January 17, 2007

A TALE OF TWO SITES: IMPACTS OF RELOCATING L.A.'S WEATHER STATION

In the classic 1859 novel, "A Tale of Two Cities," Charles Dickens spins a moral tale of dramatic contrasts between 18th century London and Paris. To modern-day climatologists, though, the story could serve as a metaphor for weather records in Los Angeles since the National Weather Service relocated the city's official downtown Civic Center weather station to the University of Southern California in July 1999.

A new comparative study of daily temperature and rainfall records in Los Angeles by NASA and university scientists finds the move -- to a location nearly 6 kilometers (almost 4 miles) closer to the coast and 27 meters (90 feet) lower in elevation -- has produced weather records reflecting conditions that are cooler, drier and less extreme than those at the previous location.

At the annual meeting of the American Meteorological Society in San Antonio, Texas, climatologist Dr. Bill Patzert of NASA's Jet Propulsion Laboratory, Pasadena, Calif., said the move has created a clear discontinuity in weather records.

"The move from downtown Los Angeles to USC in 1999 has caused a major hiccup in our local climate history," said Patzert. "Suddenly, Los Angeles became dryer and cooler, and we were denied a record rain year in 2004-2005. The magnitude of change reflected in our study strongly suggests this relocation will bias long-term climatic studies."

This is hardly the first move of the downtown Los Angeles weather station, but it is by far the largest, and the first to move the station away from the built-up city center. The station was moved from the roof of a two-story parking structure at the Department of Water and Power building near city center at an elevation of almost 90 meters (270 feet), to a park-like environment on the USC campus with tall shade trees and grass at an elevation of almost 60 meters (180 feet).

The National Weather Service moved the station as part of a nationwide effort to locate all official weather stations on ground-level sites in natural settings. The Department of Water and Power site remained operational, however, allowing the direct comparisons used by Patzert and his colleagues.

The study found that between August 1999 and June 2006, maximum temperatures averaged 1 degree Celsius (1.8 degrees Fahrenheit) higher at the Department of Water and Power site, while the average temperatures at the new site were about a half-degree Celsius (0.9 degrees Fahrenheit) warmer. Minimum temperatures were about the same at both sites. The greatest differences in maximum temperatures occurred in late summer and early fall, with the smallest differences seen in late winter and spring. The Department of Water and Power site was cooler than USC in the spring and summer and warmer in the fall and winter. The authors attribute these seasonal differences to two factors: distance from the ocean and land use.

"In southern California, the cool ocean current keeps the coast cooler in summer and milder in winter than inland areas," said co-author Dr. Steve LaDochy, a climatologist at California State University, Los Angeles. "For example, the Department of Water and Power site averages 1.7 degrees Celsius (three degrees Fahrenheit) warmer than Los Angeles International Airport at the coast, just 20 kilometers (12 miles) away."

Because of the abundance of trees and grass, the USC station tends to be cooler during the day than the urbanized Department of Water and Power site. At night, the trees can block outgoing radiation, and the moisture in the lawns tends to absorb more heat than the drier, open Department of Water and Power site. 

The USC site averaged about 25.4 millimeters (about one inch) less rainfall than the higher and more inland Department of Water and Power location. Again, elevation and distance from the coast are driving factors in the differences. In the Los Angeles basin, rainfall increases with elevation and with distance from the coast. The Department of Water and Power site averages 375 millimeters (14.77 inches) of rain a year, while Los Angeles International Airport averages just 305 millimeters (12.01 inches). Rainfall comes primarily from Pacific winter storms moving inland from a westerly direction. As these storm fronts are lifted by the coastal mountains, rain and snow tend to increase with increasing elevation on the coastal side of the mountains.

The weather station relocation kept Los Angeles from setting a new rainfall record in the 2004-5 water year (from July 1 to June 30). The USC site recorded 946.2 millimeters (37.25 inches), second only to 1883-4, which had 969.8 millimeters (38.18 inches). That same season, the Department of Water and Power site recorded 973.3 millimeters (38.32 inches). Conversely, in 2001-02, the USC station recorded the driest year ever--about 112 millimeters (4.42 inches), a record that would not have occurred without the move.

LaDochy cited the record heat wave of June and July 2006 as an example of how the move has made temperature records less extreme. At the USC station, the all-time record for the highest minimum temperature (overnight low) was set on June 4, with 20 degrees Celsius (68 degrees Fahrenheit). However, that same week, the Department of Water and Power station recorded highest overnight lows three days in a row. Similarly, for the month of July, USC station temperatures broke or tied seven all-time records, while the Department of Water and Power site broke nine.

Since the Department of Water and Power station is still operational, Patzert and his co-authors suggest its records be reinstated as the official Los Angeles downtown station, with the USC station becoming part of the National Weather Service's Cooperative Observer Program. That program has sites all over the United States, maintained by volunteers or contractors.

Other study authors include JPL's Dr. Josh Willis and Teni Mardirosian, Glendale Community College, Glendale, Calif.

For more information and images, visit:
http://www.nasa.gov/vision/earth/lookingatearth/weather-20070117.html

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Contact:

Alan Buis
NASA Jet Propulsion Laboratory
818-354-0474
alan.d.buis@jpl.nasa.gov

This text is derived from:
http://www.jpl.nasa.gov/news/features.cfm?feature=1273

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