METHODS FOR COLLECTING WIND SPEED AND DIRECTION FOR USE IN CREATING TEPHRA-FALL DISTRIBUTION MAPS DURING VOLCANO HAZARDS ASSESSMENTS

Tephra fall is both a ground and aviation hazard. Therefore, tephra-fall distribution maps are useful components of volcano hazards assessments. Before these maps can be made, however, wind speed and direction for the area surrounding a given volcano must be known. In our hazard study of Telica Volcano, Nicaragua, we were unable to find in-country wind data and had to look elsewhere. Wind data was downloaded and compiled from a NOAA website (http://orbit-net.nesdis.noaa.gov/goes/winds/) which utilizes satellite cloud drift and water vapor motion wind vectors and are collected at six hour intervals. Over the course of a year data was gathered and processed into daily, weekly, and monthly rose diagrams based on altitude. While analyzing the data it became apparent that there are seasonal variations in wind speed and direction at higher altitudes. During the wet season, which begins in late December and runs through May, our rose diagrams showed winds varying anywhere from 120 degrees to 300 degrees while the dry season (June through November) tends to range between 30 degrees and 180 degrees. The technique used in this study was compared to wind data compiled from NOAA’s Global Data Assimilation System (GDAS), which is a modeled output from numerous observational and theoretical data sources. The data, which consisted of four data points per day over a broad range of altitudes, were averaged on a monthly basis for the years 1997 to 2002. Although both methods showed similar results the latter technique has more complete data for the lower altitudes. The information that was gathered during this study will benefit both ground based and aviation users in Central America when dealing with future tephra fall hazards.