NUMERICAL STUDY OF URBAN GROWTH EFFECTS ON SURFACE ATMOSPHERIC FIELDS OVER A COASTAL TROPICAL CITY Pedro J. Mulero*, Alexander Velázquez, Jorge E. González, and Amos Winter University of Puerto Rico, Mayagüez, Puerto Rico 1. INTRODUCTION 1.1 Overview of the San Juan UHI Puerto Rico is the easternmost of the Greater Antilles. Its center is roughly located at 18.20 o north-latitude and 66 o west-longitude. The island has almost a rectangle-like shape with a length of 180 km and 56 km width for a total area of 10,080 km 2 . San Juan, its capital, has a metropolitan area of ~3,000 km 2 with over 2 million people within its boundaries. The city is located at the northeastern coast of the island and is under the quasi-perennial influence of the synoptic scale trade winds, which blow mainly from the east to the east-northeast. The mesoscale sea-breeze circulation also modulates temperatures and flow over the city, particularly during the summer, when trade winds are lighter and surface heating is greater. At smaller scales, however, observations indicate that the large urban landscape and continued expansion might influence to some extent local weather across the area. Figure 1 shows how surface temperatures for the month of January (one of the driest climatologically) have been increasing, albeit slightly (~0.035°C year -1 ), for the past several decades over the downtown section of the city. Whether this is an island-wide related trend caused by long time-scale climatological patterns or an urban-associated increase remains to be studied further. Data from the Advanced Very High Resolution Radiometer (AVHRR), onboard a NOAA-Satellite, was retrieved to calculate the Normalized Difference Vegetation Index (NDVI) and the Apparent Surface Temperature (APST). The AVHRR sensor is a broadband five-channel scanner, which senses in the visible, near infrared, P1.5 *Corresponding author address: Pedro J. Mulero, UPRM, Dept. of Mechanical Enginnering, Puerto Rico, 00680; e-mail: pmulero@me.uprm.edu. 10 14 18 22 26 30 34 1970 1974 1978 1982 1986 1990 1994 1998 2002 YEAR TEMPERATURE (deg C) Figure 1: Time-series analysis for downtown San Juan temperatures on January (in °C). Blue, pink, and yellow lines depict maximum, average and minimum temperatures, respectively. The solid black line represents a linear regression on the average temperatures. and thermal infrared portions of the electromagnetic spectrum, the radiative emission off the surface. The variables NDVI and AST are calculated combining the data collected from the different channels of the sensor using a set of simple equations (not shown here). Results from the calculation of the NDVI can be seen in Figure 2. Values range from 0.0 to 0.5, which represent non-vegetated to well-vegetated areas, respectively. An area corresponding to the urbanized area of San Juan can be identified as one of the non-vegetated areas in the image. SAN JUAN URBAN AREA Figure 2: Negative difference vegetation index (NDVI); from data of NOAA satellite using the AVHRR sensor on January-February, 1999.