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March 6, 2003
Climate Changes May Increase Extreme Rain / Snow Events in California
Increasing carbon dioxide levels in the atmosphere may lead to a rise
in the number of annual extreme precipitation events in the Sierra Nevada
Mountains, which in turn could increase the frequency of flooding in
California, a NASA-funded study finds.
One of the missions of NASA's Earth Science Enterprise (ESE), which
funded this research, is to better understand how the Earth system is
changing. Within this framework, NASA is committed to studying variability
in global precipitation, how well we can predict future changes in the
earth system, and what are the consequences of change in the Earth system
for human civilization.
Based on computer model simulations of the next 40 to 50 years, Jinwon
Kim, an atmospheric scientist at the University of California, Los Angeles
(UCLA), found that the Sierra Nevada region may experience substantial
increases in heavy precipitation (exceeding 2 inches of rain/day), and
extreme precipitation events (exceeding 4 inches of rain/day). Most of
these increases occur during the winter, currently the wettest season
in the region.
"The frequency of extreme precipitation may increase, in general, and
the most notable increase of extreme events may occur in areas characterized
by heavy winter precipitation in today's climate," Kim said. Kim recently
presented his results at the 83rd Annual Meeting of the American Meteorological
Society in Long Beach, Calif.
Existing projections from Hadley Centre for Climate Prediction and Research
(HCCPR) HadCM2 computer model suggests that increases in carbon dioxide
(CO2) are likely to substantially alter the hydrologic cycle in the Western
U.S. That's because increasing levels of CO2 in the atmosphere trap heat,
and warm the air. Warmer air holds more water, and when parcels of saturated
air rise, they tend to rain water back to Earth.
Kim used his regional computer model (MAS) to make two fine-scale precipitation
projections for the decade of 2040 to 2049 based on different values
of CO2 in the atmosphere from the coarse global projections by HCCPR,
United Kingdom.
Some of the background data input into the computer model included NASA-derived
Normalized Difference Vegetation Index (NDVI) data, which measures the
amount of solar energy reflected and absorbed by vegetation. This is
important data for computing transpiration. NDVI was created by Compton
Tucker of NASA Goddard, using data from the National Oceanic and Atmospheric
Administration's (NOAA) Geostationary Environmental Orbiting Satellite
(GOES) Advanced Very High Resolution Radiometer (AVHRR) instrument.
The first projection assumed that greenhouse gas concentrations will
stay at levels equal to those of the late 1900s. The second projection
represented the climate of the same period assuming increases in greenhouse
gas levels by 1 percent per year from the year 1990.
Compared to the first projection, the second projection showed increases
in both the number of wet days and, more importantly, large increases
in heavy precipitation events for the region during the cold season from
October to March. The model showed increases of heavy precipitation events
increased by 10 to 15 days per year. It also showed that extreme precipitation
events increased by 5 to 10 days per year.
Comparing the two projections, the average number of wet days per year
over the southern and northern Sierra Nevada basins (divided along the
area near Sacramento) increased by 37 percent for the southern basin
and 32 percent for the northern basin in the second projection. While
light precipitation events (less than 5mm or .2 inches/day), stayed the
same or decreased slightly for both basins, the occurrence of heavy precipitation
events rose from 1 percent of wet days annually to 3 percent in the second
projection. Extreme events rose from .1 percent of wet days annually
to 1 percent. Similar changes are projected for all major California
basins. These projections suggest that the intensity of the hydrologic
cycle will increase as levels CO2 continue to climb.
The second model-based projection scenario also showed that elevation
levels in the mountains where freezing occurs will rise as temperatures
rise. That means that much of the precipitation that currently falls
in higher altitudes as snow may come down as rain in future years. Snow
stores water during the cold season and releases it gradually in spring
and summer. Hence, a substantial increase of cold season rainfall at
the expense of snowfall reduces the buffering effects of snow and could
result in more flooding.
These changes, combined with more heavy rain events and steep mountain
slopes, could therefore lead to a greater frequency of flooding in the
future.
"Since the primary concern for reservoir management is to reduce flooding
risks that require maintaining the storage space to capture excessive
runoff, the reservoirs may have to maintain lower water levels," Kim
said. "This directly decreases the water resources."
The climate change signals projected in this study are based on a single
global projection and are expected to include an unknown amount of uncertainties.
Hence, the projections here must be taken as qualitative, rather than
quantitative. The author is planning additional studies using global
projections from multiple Global Climate Models.
This research was funded by NASA's ESE and NOAA. NASA's ESE Applications
Division applies the results of the nation's investment in ESE to issues
of national concern, such as water and resource management, environmental
quality, community growth, and disaster management to support policy
makers at the state and local levels.
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Contacts:
Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-3026)
Stephanie Kenitzer
American Meteorological Society
(Phone: 425/432-2192)
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 Climate
Change Signals: Frequency of Heavy and Extreme Precipitation Events
These figures show the present and projected future
changes in the intensity of daily rain/snow as a result of increasing
atmospheric carbon dioxide (CO2), according to the regional climate change
study by Kim (2001) that is based on the global projection made by Hadley
Center for the 1995 IPCC report (Johns et al. 1997).
There are six maps in the figure. The top, mid, and bottom maps (CNTL,
TRAN, TRAN-CNTL) are the estimation of the frequency for today's climate,
altered climate, and the difference between the two. The three maps in
the left-hand-side column present the 'Number of days/year on which the
daily precipitation exceeds 50.8mm or 2 inches. The three maps in the
right-hand side column show the days on which the daily precipitation
exceeds 101.6mm or 4 inches.
The main points of these plots are that 'based on the projection by this
study, it is likely that the events of heavy (2"/day) and extreme (4"/day)
precipitation will increase by 10-15 days per year and 5-10 days per
year, respectively, in the Sierra Nevada region (indicated by larger,
darker areas) as well as in the northern California Coastal Range.
Most of these increases occur during the winter, which is the wettest
season in the region. Combining the effects of local orographic characteristics
(steep slopes), wet soils during the winter and changes in the precipitation
characteristics (increased/reduced rainfall/snowfall) due to increased
CO2, the increases in the number of extreme precipitation is likely to
increase the frequency of flooding. This result suggests that it may
be necessary to revise water management/flood preparation for the region.
CREDIT: Jinwon Kim, UCLA
 Possible
Increased Extreme Precipitation Events
The frequency of extreme precipitation may increase,
in general, and the most notable increase of extreme events may occur
in the areas characterized by heavy winter precipitation in today's climate.
Credit for Image 2: Michael Phelps, Storm Photographer
Credit for Image 3: CLPX NASA Land Surface Hydrology Program
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