Monday, 15 January 2007
Will the real Los Angeles stand up: Impacts of a weather station's relocation on climatic records (and record weather)
Exhibit Hall C (Henry B. Gonzalez Convention Center)
In August 1999, the NWS moved the official downtown Los Angeles weather station to the USC campus, 3.78 miles (almost 6 km) to the southwest of its previous location near city center at the Department of Water & Power (DWP). This move resulted in a significant change in weather records which could influence climatic studies of the city. A comparative study of daily temperatures and precipitation recorded at USC and DWP, clearly shows that the move resulted in a significant decrease in both temperatures and precipitation . These comparisons show both diurnal and seasonal differences. For the 1999-2006 record, maximum temperatures averaged 1.5oF higher at DWP, but nearly the same for minimum temperatures (mean values being 0.7oF higher at DWP). Greatest temperature differences between the two stations occur in late summer and early fall, while the winter records showed the least differences. Aerial photos show the land use differences between the two sites. The USC site resembles a park, with tall shade trees close to the instrument shelter. The DWP site is located on the roof of a downtown parking structure, with no vegetation nearby. The USC site is also closer to the ocean (and sea breezes). Precipitation at USC for the study period averages about 1” (25.4 mm) less than the DWP location. In the Los Angeles Basin, precipitation increases with elevation and with distance from the coast. Rainfall comes mainly from Pacific winter storms moving inland from a westerly direction. As the air mass is lifted by coastal mountains, precipitation tends to increase with increasing elevation on the windward side of the coastal range. DWP is farther inland and approximately 100 ft (30 m) higher than USC. In the 2004-5 water year (July 1-June 30), the official rain total at USC was 37.25” (946.2 mm), second only to 1883-4, which had 38.18” (969.8 mm). At the same time, DWP recorded 38.32” (973.3 mm), which would have been the wettest year on record had the move not occurred.A heat wave in June and July 2006 broke several records throughout the state, including several in Los Angeles. At USC, the all-time record for highest temperature minimum was set on June 4th with 68 oF (previous record being 66 oF in 1997). However at DWP, highest temperature minima occured June 3-5. For July, USC temperatures broke or tied 7 all-time records, mostly for highest minima., while even more records were broken at the DWP site. By moving the official LA weather station, weather is recorded as cooler, drier and less extreme than the previous downtown location. Climatologists have noted the problems concerning station moves. By shifting the official downtown station into a park-like environment, 3.78 miles closer to the beach, there is a clear discontinuity in the records. Since the original station is still operational, we suggest using DWP records as the official LA downtown station and making USC one of the many COOP stations, such as its rival, UCLA.