Jorge M. Trabal

Remote Sensing of Weather Hazards Using a Low-Cost and Minimal Infrastructure Off-the-Grid Weather Radar Network

Operational weather radars in the U.S. and other countries in the world are challenged in providing low-altitude observations of rainfall due to the Earths curvature and their deployment in “sparse” networks spaced hundreds of km apart. Given this limitation, work is underway to explore the feasibility of “dense” networks of small X-band radars. One approach developed by a student team from the U.S. Engineering Research Center for Collaborative and Adaptive Sensing of the Atmosphere (CASA) uses low-cost networks of simple, single-polarization radars that are not dependent on existing infrastructure, operating using solar energy and ad-hoc wireless networks, providing gap-filling data with improved temporal and spatial resolution. This “off-the-grid” (OTG) concept is one that might offer a means to monitor rainfall and provide useful data where it is not feasible or cost-effective to deploy more costly and more accurate radars. This paper describes the OTG concept and design, and presents examples of collected data and respective comparisons from this OTG network with measurements from an S-band NEXRAD radar as well as rainfall data from a set of rain gauges located in Puerto Rico. Results show that CASA OTG radars can provide improved spatial and temporal rainfall estimates with consistent or smaller estimated errors when compared to the S-band radar. End user validation was demonstrated in collaboration with the U.S. National Weather Service during system deployment for the XXI Central American and Caribbean Games celebrated at Mayaguez, Puerto Rico during the Summer of 2010.

Development of an Off-The-Grid X-band radar for weather applications

The Student Led Test Bed (STB) is part of the NSF Engineering Research Center CASA and is currently focused in developing low-cost and low infrastructure radar networks to fill lower atmosphere gaps not covered by current technology. The first radar node, which is part of a small region radar network, will significantly improve the time and spatial resolution of the radar data measured for the lower atmosphere. This paper describes the development of an Off-The-Grid (OTG) X-band radar node that requires minimum infrastructure for its deployment and can operate using solar energy and wireless communication links. The OTG radar was developed for meteorological applications modifying a commercially available marine radar. Hardware modifications for meteorological purposes were performed as well as the design and implementation of a photovoltaic system to power the radar using solar energy. The system was moved to the Colorado State University (CSU)-CHILL National Weather Radar facility for a cross-calibration and system evaluation. Satisfactory results were obtained where it was demonstrated that the OTG radar can provide precipitation measurements with improved spatial and temporal resolution, both necessary to have better lower troposphere measurements. This OTG node is the first prototype of a low infrastructure X-band weather radar network to aid forecasts in the western region of Puerto Rico.

Differential Reflectivity (Z DR ) calibration for CASA radar network using properties of the observed medium

The Center for Collaborative and Adaptive Sensing of the Atmosphere (CASA) has deployed a Distributive, Adaptive and Collaborative Sensing (DCAS) network of four radars in central Oklahoma working as a closed-loop system since 2006. The radars operate at the X-band frequency and are capable of polarimetric and Doppler measurements. The radar network is being evaluated for Quantitative Precipitation Estimation (QPE). QPE algorithms based on radar power measurements (e.g. ZH and ZDR) require bias correction. ZDR calibration is required prior to any application of the self-consistency principle. Two different methods were evaluated for ZDR bias correction. The intrinsic properties of dry aggregated snow present above the melting layer and light rain measurements close to the ground are used for the study. Results show a ZDR calibration accuracy of 0.2 dB or less for both analyzed events when both methods are compared.

Cirrus Clouds Millimeter-Wave Reflectivity Comparison with In- Situ Ice Crystal Airborne Data

In an effort to evaluate scattering models for particle size distributions of ice crystals within cirrus clouds, simultaneous data was collected in March 2000 during the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Cloud Intensive operational period (Cloud IOP) at the Cloud and Radiation Testbed (CART) site in Lamont, Oklahoma. In situ measurements of ice particles were collected using the National Center for Atmospheric Research (NCAR) Video Ice Particle Sampler (VIPS), which flew on the University of North Dakota Citation research aircraft. Ground-based vertical radar profiles were collected using the University of Massachusetts (UMass) 33GHz/95GHz Cloud Profiler Radar System (CPRS). Data from both sensors was used to retrieve and compare the equivalent radar reflectivity at Ka band (33GHz). The equivalent radar reflectivity measured by the ground-based, zenith-looking, CPRS radar at Ka band and compared to the reflectivity computed from the airborne VIPS samples of particle size distribution, N(D), using Mie theory. As anticipated the equivalent reflectivity of the radar and VIPS were similar at the time the UND Citation overflew the radar.

A Drop Size Distribution (DSD)-Based Model for Evaluating the Performance of Wet Radomes for Dual-Polarized Radars

A novel analytical method is presented for evaluating the electrical performance of a radome for a dualpolarized phased-array antenna under rain conditions. Attenuation, reflections, and induced cross polarization areevaluated for different rainfallconditionsand radome types. The authors presenta modelfor estimating the drop size distribution on a radome surface based on skin surface material, area, inclination, and rainfall rate. Then,amultilayerradomemodelbasedonthetransmission-line-equivalentcircuitmodelisusedtocharacterize the radome’s scattering parameters. Numerical results are compared with radar data obtained in the Next Generation Weather Radar (NEXRAD) and Collaborative Adaptive Sensing of the Atmosphere (CASA) systems, and good agreement is found.

A concept for evaluating the performance of wet radomes for phased-array weather radars

A novel analytical method for evaluating the electrical performance of a flat, tilted radome for a dual-polarized phased-array antenna under rain conditions is presented. Attenuation, reflections and induced cross-polarization are evaluated for different rainfall conditions. A new radome model is presented which takes into account the properties of the skin surface, area, inclination, radome structure, and rainfall rate. The radome is modeled as consisting of multiple layers, including a wet layer. Attenuation and propagation effects through the radome are characterized using a transmission line equivalent circuit model. Knowledge of the rainfall rate and surface properties of the radome is used to determine the radome performance. Calculated results are compared with radar data obtained with the NEXRAD and CASA systems, where good agreement between measurements and simulations was found.

Rainfall estimation and rain gauge comparison for x-band polarimetric CASA radars

The Center for Collaborative and Adaptive Sensing of the Atmosphere (CASA) developed and deployed four polarimetric and Doppler-capable distributed, collaborative and adaptive X-band radars in Oklahoma with the capability to detect and track tornadoes and storms in the lower troposphere with high spatial and temporal resolution. This paper presents an analysis of the performance of X-band rainfall estimation composite algorithms and the conventional reflectivity-rainfall relationship. The polarimetric estimators are developed by different combinations of radar products including horizontal equivalent reflectivity (Zh), differential reflectivity (Zdr) and specific differential phase (Kdp)- The Micronet Little Washita rain gauge network is used for validating rainfall estimates of the KCYR CASA radar. In addition, NEXRAD WSR-88D KFDR radar data are analyzed and compared with the rain gauge network and the CASA radar estimates. Fourteen hours of data collected during August 2006 from three different storms are analyzed. Both convective and stratiform events are represented in the radar data analysis. An accuracy of point radar measurement is assessed for different rain intensities. Results demonstrate an improvement in hourly rainfall accumulation estimation for average rainfall intensities beyond 5 mm/hr with the use of polarimetric estimators. An increase in absolute difference standard error with distance by the use of absolute difference in radar and rain gauges estimates is also evaluated.

Low cost and minimal infrastructure Off-the-Grid XBand radar network development for the west coast of Puerto Rico

The Puerto Rico Test Bed (PRTB) is part of the CASA NSF Engineering Research Center, and is currently focused on developing Off-the-Grid (OTG) X-band low infrastructure and low cost radar networks in the west coast of Puerto Rico. These radars will fill lower atmosphere gaps (< 2 km) not covered by current technology. The radar units operate using solar energy and ad-hoc wireless networks, and provide data with improved temporal and spatial resolution. Currently, there is a concern among National Weather Service forecast meteorologists and emergency managers regarding the need for more accurate radar data from the lower layers of the atmosphere. In this particular region, the earths curvature impedes weather observations due to the long ranges covered by todays current technology. This is the case in the western region of Puerto Rico. The CASA OTG network consists of three radar nodes, whose locations are strategically selected to cover the desired area. These radars were developed from commercially available marine navigation radars. They measure precipitation with the resolution required to meet scientific needs and to also complement measurements taken with the current NWS NEXRAD radar. This low-cost, low-infrastructure X-band weather radar network will aid with forecasts for any region of the world that is in need of observing the lower atmosphere and has limited resources. This paper will provide an overview of system design and development and initial results of the CASA OTG radar network in Puerto Rico.