Related Studies Analysis
Many global and regional studies merge on the results of widespread increases in
both the intensity and frequency of heavy rainfall events in a warmer climate.
Additionally, these increases coincide with increases in the contribution of extreme
precipitation events to total rainfall. In many cases, the changes in precipitation extremes
exceed to some extent the changes in mean measures. Many studies also find that the
direction of change in mean precipitation measures is often consistent with the direction
of change in extreme precipitation measures. Finally, many studies allude to the idea that
there is much more spatial variability in trends of precipitation extremes and future
projections than there is for temperatures.
Diffenbaugh et al. 2006 studies the change in 95 percentile precipitation events in
the western part of the United States. They find widespread increases in the frequency of
95th percentile events per year, particularly across California and Oregon. Some of the
biggest differences occur across northern California and southwestern Oregon. Our
results show increases in the 95th percentile 1 day precipitation in the Pacific Northwest,
but not quite as far south as the maximum positive region shown in Diffenbaugh et al.
2006. Also, our results show an area of negative values across the southwest, which is
not backed by this study.
Diffenbaugh et al. 2005 studies changes in precipitation and precipitation
extremes for the entire United States. They show increases in mean annual precipitation,
particularly across the eastern part of the United States. Our results show increases of in
mean daily precipitation across the eastern United States, although the areas of relative
maximum differ between our results and the study. They also show increases in >95th
percentile events across much of the east and northwest. Our results agree on the
increases in the northeast and northwest. This study further finds the pattern of
statistically significant anomalies in precipitation extremes to be very similar to mean
changes. Our results generally show a great deal of consistency between positive
anomalies in the mean and precipitation extremes as well. However, there are some areas
of inconsistency present. The increases in the frequency of dry days found in this study
is modestly consistent with our findings of increases of <5th percentile 30 day periods in
parts of the east. Finally, Diffenbaugh et al. 2005 discusses reasons for the precipitation
change patterns in various regions.
Leung et al. 2004 shows widespread increases in mean daily summer precipitation
across the western US. Our results show modest increases in mean daily summer
precipitation in some parts of the west, but significant decreases (of around .4 and above
mm/day) in others. However, our results are consistent with this study’s finding of
decreases in mean winter precipitation along the west coast, particularly in California.
Furthermore, this study shows widespread decreases in 95th percentile values for summer
across the western US, some of which are significant. Our results also show widespread
decreases in this measure across the western US. Due to our lack of resolution, our
results cannot be compared in great detail to the winter changes in the 95th percentile for
this study, but there is a consistency in increases in this measure for the northwestern
parts of the region.
Bell 2004 finds little change in mean daily precipitation across much of the state
of California, with the exception of portions of the northern part of the state. Our results
show decreases in the northern part of the state, which is inconsistent with Bell 2004, but
we also find little or no change in mean daily precipitation elsewhere, which is consistent
with Bell 2004. Similarly, this study finds decreases in 95th percentile extreme
precipitation events across much of the state, with the exception of parts of the north.
Our results also show decreases in much of central and southern California, but unlike
Bell 2004, we show little or no change in this measure in the northern part of the state.
Tebaldi et al. 2006 finds increases in the contribution of > 95th percentile
precipitation events for parts of the northeastern and northwestern United States, and
little or no change in the south using annual data. This pattern resembles our results of
increases in the frequency of >95th percentile events and 95th percentile values in the
same northern areas. This study also finds widespread increases in the number of
consecutive dry days across the United States, which is consistent with our finding of
increases in <5 percentile P-E 30 day events across much of the United States. Finally,
the historical patterns in the same measures that this study analyses resemble the above
results from this study.
Raisanen 2005 studies precipitation extremes but instead uses monthly and annual
data. This study generally shows increases in mean, maximum, and minimum
precipitation for both summer and winter across northern North America, and no change
or decreases in these measures in southern North America. Our results for daily data
show increases in mean P-E across the northern USA for summer and winter, but only
agree with decreases in the south for winter. For our maximum and minimum results,
only our 99th percentile summer change vaguely resembles the results from this study, but
is inconsistent across Mexico. This study finds increases in annual mean precipitation
which become more positive further north in the United States. Our results show a
pattern that is mildly similar in the south to north trend of increasing values. The annual
maximum change in precipitation is not consistent with our results. This study also
contains a great deal of results on the changes in frequency of precipitation extreme
events. It shows increases in 4th largest precipitation events (monthly) in summer in the
eastern US, and some decreases in the west. Our results show the same pattern for >95th
percentile frequency changes for 30 day periods. This study shows a winter pattern of
decreases in the south and increases further north across the United States of 4th largest
precipitation events, which intensifies as the extreme gets narrower. Our results show the
same pattern of increasing further north, but are much more positive in nature. Our
results also agree on intensification of positive values as the extreme gets narrower. The
patterns of summer dryness in this study vaguely resemble our results for <5th and <1st
percentile frequency changes for 30 days both in geographic pattern and trend as the
extreme tail gets narrower. Our winter dryness (<5th percentile frequency change for 30
days) results agree with this study’s findings of increases in 4th smallest precipitation
events across the general southern United States. This study further shows annual 4th
largest precipitation value frequency changes that are near zero or negative in southern
North America, and increase as one moves further north. Our results essentially show
this same pattern. Finally, Raisanen 2005 merges on the idea that simulated changes in
precipitation extremes are highly correlated with the changes in the long term mean
precipitation. This generally agrees with our results.
In analyzing observed trends in precipitation extremes across the contiguous
United States, Groisman et al. 2005 find statistically significant increases by 20% in the
frequency of very heavy daily precipitation events (> 99th percentile), all of which has
occurred in the last third of the 20th century. Furthermore, this study points to results
from Semenov and Bengtsson 2002 that show an increase in upper 10% rainy day
frequency over the northeastern quadrant of the United States between 2000 and 2090,
which coincides with a simultaneous decrease in the number of wet days over this area.
Our >95 percentile frequency change maps generally agree with the first finding, and our
results projecting increases in the frequency of short term droughts (<5 percentile 30-90
day) in the same geographic region, back the second finding at a conceptual level.
Finally, this study links these changes in extreme precipitation to a fundamental increase
in atmospheric water vapor upon global warming, which manifests itself in increased
cumulonimbus clouds and thus thunderstorm activity.
Groisman et al. 2004 reference earlier studies showing a century long mean
precipitation increase across much of the US (during the 20th century), with the exception
of parts of the southeast and southwest. Much of the increase was confined to spring,
summer, and fall. Our future projected daily P-E mean change map generally agrees with
these results annually, but shows much of the increases occurring during the winter.
Furthermore, this study shows that while mean total precipitation has increased over
much of the country, the increases in heavy and very heavy (>95th and >99th percentiles
respectively) precipitation were more significant, and the proportion of total precipitation
attributed to the extreme events has also increased. They also show that these trends are
most notable in the eastern two thirds of the country, primarily in the warm season. Our
future projections show an increase in heavy (>95th percentile) and very heavy (> 99th
percentile) precipitation (P-E) in the northern and eastern parts of the country, with most
the increase in the east occurring in the cold season.
The IPCC report references studies (Emori and Brown 2005 and Meehl et al.
2005) that describe physical processes which influence the increase in mean and extreme
precipitation in a warmer climate. One process is thermodynamically related, involving
an increase in atmospheric water vapor and thus thunderstorm activity, particularly over
the subtropics. Another process is dynamically related, involving changes in circulation
contributing to the pattern of precipitation intensity in middle and high latitudes.
Emori and Brown 2005 Abstract:
Extreme precipitation has been projected to increase more than the mean under
future changed climate, but its mechanism is not clear. We have separated the 'dynamic'
and 'thermodynamic' components of the mean and extreme precipitation changes
projected in 6 climate model experiments. The dynamic change is due to the change in
atmospheric motion, while the thermodynamic change is due to the change in
atmospheric moisture content. The model results consistently show that there are areas
with small change or decreases in the thermodynamic change for mean precipitation
mainly over subtropics, while the thermodynamic change for extreme precipitation is an
overall increase as a result of increased atmospheric moisture. The dynamic changes play
a secondary role in the difference between mean and extreme and are limited to lower
latitudes. Over many parts of mid- to high latitudes, mean and extreme precipitation
increase in comparable magnitude due to a comparable thermodynamic increase.
Meehl et. Al. 2005 Abstract:
In a future climate warmed by increased greenhouse gases, increases of
precipitation intensity do not have a uniform spatial distribution. Here we analyze a
multi-model AOGCM data set to examine processes that produce the geographic pattern
of these precipitation intensity changes over land. In the tropics, general increases in
water vapor associated with positive SST anomalies in the warmer climate produce
increased precipitation intensity over most land areas. In the midlatitudes, the pattern of
precipitation intensity increase is related in part to the increased water vapor being
carried to areas of mean moisture convergence to produce greater precipitation, as well as
to changes in atmospheric circulation. Advective effects, indicated by sea level pressure
changes, contribute to greatest precipitation intensity increases (as well as mean
precipitation increases) over northwestern and northeastern North America, northern
Europe, northern Asia, the east coast of Asia, southeastern Australia, and south-central
South America.
References
Diffenbaugh NS, Bell JL, Sloan LC
Simulated changes in extreme temperature and precipitation events at 6 ka
PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY 236 (1-2):
151-168 JUN 23 2006
Diffenbaugh NS, Pal JS, Trapp RJ, et al.
Fine-scale processes regulate the response of extreme events to global climate change
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED
STATES OF AMERICA 102 (44): 15774-15778 NOV 1 2005
Leung LR, Qian Y, Bian XD, et al.
Mid-century ensemble regional climate change scenarios for the western United States
CLIMATIC CHANGE 62 (1-3): 75-113 JAN-FEB 2004
Bell JL, Sloan LC, Snyder MA
Regional changes in extreme climatic events: A future climate scenario
JOURNAL OF CLIMATE 17 (1): 81-87 JAN 2004
Tebaldi C, Hayhoe K, Arblaster JM, et al.
Going to the extremes
CLIMATIC CHANGE 79 (3-4): 185-211 DEC 2006
Räisänen, J., 2005a: Impact of increasing CO2 on monthly-to-annual precipitation
extremes: Analysis of the CMIP2 experiments. Clim. Dyn., 24, 309–323.
Groisman, P.Ya., et al., 2005: Trends in intense precipitation in the climate record. J.
Clim., 18, 1326–1350.
Groisman, P.Ya., et al., 2004: Contemporary changes of the hydrological cycle over the
contiguous United States: Trends derived from in situ observations. J. Hydrometeorol., 5,
64–85.
Meehl GA, Arblaster JM, Tebaldi C
Understanding future patterns of increased precipitation intensity in climate model
simulations
GEOPHYSICAL RESEARCH LETTERS 32 (18): Art. No. L18719 SEP 30 2005
Emori S, Brown SJ
Dynamic and thermodynamic changes in mean and extreme precipitation under changed
climate
GEOPHYSICAL RESEARCH LETTERS 32 (17): Art. No. L17706 SEP 13 2005