P3C.12 A SOLAR ALIGNMENT TECHNIQUE FOR DETERMINING MOBILE RADAR POINTING ANGLES Nettie R. Arnott* Yvette P. Richardson The Pennsylvania State University, University Park, Pennsylvania Joshua M. Wurman Center for Severe Weather Research, Boulder, Colorado Jonathan Lutz National Center for Atmospheric Research, Boulder, Colorado 1. INTRODUCTION In recent years, mobile Doppler radars have been used to collect data in phenomena ranging from torna- does to hurricanes to mountain flow due to their ability to deploy close to the target of interest and thus collect data with fine resolution. While basic editing and dual-Doppler synthesis methods are nearly identical to those used for stationary radars, mobile radars present special challenges with regard to ‘navigation’ of data to a common grid due to imperfect knowledge of the truck pointing angle. For example, the Doppler on Wheels (DOW) radars record data in truck-relative coordinates such that zero degrees refers to the front of the truck. Gridding of the data for use in most analyses, including dual-Doppler syntheses, requires precise knowledge of the location of each truck (available with GPS) and the actual earth-relative azi- muth and range of each radar gate. Thus, an azimuth correction must be applied to all DOW data such that zero degrees consistently points north. Previously, this reorientation has involved matching known tower locations to ground clutter or matching sig- nificant weather features observed by two or more mobile radars at known locations. These methods are not only time consuming, but their accuracy suffers from the subjective matching process. Attempts to objectively determine the pointing angles by computing a correlation coefficient or other measure of fit between the reflectivity patterns of two radars for an array of possible angles have been somewhat successful (Zhang et al., 2000), but their accuracy is difficult to quantify and likely varies depending on the sharpness of the reflectivity features present. Thus an objective method with sub-beam accu- racy was desired. During May and June, 2002, the Doppler on Wheels (DOW) mobile radars participated in the International H 2 O Project (IHOP). Data were collected during convec- tion initiation, quiescent boundary layer, and boundary layer evolution missions. During IHOP_2002, a solar alignment technique for determining the DOW radar headings was implemented. In a solar alignment, the radar receives microwave radiation emitted by the sun and one uses the pattern of returned power to determine the truck-relative sun loca- tion. The radar pointing angle is simply the difference between the true azimuth of the sun with respect to North and its truck-relative azimuth (Fig. 1). This analysis tech- nique is described below with examples from IHOP_2002. It will be shown that a solar alignment pro- vides a simple, objective approach for determining a mobile radar pointing angle. 2. METHOD 2.1 Data Collection While the solar alignment technique has previously been considered, the DOW radars are now equipped with NCAR-developed software that points the antenna toward the sun given an approximate heading, location, and time of day. This software makes the process fast and simple enough to implement in the field. A correct _________________________________________ *Corresponding author address: Nettie R. Arnott The Pennsylvania State University, Dept. of Meteorology University Park, PA 16803; email: narnott@met.psu.edu N φ sun φ observed Pointing angle Fig. 1. Determination of the mobile radar pointing angle given the USNO azimuth φ sun and the observed azimuth of the sun with respect to the front of the truck φ observed . The pointing angle is φ sun - φ observed .