Theme for 1999:
Meteorology in California in 1999
Sierra College, Rocklin, CA
A Review of Water Year 1999: A Classic La Niña Turns Benign
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Water year 1998-99 seemed to embody a classic La Niña pattern. In contrast to the previous water year which was one of the strongest El Niño years this century with relatively warm eastern tropical Pacific sea surface temperatures, the eastern Pacific cooled rapidly during the summer of 1998. By fall a large pool of cooler than normal water was evident east of the date line. The expected outlook was a dry American Southwest and a wet Pacific Northwest. In fact, this basic winter precipitation pattern did occur with dryness extending westward into the southern half of California. The season in the Northwest, including British Columbia, was wet with many days of rain and record snowpack at some stations.
Impact of Climate Variation on Flood Control Planning in California
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If the rainfall climate were a stationary time series, one 50-year period would be as good as any other for defining flood threats. This is not the reality. The more recent 50 years is considerably wetter than the previous 50 years with important lessons for flood control planning.
Rainfall data of the last 100 years were examined on several scales of storm duration for long term trends. These included the maximum one-day, ten and thirty consecutive days, and the annual total rain. On each time scale there is a notable increase in rain in the last half of the records.
Flood control planners need to have current and reliable hydrologic records to forecast trending flooding potential. Engineering design feeds off the data sets that are compiled for current weather and flood forecasting; but with an added burden of needing well-documented and long historic records. Knowledge of both the historic scene as well as data trends are critical to understanding flood threats.
The realities of climatic variation need be rooted in observation. The increased climate variation in California is related to sea surface temperature and ocean currents. Forces associated with ocean currents and upwelling are examined. Their impacts on both rainfall and temperature trends are studied. These forces include Solar constant variations and thermohaline cycle invigoration and length of day variations.
The impact of observed climate variation on older flood control projects is that based on current hydrologic records they are undersized for their intended level of protection.
The California Land-falling Jets Experiment (CALJET): Motivation, Strategy, and Description of a Flooding Event
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The initial objectives of the CALJET experiment were to explore the role of coastal and offshore observations of low-level jets (LLJ) and their mesoscale environment in land-falling winter storms on mesoscale quantitative precipitation and wind forecasting. New goals were added when the strong El Niño of 1997-98 developed and it became evident that there was a substantially increased risk of flooding in California.
The experiment focused on the LLJ because it plays a key role in determining coastal orographic rainfall. Although orographic rainfall is very sensitive to the speed, orientation, and moisture content of the low-level flow as it encounters the coastal mountains, these parameters are poorly known even shortly before a storm makes land fall.
This presentation will describe the motivation for the experiment and the associated observing strategy. Preliminary results will be illustrated by describing an event where up to 12 inches of rain fell in 24 hours along the Big Sur Coast in central California. The evolution of this event over 48 hours, including the incipient stage as the storm developed offshore and the devastating rainfall and flooding it caused upon land fall, will be examined.
Reprints of three recent conference papers presented at the Annual Meeting of the American Meteorological Society in Dallas, Texas during January 1999 are appended here. They provide more background on the experiment and a summary of the field phase (Ralph et al.), as well as analyses of two strong storms on 2-3 February 1998 (Persson et al.) and on 5-6 February 1998 (White et al.).
Comparing Spatial Distribution of Rainfall Derived from Rain Gages and Radar
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Traditional rainfall analyses for hydrologic modeling use spatial representations of rainfall derived from rain gage observations at a series of points. These gage-derived spatial representations of rainfall are computed using any number of techniques including inverse-distance squared weighting and more advanced methods such as Kriging. None of these techniques have any relationship with the real world or provide any information about the true spatial distribution of rainfall. They are simply methods of convenience used to interpret the spatial variation of rainfall from point data in the absence of other information or techniques.
Radar, on the other hand, offers a significant analytical improvement for rainfall analysis by providing direct data more representative of the true spatial distribution of rainfall. The differences between the spatial distributions derived from radar and those derived from rain gages are often striking and dramatic. Examples will be presented where the contours indicating general spatial trends are rotated nearly 90 degrees. These findings have significant implications for both modeling and for hydrologic standards that require data supporting design storm shapes and sizes.
The Relationship Between Rainfall and Wildland Fires
Climate and weather play an important role in determining wildland fire activity. For example, vegetation and thus fuel growth and extent as well as dead fuel and soil moisture content are a function of precipitation. The amount and relative dryness of the available fuel thus influences the starting date, duration, and variability of fire seasons. A particular analysis of climate and wildfire data for four Southern California Forests between the years of 1970-1996 demonstrates these relationships. There is a significant negative correlation between winter precipitation and the subsequent fire season extent and duration. With specific consideration of El Niño and La Niña events, the greatest increase in wildfire activity occurs, not in the immediately following fire season, but in the subsequent fire season. This lag relationship is consistent with Swetnam, et al. (1993).
A Midwestern Snow Forecasting Technique: Can It be Used in California?
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Rheinhart (Bill) Harms coined such phrases as "Alberta Clipper" and "Panhandle Hook" for Midwest snowstorms. He discussed an objective technique he developed in the 1960's for forecasting snow depth. The snow forecasting technique was described in the context of a question about whether or not it can be used in California.
- Sierra College Science Center
- Floodplain Management Association
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