DEVELOPMENT AND IMPLEMENTATION OF AN ATTENUATION CORRECTION ALGORITHM FOR CASA OFF THE GRID X-BAND RADAR S98 NETWORK Keyla M. Mora 1 , Leyda León 1 , Sandra Cruz-Pol 1 University of Puerto Rico, Mayaguez Campus 1 , Mayaguez, PR, ABSTRACT The Collaborative Adaptive Sensing of the Atmosphere (CASA) Engineering Research Center (ERC) at UPRM developed an Off-The-Grid (OTG) X-band single polarized radar network deployed in the Puerto Rico west coast. With this kind of radar it is possible to observe some meteorological events that might not be detected by NEXRAD radar due to earth curvature. The main goal of this research is to break with the paradigm and limitations of using X- band OTG radars as a reliable forecast system, where the principal issue is the attenuation due to rain along the path. To overcome this problem the attenuated signal could be rectified through a correction algorithm based on reflectivity and also an ideal polarization (vertical or horizontal) will be identified to certain effects of rain attenuation. To correct for attenuation a hybrid between Hitschfeld – Bordan (HB) method and a Surface Reference Technique (SRT) would be used where the reference signal is the NEXRAD radar data because is considered negligible to attenuation. 1 Corresponing author address: Keyla M. Mora, University of Puerto Rico, Mayaguez Campus Dept., Mayaguez,PR00680 Email:keyla.mora@upr.edu 1. INTRODUCTION Weather radars as part of forecasting systems, are useful tools for meteorology and hydrology purposes. Meteorological trends and patterns can be analyzed with the highest precision possible provided by these types of radars. In particular X-band weather radars have various advantages such as high temporal and spatial resolution and low cost as compared to S-band systems. Yet the main X-band radar limitation is the attenuation caused by rain and others hydrometeors. Several attenuation correction algorithms have been proposed for both single and dual-polarized X-band radars. Here we present the development and validation of a modified algorithm to correct for atmospheric attenuation for low-cost single-pol X-band weather radars, which will greatly improve the data provided by these systems. Previous studies have focused on the limitation (attenuation) of X-band weather radars and possible solutions to this limitation. These studies were the motivation from the NSF CASA-ERC at UPRM to develop and implement an attenuation correction algorithm for Off-The-Grid (OTG) X-band radar network installed in the western region of Puerto Rico. This paper describes the process of the development of this attenuation correction algorithm as well as validation of such. This algorithm is then implemented to the data provided by the CASA OTG X-band radar. The method applied to estimate and correct the attenuation caused mainly by rain was based on a hybrid between Hitschfeld – Bordan (HB) method and Surface Reference Technique (SRT). A similar method was first adapted and tested by Colorado State University (CSU) with CASA radars but for wet ice attenuation correction. The method was refered to as the SRT- modified correction method. This mentioned method uses the measured reflectivity (Zv) to estimate true reflectivity and specific attenuation (Av) at each range gate based on a power law Av-Zv relationship. The coefficient β is fixed, and α is adjusted so that the path – integrated attenuation estimated with Hitschfeld – Bordan (HB) method matches with the path – integrated attenuation (PIA) estimated with Surface Reference Technique method as specified in the SRT modified method. SRT modified method requires computing ∆Z value that is the difference between the un-attenuated S-band reflectivity (WSR-88D S-band NEXRAD located at Puerto Rico) and X-band measured reflectivity (OTG X-band radar data). The corrected radar reflectivity data was finally compared and the method validated using X-band TropiNet radar data, an NSF funded network of dual- pol, Doppler weather radars also located at UPRM.