Los Angeles Urban Heat Islands: Land Use, Pacific and Climate Change Influences

The Los Angeles urban heat island (UHI) is a complex entity that is changing in time, space and intensity. The major influences on its characteristics appear to be population, land use, and Pacific Ocean variability. From 1950 to 2010, the county of Los Angeles has nearly tripled in population from 1,333,300 to 3,792,621. The downtown skyline also changed as more highrises replaced lower density buildings and parking lots. Downtown average temperatures have increased rapidly, rising over 3oC in the last century. Los Angeles Tmin values have increased faster than Tmax similar to other cities. However the Los Angeles UHI is unique among cities, with complex terrain and dominant land/sea breeze circulations. Also, the city is part of a regional megalopolis, whereas the surrounding rural areas are distant and ill-defined, in contrast to most UHIs. Our study looks at the diurnal and seasonal patterns in the urban thermal regime and how they have changed over recent decades. Temporal changes in land use (particularly vegetation), coastal sea surface temperatures, Pacific climatic indices such as the Pacific Decadal Oscillation (PDO) and coastal upwelling all contribute to the changes in city temperatures. The PDO especially correlates well with Los Angeles temperatures. The spatial changes in the UHI are described combining surface met data and aircraft remote sensing, using the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) and the MODIS/ASTER Airborne Simulator (MASTER) sensors at spatial resolutions of 30 and 50 m, respectively. In our study recent sea breeze enhancement will be investigated in its influence on coastal cooling. Implications of the role of the intensifying UHI in the increases in LA heat waves will also be discussed.