁻² with a diurnal amplitude of ∼20 W m⁻². This is primarily because a high surface reflectance (0.86) reduces the solar cooling influence. The net cloud forcing is primarily sensitive to cloud optical depth for the low-optical-depth cloudy columns and to the surface reflectance for the high-optical-depth cloudy columns. Any projected increase in the springtime cloud optical depth at this location (76°N, 165°W) is not expected to significantly alter the surface radiation budget, because clouds were almost always present, and almost 60% of the cloudy columns had optical depths >6.

Abstract

Simultaneous multiwavelength measurements of a developing cloud system were obtained by NOAA Doppler lidar, Doppler radar, Fourier transform infrared interferometer, and microwave and infrared radiometers on 26 November 1991. The evolution of the cloud system is described in terms of lidar backscatter, radar reflectivity and velocity, interferometer atmospheric spectra, and radiometer brightness temperature, integrated liquid water, and water vapor paths. Utilizing the difference in wavelength between the radar and lidar, and therefore their independent sensitivity to different regions of the same cloud, the cloud top, base, depth, and multiple layer heights can he determined with better accuracy than with either instrument alone. Combining the radar, lidar, and radiometer measurements using two different techniques allows an estimation of the vertical profile of cloud microphysical properties such as particle sizes. Enhancement of lidar backscatter near zenith revealed when highly oriented ice crystals were present. The authors demonstrate that no single instrument is sufficient to accurately describe cirrus clouds and that measurements in combination can provide important details on their geometric, radiative, and microphysical properties.

Abstract

Observations from a wide variety of instruments and platforms are used to validate many different aspects of a three-dimensional mesoscale simulation of the dynamics, cloud microphysics, and radiative transfer of a cirrus cloud system observed on 26 November 1991 during the second cirrus field program of the First International Satellite Cloud Climatology Program (ISCCP) Regional Experiment (FIRE-II) located in southeastern Kansas. The simulation was made with a mesoscale dynamical model utilizing a simplified bulk water cloud scheme and a spectral model of radiative transfer. Expressions for cirrus optical properties for solar and infrared wavelength intervals as functions of ice water content and effective particle radius are modified for the midlatitude cirrus observed during FIRE-II and are shown to compare favorably with explicit size-resolving calculations of the optical properties. Rawinsonde, Raman lidar, and satellite data are evaluated and combined to produce a time–height cross section of humidity at the central FIRE-II site for model verification. Due to the wide spacing of rawinsondes and their infrequent release, important moisture features go undetected and are absent in the conventional analyses. The upper-tropospheric humidities used for the initial conditions were generally less than 50% of those inferred from satellite data, yet over the course of a 24-h simulation the model produced a distribution that closely resembles the large-scale features of the satellite analysis. The simulated distribution and concentration of ice compares favorably with data from radar, lidar, satellite, and aircraft. Direct comparison is made between the radiative transfer simulation and data from broadband and spectral sensors and inferred quantities such as cloud albedo, optical depth, and top-of-the-atmosphere 11-µm brightness temperature, and the 6.7-µm brightness temperature. Comparison is also made with theoretical heating rates calculated using the rawinsonde data and measured ice water size distributions near the central site. For this case study, and perhaps for most other mesoscale applications, the differences between the observed and simulated radiative quantities are due more to errors in the prediction of ice water content, than to errors in the optical properties or the radiative transfer solution technique.

Many of the clouds important to the Earth's energy balance, from the Tropics to the Arctic, contain small amounts of liquid water. Longwave and shortwave radiative fluxes are very sensitive to small perturbations of the cloud liquid water path (LWP), when the LWP is small (i.e., < 100 g m⁻²; clouds with LWP less than this threshold will be referred to as “thin”). Thus, the radiative properties of these thin liquid water clouds must be well understood to capture them correctly in climate models. We review the importance of these thin clouds to the Earth's energy balance, and explain the difficulties in observing them. In particular, because these clouds are thin, potentially mixed phase, and often broken (i.e., have large 3D variability), it is challenging to retrieve their microphysical properties accurately. We describe a retrieval algorithm intercomparison that was conducted to evaluate the issues involved. The intercomparison used data collected at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site and included 18 different algorithms to evaluate their retrieved LWP, optical depth, and effective radii. Surprisingly, evaluation of the simplest case, a single-layer overcast stratocumulus, revealed that huge discrepancies exist among the various techniques, even among different algorithms that are in the same general classification. This suggests that, despite considerable advances that have occurred in the field, much more work must be done, and we discuss potential avenues for future research.)