Doppler lidar measurements of oriented planar ice crystals falling from supercooled and glaciated layer clouds

The properties of planar ice crystals settling horizontally have been investigated using a vertically-pointing Doppler lidar. Strong specular reflections were observed from their oriented basal facets, identified by comparison with a second lidar pointing 4deg from zenith. Analysis of 17 months of continuous high-resolution observations reveal that these pristine crystals are frequently observed in ice falling from mid-level mixed-phase layer clouds (85% of the time for layers at -15C). Detailed analysis of a case study indicates that the crystals are nucleated and grow rapidly within the supercooled layer, then fall out, forming well-defined layers of specular reflection. From the lidar alone the fraction of oriented crystals cannot be quantified, but polarimetric radar measurements confirmed that a substantial fraction of the crystal population was well oriented. As the crystals fall into subsaturated air, specular reflection is observed to switch off as the crystal faces become rounded and lose their faceted structure. Specular reflection in ice falling from supercooled layers colder than -22C was also observed, but was much less pronounced than at warmer temperatures: we suggest that in cold clouds it is the small droplets in the distribution that freeze into plates and produce specular reflection, whilst larger droplets freeze into complex polycrystals. The lidar Doppler measurements show that typical fall speeds for the oriented crystals are 0.3m/s, with a weak temperature correlation; the corresponding Reynolds number is Re~10, in agreement with light-pillar measurements. Coincident Doppler radar observations show no correlation between the specular enhancement and eddy dissipation rate, indicating that turbulence does not control crystal orientation in these clouds.Comment: revised manuscript for Q. J. Royal Met So

Doctor of Philosophy

dissertationInterpretations of remote sensing measurements collected in sample volumes containing ice-phase hydrometeors are very sensitive to assumptions regarding the distributions of mass with ice crystal dimension, otherwise known as mass-dimensional (m-D) relationships. The uncertainties from these assumptions extend to backscattered cross-sections and radar forward modeled reflectivity factors. These uncertainties and m-D variability were derived using an optimal estimation (OE) algorithm applied to reflectivity factors measured by CloudSat and combined with particle size distributions (PSDs) collected by coincident in-situ aircraft during SPartICus. This OE algorithm minimized the difference between observed radar reflectivity and PSD calculated reflectivity, to output optimal m-D relationships per PSD. I found that ice crystal populations tend to be distributed over a continuum-defying simple categorization. Also, the quantified uncertainties in backscatter cross-section and reflectivity factors can be appropriately applied to remote sensing algorithms. Further investigation of the ice particle m-D relationship was studied with in-situ measurements collected during TC4. Two OE algorithms were used -- one algorithm minimized radar reflectivity (MZ), the other minimized observed ice water content (IWC) and PSD calculated IWC (XIWC). The XIWC results show that both parameters in the m-D relationship increase with temperature. With the prefactor varying by a factor of 5 and the exponent varying by some 16% over a typical range of ice cloud temperatures, forward modeling errors in radar reflectivity could be in excess of 5 dB, further suggesting that retrievals of precipitation rates from radar measurements in ice clouds be in error by factors easily exceeding 3. The MZ algorithm, adjusted for slant radar incidence, was applied to in-situ and radar data collected in mountainous terrain during StormVEx. The outputs of the MZ algorithm here were analyzed along with the enhancement of backscatter (EB) cross-section in the zenith and slant 45º depolarization ratio (DR). Statistics of the results show that forward model errors can create reflectivity differences around 7 dB compared to using fixed m-D relationships, resulting in snowfall rate differences of 1.7 mm per hour. An inverse (direct) relationship between the m-D prefactor and slant 45º DR (zenith EB) can help improve radar-based retrievals by reducing forward model errors

Cloud radar with hybrid mode towards estimation of shape and orientation of ice crystals

This paper is devoted to the experimental quantitative characterization of the shape and orientation distribution of ice particles in clouds. The characterization is based on measured and modeled elevation dependencies of the polarimetric parameters differential reflectivity and correlation coefficient. The polarimetric data are obtained using a newly developed 35 GHz cloud radar MIRA-35 with hybrid polarimetric configuration and scanning capabilities. The full procedure chain of the technical implementation and the realization of the setup of the hybrid-mode cloud radar for the shape determination are presented. This includes the description of phase adjustments in the transmitting paths, the introduction of the general data processing scheme, correction of the data for the differences of amplifications and electrical path lengths in the transmitting and receiving channels, the rotation of the polarization basis by 45°, the correction of antenna effects on polarimetric measurements, the determination of spectral polarimetric variables, and the formulation of a scheme to increase the signal-to-noise ratio. Modeling of the polarimetric variables is based on existing back-scattering models assuming the spheroidal representation of cloud scatterers. The parameters retrieved from the model are polarizability ratio and degree of orientation, which can be assigned to certain particle orientations and shapes. The developed algorithm is applied to a measurement of the hybrid-mode cloud radar taken on 20 October 2014 in Cabauw, the Netherlands, in the framework of the ACCEPT (Analysis of the Composition of Clouds with Extended Polarization Techniques) campaign. The case study shows the retrieved polarizability ratio and degree of orientation of ice particles for a cloud system of three cloud layers at different heights. Retrieved polarizability ratios are 0.43, 0.85, and 1.5 which correspond to oblate, quasi-spherical, and columnar ice particles, respectively. It is shown that the polarizability ratio is useful for the detection of aggregation/riming processes. The orientation of oblate and prolate particles is estimated to be close to horizontal while quasi-spherical particles were found to be more randomly oriented

The FIRE Cirrus Science Results 1993

FIRE (First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment) is a U.S. cloud-radiation research program that seeks to improve our basic understanding and parameterizations of cirrus and marine stratocumulus cloud systems and ISCCP data products. The FIRE Cirrus Science Conference was held in Breckenridge, CO, 14-17 Jun. 1993, to present results of cirrus research for the second phase of FIRE (1989-present) and to refine cirrus research goals and priorities for the next phase of FIRE (1994-future). This Conference Publication contains the text of short papers presented at the conference. The papers describe research analyses of data collected at the Cirrus Intensive Field Observations-2 field experiment conducted in Kansas, 13 Nov. - 7 Dec. 1991

Midlatitude Cirrus Cloud Structural Properties Analyzed From The Extended Facility For Atmospheric Remote Sensing Dataset

Thesis (Ph.D.) University of Alaska Fairbanks, 2004The knowledge on cirrus inhomogeneous structural properties is important not only in radiation calculations, but also in deeply understanding the dynamics mechanism including the formation, development, and dissipation of cirrus clouds. The midlatitude cirrus inhomogeneous structural properties have been evaluated by analyzing the 10-year high cloud datasets obtained at the University of Utah, Facility for Atmospheric Remote Sensing in Salt Lake City, UT. Three goals have been reached in this research. First, the means to analyze lidar data using wavelet analysis, an advanced approach to obtain information on the structure of cirrus clouds, has been successfully developed. And then, typical cirrus structures including Kelvin-Helmholtz instabilities, cirrus mammata, and the uncinus cells have been analyzed by case studies and statistical survey. Their dynamical mechanisms, environmental characteristics, and vertical and horizontal length scale have been studied. Thirdly, using the method based on the wavelet transform and other methods, a climatology of midlatitude cirrus horizontal inhomogeneous properties is developed from the FARS lidar backscattered power data, the proxies of real cirrus clouds

Ice clouds over Fairbanks, Alaska

Thesis (M.S.) University of Alaska Fairbanks, 2014Arctic clouds have been recognized long ago as one of the key elements modulating the global climate system. They have gained much interest in recent years because the availability of new continuous datasets is opening doors to explore cloud and aerosol properties as never before. This is particularly important in the light of current climate change studies that predict changing weather scenarios around the world. This research investigates the occurrence and properties of a few types of ice clouds over the Arctic region with datasets available through the Arctic Facility for Atmospheric Remote Sensing (AFARS; 64.86° N, 147.84° W). This study exclusively focuses on ice clouds that form in the upper (cirrus clouds) and midlevels of the troposphere, and that are transparent to laser pulses (visible optical depth τ< 3.0 - 4.0). Cirrus clouds are icedominated clouds that are formed in the upper levels of the troposphere and are relatively thin such that their visual appearances range from bluish to gray in color. Mid-level ice clouds are those clouds primarily composed of ice crystals forming in the midlevels of the troposphere. It is hypothesized that unlike the basic midlevel cloud type (altostratus), other varieties of midlevel ice clouds exist at times over the Arctic region. The midlevel ice clouds studied here are also transparent to laser pulses and sometimes appear as a family of cirrus clouds to a surface observer. Because of their intermediate heights of occurrence in the troposphere, these could have microphysical properties and radiative effects that are distinct from those associated with upper level ice clouds in the troposphere. A ground-based lidar dataset with visual observations for identifying cloud types collected at AFARS over eight years is used to investigate this hypothesis. Cloud types over AFARS have been identified by a surface observer (Professor Kenneth Sassen) using established characteristics traits. Essential macrophysical properties of the clouds are derived from the lidar data, which serves as a climatological representation for the visually identified cirrus and mid-level ice clouds over a typical sub-Arctic location. Synoptic-scale weather patterns conducive for such cloud type formations are derived using a clustering technique applied to a re-analysis dataset. The cloud properties derived from ground-based lidar over AFARS are used to assess the cloud observations from the CALIPSO satellite.Chapter 1. Introduction -- Chapter 2. Ice clouds over the AFARS site (64.86° N, 147.84° W) -- Chapter 3. Ice clouds over Fairbanks: influence of weather patterns -- Chapter 4. Ice clouds over Fairbanks: ground-based and satellite view -- Chapter 5. Conclusions -- References

Abstracts of papers presented at the Eleventh International Laser Radar Conference

Abstracts of 39 papers discuss measurements of properties from the Earth's ocean surface to the mesosphere, made with techniques ranging from elastic and inelastic scattering to Doppler shifts and differential absorption. Topics covered include: (1) middle atmospheric measurements; (2) meteorological parameters: temperature, density, humidity; (3) trace gases by Raman and DIAL techniques; (4) techniques and technology; (5) plume dispersion; (6) boundary layer dynamics; (7) wind measurements; visibility and aerosol properties; and (9) multiple scattering, clouds, and hydrometers

Polarized signals from oriented frozen hydrometeors using passive microwave radiometry

Ice clouds play a significant role in energy budget of the earth-atmosphere system, and they also participate in global hydrological cycle. Thick ice clouds which are associated with precipitation transfer energy and water between the atmosphere and the earth. The net effects of ice clouds on the earth-atmosphere system highly depend on their microphysical properties. However, the complex and variable structure of ice clouds makes it difficult to capture them well in models. The oversimplified microphysical properties of ice clouds in retrievals introduce significant uncertainties in weather and climate studies. The knowledge on the orientation of ice particles is very limited. The orientation of frozen hydrometeors which induces polarization signatures determines the magnitude of polarized signals. In order to investigate the potential polarized signatures induced by the oriented frozen hydrometeors, ground-based polarization observations have been performed at “Umweltforschungsstation Schneefernerhaus” (UFS) on Mount Zugspitze (German Alps) at 2650 m above sea level. In this study, the polarization observations carried out by a ground-based dual polarized microwave radiometer (DPR) at 150 GHz are investigated together with auxiliary instruments deployed at UFS, i.e., a second microwave radiometer (HATPRO) and a K-band micro rain radar (MRR). HATPRO measures liquid water path (LWP) and integrated water vapor (IWV) during snowfall, and MRR operating at 24.1 GHz provides indirect snow water path (SWP) information. Based on the observations, the analysis of a single snow case and one-year snowfall data show that the brightness temperature (TB) differences between the vertical and horizontal polarizations reach up to −10 K at an elevation angle of 34.8^o during snowfall. The polarized signals during snowfall can be explained well by the occurrence of oriented snow particles. The analysis of the synergic observations shows the effects of snowfall parameters on polarization differences (PDs) observed with DPR at 150 GHz. The dependencies of the measured PD and TB on MRR integrated radar reflectivity and independently derived LWP are discussed. It shows that the high SWP indicated by high values of MRR integrated reflectivity enhances both TB and PD due to the scattering effects of snow particles. Meanwhile, TBs are found to be enhanced during snowfall when supercooled liquid water is present, while PD resulting from oriented snow particles is damped by the increase of LWP. The polarization observations support the potential role of polarization measurements in improving retrievals of snowfall microphysical parameters. To evaluate the effects of SWP and LWP on PD and TB, radiative transfer (RT) simulations assuming horizontally aligned snow oblates using a polarized RT model have been performed. PD and TB observations can be captured well by the RT model with given reasonable assumptions on the microphysical parameters of oriented snow oblates. Additionally, the uncertainties of PD and TB caused by snow microphysical properties are fully examined in the RT simulations. The “damping (enhancing)” effects of supercooled liquid water on PD (TB) are further interpreted by a simple physical model where the height of cloud liquid varies with respect to the dichroic snow layer. From the ground-based observations, it is found that PD resulting from oriented snow particles is absorbed by supercooled liquid below snow layers. When supercooled liquid water is located above snow layers, PD is damped since it is compensated by the emission of supercooled liquid water penetrating the snow layer. TB is generally enhanced by the presence of LWP: the warmer the supercooled liquid water, the larger the TB. The polarization observations promote the design of new instruments further. Under an assumption that ice particles are oriented, RT simulations are performed for the space-borne satellite FengYun-4 (FY-4) channels to examine polarization information content for ice cloud characterization. The results show that polarization can be beneficial for ice cloud retrievals and additional information can be provided by polarized signals to quantify ice cloud parameters, especially at high frequency channels. Therefore, the present work strongly suggests the deployment of microwave polarization channels for ice cloud observations

Lidar Observations of the Optical Properties and 3-Dimensional Structure of Cirrus Clouds

The scientific research conducted under this grant have been reported in a series of journal articles, dissertations, and conference proceedings. This report consists of a compilation of these publications in the following areas: development and operation of a High Spectral Resolution Lidar, cloud physics and cloud formation, mesoscale observations of cloud phenomena, ground-based and satellite cloud cover observations, impact of volcanic aerosols on cloud formation, visible and infrared radiative relationships as measured by satellites and lidar, and scattering cross sections