A VALIDATION STUDY OF THE URBAN HEAT ISLAND IN THE TROPICAL COASTAL CITY OF SAN JUAN, PUERTO RICO

In order to study the existence and characteristics of an Urban Heat Island (UHI) in the San Juan Metropolitan Area (SJMA) and its surrounding environment, a field campaign was conducted in February 2004 referred as the San Juan Atlas Mission. The campaign incorporated an airborne remote sensor equipped with fifteen channels, most of them in the infrared range (ATLAS) along with a number of surface weather stations and temperature sensors deployed in the area of study. The stations and the sensors were installed in strategic localities to observe climatic variations through the urban and rural landscapes. Time series diagrams of the data collected by the stations and sensors demonstrate that heavy urbanized commercial areas (with greater concrete density and other construction materials) have air temperatures higher than other urban and suburban residential areas, and much higher temperatures than in rural areas. The temperature differences [dT(U-R)] were obtained by subtracting the temperature recording of several stations/censors from that of an urban station taken as reference, this latter located in the commercial area of San Juan. The dT(U-R) time series demonstrates that the Urban Heat Island peaks during the morning between 10:00am and noon, with an average of 4.5ºC difference, a temporary pattern not observed previously in similar studies for continental cities. A high variability of the UHI with the precipitation pattern was also observed, even for short and relatively weak rainfall events. These results can be a reflection of the large low rise building density shown by the absence of any significant effects of heat storage in the urban areas, and the importance of the surrounding humidity of the ground and the vegetation in controlling the urban climate. Mesoscale atmospheric modeling results replicate the observed diurnal patterns of the UHI and demonstrate the impact of the local land use in the air temperature, wind field, and vertical profiles of the lower atmosphere. The mentioned environmental change is simulated as the substitution of the original San Juan vegetation by the current urban landscape, and the ongoing urban sprawl. The presence of the SJMA produces air temperatures of up to 3.5ºC higher than in the primitive scenario. The numerical experiments also show an increase of the land and sea breezes as a function of the San Juan UHI. On-going works are focusing in the impact of precipitation and the water cycles, features of interest for tropical coastal cities.