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Climatic implications of a weather station's relocation: Bring back Los Angeles

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Monday, 5 January 2015
William C. Patzert, JPL, Pasadena, CA; and P. Ramirez, S. LaDochy, and J. Willis
Manuscript (254.1 kB)

Handout (273.2 kB)

For climate records at individual weather stations one assumes that weather readings take place at a relatively constant location, with minimal temporal and spatial disruption. This study examines the station relocation for the official Los Angeles downtown weather station and builds on previous research. 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). The USC site, which is closer to the ocean (and sea breezes), 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. This move resulted in a significant change in weather records which could influence climatic studies of the city. A comparative analysis of daily temperatures and precipitation recorded at USC and DWP clearly shows that the move resulted in a perceptible decrease in both temperatures and precipitation. These comparisons show both diurnal and seasonal temperature differences. For the 1999-2014 record, maximum temperatures averaged 1.5oF (0.9oC) higher at DWP (previously 1.0oF, or 0.6oC, higher with the shorter record), but nearly the same for minimum temperatures (mean values being 0.2oF, or 0.1oC, higher at USC, previously the same). The greatest temperature differences between the two stations occur in late summer and early fall, while the winter records showed the least differences. More record temperatures occurred at DWP than USC. For example 10 records mostly of highest minima were set at DWP, compared with 7 at USC, for the June-July 2006 heat wave. Precipitation at DWP for the study period averages about 0.6 (15.2 mm) more than the USC location for water years and 0.9 (22.9 mm) for calendar years. Previous values for the shorter period were 1.0 (25.4 mm) and 1.2 (30.5 mm), respectively. For extreme water year precipitation, DWP was both wetter (2004-5) and drier (2006-7) than USC. This recent analysis using a longer record supports the conclusions found earlier that by moving the official LA weather station, weather is recorded as cooler, drier and less extreme than the previous downtown location producing 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. The other possibility is introducing a correction factor for the differences, which would require a much longer record to accurately generate.