MODIS land surface retrieval in San Juan, Puerto Rico during the ATLAS field campaign

The Airborne Thermal and Land Applications Sensor (ATLAS) from NASA/Stennis that operates in the visual and IR bands was used as the main sensor for a field campaign in San Juan, Puerto Rico with the main objective of investigating the Urban Heat Island (UHI) in tropical cities. The UHI represents the temperature difference between urban areas and surrounding vegetated areas and is a good indicator of the impact of the land use in the climate. The sensor was flown over Puerto Rico in a Lear 23 jet plane during February 2004. One of the efforts to support the data gathered by the ATLAS sensor was the acquisition of remote sensing observations for the flight period. A temporal analysis of this complementary data for the Atlas San Juan Mission is presented with the objectives of calibrating the response of satellite sensors such as MODIS (Moderate Resolution Imaging Spectroradiometer) to study the UHI effect in tropical and subtropical regions. An analysis of the surface temperature variation was performed using remote sensing images from MODIS for the same days of the ATLAS field campaign. Surface temperatures were estimated for San Juan using the land surface temperature product MOD11_L2 distributed by Land Process Distributed Active Archive Center (LP DAAC). These results show the maximum, minimum and averages temperatures in San Juan and in the rest of the Island of Puerto Rico as measured by MODIS. A comparison of temperatures between El Yunque rain forest and San Juan reflects a tendency of higher temperatures for the San Juan area, an indication of the presence of a UHI. The information retrieved from MODIS for land surface temperatures was compared with weather stations temperature measurements spread over San Juan reflecting similar temporal and spatial variations with absolute offsets of about 3.71°C due to the differences between surface and air temperatures. Observations from the weather stations and MODIS suggest an increase in temperature in urban areas during daytime over rural areas. One of the flight lines from the ATLAS sensor coincides with the pass of MODIS. Because ATLAS flight line spatial resolution is 10 meters and MODIS swath spatial resolution is 1 kilometer, an averaging of every 100 x 100 pixels in the ATLAS flight line was performed. Highest difference between ATLAS and MODIS was 12.33 °C where the ATLAS sensor had estimated 47.82°C for an area of 1 kilometer by 1 kilometer. This high difference is less than 1% of the errors between the two sensors. About 60% of the errors between surface temperatures is less than 4°C. Some differences between MODIS and other field sensors may be related to the total retrieval for Puerto Rico which in most cases was about 50%. Based on the available data from the MODIS product and the comparisons with other instrument measurements, some suggestions about calibrating the algorithm will be made for applications in tropical regions.