Drop Axis Ratio Distributions in Stratiform and Convective Rain

A fully calibrated low profile 2D video disdrometer (2DVD) has been recording many different rainfall events in Northern Alabama (USA) since June 2007. An earlier publication reported drop shapes and axis ratio distributions determined for some of the events. For one of the cases examined, a noticeable shift in the 3.5 - 3.75 mm drop axis ratio distribution was noted. In this paper, we extend the earlier work by separating the 2DVD measurements into stratiform and convective rain. The separation is made possible by using the minute-by-minute drop size distribution (DSD) measured by the 2DVD. The 1-minute DSDs are fitted to a gamma distribution, and using a simple indexing technique which involves two of the fitted parameters, periods of convective and stratiform rain are separated for a given event. The output of the DSD indexing technique is qualitatively confirmed by comparing with simultaneous time series observations from a co-located UHF profiler which continuously records height profiles of reflectivity, Doppler mean and spectral width, all of which enable the identification of bright-band periods and, furthermore, periods of moderate and deep convection. Excellent consistency is found between the output of the DSD-based separation method and the profiler observations. Next, we utilize the output of DSD index-based separation method to flag the periods of severe convection for a given event. Drop axis ratios during the flagged periods are derived and compared with those during stratiform rain periods. Five cases have been considered. Axis ratio distributions do not show appreciable differences between stratiform and convective periods for four of the cases. The fifth case (the same case as reported earlier) shows a shift in the 3.5 - 3.75 mm drop axis ratios during a prolonged period of convection. The contoured shapes for these drops determined from the 2DVD camera data indicate the possibility of non-axisymmetric oscillations, compared with the contoured images for other events which fit well to our reference drop shapes. For all of above cases, observations from a C-band polarimetric radar - situated 15 km away are examined. The variations between the co-polar radar reflectivity and the differential reflectivity as well as the specific differential phase are compared with the 2DVD data based scattering calculations for the 5 events. The implications will be discussed

Drop Shapes Versus Fall Velocities in Rain: 2 Contrasting Examples

Rainfall retrievals from polarimetric radar measurements require the knowledge of four fundamental rain microstructure parameters, namely, drop size distribution, drop shape distribution, canting angles and drop fall velocities. Some recent measurements of all four parameters in natural rain are summarized in [1]. In this paper, we perform an in-depth analysis of two events, using two co-located 2D video disdrometers (2DVD; see [2]) both with high calibration accuracy, and a C-band polarimetric radar [3], located 15 km away. The two events, which occurred 7 days apart (on the 18th and the 25th of Dec 2009), had moderate-to-intense rainfall rates, but the second event had an embedded convection line within the storm. The line had passed over the 2DVD site, thus enabling the shapes and fall velocities to be determined as the line crossed the site. The first event was also captured in a similar manner by both the 2DVDs as well as the C-band radar. Drop fall velocity measurements for, say, the 3 mm drops show noticeable differences between the two events. Whereas for the first event, the velocity distribution showed a narrow and symmetric distribution, with a mode at the expected value (7.95 m/s, as given by the formula in [4]), the second event produced a wider distribution with a significant skewness towards lower velocities (although its mode too was close to the expected value). Moreover, the slower 3 mm drops in the second event occurred when the convection line was directly over the 2DVD site (03:35-03:45 utc), and not before nor after. A similar trend was observed in terms of the horizontal dimensions of the 3 mm drops, i.e. large fluctuations during the same time period, but not outside the period. Vertical dimensions of the drops also fluctuated but not to the same extent. Interestingly, the horizontal dimensions tended towards larger values during the 10-minute period, implying an increase in drop oblateness, which in turn indicates the possibility of the horizontal mode oscillation, one of the three fundamental modes of drop oscillations [5], albeit the most difficult one to excite

RELAMPAGO-CACTI drop size distribution observations and the implications for cloud and hydrologic studies

Comparisons of drop size distributions (DSD) properties and rainfall modes among three deep convective regions (the U.S. Southern Great Plains, in Córdoba Province in subtropical South America, and Manacapuru in central Amazonia) where heavy rain-producing systems contribute the majority of rainfall in the largest river basins of the Americas, are conducted. Measurements from two types of disdrometers sampled at two of the three sites were considered, and subsequently separated into a light precipitation mode and a precipitation mode using a cutoff at 0.5 mm hr-1. The distributions of physical parameters (such as rain rate R, mass-weighted mean diameter Dm, and normalized droplet concentration Nw) for the raindrop spectra without classification appear to be similar amongst the sites, except for much broader distributions of Nw at the Córdoba site. In the light precipitation and precipitation modes, the dominant higher observed frequency of Nw in both types of disdrometers, as well as the identification of shallow, light precipitation in vertically-pointing cloud radar data represent a unique characteristic of the Córdoba site relative to the other sites. As a result, the co-variability between DSD physical parameters indicates that the precipitation observed at Córdoba may confound existing drop size distribution methods of determining rain type

Polarimetric Measurements of Ice Pellets and Aggregated Snow

The recent upgrade of the National Weather Service WSR-88D radar network to polarimetric capabilities provides the abundance of information about the precipitation microphysics. Even with the plethora of polarimetric measurements at our disposal, use of this essential information regarding the microphysical processes is underutilized. For example, there is no polarimetric relation for snow estimation. The focus of this study is to improve the existing state of polarimetric data usage for discrimination between the ice pellets and freezing rain if their occurrence is away from the radar (patchy, no radar-centric structure), and more importantly the development of the polarimetric relations for snow quantification. Observations and analysis of an ice-liquid phase precipitation event, collected with an S-band polarimetric KOUN radar and a two-dimensional video disdrometer in central Oklahoma are presented. Using the disdrometer measurements, precipitation is classified either as ice pellets or rain/freezing rain. The ice pellets were challenging to detect by looking at conventional polarimetric radar data due to the localized and patchy nature of the ice phase and occurrence close to the ground. In this study, a new, unconventional way of looking at polarimetric radar data is introduced: Slanted Vertical Profiles SVPs at low (0° - 1°) radar elevations. From the analysis of the localized and patchy structures using SVPs, the polarimetric refreezing signature, reflected in local enhancement in ZDR and reduction in ZH and ρhv, became much more evident. Model simulations of sequential drop freezing using Marshal-Palmer DSDs along with the disdrometer observations suggest that preferential freezing of small drops may be responsible for the refreezing polarimetric signature. Accurate measurements of snow amounts by radar are very difficult to achieve. The inherent uncertainty in radar snow estimates based on the radar reflectivity factor Z is caused by the variability of snow particle size distributions and snow particle density as well as large diversity of snow growth habits. In this study, a novel methodology for snow quantification based on the joint use of radar reflectivity Z and specific differential phase KDP is introduced. An extensive dataset of 2D video disdrometer measurements of aggregated snow in central Oklahoma is used to derive polarimetric relations for liquid-equivalent snowfall rate S and ice water content IWC in the forms of bivariate power-law relations. The physical basis of these relations is explained. Their multipliers are sensitive to variations in the width of the canting angle distribution, and to lesser extent particles’ aspect ratios and densities, whereas the exponents are practically invariant. This novel approach is tested against the S(Z) relation using snow disdrometer measurements in three geographical regions (Oklahoma, Colorado, and Canada). Similarly, the new approach is tested on polarimetric radar data at three localities, Oklahoma, Virginia, and Colorado. Polarimetric relations for snow quantification demonstrated significant improvement in snow estimates compared to the traditional Z-based methods

The impact of precipitation physical processes on the polarimetric radar variables

The ongoing upgrade of the National Weather Service WSR-88D radar network to polarimetric capabilities, as well as similar upgrades worldwide, will soon provide a wealth of data and information regarding storm precipitation physics. Fully understanding how a variety of microphysical processes are revealed in polarimetric data is necessary for the best use of these new data by operational and research meteorologists. The focus of this study is to quantify a number of these precipitation physics "fingerprints" in the polarimetric radar variables by using a synthesis of explicit microphysical modeling, electromagnetic scattering calculations, thought experiments, and polarimetric radar observations.The complete set of polarimetric variables available from linearly-orthogonal dual-polarization radars are derived from basic electromagnetic scattering principles. A detailed physical description of these variables is then provided for applications in precipitation and other atmospheric scatterers, as is a summary of common data artifacts. The impact of various precipitation physical processes on these radar variables is then quantified. Using explicit microphysical models, scattering calculations, observations, and thought experiments, the microphysical fingerprints are determined and quantified for raindrop thermal conduction, raindrop size sorting by sedimentation, updrafts, and vertical wind shear, evaporation of raindrops, coalescence of raindrops, and freezing of raindrops in deep convective storm updrafts. A catalogue of the qualitative fingerprints of a number of precipitation processes is summarized

Measurements of Differential Reflectivity in Snowstorms and Warm Season Stratiform Systems

The organized behavior of differential radar reflectivity (ZDR) is documented in the cold regions of a wide variety of stratiform precipitation types occurring in both winter and summer. The radar targets and attendant cloud microphysical conditions are interpreted within the context of measurements of ice crystal types in laboratory diffusion chambers in which humidity and temperature are both stringently controlled. The overriding operational interest here is in the identification of regions prone to icing hazards with long horizontal paths. Two predominant regimes are identified: category A, which is typified by moderate reflectivity (from 10 to 30 dBZ) and modest +ZDR values (from 0 to +3 dB) in which both supercooled water and dendritic ice crystals (and oriented aggregates of ice crystals) are present at a mean temperature of −13°C, and category B, which is typified by small reflectivity (from −10 to +10 dBZ) and the largest +ZDR values (from +3 to +7 dB), in which supercooled water is dilute or absent and both flat-plate and dendritic crystals are likely. The predominant positive values for ZDR in many case studies suggest that the role of an electric field on ice particle orientation is small in comparison with gravity. The absence of robust +ZDR signatures in the trailing stratiform regions of vigorous summer squall lines may be due both to the infusion of noncrystalline ice particles (i.e., graupel and rimed aggregates) from the leading deep convection and to the effects of the stronger electric fields expected in these situations. These polarimetric measurements and their interpretations underscore the need for the accurate calibration of ZDR.United States. Federal Aviation Administration (Air Force Contract FA8721-05-C-0002

TOWARDS IMPROVED QPE BY CAPITALIZING GROUND- AND SPACE- BASED PRECIPITATION MEASUREMENTS

There are primarily two types of weather radar systems offering precipitation measurements covering relatively large areas: (1) Ground-based Radar (GR) networks such as the NEXRAD and (2) Spaceborne radars onboard meteorological satellites. Ground-based polarimetric weather radar is arguably the most powerful validation tool that provides physical insight into the development and interpretation of space-borne weather radar algorithms and observations. To achieve the synergy between ground- and space-borne weather radar, this study first aims to compare and resolve discrepancies in hydrometeor retrievals and reflectivity observations between the NOAA/National Severe Storm Laboratory (NSSL) “proof of concept” polarimetric WSR-88D radar (KOUN) and the space-borne precipitation radar (PR) onboard NASA’s Tropical Rainfall Measuring Mission (TRMM) platform. The comparisons reveal an overall bias <0.2% between PR and KOUN. The bias is hypothesized to be from non-Raleigh scattering effects and/or errors in attenuation correction procedures applied to Ku-band PR measurements. Provided the upgrade of the U.S. national weather radar network to include polarimetric capabilities, the findings in this study will potentially serve as the basis for nation-wide validation of space precipitation products and also invite synergistic development of coordinated space/ground multisensor precipitation products. On the other hand, due to inadequate radar coverage from intervening terrain blockages, ground QPE needs enhancement aided by spaceborne radars. In the second part of the talk, I will introduce an approach that identifies and corrects for vertical profile of reflectivity (VPR) by using TRMM PR measurements in the region of Arizona and southern California, where ground-based NEXRAD radars are difficult to obtain reliable ground precipitation estimation due to complex terrain and limited radar coverage. A VPR Identification and Enhancement (VPR-IE) method based on the modeling of the vertical variations of the equivalent reflectivity factor using a physically-based parameterization and climatological information is employed to obtain VPRs at S-band from the TRMM PR measurement at Ku-band. The VPR-IE methodology is comprehensively evaluated with all stratiform precipitation events in cold season in the year of 2011. The results show that the VPR-IE has overall good performance and provides much more accurate surface rainfall estimates than original radar QPE in NMQ system. With the recent availability of GPM Dual-frequency PR, the VPR-IE approach is anticipated to be more robust and more useful by extending to higher latitude mountainous regions

Effects of spatial resolution on radar-based precipitation estimation using sub-kilometer X-band radar measurements

Known for the ability to observe precipitation at spatial resolution higher than rain gauge networks and satellite products, weather radars allow us to measure precipitation at spatial resolutions of 1 kilometer (typical resolution for operational radars) and a few hundred meters (often used in research activities). In principle, we can operate a weather radar at resolution higher than 100m and the expectation is that radar data at higher spatial resolution can provide more information. However, there is no systematic research about whether the additional information is noise or useful data contributing to the quantitative precipitation estimation. In order to quantitatively investigate the changes, as either benefits or drawbacks, caused by increasing the spatial resolution of radar measurements, we set up an X-band radar field experiment from May to October in 2017 in the Stuttgart metropolitan region. The scan strategy consists of two quasi-simultaneous scans with a 75-m and a 250-m radial resolution respectively. They are named as the fine scan and the coarse scan, respectively. Both scans are compared to each other in terms of the radar data quality and their radar-based precipitation estimates. The primary results from these comparisons between the radar data of these two scans show that, in contrast to the coarse scan, the fine scan data are characterized with losses of weak echoes, are more subjected to external signals and second-trip echoes (drawback), are more effective in removing non-meteorological echoes (benefit), are more skillful in delineating convective storms (benefit), and show a better agreement with the external reference data (benefit)

Remote sensing and electromagnetic modeling applied to weather and forward scatter radar

This dissertation deals with electromagnetic modelling and data analysis, related to radar remote sensing and applied to forward scatter and meteorological polarimetric systems. After an overview of radar fundamentals to introduce the general terminology and concepts, results are presented at the end of each chapter. In this respect, a generalized electromagnetic model is first presented in order to predict the response of forward scatter radars (FSRs) for airtarget surveillance applications in both near-field and far-field regions. The model is discussed for increasing levels of complexity: a simplified near-field model, a near-field receiver model and a near-field receiver and transmitter model. FSR results have been evaluated in terms of the effects of different target electrical sizes and detection distances on the received signal, as well as the impact of the trajectory of the moving objects and compared with a customized implementation of a full-wave numerical tool. Secondly, a new data processing methodology, based on the statistical analysis of ground-clutter echoes and aimed at investigating the monitoring of the weather radar relative calibration, is presented. A preliminary study for an improvement of the ground-clutter calibration technique is formulated using as a permanent scatter analysis (PSA) and applied to real radar scenarios. The weather radar relative calibration has been applied to a dataset collected by a C-band weather radar in southern Italy and an evaluation with statistical score indexes has drawn through the comparison with a deterministic clutter map. The PSA technique has been proposed using a big metallic roof with a periodic mesh grid structure and having a hemispherical shape in the near-field of a polarimetric C-band radar and evaluated also with an ad-hoc numerical implementation of a full-wave solution. Finally, a radar-based snowfall intensity retrieval is investigated at centimeter and millimeter wavelengths (i.e., at X, Ka and W band) using a high-quality database of collocated ground-based precipitation measurements and radar multi-frequency observations. Coefficients for the multifrequency radar snowfall intensity retrieval are empirically derived using multivariate regression techniques and their interpretation is carried out by particle scattering simulations with soft-ice spheroids. For each topic, conclusions are proposed to highlight the goals of the whole work and pave the way for future studies