Patrick C. Kennedy

S-Band Dual-Polarization Radar Observations of Winter Storms

AbstractThis study is based on analyses of dual-polarization radar observations made by the 11-cm-wavelength Colorado State University–University of Chicago–Illinois State Water Survey (CSU–CHILL) system during four significant winter storms in northeastern Colorado. It was found that values of specific differential phase KDP often reached local maxima of ∼0.15°–0.4° km−1 in an elevated layer near the −15°C environmental temperature isotherm. The passage of these elevated positive KDP areas is shown to be linked to increased surface precipitation rates. Calculations using a microwave scattering model indicate that populations of highly oblate ice particles with moderate bulk densities and diameters in the ∼0.8–1.2-mm range can generate KDP (and differential reflectivity ZDR) values that are consistent with the radar observations. The persistent correlation between the enhanced KDP level and the −15°C temperature regime suggests that rapidly growing dendrites likely played a significant role in the product...

A Description of the CSU-CHILL National Radar Facility

The subject of this paper is the Colorado State University‐University of Chicago‐Illinois State Water Survey (CSU‐CHILL) National Radar Facility’s S-band polarimetric research radar. Key features of this system include polarization agility (provided by the dual-transmitter, dual-receiver design), a recently updated signal processor, and a low (234 dB, two way) integrated cross-polar ratio (ICPR 2) antenna system. After reviewing the technical description of the radar, the authors present a new differential reflectivity ( ZDR) calibration technique and data examples collected in different polarization modes. Although the CSU‐CHILL radar is transportable, it can also be operated in a dual-Doppler configuration with the CSU‐Pawnee radar, an 11-cm Doppler radar system situated 48 km north of the CSU‐CHILL Greeley field site. Used together, these radars provide three-dimensional kinematic and hydrometeor information in precipitating cloud systems.

The Community Collaborative Rain, Hail, and Snow Network Informal Education for Scientists and Citizens

T he Community Collaborative Rain, Hail, and Snow Network (CoCoRaHS) originated in the aft ermath of a fl ash-fl ood storm that dropped more than 12-in. of rain over a small portion of Fort Collins, Colorado, on 28 July 1997, and a similar storm the following evening over the grasslands of northeastern Colorado. Th ese fl oods were responsible for several fatalities and at least

High-Resolution Polarimetric Radar Observations of Snow-Generating Cells

200 million in property damage. Neither event would have been accurately recorded by existing networks of offi cial weather stations. National Weather Service (NWS) radar also failed to assess the severity of these storms (Kelsch 1998a,b; Petersen et al. 1999). However, when the citizens of these areas were asked to help, they enthusiastically provided scientists with a wealth of information from their own backyard observations. Th ese data eventually resulted in the very accurate mapping of precipitation from the Fort Collins fl ash fl ood that continues to be used by engineers, hydrolo-gists, weather forecasters, city planners, emergency managers, attorneys, teachers, historians, and many others for such applications as flood forecasting, drought monitoring, verifi cation of radar-estimated precipitation, and climate trends. Encouraged by the enthusiastic public response, versity (CSU) in Fort Collins began mobilizing a network of citizen rain and hail observers in the spring of 1998. Th e initial plans targeted teachers and students from all grades in Fort Collins. However, unexpected free publicity from local media spread the word to the general public. Within a matter of days over 150 people volunteered, including students from nearly 40 schools in the region and interested adults. What started as a collaborative data-collection venture has developed into an exciting community based science education program that includes classroom presentations, fi eld trips, training sessions, picnics, and informal seminars. CoCoRaHS now includes over 1000 active volunteers across Colorado (see Table 1), as well as several hundred in Nebraska, Wyoming, Kansas, and New Mexico. They range in age from 6 to 80-plus years. Th is diversity of age and background of participants is one of the true TABLE 1. Level of participation in the CoCoRaHS network (Colorado only) from 1998 through Oct 2004. Active volunteers are defined as observers who submit at least five observations between 1 Jun and 31 Oct for the calendar year. In addition to the numbers below, several hundred more observers participated for a few weeks but have not remained active.

REFRACTT 2006: Real-time retrieval of high-resolution, low-level moisture fields from operational NEXRAD and research radars

AbstractHigh-resolution X-band polarimetric radar data were collected in 19 snowstorms over northern Colorado in early 2013 as part of the Front Range Orographic Storms (FROST) project. In each case, small, vertically erect convective turrets were observed near the echo top. These “generating cells” are similar to those reported in the literature and are characterized by ~1-km horizontal and vertical dimensions, vertical velocities of 1–2 m s−1, and lifetimes of at least 10 min. In some cases, these generating cells are enshrouded by enhanced differential reflectivity ZDR, indicating a “shroud” of pristine crystals enveloping the larger, more isotropic particles. The anticorrelation of radar reflectivity factor at horizontal polarization ZH and ZDR suggests ongoing aggregation or riming of particles in the core of generating cells. For cases in which radiosonde data were collected, potential instability was found within the layer in which generating cells were observed. The persistence of these layers sug...

The Collaborative Colorado–Nebraska Unmanned Aircraft System Experiment

High-resolution moisture fields retrieved for the first time from both operational and research radars illustrate the low-level moisture variability associated with boundary layer processes and the prethunderstorm environment.

A Comparative Study of Rainfall Retrievals Based on Specific Differential Phase Shifts at X- and S-Band Radar Frequencies

The Collaborative Colorado–Nebraska Unmanned Aircraft System Experiment (CoCoNUE) was executed on 1 March and 30 September 2009. The principal objective of this project was to examine the feasibility of using a small unmanned aircraft operating semi-autonomously with an onboard autopilot to observe atmospheric phenomena within the terrestrial boundary layer covered by the United States National Airspace System. The application of an unmanned aircraft system (UAS; the aircraft along with the communications and logistics infrastructure required for operation) is beset by a number of engineering and regulatory challenges. This article discusses the strategies implemented to meet these challenges. Airmass boundaries served as the target of the flights conducted. These atmospheric phenomena have the fortuitous combination of an across-boundary scale that yields a coherent signal in the in situ meteorological data that can be collected by a UAS and an along-boundary scale that can be easily tracked via the exis...

Single-Doppler Radar Observations of a Mini-Supercell Tornadic Thunderstorm

Abstract A comparative study of the use of X- and S-band polarimetric radars for rainfall parameter retrievals is presented. The main advantage of X-band polarimetric measurements is the availability of reliable specific differential phase shift estimates, KDP, for lighter rainfalls when phase measurements at the S band are too noisy to produce usable KDP. Theoretical modeling with experimental raindrop size distributions indicates that due to some non-Rayleigh resonant effects, KDP values at a 3.2-cm wavelength (X band) are on average a factor of 3.7 greater than at 11 cm (S band), which is a somewhat larger difference than simple frequency scaling predicts. The non-Rayleigh effects also cause X-band horizontal polarization reflectivity, Zeh, and differential reflectivity, ZDR, to be larger than those at the S band. The differences between X- and S-band reflectivities can exceed measurement uncertainties for Zeh starting approximately at Zeh > 40 dBZ, and for ZDR when the mass-weighted drop diameter, Dm,...

Performance of the hail differential reflectivity (HDR) polarimetric radar hail indicator

Abstract During the early evening hours of 19 May 1989, the CHILL 10-cm Doppler weather radar observed most of the lifetime of an unusually small tornadic thunderstorm. Throughout the event, the parent thunderstorm echo top remained below 6.7 km MSL The low-altitude echo diameter, as defined by the 25-dBZ contour, was only 15 km. Despite its small size, both visual and radar observations indicated that this storm contained many of the organizational features often noted in large, “classical” southern Great Plains supercells. The synoptic setting in which this storm occurred was atypical for supercell development in that both the thermodynamic instability and vertical wind shear magnitudes were limited. This documentation of a tornadic storm that developed in a nonthreatening environment mid that presented a small, seemingly inconsequential radar appearance demonstrates some of the challenges that will be faced by automated Doppler radar-based severe weather detection algorithms.

Polarimetric Radar Observations of Hail Formation

Abstract A series of poststorm surveys were conducted in the wake of hailstorms observed by the Colorado State University–University of Chicago–Illinois State Water Survey (CSU-CHILL) S-Band polarimetric radar. Information on hail characteristics (maximum diameter, building damage, apparent hailstone density, etc.) was solicited from the general-public storm observers that were contacted during the surveys; the locations of their observations were determined using GPS equipment. Low-elevation angle radar measurements of reflectivity, differential reflectivity ZDR, and linear depolarization ratio (LDR) were interpolated to the ground-observer locations. Relationships between the hail differential reflectivity parameter HDR and the observer-reported hail characteristics were examined. It was found that HDR thresholds of 21 and 30 dB were reasonably successful (critical success index values of ∼0.77) in respectively identifying regions where large (>19 mm in diameter) and structurally damaging hail were obse...